Early development of the human embryonic testis.

Human gonadal development culminating in testicular differentiation is described through analysis of histologic sections derived from 33-day to 20-week human embryos/fetuses, focusing on early development (4-8 weeks of gestation). Our study updates the comprehensive studies of Felix (1912), van Wagenen and Simpson (1965), and Juric-Lekic et al. (2013), which were published in books and thus are unsearchable via PubMed. Human gonads develop from the germinal ridge, a thickening of coelomic epithelium on the medial side of the urogenital ridge. The bilateral urogenital ridges contain elements of the mesonephric kidney, namely the mesonephric duct, mesonephric tubules, and mesonephric glomeruli. The germinal ridge, into which primordial germ cells migrate, is initially recognized as a thickening of coelomic epithelium on the urogenital ridge late in the 4th week of gestation. Subsequently, in the 5th week of gestation, a dense mesenchyme develops sub-adjacent to the epithelium of the germinal ridge, and together these elements bulge into the coelomic cavity forming bilateral longitudinal ridges attached to the urogenital ridges. During development, primordial cells migrate into the germinal ridge and subsequently into testicular cords that form within the featureless dense mesenchyme of the germinal ridge at 6-8 weeks of gestation. The initial low density of testicular cords seen at 8 weeks remodels into a dense array of testicular cords surrounded by α-actin-positive myoid cells during the second trimester. Human testicular development shares many features with that of mice being derived from 4 elements: coelomic epithelium, sub-adjacent mesenchyme, primordial germ cells, and the mesonephros.