Apoptosis during spontaneous luteolysis in the cyclic golden hamster: biochemical and morphological evidence

The corpora lutea (CL) of the cyclic hamster are destroyed between Days 2 and 3 of the 4-day estrous cycle so that only one set is ever present (Day 1 = estrus, Day 4 = proestrus). The possibility that luteal cell death in the cyclic hamster is attributable to apoptosis was explored. The earliest histological signs of structural luteolysis were detected at 0600 h of Day 3 as evidenced by a few scattered luteal cells displaying the characteristic morphology of apoptotic cells and by a massive infiltration of neutrophils. The peaks of neutrophil influx and luteal apoptosis were reached on Day 3, 1200 h, and Day 3, 2400 h, respectively. Thus, the increase in neutrophils occurs before the major onset of luteolysis. By Day 3, 2400 h, the CL had already shrunken one third by weight, and they virtually vanished by the next Day 1. Apoptosis ultimately destroyed luteal endothelial cells, luteal cells, and neutrophils. Electrophoretic analysis of low-molecular weight DNA in luteal cell lysates revealed a definite ladder pattern of oligonucleosomal-length DNA fragments--characteristic of apoptosis--on Day 3 beginning at 1200 h. The pattern was not detectable in CL collected on Day 2. Comparing Day 3 CL collected at 0900-1200 h with those at 1500-1800 h showed that only the latter group exhibited inter-nucleosomal cleavage activity. The minimal number of CL on Day 3, 1500 h, needed to demonstrate DNA laddering was six. In summary, the electrophoretic separation of oligonucleosomal fragments and histology indicated that apoptosis occurs during spontaneous luteal regression on Day 3 of the hamster cycle. The initiation of apoptosis is not apparent until several hours after the onset of functional luteolysis. The rapidity with which apoptosis eliminates the CL over a very precise time schedule makes the cyclic hamster an ideal model to analyze the factors involved in structural luteolysis.