Platelet Biogenesis in the Lung Circulation

Abstract

Megakaryocytes are normal cellular components of the blood returning to the heart and entering the lungs, and historical data has pointed to a role of the lungs in platelet production. Recent studies using intravital microscopy have demonstrated that platelet release occurs in the lung from bone marrow megakaryocytes that embolize into the lung circulation.

Introduction

Platelets are small, anucleate blood cells with active roles in a wide range of physiological responses, including hemostasis, thrombosis, wound healing, and immunity. In humans, 150–450 billion platelets circulate per liter of blood, with a lifespan of 8–10 days, requiring a daily turnover of 100 billion platelets (70 million/min) to maintain a physiological platelet concentration.

Platelets originate from megakaryocytes, large cells described for the first time in 1890 by Howell (12). Although the role of megakaryocytes in platelet formation proposed by Wright in 1906 was rapidly accepted (55), the site of platelet production has been a matter of debate for at least 100 years. Megakaryocytes develop primarily in the bone marrow, a place where the majority of platelets are presumed to be produced in adults. Megakaryocytes differentiate from hematopoietic stem cells in a process highly dependent on thrombopoietin (TPO), and, in the bone marrow, progenitors undergo a series of transformational stages to prepare for platelet production (28). In the final phases of differentiation, megakaryocytes migrate to the perivascular microenvironment where they interact with endothelial cells and elongate their cytoplasm to form branched pseudopodial extensions called proplatelets. These proplatelets extend into the lumen of bone marrow sinusoids and, under shear and turbulent flow, are released into the blood stream (FIGURE 1).

FIGURE 1.

Platelet production in the lung circulation and its regulation

Platelet production in the lung circulation is the result of proplatelets and megakaryocytes released in spleen and bone marrow sinusoids. We can thus differentiate two levels of regulation: 1) proplatelet release and megakaryocyte egress from spleen and bone marrow, and 2) platelet shedding in the lung vasculature. 1: proplatelet release and megakaryocyte egress depend on megakaryocyte maturation (driven by thrompopoietin) and translocation to the perivascular environment (driven by SDF-1 and FGF-4). Sensing the microenvironment (collagen-rich osteoblastic niche or endothelial cells) regulates proplatelet formation, and transendothelial proplatelet migration and proplatelet shedding into the circulation is regulated by a S1P gradient. Thin proplatelets, large protrusions, and whole megakaryocytes can be released into the circulation. The different types of fragments or cells released is dependent on platelet demand, inflammatory signals, or site of origin. 2: in the lung vasculature, megakaryocytes and cytoplasmic fragments are trapped in vascular bifurcations where shear stress and turbulence must be ideal for final platelet shedding. The importance of other signals, such as the interaction with pulmonary endothelial cells, needs to be further elucidated.

Challenging the dogma that platelets are produced in the bone marrow sinusoids, there has been mounting evidence suggesting that the lungs are a possible birthplace for platelets. Scattered reports of the latter observation have appeared in the literature since the 1930s. In 1937, Howell and Donahue observed the existence of two populations of megakaryocytes, one in the bone marrow and the other in the lungs, and proposed that the latter produce platelets by fragmentation in the pulmonary circulation (13). This hypothesis is supported by a significant number of megakaryocyte observations in human and animal lungs (42, 60), including the observation of higher platelet counts in postpulmonary vessels compared with pulmonary arteries (18, 25, 38, 44, 49). However, because of the lack of direct evidence, the concept of the lung as an active site of thrombopoiesis has not been universally accepted.

This review will focus on recent findings that the lung is also a primary site for platelet biogenesis and will discuss how platelet production in the lung can be regulated under normal circumstances and how this process can be influenced by physiological challenges such as inflammation, infection, or during lung pathologies.

Direct Evidence of Platelet Production in Lung Capillaries

With the development of intravital microscopy, direct observation of dynamic events occurring in mouse lungs is now possible (27), providing answers to the platelet production controversy. Using platelet/megakaryocyte reporter mice (PF4-Cre x mTmG), the first direct evidence that platelet release occurs physiologically in the lung was provided (23). Nucleated megakaryocytes and large cytoplasmic fragments are observed in the pulmonary circulation, often trapped at vascular bifurcations (FIGURE 1). During a process ranging from 20 to 60 min, these events result in the release of 150 to 2,500 single platelets (average = 700). These events are not rare since they are visualized twice per hour in only 1/10,000 of the total lung volume. By extrapolating to the total lung volume, it is estimated that half of the platelet production in mice is occurring in the lung circulation.

Origin of Megakaryocytes and Proplatelets Found in the Lung Circulation

What is the origin of the megakaryocytes and proplatelets found in the pre-capillary lung circulation? By examining lungs of non-fluorescent mice transplanted into megakaryocyte/platelet fluorescent mice, fluorescent megakaryocytes were observed in the lung vasculature, indicating that platelet-producing megakaryocytes in the pulmonary circulation are produced from other organs (23). Indeed, direct visualization of the bone marrow and spleen confirm the presence of resident megakaryocytes releasing cytoplasmic fragments much larger than single platelets. Other groups have observed proplatelet protrusion and release into marrow sinusoids (17, 34). Megakaryocyte fragments exceeding platelet size have been visualized, suggesting that final platelet production occurs elsewhere in the circulation (17, 34, 59). In most cases, megakaryocyte fragments are released, but occasionally an intact megakaryocyte leaving the bone marrow to enter the circulation has been visualized (17, 21, 23). These observations confirm the model whereby megakaryocytes and large cytoplasmic fragments emerge from the bone marrow and spleen, and are trapped by size exclusion in the lung vasculature, where final platelet release occurs (FIGURES 1 AND 2).

FIGURE 2.

Circulating and resident megakaryocytes in the lung

Two types of megakaryoctyes are found in the lung, differing in their location, origin, maturation, shape, and role in thrombopoiesis. Circulating megakaryocytes arrest at vascular bifurcations. They are of extrapulmonary origin, likely released in the bone marrow and spleen sinusoids. These megakaryocytes produce several pseudopodial extensions and release individual platelets in the lung circulation. Resident megakaryocytes are found in the lung interstitium. Their origin is unknown, and they present a more immature profile. These cells are sessile, and transcriptomic data point to a potential role in lung immunity. Further studies should determine their exact function and their capacity to migrate and produce platelets.

Regulation of Platelet Production in the Lung

The regulation of platelet production in the lung is almost entirely unknown. Two levels of regulation could take place in distinct anatomic locations. The first level of regulation occurs in the bone marrow, with control of megakaryocyte maturation, migration to the sinusoidal niche, and proplatelet formation and release in the blood stream (28). The second level of regulation may occur in the lung circulation with terminal platelet shedding (FIGURE 1). Among the factors that control megakaryocyte differentiation from hematopoietic progenitor cells is the binding of TPO to its receptor, Mpl. Indeed, TPO treatment increases peripheral blood platelet counts and is associated with an increased number of megakaryocytes and proplatelets in lung vascular beds observed by electron microscopy (60) and intravital microscopy (23). Another important regulator of megakaryocyte relocation to the bone marrow perivascular niche is the activation of CXCR4 by the chemokine stromal-derived factor-1 (SDF-1/CXCL12) (1, 35). SDF-1 and fibroblast growth factor-4 (FGF-4) drive megakaryocyte progenitors to the sinusoidal vascular niche (FIGURE 1), where interaction with the endothelium promotes megakaryocyte maturation and restores platelet production in thrombocytopenic mice.

After maturation and relocation to bone marrow sinusoidal interface, megakaryocytes and proplatelets must gain vascular access to reach the lung circulation. This process is regulated by chemical guidance, interaction with endothelial cells, extracellular matrix, and blood shear flow, leading to membrane and cytoskeleton reorganization and modification of microtubules, actin filaments and spectrin-based internal membrane. Among the signals required for transendothelial proplatelet migration and proplatelet shedding into the circulating blood is sphingosine-1 phosphate (S1P). Higher concentrations of S1P are found in the blood compared with the bone marrow interstitium, and S1P activates S1PR on megakaryocytes, guiding proplatelet elongation and shedding into the blood stream (FIGURE 1) (59). Sensing of the chemokine CCL5 through CCR5 has also been involved in increased proplatelet formation in vitro (29).

There is also evidence that megakaryocyte interactions with the bone marrow matrix regulates proplatelet production. For example, type I collagen is enriched in the osteoblastic niche and signals through integrin alpha2beta1 to inhibit proplatelet formation (41). Conversely, direct contact with endothelial cells has been reported in vivo (4), and, in vitro, von Willebrand factor (vWF) and fibrinogen, two factors localized to the endothelium, induce proplatelet formation (22, 39).

Megakaryocyte Release Events in the Bone Marrow

Mechanisms of proplatelet formation in the bone marrow environment may not necessarily translate to lung thrombopoiesis. However, megakaryocyte/proplatelet events released from the bone marrow (or other sites) will ultimately impact the lung production. Interestingly, differences have been observed in the type of pseudopods formed by megakaryocytes in vitro or in vivo. In vitro, megakaryocytes generate fine elongated proplatelets characterized by thin cytoplasmic elongation and platelet size beads. In vivo, several groups (4, 21, 23, 34) described in bone marrow or spleen two types of megakaryocyte protrusions—thin proplatelets and thick protrusions, with some discrepancy in their relative abundance. Analyzing histological sections, Brown et al. conclude that most megakaryocytes (90%) enter the sinusoidal space as large protrusions (4–10 µm) rather than extruding fine proplatelet extensions (2–3 µm). Using correlative light electron microscopy, the authors also showed that the thick pseudopodia observed in vivo are structurally different from proplatelets formed in vitro. The microtubules are not in close proximity to the plasma membrane, and the typical microtubule accumulation and loop at the leading edge is not present (4).

In another study, Kowata et al. also observed that microtubules in thick protrusions are randomly organized, suggesting that their intracellular remodeling had not yet been accomplished, thus representing more immature platelet progenitors. However, they found a different relative abundance with thin proplatelets predominating in the physiological state (84%). Interestingly, during acute thrombocytopenia, thick protrusions were dominant, accounting for 56% and 70% of events at 24 h and 48 h after phlebotomy or anti-platelet serum, respectively (21). Difference in megakaryocyte fragment shapes were also described by Nishimura et al. (34), with short (<100 µm) and long proplatelet (>100 µm) fragmenting events. According to Kowata et al. (21), less elongated proplatelets were observed following platelet depletion or peritoneal inflammation with thioglycolate. A new IL-1α-dependent rupture mechanism was also described, giving rise rapidly to higher platelet numbers (34). Taken together, these results strongly suggest that platelet demand and extracellular signals modulate the type of megakaryocyte fragments that are released, with thicker and more immature protrusions during stress. These protrusions are likely involved in further fragmentation in the lung, and these pathways should be considered in investigations of lung thrombopoiesis.

Most of the in vivo studies on proplatelet formation have been done in the mouse calvarium or long bones. In mice, proplatelets are also released by spleen megakaryocytes, and further studies should examine the different mechanisms of release in the spleen versus bone marrow (23). For example, after bone marrow transplantation, larger megakaryocytes developed in the spleen (45), and, when observed with similar methods and conditions, thicker cytoplasmic fragments were released from megakaryocytes in the spleen than from bone marrow megakaryocytes. Intravital imaging in the spleen has revealed large megakaryocytes that release long, elastic proplatelet extensions into the splenic sinusoids (23). These release events are strikingly different from the bone marrow, potentially related to the sinusoidal anatomy or shear forces (see below).

Mechanisms of Proplatelet Formation From Intravascular Megakaryocytes in the Lung

Why and how are platelets produced in the lung circulation? The large size of megakaryocytes naturally impedes transit through the lung, which can have capillaries that are <5 µm in diameter. The physical trapping of megakaryocytes may allow for intimate interactions with the lung endothelium, which could facilitate platelet release (discussed below). Increasing in vitro (7) and in vivo evidence (15) suggests that blood flow-dependent shear stress and turbulence are crucial for platelet biogenesis. By coupling skull intravital imaging with particle image velocimetry, Ito et al. showed that megakaryocytes displaying proplatelet protrusion and platelet release were adjacent to areas of high vascular turbulence (15). In contrast, resting megakaryocytes that did not release platelets were exposed to continuous laminar flow with no turbulence. In vitro, the optimized levels of turbulent energy and shear stress was determined to be between 0.002 and 0.014 m²/s², and 0.4 and 3.0 Pa, respectively. Pulmonary vessels, the first capillary bed encountered by megakaryocytes and proplatelets leaving the bone marrow, may provide an optimal level of turbulence and shear stress required for efficient platelet generation. Corroborating this hypothesis, murine and human megakaryocytes grown in vitro and injected intravenously are trapped in mouse lungs and shed platelets with normal size distribution, circulating half-life, and functionality (8, 50).

In addition to size exclusion and blood flow-induced mechanical forces, the pulmonary circulation may also provide additional signals promoting platelet generation through specific lung endothelial-megakaryocyte interactions. Among the possibilities, vWF and its receptor on megakaryocytes and platelets, GPIb, could be a mechanism for megakaryocytes to sense shear (10) and an important regulator of proplatelet formation (39). vWF expression in endothelial cells is heterogeneous throughout the vascular tree, and its high level in lung endothelial cells could be another reason for efficient platelet production in the lung (33, 37).

Lung Contribution to Overall Thrombopoiesis

The contribution of the lung to overall thrombopoiesis has been a matter of controversy for decades. Attempts to estimate the percentage of platelets produced in the lung, both in human and mouse, are summarized in Table 1 and vary from 7% to almost 100% (19, 23, 25, 38).

Table 1.

Calculation of the lung contribution to overall platelet production

			Calculated Lung Platelet Production					Values Used for Calculation
Reference	Species	Measured Value	MK/ml	Active MK/h	% of MK reaching the lung (estimated)	MK in lung vasculature/min	Platelets produced/lung/day (billion)	Fraction of the lung observed	Cardiac output, ml/min	Blood volume, ml	Platelet/MK	Platelet/mm3	Platelet lifespan, days	Spleen fraction	Overall Production, platelets produced/day (billion)	Lung Contribution, %
19	Human	Nucleated MK before lung (right heart)	2.4		100	12,000	75		5,000	5,000	4,000	300,000	10, 4	No	150, 375*	52, 20*
					33	4,000	24						10, 4		150, 375*	17, 7*
38	Human	Nucleated MK before lung (vena cava)	11.9			40,000 (18,000 copious cyto)	103		5,000	5,000	4,000	300,000	10	No	150	69
		Nucleated MK after lung (femoral artery)	3.8
25	Human	Nucleated MK before lung (pulmonary artery)	5			25,200	133		5,000	5,370	2,870	251,000	10	No	135	98
		Nucleated MK (after lung) (aortic blood)	0.5
		Enucleated MK(before lung)	7.2
		Enucleated MK (after lung)	3.4
23	Mouse	MK and proplatelets releasing platelet in lung vasculature		2.3			0.287	1/10,000		1.5	500	1,000,000	4	1/3	0.5	56

In humans, the number of platelets produced in the lung vasculature was estimated from the number of circulating megakaryocytes reaching the lung, and more precisely using the difference before and after lung filtration. These measurements required an estimation of number of platelets produced per megakaryocyte and measurement of cardiac output. In mice, the number of platelets produced in the lung was estimated by extrapolation to whole lung of direct visualization and quantification of platelet release in the lung vasculature. The number of platelets produced in the lung was then compared with the total number of platelets produced daily, which required calculation of blood platelet counts, blood volume, an estimation of platelet lifespan, and platelets sequestered in the spleen. Platelets produced/lung/day human = (MK/ml before lung – MK/ml after lung) × platelet/MK × cardiac output × 60 × 24. Platelets produced/lung/day mouse = releasing MK/hour × platelet/MK × lung fraction × 24. Overall platelet production = (platelets/ml × blood volume/platelet lifespan × 1,000) × (1 + spleen sequestration). Lung contribution = platelets produced/lung/day/overall platelet production. *In Ref. 19, four different values for the overall production and lung contribution are presented, depending on different estimates for the percentage of MK reaching the lung (column 6) and the platelet lifespan (column 14).

The differences in estimates are partly explained by the calculation of total platelets produced per day, estimates of platelet lifespan, and the contribution of splenic sequestration of platelets. Differences also arise from how investigators considered enucleated megakaryocytes or megakaryocytes with small cytoplasm. Important limitations of the lung intravital microscopy technique should also be acknowledged. The principle limitations are 1) only the subpleural lung can be visualized with this technique, and it is assumed that the megakaryocyte distribution in the interior of the lung is similar; 2) anesthesia and mechanical ventilation are necessary, which could influence lung hemodynamics; and 3) immobilization of the lung using a surgically implanted thoracic window could similarly influence local lung mechanics. Related to the last two limitations, blood platelet counts measured after the lung intravital imaging procedure are not significantly altered (23). An important remaining question is whether the contribution of the lung is constant or whether it can be modulated according to physiological or pathophysiological events. As suggested by different studies, during periods of great platelet demand, immature megakaryocytes and larger proplatelets leave the bone marrow (21, 34), and an even greater percentage of the platelet release may take place in the lungs.

It should be noted that not all studies support a role of the lung in platelet biogenesis. For example, in a study in mice under basal conditions or during recovery from thrombocytopenia (induced by chemotherapy, platelet antiserum, or radiation), megakaryocytes were only rarely found in pulmonary blood vessels by using an immunostaining technique (6). Also, the production of platelets in the lung circulation is dependent on megakaryocytes exiting the bone marrow and traveling to the lung. However, a recent study using intravital microscopy found that only 1–2% of megakaryocytes in bone marrow were positioned in the vascular space (46). Although it is undeniable that megakaryocytes are normal constituents of venous blood, the process of megakaryocytes traversing the sinusoidal endothelium, entering the bloodstream, and exiting the bone marrow space has not been directly visualized.

Potential Phenotypic Differences in Platelets Produced in the Bone Marrow Versus the Lung

Further studies should determine whether platelets produced in the lung are different from those produced in the bone marrow. These experiments should investigate whether external signals in the lung can affect megakaryocytes/proplatelets, altering platelet content, size, or reactivity. Even if platelets lack nuclear DNA, they carry a pool of ribosomes, messenger RNAs (mRNAs), and microRNAs (miRNAs), allowing platelets to synthesize protein and to adjust their function (40). Studies have shown that megakaryocytes specifically sort, rather than randomly transfer, mRNA to platelets during megakaryocyte development and platelet formation (5, 40). It is not known whether larger pseudopods and immature megakaryocytes are more likely to be further fragmented in the lung, but, if so, entrapment of megakaryocytes and ribosome-rich proplatelets in the lung, one of the largest surfaces in contact with the environment, may be important to adapt platelets locally for specific cellular and immunological responses. In favor of a role of the microenvironment in platelet modification, platelet mRNA and miRNA profiles have been shown to reflect disease and human clinical phenotypes (2). Further studies should investigate whether the site of production can leave an imprint on platelet quality and specificity.

Lung-Resident Megakaryocytes

In addition to the population of platelet-producing intravascular megakaryocytes in the lung, a second population of megakaryocytes residing in the pulmonary interstitium has been identified in the mouse lung (23) (FIGURE 2). Observation of these cells and transcriptomic analysis revealed that lung-resident megakaryocytes are smaller and more immature than megakaryocytes present in the bone marrow. Using intravital imaging, these extravascular megakaryocytes are sessile and have not been observed to release proplatelets/platelets during homeostasis. Rather, early evidence points to a potential immune role for lung extravascular megakaryocytes, since these cells differentially express many genes associated with innate immunity, inflammation, and pathogen recognition compared with bone marrow megakaryocytes. These results may suggest a function in inflammation for these strategically positioned cells in the lung where they could be early responders to pathogens.

To determine a possible role of extravascular lung megakaryocytes in platelet production, perfused lungs were transplanted into thrombocytopenic animals deficient in platelets and hematopoietic stem cells. It was discovered that lung hematopoietic progenitors are able to migrate to the bone marrow, reconstitute normal platelet counts, and contribute to the hematopoiesis of several cell lines several months after transplantation (23). These results indicate that the lung contains not only megakaryocytes but also progenitors of blood cells further upstream in hematopoietic development. The origin of these cells, the mechanisms of migration, and their exact role in the lung in physiological or pathological conditions remain to be elucidated.

Platelet Biogenesis in the Human Lung

Is the contribution of the lung to platelet biogenesis also important in humans? It is not possible to image the human lung circulation for the real-time observation of platelet biogenesis, but several lines of indirect evidence point to an important role of the human lung in thrombopoiesis. For example, blood entering the human lung is enriched in megakaryocytes, and megakaryocytes seem to be filtered in the lung, since the blood exiting the lung is largely devoid of megakaryocytes (25) but enriched in platelets (13). These measurements were used to calculate and estimate that the lung production represents >60% of total platelet production in humans (25, 38) (Table 1). In addition, patients receiving cardiopulmonary bypass commonly develop thrombocytopenia, likely from a combination of factors including surgical blood loss, platelet activation, and aggregation from extracorporeal circulation, and sometimes related to heparin administration (51). However, several studies have found a large increase in the number of circulating megakaryocytes in the venous and arterial blood during cardiopulmonary bypass, potentially from the lack of lung filtration/trapping of megakaryocytes (53, 54, 56). Thus the mechanism of thrombocytopenia in cardiopulmonary bypass may involve consumption and potentially a deficiency in lung platelet production. Finally, Fuentes et al. (8) showed that human megakaryocytes injected into mice are transiently trapped in the lungs and release a platelet wave within a similar timeframe as described (50), arguing that trapping of the megakaryocytes is potentially a mechanism of platelet formation in the lung.

Clinical Relevance of Lung Platelet Production

Lung Pathologies and Platelet Numbers

What is the impact of lung pathologies on platelet production? Common pulmonary diseases such as asthma, chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis, and pulmonary hypertension are not typically associated with thrombocytopenia. Extrapolating from intravital lung imaging in mice, only a small fraction of the vast lung vasculature is involved at any given time in platelet-forming events. The great vascular redundancy in the lung likely allows for platelet biogenesis to occur unimpeded even in the face of significant lung pathology. In the following sections, several case examples will be discussed regarding the clinical relevance of lung platelet production.

First, in individuals with cyanotic congenital heart disease, there is an ~25% incidence of thrombocytopenia, usually in the setting of Eisenmenger syndrome where pulmonary blood is shunted into the systemic circulation (11, 26). There is an inverse relationship between platelet counts and the severity of right-to-left shunting. With greater shunting of blood away from the lungs, platelet counts decrease. This result also points to a specialized function of the lung circulation in platelet formation, since megakaryocytes shunted into systemic vascular beds do not seem capable of normal platelet production.

Second, as previously discussed, patients receiving cardiopulmonary bypass or veno-arterial extracorporeal oxygenation have a high incidence of thrombocytopenia classically attributed to consumption of platelets activated during the extracorporeal circulation. However, an intriguing finding is that megakaryocytes increase threefold in these patients who have the lung circulation bypassed (53). Therefore, it is reasonable to hypothesize that thrombocytopenia in these settings could be due to underproduction of platelets in the lung, in addition to consumption. Again, delivery of megakaryocytes to systemic organs does not seem sufficient to maintain platelet counts.

Third, it is reasonable to speculate that, if the lung is important in platelet biogenesis, oxygen tension may regulate platelet production. This issue has been studied clinically and experimentally in the setting of hypoxia with conflicting results. The acute exposure to high altitude in healthy individuals decreased platelet counts with evidence of platelet activation and aggregation (24). However, another study found that ascending from 600 to 3,600 m increased platelet counts at 2 and 7 days (14). Additionally, platelet counts were higher in residents at 4,200 m compared with residents at 600 m (14). Experimentally, a biphasic response to hypoxia has been observed in which rats or mice exposed to acute hypoxia increase platelet counts in the first few days of exposure, but platelet counts decrease after the first week (16, 31). The effects of hemoconcentration, platelet activation, and potentially hypoxic vasoconstriction (3) are likely contributing to these findings. In hyperoxic exposure in rats, no difference was observed in the number of megakaryocytes in the blood entering and exiting the lung, suggesting that lung filtration of circulating megakaryocyte was reduced (57). More work is needed to determine the effects of oxygen tension on megakaryocyte development and platelet release.

A fourth example is the acute respiratory distress syndrome (ARDS) that is associated with thrombocytopenia in severe cases—in fact, thrombocytopenia is an independent risk factor for mortality in ARDS (52). In ARDS, an excessive inflammatory response is present that is likely driving platelet consumption (48), but underproduction of platelets in the lung could also contribute. In severe cases of ARDS, the majority of the lung is injured, which may limit sites with preserved endothelium that could facilitate lung platelet production from intravascular megakaryocytes. Clinical and experimental studies are needed to determine the role of consumption and underproduction of platelets in ARDS.

Lung Pathologies and Platelet Quality

Lung diseases could quantitatively and/or qualitatively affect platelets. As previously discussed, it is possible that the lung environment (e.g., inflammation, injury, etc.) can be sensed by intravascular and resident megakaryocytes, and local signaling could influence the packaging of granules, mRNA, and the release of microparticles from locally produced platelets. Altered platelet size, metabolism, and reactivity have been described in several lung diseases (32) such as cystic fibrosis (36), asthma (9, 20, 47), acute lung injury (58), and COPD (30). Platelet activation in the blood and platelet consumption could explain these observations, but further studies should investigate whether modification of lung megakaryocytes (resident or intravascular) or alteration of the production site (pulmonary endothelium, lung biochemical environment) during lung pathologies could also modify platelet content, reactivity, and functionality.

Lung Platelet Production and Thrombocytopenia Treatments

Embracing the concept of lung platelet production should open new avenues for improving the treatment of thrombocytopenia, which affects millions of patients worldwide. There has been much interest in the development of platelet transfusions from megakaryocytes grown in vitro, and integrating concepts such as turbulent flow have improved production capacity (43). A better understanding of how the pulmonary vasculature contributes to the release of platelets could lead to improved platelet bioreactors. Alternatively, the demonstration that megakaryocytes move physiologically to the lungs could lead to direct infusions of megakaryocytes in patients, allowing the lung to act as a bioreactor for the production of mature platelets.

Conclusions

A large number of reports have now demonstrated that megakaryocytes are normal constituents of the venous blood returning to the lungs and that megakaryocytes are found in the lung vasculature. In humans, circulating megakaryocytes are filtered in the lung, and in the mouse lung, megakaryocytes have been directly observed producing platelets with estimates of at least 50% of daily platelet production. The pulmonary vasculature likely offers ideal shear and turbulence to promote individual platelet shedding from megakaryocytes and proplatelets released from the spleen (mice) and bone marrow. Further investigations should determine whether the lung endothelium provides unique molecular signals that could facilitate platelet release from megakaryocyte/proplatelets and whether the platelets produced in the lung differ from the bone marrow. Studies are needed to decipher whether the lung, through its environment or external signals (exposure to pathogens and inflammatory signals), influences platelet formation and ultimately platelet function.