APOPTOSIS DURING SPONTANEOUS LUTEOLYSIS IN THE CYCLIC GOLDEN-HAMSTER - BIOCHEMICAL AND MORPHOLOGICAL EVIDENCE

The corpora lutea (CL) of the cyclic hamster are destroyed between Day s 2 and 3 of the 4-day estrous cycle so that only one set is ever pres ent (Day 1 = estrus, Day 4 = proestrus). The possibility that luteal c ell death in the cyclic hamster is attributable to apoptosis was explo red. The earliest histological signs of structural luteolysis were det ected at 0600 h of Day 3 as evidenced by a few scattered luteal cells displaying the characteristic morphology of apoptotic cells and by a m assive infiltration of neutrophils. The peaks of neutrophil influx and luteal apoptosis were reached on Day 3, 1200 h, and Day 3, 2400 h, re spectively. 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. Apopt osis ultimately destroyed luteal endothelial cells, luteal cells, and neutrophils. Electrophoretic analysis of low-molecular weight DNA in l uteal cell lysates revealed a definite ladder pattern of oligonucleoso mal-length DNA fragments-characteristic of apoptosis-on Day 3 beginnin g 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 internucleosomal cleavage activity. The minimal number of CL on Day 3, 1500 h, needed to demonst rate 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 cy cle. The initiation of apoptosis is not apparent until several hours a fter the onset of functional luteolysis. The rapidity with which apopt osis eliminates the CL over a very precise time schedule makes the cyc lic hamster an ideal model to analyze the factors involved in structur al luteolysis.