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Abstract
The corpus luteum, one of the biological clocks of the estrous cycle and pregnancy,
is known foremost for its production of progesterone that blocks the pituitary release
of gonadotropins and prepares the uterus for a pregnancy. The cellular sources of
this progesterone are the steroidogenic small and large luteal cells. Other luteal
cells that are not steroidogenic, but are believed to have an important role in the
function of this gland are the fibroblast, macrophages and endothelial cells. The
most prominent luteal cell is the large steroidogenic cell characterized by an abundance
of smooth endoplasmic reticulum and densely packed spherical mitochondria that are
indicative of its contribution to most of the circulating progesterone believed to
be constitutively secreted and not under the control of LH. Other distinguishing features
of the large luteal cell are the presence of rough endoplasmic reticulum, prominent
Golgi, and secretory granules that are indicative of endocrine cells. This cell undergoes
dynamic changes across the estrous cycle and pregnancy, believed to reflect a change
in progesterone and protein secretion that will eventually influence a successful
pregnancy or another ovulation if pregnancy fails. The morphological characteristics
of the bovine luteal cells are the focus of this review.