Differentiation of the gonad rudiment into ovary and testis in the solitary ascidian, Ciona intestinalis

In the early juveniles of Ciona intestinalis, primordial germ cells arise on the degenerated mass of the resorbed tadpole tail, and assemble to form a discrete gonad rudiment. The present study elucidated the morphological sequences during differentiation of the gonad rudiment into the testis and ovary. In 11- to 12-day juveniles, the gonad rudiment, an elongate sac, divided into the testicular and ovarian rudiments. The testicular rudiment separated as a round vesicle from the thickened wall of the elongate sac. The original sac, after separation of the round vesicle, developed into the ovary. In the testicular rudiment, germ cells formed a continuous central mass without association of somatic cells, while in the ovarian rudiment, each germ cell was associated with somatic cells within the epithelium composing the wall of the rudiment. In 13- to 15-day juveniles the testicular rudiment changed into branched tubes ending in club-shaped follicles. Cells characterized by many flattened cisternae of rough endoplasmic reticulum (distal cells) constituted the distal wall of each follicle. Spermatogenic cells were freely present in the follicular lumen, but the largest spermatogonia were in contact with the distal cells. Both in the testicular and ovarian rudiments, germ cells entered meiosis in 18-day juveniles. A novel body (periesophageal body) was found just beneath the ventral margin of the esophageal opening. It comprised irregular follicles made up of one cell type whose cytoplasm, filled with round vesicles and Golgi complexes, was suggestive of an endocrine function. Fragments derived from the periesophageal body were present around the developing ovary.

Members of the ErbB receptor tyrosine kinases are involved in germ cell development in fetal mouse gonads

To isolate the genes involved in mouse primordial germ cell (PGC) development, we carried out subtraction cDNA cloning between PGC-derived embryonic germ (EG) cells and inner cell mass-derived embryonic stem cells. Among the genes preferentially expressed in EG cells, we found a gene encoding a receptor tyrosine kinase ErbB3. By in situ hybridization and immunohistochemical staining, the expression of ErbB3 as well as that of ErbB2, a coreceptor for ErbB3, was detected in PGCs in genital ridges at 12.5 dpc (days postcoitum). The expression was, however, downregulated at 14.5 dpc when the PGCs underwent growth cessation. Neuregulin-beta, a ligand for ErbB2 and ErbB3, was also expressed in genital ridges. In addition, a recombinant Neuregulin-beta enhanced the number of PGCs in 12.5-dpc embryos in culture. Taken together, these observations suggest that ErbB signaling controls the growth or survival of PGCs in genital ridges.

Differentiation of donor primordial germ cells into functional gametes in the gonads of mixed-sex germline chimaeric chickens produced by transfer of primordial germ cells isolated from embryonic blood

This study was carried out to elucidate whether primordial germ cells, obtained from embryonic blood and transferred into partially sterilized male and female recipient embryos, could differentiate into functional gametes and give rise to viable offspring. Manipulated embryos were cultured until hatching and the chicks were raised until maturity, when they were mated. When the sex of the donor primordial germ cells and the recipient embryo was the same, 15 out of 22 male chimaeric chickens (68.2%) and 10 out of 16 female chimaeric chickens (62.5%) produced donor-derived offspring. When the sex of the donor primordial germ cells and the recipient embryo was different, 4 out of 18 male chimaeric chickens (22.2%) and 2 out of 18 female chimaeric chickens (11.1%) produced donor-derived offspring. The rates of donor-derived offspring from the chimaeric chickens were 0.6-40.0% in male donor and male recipient and 0.4-34.9% in female donor and female recipient. However, the rates of donor-derived offspring from the chimaeric chickens were 0.4-0.9% in male donor and female recipient and 0.1-0.3% in female donor and male recipient. The presence of W chromosome-specific repeating sequences was detected in the sperm samples of male chimaeric chickens produced by transfer of female primordial germ cells. These results indicate that primordial germ cells isolated from embryonic blood can differentiate into functional gametes giving rise to viable offspring in the gonads of opposite-sex recipient embryos and chickens, although the efficiency was very low.

Emergence of T, B, and myeloid lineage-committed as well as multipotent hemopoietic progenitors in the aorta-gonad-mesonephros region of day 10 fetuses of the mouse

We investigated the developmental potential of hemopoietic progenitors in the aorta-gonad-mesonephros (AGM) region, where the definitive type hemopoietic progenitors have been shown to emerge before the fetal liver develops. By using an assay system that is able to determine the developmental potential of individual progenitors toward the T, B, and myeloid lineages, we show that not only multipotent progenitors but also progenitors committed to the T, B, or myeloid lineage already exist in this region of day 10 fetuses. Bipotent progenitors generating myeloid and T cells or those generating myeloid and B cells were also detected, suggesting that the commitment to T and B cell lineages is in progress in the AGM region. The numbers of these progenitors, however, were only 1/200-1/1000 of those in fetal liver of day 12 fetuses. Such small numbers of progenitors suggest that hemopoiesis has just started in the AGM region of day 10 fetuses. Although most of T cell lineage-committed progenitors in the AGM region generated only a small number of immature T cells, some were able to generate a large number of mature T cells. The detection of various types of lineage-committed progenitors strongly suggests that the AGM region is not only the site of stem cell emergence, but also the site of hemopoiesis, including lineage commitment. The T cell progenitors found in the AGM region may represent the first immigrants to the thymus anlage.

Identification of podocalyxin-like protein 1 as a novel cell surface marker for hemangioblasts in the murine aorta-gonad-mesonephros region

Recent studies with avian embryos and murine embryonic stem cells have suggested that hematopoietic cells are derived from hemangioblasts, the common precursors of hematopoietic and endothelial cells. We molecularly cloned podocalyxin-like protein 1 (PCLP1) as a novel surface marker for endothelial-like cells in the aorta-gonad-mesonephros (AGM) region of mouse embryos, where long-term repopulating hematopoietic stem cells (LTR-HSCs) are known to arise. PCLP1+ CD45 cells in the AGM region incorporated acetylated low-density lipoprotein and produced both hematopoietic and endothelial cells when cocultured with OP9 stromal cells. Moreover, multiple lineages of hematopoietic cells were generated in vivo when PCLP1 +CD45-cells were injected into neonatal liver of busulfan-treated mice. Thus, PCLP1 can be used to separate hemangioblasts that give rise to LTR-HSCs.

Expression of anti-Müllerian hormone during normal and pathological gonadal development: association with differentiation of Sertoli and granulosa cells

The ontogeny of expression of anti-Müllerian hormone (AMH) was examined by immunohistochemistry in 135 human gonadal tissue specimens of various developmental age, ranging from 6 weeks of fetal development to 38 yr of postnatal age. The series included specimens from normal testes and ovaries and from individuals either with pathological conditions affecting gonadal development or with idiopathic infertility manifested as azoospermia or severe oligozoospermia. AMH expression was found only in Sertoli and granulosa cells. A 6-week-old fetal testis at the indifferent gonad stage did not yet express AMH. The protein was first visible at 8.5 weeks of development, when sex cords have not yet been formed. Afterward, a majority of testicular specimens, including those from pathological conditions, strongly expressed AMH through fetal development and childhood until puberty. Markedly prolonged expression of AMH was observed in a 20-yr-old 46,XY female with androgen insensitivity syndrome, who retained prepubertal testicular morphology. In normal testes, the switch-off of AMH expression was usually associated with the appearance of primary spermatocytes, suggesting that their presence had an inhibitory effect on AMH. However, in adolescent boys lacking germ cells because of cancer treatment and in a majority of infertile adult men with idiopathic germ cell aplasia, AMH expression was also down-regulated despite the complete lack of spermatogenesis. The decrease in AMH expression thus reflects the terminal differentiation of Sertoli cells and is probably only partially dependent upon a regulatory factor associated with the onset of meiosis. In fetal ovaries, AMH was first detected at 36 weeks gestation in granulosa cells of preantral follicles. Thus, the onset of ovarian expression is at the end of fetal life and not in infancy as previously reported.

Effects of embryonic treatment with fadrozole on phenotype of gonads, syrinx, and neural song system in zebra finches

Previous studies have found that treatment of zebra finch embryos with an aromatase inhibitor on Day 5 or 8 of incubation caused partial sex reversal of gonadal phenotype in females. These females possessed both testicular and ovarian tissue, and the development of the neural circuit for song remained feminine. The present study attempted more complete gonadal reversal by treating zebra finch embryos earlier, on Day 3 of incubation, with Fadrozole (CGS 16949A), an aromatase inhibitor, or with saline. We examined the phenotype of the syrinx (androgen-dependent vocal organ), the gonads, and the telencephallic neural song system in 100-day-old birds. Treated females typically possessed a left ovotestis and a right testis, and significantly larger syringes than control females. The histology and steroid synthetic enzyme activity of the testicular tissue in treated females were quite masculine and similar to that of control males. At the time of sacrifice, the plasma concentrations of testosterone and estradiol for fadrozole-treated females did not differ from those of control females, but dihydrotestosterone was lower in treated females. Despite the large amount of functional testicular tissue and a masculine syrinx, the volumes and soma sizes of song system nuclei (HVC, RA) in treated females remained feminine. These results suggest that testicular secretions masculinize the syrinx, but are not sufficient to masculinize the song system in zebra finches.

Functional and molecular analysis of hematopoietic progenitors derived from the aorta-gonad-mesonephros region of the mouse embryo

Herein, we show that CD34, c-kit double-positive (CD34(+)c-kit(+)) cells from the aorta-gonad-mesonephros (AGM) region of the developing mouse are multipotent in vitro and can undergo both B-lymphoid and multimyeloid differentiation. Molecular analysis of individual CD34(+)c-kit(+) cells by single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) shows coactivation of erythroid (beta-globin) and myeloid (myeloperoxidase [MPO]) but not lymphoid-affiliated (CD3, Thy-1, and lambda5) genes. Additionally, most cells coexpress the stem cell-associated transcriptional regulators AML-1, PU.1, GATA-2 and Lmo2, as well as the granulocyte colony-stimulating factor receptor (G-CSF-R). These results show that the CD34(+)c-kit(+) population from the AGM represents a highly enriched source of multipotent hematopoietic cells, and suggest that limited coactivation of distinct lineage-affiliated genes is an early event in the generation of hematopoietic stem and progenitor cells during ontogeny.

Reverse transcription-polymerase chain reaction analysis of genes involved in gonadal differentiation in pigs

In mammals, testis development is initiated in the embryo as a response to the expression of the sex-determining gene, SRY. The time course of SRY expression during gonadal differentiation in the male has been described in detail only in mice and sheep. In this study, we used reverse transcription-polymerase chain reaction analysis to define the SRY transcription profile in pig genital ridges. SRY transcripts were first detectable from 23 days postcoitum (dpc), then declined sharply after 35 dpc. None were detected at 60 dpc. In addition, we analyzed temporal expression of other genes known to be involved in mammalian sex determination: WT-1, SF-1, SOX9, and AMH. A key stage seems to be 28 dpc, in which SOX9 expression switches between the male and female, and AMH expression begins to attest to Sertoli cell differentiation and to correspond to seminiferous cord formation in the male. Expression of gonadotropin receptors and aromatase was also investigated in porcine gonads, and we showed that their transcripts were detected very early on, especially in the male: 25 dpc for the LH receptor (rLH) and aromatase, and 28 dpc for the FSH receptor (rFSH). In the female, aromatase transcripts were not detected until 70 dpc, and rFSH expression occurred later: at 45 dpc at the onset of meiosis. Moreover, no difference was observed between the sexes for the onset of rLH transcription at 25 dpc. Such a thorough study has never been performed on pigs; developmental analysis will be useful for investigating sex-reversed gonads and determining ontogeny in intersexuality, a common pathology in pigs.

Molecular cloning and genomic organization of mouse homologue of Drosophila germ cell-less and its expression in germ lineage cells

Primordial germ cells (PGCs) are founder cells of all gametes. A number of genes which control PGCs development have been identified in invertebrates, whereas such genes are by and large unelucidated in mammals. Here we describe cloning, genomic structure and expression of mouse homologue of germ cell-less (gcl) gene which is required for PGCs formation in Drosophila. The mouse gcl shows 34% identity compared with Drosophila gcl protein and contains BTB/POZ domain. The gcl gene consists of 14 exons and spans more than 50 kb. The CpG islands are found around exon 1 of the gene. Putative promoter region contains potential binding sites for various transcription factors. Northern blot analysis showed that its mRNA is highly expressed in adult testis with lower expression in ovary, ES (embryonic stem) cells, and various other organs. In situ hybridization analysis revealed strong expression of the gcl gene in the pachytene stage spermatocytes. The expression was also observed in post-migratory PGCs, but was not apparent in migratory and pre-migratory PGCs. Further studies including gene disruption analysis would provide an important insight into mammalian germ lineage development.

Irradiation of fish embryos prior to blastomere transfer boosts the colonisation of their gonads by donor-derived gametes

Blastomere transplantation into fish blastula embryos results in somatic chimeras, which generally provide null or a small proportion of gametes derived from the donor. This may partly explain why none of the ES-like cell lines established from fish embryos has contributed to the germline of chimeras when transplanted at the blastula stage. Here, we report that a moderate gamma-irradiation of recipient embryos, followed by transplantation of dispersed blastomeres, considerably enhances the proportion of donor-derived gametes (53% versus 5% in average). In fish, the resulting protocol should maximise the pluripotency level measured in vivo for embryonic cell lines and for cultured germ cells.

Vulva formation in Pristionchus pacificus relies on continuous gonadal induction

One of the best known features of vulva development in Caenorhabditis elegans is the induction of vulval precursor cells by the gonadal anchor cell. Induction is crucial for the initiation of pattern formation within the C. elegans vulva equivalence group, and it is therefore surprising to find that this aspect of vulva formation, in particular, varies greatly among nematodes. In some species which form vulvae in the posterior body region, no gonadal signal is necessary for vulva induction. In other nematodes, such as Panagrolaimus, Oscheius, and Rhabditella, vulva formation depends on two temporally distinct gonadal inductions which specify the different cell fates. Here we report our analysis of vulva induction in Pristionchus pacificus, a species which has recently been used as a genetic system to analyze the evolution of vulva development. Cell ablation studies in P. pacificus show that another mode of vulva induction exists. P. pacificus vulva formation depends on a continuous gonadal induction that starts several hours after hatching and continues until the birth of the anchor cell, some 20 h later. Mutations defective in gonadal induction result in the absence of vulva differentiation, suggesting that only one signaling system is involved in the gonadal-epidermal interaction. This new mode adds further to the great variety of gonadal inductions among nematode species.

Hematopoietic cells in cultures of the murine embryonic aorta-gonad-mesonephros region are induced by c-Myb

Definitive hematopoiesis begins in the para-aortic, splanchnopleural (P-Sp) and aorta-gonad-mesonephros (AGM) regions of mouse embryos and then switches to the fetal liver [1] [2] [3]. Gene-targeted mice lacking the c-Myb transcription factor have severe hematopoietic defects in the fetal liver [4]. The role of c-Myb, if any, in P-Sp/AGM hematopoiesis has not been examined, however. Recently, we reported that oncostatin M can effectively expand both hematopoietic and endothelial-like cells from in vitro cultures of the AGM region [5]. Using this cell culture system, we examined the involvement of c-Myb in definitive hematopoiesis in the P-Sp and AGM regions. When primary cultures from the P-Sp or AGM regions of wild-type mouse embryos were probed with an anti-c-Myb antibody, hematopoietic cells but not endothelial-like cells showed positive staining. In contrast, in the P-Sp/AGM culture from c-myb(-/-) embryos, no hematopoietic cells were generated and endothelial-like cells predominated, indicating that the impairment of hematopoiesis in the liver of c-myb(-/-) embryos is actually preceded by a defect in P-Sp/AGM hematopoiesis. Hematogenic precursor cells were, however, still present in an inert but competent form among the endothelial-like, adherent cell population of c-myb(-/-) P-Sp/AGM cultures. When infected with a retrovirus carrying c-myb cDNA, these cultures gave rise to a significant number of hematopoietic cells. The rescued cells, unlike wild-type hematopoietic cells, were negative for c-Kit (a marker of hematopoietic progenitors), but did express other hematopoietic cell surface markers such as Mac-1, Gr-1 (myeloid markers), CD19, B220, Thy-1.2 (Iymphoid markers), and Ter119 (an erythroid marker). Thus, c-Myb plays a role in the generation of hematopoietic cells in the embryonic P-Sp and AGM regions.

Acetylcholinesterase-positive innervation is present at undifferentiated stages of the sea turtle Lepidochelis olivacea embryo gonads: implications for temperature-dependent sex determination

In embryos of different reptile species, incubation temperature triggers a cascade of endocrine events that lead to gonad sex differentiation. The cellular and molecular mechanisms by which temperature sets in motion this process are still controversial. Here, we begin evaluating the possible participation of the nervous system in temperature-dependent sex determination by showing the existence and origin of acetylcholinesterase (AchE)-positive nerve fibers in undifferentiated gonads of the Lepidochelys olivacea (L. olivacea) sea turtle putative male and female embryos, along the thermosensitive period for sex determination (TPSD; stages 20-27). AChE-positive nerve bundles and fibers were readily visualized until developmental stage 24 and thereafter. DiI injections and confocal imaging showed that some of these gonadal nerves arise from the lower thoracic and upper lumbar spinal cord levels, and might thus be sensory in nature. Because the vertebrate spinal cord is capable of integrating by itself thermoregulatory responses with no intervention of uppermost levels of the central nervous system, we also evaluated spinal cord maturation during the TPSD. The maturation of the spinal cord was more advanced in putative female than in male embryos, when sex determination is taking place for each sex; this process starts and ends earlier in male than in female embryos. Together these observations open the possibility that the spinal cord and the innervation derived from it could play a direct role in driving or modulating the process of temperature-dependent gonad sex determination and/or differentiation, particularly in female L. olivacea embryos.

Gene expression during gonadogenesis in the chicken embryo

Genes implicated in vertebrate sex determination and differentiation were studied in embryonic chicken gonads using reverse transcription and the polymerase chain reaction (RT-PCR). Expression profiles were obtained during gonadal sex differentiation for AMH, SOX9, SOX3, the Wilm's Tumour gene, WT1, and the orphan nuclear receptor genes, SF1 and DAX1. Some of these genes showed sexually dimorphic expression profiles during gonadal development, whereas others were expressed at similar levels in both sexes. The gene encoding Anti-Müllerian hormone (AMH) was expressed in both sexes prior to and during sexual differentiation of the gonads, with levels of expression consistently higher in males than in females. SOX9 expression was male-specific, and was up-regulated after the detection of AMH transcripts. SOX3 expression was observed prior to clear SOX9 expression and was up-regulated in both sexes at the onset of gonadal sex differentiation (but declined later in development). The WT1 gene was highly expressed in both sexes, whereas SF1 expression was clearly higher in developing ovaries compared to testes. DAX1 transcripts were observed in both sexes at all stages examined, but expression appeared somewhat higher in developing ovaries. These expression profiles are analysed in terms of current theories of vertebrate sex determination.

Germline development in vertebrates and invertebrates

In all animals information is passed from parent to offspring via the germline, which segregates from the soma early in development and undergoes a complex developmental program to give rise to the adult gametes. Many aspects of germline development have been conserved throughout the animal kingdom. Here we review the unique properties of germ cells, the initial determination of germ cell fates, the maintenance of germ cell identity, the migration of germ cells to the somatic gonadal primordia and the proliferation of germ cells during development invertebrates and invertebrates. Similarities in germline development in such diverse organisms as Drosophila melanogaster, Caenorhabditis elegans, Xenopus laevis and Mus musculus will be highlighted.

Migratory mechanisms of chick primordial germ cells toward gonadal anlage

After appearing at the germinal crescent region, chick primordial germ cells (PGCs) migrate toward the presumptive gonads (pG) till stage 19 (Hamburger and Hamilton, 1951). This study seeks to elucidate the roles of passive and active factors in the PGC-migration, physical trapping of circulating PGCs by the capillary network and PGC attraction by chemotactic factor from presumptive gonads. Firstly, latex beads/pollens (the same size or larger than PGCs) were injected into the embryonic bloodstream at stage 13-19 (when PGCs are in the migrating and settlement phase to the presumptive gonad) in ovo in order to determine whether the PGCs passively reach pG. Most of such particles accumulated in the head region (60%), whereas the remainder did the same in the gonadal region (23% at the peak) at stage 16 when both the head and gonadal regions are rich in capillary plexus. After 3 days, most particles in the gonadal region were located at the angles of dorsal mesentery near the developing gonads where many extra-gonadal PGCs had been located, and a few particles were detected close to the gonad. These results suggest that one of the mechanisms of PGC-migration to the developing gonads is an autonomous trapping of PGCs by the capillary network quite close to the germinal epithelium (GE) and passive translocation by morphogenetic movement. Secondly, the attraction for PGCs by the gonadal anlage proper was examined in ovo using chick and quail embryos. Grafts of quail gonadal anlage containing gonadal epithelium and neighbouring mesenchymal tissue were excised from the quail embryo at stages 12 to 16 (staging by Zacchei, 1961). With the aims of eliminating the influence of surrounding tissue, the quail graft was ectopically transplanted into the posterior to the optic vesicle of 8 to 17 somite chick embryo from the point of a posterior region to the auditory vesicle by a fine tungsten needle under the illumination by the method of Hara (1971). Then the region posterior to the level of presumptive vitelline arteries was surgically excised in ovo. After a 48 hrs.-incubation, the host PGCs which lost their own gonadal anlage as a target organ accumulated in the transplanted quail gonadal anlage originating from the embryo at PGC-migrating periods. This result strongly suggested the presence of some attractive factor that may be emitted from the gonadal anlage proper. Furthermore, it was demonstrated that the PGCs in vitro showed no contact inhibition in relation to other PGCs or fibroblasts in their moving pathway.

Stem cell emergence and hemopoietic activity are incompatible in mouse intraembryonic sites

In the mouse embryo, the generation of candidate progenitors for long-lasting hemopoiesis has been reported in the paraaortic splanchnopleura (P-Sp)/ aorta-gonad-mesonephros (AGM) region. Here, we address the following question: can the P-Sp/AGM environment support hemopoietic differentiation as well as generate stem cells, and, conversely, are other sites where hemopoietic differentiation occurs capable of generating stem cells? Although P-Sp/AGM generates de novo hemopoietic stem cells between 9.5 and 12.5 days post coitus (dpc), we show here that it does not support hemopoietic differentiation. Among mesoderm-derived sites, spleen and omentum were shown to be colonized by exogenous cells in the same fashion as the fetal liver. Cells colonizing the spleen were multipotent and pursued their evolution to committed progenitors in this organ. In contrast, the omentum, which was colonized by lymphoid-committed progenitors that did not expand, cannot be considered as a hemopoietic organ. From these data, stem cell generation appears incompatible with hemopoietic activity. At the peak of hemopoietic progenitor production in the P-Sp/AGM, between 10.5 and 11.5 dpc, multipotent cells were found at the exceptional frequency of 1 out of 12 total cells and 1 out of 4 AA4.1+ cells. Thus, progenitors within this region constitute a pool of undifferentiated hemopoietic cells readily accessible for characterization.

Control of organ shape by a secreted metalloprotease in the nematode Caenorhabditis elegans

The molecular controls governing organ shape are poorly understood. In the nematode Caenorhabditis elegans, the gonad acquires a U-shape by the directed migration of a specialized 'leader' cell, which is located at the tip of the growing gonadal 'arm'. The gon-1 gene is essential for gonadal morphogenesis: in gon-1 mutants, no arm elongation occurs and somatic gonadal structures are severely malformed. Here we report that gon-1 encodes a secreted protein with a metalloprotease domain and multiple thrombospondin type-1-like repeats. This motif architecture is typical of a small family of genes that include bovine procollagen I N-protease (P1NP), which cleaves collagen, and murine ADAMTS-1, the expression of which correlates with tumour cell progression. We find that gon-1 is expressed in two sites, leader cells and muscle, and that expression in each site has a unique role in forming the gonad. We speculate that GON-1 controls morphogenesis by remodelling basement membranes and that regulation of its activity is crucial for achieving organ shape.

Detailed characterization of the human aorta-gonad-mesonephros region reveals morphological polarity resembling a hematopoietic stromal layer

The definitive long-term repopulating human hematopoietic stem cell, which seeds the adult blood system, was previously thought to derive from the extra-embryonic yolk sac. However, there is now considerable evidence that in both avian and murine systems, yolk sac hematopoietic cells are largely a transient, embryonic population and the definitive stem cell, in fact, derives from a distinct region within the embryonic mesoderm, the aorta-gonad-mesonephros region. In the human embryo, an analogous region has been found to contain a cluster of cells distinct from, but closely associated with, the ventral endothelium of the dorsal aorta, the appearance of which is restricted both spatially and temporally. We have used antibodies recognising hematopoietic regulatory factors to further characterise this region in the human embryo. These studies indicate that all factors examined, including vascular endothelial growth factor and its receptor FLK-1, Flt-3 ligand and its receptor STK-1, and stem cell leukemia transcription factor, are expressed by both hematopoietic cells in the cluster and endothelial cells. However, there is some discontinuity in cells directly underlying the cluster. Furthermore, we have identified a morphologically distinct region of densely-packed, rounded cells in the mesenchyme directly beneath the ventral wall of the dorsal aorta, and running along its entire length. In the preumbilical AGM region, directly underlying the hematopoietic cluster, but not at more rostral and caudal levels, this region of mesenchyme expresses tenascin-C, an extracellular matrix glycoprotein known to facilitate cell-cell interactions and migration. This region of cells may therefore provide the microenvironmental support for the intraembryonic development of definitive hematopoietic stem cells, a process in which tenascin-C may play a pivotal role.

Amphibian sex determination and sex reversal

Amphibians employ a genetic mechanism of sex determination, according to all available information on sex chromosomes or breeding tests. Sex reversal allows breeding tests to establish which sex is heterogametic and provides an indication of the mechanism of sex determination. Cases of spontaneous and experimental sex reversal (by temperature, hormones or surgery) are reviewed and illustrated by previously unpublished studies on crested newts. These newts respond conventionally to temperature and hormone treatment but provide anomalous results from breeding tests. It is suggested that both the evolution from temperature dependency to a genetic switch and from ZZ/ZW to XX/XY are superimposed on a generally uniform mechanism of sex determination in all vertebrates.

Migration of mesonephric cells into the mammalian gonad depends on Sry

In mammals, the primary step in male sex determination is the initiation of testis development which depends on the expression of the Y-linked testis determining gene, Sry. The mechanisms by which Sry controls this process are unknown. Studies showed that cell migration from the adjacent mesonephros only occurs into XY gonads; however, it was not known whether this effect depended on Sry, another Y-linked gene, or the presence of one versus two X chromosomes. Here we provide genetic proof that Sry is the only Y-linked gene necessary for cell migration into the gonad. Cell migration from the mesonephros into the differentiating gonad is consistently associated with Sty's presence and with testis cord formation, suggesting that cell migration plays a critical role in the initiation of testis cord development. The induction of cell migration represents the earliest signaling pathway yet assigned to Sry.