Fine structure of the corpus luteum of pregnancy in the gerbil (Meriones unguiculatus).

Corpora lutea from gerbils on days 1, 4, 8, 12, 16, 20, and 24 of pregnancy were studied electron microscopically. Similarly, luteal tissue from animals on the day of parturition and one day postpartum was studied (gestation: 24 days +/- 8-24h). Agranular endoplasmic reticulum increases in quantity through day 16 and thereafter is somewhat reduced. Granular endoplasmic reticulum and a population of small granules (type I) become abundant during late pregnancy and their possible role in the production and storage of relaxin is discussed. Luteal tissue undergoes a relatively rapid regression which begins on the day of parturition. Conspicuous in the regressing luteal tissue are large (type II) granules (possibly lysosomes), lipid droplets, leucocytic elements and macrophages. Functional correlates of these morphological findings are discussed.