Morphogenesis of the bovine rete testis: the intratesticular rete and its connection to the seminiferous tubules

The development of the intragonadal rete testis and the establishment of th e connection between seminiferous and straight testicular tubules was studi ed using ultrastructural and histochemical methods in 60 bovine embryos and fetuses ranging from day 39 through day 225 post conceptionem. The methodo logy included a modified acetylcholinesterase (AChE) reaction as a selectiv e marker for pre-Sertoli cells and a modified microsomal aminopeptidase (MA P) reaction as a selective marker for the epithelia of rete testis and stra ight testicular tubules. Between 40 and 45 days, the rete testis is predomi nantly an extratesticular rete situated in the cranial peduncle of the gona dal fold and in broad contact with the pro/mesonephric giant corpuscle. Dur ing this period, the intragonadal rete enters the gonad proper from its cra niodorsal pole and extends into the cranial fourth of the testis. Between 6 0 and 110 days the rete testis attains its definitive position, extending i nto the central longitudinal axis as far as to the caudal fourth of the tes tis. For the caudal expansion of the rete testis the preceding proliferatio n of the mediastinal stroma is an important prerequisite. In the 40 to 45-d ay-old embryo the area of the testicular cords may be divided into two zone s. A narrow outer zone contains plate-like cords with a thick diameter, and a larger central zone is filled with a network of thinner cords. Only the thick outer cords transform into the permanent seminiferous tubules, wherea s the thinner cords in the central zone are transitory structures that disa ppear between 45 and 110 days. One important function of these transitory c ords is to establish a continuous system of basal laminae that allows a dir ect connection between the central ends of the growing seminiferous tubules and the peripheral extensions of the rete testis (future straight testicul ar tubules). The first true straight testicular tubules become visible betw een 85 and 110 days. Due to a strong proliferation of the tubulus rectus-ce lls the straight testicular tubules elongate continuously, and the border b etween the rete system and the seminiferous tubules is slowly shifted towar ds the testicular periphery. This shift is not restricted to the prenatal p eriod, but proceeds until after birth. At the cytological level, the format ion and elongation of the straight testicular tubules is effected by prolif erating cells that advance along the continuous basal lamina into the area of the seminiferous tubules. The preSertoli and germ cells in this zone of invasion are separated from each other and overgrown by the tubulus rectus- cells. Exposed to the special milieu of the straight testicular tubules, pr eSertoli and germ cells apparently cannot survive and finally disappear.