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The present work reviews the latest information on the cellular, molecular and physiological aspects of sexual determination and differentiation in teleost fish. The group exhibits a large variety of mechanisms of sexual determination. These may be genetic, or depend on environmental conditions such as temperature, pH, and social factors, all of which can influence the proportion of the sexes. Additionally, sex steroids play an important role in the regulation of sexual differentiation. The patterns of gonadal sexual differentiation are diverse, and species may be hermaphroditic or gonochoristic, some of the latter displaying juvenile hermaphroditism. In recent years, several genes involved in the sexual determination and differentiation pathways in vertebrates, particularly in mammals, have also been characterized in teleosts. Conserved as well as diversified functions have been proposed.

Different studies in ovariectomized estrogen treated animals support the idea that cfos plays a role in the proliferation of uterine epithelial cells. However, these studies invite us to reassess the role played by c-fos in epithelial cell types of the endometrium during the estrous cycle. The present study was undertaken to determine the c-fos and estrogen receptor (ER) gene expression pattern in the rat uterine epithelium during the estrous cycle in which natural and cyclic changes of steroid hormones occur, and correlate these changes with the proliferation status of this cellular types. Proliferation was assessed during the estrous cycle using bromodeoxyuridine incorporation to DNA. ERa and b proteins were assessed by immunohistochemistry. The regulation of c-fos gene expression in the uterus of intact animals during the estrous cycle was evaluated using both in situ hybridization and immunohistochemistry. Estradiol (E 2 ) and progesterone (P 4 ) plasma levels were assessed by radioimmunoassay. The results indicated that luminal (LE) and glandular epithelia (GE) presented maximal proliferation during the metestrus (M) and the diestrus (D) days. However, during the proestrus (P) day only LE presented proliferation, and during the estrus (E) day only the stromal cells proliferated. A marked immunostaining for ERa was detected in both LE and GE cells during the early phases of the cycle but diminished on the P and the E day. In contrast, ERb was undetectable in both epithelia during all stages of the cycle. The highest cfos mRNA level was detected in both epithelia on the M day, followed by a significant reduction during the other days of the cycle. The highest protein content was observed on the M and D days, and the minimal value was detected on the E day. The c-Fos protein level in LE was increased during M and D days, presenting a high correlation with the cellular proliferation pattern of this cell type. In conclusion, the overall results indicate that c-Fos protein presented a good correlation with uterine epithelial cell proliferation of LE. In the case of GE, the same tendency was observed, although no significant correlation was found. Both in LE and GE, c-fos mRNA did not strictly correlate with its protein levels. c-fos seems to have a postranscriptional regulation in uterine epithelial cells during the rat's estrous cycle. Mol. Reprod. Dev. 64: 379-388,

BDNF retrograde axonal transport is substantially inhibited by intraocular pressure elevation. TrkB accumulation at the ONH in glaucoma suggests a role for neurotrophin deprivation in the pathogenesis of RGC death in canine glaucoma, as well as a possible paracrine and/or autocrine signaling within the lamina cribosa. Neurotrophin signaling may regulate more than neuronal development, survival and differentiation. BDNF neurotrophin and its TrkB receptor expression by lamina cribosa cells and ONH astrocytes in glaucomatous eyes may help to determine the role of these cells as a paracrine source in terms of retinal ganglion cell survival, during episodes of elevated intraocular pressure.

Both matrotrophy, the maternal provisioning of nutrients to developing embryos after fertilization, and superfetation, the simultaneous presence of two or more groups of embryos at different stages of development, occur at varying degrees among species of the fish family Poeciliidae. However, it is still unclear if these two reproductive modes depend on the presence of relatively complex placentas. We describe the ultrastructure of the maternal follicular placenta of 11 poeciliid fishes using electron microscopy. In addition, we quantified six ultrastructure characteristics that reflect the degree of complexity (number of vesicles, area of vesicles, number of microvilli, microvilli length, thickness of the maternal follicle and follicular area). Using phylogenetic comparative methods, we evaluated the relationship between degree of matrotrophy and placental characteristics. We also analysed the potential effect of the presence of superfetation on placental complexity. We found a positive relationship between the degree of matrotrophy and follicular area, number of microvilli and number and area of vesicles. Similarly, follicular area and number of microvilli were larger in species with superfetation than in those without superfetation. We conclude that high degrees of matrotrophy and superfetation are associated with placental characteristics that increase the efficiency of nutrient transfer between mother and embryos. K E Y W O R D Sfollicular placenta, matrotrophy, placentation, Poeciliidae, superfetation

In mouse and chick embryos, the SOX9 gene is down-regulated in genetic females whereas in genetic males it remains in the Sertoli cells. We studied the distribution of SOX9 protein in developing genital ridges of embryos of the sea turtle Lepidochelys olivacea incubated at male- or female-promoting temperatures, using the antibody for detection. At stages 22-24, cells in medullary cords show SOX9 positive nuclei, while coelomic epithelial cells appear negative. At stage 25 however, most medullary cells are SOX9 negative and at the female-promoting temperature, and from stage 26 onwards, SOX9 protein is not detected. At the male-promoting temperature, medullary cords remain SOX9-positive at all stages. These results suggest that SOX9 is up-regulated in Sertoli cells irrespective of primary sex-determining switch. Sex is irreversibly determined at stage 24 or 26 at the male- or female-promoting temperature, respectively (Merchant-Larios et al.,'97). The present results suggest that there is a correlation between SOX9 expression and sex determination in the olive ridley. At the male-promoting temperature, Sertoli cells expressing SOX9 become committed at stage 24 and male sex is determined, whereas at the female-promoting temperature, SOX9 is down-regulated at stage 26 and female sex is determined. J. Exp. Zool. 284:705-710, 1999.

Although sex determination starts in the gonads, this may not be the case for species with temperature sex determination (TSD). Since temperature affects the whole embryo, extragonadal thermosensitive cells may produce factors that induce gonadal sex determination as a secondary event. To establish if gonads of a species with TSD respond directly to temperature, pairs of gonads were cultured, one at female-promoting temperature (FPT) and the contralateral at male-promoting temperature (MPT). Histological and immunohistochemical detection of SOX9 revealed that the response to temperature of isolated gonads was similar to that of the gonads of whole embryos. While gonads cultured at MPT maintained SOX9 expression, it was downregulated in gonads at FPT. Downregulation of SOX9 took longer in gonads cultured at stage 23 than in gonads cultured at stage 24, suggesting that a developmental clock was already established at the onset of culture. To find out if sex commitment occurs in vitro, gonads were switched from FPT to MPT at different days. Results showed that the ovarian pathway was established after 4 days of culture. The present demonstration that gonads have an autonomous temperature detector that regulates SOX9 expression provides a useful starting point from which the molecular pathways underlying TSD can be elucidated.