Exposure in utero to di(n-butyl) phthalate alters the vimentin cytoskeleton of fetal rat Sertoli cells and disrupts Sertoli cell-gonocyte contact

Di(n-butyl) phthalate (DBP) is commonly used in personal care products and as a plasticizer to soften consumer plastic products. Male rats exposed to DBP in utero have malformations of the male reproductive tract and testicular atrophy characterized by degeneration of seminiferous epithelium and decreased sperm production. In the fetal testis, in utero exposure to DBP reportedly resulted in reduced testosterone levels, Leydig cell aggregates, and multinucleated gonocytes (MNG). We investigated whether exposure in utero to DBP affects rat fetal Sertoli cells and compromises interactions between Sertoli and germ cells in the developing testis. Histological examination showed that MNG occurred at low frequency in the normal fetal rat testis. Exposure in utero at the dose level of DBP above estimated environmental or occupational human exposure levels significantly increased the number of these abnormal germ cells. Postnatally, MNG exhibited aberrant mitoses and were detected at the basal lamina. MNG were not apoptotic in the fetal and postnatal rat testes, as indicated by TUNEL. Sertoli cells in DBP-exposed fetal testis had retracted apical processes, altered organization of the vimentin cytoskeleton, and abnormal cell-cell contacts with gonocytes. The effect of DBP on Sertoli cell morphology at the level of light microscopy was reversed after birth and cessation of exposure. Our data indicate that fetal Sertoli cells are targeted by exposure in utero to DBP and suggest that abnormal interactions between Sertoli and germ cells during fetal life play a role in the development of MNG.     

Involvement of actin filaments in mouse testicular cord organization in vivo and in vitro.

Involvement of actin filaments in mouse fetal testicular differentiation was examined in vivo and in vitro. During testicular cord formation in vivo, actin filaments accumulated in the basal cytoplasm of Sertoli cells. Addition of cytochalasin D (CD) to organ cultures of undifferentiated gonadal primordia significantly inhibited testicular cord formation. In brief, treatment with 25 ng/ml CD induced the formation of slender testicular cords, and treatment with 50 ng/ml largely inhibited cord formation in the explants. However, development and growth of the testicular parenchyma and Leydig cell differentiation occurred in the presence of CD. By electron microscopic and immunohistochemical examinations, it became clear that CD also affected formation of the basal lamina and accumulation of vimentin filaments in Sertoli cells. On the other hand, treatment with colcemid at 12.5 or 15 ng/ml prevented growth of the testicular parenchyma and development of interstitial regions. Interestingly, testicular cords formed under this condition. These results indicate that the basal actin filaments of Sertoli cells may play an important role in testicular cord formation, especially Sertoli cell polarization. Cell mitosis and/or microtubules, on the other hand, may not be directly involved in this process.     

Cell type-autonomous and non-autonomous requirements for Dmrt1 in postnatal testis differentiation

Genes containing the DM domain, a conserved DNA binding motif first found in Doublesex of Drosophila and mab-3 of Caenorhabditis elegans, regulate sexual differentiation in multiple phyla. The DM domain gene Dmrt1 is essential for testicular differentiation in vertebrates. In the mouse, Dmrt1 is expressed in pre-meiotic germ cells and in Sertoli cells, which provide essential support for spermatogenesis. Dmrt1 null mutant mice have severely dysgenic testes in which Sertoli cells and germ cells both fail to differentiate properly after birth. Here we use conditional gene targeting to identify the functions of Dmrt1 in each cell type. We find that Dmrt1 is required in Sertoli cells for their postnatal differentiation, and for germ line maintenance and for meiotic progression. Dmrt1 is required in germ cells for their radial migration to the periphery of the seminiferous tubule where the spermatogenic niche will form, for mitotic reactivation and for survival beyond the first postnatal week. Thus Dmrt1 activity is required autonomously in the Sertoli and germ cell lineages, and Dmrt1 activity in Sertoli cells is also required non-autonomously to maintain the germ line. These results demonstrate that Dmrt1 plays multiple roles in controlling the remodeling and differentiation of the juvenile testis.     

Testicular cell differentiation in fetal mouse ovaries following transplantation into adult male mice

Testicular development is a complicated process involving differentiation and arrangement of several cell types. To analyze the process of testicular organization we examined the sequence of the appearance of testicular structures induced in fetal ovaries following transplantation. Fetal mouse ovaries on the twelfth day of gestation were transplanted beneath the kidney capsules of adult male mice. They continued to develop morphologically as ovaries until the eleventh day after transplantation, when seminiferous cord formation and testosterone production began in addition to follicle development (ovotestes). Between the eleventh and fourteenth day after transplantation, ovarian grafts frequently contained transitional structures consisting of Sertoli cells, pregranulosa cells, a third type of cells which show intermediate characteristics between Sertoli and pregranulosa cells, and oocytes enclosed by common basal lamina. Leydig cells or peritubular myoid cells were not found in the transitional area, whereas these cells were present around seminiferous cords composed only of Sertoli cells. Oocytes were absent or degenerated in the well-developed seminiferous cords. The present findings suggest that, in ovarian grafts, pregranulosa cells can differentiate into Sertoli cells, which are responsible for the organization of the seminiferous cords, degeneration of oocytes, and differentiation of other testicular somatic cell types.     

Expression of Connexin 43 in normal canine testes and canine testicular tumors

In human testis, gap junctions containing connexin(Cx)43 are located within the seminiferous epithelium between Sertoli cells and between Sertoli and germ cells. Cx43 is known to play a role in the differentiation and proliferation of these cell types. It can further be associated with human seminoma development. The dog has been proposed as a model for studies of the male reproductive system, because of the frequent occurrence of testicular neoplasms. Thus, we investigated Cx43-mRNA and -protein expression in testes of normal prepubertal dogs, adult dogs, and in canine testicular tumors. Sertoli cells in prepubertal cords express Cx43 mRNA, but do synthesize only less Cx43 protein. Within the seminiferous tubules, Cx43 mRNA was detected in Sertoli cells, spermatogonia, and spermatocytes. Cx43 protein was mainly present in the basal compartment. In canine testicular tumors Cx43 mRNA was detectable in both seminoma and neoplastic Sertoli cells, whereas Cx43 protein was only found in neoplastic Sertoli cells. Our data indicate that Cx43 is regulated differentially in testicular tumors and that alterations of Cx43 expression may be involved in the pathogenesis of canine testicular malignancies. This study represents the first morphological work on the spatiotemporal expression pattern of Cx43 in normal and neoplastic canine testis.     

Role of β-catenin in post-meiotic male germ cell differentiation

Though roles of β-catenin signaling during testis development have been well established, relatively little is known about its role in postnatal testicular physiology. Even less is known about its role in post-meiotic germ cell development and differentiation. Here, we report that β-catenin is highly expressed in post-meiotic germ cells and plays an important role during spermiogenesis in mice. Spermatid-specific deletion of β-catenin resulted in significantly reduced sperm count, increased germ cell apoptosis and impaired fertility. In addition, ultrastructural studies show that the loss of β-catenin in post-meiotic germ cells led to acrosomal defects, anomalous release of immature spermatids and disruption of adherens junctions between Sertoli cells and elongating spermatids (apical ectoplasmic specialization; ES). These defects are likely due to altered expression of several genes reportedly involved in Sertoli cell-germ cell adhesion and germ cell differentiation, as revealed by gene expression analysis. Taken together, our results suggest that β-catenin is an important molecular link that integrates Sertoli cell-germ cell adhesion with the signaling events essential for post-meiotic germ cell development and maturation. Since β-catenin is also highly expressed in the Sertoli cells, we propose that binding of germ cell β-catenin complex to β-catenin complex on Sertoli cell at the apical ES surface triggers a signaling cascade that regulates post-meiotic germ cell differentiation.     

Desert hedgehog (Dhh) gene is required in the mouse testis for formation of adult-type Leydig cells and normal development of peritubular cells and seminiferous tubules

Testes from adult and prepubertal mice lacking the Desert hedgehog (DHH:) gene were examined in order to describe further the role of Dhh in spermatogenesis because, in a previous report, DHH:-null male mice were shown to be sterile. Dhh is a signaling molecule expressed by Sertoli cells. Its receptor, patched (Ptc), has been previously localized to Leydig cells and is herein described as being localized also to peritubular cells. Two phenotypes of the mice were observed: masculinized (7.5% of DHH:-null males) and feminized (92.5%), both of which displayed abnormal peritubular tissue and severely restricted spermatogenesis. Testes from adult feminized animals lacked adult-type Leydig cells and displayed numerous undifferentiated fibroblastic cells in the interstitium that produced abundant collagen. The basal lamina, normally present between the myoid cells and Sertoli cells, was focally absent. We speculate that the abnormal basal lamina contributed to other characteristics, such as extracordal gonocytes, apolar Sertoli cells, and anastomotic seminiferous tubules. The two DHH:-null phenotypes described have common peritubular cell defects that may be indicative of the essential role of peritubular cells in development of tubular morphology, the differentiation of Leydig cells, and the ultimate support of spermatogenesis.     

Male hypogonadism as a candidate of deficiency of postnatal testicular growth or differentiating factor(s): a new autosomal recessive mutation in the rat

A new rat mutant showing severe male hypogonadism (hgn for the gene symbol) was found and isolated from the hereditary hydronephrosis rat strain originating from the Wistar-Imamichi rat. The affected rats have very tiny testes that weigh 26 mg in the adult, and contain small numbers of seminiferous tubules with degenerated Sertoli cells. It is difficult to identify the gonocyte in the seminiferous tubules in the mutant testis. Very small male accessory sex organs are all present in the mutant. The number of chromosomes is 42 (40A + XY). The structure of each chromosome is normal, showing that the mutant has male sex genes. Thus, it was considered that this status is not due to either lack of the H-Y antigen or Muellerian inhibiting factor or expression of the Tfm. By analyzing the pedigree of the matings producing the mutant, it was concluded that the status was expressed only in the male, but inherited with a single autosomal recessive trait with existence of the phenotypically normal fertile female recessive homozygote. A possible deficiency of certain known or unknown testicular growth or differentiating factor(s) in the mutant is suggested.     

Sertoli cells in the testis of caecilians, Ichthyophis tricolor and Uraeotyphlus cf. narayani (Amphibia: Gymnophiona): light and electron microscopic perspective

The caecilians have evolved a unique pattern of cystic spermatogenesis in which cysts representing different stages in spermatogenesis coexist in a testis lobule. We examined unsettled issues relating to the organization of the caecilian testis lobules, including the occurrence of a fatty matrix, the possibility of both peripheral and central Sertoli cells, the origin of Sertoli cells from follicular cells, and the disengagement of older Sertoli cells to become loose central Sertoli cells. We subjected the testis of Ichthyophis tricolor (Ichthyophiidae) and Uraeotyphlus cf. narayani (Uraeotyphliidae) from the Western Ghats of Kerala, India, to light and transmission electron microscopic studies. Irrespective of the functional state of the testis, whether active or regressed, Sertoli cells constitute a permanent feature of the lobules. The tall Sertoli cells adherent to the basal lamina with basally located pleomorphic nuclei extend deeper into the lobule to meet at the core. There they provide for association of germ cells at different stages of differentiation, an aspect that has earlier been misconceived as the fatty matrix. Germ cells up to the 4-cell stage remain in the intercalating region of the Sertoli cells and they are located at the apices of the Sertoli cells from the 8-cell stage onwards. The developing germ cells are intimately associated with the Sertoli cell adherent to the basal lamina until spermiation. There are ameboid cells in the core of the lobules that appear to interact with the germ cells at the face opposite to their attachment with the Sertoli cells. Adherence of the Sertoli cells to the basal lamina is a permanent feature of the caecilian testicular lobules. The ameboid cells in the core are neither Sertoli cells nor their degeneration products.     

Chemotactic role of neurotropin 3 in the embryonic testis that facilitates male sex determination

The first morphological event after initiation of male sex determination is seminiferous cord formation in the embryonic testis. Cord formation requires migration of pre-peritubular myoid cells from the adjacent mesonephros. The embryonic Sertoli cells are the first testicular cells to differentiate and have been shown to express neurotropin-3 (NT3), which can act on high-affinity trkC receptors expressed on migrating mesonephros cells. NT3 expression is elevated in the embryonic testis during the time of seminiferous cord formation. A trkC receptor tyrophostin inhibitor, AG879, was found to inhibit seminiferous cord formation and mesonephros cell migration. Beads containing NT3 were found to directly promote mesonephros cell migration into the gonad. Beads containing other growth factors such as epidermal growth factor (EGF) did not influence cell migration. At male sex determination the SRY gene promotes testis development and the expression of downstream sex differentiation genes such as SOX-9. Inhibition of NT3 actions caused a reduction in the expression of SOX-9. Combined observations suggest that when male sex determination is initiated, the developing Sertoli cells express NT3 as a chemotactic agent for migrating mesonephros cells, which are essential to promote embryonic testis cord formation and influence downstream male sex differentiation.     

Adenomatous polyposis coli (APC) is essential for maintaining the integrity of the seminiferous epithelium

Sertoli cells provide the microenvironment necessary for germ cell development and spermatogenesis; disruption of Sertoli cell morphology or function can lead to germ cell aplasia, which is observed in testicular dysgenesis syndrome. Mutation of the adenomatous polyposis coli (APC) gene has been associated with various human cancers, including testicular cancer, but its involvement in nonmalignant testicular pathologies has not been reported. We have developed a mouse model (APC(cko)) that expresses a truncated form of APC in Sertoli cells. Despite normal embryonic and early postnatal testicular development in APC(cko) mice, premature germ cell loss and Sertoli cell-only seminiferous tubules were observed in mutant testes without affecting Sertoli cell quiescence, apoptosis, or differentiation, which were confirmed by the absence of both proliferating cell nuclear antigen, DNA strand breaks, and anti-Müllerian hormone, respectively. We show that mutant Sertoli cells lose their apical extensions, which would normally enclose germ cells during various stages of spermatogenesis, and were unable to maintain the blood-testis barrier because of disrupted expression of junctional proteins. We also observed an up-regulation of Snail and Slug, markers suggestive of epithelial-mesenchymal transition in the Sertoli cells, but tumorigenesis was not observed. No comparable phenotype was observed with Sertoli cell-specific loss-of-function mutations in β-catenin, leading us to speculate that truncation of APC in Sertoli cells results in progressive degeneration of the seminiferous tubules by a mechanism that disrupts the integrity of Sertoli cell junctions independently of APC-regulated β-catenin activities and leads to development of a Sertoli cell-only phenotype.     

The pesticide methoxychlor given orally during the perinatal/juvenile period,reduced the spermatogenic potential of males as adults by reducing their Sertoli cell number

Perinatal and juvenile oral treatment of rats with the insecticide, methoxychlor (MXC), reduced testicular size and other reproductive indices including the number of epididymal spermatozoa in those animals as adults 161. The objective was to determine if these males exposed during development had fewer Sertoli cells which might explain these testicular effects. Rat dams were gavaged with MXC at 0, 5, 50, or 150 mg x kg(-1) x day(-1) for the week before and after they gave birth. Resulting male pups (15/group) then were dosed directly from postnatal day 7 to 42. Testes were fixed in Bouin's and in OsO4, embedded in Epon and sectioned at 0.5 microm, stained with toluidine blue, and evaluated stereologically or cut at 20 microm to measure Sertoli cell nuclei with Nomarski optics. Sertoli cell number was calculated as the volume density of the nucleus times the parenchymal weight (90% of testicular weight) divided by the volume of a single Sertoli cell nucleus. Across dose groups, there were no changes in the nuclear volume density, the volume of a single nucleus, or the number of Sertoli cells per g parenchyma. There were highly significant dose-related changes in the volume of Sertoli cell nuclei per testis and the number of Sertoli cells per testis. Reduced testicular weight (r = 0.94) and reduced numbers of epididymal spermatozoa (r = 0.43) were significantly (p < 0.01) correlated to reduced number of Sertoli cells per testis. Hence, perinatal and juvenile oral exposure to MXC can reduce spermatogenic potential of males as adults by reducing their number of Sertoli cells.     

Laminin ultrastructural immunolocalization in rat testis during ontogenesis

Summary The synthesis of one of the main glycoproteins of the basement membrane, the laminin, was demonstrated by ultrastructural immunolocalization during rat foetal (16th day to 20th day of gestation) and postnatal development of the testis. The lamina densa, part of seminiferous tubular basement membrane, is labeled uniformly at all studied stages. The lamina lucida is not well defined before the postnatal stages, at which times discrete immunostaining extends from the lamina densa to the adjacent seminiferous epithelial cells (spermatogonia and Sertoli cells). The extracellular matrix around the peritubular cells is not labeled before birth. Intracellular immunostaining was detected as early as the 16th day of gestation in both Sertoli cells and cells around the seminiferous tubules which will transform later into peritubular cells. It was located in rough endoplasmic reticulum (RER) cisternae and secretory vesicles. After 18–20 days of postnatal life, the immunostaining faints progressively. Some positive material is seen in the RER of the gonocytes at all studied stages.Sertoli cells and peritubular cells are the main producing cells of laminin after the 16th of gestation. The laminin secreted by gonocytes may play an important role in adhesion of gonocytes to the lamina densa and adjacent Sertoli cells before their transition from basal compartment to adluminal compartment.     

Laminin ultrastructural immunolocalization in rat testis during ontogenesis

The synthesis of one of the main glycoproteins of the basement membrane, the laminin, was demonstrated by ultrastructural immunolocalization during rat foetal (16th day to 20th day of gestation) and postnatal development of the testis. The lamina densa, part of seminiferous tubular basement membrane, is labeled uniformly at all studied stages. The lamina lucida is not well defined before the postnatal stages, at which times discrete immunostaining extends from the lamina densa to the adjacent seminiferous epithelial cells (spermatogonia and Sertoli cells). The extracellular matrix around the peritubular cells is not labeled before birth. Intracellular immunostaining was detected as early as the 16th day of gestation in both Sertoli cells and cells around the seminiferous tubules which will transform later into peritubular cells. It was located in rough endoplasmic reticulum (RER) cisternae and secretory vesicles. After 18-20 days of postnatal life, the immunostaining faints progressively. Some positive material is seen in the RER of the gonocytes at all studied stages. Sertoli cells and peritubular cells are the main producing cells of laminin after the 16th of gestation. The laminin secreted by gonocytes may play an important role in adhesion of gonocytes to the lamina densa and adjacent Sertoli cells before their transition from basal compartment to adluminal compartment.     

Transforming growth factor-alpha and epidermal growth factor receptor gene expression and action during pubertal development of the seminiferous tubule.

The potential role of transforming growth factor-alpha (TGF-alpha) as a mediator of cell-cell interactions in the growth and development of the testis was examined. Developing rat testes were collected, and preparations of mesenchymal-derived peritubular cells and epithelial-like Sertoli cells were isolated from prepubertal, midpubertal, and late pubertal rat testes. The developmental expression of TGF-alpha and its receptor, the epidermal growth factor receptor (EGFR), in whole testis and isolated cell types was determined using a nuclease protection assay. TGF-alpha and EGFR gene expression were predominant early in testis development and decreased during pubertal development. TGF-alpha expression was greatest in prepubertal peritubular cells. Sertoli cell TGF-alpha expression remained relatively constant during development, with a slight decline at the later pubertal stages. EGFR gene expression was predominant in peritublar cells throughout development. A low level of EGFR expression was detected in Sertoli cells. Scatchard analysis confirmed the presence of high affinity receptors on peritubular cells; however, no functional receptors were detected on Sertoli cells from any stage of development examined. Interestingly, low-level EGFR gene expression was also detected in pachytene spermatocytes and round spermatids. TGF-alpha was found to stimulate [3H] thymidine incorporation into DNA and increase cellular proliferation of peritubular cells from each developmental stage, while having no effect on Sertoli cells. The in vivo physiological significance of TGF-alpha was evaluated in a line of transgenic mice which overexpress TGF-alpha in the mature testis. These transgenic animals had no abnormal testicular morphology or alterations in spermatogenesis. Observations demonstrate that gene expression of TGF-alpha and its receptor is high during early pubertal stages when somatic cell growth is predominant and low at late pubertal stages when somatic cell proliferation is reduced. TGF-alpha can act as an autocrine/paracrine mitogen for the mesenchymal-derived peritubular cell, while actions on the Sertoli cell population are not evident. The observation that spermatogenic cells express the EGFR gene, although the protein remains to be identified, implies that TGF-alpha may potentially mediate Sertoli-germinal cell interactions.     

Application of morphometric techniques to postnatal rat testes in organ culture: insights into testis growth

The extent of Sertoli cell proliferation during fetal and neonatal development determines the final adult testis size and potential for sperm output. To gain further knowledge of the factors that regulate Sertoli cell proliferation, the present study used a new approach to analyse changes in morphology and proliferation in the postnatal testis by combining organ culture with morphometric analysis. Fragments of rat testes from days 0 to 10 postpartum were cultured in contact with DMEM for 6 h or 72 h and fixed. The effects of ovine follicle-stimulating hormone (FSH) and activin were studied in an additional 72-h organ culture experiment using day 9 testes. Bromodeoxyuridine (BrdU) was added for the last 6 h of culture to mark proliferating cells. Two-microm sections of the fragments were analysed for morphological changes of the seminiferous cords, and the proportion of BrdU-labelled Sertoli and germ cells was determined. Assessment of 6-h samples revealed growth characteristics consistent with those observed in vivo during days 1-10 of postnatal development. From day 2 onwards, the volume fraction of seminiferous cords began to increase, while significant growth in cross-sectional area of the cords occurred only after day 6. In these culture conditions, germ cell proliferation and testicular architecture was consistent with that expected for the age of the tissue at time of explant. The proportion of dividing Sertoli cells declined from 15-20% at days 0-4 postpartum to below % at day 10 postpartum in the 6-h culture, and it was low or abolished in the 3-day culture at all time points. Activin and FSH together, but not singly, stimulated Sertoli cell proliferation in the 72-h culture. This paper presents a new approach to analysis of in vitro testis development. The combination of fragment culture and stereological analysis permits rigorous and detailed assessment of developmental changes in the postnatal testis.     

Effects of hypothyroidism on anti-mullerian hormone expression in the prepubertal rat testis

Differentiation of adult Leydig cells (ALC) in the prepubertal rat testis is stimulated by thyroid hormone (Thy) and inhibited by the Anti-Mullerian Hormone (AMH) produced by the immature Sertoli cell (SC). As Thy induces SC maturation in the prepubertal rat testis, we hypothesized that Thy stimulation of ALC differentiation is mediated via inhibition of AMH production by the SC with their maturation. If this hypothesis is true, AMH production by the prepubertal Sertoli cells in hypothyroid rats should not decline immediately after birth as in euthyroid rats, but should be maintained throughout the hypothyroid period at a similar or higher level to that of day 1 rats. This concept was tested using control rats of postnatal days (pd) 1, 7 and 14 and hypothyroid (fed 0.1% propyl thiouracil/PTU to lactating mothers) rats of pd7 and pd14. Presence of AMH in SC was examined by immunocytochemistry for AMH. Results demonstrated that testes of pd1 rats had intense AMH positive labeling exclusively in cytoplasm of SC. In testes of pd7 and pd14 control and PTU rats, a positive but weak labeling was also observed in cytoplasm of some SC; Germ cells and testicular interstitial cells were negative for AMH at all tested ages in both experimental groups. These findings suggest that AMH production by the prepubertal SC is independent of Sertoli cell maturation and not regulated by Thy. Therefore, Thy regulation of ALC differentiation in the prepubertal rat testis is unlikely to be mediated via inhibition of AMH produced by the SC with their maturation.     

Postnatal development of the connexion between tubulus seminiferus and tubulus rectus in the bovine testis

Summary Histology and ultrastructure of the connexion of seminiferous and straight testicular tubules were studied in 58 bovine testes of 29 animals ranging from 4 to 52 weeks of postnatal development. In the 4th and 8th week seminiferous tubules are solid. Their non-germinal supporting cells possess spherical nuclei in a basal location and a great amount of granular endoplasmic reticulum. The straight tubules have a narrow lumen and a stratified epithelium rich in intercellular canaliculi. Between 20 and 25 weeks the seminiferous tubules acquire a lumen and develop a terminal segment, the tip of which (terminal plug) protrudes into the cup-shaped modification of the adjacent straight tubule. At 30 weeks the structural differentiation between seminiferous tubule proper and its terminal segment has proceeded: in the former spermatocytes and spermatids make their first appearance, and the supporting cells have transformed to Sertoli cells. In the latter the morphology of the supporting cell preserves a more primitive state. Starting from the 16th week and proceeding through the 30th week and further, the epithelium of the tubulus rectus close to the connexion with the seminiferous tubule becomes monolayered by rearrangement of its cells and advances along the basal lamina into the area of the seminiferous tubule. Those cells of the seminiferous tubule that are cut off from the basal lamina by invading rectus cells degenerate. Between 40 and 52 weeks the adult situation is principally achieved. The terminal segment of the seminiferous tubule is tripartite consisting of transitional region, intermediate portion, and terminal plug. The terminal segment is surrounded by a vascular plexus. The straight testicular tubule adjacent to the terminal segment is modified into a cup region encompassing the terminal plug, followed by a narrow stalk region, which is lined by simple columnar epithelium. Mononuclear free cells are a constant feature of the tubulus rectus epithelium in all stages of postnatal development.Supported by grant Wr 7/6-6 from the Deutsche Forschungsge-meinschaft