Discipline: Veterinary Medicine
The corpus luteum of the Ph1hppine water buffalo (Bubalus bubalis L.) during the early, middle and late stages of pregnancy was studied using hematoxylin and eosin stained sections. Grossly, the corpus luteum appeared pinkish red in color at the early stage of pregnancy and was almost entirely embedded in the ovary, except for the crown which protruded in the ovarian stroma. During middle and late stages of pregnancy, the corpus luteum turned brownish in color and as pregnancy progressed, it decreased in size. Microscopically, a well-developed connective tissue layer composed mainly of collagen fibers surrounded the corpus luteum and divided it into lobules. Large and small luteal cells made up the cell population of the corpus luteum. The large cell was polygonal, with a large eccentric euchromatic nucleus and eosinophilic cytoplasm. Cytoplasmic lipid droplets were observed in large cells during the early stage of pregnancy and appeared to decrease in amount in the middle and late stages of pregnancy. Eosinophilic granules were visible in the cytoplasm of large cells in the mid stage and were most numerous in the late stage of pregnancy. Small cells were darker with a dense heterochromatic nucleus. Small cells and large cells were largest during mid-pregnancy and early pregnancy, respectively (P<0.05). Small cell population was lowest at early pregnancy and highest at late pregnancy (P<0.05). Large cell population was lower (P<0.05) in early pregnancy than in middle and late pregnancy. Several studies on the microscopic appearance of luteal cells in pregnant cows, ewes and goats have been published, however such studies do not specify the different stages of pregnancy. A thorough description of the histology of luteal cells during the three stages of pregnancy remains limited. The present study was conducted to describe the light microscopic features of the small and large luteal cells in the Philippine water buffalo during the three stages of pregnancy.