Research progress on regulating factors of muscle fiber heterogeneity in poultry: a review

Abstract

Control of meat quality traits is an important goal of any farm animal production, including poultry. A better understanding of the biochemical properties of muscle fiber properties that drive muscle development and ultimately meat quality constitutes one of the major challenging topics in animal production and meat science. In this paper, the existing classification methods of skeletal muscle fibers in poultry were reviewed and the relationship between contractile and metabolic characteristics of muscle fibers and poultry meat quality was described. Finally, a comprehensive review of multiple potential factors affecting muscle fiber distribution and conversion is presented, including breed, sex, hormones, growth performance, diet, muscle position, exercise, and ambient temperature. We emphasize that knowledge of muscle fiber typing is essential to better understand how to control muscle characteristics throughout the life cycle of animals to better manage the final quality of poultry meat.

INTRODUCTION

Poultry, which are the largest group of domesticated animals in the world, are farmed in vast numbers across the globe for their meat, eggs, and feathers. One of the most important physiological structures in the connective tissue of the poultry carcass is the skeletal muscle, which is the source of high-quality meat protein for humans. The selection of poultry for meat has always focused on growth rate and muscle yield, and in less than 40 y, the production time to grow a 1.8 kg and 30 kg broiler and turkey has been reduced to 5 wk and 20 wk, respectively (Zuidhof et al., 2014). However, changes in consumer attitudes in most countries have shifted meat consumption from “survival” to “enjoyment”. Therefore, the meat industry should always produce and supply to meet consumer demand for cheap, flavorful, and high-quality meat to ensure the healthy and sustainable development of the meat industry (Caldas-Cueva and Owens, 2020). To produce quality meat, it is necessary to understand the characteristics of muscle traits and the factors that influence them.

For a long time, different muscles were classified as white or red based on their appearance and color, and as slow or fast based on their contractile properties (Zierath and Hawley, 2004). Currently, market demand and breeding practices tend to favor a greater proportion of “red” meat with slow contractile properties. Changes in muscle adaptation, known as muscle plasticity, reflect alterations in the structure and function of various muscle fiber components, which in turn determine the characteristics of the muscle fibers. Both specialization and plasticity are rooted in the heterogeneity of muscle fibers and their dynamic and changeable states (Pette and Staron, 1997). The functional properties of a muscle depend on its fiber type composition (FTC), and the muscle adapts to new functional requirements as its composition changes. However, due to differences in food culture, consumption habits, social development, or cooking methods, there is a lack of comprehensive reviews focusing on the influencing factors and conversion mechanisms of poultry muscle fiber characteristics. In this context, this paper attempts to review the related aspects of muscle fiber and its plasticity in poultry from the perspective of individual differences, breed differences, living environment, pathological conditions and dietary interventions.

In addition to the major differences in reproductive patterns, there are also differences in genome evolution (Kapusta et al., 2017), embryonic development (Mittwoch, 1998), muscle development and regeneration(Maier, 1997), and hormone regulation (Kriegsfeld et al., 2015) between birds and mammals. Therefore, the database for this review was prepared using computerized searches of various databases, including PubMed, Google Scholar, and Web of Science. We focused our search on poultry within the numerous papers on muscle fiber types and meat quality associated with domesticated animals. Our search employed several keywords, including “fibre” or “fiber”, “muscle fiber type” or “muscle myofiber”, “myofiber transformation” or “myofiber conversion”, “poultry”, “broiler” or “birds”, “chicken” or “gallus”, “feed restriction” or “dietary intervention” or “feed deprivation”. The literature search was limited to full-text articles published in peer-reviewed journals.

MUSCLE FIBER TYPE

Skeletal muscle is composed of a variety of tissues, including muscle fibers, connective tissue, and adipose tissue (Listrat et al., 2016). Muscle fibers are long, cylindrical, mitotic, multinucleated cells that serve as the primary structural unit of skeletal muscle, making up nearly 90% of its volume (Dumont et al., 2015). The diversity in the structure, function, and metabolic characteristics of muscle fibers within the same muscle or between different muscles is termed muscle fiber heterogeneity. For a long time, the skeletal muscles of vertebrates have been divided into dark red slow muscles and light white fast muscles (Matarneh et al. 2021). In general, muscle fibers are mainly classified by comparing their contraction rates and metabolic types using histological and physiological methods. During skeletal muscle contraction, the chemical energy of adenosine-triphosphate (ATP) is converted into mechanical energy under the action of ATP hydrolase, which drives the cross-bridge of actin to complete muscle contraction. The ATP hydrolysis rate of the myosin heavy chain (MyHC) subtype closely determines its contraction rate (Bárány, 1967). Currently, the most widely accepted muscle fiber type markers (MyHC subtypes) are MyHC type I, IIA, IIX(D), and IIB, which are encoded by MYH7, MYH2, MYH1, and MYH4 genes, respectively (Brooke and Kaiser, 1970; Schiaffino et al., 1989). The classification of muscle fibers in poultry is somewhat unique, with their IIX(D) type muscle fibers being relatively rare (Bandman and Rosser, 2000). In addition, studies have also reported other types of muscle fibers unique to poultry, such as type IIIA and type IIIB multitonic innervation slow fibers (Barnard et al., 1982). These are distributed in the anterior latissimus dorsi muscle, deep adductor muscle, and plantar muscle. Beyond the differences in contractile characteristics dominated by MyHC subtypes, muscle fibers also vary in their types of energy metabolism. The oxidative and glycolytic pathways in muscle fibers are tightly regulated to ensure that ATP production meets the tissue's needs (Bourdeau Julien et al., 2018). There are two main pathways for ATP regeneration in muscle, the aerobic and anaerobic pathways of pyruvate metabolism. In simple terms, the aerobic pathway oxidizes pyruvate in the mitochondria and produces the characteristics of oxidized muscle fiber types (lean; red; rich in myoglobin, the sarcoplasmic heme protein responsible for flesh color); the anaerobic pathway produces lactic acid through glycolysis in the sarcoplasm and produces the characteristics of glycolytic muscle fibers (thick, white; almost no myoglobin) (Listrat et al., 2016; Pette and Staron, 2001; Schiaffino and Reggiani, 1996). Existing studies have classified fibers into slow oxidation, fast oxidative glycolysis, and fast glycolysis, corresponding to type I, IIA, and IIB fibers, respectively (Gauthier, 1969; Peter et al., 1972). However, no studies have clearly defined whether the type III fibers in chickens are oxidative or glycolytic (Remignon et al., 1994). Recent research strongly suggests that type III fibers exhibit characteristics of oxidative fibers based on the expression of glycolysis and lipolysis genes (Saneyasu et al., 2015). The succinate dehydrogenase (SDH) activity method is an unconventional approach for identifying fast muscle fiber types, with IIR corresponding to IIA and IIW corresponding to IIB (Sakakibara et al., 2000). Additionally, mitochondrial phenotype and structure can indirectly distinguish muscle fiber types. For instance, in type I muscle fibers, mitochondria are large, densely packed, and contain lipid droplets, whereas in type IIB muscle fibers, mitochondria are small, sparse, and lack lipid droplets are absent (Hosotani et al., 2021). With advancements in detection methods and technological innovations, new specific MYH subtype fibers are slowly emerging in addition to these conventional muscle fiber types. Several excellent reviews have discussed emerging MHC subtypes such as MHCeom, MHCIIm, and MHCIton (Pette and Staron, 1997, Pette and Staron, 2000; Schiaffino and Reggiani, 2011), which will not be detailed here. In our previous studies, chicken muscle fiber properties were characterized using different slice staining methods (Figure 1).

Figure 1.

Cross section staining of muscle fibers of chicken soleus muscle. H&E, Immunofluorescence, ATPase, and SDH staining. The arrows point to different types of muscle fibers. (Dong hao Zhang unpublished data).

MUSCLE FIBER CHARACTERISTICS AFFECT MEAT QUALITY

Meat quality refers to the comprehensive embodiment of physical and chemical properties related to the appearance, palatability, and nutritional value of fresh or processed meat (Matarneh et al., 2021). Important factors determining the quality of poultry meat include its complex ingredients, but also the degree of consumer preference for meat products (Ismail and Joo, 2017). It is widely accepted that muscle fiber type composition is a significant source of variation in fresh meat quality, especially in large livestock (Lefaucheur, 2010). Like mammals, the skeletal muscles of poultry are inherently heterogeneous. The heterogeneity of muscle fibers and the proportion of each fiber type in muscle tissue greatly influence the intrinsic characteristics of poultry meat. Consumers heavily rely on the color of fresh meat as an indicator of freshness and health, with a brighter cherry red color increasing the appeal of fresh meat products and boosting sales (Suman and Joseph, 2013). Poultry leg muscles have a redder color than breast muscles, attributed to the significant differences in muscle fiber composition between various muscle tissues. The proportion of oxidative fibers in the soles (SOL) of broilers' legs is as high as 76 to 79% (Du et al., 2017). Other studies have found that the pectoralis major (PM) muscle of broilers is composed only of type IIB muscle fibers, with no variation between populations (Roy et al., 2006; Verdiglione and Cassandro, 2013; Lilburn et al., 2019). Recent findings by Weng et al. (2022) also supported this conclusion.

Meat consumables are products of transportation and processing. Under poor preservation conditions, a higher proportion of type I fibers will reduce color stability and may change to brown, iron-rich red protein (Joo et al., 2013). The rate and extent of pH decrease after slaughter are the two most influential factors in determining overall meat quality (Schiaffino and Reggiani, 2011). The pH changes not only impact meat color but also affect muscle water content and proteolytic rate, serving as indicators of meat quality. Postmortem pH change and final value are regulated by the rate of ATP hydrolysis by muscle ATPase, with muscle fiber type significantly influencing this process (Scopes, 1974). In general, muscle pH declines more slowly after death in ruminants than in swine and chickens, attributed to the higher abundance of slow-oxidizing muscle fibers in ruminants (Matarneh et al., 2021). Recent studies have shown that the pH of the iliotibial (IL) muscle is significantly higher than that of the PM muscle at all stages after slaughter (Cheng et al., 2022). Notably, the PSE meat phenomenon, which has garnered significant attention in poultry, is largely due to a rapid decrease in pH of the PM muscle after death, accompanied by specific characteristics (Karunanayaka et al., 2016; Owens et al., 2000). The PSE phenomenon in domestic turkeys is also mainly attributed to the higher lactic acid concentrations and lower hydraulic capacity associated with muscle composed of type IIB fibers (Pietrzak et al., 1997).

The content of connective tissue, including intramuscular fat and collagen, also varies with muscle fiber characteristics (Klont, Brocks, and Eikelenboom, 1998). Weng et al. (2022) reported that the proportion of type I fibers was positively correlated with intramuscular fat content, whether comparing pectoralis major and leg muscles with a large difference in the proportion of muscle fiber types or comparing chickens with different growth rate. They also showed that slow-growing broilers with a high proportion of type I fibers had higher protein and collagen content in their pectoral muscles. However, it remains to be seen whether red oxidized muscle contains more collagen than white glycolytic muscle. In addition, the relationship between muscle fiber type composition and meat tenderness is still controversial, and no clear relationship between collagen content and FTC has been reported in livestock species (Lefaucheur, 2010). Sakakibara et al. (2000) made a comprehensive comparison using several indicators and concluded that total collagen content was affected by muscle location in the body and sex rather than fiber composition. In recent years, other reviews have addressed various other aspects of muscle fiber characters-mediated meat quality traits, including tenderness, cooking loss, flavor, fatty acid composition, and chemical composition. (Maltin et al., 2003; Joo et al., 2013; Listrat et al., 2016; Ismail and Joo, 2017; Matarneh et al., 2021). The relative composition of fiber types in muscle determines the overall metabolism of muscle during postmortem transformation and ultimately its quality as fresh meat. For this reason, the pursuit of meat quality will ultimately be based on genetic selection and environmental control that affect the transformation of fiber types.

REGULATING FACTORS OF MUSCLE FIBER HETEROGENEITY

Individual Characteristics

Breed

As a result of long-term domestication and selection processes, the muscle phenotypes of various carnivores have changed, resulting in extreme differences in muscle fiber characteristics. Generally, the skeletal muscle of domesticated birds contains more and coarser glycolytic fibers and fewer oxidative fibers compared to that of wild birds. While the effect of exercise differences caused by survival pressure on muscle fiber transformation is not the focus here, growth rate is a more compelling influence. Dransfield and Sosnicki previously reported that higher growth rates in poultry might induce muscle morphological abnormalities, resulting in larger fiber diameters and a higher proportion of glycolytic fibers (Dransfield and Sosnicki, 1999). Studies have shown that the proportion of oxidative fibers in the leg of slow-growing broilers is significantly higher than in fast-growing broilers, accompanied by increased firmness, intramuscular fat content, and redness score (Huo et al., 2022; Weng et al., 2022). At the same dietary energy level, the Beijing You Chicken, a local high-quality broiler breed, also has more red and intermediate fibers and less white fibers compared to mature commercial Arbor Acres (AA) broilers (Zhao et al., 2012). These shifts in muscle metabolism from oxidation to glycolysis may reflect breeding selection for higher muscle growth rates in modern livestock production. However, rate-dependent crossbreeding selection in broilers has significantly increased muscle mass while decreasing skeletal muscle glycogen reserve and pH, leading to degenerative diseases in fast-growing broilers.

Similar to the differences between fast and slow growing chickens, broilers and laying hens differ in fiber size and fiber type composition, with Type II fibers being more prevalent than Type I fibers (Aberle and Stewart, 1983). However, for the same body weight, broilers and laying hens have similar proportions of various muscle fiber types. In addition, although the proportion of fast muscle fibers in the sartorius muscle of broilers is lower than that of laying hens, while the proportion of glycolytic fiber (IIB) in the fast muscle fibers of broilers is significantly higher than that of laying hens, resulting in the muscle likely to be more anaerobic. In Japan, there is a large difference proportion of IIR fiber in IL muscle of common broiler chickens and several local breeds (Shamo; Kumanmoto Cochin; Silkie; White Leghor) (Sakakibara et al., 2000). However, it is worth considering that the age of each breed of chickens in these studies were not uniform, so differences in age and development stage may also contribute to the variations in the proportion of muscle fiber types. A study by Remignon et al. (1995) presents an opposing view, claiming that fiber type is primarily related to muscle type and function is not influenced by growth rate. They reject the idea that muscle gain in mammals necessitates a switch in muscle fiber types, which warrants further discussion. Of course, the differences in muscle fiber characteristics between different bird species are also large. For example, duck meat is classified as red meat because its pectoral muscle contains 73.3% Type IIA fiber in addition to 26.7% Type IIB fiber, unlike chicken PM muscle, which is composed almost entirely of Type IIB fibers (Janiszewski et al., 2018). Furthermore, there are notable differences among duck breeds. Mallard ducks and Pekingese ducks have lower proportions of red muscle fiber (85-84% vs. 90% for Muscovy ducks) and higher proportions of white muscle fiber (15-16% vs. 10% for Muscovy ducks) (Torrella et al., 1998).

Sex and Hormones

Sex selection in domestic animals is an unavoidable issue in the development of animal husbandry, particularly in dairy cows and laying hens (Gautron et al., 2021). There are consistent sex differences in muscle fiber characteristics. For instance, in quail, males have heavier muscles and a higher percentage of type IIB fibers compared to females (64 vs. 51%) (Choi et al., 2016b). Type IIB fibers grow about twice as fast as type I fibers after birth, which may partly explain the greater muscle mass observed in adult males. Sakakibara et al. (2000) demonstrated that the frequency of IIR fibers in the PM muscle and IL muscle of cocks was significantly higher than that of hens in two breeds of broilers evaluated. However, the PM muscle in chickens, which has strong muscle fiber type specificity, appears to be unaffected by genotypic and sex interactions (Verdiglione and Cassandro, 2013). A study examining the histochemical characteristics of sternal (ST) muscle fibers from several different sexually dimorphic anseriformes (mallard, pekingese, muscovy, and goose) revealed sexual dimorphism in muscle fiber types (Lalatta-Costerbosa et al., 1990). For example, type I, IIA, IIB, and IIIA fibers were found in the ST muscles of male mallard ducks and Pekingese ducks, whereas type IIIA fibers were not observed in females. Female Muscovy ducks have more type I fibers than males, and the proportion of each fiber type also differs significantly between sexes. Baéza et al. (1999) claimed that sex had no significant effect on the percentage of each fiber type, and that the difference in muscle weight between the sexes could be explained by the larger size and/or greater total number of fibers in male muscle.

Some hormones have a profound effect on the muscle fiber characteristics of specific muscles. Differences in hormones, particularly testosterone, can lead to sex differences in the size of specific fiber types, ultimately affecting the relative proportions of MHC subtypes (Staron et al., 2000). Recent studies have shown that testosterone supplementation can stimulate myoblast proliferation in chicken embryos, indirectly or directly promotes skeletal muscle growth and hypertrophy (Li et al., 2020). Similar to sex hormones, anabolic androgenic steroids can reduce the oxidative capacity of skeletal muscle by inducing the conversion of muscle fibers from the slow oxidative type to the fast glycolytic type (Holmäng et al., 1990). Oral methadone increased carcass and muscle weight and induced hypertrophy of glycolytic PM muscle fibers in adult female white steroid-horned chickens, but had no significant effect on anterior latissimus dorsi muscle fibers (Asfour et al., 2021). It is widely believed that thyroid hormone can significantly alter muscle fiber phenotypes. Hypothyroidism leads to the preferential expression of slow fiber types, whereas hyperthyroidism increases the content of fast muscle fibers in rats (Vadászová et al., 2004). The study by Li et al. (2007) showed that a decrease in thyroid hormone levels in broilers at two different developmental stages resulted in a reduction in the cross-sectional area of all muscle fiber types in the gastrocnemius, though the difference in slow fiber was not significant. Differences in thyroid hormone receptor content between sexes also provide an explanation for sex differences in fiber types (Dainat et al., 1986). Insulin-like growth factor-1 (IGF-1) is a crucial hormone for mammalian skeletal muscle growth, with its concentration negatively correlated with the proportion of type I fibers and positively correlated with type IIB fibers (Lamberson et al., 1996). Unfortunately, specific studies on whether IGF-1 is involved in skeletal muscle fiber conversion in poultry are lacking. Current research has focused more on how IGF-1 affects skeletal muscle growth rate (Guernec et al., 2003) and regulates myogenesis in vitro (Yu et al., 2015). Oudin et al. (1998) demonstrated a low level of IGF-1R in the pectoral muscles (type IIB) and leg muscles (types I and IIA) of broilers from 1 to 7 wk of age. This finding suggests that IGF-1′s role may depend on the growth cycle of chickens. However, Abdalhag et al. (2016) observed specific IGF-1 expression patterns in the pectoral and leg muscles of male and female chickens, with relatively higher IGF-1 expression in leg muscles with a high type I fiber content compared to pectoral muscles. This differs from the pattern of IGF-1 expression in mammalian muscle fibers. It is important to consider that the IGF-1 receptor regulates muscle growth in a paracrine/autocrine manner, so the detected high expression of IGF-1 may act on other parts of the muscle. A recent study found that type I fibers in chicken lateral femur muscle express both IGF-1 and IGF-1R, promoting type I fiber hypertrophy under common conditions (Nagasao et al., 2022). Therefore, it is likely that IGF-1 regulates the type conversion of poultry muscle fibers, but the specific regulatory relationship and mechanism remain to be discovered.

Caponization, the surgical removal of the testicles (castration), has been used to produce larger birds and better meat products (Gesek et al., 2017). The removal of the testicles leads to a lack of androgens and changes in lipid metabolism, resulting in alterations in poultry phenotype and meat quality. However, existing studies indicate that castration has no significant effect on the conversion of muscle fiber types in male chickens. For instance, a study by Zeng et al. (2020) demonstrated that castration altered fat deposition and muscle fiber diameter in both breast muscle and thigh. The expression level of genes related to muscle fiber types were also affected to some extent, but the overall muscle fiber ratio remained unchanged. Similarly, research on castrated Leghorn cocks (Gesek et al, 2019) and male partridges (Gesek et al., 2017) showed that these birds still retained only type IIB muscle fibers in their pectoral muscles despite castration. Despite its potential value, castrated meat is not widely available in the market due to high production costs (purchase of roosters, long production cycle, and high castration costs) and only some local varieties have been applied.

Age

The differential expression patterns of these MyHC gene subtypes are established early in muscle formation, thus determining the regulatory mechanisms of muscle fiber types. The embryonic period of poultry is crucial for muscle development and muscle fiber type determination. During this period, primary muscle fibers expressing specific MyHC subtypes develop between d 3 and 7 of embryonic phase, while secondary muscle fibers form between d 8 and 14 and attach to the primary fibers (Hammond et al., 2007).Primary muscle fibers mature into type I fibers in most muscles, such as the PM muscle of birds, while secondary muscle fibers primarily mature into type II fibers. Studies have shown that the intrafusal fibers in chicken leg muscle are fast-twitch, the percentage of slow-twitch fibers increases with development until the they are approximately equal before birth (Maier, 1993). After birth, the proportion of fast-twitch fibers gradually increases, indicating a shift from slow to fast muscle fibers. Saneyasu et al. (2017) also found significant changes in the MyHC subtype gene and Tn1 gene from oxidative to glycolytic types in the PM muscle of hens during the first week post-hatching, revealing the basis for physiological and chemical changes in the muscle fibers post-hatching. It is generally believed that increasing age after birth is also crucial for fiber type transitions. Intermediate fibers (IIA) typically transform into white fibers (IIB) (Ashmore and Doerr, 1971). Smith suggested that both the type and diameter of chicken breast muscle fibers are significantly affected by age and intramuscular location (Smith and Fletcher, 1988). Additionally, in both layers and broilers, the proportion of type IIA muscle fibers decreases with growth, while the proportion of type IIB muscle fibers increases (Aberle and Stewart, 1983). Ono et al. (1993) also showed that from 1 wk to 35 wk of age, the ratio of type IIB fibers to type IIA fibers in chicken breast muscle changed from 1:1 to 4:1.

Therefore, from the embryonic stage to the transformation of chicken muscle fiber types after birth, the general trend can be summarized as: fast → slow → fast. This progression explains many biological phenomena. The increase in the number of slow muscle fibers before birth implies the rapid development of leg muscles, which is crucial for foraging and escape behaviors post-hatching. The subsequent increase in the proportion of fast muscle fibers after birth can be attributed to growth rate-dependent selection pressure, as the rate and diameter of muscle fiber growth vary between muscles and fiber types. In general, the greatest growth occurs in type IIB fibers of hindlimb muscles (Ono et al., 1993).

Muscle Position in Body

Different muscle tissues, and even different positions within the same muscle tissue, exhibit distinct physiological functions, leading to heterogeneity of muscle fibers (Figure 2). Functional differences between different regions of the vertebrate body result in different muscle fiber compositions, which function at different rates of shortening and contraction to maximize muscle force output and efficiency (Rome et al., 1988). For example, muscles involved in fast, precise, but short-duration movements are typically composed of fast muscle fibers, while those used in prolonged but slow or stationary postures primarily consist of slow muscle fibers (Schiaffino and Reggiani, 1994). The PM muscle is the largest component of the shoulder girdle muscles in birds and serves as the primary propulsive organ for flight, enabling rapid and forceful wing movements. A large number of studies have shown that the PM muscle of poultry is predominantly composed of glycolytic fibers, including chicken (Ono et al., 1993; Roy et al., 2006), duck (Huo et al., 2021), goose (Weng et al., 2021), turkey (Lilburn et al., 2019), and quail (Choi et al., 2013). The superficial adductor muscle mainly consists of oxidized type IIA fibers, and varying proportions of fiber types have been observed in the other muscles studied (Barnard et al., 1982). In contrast, the IL muscles are part of the thigh muscles, aiding in terrestrial locomotion. Differences in muscle fiber composition between PM (100% fast type) and IL (88.85% fast type and 11.15% slow type) result in differences in proteolysis (Cheng et al., 2022). In addition, SOL muscle is composed of 76 to 79% oxidized fibers (Du et al., 2017). Another study indicated that the lateral IL muscle, found in chicken leg muscles, is a typical fast muscle composed exclusively of type II muscle fibers (Iwamoto et al, 1993). Moreover, previous studies by Suzuki et al. (1985) distinguished various parts of the chicken leg, such as the lateralis femoris tibialis muscle, the pectineus muscle, and the ilioperoneus muscle, showing significant differences in muscle fiber types across different regions.

Figure 2.

Anatomical and histological analysis of chicken skeletal muscle. (A) Representative images of ten chicken skeletal muscles. (B) Succinate dehydrogenase staining of muscles (Kui et al., 2022).

In general, deep muscles involved in maintaining posture are more susceptible to oxidation and contain more type I muscle fibers than surface muscles involved in rapid movement (Rosser et al., 1992). Horák et al. (1989) identified the differences in muscle fiber types caused by the varying depths of the sartorius and hemimembrane muscles in chicken legs. In the superficial areas, only the fast oxidative glycolytic and fast glycolytic types were present, while in the deep muscle, the slow oxidative type appeared. This reflects the influence of common muscle functions on muscle fiber types, as deep leg muscles are often used for maintaining posture and sustained contraction. This is similar to the SOL muscles, which have a high proportion of oxidative muscle fibers. In contrast, superficial muscles are more involved in flexible movement and explosive power, such as supporting take-off. In chickens, the puboischiomedial femur (PIF) muscle is one of the deeper muscles of the trunk. Since the histochemical properties of type I fibers effectively respond to the relative development of PIF muscles of different varieties, it is concluded that PIF muscles play a role in supporting body weight and maintaining posture (Iwamoto et al., 1993). In geese, Weng et al. (2021) found that the distribution of slow muscle fibers in the gastrocnemius was significantly higher than that in the SOL muscle at birth (16.69 vs. 4.74%), while only fast muscle fibers were present in the extensor digital longus.

These findings underscore the intricate relationship between muscle function, depth, and fiber type composition. Deep muscles, tasked with maintaining posture and supporting sustained activity, predominantly feature slow oxidative fibers. In contrast, surface muscles, which are designed for rapid and powerful movements, are rich in fast glycolytic fibers. This functional specialization highlights the adaptability of muscle tissue in response to the physiological demands placed upon it.

Although the PM muscle of poultry is commonly believed to be composed mainly of fast muscle fibers, the proportion of MHC subtypes varies among different breeds. Research has shown that, at a certain growth stage, duck breast muscle contains 73.3% type IIB and 26.7% type IIA muscle fibers, while chicken breast muscle only contains type IIB muscle fibers (Ismail and Joo, 2017). The focus on a specific stage is due to the variability of fiber types, which can differ depending on the age of the animal. It is important to note that Smith et al. (1992) found a significant proportion of slow muscle fibers in the pectoral muscle of Peking ducks, which may be attributed to their method of fiber type classification. Additionally, various skeletal muscles display significant transcriptional heterogeneity. A recent study analyzed the transcriptome profiles of ten different skeletal muscles in white Plymouth Rock chickens and measured the proportion of muscle fiber types at each site (Kui et al., 2022). The study found that differences in muscle fiber types between different parts of the chicken resulted in regional differentiation in growth rates. From hatching until one week of age, chickens experience the most significant muscle growth in their forelimbs. This can be attributed, in part, to the large difference in muscle fiber ratio between the pectoral and leg muscles. The proportion of muscle fibers varies within different parts of the same muscle type that perform different functions. For instance, the gastrocnemius and the PIF muscle are divided into the lateral head and the medial head. The medial head contains a larger proportion of slow fibers than the lateral head due to their different functions in helping birds take off or stand (Iwamoto et al., 1993; Shelton and Bandman, 1985).

Hummingbirds represent an extreme case in muscle fiber adaptation, as they have lost type IIB muscle fibers in the gastrocnemius due to the degeneration of hind limb function (Welch and Altshuler, 2009). This adaptation is likely due to the evolutionary pressures favoring flight over ground locomotion. In contrast, the extraocular muscles are a unique group of specialized muscles that differ from other skeletal muscles in anatomy, physiology, metabolism, and fiber type classification (Fischer et al., 2005). These muscles are specialized for rapid, precise eye movements and exhibit a unique composition. Baryshnikova et al. (2007) investigated the development of the extraocular muscle of the upper strabismus in chickens and identified 4 types of muscle fibers (type I-IV) similar to those found in mammals. The dorsal longus muscles contain all three types of fibers. The tail end of the femoris tibialis medius is composed of type I and type IIA fibers, while the tail of the lateral iliotibial muscle and the femoral mediotibial muscle are composed of type IIA and type IIB fibers (Ono et al., 1993). This diversity in muscle fiber composition is essential for optimizing muscle function and efficiency.

Understanding the properties of muscle fibers in individual muscles is critical for producing high-quality meat, which is increasingly relevant in the meat market. The industry in many countries has shown a trend towards selling individual muscle parts to meet consumer preferences (Park et al., 2021; Singh et al., 2021).

Feeding System

Currently, caging is the primary method of managing poultry production due to its cost-saving benefits, ease of management, and high efficiency. However, high-density confinement can cause significant stress to birds, resulting in fatty liver syndrome, panic disorder, and pecking (Buijs et al., 2009). Consequently, the use of a free-range feeding system in the modern broiler industry has become a popular method to increase exercise and improve the quality of poultry meat. Skeletal muscle has the ability to undergo significant adaptive changes during use and disuse, including changes in fiber size (such as muscle hypertrophy) and fiber type (such as fast to slow fiber type switching), which can affect muscle strength and endurance (Pette and Staron, 1997). Endurance training can change the type of muscle fiber from fast to slow (Díaz-Herrera et al., 2001). For example, the lateral IL muscle is primarily composed of type IIA and type IIB fibers, but exercise can increase the IIA:IIB fiber ratio from 0.77:1 to 1.1:1 (Brackenbury and Holloway, 1991). Type I muscle fibers were identified using SDH. Research has demonstrated that treadmill training can considerably increase the proportion of SDH-positive (slow) fibers in the lateral IL muscle of chickens and this change has a significant correlation with exercise duration (Brackenbury and Williamson, 1989). In ducks, artificial swimming training not only increased the activity of oxidase citrate synthase in tufted ducks but also changed the fiber type in the lateral gastrocnemius from slow to fast (Butler and Turner, 1988). This indicates a remarkable plasticity in muscle fiber types in response to different types of exercise.

For poultry managed under free-range systems, similar adaptive changes in muscle fibers have been observed. For example, the proportion of fast muscle fibers in the thigh muscle samples of free-range Beijing oil chickens decreased significantly at both 35 and 40 weeks of age, indicating that the free-range system can promote the conversion of fast muscle fibers into slow muscle fibers (Fu et al., 2015). The free-range model mainly improves meat quality by promoting the formation of slow muscle fibers and intermuscular fat in chicken thigh muscles (Cheng et al., 2023). Under the free-range condition, the proportion of slow muscle fibers also increased with the increase in the time spent outdoors. However, while it is clear that free-range systems and exercise can promote the conversion of fast muscle fibers into slow muscle fibers, no study has clearly demonstrated a linear relationship between the duration of free-range activity of poultry and the proportion of slow muscle fibers. It is worth considering that the increase in exercise may have indirectly contributed to the increase in feed to meat ratio, which has a huge impact on the economic traits of poultry. Jin et al. (2019) noted that while the free-range system offers advantages in terms of pectoralis muscle yield, meat quality, and serum biochemical indicators of Wannan yellow chickens, it negatively effects growth performance. Additionally, although there is evidence that exercise enhances muscle oxidative capacity, lower intensity and shorter duration exercise have minimal impact on skeletal muscle growth and development in birds (Brackenbury and Holloway, 1991). In general, exercise-induced muscle fiber type switching in poultry is a long-term and complex process, influenced by factors such as increasing age and capillary density. Understanding the mechanisms of fiber type switching and the manipulation of myofiber type composition in individual muscles through exercise has become increasingly important to the meat industry. This is due to the growing development of “organic” animal diets and rising consumer concerns about animal welfare.

Ambient Temperature

Low temperature is a significant environmental factor that restricts animal growth and may endanger their survival (Xu et al., 2021). In cold environments, mammals and birds maintain body temperature by increasing thermogenesis (Chaffee and Roberts, 1971). Since birds lack brown adipose tissue, which is the primary thermogenic organ in mammals, skeletal muscle becomes a crucial thermogenic tissue in birds (Bicudo et al., 2001; Argyropoulos and Harper, 2002). Birds have developed the adaptive ability to increase skeletal muscle mass and metabolic rate to cope with cold temperatures (Cooper, 2002). For example, Muscovy ducks increase both their skeletal muscle mass and convert their fast muscle fibers to slow muscle fibers to enhance muscle heat production (Barre et al., 1985; Duchamp et al., 1992). Cold-adapted chickens showed an increase in the number of type IIA fibers in their pectoral muscles, while control chickens only had primitive type IIB fibers (Ueda et al., 2005). Peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) plays a central role in the regulation of cellular energy metabolism and can reduce the expression of fast muscle fiber specific genes in both mammals and birds (Mortensen et al., 2006; Shu et al., 2014). Studies have demonstrated that exposure to cold temperatures can induce the transition from rapid twitch to slow twitch muscle fibers by up-regulating PGC-1α expression in chicken skeletal muscle fibers (Hirabayashi et al., 2005; Ueda et al., 2005; Ijiri et al., 2009a; Ijiri et al, 2009b). Mechanistically, PGC-1α promotes mitochondrial biogenesis, enhances muscle fiber metabolism, reduces glycolysis, and regulates carbohydrate and lipid metabolism during cold exposure. Nie et al. conducted a comprehensive review of the specific mechanism by which PGC-1α regulates mitochondrial biogenesis and skeletal muscle development (Kong et al., 2022). Myostatin is another important co-regulator (Ijiri et al., 2009a; Ijiri, Miura, et al., 2009), and its expression shows an opposite trend to PGC-1α under cold stimulation. Cold adaptation in poultry is regulated not only by muscle fiber turnover but also by the synergistic effect of lipid metabolism, non-shivering thermogenesis, body temperature maintenance, while there would be other factors. Short-term cold stimulation and long-term cold exposure have distinct effects on cold adaptation in birds. Specifically, long-term exposure to cold may enhance cold stress tolerance in chicks by increasing in vivo lipid oxidation (Dumonteil et al., 1994), accelerating feather growth (Ozkan et al., 2002), and facilitating age-dependent cold tolerance acquisition. The average altitude of the Qinghai-Tibet Plateau is over 4000 meters. The low temperature and oxygen levels in this environment have led to unique genetic adaptations in local animals. Zhang et al. (2019) identified MYH7 as a candidate gene for plateau adaptation in Tibetan chickens, which may explain the metabolic type conversion of muscle fiber in these chickens under hypoxia.

Hatching is a distinctive embryonic development mode observed in birds and other egg-laying animals, significantly influenced by changes in environmental temperature. Mild temperature increases due to environmental factors can induce the transformations in muscle fiber types in mammals, birds, and various cell lines derived from satellite cells. Research on quail has demonstrated that the metabolic state of quail myogenic cells changes significantly when incubated at higher temperatures, enabling them to maintain their slow-twitch fiber properties (Choi et al., 2016a). Similarly, C2C12 cells subjected to appropriate heat stress conditions exhibited a higher myotube area and percentage of slow muscle fibers three days after induction of differentiation than cells without heat stress (Yamaguchi et al., 2010). Additionally, postnatal heat stress can alter the density and diameter of chicken PM muscle fibers and cause damage to satellite cells through myogenesis (Piestun et al., 2017; Patael et al., 2019). Therefore, modifying the feeding system to increase exercise and implementing precise environmental temperature control has become a crucial method for regulating the fiber type composition of individual muscles in traditional feeding systems.

Dietary Regulation

Dietary nutrient levels are closely related to endocrine or paracrine mechanisms that regulate muscle growth and protein deposition, making them a primary environmental factor affecting muscle fiber development (Burke and Henry, 1997). White muscle fibers are significantly responsive to changes in nutrient composition or nutrient density, while red muscle fibers are less sensitive (Tesseraud et al., 1996). In the case of excess nutrients, especially lipids, adipose tissue may become overloaded, redirecting lipids to other tissues such as the heart, liver, and skeletal muscle, leading to lipotoxicity and significant interference with glucose utilization (Kalinkovich and Livshits, 2017). As a result, muscle fibers may become more glycolytic, experiencing rapid growth due to high nutrient feeding (Roy et al., 2006). Zhao et al. (2012) studied the changes in muscle fiber characteristics of broilers from two breeds under three different dietary nutrient densities: high, medium and low. They found that with the increase of nutrient density, the proportion of type I and type IIA muscle fibers decreased significantly, while the proportion of type IIB muscle fibers increased significantly. Other studies have shown that the hypertrophy of IL muscle in chickens fed a high-nutrient diet is caused by the transformation of smaller IIA or IIC muscle fibers into larger IIB muscle fibers, without affecting the collagen structure (Das et al., 2009; Roy et al., 2007). However, this effect appears to be breed-specific, as the results for AA broilers showed a completely opposite trend. This may be related to the increased fat, decreased muscle fiber diameter, and increased muscle fiber density caused by reduced nutrient density in AA broilers. However, the complete effect of dietary nutrient density on muscle metabolic state and fiber properties cannot be accurately explained due to the lack of oxidative substrates or enzymes of the complete Krebs cycle. Glycolytic muscle prefers glucose as an energy source, while oxidative muscles have a higher rate of fatty acid (FA) metabolism due to their higher mitochondrial density and higher levels of fatty acid transferase/differentiation cluster 36 (FAT/CD36) protein expression level (Schiaffino and Reggiani, 2011).

Feed restriction, an alternative method for controlling dietary nutrient levels, has been reported to impact the mitotic activity of satellite cells (Moore et al., 2005), suggesting its potential influence on skeletal muscle growth and muscle fiber properties in poultry. Li et al. (2007) discovered that early feed restriction delayed the transition of muscle fibers from slow to fast type in hybrid broilers, resulting in retarded muscle growth with long-term effects. Excessive muscle hypertrophy in fast-growing broilers is associated with both white striated muscle and lignified muscle myopathies, leading to reduced capillary density and hypoxia near the muscle fibers (Hoving-Bolink et al., 2000; Joiner et al., 2014). Moreover, individuals affected by these disorders exhibit overexpression of Ca-ATPase in their fast-twitch skeletal muscles (Soglia et al., 2016). A study investigating the regulation of muscle fiber degeneration through feed restriction demonstrated that appropriate implementation of this practice partially alleviated the occurrence of these conditions in broilers (Radaelli et al., 2017). This relief can be attributed to local hypoxia mitigation, increased intracellular calcium content, and facilitation of muscle fiber type conversion (Mutryn et al., 2015).

Dietary carnosine supplementation has been shown to positively influence the antioxidant capacity of chicken thigh muscle, with a linear increase in the proportion of MyHC I and IIA as carnosine levels increased. Conversely, there is a linear decrease in the proportion of MyHC IIB (Cong et al., 2017). Mechanistically, both carnosine and histidine regulate sarcoplasmic reticulum calcium channel activity in skeletal muscle (Batrukova and Rubtsov, 1997). Additionally, carnosine inhibits calcium pump uncoupling and Ca-ATPase inactivation, leading to an increase in Ca-ATPase activity and subsequent elevation of calcium ion concentrations (Gariballa and Sinclair, 2000). Higher concentrations of calcium ions can bind to calmodulin-calmodulin complexes and transmit specific signals to transcription factors responsible for type I fiber-specific gene expression. Furthermore, dietary L-arginine supplementation can selectively modulate protein turnover within specific muscle fibers, thereby enhancing protein concentrations and promoting muscle development specifically in the fast glycolytic muscles of chickens (R. Wang et al., 2022). Therefore, diet control represents a potential tool that can be combined with different feeding systems to manipulate the conversion of muscle fiber properties associated with meat quality.

The Internal and External Regulation Mechanism of Fiber Classification

Studies have demonstrated that both intrinsic and extrinsic mechanisms contribute to the classification of primary and secondary muscle fibers. The intrinsic mechanism, regulated by the MyHC isoform gene, is evident in the characteristics of muscle fibers formed by myoblast clones during early embryonic stages in chickens or quails (Miller and Stockdale, 1986). This is because primary myotubes, generated by myoblasts independently of the nervous system, exhibit different MyHC content. Clonal populations of single embryonic myoblasts differentiate into fiber populations expressing either a single fast MHC gene or both fast and slow MyHC genes. In vivo studies on the intrinsic mechanism of embryonic myofiber type formation involve reintroducing cloned myoblasts into limb buds of chicken embryos. The differentiated fibers formed express the same MyHC gene as those expressed during these myoblasts' differentiation process (DiMario et al., 1993). However, at later stages, the maturation and maintenance of fiber diversity in secondary myotubes, also produced by myoblasts, rely on innervation. For instance, denervated quail slow muscles show delayed or absent expression of slow MyHC subtype genes in vivo, indicating that neural tubes influence is crucial for primary and secondary myotube differentiation in avian limbs (Lefeuvre et al., 1996).

It is widely acknowledged that motor nerves play a crucial role as extrinsic determinants in muscle fiber growth, differentiation, and morphogenesis. Following partial denervation by the motor nerve, the chicken anterior latissimus dorsi (ALD) muscle, known for its slow contraction, and the fast-contracting posterior latissimus dorsi muscle can maintain their respective contractile and membrane properties for a short period (Gordon et al., 1981). Even in adult animals during natural development, the properties of muscle fibers are influenced by motor nerves. Similarly, in mammals after birth, the physiological and type characteristics of regenerated muscle fibers are believed to be determined by the innervating nerve type (Carlson, 1968), a concept that has been experimentally validated through cross-nerve reinnervation methods. In chickens, however, fast muscle fibers quickly convert into slow-twitch fibers when re-innervated by slow muscle nerves, while slow fibers remain completely resistant to influence of fast muscle nerves (Kikuchi et al., 1986). Syrový and Zelená (1975) also demonstrated that approximately two-thirds of ALD muscle fibers innervated by cross nerves change to the fast type under nerve influence, while about one-third remain slow. This phenomenon may be attributed to different responses exhibited by the intrinsic characteristics of muscles from various species towards foreign nerves. Similar findings have also been observed in frogs (Schmidt and Emser, 1985). As noted above, cross reinnervation and denervation showed a lack of complete fiber type conversion, suggesting that an intrinsic component of MyHC gene expression remains among fiber-specific regulators. A comprehensive study by Dimario et al. combined the intrinsic and extrinsic mechanisms of myofiber formation and, demonstrating that these two factors are inseparable in determination myofiber specificity. Additionally, myofiber subtype differentiation may also be influenced by the environment in which myofiber formation occurs (DiMario and Stockdale, 1997). The expression of MyHC isoforms in myotubes can be modulated by supplementing the in vitro culture medium of chicken myoblasts with extracts from fast or slow muscle. This phenomenon is significant for determination of fiber types affected by myoblast differentiation caused by in vivo microenvironment differences during muscle fiber regeneration after injury or atrophy. Moreover, low frequency electrical stimulation increased oxidative activity in all muscle fiber types, whereas high-frequency stimulation resulted in decreased oxidative activity (Khaskiye et al., 1987).Therefore, the muscle fiber types of normally innervated chicks changed with different electrical stimulation frequencies, indirectly proving that the activity pattern of the birds affected the difference between slow and fast muscle fiber types.

Genetic Regulatory Factors

Skeletal muscle fiber type is determined by various genetic factors. Although the physiological and functional differences between muscle fiber types have been extensively studied, the molecular regulatory mechanisms associated with the different muscle fiber types in the chicken remain poorly understood. Interestingly, the regulatory factors for chicken muscle fiber type conversion that have been screened and validated thus far are predominantly non-coding RNAs (ncRNAs). Research has demonstrated that ncRNAs, as regulators of muscle growth and development, play a crucial role in regulating skeletal muscle development and fiber type specification in chickens (Shi et al., 2022). Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PPARGC1A), a transcriptional coactivator, is widely recognized as a marker gene for oxidative muscle fibers (Lin et al., 2002). The miR-193b-3p, highly expressed in fast muscle fibers, inhibited PPARGC1A expression by directly binding to its 3′ UTR (Ma et al., 2022). Generally, miRNA regulates post-transcriptional gene expression primarily through targeted inhibition. Liu et al. (2020) found that miR-499-5p and miR-196-5p were highly abundant and significantly up-regulated in oxidized muscles (sartorius). These miRNAs target SOX6, a repressor of slow muscle-specific gene expression, and CALM1, a key component of cGMP-PKG and calcium signaling pathways, respectively, to regulate the formation of slow muscle fibers. Recent transcriptome sequencing analysis also confirms this conclusion at the omics level (Liu et al., 2022). One such regulator, miR-499-5p, is a well-known positive regulator for type I muscle fibers across species and has been found to influence muscle fiber type transformation in chickens by targeting multiple genes, including cysteine and glycine protein 3 (CSRP3) and calpain 3 (CAPN3) (Felício et al., 2013). The miR-1611 exhibits high expression levels in slow muscle fibers of chickens, facilitates the transition from fast to slow muscle fibers by mediating the regulatory effects of lncRNA-Six1 on Six1 (Ma et al., 2018). Zhang et al. (2022) identified a novel lncRNA, SMARCD3-OT1, in chicken skeletal muscle that promotes the conversion of slow muscle fibers. This conversion is achieved through the upregulation of genes associated with fast muscle fiber development and downregulation of genes related to slow muscle fiber development in myoblasts, as confirmed in vitro. Furthermore, circPTPN4, a recently identified circular RNA, functions as a molecular sponge for miR-499-3p, inhibiting mitochondrial biogenesis and activating the phenotype characteristic of fast-twitch muscles (Cai et al., 2022). Over the past three years, multi-omics sequencing studies focusing on oxidative and glycolytic muscle fibers in poultry have revealed numerous potential regulators (Liu et al., 2020; Wang et al., 2020; Ju et al., 2021; Liu et al., 2022) However, most of these findings require validation at the single-cell level to fully understand their roles and mechanisms in muscle fiber type regulation.

The striated muscle signaling protein and transcriptional regulator ANKRD2 is abundantly expressed in skeletal muscle and plays a crucial role in myogenesis, myogenic differentiation, and muscle stress adaptation (Cenni et al., 2019). Comprehensive analysis of ANKRD2 gene and protein expression in adult chickens has revealed its predominant expression in the red slow-oxidizing muscles of the leg, compared to the white fast-glycolytic muscles (Stamenkovic et al., 2020). Similarly, a whole transcriptome analysis of the Chinese Qingyuan partridge chicken showed that ANKRD2 expression in red oxidized SOL muscle was approximately 23-fold higher than in white glycolytic EDL muscle (Jingting et al., 2017). ANKRD2 actively participates in muscle remodeling, as evidenced by its significant downregulation when denervated soleus muscles shift towards a fast-muscle phenotype (McKoy et al., 2005). Previous studies have shown that exercise can induce conversion from fast to slow muscle fibers, with literature suggesting that chicken ANKRD2 expression responds to physical activity similarly to mammals (Lehti et al., 2009). Guo et al. (2019) discovered that ANKRD2 expression significantly increased only in the white glycolytic muscles of highly exercised chickens, but not in the PM muscles. The elevated ANKRD2 expression specifically in the skeletal muscles of athletic chickens may be associated with muscular adaptation and changes in fiber type composition, which are reflected in alterations in meat color.

The neuromuscular junction is a crucial link in the transmission of signals for muscle contraction, with acetylcholinesterase (AChE) terminates synaptic signal transmission by catalyzing the hydrolysis of acetylcholine (Massoulié et al., 2008). The expression and distribution of AChE in muscles depend on factors such as muscle fiber type, motor nerve contacts, and contractile activity. In chickens, slowly twitching muscles express G1, G4, and A12 forms of AChE, while fast muscles predominantly express only the A12 form (Barnard et al., 1982). The proline-rich membrane anchor (PRiMA), which organizes AChE into tetrameric spheroids, is anchored similarly to AChE (Perrier et al., 2002). Mok et al. (2009) demonstrated that two PRiMA isoforms - PRiMA I and II - are expressed in chicken muscles, with higher levels observed in slow muscle compared to fast muscle. These findings strongly suggest that PRiMA and G4AChE regulate fiber properties concurrently in muscle tissue and imply a role for PRiMA in directing G4AChE formation. Collagen (ColQ), the tail form of AChE collagen, consists of two different subunits encoded by transcripts Colq-1 and Colq-1A respectively. Their promoters exhibit differential expression patterns between slow (SOL) and fast (tibialis anterior) muscles (Ting et al., 2005) The myofiber type-specific expression pattern of ColQ transcripts is regulated by slow regulatory element (SURE) and the fast intronic regulatory element (FIRE), thereby explaining the specific expression pattern observed for the collagenous tail form of AChE within slow and fast muscle fibers.

CONCLUSION AND PROSPECT

In summary, extensive literature demonstrates the crucial significance of avian muscle fiber contractile and metabolic properties. Furthermore, it reveals that these muscle properties (i.e., oxidation and glycolysis) undergo dynamic changes throughout an animal's lifespan and along the continuum from farm to market for meat consumption due to a multitude of factors (Figure 3). By manipulating the properties of muscle fibers in birds, a gradual enhancement in meat quality can be achieved. This can be accomplished through poultry breeding techniques, utilization of specific genetic or transcriptional regulators, and control over growth performance and sex of birds. Significant alterations in muscle fiber type composition can also be attained by administering specific hormones and regulating energy levels and nutrient ratios during poultry growth. Moreover, exercise combined with ambient temperature control in conventional feeding systems can effectively regulate fiber type composition within individual muscles. In conclusion, manipulating muscle fiber properties by considering all the potential factors mentioned above may be the best way to achieve gradual improvement of meat quality.

Figure 3.

Influencing factors of muscle fiber heterogeneity.

At present, most studies have focused on poultry meat quality have not detailed muscle fiber characteristics, focusing primarily on different strains of fast-growing broilers. Poultry meat quality remains a vague and difficult-to-define concept, hindering the establishment of uniform evaluation criteria and subsequent genetic selection. Therefore, focusing on the control of muscle fiber type composition will enhance the precision and focus of breeding efforts. Unlike the more mature classifications of muscle fibers in mammals (I, IIA, IID, and IIB), there is no unified classification system for birds. The fiber typing method using specific monoclonal antibodies produced by each MyHC subtype, commonly used in mammals, may not be fully applicable to birds. Therefore, identifying and unifying biomarkers for different muscle fiber types in poultry is crucial. Our understanding of myofiber heterogeneity is limited by basic biochemical techniques, such as fast/slow myosin ATPase immunostaining and enzyme histochemistry of succinate dehydrogenase and cytochrome c oxidase. These methods are non-quantitative and low-dimensional analyzing only a few enzymes at a time. Although several studies have begun to explore the metabolic analysis of slow and fast muscle fibers, and even study isolated individual muscle fibers in vitro, many technical hurdles remain. In the future, the application of single-cell technologies and spatial omics technologies such as spatial transcriptomics, proteomics and metabolomics may help to achieve a more comprehensive understanding of muscle fiber types. Lastly, the study of key regulators of chicken muscle fiber type is largely limited to the in vitro primary cell level. Developing individual experimental systems will be valuable for studying the genes involved in determining muscle fiber types.

DISCLOSURES

The authors declare no conflicts of interest.