The temporospatial distribution of bovine primordial germ cells (PGC) was studied in 114 embryos 18- to 60-day-old. From these, 34 embryos were from the period prior to gonadal differentiation. For a reliable identification of PGC the alkaline phosphatase (AP)-reaction in combination with lectin histochemistry (WGA, WFA, STA) is recommended as the method of choice. The use of the acetylcholinesterase- (AChE) reaction is of limited importance. The first potential PGC are identified in a 18-day-old trilaminar embryo in the caudal wall of the proximal yolk sac at a distance of less than 100 µm from the germ disc. AP- and lectin-positive putative PGC are observed in intraembryonic location close to the mesonephros in 23- to 25-day-old embryos, that is several days before a gonadal ridge and a typical dorsal mesentery are developed. When the gonadal ridge appears (about day 27), it contains a certain number of PGC from the very beginning. This finding is at variance with the tenet of an active immigration from an extraembryonic or extragonadal localization: from our observation follows, only perigonadal PGC are prospective PGC. At the cranial end of the gonadal ridge, PGC prefer a localization close to the capsule of the mesonephric giant corpuscle. Further caudally, in the region of the prospective gonadal ridge, the PGC are found in a subcoelothelial position in contact with a heparansulfate-, AChE-, and WFA- positive basal lamina-like matrix. In the sexually indifferent gonadal fold (32 to 39 days) the PGC are also unevenly distributed. They concentrate at the caudal end and at the abmesogonadal side and are seldom seen in the hilus region. After the differentiation of the gonad, the female PGC are mostly located in the gonadal cortex in laminin-delineated areas (Keimballen). Immediately after gonadal differentiation, significant numbers are also found around the medullary cords in the neighbourhood of a steroid hormone producing zone described here for the first time. The male PGC after gonadal differentiation are generally located in the peripheral mesenchyme (future tunica albuginea) as they were also seen in the indifferent situation. Somewhat later in the young male gonad their number has increased, and the cells lie equally scattered within aggregations of pre- Sertoli cells. Extragonadal potential PGC are regularly encountered in bovine embryos with indifferent gonads, seldom afterwards. Such cells occur predominantly in the paraaortal tissue (especially in the areas of somite desintegration), but also in the liver, in blood vessels and in periadrenal site. In the light of recent evidence that germ cells and hematopoietic cells share a common ancestor, the results enforce two conclusions: a) Lineage restriction of PGC does not take place before these cells have settled in the gonad and establish contact with pre-Sertoli cells. b) Extragonadal embryonic PGC are multifunctional and more than only ectopic candidates for degeneration or pathologic developments. The sudden stop of both, AP-positivity and the typical lectin binding, about day 80 p.i. coincide with the appearance of a new germ cell typical antigen (protein gene product 9,5) in the testis and characterize a decisive step of differentiation from PGC to prespermatogonia. Similar changes as seen in the male line very likely are also occur in female development; however, additional investigations of the time span immediately following female gonadal differentiation are necessary.
