Overview of Embryological and Fetal Development of the Ovary and Testis

Abstract

2The events that take place in the urogenital region during mammalian embryonal and fetal life represent ideal subjects of study for those interested in cellular differentiation and organogenesis. Within a short period of time, from a few hours in species with short gestational periods to no more than several days in those with longer ones, the urogenital region witnesses the arrival of the primordial germ cells (PGCs), their colonization of the genital folds, and the concomitant "activation" of the cells preexisting in the area, major phenomena that culminate in the development of a sexually indifferent gonad. The subsequent differentiation of the latter into a testis or an ovary is accompanied by the maturation of the PGCs into germinal elements with distinct sexually dimorphic characteristics, the development of a complex system of blood vessels and nerves, the differentiation of interstitial elements with important endocrine functions, the sexually dimorphic involution/transformation of the mesonephros, and the organization of the metanephros as the definitive excretory organ.Of these phenomena, the migration of the PGCs to, and their colonization of, the genital ridges, the origin of the somatic cells which at first contribute to the formation of the blastema of the sexually indifferent gonad together with the germinal cells and later differentiate into the definitive testicular and ovarian sustentacular elements, and the sexually dimorphic patterns of male and female germinal cell differentiation with emphasis on the time of initiation of meiosis are those that have been most intensively studied. While many of the issues related to the evolution of these phenomena and their regulatory mechanisms have been clarified, others remain unsettled and require further study.In all mammals, onset of gonadal development is heralded by the arrival of the PGCs to the gonadal folds following a migratory journey that initiates in the yolk sac and takes the germ cells through the primitive intestine and its mesentery. In mammals, PGC migration is exclusively extravascular, sporadic reports to the contrary (1, for example) being based on the erroneous interpretation, mostly due to similarities in size and overall appearance, of circulating megakaryocytes as primordial germinal cells. It has been firmly established that the PGCs cover the distance separating the yolk sac from the gonadal folds primarily by