Ultrastructural dynamics of human testicular cords from 6 to 16 weeks of embryonic development. Study by transmission and high resolution scanning electron microscopy

When the embryo is 6-week-old the gonad is composed mostly of migrating pri mordial germ cells, surface coelomic mesothelium and mesenchymal cells. At 7 weeks of gestation (wg) testicular cords consist of prespermatogonia, lar ger, with a more regular outline and higher mitotic activity than primordia l germ cells, embraced by somatic pre-Sertoli cells. The morphofunctional d evelopment of Sertoli cells defines testicular differentiation. Towards 7.5 wg the gonad is finally in its differentiated stage; the basal lamina of t he cords becomes distinguishable, testicular cords radially branch and elon gate, and Leydig cells are clearly recognized. Primordial germ cells are la rge and spherical, with rounded and eosinophilic nuclei and large nucleoli. Pre-Sertoli cells, in turn, show round or columnar nuclei and rough endopa smic reticulum. Prespermatogonia and mostly pre-Sertoli cells actively prol iferate. Many interdigitations and cytoplasmic processes are observed betwe en neighboring pre-Sertoli cells and between pre-Sertoli cells and presperm atogonia. A sort of compartmentalization is established inside the cords in which pre-Sertoli cells tend to localize closer to the basal membrane embr acing the prespermatogonia with long and thin cytoplasmic processes. Betwee n 14 and 16 wg the most significant changes besides maximum development of Leydig cells are differentiation of mesenchymal cells around the cords into future peritubular cells and maturational changes of pre-Sertoli cells. Th ese likely reveal a peak coinciding in time with reported increases in eith er testosterone production and Mullerian Inhibiting Substance secretion. Du ring the period herein considered testicular cords show no lumen neither an y sign of canalization so they cannot be termed "tubules".