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Immunolocalization of CD18-positive cells in the bovine ovary

The life cycle of follicles and of corpora lutea may be finely tuned by resident leucocytes in control of the angiogenesis, dilatation and permeability of microvessels, remodelling of the extracellular matrix, and phagocytosis of degenerated cells. Since the overall leucocyte distribution has not been studied in the bovine ovary, we located the CD18 molecule in this organ using indirect immunohistochemistry. This molecule represents a subtype of β2 integrin and is expressed by all subsets of leucocytes. The microvessels were also identified with an antibody against factor VIIIr antigen. The outer layer of the interstitial cortex, where the primordial follicles were located, contained a few CD18-positive cells and a few microvessels. Intact follicles displayed the CD18-positive cells in the theca. They were aligned in the zone of the basal membrane of preovulatory follicles. This was associated with a dilated inner microvessel layer. CD18-positive cells and probably capillary sprouts were seen in the granulosa layer of freshly ovulated follicles. Regressing follicles exhibited CD18-positive cells in the hypertrophied theca and in the degenerating granulosa layer, together with the pattern of capillary sprouts. At the developmental stage, corpora lutea exhibited many CD18-positive cells in the septa. The microvascular bed was expanding. At the secretory stage, a regular and moderate appearance of CD18-positive cells corresponded to a well-developed microvascular bed. Abundant positive cells were seen at the regressive stage, together with the development of shunt-like arterioles. The semiquantitative evaluation of leucocyte subsets in corpora lutea revealed an increase in the CD18-positive cells between stages of development and regression, which was not observed in the corresponding interstitial cortex. Lymphocytes with the expression of CD2, CD3 or both molecules displayed a similar behaviour. In conclusion, the CD18-positive leucocytes represent a heterologous pool with a distribution that varies in accordance with morphological changes in the follicles and corpora lutea.