Expression and localization of thrombospondin-1 and -2 and their cell-surface receptor, CD36, during rat follicular development and formation of the corpus luteum.

Thrombospondin (TSP)-1 and -2 are extracellular matrix glycoproteins that are both antiangiogenic and important in regulating cellular development, differentiation, and function. To evaluate the expression of TSP in follicular and luteal development, ovarian cycles of Sprague-Dawley rats were synchronized and tissues collected daily at stages corresponding to the early antral, ovulatory, early luteal, and late luteal phases of the ovarian cycle. Immunohistochemistry and Western blot analyses demonstrated that TSP-1 protein and its receptor, CD36, were present in the early antral phase and were localized primarily to the granulosa cells of antral follicles. Both proteins were also present immediately after ovulation and were localized to the developing corpus luteum. Messenger RNA for TSP-1 showed a similar pattern, with expression at the early antral and ovulatory phases. Protein and mRNA expression for TSP-2 was relatively delayed compared to TSP-1, although TSP-2 also was expressed in granulosa cells. Both TSP-1 and -2 were increased in response to LH stimulation in vitro, whereas TSP-2 was suppressed by FSH. The temporal pattern of expression of TSP-1, -2, and CD36, which mirrors the active phases of angiogenesis in this experimental model, is compatible with a role for these proteins in the control of ovarian vascularization.