The Sertoli cell: what can we learn from different vertebrate models?

Besides having medical applications, comparative studies on reproductive biology are very useful, providing, for instance, essential knowledge for basic, conservation and biotechnological research. In order to maintain the reproductive potential and the survival of all vertebrate species, both sperm and steroid production need to occur inside the testis. From the approximately fifty thousand vertebrate species still alive, very few species are already investigated; however, our knowledge regarding Sertoli cell biology is quite good. In this regard, it is already known that since testis differentiation the Sertoli cells are the somatic cells in charge of supporting and orchestrating germ cells during development and full spermatogenesis in adult animals. In the present review, we highlight key aspects related to Sertoli cell biology in vertebrates and show that this key testis somatic cell presents huge and intrinsic plasticity, particularly when cystic (fish and amphibians) and non-cystic (reptiles, birds and mammals) spermatogenesis is compared. In particular, we briefly discuss the main aspects related to Sertoli cells functions, interactions with germ cells, Sertoli cells proliferation and efficiency, as well as those regarding spermatogonial stem cell niche regulation, which are crucial aspects responsible for the magnitude of sperm production. Most importantly, we show that we could greatly benefit from investigations using different vertebrate experimental models, mainly now that there is a big concern regarding the decline in human sperm counts caused by a multitude of factors.

Comparative testis structure and function in three representative mice strains.

Mice are widely used as experimental models due to several positive characteristics and in particular their suitability for studies involving molecular biology and transgenesis. Despite the large number of mice strains currently available, the literature regarding their basic reproductive biology is still relatively scarce. Herein, we comparatively evaluated several important and correlated parameters related to testis structure and function in sexually mature male mice of inbred (C57BL/6, n = 19; BALB/c, n = 17) and outbred (Swiss, n = 17) strains, frequently utilized in research. Swiss mice presented significant variation for many parameters evaluated, including higher sperm production, mainly when compared to the C57BL/6 strain. However, some key parameters such as the duration of spermatogenesis, the Sertoli cell number per testis, and the spermatogenic efficiency were similar among the different strains. Although presenting significantly higher Leydig cell (LC) proportion and numbers per testis gram and per testis, the anogenital index was smaller in Swiss mice. Estradiol levels were lower in C57BL/6, whereas testosterone levels and 3ß-HSD expression were similar among strains. Regarding the LC/macrophages relationship, in comparison to the literature, we reported a much higher contribution of macrophages to the mouse intertubule. Thus, we estimated that there are around 1.6 macrophages per LC in BALB/c mice and this intriguing finding could be relevant to testis function in overall and spermatogonial biology in particular. Taken together, our results highlight the importance of knowing more accurately the testis structure and function in the different mice strains available for research, particularly when a specific testis parameter is being investigated.

Prepubertal PTU treatment in rat increases Sertoli cell number and sperm production.

The number of Sertoli cells (SCs) ultimately determines the upper limit of sperm production in the testis. Previous studies have shown that thyroid hormones (TH) receptors are abundantly expressed in developing SCs; therefore, it was highly significant to discover that transient neonatal hypothyroidism induced by the goitrogen 6-n-propyl-2-thiouracil (PTU) can extend SCs proliferation beyond the first 2 weeks postnatal and increase testis weight and sperm production. Further studies concluded that treatment must begin before day 8 post birth in rats. Recent studies, however, showed that SCs present in the transition region at the rete testis exhibit a more immature phenotype and have prolonged mitotic activity, which led to the hypothesis that SCs in this region will retain the capacity to respond to PTU treatment over a longer period of time. In the present study, male Wistar rats were treated with PTU from days 21 to 40 and were evaluated at 40 and 160 days of age. Similar to neonatal rat SCs, it was demonstrated that prepubertal SCs in the transition region have a high mitotic activity and are highly sensitive to TH levels. This delayed, transient hypothyroidism resulted in significantly increased testis weight, SCs number and daily sperm production. The results demonstrate for the first time that Sertoli cells showing plasticity in the transition region can be stimulated to increase proliferation and contribute to a late stage surge in testis weight and sperm output.

Germ cell-less hybrid fish: ideal recipient for spermatogonial transplantation for the rapid production of donor-derived sperm†.

An interspecific hybrid marine fish that developed a testis-like gonad without any germ cells, i.e., a germ cell-less gonad, was produced by hybridizing a female blue drum Nibea mitsukurii with a male white croaker Pennahia argentata. In this study, we evaluated the suitability of the germ cell-less fish as a recipient by transplanting donor testicular cells directly into the gonads through the urogenital papilla. The donor testicular cells were collected from hemizygous transgenic, green fluorescent protein (gfp) (+/-) blue drum, and transplanted into the germ cell-less gonads of the 6-month-old adult hybrid croakers. Fluorescent and histological observations showed the colonization, proliferation, and differentiation of transplanted spermatogonial cells in the gonads of hybrid croakers. The earliest production of spermatozoa in a hybrid recipient was observed at 7 weeks post-transplantation (pt), and 10% of the transplanted recipients produced donor-derived gfp-positive spermatozoa by 25 weeks pt. Sperm from the hybrid recipients were used to fertilize eggs from wild-type blue drums, and approximately 50% of the resulting offspring were gfp-positive, suggesting that all offspring originated from donor-derived sperm that were produced in the transplanted gfp (+/-) germ cells. To the best of our knowledge, this is the first report of successful spermatogonial transplantation using a germ cell-less adult fish as a recipient. This transplantation system has considerable advantages, such as the use of comparatively simple equipment and procedures, and rapid generation of donor-derived spermatogenesis and offspring, and presents numerous applications in commercial aquaculture.

Progress and biotechnological prospects in fish transgenesis.

The history of transgenesis is marked by milestones such as the development of cellular transdifferentiation, recombinant DNA, genetic modification of target cells, and finally, the generation of simpler genetically modified organisms (e.g. bacteria and mice). The first transgenic fish was developed in 1984, and since then, continuing technological advancements to improve gene transfer have led to more rapid, accurate, and efficient generation of transgenic animals. Among the established methods are microinjection, electroporation, lipofection, viral vectors, and gene targeting. Here, we review the history of animal transgenesis, with an emphasis on fish, in conjunction with major developments in genetic engineering over the past few decades. Importantly, spermatogonial stem cell modification and transplantation are two common techniques capable of revolutionizing the generation of transgenic fish. Furthermore, we discuss recent progress and future biotechnological prospects of fish transgenesis, which has strong applications for the aquaculture industry. Indeed, some transgenic fish are already available in the current market, validating continued efforts to improve economically important species with biotechnological advancements.

Gene delivery to Nile tilapia cells for transgenesis and the role of PI3K-c2α in angiogenesis.

Microinjection is commonly performed to achieve fish transgenesis; however, due to difficulties associated with this technique, new strategies are being developed. Here we evaluate the potential of lentiviral particles to genetically modify Nile tilapia cells to achieve transgenesis using three different approaches: spermatogonial stem cell (SSC) genetic modification and transplantation (SC), in vivo transduction of gametes (GT), and fertilised egg transduction (ET). The SC protocol using larvae generates animals with sustained production of modified sperm (80% of animals with 77% maximum sperm fluorescence [MSF]), but is a time-consuming protocol (sexual maturity in Nile tilapia is achieved at 6 months of age). GT is a faster technique, but the modified gamete production is temporary (70% of animals with 52% MSF). ET is an easier way to obtain mosaic transgenic animals compared to microinjection of eggs, but non-site-directed integration in the fish genome can be a problem. In this study, PI3Kc2α gene disruption impaired development during the embryo stage and caused premature death. The manipulator should choose a technique based on the time available for transgenic obtainment and if this generation is required to be continuous or not.

Fsh Stimulates Spermatogonial Proliferation and Differentiation in Zebrafish via Igf3.

Growth factors modulate germ line stem cell self-renewal and differentiation behavior. We investigate the effects of Igf3, a fish-specific member of the igf family. Fsh increased in a steroid-independent manner the number and mitotic index of single type A undifferentiated spermatogonia and of clones of type A differentiating spermatogonia in adult zebrafish testis. All 4 igf gene family members in zebrafish are expressed in the testis but in tissue culture only igf3 transcript levels increased in response to recombinant zebrafish Fsh. This occurred in a cAMP/protein kinase A-dependent manner, in line with the results of studies on the igf3 gene promoter. Igf3 protein was detected in Sertoli cells. Recombinant zebrafish Igf3 increased the mitotic index of type A undifferentiated and type A differentiating spermatogonia and up-regulated the expression of genes related to spermatogonial differentiation and entry into meiosis, but Igf3 did not modulate testicular androgen release. An Igf receptor inhibitor blocked these effects of Igf3. Importantly, the Igf receptor inhibitor also blocked Fsh-induced spermatogonial proliferation. We conclude that Fsh stimulated Sertoli cell production of Igf3, which promoted via Igf receptor signaling spermatogonial proliferation and differentiation and their entry into meiosis. Because previous work showed that Fsh also released spermatogonia from an inhibitory signal by down-regulating anti-Müllerian hormone and by stimulating androgen production, we can now present a model, in which Fsh orchestrates the activity of stimulatory (Igf3, androgens) and inhibitory (anti-Müllerian hormone) signals to promote spermatogenesis.

Androgens directly stimulate spermatogonial differentiation in juvenile Atlantic salmon (Salmo salar).

We studied the effects of androgens on early stages of spermatogenesis along with androgen receptor binding characteristics and the expression of selected testicular and pituitary genes. To this end, immature Atlantic salmon postsmolts received testosterone (T), adrenosterone (OA, which is converted in vivo into 11-ketotestosterone, 11-KT) or a combination of the two androgens (T+OA). Treatment with OA and T elevated the plasma levels of 11-KT and T, respectively, and co-injection of OA with T lead to high 11-KT levels but prevented plasma T levels to reach the levels observed after injecting T alone. Clear stimulatory effects were recorded as regards pituitary lhb and gnrhr4 transcript levels in fish receiving T, and to a lesser extent in fish receiving OA (but for the lhb transcript only). The two androgen receptors (Ara1 and Ara2) we cloned bound T and 11-KT and responded to these androgens in a similar way. Both androgens down-regulated testicular amh and increased igf3 transcript levels after 1 week of treatment, but effects on growth factor gene expression required sustained androgen stimulation and faded out in the groups with the decreasing T plasma levels. In fish exhibiting a sustained elevation of 11-KT plasma levels (OA and T+OA groups) for 2 weeks, the number of differentiating spermatogonia had increased while the number of undifferentiated spermatogonia decreased. Previous work showed that circulating gonadotropin levels did not increase following androgen treatments of gonad-intact immature male salmonids. Taken together, androgen treatment of immature males modulated testicular growth factor expression that, when sustained for 2 weeks, stimulated differentiation, but not self-renewal, of undifferentiated type A spermatogonia.

Biology and identity of fish spermatogonial stem cell.

Although present at relatively low number in the testis, spermatogonial stem cells (SSCs) are crucial for the establishment and maintenance of spermatogenesis in eukaryotes and, until recently, those cells were investigated in fish using morphological criteria. The isolation and characterization of these cells in fish have been so far limited by the lack of specific molecular markers, hampering the high SSCs biotechnological potential for aquaculture. However, some highly conserved vertebrate molecular markers, such as Gfra1 and Pou5f1/Oct4, are now available representing important candidates for studies evaluating the regulation of SSCs in fish and even functional investigations using germ cells transplantation. A technique already used to demonstrate that, different from mammals, fish germ stem cells (spermatogonia and oogonia) present high sexual plasticity that is determined by the somatic microenvironment. As relatively well established in mammals, and demonstrated in zebrafish and dogfish, this somatic environment is very important for the preferential location and regulation of SSCs. Importantly, a long-term in vitro culture system for SSCs has been now established for some fish species. Therefore, besides the aforementioned possibilities, such culture system would allow the development of strategies to in vitro investigate key regulatory and functional aspects of germline stem cells (ex: self-renewal and/or differentiation) or to amplify SSCs of rare, endangered, or commercially valuable fish species, representing an important tool for transgenesis and the development of new biotechnologies in fish production.

Spermatogenic cycle length and sperm production in the freshwater turtle Kinosternon scorpioides.

Kinosternon scorpioides is a Brazilian freshwater turtle that belongs to the class Reptilia, encompassing almost 10,000 species. Nevertheless, very little is known about the testicular quantitative parameters, particularly those related to spermatogenesis, in this vertebrate class. Our main objectives were to investigate in detail the structure and function of the testis in K. scorpioides, particularly the aspects related to spermatogenic cycle length and Sertoli cell (SC) and spermatogenic efficiencies. Nine sexually mature turtles were examined, and intraperitoneal bromodeoxyuridine injections were administered to estimate duration of spermatogenesis. Based on the acrosome development in spermatids and the overall germ cell associations, 10 stages of the seminiferous epithelium cycle were characterized. Similar to birds, humans, and some primate species, several stages were observed per seminiferous tubule cross-sections. One spermatogenic cycle and the entire spermatogenic process lasted, respectively, 12 and 53 days. The SC efficiency (number of round spermatids per SC) and daily sperm production per gram of testis were, respectively, 20 and 40 million spermatids. As established for mammals, our findings suggest that SC efficiency is also a critical determinant of sperm production in reptiles. To our knowledge, this is the first study to investigate the kinetics of spermatogenesis and testis function in any reptilian species. Besides allowing a better understanding of reproductive biology in reptiles, these data will be useful in comparative studies. Moreover, these results could provide the basis for investigations related to the evaluation of spermatogonial stem cell physiology niche in Kinosternon scorpioides.

Derivation of sperm from xenografted testis cells and tissues of the peccary (Tayassu tajacu).

Because the collared peccary (Tayassu tajacu) has a peculiar Leydig cell cytoarchitecture, this species represents a unique mammalian model for investigating testis function. Taking advantage of the well-established and very useful testis xenograft technique, in the present study, testis tissue and testis cell suspensions from immature collared peccaries (n=4; 3 months old) were xenografted in SCID mice (n=48) and evaluated at 2, 4, 6, and 8 months after grafting. Complete spermatogenesis was observed at 6 and 8 months after testis tissue xenografting. However, probably due to de novo testis morphogenesis and low androgen secretion, functionally evaluated by the seminal vesicle weight, a delay in spermatogenesis progression was observed in the testis cell suspension xenografts, with the production of fertile sperm only at 8 months after grafting. Importantly, demonstrating that the peculiar testicular cytoarchitecture of the collared peccary is intrinsically programmed, the unique Leydig cell arrangement observed in this species was re-established after de novo testis morphogenesis. The sperm collected from the xenografts resulted in diploid embryos that expressed the paternally imprinted gene NNAT after ICSI. The present study is the first to demonstrate complete spermatogenesis with the production of fertile sperm from testis cell suspension xenografts in a wild mammalian species. Therefore, due to its unique testicular cytoarchitecture, xenograft techniques, particularly testis cell suspensions, may represent a new and very promising approach to evaluate testis morphogenesis and to investigate spermatogonial stem cell physiology and niche in the collared peccary.

Morphometric evaluation of the spermatogonial stem cell distribution and niche in vertebrates.

Morphometry is a classical quantitative method often used in biology to provide a data basis for functional interpretations/interactions of a particular organ or system. Herein we took advantage of this valuable approach to evaluate the spermatogonial stem cell niche using the horse testis and immunocytochemical localization of GFRA1 [glial cell line-derived neurotrophic factor receptor produced by Sertoli cells)] as an example. Using the NIH ImageJ free software, we describe in detail all the necessary steps to investigate this specific and crucial microenvironment. Based on several recently published papers from our research group, this approach has proved to be fast, simple, and adaptable to a wide range of species and has the potential to be easily reproducible in different laboratories.

Phenotypic characterization and in vitro propagation and transplantation of the Nile tilapia (Oreochromis niloticus) spermatogonial stem cells.

In association with in vitro culture and transplantation, isolation of spermatogonial stem cells (SSCs) is an excellent approach for investigating spermatogonial physiology in vertebrates. However, in fish, the lack of SSC molecular markers represents a great limitation to identify/purify these cells, rendering it difficult to apply several valuable biotechnologies in fish-farming. Herein, we describe potential molecular markers, which served to phenotypically characterize, cultivate and transplant Nile tilapia SSCs. Immunolocalization revealed that Gfra1 is expressed exclusively in single type A undifferentiated spermatogonia (Aund, presumptive SSCs). Likewise, the expression of Nanos2 protein was observed in Aund cells. However, Nanos2-positive spermatogonia have also been identified in cysts with two to eight germ cells that encompass type A differentiated spermatogonia (Adiff). Moreover, we also established effective primary culture conditions that allowed the Nile tilapia spermatogonia to expand their population for at least one month while conserving their original undifferentiated (stemness) characteristics. The maintenance of Aund spermatogonial phenotype was demonstrated by the expression of early germ cell specific markers and, more convincingly, by their ability to colonize and develop in the busulfan-treated adult Nile tilapia recipient testes after germ cell transplantation. In addition to advancing our knowledge on the identity and physiology of fish SSCs, these findings provide the first step in establishing a system that will allow fish SSCs expansion in vitro, representing an important progress towards the development of new biotechnologies in aquaculture, including the possibility of producing transgenic fish.

Spermatogonial stem cell niche and spermatogonial stem cell transplantation in zebrafish.

BACKGROUND: Spermatogonial stem cells (SSCs) are the foundation of spermatogenesis, and reside within a specific microenvironment in the testes called "niche" which regulates stem cell properties, such as, self-renewal, pluripotency, quiescence and their ability to differentiate. METHODOLOGY/PRINCIPAL FINDINGS: Here, we introduce zebrafish as a new model for the study of SSCs in vertebrates. Using 5'-bromo-2'-deoxyuridine (BrdU), we identified long term BrdU-retaining germ cells, type A undifferentiated spermatogonia as putative stem cells in zebrafish testes. Similar to rodents, these cells were preferentially located near the interstitium, suggesting that the SSC niche is related to interstitial elements and might be conserved across vertebrates. This localization was also confirmed by analyzing the topographical distribution of type A undifferentiated spermatogonia in normal, vasa::egfp and fli::egfp zebrafish testes. In the latter one, the topographical arrangement suggested that the vasculature is important for the SSC niche, perhaps as a supplier of nutrients, oxygen and/or signaling molecules. We also developed an SSC transplantation technique for both male and female recipients as an assay to evaluate the presence, biological activity, and plasticity of the SSC candidates in zebrafish. CONCLUSIONS/SIGNIFICANCE: We demonstrated donor-derived spermato- and oogenesis in male and female recipients, respectively, indicating the stemness of type A undifferentiated spermatogonia and their plasticity when placed into an environment different from their original niche. Similar to other vertebrates, the transplantation efficiency was low. This might be attributed to the testicular microenvironment created after busulfan depletion in the recipients, which may have caused an imbalance between factors regulating self-renewal or differentiation of the transplanted SSCs.

Postnatal testis development, Sertoli cell proliferation and number of different spermatogonial types in C57BL/6J mice made transiently hypo- and hyperthyroidic during the neonatal period.

The role of thyroid hormones in testis structure and function has been fairly well studied in laboratory rodents. However, there are no comprehensive data in the literature for mice regarding the effects of transiently induced neonatal hypo- and hyperthyroidism on testis and spermatogonial cell development from birth to adulthood. Our goals were to evaluate the effects of propylthiouracil (PTU) and triidothyronine (T3) on Sertoli cell proliferation/differentiation and to correlate these events with the evolution of the spermatogenic process, tubular lumen formation, blood vessel volume density, and size and number of different spermatogonial types. Although Sertoli cell maturation was accelerated or delayed, respectively, in T3- and PTU-treated mice, the pace of the germ cell maturation was only slightly altered before puberty and the period of Sertoli cell proliferation was apparently not affected by the treatments. However, compared with controls, the total number of Sertoli cells per testis from 10 days of age to adulthood was significantly increased and decreased in PTU- and T3-treated mice, respectively. In comparison to all other spermatogonia, type A(2) was the largest cell in all ages and groups investigated. The PTU-treated mice had a significantly increased total number of undifferentiated spermatogonia as well as volume and percentage of vessels/capillaries, probably due to the higher number of Sertoli cells, particularly at 10 days of age. Taken together, our results suggest that neonatal hypothyroidism may be a valuable tool for studying spermatogonial biology as well as a means for providing more spermatogonial stem cells that could potentially be used for spermatogonial transplantation, thereby optimizing the efficiency of this technique when young mice are used as donors.

Spermatogenesis in fish.

Spermatogenesis is a developmental process during which a small number of diploid spermatogonial stem cells produce a large number of highly differentiated spermatozoa carrying a haploid, recombined genome. We characterise morphologically the different germ cell stages with particular attention for the spermatogonial generations, including the stem cells and their specific capacity to colonise a recipient's testis after transplantation. We propose a nomenclature for fish germ cells to improve the comparability among different teleost fish but also to higher vertebrates. Survival and development of germ cells depends on their continuous and close contact to Sertoli cells, and we review their multiple roles in the cystic mode of spermatogenesis seen in fish. We then discuss gene expression patterns associated with testis maturation. The endocrine system of vertebrates has evolved as master control system over spermatogenesis. In fish, both pituitary gonadotropins LH and FSH stimulate gonadal sex steroid hormone production directly by activating Leydig cells. Information is reviewed on the effects of progestin, androgens, and estrogens on global testicular gene expression patterns (microarray analysis), and on the molecular mechanisms by which steroids regulate specific candidate genes (identified by subtractive hybridization approaches) during early stages of testis maturation. Moreover, progestin and androgen effects on spermiation and milt hydration are discussed. Sex steroids mainly act via receptors expressed by Sertoli cells. One type of response is that Sertoli cells change growth factor expression, which subsequently modulates germ cell proliferation/differentiation via mechanisms yet to be characterised. Finally, we review data on germ cell autonomous processes, mainly derived from loss-of-function mutant fish lines, before identifying a number of focus areas for future research activities.

Effects of different temperatures on testis structure and function, with emphasis on somatic cells, in sexually mature Nile tilapias (Oreochromis niloticus).

The Nile tilapia (Oreochromis niloticus) is economically one of the most important freshwater fish and is an excellent model for studies under laboratory conditions. Temperature is considered a very important modulator of reproductive activity in fish, although few studies have specifically addressed the effects of this key factor on morphological and functional aspects of teleost testes. Therefore, our main objectives in the present study were to analyze the effects of different temperatures (20, 25, 30, and 35 degrees C) on testicular somatic and germ cells in sexually mature Nile tilapias. Compared with fish kept at other temperatures, tilapias maintained at 20 degrees C demonstrated increased (P < 0.05) Sertoli cell and Leydig cell proliferation, volume density and frequency of most type B spermatogonia, and germ cell apoptosis. Conversely, tubular fluid secretion was decreased (P < 0.05) in the same animals. Although not significant, type A spermatogonia proliferation followed the pattern established for Sertoli cell and Leydig cell mitotic activity, suggesting that they preferentially would proliferate at lower temperatures. Based on most results found in our study and considering that tilapias are nonseasonal breeders, we suggest a model for temperature action on tilapia testes in which lower temperature (20 degrees C) would favor type A spermatogonial renewal, Sertoli cell and Leydig cell proliferation, and germ cell apoptosis, whereas higher temperatures (30-35 degrees C) would trigger rapid germ cell differentiation. Thus, tilapias could potentially be utilized in studies involving hormones and factors related to Sertoli cell and Leydig cell proliferation and spermatogonial self-renewal or differentiation.

Development and function of the adult generation of Leydig cells in mice with Sertoli cell-selective or total ablation of the androgen receptor.

It is established that androgens and unidentified Sertoli cell (SC)-derived factors can influence the development of adult Leydig cells (LC) in rodents, but the mechanisms are unclear. We evaluated adult LC development and function in SC-selective androgen receptor (AR) knockout (SCARKO) and complete AR knockout (ARKO) mice. In controls, LC number increased 26-fold and LC size increased by approximately 2-fold between 12 and 140 d of age. LC number in SCARKOs was normal on d 12, but was reduced by more than 40% at later ages, although LC were larger and contained more lipid droplets and mitochondria than control LC by adulthood. ARKO LC number was reduced by up to 83% at all ages compared with controls, and LC size did not increase beyond d 12. Serum LH and testosterone levels and seminal vesicle weights were comparable in adult SCARKOs and controls, whereas LH levels were elevated 8-fold in ARKOs, although testosterone levels appeared normal. Immunohistochemistry and quantitative PCR for LC-specific markers indicated steroidogenic function per LC was probably increased in SCARKOs and reduced in ARKOs. In SCARKOs, insulin-like factor-3 and estrogen sulfotransferase (EST) mRNA expression were unchanged and increased 3-fold, respectively, compared with controls, whereas the expression of both was reduced more than 90% in ARKOs. Changes in EST expression, coupled with reduced platelet-derived growth factor-A expression, are potential causes of altered LC number and function in SCARKOs. These results show that loss of androgen action on SC has major consequences for LC development, and this could be mediated indirectly via platelet-derived growth factor-A and/or estrogens/EST.