The lack of genital ridge vascularization in the early chick embryo: implications in the migration of the primordial germ cells.

It is known that chick primordial germ cells (PGCs) in early embryonic development migrate via the blood vascular system to colonize the gonadal anlagen. Classically, two factors have been involved in the extravasation of PGCs from the blood stream: chemotactic and mechanical factors. An accurate knowledge of the vascular system of the genital ridge is therefore necessary. However, development of gonadal vascularization in bird embryos has been scarcely studied. Our previous studies have shown that the gonadal arteries develop from the mesonephric arteries. The purpose of this work was to study the implications of the development of the vascular system of the chick genital ridge on PGCs colonization. We selected the Hamburger and Hamilton (H-H) stage 18, since the genital ridge is well developed and PGCs actively extravasate. Forty chick embryos of this stage were processed for scanning electron microscopy of vascular corrosion casts and of critical point-dried specimens as well as light microscopy. Our results are conclusive. We could not find any vessel or capillary network supplying the genital ridge; the dorsal aorta and the primordia of the mesonephric arteries were the closest vessels. However, numerous interendothelial spaces were found in the dorsal aorta at the level of the genital ridge. It is suggested that the interendothelial gaps may be very important in the exchange of substances between the avascular genital ridge and the aortic endothelium at this developmental stage. Two different routes are thought to be involved in PGC migration to the gonadal anlage at this stage: the aortic endothelium and the mesonephric arteries. Whereas mechanical factors may be important for extravasation of PGCs in the mesonephric arteries, no reasons have been found from the morphological point of view to support a slowness of the blood flow in the dorsal aorta at the level of the genital ridge facilitating the extravasation.