Gonadal development begins in the x day fetal rat with the appearance of two gonadal ridges. The epithelium covering the ridges grows inward to form the primordia of seminiferous tubules, consisting at this time only of epithelial cells that will become the future Sertoli cells. Primordial gonocytes (germ cell precursors) arise in the yolk sac and migrate to the developing gonad, where they accumulate in the primordia of the seminiferous tubules. Leydig cells differentiate in the mesenchyme between the tubules. In the testes of 18-19 day fetuses (see photos below), the seminiferous tubules are packed with primordial gonocytes, the nuclei of Sertoli cell precursors are visible here and there at the periphery of the tubules, and abundant Leydig cells fill the space between the tubules. At this time the Leydig cells are secreting androgen to stimulate the development of the male reproductive tract and the formation of male behavioral centers in the brain. Birth occurs at about fetal day 21. In the newborn testis (see photo below), the seminiferous tubules have expanded greatly, primarily by Sertoli cell division, and the primordial gonocytes (still large, round and pale in appearance) are seen here and there in the tubules. The Leydig cells, having completed their initial responsibilities, are not keeping up with the growth, and will continue to decline in relative number until their resurgence at puberty. The appearance of the seminiferous tubules stays approximately the same for a few days, until the primordial gonocytes begin dividing and transforming into spermatogonia, a process that is completed by about postnatal day 10 (see photo below). Testicular development over the next 40 or so days consists of a gradual establishment of the successive layers of developing germ cells -- (1) spermatogonia and early primary spermatocytes, (2) pachytene primary spermatocytes, (3) early to partially developed spermatids, and finally (4) older spermatids -- seen in the cycle of the seminiferous epithelium in seminiferous tubules of mature rat testes. This successive layering greatly expands the diameter of the seminiferous tubules (as can be seen in the photos below), and reaches completion by about 50 days of postnatal age. The rat will be fertile once the testis is producing sperm and the sperm have had time to traverse the epididymis (which takes several days). To understand rat testicular development, one must have some acquaintance with the cycle of the seminiferous epithelium in the mature rat testis (see companion set of photos on that subject in this web page).

Bibliography of rat testicular development.

Photographs of rat testicular development
Newborn-61 day taken at the same magnification (10X, 1.6 Optivar, Zeiss)
(Click a thumbnail to see a larger version of the picture,
and scroll down to see all of it)

emb D18	emb D19	newborn	1 day	2 day	5 day	10 day
16 day	20 day	30 day	35 day	40 day	50 day	61 day

The rat testes used for the photographs of histological slides were obtained and fixed by A. K. Christensen from the rat breeding colony in the Biological Laboratories (Biology Department) of Harvard University, Cambridge, Massachusetts, in 1958. The manager, Richard Stafford, ran the rat colony with great care, recording the timing of pregnancies and the postnatal age of litters. Sprague-Dawley strain. Stieve fixation. The tissues were prepared for histological slides by A.K.C.

The light microscope semithin sections of testes that had been prepared for electron microscopy (links from newborn, 5 day and 10 day pictures) were prepared by Frances Thomas, who had worked in the A.K.C. lab in the Department of Anatomy at Stanford School of Medicine during 1969-71.