Observations on the corpus luteum during pregnancy and lactation in the marsupials Isooson macrous and Perameles nasuta

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Abstract
Ovaries from the bandicoots I. macrourus and P. nasuta, collected during pregnancy and lactation, were examined macroscopically and with the light microscope. The mean number of corpora lutea (CL) was greater in I. macrourus (5.13) than in P. nasuta (3.31). This difference in ovulation rate was reflected in the mean number of embryos and the mean number of pouch young respectively (I. macrourus 4.05, 3.07; P. nasuta 2.63, 2.65). During the first few days of pregnancy, the transformation of follicle granulosa cells into luteal cells was accompanied by the intrusion of the theca, which produced a network of blood vessels and connective tissue. A conspicuous feature of the developing CL was the presence of pools of blood between the luteal cells. By 5-6 days of pregnancy, the CL was a fully formed structure, the central cavity having been filled in by hypertrophy of the luteal cells and an increase in the connective tissue which formed a central core. The luteal cells had almost reached their maximum size and they contained numerous lipid droplets. Mitoses of luteal cells were not evident at any stage in the formation of the CL or later, though some of the luteal cells had two or more nuclei. The structure of the CL and the diameter of the luteal cells and their nuclei remained almost constant until regression occurred after the 45th day of the 60-day lactation period. Regression of the CL was characterized by a marked reduction in the size of the luteal cells and their nuclei as well as an increase in the connective tissue component. Diameters and volumes of the CL were similar in the two species, reaching maxima during the second half of pregnancy and changing very little until the 45th day of lactation, after which they decreased rapidly. This decline occurred whether or not the animals subsequently ovulated during the last quarter of the lactation period.