Leucocyte proliferation in the bovine corpus luteum

Leucocytes vary in type and number during the lifespan of a corpus luteum. The aim of this study was to determine whether there is an increase in the number of lymphocytes and macrophages as a result of local proliferation. Bovine corpora lutea were classified into stages of development, secretion and regression. A new double immunolabelling method was established for nuclear Ki-67 antigen (a marker for cell proliferation) and for leucocyte surface antigens (detection of CD2-, CD3-, CD4-, CD8-positive lymphocytes and CD14-positive monocytes). Differential cell counting was performed. Between the stages of development and regression there was an increase in the number of T-lymphocytes and macrophages. The percentage of proliferating leucocytes in relation to the total number of proliferating cells was approximately 20% at the stage of advanced secretion and 70% at late regression. The increase in the number of proliferating leucocytes at late regression was due to CD14-positive macrophages. These macrophages migrated from small blood vessels into the septa of corpora lutea at the early stage of regression. Macrophages showed local proliferation in the late stage of regression when capillaries were no longer present. It is concluded that the physiological involution of the corpus luteum is an inflammatory-like condition, which includes local proliferation of monocytes.