The fine structure of the lutein cells in the mink (Mustela vison) with special reference to the secretory activity during pregnancy

Summary The ultrastructure of the lutein cells in the mink throughout pregnancy and the regression periodpost partum is described. To correlate the fine structure with the changes in the peripheral plasma progesterone levels, the concentrations of progesterone were measured by a rapid competitive protein-binding assay.Even during the delay period (e.g. as long the plasma progesterone levels remain at the basal level, <8ng/ml), the lutein cells in the mink exhibit structural criteria of functional activity. However, the increase in progesterone secretion is accompanied by some morphological transformations, characterized by the presence of more and more small dense homogenous bodies in the cytoplasm, which become irregular and scalloped during the stage with maximum release of progesterone. At this stage the agranular endoplasmic reticulum is often cisternal or vesicular.During the decline of the progesterone levels, typical and moderate electron-dense lipid droplets are found increasingly more within the lutein cells. The expanded agranular ER is now more sparse, while the granular ER becomes more pronounced, often forming parallel arrays. During this phase the mitochondria become elongated, dumb-bell, or cup shaped. After parturition the corpora lutea consist of cells in various stages of degeneration. At day 14post partum only a few lutein cells are still identifiable.Evidently the observed morphological changes take place in the lutein cells during the life span of corpora lutea. This feature lends further support to the concept that the mink lutein cells are steroid-producing cells and furthermore, that the corpora lutea may be the main sites of gestagen production during pregnancy.