Characterization of Granule Types in Luteal Cells of Sheep at the Time of Maximum Progesterone Secretion1

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Abstract
Luteal cells of sheep contain small, densely staining granules which are suggested to have a role in progesterone storage and secretion, and have therefore been called "secretory granules." Some confusion exists, however, because these granules overlap in size and appearance with other cytoplasmic granules, including lysosomes and peroxisomes. It is thus important to determine whether the purported secretory granules constitute a separate granule population. To this end, we have studied sheep luteal cells fixed by vascular perfusion at the time of maximal progesterone secretion, when these granules are most abundant. Nonfrozen sections of perfusion-fixed corpora lutea were incubated for the fine structural localization of either catalase, a peroxisomal marker enzyme, or acid phosphatase (ACPase), a lysosomal marker enzyme. Appropriate controls verified the specificity of the cytochemical reactions. The results indicate that luteal cells of midcycle sheep have at least four morphologically distinct granule types: 1) The so-called secretory granule, which is round, ∼0.25 µm in diameter, and commonly has a dense central patch surrounded by a lighter periphery. These granules appear to undergo exocytosis, and they as well as their released contents are unreactive for both catalase and ACPase. 2) Microperoxisomes, which are irregularly oval or round, ∼0.25 µm in diameter, and filled with a pale, finely granular matrix. Cytochemical incubation reveals that these granules contain catalase. 3) Lysosomes (dense bodies), which vary in shape, contents, and density. These granules are strongly ACPase-positive. 4) Multivesicular bodies (MVBs), which are round, ∼0.4 µm in diameter, and contain small vesicles in a pale to dense matrix. Although usually negative after cytochemical incubation for ACPase, rare MVBs show scant reaction product. On Day 8 of the estrous cycle, the granule population in luteal cells consists of 55% secretory granules, 39% microperoxisomes, 3% lysosomes, and 3% MVBs. This study supports the claim that the so-called secretory granules of sheep luteal cells are a separate population of granules, distinct from both lysosomes and peroxisomes.