Comparative study on gonadal development and sex differentiation in teleost fishes and anuran amphibians

F. MEIJIDE; Y. PIAZZA; G. GUERRERO

Buenos Aires, Argentina

Congreso; 4th Internacional Meeting of the Latin American Society of Developmental Biology; 2008

Latin American Society of Developmental Biology

The origin of the somatic cells that compose the vertebrate gonadal primordium remains obscure and, frequently, information about this issue in the literature is confusing or incomplete. It is generally stated that the undifferentiated gonads of amphibians and other higher vertebrates consist of two somatic cell components with different embryological origin: the cortex, which originates from the peritoneal wall, and the medulla, derived from the mesonephric blastema. The cortex develops and the medulla degenerates during ovarian differentiation. Conversely, the cortex regresses and the medulla develops during testicular differentiation. The gonads of teleost fishes are composed of somatic tissues derived exclusively from the peritoneal wall, i.e. they correspond to only the cortex of other vertebrates. Histological observations of the process of morphogenesis of the gonads are of primary importance for the understanding of the mechanism of gonadal sex differentiation. Vertebrates with external fertilization, such as most teleost fishes and anuran amphibians, are useful as basic models in which to study developmental processes since it is simple to obtain hundreds of synchronized embryos and larvae that develop externally, allowing direct examinations. The freshwater cichlid fish Cichlasoma dimerus and the common toad Rhinella (Bufo) arenarum are two native vertebrate species that have been extensively used in our laboratory as models for developmental studies. In the present work, gonadal development was analysed in these species by means of light and transmission electron microscopy. Early developmental stages were sampled periodically, covering the period of histologically discernible sex differentiation. The ultrastructural analysis revealed that in C. dimerus, germ cells of the primordial gonad are located inside an inpocketing of the coelomic epithelium. Although a cortico-medullary structure was evidenced in the indifferent gonad of R. arenarum, observations showed that medullary somatic cells have an epithelial origin. In both species, ovarian differentiation is marked by the formation of the ovarian cavity, and the onset of germ cell meiotic activity occurs earlier in females than in males. Spatial relationships between cellular elements and the basement membrane must be taken into account when studying the origin of the nongerminal components in the developing gonad. Right from the beginning of gonadal development, the basement membrane subjacent to the coelomic epithelium (mesothelium) of the genital ridge separates the germinal and the interstitial compartments of the gonad, both in teleosts and anurans. Somatic cells within the germinal compartment (namely support cells in the undifferentiated gonad, follicle cells in the ovary and Sertoli cells in the testis) have an epithelial origin, since they are derived from the mesothelium. In contrast, somatic cells within the interstitial compartment (namely fibroblasts in the undifferentiated gonad, thecal cells in the ovary and Leydig and myoid cells in the testis) have a mesenchymal origin, being derived from the subepithelial connective tissue. We hypothesize that the significance of the basement membrane in establishing the separation between both tissue compartments within the gonad is that it would be apparently a constant throughout vertebrate evolution. The results of this study are discussed in relation with observations on the early gonadal development in mammalian embryos. Taken together, these findings indicate that some of the concepts regarding the embryological origin of the vertebrate gonad need to be revised.