Early Development of the Human Testis

The gonads (testes and ovaries) are derived from three sources: the mesothelium (mesodermal epithelium) lining the posterior abdominal wall, underlying mesenchyme (embryonic connective tissue), and primordial germ cells. The initial stages of gonadal development occur during the fifth week, when a thickened area of mesothelium develops on the medial side of the mesonephros. Proliferation of this epithelium and the underlying mesenchyme produces a bulge on the medial side of the mesonephros, which is called the genital or gonadal ridge [1]. At first, independently from the genetic sex, the gonadal primordium is colonized by the primordial germ cells [2]. They originate from the epiblast, migrate through the primitive streak, and, by the third week, reside among endodermal cells in the wall of the yolk sac close to the allantois. They migrate along the dorsal mesentery of the hindgut during the fourth week, arrive at the primitive gonad at the beginning of the fifth week, and invade the genital ridges in the sixth week [3]. Fingerlike epithelial (gonadal) cords grow into the underlying mesenchyme. The indifferent gonad now consists of an external cortex and an internal medulla [1]. If the germ cells fail to reach the ridges, the gonads do not develop. Hence, the primordial germ cells have an inductive influence on development into ovaries or testes [3]. As the embryo grows, the gonadal ridge gradually becomes pinched off from the mesonephros. However, some cells of mesonephric origin join the gonadal ridge. Furthermore, the gonadal ridge remains connected to the remnant of mesonephros by a fold of peritoneum, the mesorchium, or the mesovarium [4]. During the seventh week, sexual distinction in the gonadal ridge becomes perceptible. The differentiation of the gonadal ridge into a testis is a rapid phenomenon, in contrast to the slow and late development of the ovary. Seminiferous tubules originate from the mesonephros, the “medulla, ” whereas ovarian tissue originates from the secondary sex cords formed from the germinal epithelium, the “cortex.” Inside the seminiferous tubules, the germ cells are large. They divide actively but do not enter meiosis. The Sertoli cells are smaller than the germ cells, tend to surround them and prepare the future seminiferous tubules. A basal membrane is formed, isolating the tubules from the surrounding mesenchymal tissue. The individualization of tubules and the synthesis of anti-Mullerian hormone precede Leydig cell differentiation. Between the eighth and ninth weeks, the Leydig cells differentiate from interstitial tissue and then spread progressively in the intertubular spaces between the 14th and 18th weeks. They secrete testosterone from the eighth week of development [2]. The following microphotographs, which show human testicular development up to the tenth week, are from the image collection of human embryos and fetuses from the Archive of the Department of Histology and Embryology, School of Medicine, University of Zagreb. This collection consists of histologic slides with serial sections of celloidin-embedded material, routinely processed and stained.