Ultrastructural Evidence that Chick Primordial Germ Cells Leave the Blood‐Vascular System Prior to Migrating to the Gonadal Anlagen

Abstract

It is known that chick primordial germ cells (PGCs), after separation from the endoderm in early embryonic development, temporarily circulate via the blood-vascular system and eventually migrate to the gonadal anlagen. However, direct evidence that circulating PGCs leave the blood vessels is lacking. The pdrpose of present study is to describe the ultrastructural features of PGCs as they emerge from the blood vessels. PGCs leaving the blood vessels were first examined with semi-thin sections stained with toluidine blue. Then, some of the sections were re-embedded in Epon 812, and sectioned for electron microscopy. PGCs were observed emerging from the capillaries in the region posterior to the omphalomesenteric arteries of the embryo, between the splanchnic mesoderm and open-gut endoderm, at stages 15-1 8 (about 2.5 days of incubation). Ultrastructurally, PGCs exhibited the protruding, bulge-like cytoplasmic processes through the endothelial gaps in the capillary walls. Prior to emerging, intravascular PGCs seemed to stick to the endothelium of the blood vessels. Thus, our results offer ultrastructural evidence that the circulating PGCs exit the blood vessels prior to migrating to the gonadal anlagen.It is now generally accepted that primordial germ cells (PGCs) in amniotes first appear in the endoderm of the yolk sac in early stages of embryonic development and subsequently migrate to the gonadal anlagen during embryogenesis. In birds, PGCs having separated from the endoderm circulate temporarily via the blood-vascular system prior to migrating to the gonadal primordium (1-4). However, it is unknown where and how the circulating PGCs leave the blood stream to enter the gonadal region. Direct evidence for the emergence of PGCs from the blood vessels has not been demonstrated, although a few investigators suggested that the PGCs left the dorsal aortae or the capillaries in the area posterior to the omphalomesenteric arteries of the embryo (3, 5-7).The present study describes the ultrastructural features of PGCs emerging from the blood vessels prior to their migration to the gonadal anlagen and provides the first morphological evidence that PGCs leave the circulation. MATERIALS AND METHODSWhite Leghorn chick embryos at 2 4 days of incubation (stages 15-24 of HAMBURGER and HAMILTON (8)) were processed in the following ways.(1) For the change of PGC-distribution in the blastoderm, whole mounts stained with the periodic-acid Schiff (PAS) technique by UKESHIMA and FUJIMOTO (9) were prepared. 345

Relationship between genital ridge formation and settlement site of primordial germ cells in chick embryos

“…In this study, particularly careful observations were performed in a n area posterior to the vitelline artery, because it is indicated by Ando and Fujimoto (1983) that PGCs concentrically come out of the blood vessels in this area (Fig. 1).…”

“…Prior to 3 days of incubation when the epithelial thickening appears at the gonadal site, the PGCs after extravasation are found to penetrate into the coelomic epithelium of the splanchnopleure (Ando and Fujimoto, 1983;Ukeshima et al, 1985).…”

A method to obtain avian germ-line chimaeras using isolated primordial germ cells

“…PGCs appear in the blood circulation at stages 10-12 and concentrate in the area where gonads are developing, leave the blood (Ando & Fujimoto, 1983;Ukeshima et al, 1991), and finally colonize the gonadal anläge (reviewed in Nieuwkoop & Sutasurya, 1979). These features of PGC migration facilitate their isolation and transfer.…”

Attraction of chick primordial germ cells by gonadal anlage in vitro

“…Subsequently, a t stages 15-18, they leave the blood vessels in the splanchnic mesoderm posterior to the omphalomesenteric artery of the embryo (Ando and Fujimoto, 1983). The area from which the PGCs leave the blood vessels coincides with the future dorsal mesentery through which PGCs finally migrate into the gonadal anlage (germinal ridge; GR) by stages 20-24.…”