Timing and sequence between a thick fascia covering muscles and a cutaneous nerve perforating it: a study using histological sections of human fetuses

Abstract

A deep fascia is believed to develop after a cutaneous nerve (CN) reaches the subcutaneous tissue. The present study aimed to examine a developmental sequence of the CN perforation of the deep fascia. We examined the occipital and posterior neck region as well as the anterior abdominal wall using histological sections from 12 human early fetuses at approximately 7–9 weeks of gestation age (GA) and 10 midterm fetuses at approximately 12–18 weeks GA. When the anterior abdominal CN reached the anterior aspect of the rectus abdominis muscle at 7 weeks, the rectus sheath was still loose. However, the aponeurosis of the obliquus externus abdominis was relatively tight. Later, depending on the rectus sheath became evident, a “window” for the CN also became clear. After the anterior CN took a wavy course along the aponeurosis, it grew to the skin by 12 weeks. The occipital CNs reached the superficial surface of the semispinalis and ran superiorly along the fascia by 8 weeks. Later, depending on the superior extension of the splenius and trapezius muscles, the occipital CNs issued multiple twigs to perforate the superior margin of the upward extending fascia or muscle. Both a deep fascia and the window for CNs develop simultaneously. However, during several weeks, the CN likely stopped its further growth to the skin and remained on and along the fascia. Likewise, after perforation of the deep fascia, another fascia or muscle likely interferes with the straight course of CNs to the skin.

Introduction

Cutaneous nerves (CNs) perforate the deep fascia, i.e., a thick fascia covering muscles. The CN can perforate a very thick fascia attaching to the acromion of the scapula [1] as well as the central aponeurosis (=speculum rhomboideum) of the trapezius muscle [2]. The nerve early innervates the muscles and further issues the CNs. In the extremities, the CN reaches the future subcutaneous tissue at midterm and, later, the deep fascia develops with an aid of growth and contraction of the muscle [3, 4]. Thus, even if the deep fascia finally becomes thick and tight, the CN can make a window of the deep fascia to pass through it. The developing collagenous bundles of the fascia seem to avoid the already-established CN and the fascia allows an almost straight course of the nerve. This might be the usual understanding of the developmental sequence of a fascia and the CN perforating it (Fig. 1).

Fig. 1.

The usual understanding of a developing cutaneous nerve and fascia. At the early stage, a mother nerve issues cutaneous and muscular branches. The muscular branch obtains the innervation to a muscle, while the cutaneous branch reaches the subcutaneous tissue before development of the deep fascia (A). Later, when the deep fascia covers the muscle, the cutaneous nerve branch passes through a window of the fascia (semicircle in B) and reaches the dermis of the skin.

The greater and third occipital nerves of the dorsal rami of the second and third cervical nerves as well as the anterior abdominal CNs of the ventral rami of the thoracolumbar nerves are known well as CNs perforating the thick fasciae in adults. The occipital nerves develop as early as posterior neck skeletal muscles at 6–8 weeks of gestational age (GA) in human embryos and fetuses [5, 6]. Likewise, in the anterior abdominal wall, the rectus sheath [7] as well as the anterior CN [8] develops at 6–8 weeks of GA much earlier than the recovery of physiological herniation. However, to our knowledge, the previous study did not demonstrate a stage when the CN just perforates the deep fascia. Consequently, the present study aimed to examine a developmental sequence of the CN perforation of the deep fascia in the anterior abdominal wall as well as the occipital and posterior neck region.

Materials and Methods

The present study was performed in accordance with the provisions of the Declaration of Helsinki 1995 (revised in 2024). We examined CNs in the occipital and posterior neck region as well as the anterior abdominal wall using serial histological sections from 12 human early fetuses at approximately 7–9 weeks GA and 10 midterm fetuses at approximately 12–18 weeks GA. The size of the former group ranged from 24–35 mm of the crown-rump length (CRL), while the latter ranged from 58–170 mm of CRL. Sectional planes were all sagittal. The sections of the early fetuses covered the entire head and neck, while the other sections of the midterm fetuses contained the upper posterior neck and lower occipital regions. In spite of the long preservation, whether a fascia is loose or tight is clearly seen in the present materials [3, 4].

These specimens were part of the collection of the Department of Anatomy, Akita University, Akita, Japan, and had been donated by the families concerned to the Department in 1975–1985 and preserved in 10% v/v neutral formalin solution for more than 30 years. Data on these specimens included the date of donation and the GA estimated gynecologically, but did not include the name of the family, obstetrician or hospital or the reason for abortion. Use of the specimens for research by one of the authors (G.M.) was approved by the Akita University Ethics Committee (No. 1428). All sections had been stained with H&E and most sections had been used for our recent studies [6, 9]. All the photographs of histology were captured using a Nikon Eclipse 80 (Nikon).

Results

Observations of the anterior abdominal CNs

In early fetuses at approximately 7–8 weeks of GA, the rectus sheath was underdeveloped, whereas the aponeurosis of the obliquus externus abdominis muscle was well developed (Fig. 2). The anterior abdominal CN reached the superficial surface of the rectus abdominis muscle, but it did not yet grow into a mesenchymal tissue corresponding to the future subcutaneous tissue (Fig. 2B, G). At this early stage, the future umbilicus was opened widely for physiological herniation and the stomach and liver extended inferiorly. Thus, we observed the lower abdominal wall below the opening. In the lateral abdominal wall, the obliquus externus muscle directly attached to the rectus abdominis muscle without an interposed aponeurosis (Fig. 3). The CN reached the rectus muscle surface and a window of the rectus sheath was developing (Fig. 3D). However, the CN did not extend into the future subcutaneous tissue.

Fig. 2.

Anterior abdominal cutaneous nerve and the underdeveloped rectus sheath (RS) at approximately 7 weeks of gestational age. Sagittal sections. H&E staining. Panels (A, B), a fetus of 24 mm crown-rump length (CRL); panels (C–G), a fetus of 28 mm CRL. In panels (A, B), the right-hand side of each panel corresponds to the inferior side of the body. Panel (B) displays a square in panel (A) at the higher magnification. In panels (C–G), the lower side of each panel corresponds to the inferior side of the body. Panels (D, F) are higher magnification views of squares in panels (C, E), respectively. Panel (C) (or G) displays the most lateral (medial) planes in panels (C–G), Panel (G) displays a plane 0.2 mm medial to panel (C). In both specimens, the umbilicus is widely open and the RS is underdeveloped in the medial site (F, G). An anterior cutaneous nerve (arrows) reaches the superficial surface of the rectus abdominis muscle (RA) (B, G). The obliquus externus aponeurosis (OEap) is relatively tight (D). Panels (A, C, E) or panels (B, D, F, G) were prepared at the same magnification. (A) Scale bar=1 mm; (B) scale bar=0.1 mm. OE, obliquus externus abdominis muscle; TA, transversus abdominis muscle; PE, peritoneum; PU, pubis.

Fig. 3.

A window of the rectus sheath (RS) for the anterior abdominal cutaneous nerve at approximately 8 weeks of gestational age. Sagittal sections. H&E staining. A fetus of 35 mm crown-rump length. This figure contains the abdominal wall lateral to Figs. 2, 4, and 5 to show the nerve without covering of the obliquus externus aponeurosis. The lower side of each panel corresponds to the inferior side of the body. Panels (B, D) are higher magnification views of squares in panels (A, C), respectively. Panel (C) exhibits a plane 0.2 mm medial to panel (A). The anterior cutaneous nerve (arrows) runs through the rectus abdominis muscle (RA) (B) and penetrates the RS (D). A window (arrowhead in D) for the nerve is evident in the sheath (RS). Panels (A, C) or panels (B, D) were prepared at the same magnification. (A) Scale bar=1 mm; (B) scale bar=0.1 mm. TA, transversus abdominis muscle; OE, obliquus externus abdominis muscle; PE, peritoneum; OI, obliquus internus abdominis muscle.

At 12 weeks of GA or later, we found the anterior CN growing into the subcutaneous tissue. Notably, immediately after passing through the rectus sheath, the CN showed an angulation to change the direction (Fig. 4). After angulation, the nerve took a wavy course between the rectus sheath and obliquus externus aponeurosis. Thus, sagittal sections exhibited a series of “nerve fragments” (Fig. 4D, G). Later, after the aponeurosis became incorporated into the rectus sheath, the angulation and wavy course was unclear or lost and the CN exhibited an almost straight course in the subcutaneous tissue (Fig. 5).

Fig. 4.

The anterior abdominal cutaneous nerve running inferiorly along the rectus sheath (RS). Sagittal sections. H&E staining. A fetus of 61 mm crown-rump length. The lower side of each panel corresponds to the inferior side of the body. Panels (A–D) (or E–G) display the upper (or lower) abdominal wall. Panels (B, F) are higher magnification views of squares in panels (A, E), respectively. Panel (D) exhibits a plane 0.6 mm medial to panel (A), while panel (G) is 0.1 mm medial to panel (E). The anterior cutaneous nerve (arrows) runs through a window of the RS (B, F), makes an angulation (arrowhead in F) and takes a long inferior course along the sheath (D, F). The obliquus externus aponeurosis (OEap) is thick and tight. Panels (A, E) or panels (B–D, F, G) were prepared at the same magnification. (A) Scale bar=1 mm; (B) scale bar=0.1 mm. PL, pleura; TA, transversus abdominis muscle; RA, rectus abdominis muscle; PE, peritoneum; PY, pyramidalis muscle.

Fig. 5.

The anterior abdominal cutaneous nerve extending in the subcutaneous tissue. Sagittal sections. H&E staining. A fetus of 125 mm crown-rump length. The lower side of each panel corresponds to the inferior side of the body. Panels (B, D) are higher magnification views of squares in panels (A, C), respectively. Panel (C) exhibits a plane 0.1 mm medial to panel (A). The anterior cutaneous nerve (arrows) runs through a window of the rectus sheath (RS) (B) and extends superiorly in the subcutaneous tissue (B, D). The obliquus externus aponeurosis (OEap) is incorporated into the RS at a site of the window (A, C). Panels (A, C) or panels (B, D) were prepared at the same magnification. (A) Scale bar=1 mm; (B) scale bar=0.1 mm. IEA, inferior epigastric artery; PE, peritoneum; PY, pyramidalis muscle; RA, rectus abdominis muscle.

In the lower abdominal wall below the umbilicus at midterm, Fig. 4E–G shows a downward course of the CN, while Fig. 5 exhibits an upward course. In 10 midterm fetuses examined, a candidate of the lowest CN ran superiorly in 3 fetuses (CRL: 75, 105, and 125 mm) and inferiorly in 5 fetuses (61, 81, 85, 155, and 170 mm). The other 2 specimens carried a relatively high-positioned CN that ran superiorly (58 and 65 mm). All these lower abdominal CN passed through the rectus sheath in the superior or posterolateral side of the joining between the obliquus externus aponeurosis and the rectus sheath. The obliquus externus aponeurosis appeared to be an obstacle that interfered with the straight nerve course to the skin.

Observations of the greater and third occipital nerves

Even in the smallest specimen in the 12 early fetuses (24 mm CRL), the semispinalis capitis muscle as well as the suboccipital muscles had already developed well, but the splenius capitis and trapezius muscles did not fully extend toward the superolateral area. Twigs of the greater occipital nerve reached the subcutaneous tissue at 8–9 weeks of GA (Fig. 6), but the nerve trunk remained immediately inside of a thin fascia connecting to the splenius capitis muscle (Fig. 6C). Since the fascia was thin, a window of the fascia was not evident. Later, we found a window of a fascia connecting to the trapezius muscle, not to the splenius muscle at 12 weeks of GA (Fig. 7C). When it passed through the semispinalis capitis muscle, the greater occipital nerve was composed of multiple nerve bundles most of which appeared to correspond to CNs (Fig. 7A). These cutaneous branches (CBs) perforated the expanding splenius muscle near or at the superior margin (Fig. 7B). Each of the CBs reached the external surface of the splenius at different heights. Finally, the multiple CNs appeared not to perforate the muscular part of the trapezius but a superior aponeurosis connecting to the future galea aponeurotica. All CNs ran superiorly.

Fig. 6.

The greater occipital nerve that had already reached the subcutaneous tissue. Sagittal sections. H&E staining. A fetus of 35 mm crown-rump length. The lower side of each panel corresponds to the inferior side of the body. Panel (E) exhibits a plane 2.5 mm lateral to panel (A). The greater occipital nerve (GON) penetrates the semispinalis capitis muscle (SSCA) (B) and runs superolaterally along the surface of the muscle (C). It issues a twig (arrows in D). Finally, the occipital nerve reached the subcutaneous tissue (arrows in E). A window at the nerve perforation is not evident in the thin fascia (stars in A–C) of the splenius capitis muscle (SPCA) (A). Asterisks in panels (C, D) indicate artifact tissue clefts in the histological procedure. (A, B) Scale bars=1 mm; (C–E) scale bars=0.1 mm. AT, atlas; OC, occipital bone; OCI, obliquus capitis inferior muscle; RCM, rectus capitis posterior major muscle.

Fig. 7.

The greater and third occipital nerves (TON) penetrating or running along deep fasciae. Sagittal sections. H&E staining. A fetus of 150 mm crown-rump length. The lower side of each panel corresponds to the inferior side of the body. Panels (A–C) display the greater occipital nerve (GON) and panels (D–F) exhibit the TON. Panel (C) exhibits a plane 0.5 mm lateral to panel (A), while panel (F) is 0.1 mm medial to panel (D). The GON passes through the semispinalis capitis muscle (SSCA), reaches the superior margin of the splenius capitis muscle (SPCA) and provides a window (arrowhead in C) of the fascia connecting to the trapezius muscle (TR). Stars indicate a thin fascia extending from the TR upper end in panel (B, C). In panel (E), the TON makes two angulations (arrowheads in E) at crossing the SPCA and takes an upward course (arrows in E) along the surface of the muscle. A branch of the third nerve penetrates the trapezius muscle (arrowhead in F). All panels were prepared at the same magnification. (A) Scale bar=1 mm. RCM, rectus capitis posterior major muscle; AT, atlas.

The third occipital nerve made a pair of angulations when it crossed the splenius capitis muscle at a right angle (Fig. 7E). After the crossing, the nerve ran upward through a narrow space between the semispinalis and splenius muscles. During the upward course, the third occipital nerve issued multiple cutaneous twigs that penetrated the expanding trapezius muscle near or at the superior margin. Finally, the third nerve reached the external surface of the splenius at a single superior site and, after branching, these nerve twigs penetrated the trapezius at multiple heights (Fig. 7F): the topographical anatomy with the trapezius was different from the greater occipital nerve.

Fig. 8 summarizes the present observations. The anterior abdominal CN reached the external surface of the rectus abdominis muscle before the rectus sheath was completed (Fig. 8A). However, during several weeks, the CN likely stopped its further growth to the skin on and along the fascia (Fig. 8B). After the obliquus externus aponeurosis was incorporated into the sheath, the nerve grew to the skin at the superior margin of the aponeurosis (Fig. 8C). The occipital nerves issued a CB reaching a fascia connecting to the trapezius or splenius capitis muscle, but the main trunk ran upward along the semispinalis capitis muscle (Fig. 8D). As the superficial muscle grew, the nerve issued another CB at the superior muscle margin and further extended upward (Fig. 8E).

Fig. 8.

Summary of the present observations. The anterior abdominal cutaneous nerve perforates the rectus abdominis muscle (RA) before development of a deep fascia (A). The nerve runs beneath another fascia (B) and, later, the nerve grows to the skin at the superior margin of the fascia (C). The nerve (arrowhead in B) runs beneath another fascia and, later, the nerve grows. The occipital nerve issues a cutaneous branch reaching a deep fascia. The main trunk (arrowhead in D) runs upward and issues another cutaneous branch at the superior muscle margin (E). RS, rectus sheath; TR, trapezius muscle; SPCA, splenius capitis muscle; SSCA, semispinalis capitis muscle; OEap, obliquus externus aponeurosis.

Discussion

The present histology ensured the usual understanding as to how a CN penetrates the deep fascia (Fig. 1). At 7 weeks of GA when the CN reached the muscle surface, the deep fascia was still underdeveloped in the abdominal and occipital regions. In particularly, the anterior abdominal wall provided a good sample showing the CN growth because of the developing rectus sheath. After passing through the rectus abdominis or semispinalis capitis muscle, the CN stopped its further growth to the skin for several weeks on the muscle surface (Figs. 3D, 6C). We termed this morphology a “transit-stay.” Thus, after perforation through the deeper fascia, these CNs did not take an external course but took a superior or inferior course inside of and along the fascia or muscle. The transit-stay was also seen in the lateral abdominal wall in which the obliquus externus aponeurosis was absent or underdeveloped. Thus, the aponeurosis did not stop the CN growth on the muscle. Why did not the growing CN take a straight course to the skin? One answer seemed to be found in a relatively delayed differentiation of the skin dermis: the CN might wait for a chemical cue from the matured skin.

The present histology also showed that factors other than the deep fascia likely determine the nerve growth and course. The anterior abdominal wall carried the early-developed obliquus externus aponeurosis, while the delayed superolateral expansion of the splenius and trapezius muscles was seen in the future occipital subcutaneous tissue. After perforation of the deep fascia, the CNs find an obstacle against growth to the skin. When the obliquus externus aponeurosis became incorporated into the rectus sheath, the anterior abdominal CN seemed to be free from the obstacle from the superolateral site. The incorporation advances from the upper site to the lower site. Therefore, the upper nerve might reach the skin earlier than the lower nerve. When the CN takes an inferior course at the early stage, the obliquus aponeurosis is liable to interfere with the nerve course to the skin. Herein, we considered the early development. Therefore, the hypothesis does not directly connect to the present counting of the superior or inferior course of the lowest abdominal CN at “midterm.” A further study using greater numbers of specimens was required to evaluate the superior or inferior course in relation to growth of the covering aponeurosis.

To search a “weak point” at the superior margin of the expanding muscles, the occipital nerves might issue multiple CNs. The present morphology (Figs. 6, 7) suggested that multiple trials by CNs occurred for passing through the expanding superficial muscles. Accordingly, a thicker CN seemed to penetrate the muscle or fascia in the lower site, whereas a thinner CN seemed to pass through the upper site. Another striking feature was found in a delayed change of the nerve morphology: the wavy course of the anterior abdominal CN was lost and the angulations became less evident at midterm (Fig. 4F, G). The wavy and angulated course might be corrected by a differential growth. Simultaneously, the increased nerve thickness might mask the slightly tortuous nerve.

The present study was based on observations of sagittal sections. The sagittal sections provided us an easy understanding of the CN course. However, an accurate evaluation of the shape requires observations of both sagittal and horizontal sections. Thus, this is a limitation of the present study, perhaps bringing a greater disadvantage than a limited number of specimens. In addition, the present nerve identification without immunohistochemistry might mislead readers to regard our-identified twigs as a fibrous tissue or even an artifact.

The lesser occipital nerve sometimes perforates the trapezius muscle and Fujita [10] considered the morphology as a result of the delayed separation of the sternocleidomastoideus and trapezius muscles from the common primordium. The musculocutaneous nerve usually passes through the coracobrachialis muscle, but rarely the nerve passes alongside the muscle [11]. Koizumi and Sakai [12] considered that whether it perforates or not depends on nerve segments of the muscle innervation. Irrespective of the origin and timing of developing muscles and nerves, according to the present observation, a slight variation in fetal topographical relation between a fascia (or muscle) and the CN also seemed to determine the adult nerve morphology.