Ultrastructure of the Corpus Luteum of the Cycling Mare

Corpora lutea of 12 cycling pony mares were studied by electron microscopy on Days 3, 9 and 15 after ovulation. In the early part of the secretory phase (Day 3), luteal cells were fully luteinized and contained numerous lipid droplets, round to elongate mitochondria with tubular cristae, perinuclear Golgi elements, dense bodies, multivesicular bodies and regions of extensively developed smooth endoplasmic reticulum (SER). On Day 9, luteal cells contained few lipid droplets, round to pleomorphic mitochondria with tubular cristae, dispersed Golgi elements, increased numbers of dense bodies, rare multivesicular bodies and an increased complement of SER. Decreased numbers of lipid droplets and hypertrophy of the SER correlated with known peak progesterone levels associated with Day 9. By Day 15, luteal cells displayed symptoms of degeneration including rarefied and swollen mitochondria, condensed whorls of SER, vesiculated SER, irregular or pycnotic nuclei, accumulations of pale lipid droplets, myelin figures and autophagic vacuoles.β -glycogen particles were interspersed between SER cisternae in all time periods studied. Rarefied mitochondria were observed in otherwise nondegenerate cells and since mitochondrial enzymes catalyze the rate limiting step in progesterone synthesis, it is suggested that mitochondrial disruption may be an efficient way to terminate luteal progesterone synthesis.