Localization of thrombospondin-1 and its receptor CD36 in the ovary of the ostrich (Struthio camelus)

Angiogenesis, the formation of new blood vessels from pre-existing vasculature, plays a decisive role for the rapid growth of avian follicles. Compared to mammals, few data on the angiogenesis in the avian ovary are available. However, whereas several pro-angiogenic factors in the avian ovary have been recently studied in detail, little information is available on the localization of anti-angiogenic factors. The aim of this study was to determine the localization and possible function of the anti-angiogenic factor thrombospondin-1 (TSP-1) and its receptor CD36 in the ovary of the ostrich using immunohistochemistry and to correlate the results with ultrastructural data. Whereas the oocytes and granulosa cells of all follicular stages were negative for TSP-1, myofibroblasts of the theca externa and smooth muscle cells of blood vessels showed distinct reactions. A distinctly different staining pattern was observed for CD36. The oocytes were CD36 negative. No immunostaining for CD36 could be observed neither in the granulosa cells nor in the adjacent theca interna of vitellogenic follicles. In the theca externa, blood vessels protruding towards the oocyte showed CD36-positive endothelial cells. In conclusion, a fine balance between angiogenic and anti-angiogenic processes assures that a dense net of blood vessels develops during the rapid growth of a selected follicle. Anti-angiogenic molecules, such as TSP-1 and its receptor CD36 may, after the oocyte has reached its final size, inhibit further angiogenesis and limit the transport of yolk material to the mature oocyte. By this mechanism, the growth of the megalecithal oocyte during folliculogenesis may cease.