Undifferentiated Gonads Research Articles

Origin, proliferation and differentiation of Leydig cells.

Mammalian sex determination involves complex interacting networks of cellular and hormonal signals leading to the development of male or female phenotypes. Three main sequential processes are involved: first, the establishment of chromosomal sex at fecundation (genetic sex); second, the development of the undifferentiated gonad into either testes or ovaries (gonadal sex); and third, differentiation of male or female internal and external genitalia (phenotypic sex). Male phenotype is controlled by two testicular hormones, the anti-Mullerian hormone (AMH) secreted by fetal Sertoli cells which induces regression of the Mullerian ducts, and testosterone produced by Leydig cells which induces differentiation of the Wolffian ducts into male reproductive organs, although conversion of testosterone into dehydrotestosterone is required for masculinization of the external genitalia. In the absence of testes, and therefore in the absence of both AMH and testosterone, the Wolffian ducts regress, creating a permissive environment for the differentiation of the Mullerian ducts and, thereby, female reproductive organs.

Patterns of keratins 8, 18 and 19 during gonadal differentiation in the mouse: sex- and time-dependent expression of keratin 19

The acidic keratins K18 and K19 have been shown to display a sex-specific expression during gonadal differentiation in the rat. To extend these findings, we have undertaken a study of the expression of genes encoding for K18 and K19 and their basic partner K8 in the mouse from 10.5 days of gestation until adulthood, using immunofluorescence, in situ hybridization, and reverse transcriptase polymerase chain reaction (RT-PCR). In the urogenital ridge at 10.5 days of gestation, K18, K19, and K8 are present, in both sexes, in coelomic epithelium in the area of the prospective gonad. At 11 days and 10 h of gestation, they are detected in differentiating gonadal blastema. In male gonads at 11 days and 16 h of gestation the first Sertoli cells differentiate. They are stained for anti-Müllerian hormone by immunofluorescence and appear as dispersed cells throughout the blastema. Progressively, they adhere to each other and form differentiating seminiferous cords. K19 disappears as Sertoli cells differentiate. K18 and K8 continue to be detected in Sertoli cells during fetal life and after birth until 14 days postpartum. In the adult testis, no keratin is observed. In differentiating ovaries, the three keratins are present in somatic cells of the ovigerous cords during fetal life and in primordial follicles differentiating from 1–2 days postpartum. In the course of follicular development, K19 is no longer detected as primordial follicles differentiate into growing follicles. K18 and K18 are present in all stages of follicular development. These results show both differences and similarities with the results previously obtained in the rat. In the mouse, in contrast to the rat, keratins are detected in adult ovaries, and K18 is found in undifferentiated gonads and in ovaries. K18 is, thus, not specific to the testis in the mouse, as it is in the rat. In both species, K19 ceases to be expressed in male gonads as Sertoli cells differentiate and form seminiferous cords. The present observations confirm that downregulation of K19 gene expression in the fetal testis is one of the earliest molecular events attesting the commitment of the undifferentiated gonad to the male differentiative pathway.

Delivery after surgical treatment of ovarian dysgerminoma

Dysgerminoma is a malignant tumor that arises from elements of undifferentiated gonads preserved in the ovarian portal from the embryonic period of development. Its frequency varies from 0.5 to 7% of all malignant ovarian tumors. Young women are more often affected, and large tumors occur in patients aged 13 to 18 years who have signs of infantilism.

Early transcription of the SRY gene by bovine preimplantation embryos

We have examined mRNA expression of two genes located on the Y chromosome, the sex-determining region Y gene (SRY) and the linked zinc finger gene (ZFY), using in vitro fertilized-in vitro cultured bovine embryos. Expression of the SRY gene, implicated in sex determination in mammals, has been reported to occur both for a short time at the sex-determining stage of development around the period of the primitive undifferentiated gonad and in the adult testis. In this study, using a sensitive reverse transcriptase-polymerase chain reaction (RT-PCR) assay, we detected SRY but not ZFY mRNA expression as early as the 4- to 8-cell stage and through to the blastocyst stage in bovine embryos. The expression of SRY at these early stages and the previous observation that in vitro-produced male bovine embryos develop faster in culture than female embryos suggest that sex differences are evident prior to gonadal differentiation and that preimplantation bovine embryos have sexually dimorphic gene expression at least with respect to SRY transcripts.

Early transcription of the SRY gene by bovine preimplantation embryos

We have examined mRNA expression of two genes located on the Y chromosome, the sex-determining region Y gene (SRY) and the linked zinc finger gene (ZFY), using in vitro fertilized–in vitro cultured bovine embryos. Expression of the SRY gene, implicated in sex determination in mammals, has been reported to occur both for a short time at the sex-determining stage of development around the period of the primitive undifferentiated gonad and in the adult testis. In this study, using a sensitive reverse transcriptase-polymerase chain reaction (RT-PCR) assay, we detected SRY but not ZFY mRNA expression as early as the 4- to 8-cell stage and through to the blastocyst stage in bovine embryos. The expression of SRY at these early stages and the previous observation that in vitro-produced male bovine embryos develop faster in culture than female embryos suggest that sex differences are evident prior to gonadal differentiation and that preimplantation bovine embryos have sexually dimorphic gene expression at least with respect to SRY transcripts. Mol. Reprod. Dev. 48:246–250, 1997. © 1997 Wiley-Liss, Inc.

Correlation among Thermosensitive Period, Estradiol Response, and Gonad Differentiation in the Sea TurtleLepidochelys olivacea

Reptile embryos with temperature sex determination have a thermosensitive period (TSP). The finding that exogenous estradiol (E2) overcomes the effect of male-promoting temperature led to the idea that temperature may regulate estrogen concentration in the gonad during TSP. Since interspecific variations in TSP and in the effect of exogenous E2 exist, we undertook a study in the olive ridleyLepidochelys olivacea.Four parameters were correlated: the TSP (time dimension), the thermosensitive stages (rate of development), gonad development (histological aspect), and the estradiol response. Two kinds of experiments were performed: (1) Eggs were shifted once, at different stages of development, from a male-promoting temperature to a female-temperature (or vice versa) for the remainder of development. (2) Eggs at male-promoting temperature were treated once with 6 or 12 μg of estradiol (E2) at various times of incubation. Sex ratio was established around hatching in each experimental series. We found that the temporal dimension of the TSP was around 7 days (Days 20–27 of incubation) at a male-promoting or a female-promoting temperature. The rate of development of the whole embryo and gonadal growth was faster at female-promoting temperature than at male-promoting temperature. Formation of the genital ridge began at stage 21–22 and histological differentiation of the gonads occurred around stage 26–27. Although these stages coincided with the TSP, at male-promoting temperature the thermosensitive stages occurred earlier (from stages 20–21 to stages 23–24) than at female-promoting temperature (from stages 23–24 to stages 26–27). Thus, at male promoting-temperature, sex was determined in embryos with incipient or undifferentiated gonads. In contrast, E2 treatment continued to feminize the gonads of embryos at a male-promoting temperature beyond the TSP up to stage 25–26, but the E2-induced ovaries were significantly smaller than temperature-induced ovaries. It is suggested that the doses of E2 used were higher than the concentration of endogenous E2 required for normal sex determination. The lack of correlation between sex determination and gonad differentiation suggests that irreversible molecular processes underlying sex determination occur earlier at male- than at female-promoting temperature. Results suggest that the male sex may be the default state and that the female condition must be imposed upon it.

Données nouvelles sur le rôle des glycoprotéines dans le chimiotactisme des cellules germinales chez l'embryon de poulet

Usingperoxidase-coupled Concanavalin A during the organogenesis of embryonic gonads, we show that there is a positive correlation between the expression of specific glycoprotein complexes by germ cells and the migratory activity of these cells. An experimental artifice enabled us to subject ‘sedentary’ primary gonocytes in the undifferentiated gonad to the attractive influence of a young germinal epithelium. Under these conditions, chemotactism was reactivated in the primary gonocytes, in conjunction with a stimulation of their expression of glycosylated complexes.

Early gonadal development and sexual differentiation in muskellunge (Esox masquinongy)

Primordial germ cells (PGCs) were first identified in muskellunge (Esox masquinongy) of 14 mm total length (TL) 3 weeks post fertilization. At 32 mm TL, gonad strings were complete and formed a typical gonad shape in cross section. Blood vessels were first found in the gonads with Crossmon staining at 46 mm TL. Some of the PGCs underwent mitotic division at this stage. The ovarian sac started to develop in a fish of 82 mm TL, while the germ cells were still considered to be undifferentiated. In a fish of 138 mm TL, female gonads could be clearly identified from the ovarian sac and groups of oogonia, whereas in another type of gonad, the morphology of undifferentiated gonads was maintained. Germ cells became numerous in both sexes at 211 mm TL. Female gonads contained lobes with germ cells, including oogonia, early-prophase oocytes, and large oocytes. Spermatogonia and cells undergoing mitosis were observed in the testis. Ovaries in a fish of 250 mm TL were at the early stage of perinucleolus (early diplotene). Our observations indicate that in muskellunge (i) the PGCs remained in a resting state for up to 8 weeks post fertilization, (ii) gametogenesis occurred earlier in females than in males, (iii) the gonads developed from an undifferentiated stage directly into an ovary or testis, and (iv) the somatic elements in the gonads differentiated prior to the germ cells.

Gonadal differentiation, intersexuality and sex ratios of European eel ( Anguilla anguill L.) maintained in captivity

Migrating European eel ( Anguilla anguilla L.) glass eels were collected annually in the Garonne Basin (France) during 1987–1989 and subsequently raised under freshwater conditions. In 1990 gonadal development was determined and after about 1 year it was re-examined to check if sex ratios had changed. Undifferentiated gonads were seldom found, and were mainly present in the youngest year sample 89-1 + (numbers refer to year of catch followed by freshwater age). This gonadal stage can develop directly into an ovary. Differentiation of male sex proceeded via an intersexual stage with male and female sex cells. Phenotypic expression of female gonadal sex was considered as being gonochoristic and differentiated. The female gonad gradually developed towards a fourth stage ovary, characterised by the presence of follicles containing oocytes arrested in the late prophase of meiosis. Yolk formation in oocytes was not observed. The intersexual progression to males was termed pre-maturational sex change. The testes of full grown males contained numerous tubules with spermatogonia in mitotic arrest. The percentage of intersexual eels in 1990 of the youngest year sample (89-1+) was very high (91%). Most of these animals differentiated into males in 1991 (89-2+). The percentages of undifferentiated and intersexual eels in the older year samples of 1988 and 1987 examined in 1990 were low, showing that sex differentiation in most animals had occurred after 2–3 years. The sex ratios expressed as males/(males + females) were 64% (88-2+) and 76% (88-3+); 84% (87-3+) and 83% (87-4+). Comparison of these sex ratios suggests that sex reversal did not occur. Selective mortality and migration, population density, feeding conditions and temperature are reviewed as possible factors controlling sex differentiation in European eel.

Distribution of desmin and fibronectin in chick embryo gonad during testicular cord formation.

The distribution of desmin and fibronectin in the chick embryo gonad in and after sexual differentiation was investigated immunohistochemically. In the undifferentiated gonad (stage 27), fibronectin was detected in the dorsal mesentery, basement membrane and part of the parenchyma, while desmin was not recognized. After sexual differentiation (stages 31 to 34), desmin-positive cells gradually increased in number with the progress of development. They were obviously seen in developing interstitial regions between testicular cords. However, fibronectin was hardly observed at the same stages. At stage 37, desmin-positive cells were present only in a part of the Leydig cells and myoid cells, while fibronectin was discontinuous in the basement membrane surrounding testicular cords. In addition, the expressions of desmin and fibronectin were simultaneously and adjacently recognized. These results suggest that myoid cells may produce fibronectin protein and that fibronectin mainly contributes to the formation of testicular cord basement membrane in chick embryos. Thus, the cell-to-cell and cell-to-matrix interactions may be important in testicular morphogenesis of chick embryos.

Molecular determinants of mammalian sex

Mammalian male sex determination is controlled by a complex hierarchy of gene regulatory proteins and hormones, which promote male gonadal development and regression of the female primordia. At the core of this pathway lies the SRY protein, the master developmental switch for testicu- lar differentiation and hence, the male sex. The three-dimensional struc- ture of the SRY-DNA complex suggests a model of developmental gene regulation in which proteins that alter DNA structure and promote the as- sembly of higher-order nucleoprotein complexes play an essential role in the timing of cell specialization events. leads directly to the development of the testes, and thus, to male sex. Regulated expression in the undifferentiated gonad Three gene products appear to play regulatory roles in the development of the undifferentiated gonad, namely the Wilms' tumor suppressor protein WT1 ~ef. 1), the orphan nuclear receptor protein SF-1 ~ef. 2) and a high mobility group (HMG) family member, which is closely related to SRY and known as SOX9 ~ef. 3)(Fig. 1). Each of these proteins possesses a sequence-specific DNA-binding domain and a putative protein-protein interaction domain, attributes that suggest that these pro- teins regulate transcription. Expression patterns for the genes encoding these proteins indicate that their influence is exerted before the appearance of morphological differences between the sexes in the developing gonad. IN MAMMALIAN SEXUAL differenti- ation, male and female developmental pathways are controlled by the presence or absence of hormones, which are pro- duced by the male and female gonads in the developing fetus. Before hormonal secretion, XX and XY embryos develop two pairs of genital ducts associated with the undifferentiated gonads. The Miillerian ducts have the potential to differentiate into the fallopian tubes, uterus, cervix and upper vagina of the female reproductive tract. The Wolffian ducts become the vas deferens, epi- didymis and seminal vesicles of the male reproductive tract. Consequently, for normal male or female development to occur, one genital duct system must differentiate while the other must regress (Fig. 1). Regression of the female pathway is achieved by the production of Miillerian inhibitory substance (MIS) (also known as the anti-M011erian hormone) in the fetal testicular Sertoli cells. Subse- quently, testosterone, produced by the Leydig cells of the testes, induces dif- ferentiation of the Wolffian ducts into male reproductive organs. In the ab- sence of testes, and therefore in the ab- sence of both MIS and testosterone, the Wolffian ducts passively regress, creat- ing a permissive environment for the M. H. Werner, J. R. Huth, A. M. Gronenborn and G. M. Clore are at the Laboratory of Chemical Physics, Building 5, National institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-0520, USA.

The role of SRY in mammalian sex determination.

The gene SRY (sex determining region of the Y), located at the distal region of the short arm of the Y chromosome, is necessary for male sex determination in mammals. SRY initiates the cascade of steps necessary to form a testis from an undifferentiated gonad. The SRY gene encodes an HMG (High Mobility Group) protein which may act as a transcription factor by binding to double stranded DNA and then bending the DNA. Mutations in SRY have been identified in some subjects with 46,XY pure gonadal dysgenesis. However the role for other autosomal and X-linked genes in testis determination is evident by the presence of a normal SRY gene in the majority of females with 46,XY pure gonadal dysgenesis and the lack of SRY in a minority of males with 46,XY maleness.

Effects of oestradiol—enriched diet and of feeding with porcine testicular tissue on macroscopic gonadal sex in European eels

Oestradiol–enriched food (25 mg kg‐1 oestradiol) fed for 91 days to small eels (2 or 7 g) with undifferentiated gonads (UD) significantly favoured development of gonads with the macroscopic appearance of ovaries (♀‐gonads) during the next 8 months. Fifty‐four and 51% of control eels and 75 and 78% ofoestradiol‐treated eels developed ♀‐gonads. Minced porcine testicular tissue fed to 2‐g eels for up to 371 days had effects similar to oestradiol‐enriched food (68% with ♀‐gonads), whereas feeding for only 91 days had no effect during the next 8 months. Oestradiol‐enriched food was also fed to larger eels with already macroscopically differentiated gonads (30–70 g; 95% with ♀‐gonads, 5% with ♀‐gonads). After 27 days significantly more eels with a ♀‐gonad or with a mixture of ovary‐like and testis‐like regions in the gonads (♂+♀‐gonads) were found. After 96 days there were 44% with ♀‐gonads, 40% with ♂+♀‐gonads and 16% with ♀‐gonads. Oestradiol thus had a feminizing effect, not only on morphologically undifferentiated gonads but also on morphologically differentiated ♀‐gonads. The presence of sex steroid hormones or their precursors in porcine testicular tissue may also exert a feminizing influence. In all experiments the hormone‐fed groups showed a tendency (not significant) towards increased growth rate. In small eels early rapid growth and differentiation of ♀‐gonads were clearly correlated, both in hormone treated and in control eels. Otherwise no correlation was found between growth rate and gonadal sex.

Histological study of the development and sex differentiation of the gonad in the European eel

The histological structure of the gonads was studied in yellow eels sampled from a coastal lagoon and from stocks reared in an aquaculture plant showing different sex ratios. Gonad development related to body size rather than to age and underwent an intermediate stage characterized by a structure of an early testis but containing oogonia and oocytes. This gonad was called the Syrski organ and the stage juvenile ambisexual. Ovaries were found in eels from 22–30 cm in length, possibly derived from undifferentiated gonads or from Syrski organs. Fully differentiated testes were found in eels >35 cm, derived from Syrski organs. These observations support the results of previous research. From elvers and in eels up to 15–16 cm in length, growth of the gonadal primordium is due to primordial germ cell migration. In eels > 15 cm multiplication of primordial cells begins. Oogonial clones were found in eels > 18 cm in length, whilespermatogonium B clones were observed in eels >30 cm in length. The dynamics of sex differentiation was different among stocks with different ultimate sex ratios: ovaries were found in shorter eels in stocks with a prevalence of females, in longer eels in stocks with a prevalence of males. This result supports the hypothesis of a metagametic (environmental) sex determination. The somatic cells in contact with germ cells and those in the interstitium appeared early during gonad development and preceded germ cell differentiation. This suggests that somatic cells are the targets of the environmental factors influencing sex differentiation.

Switch in the expression of the K19/K18 keratin genes as a very early evidence of testicular differentiation in the rat

It has been shown previously that acidic K18 and K19 keratins display a differential immunohistochemical pattern of expression during sexual differentiation of the gonads in the rat (Fridmacher et al. (1992) Development 115, 503–517). The present results indicate that K18 and K19 gene expression is regulated at the transcriptional level. The analysis was performed by Northern Blot, reverse transcriptase polymerase chain reaction (RT-PCR) and in situ hybridization. PCR products were cloned, sequenced and used as species-specific K18 and K19 riboprobes for in situ hybridization. K19 mRNA but not K18 mRNA was detected in undifferentiated gonads and in somatic cells of ovarian cords throughout the fetal ovary development. K18 mRNA expression appeared in male gonads, at 13.5 days of gestation, at the onset of testicular differentiation, as the first Sertoli cells differentiated and aggregated to form seminiferous cords. As testicular differentiation progressed, K19 mRNA disappeared and, from 14.5 days of gestation on, fetal Sertoli cells expressed exclusively K18 mRNA. The changes in the transcriptional activity of K19 and K18 genes, observed in male gonads, occur characteristically at the very beginning of testicular differentiation. In the male pathway of sexual differentiation, the switch in K19/K18 gene expression is, in addition to the activation of the anti-Müllerian hormone gene, the most precocious regulative event occurring after the expression of the testis determining factor SRY.

Immunohistochemical localization of GnRH in the crested newt (Triturus carnifex) brain and terminal nerve: a developmental study.

Localization of GnRH-immunoreactive neuronal system was studied by immunohistochemistry in the nasal-brain area of the crested newt, Triturus carnifex. Besides adults, developmental stages were those from hatchlings up to complete metamorphosis. Neurons containing immunoreactive GnRH were first detected in the nasal area of larvae with yet undifferentiated gonads. Subsequently, in prometamorphic stages, GnRH-immunoreactive cell bodies and fibers were detected in the proximal part of the terminal nerve as well as along the ventromedial surface of the olfactory bulbs. In older larvae with sexually differentiated gonads and up to the metamorphic climax GnRH-neurons were detected, as a rostral to caudal continuum, along the ventromedial surface of the olfactory bulbs and midtelencephalon. This is exactly the route followed by the terminal nerve. In the adult brain, besides the presence of occasional GnRH-neurons and fibers in the terminal nerve proximal to olfactory bulbs, olfactory bulbs and the mid-basal telencephalon, another aggregate of immunoreactive neurons was present in the anterior preoptic area, and a greater number of fibers in the habenular area as well as in the infundibular floor, median eminence and pars nervosa. These data suggest the nasal area to forebrain migration (along the course of the terminal nerve) of GnRH-neurons during development in the crested newt.

EFFECTS OF THE HEAVY METAL, ZINC, ON THE FRESHWATER FISH TILAPIA NILOTICA L.

Gills, gonads, and blood of Tilapia nilotica exposed to different concentrations of zinc sulfate (ZnSO4. 7H2O) exhibited histological effects. Gills of posthatch larvae exposed chronically for 21 days to 2 ppm zinc sulfate and fingerlings to 10 ppm sublethal zinc concentrations exhibited hyperplasia that resulted in fusion of adjacent secondary gill lamellae. The same effects were observed in 4-hour short-term exposure to 30 ppm lethal dose. Posthatch larvae subjected to 2 and 5 ppm sublethal levels of zinc for 30 days retained undifferentiated gonads with differentiation with oogonial proliferation. Ovaries of control fish demonstrated healthy oocyte growth and other normal histological features after 57 days. In contrast, ovaries in treated groups exhibited excessive amounts of connective tissue, hyperemia and markedly reduced oocyte number. Oocytes had wavy irregular surface outlines. Deviation from normal was observed to be dose dependent. In juvenile tilapia, spermatogenesis was observed in control testes. Testes of zinc-exposed fish, on the other hand, remained immature. Hyperemia was markedly pronounced in both testes and ovary after 90 days exposure to zinc. Blood of Tilapia nilotica fingerlings exposed to sublethal concentrations of 2, 5, and 10 ppm zinc for 30, 60, and 90 days exhibited anisocytosis and poikilocytosis. There was an increase in hematocrit values in zinc-reared fish which, however, reverted to control/near control levels at day 90. Hemoglobin values were inversely proportional to the level of zinc in the rearing water. The marked reduction in hemoglobin values in fish reared at the higher zinc concentrations of 5 and 10 ppm suggests the development of some degree of anemia which is also supported by the observations of anisocytosis and poikilocytosis.

Analysis of the sex-determining region of the Y chromosome (SRY) in sex reversed patients: point-mutation in SRY causing sex-reversion in a 46,XY female.

The first and essential step in normal sexual differentiation takes place during the 5th-6th week of gestation. The testis determining factor (TDF) directs the undifferentiated gonad into a testis, which secretes hormones responsible for normal male development. A new candidate for TDF has recently been reported, and it has been called the sex determining region of the Y (SRY). The hypothesis has been supported by the finding of XX individuals with SRY, and two females with 46,XY karyotype and a mutation in SRY. However, XX males without SRY has been reported, and the role of SRY still has to be determined. We have tested three human females with 46,XY karyotype and gonadal dysgenesis and two 46,XX males for the presence of SRY using the polymerase chain reaction and subsequent DNA sequencing. Both 46,XX males contained SRY, whereas one of the 46,XY females had suffered a point mutation in SRY changing a codon for lysine to a stop codon. This information supports the hypothesis that SRY is significant in normal male sex differentiation. The two remaining 46,XY individuals had an intact HMG box, but it is possible that a mutation may be found in a regulatory gene or further downstream in the gene regulatory cascade. Two patients including the one with a mutation in SRY had gonadoblastomas supporting the hypothesis that another gene on the Y-chromosome is involved in the pathogenesis of this neoplasia.

Changes in Intracellular and Cell Surface Localization of Lex Epitope during Germ Cell Differentiation in Fetal Mice.

Changes in the intracellular and cell surface localization of Lewis x (Le(x)) determinants in germ cells during fetal development in mice were examined by light and electron microscopy. Light microscopically, in undifferentiated gonads on day 12 post coitum (p.c.), the anti-Le(x) monoclonal antibody (MAb) was specifically bound to the plasma membranes and to the cytoplasmic granule-like structures of germ cells. In the testes on day 13 p.c., most of the germ cells were enclosed within the testicular cords and showed an MAb-positive reaction which was restricted mainly to the cytoplasmic granule-like structures. The reaction on the plasma membranes almost disappeared. On the other hand, the ectopic germ cells still showed a positive reaction on their plasma membranes. In the ovaries on day 13 p.c., the germ cells also exhibited positive reactions both on the plasma membranes and on the granule-like structures. Immunoelectron microscopic observations agreed well with these light microscopic observations in such a way that both the plasma membranes and the "small dense bodies" (SDB) were positive in undifferentiated gonads on day 12 p.c. In the germ cells organized into the testicular cords, the reaction to anti-Le(x) MAb became restricted to the SDB. These results may indicate that such intracellular changes in Le(x) determinants during germ cell differentiation are associated with the enclosure of germ cells within the testicular cords.

Testicular differentiation in mammals under normal and experimental conditions

Gonadal differentiation begins with the establishment of a sexually undifferentiated gonad, in which gonadal cords are formed by condensation of somatic cells and deposition of basal laminar components around the cluster of epithelial-like cells. The first event of sexual differentiation is the invasion of mesenchymal and endothelial cells into the genital ridge in the XY gonad. As a consequence of this event, the gonadal cords become conspicuous, recognized as seminiferous cords (or testis cords). Cytological differentiation of Sertoli cells follows these stromal changes. In the XX gonad, by contrast, the invasion of the mesenchyme is absent and gonadal cords remain associated with the surface epithelium. In the B6.YDOM XY ovotestis, seminiferous cords and ovarian gonadal cords are often enveloped by common basal laminae, confirming that both structures share the embryonic origin. It has been recently reported that seminiferous-like cords are formed after loss of oocytes in the rat XX ovary cultured in the presence of Müllerian inhibiting substance or after long-term culture in the basic medium alone. These results are comparable with our observation on the persistent gonadal cords in the ovary of busulphan-treated rats or W/WV mutant mice, in which oogonia are absent or scarce. Ultrastructural evidence for Sertoli cell differentiation from XX cells has been presented, so far, only in the fetal mouse ovary that has been grafted beneath the kidney capsule of adult male mice. Possible mechanism of gonadal sex determination is discussed based on these morphological studies.