Is extraembryonic angiogenesis in the chick embryo controlled by the endoderm? A morphology study.

The area vasculosa of the chick embryo is subdivided into two concentric zones: the inner transparent area pellucida vasculosa (AVP) and the less transparent surrounding area opaca vasculosa (AOV). The different optical properties of these zones are caused by the different morphology of the endoderm, which consists of flat cells in the APV and of high-prismatic cells containing large yolk vacuoles in the AOV. The present study describes how this endodermal subdivision of the area vasculosa is related to the development of the extraembryonic vascular pattern. By injection of ink into the vascular system of chick embryos at stages 12 to 20 (Hamburger and Hamilton 1951 "HH"), it has been demonstrated that the vascular net of the area vasculosa from stage 14 (HH) onwards develops into different patterns in APV and AOV. The small loops of uniform capillary vessels of stage 13 (HH) are widened due to the rapid expansion of the extraembryonic mesoderm. In the AOV from stage 14 (HH) onwards numerous small blood vessels sprout into the enlarged intervascular spaces. This process is maximal at stage 17 (HH). In contrast, the blood vessels of the APV remain largely unbranched. These findings suggest that the development of the extraembryonic vascular pattern is controlled by the endodermal pattern. To test this hypothesis, both zones (APV and AOV) were examined by light microscopy, transmission and scanning electron microscopy, in vivo observations and by treatment with bromodeoxyuridine (BrdU). TEM examinations show that the ultrastructural organization of the APV mesoderm is different from that of the AOV: The splanchnopleuric cells of the APV form a continuous cover around the endothelial cells connected by numerous desmosomes, whereas the splanchnopleuric cells of the AOV are frequently separated by gaps. The largest gaps are seen in the small blood vessels at stage 17 (HH). These results should be considered in relation to the dynamic changes in the vascular pattern of the AOV. The endodermal cells of APV and AOV are two different populations. In vivo observation of the endodermal transition from APV to AOV detected no transformations of APV cells into AOV cells or vice versa. The borderline between the zones is stable. The AOV endoderm, having been overgrown by the expanding mesoderm, stops proliferating almost completely, whereas the proliferation of the APV endoderm is unaffected by contact with the mesoderm. The rate of its proliferation is approximately as high as that of the AOV prior to contact with the expanding mesoderm (results after treatment with BrdU).(ABSTRACT TRUNCATED AT 400 WORDS)