Fine structure of the corpus Luteum of the raccoon during pregnancy

Summary Ultrastructure of the granulosa lutein cells of the raccoon from throughout pregnancy has been described. The lutein cells often from epithelial cords which are separated by the connective tissues, capillaries and lymphatics. Based on the arrangements and modifications of the cytoplasmic organelles and inclusions, three types of lutein cells have been recognized. The type I lutein cells predominantly contain tubular, agranular endoplasmic reticulum, juxtanuclear Golgi complexes, a few round to rod-shaped mitochondria, some free ribosomes, and occasional lipid droplets. Occasionally the tubular cristae of mitochondria and tubular smooth endoplasmic reticulum appear contiguous. The type II cells contain abundant lace-like and/or stacked fenestrated endoplasmic reticulum cisternae that frequently form membranous whorls, some tubular, agranular endoplasmic reticulum, mitochondria, and lipid droplets. Mitochondria are usually small, but unusual large ones also occur. The small, rod-to round-shaped mitochondria usually have tubular cristae; but the large, oval, elongate, and cup shaped mitochondria possess tubular, lamellar, plate like, and whorl-like cristae. The plasma membranes of the cells are complexly elaborated and folded, especially when apposing each other. In favorable sections, strands of fenestrated cisternae appose the folds of the plasma membranes. In general, the amount of cytoplasmic organelles and inclusions vary greatly in the cells. The type III cells predominantly contain lipid droplets and sparse cytoplasmic organelles. The type I and II cells are found throughout pregnancy, but the type III cells are observed from mid gestation to term. The cytological features of type I and II cells suggest that they probably secrete most of the steroids, whereas the type III cells primarily store lipids.This research was supported by UPSHS grant AM-11376 and NIH contract 69-2136.     

Progesterone exposure of the preovulatory follicle in the seasonally anestrous ewe alters the expression of angiogenic growth factors in the early corpus luteum

Gonadotrophin releasing hormone (GnRH)-induced ovulation in seasonally anestrous ewes is associated with a high incidence of defective corpora lutea (CL), which can be completely eliminated by priming ewes with progesterone before GnRH treatment, but the physiological basis of this has remained elusive. This study tested the hypothesis that progesterone priming eliminates defective luteal function by altering the expression of Vascular Endothelial Growth Factor (VEGF), its receptor VEGFR-2, and angiopoietin (ANG)-1, ANG-2 and their receptorTIE-2 in the early CL. Fifteen seasonally anestrous ewes were treated by i.m. injection with 20 mg of progesterone 3 days before the start of GnRH treatment, while another 15 animals served as controls. Intravenous injections of 500 ng GnRH were given to all the ewes every 2 h for 28 h, followed by a 300 μg GnRH bolus injection to synchronize the preovulatory luteinizing hormone (LH) surge. Corpora lutea were collected 1, 2 and 4 days after ovulation and analyzed for protein and mRNA expression of VEGF, VEGFR-2, ANG-1, ANG-2 and Tie-2 using Western Immunoblotting and in situ hybridization. VEGF, VEGFR-2 and ANG-1 expression was significantly higher (P ≤ 0.05) in the CL of progesterone-primed animals compared to non-primed ones. However, no differences were observed in the ANG-2 or Tie-2 expression levels between the two treatment groups. These data suggest that progesterone priming of the preovulatory follicle alters the expression of some angiogenic growth factors in the early CL, leading to greater vascular stability and thereby normal luteal function.     

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

Summary 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.The author would like to thank Mrs. Yvis Tablada, Miss Mary Ann Anderson and Mr. Garbis Kerimian for their technical, secretarial and photographic assistance, respectively     

Fine structure of seminiferous tubules in antarctic seals

Summary The fine structure of seminiferous tubules from 5 crabeater, 2 leopard and 2 Ross seals showed that during the nonbreeding season the tubules were essentially similar in possessing spermatogenic and Sertoli cells. However, the tubules of leopard and Ross seals had more primary and secondary spermatocytes and spermatids than the crabeater seals. In general, the tubules were devoid of spermatozoa. The spermatids showed stages of maturation such as Golgi phase of acrosome formation, acrosomal cap formation and condensation of nuclei. Some spermatids degenerated in tubules. Both maturing and degenerating spermatids were closely associated with Sertoli cells. Junctional complexes with plaques of filaments were observed between Sertoli cells and the spermatogenic cells. Sertoli cells, irregular and polygonal, contained highly convoluted nuclei, strands of rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi complexes, small mitochondria, variable amounts of lipid droplets, lysosomes, lipofuscin granules and highly plicated plasma membranes. In brief, the spermatogenic activity had practically ceased in the testes and the animals probably secreted low levels of testosterone during the nonbreeding season.This research was supported in part by National Science Foundation Grants G.U. 30270 and G.U. 29829X from the Office of Polar Program and by NIH Grant 5 R01 AM11-376     

Rescue of the corpus luteum in human pregnancy

Rescue of the corpus luteum from its programmed senescence maintains progesterone production required for pregnancy. In primates, chorionic gonadotropin produced by the developing conceptus acts as the primary luteotrophic signal. The purpose of this research was to assess corpus luteum rescue by examining changes in daily urinary progesterone metabolite levels during the first week after implantation. We determined the variability in progesterone metabolite profiles and evaluated its relationship to early pregnancy loss in 120 naturally conceived human pregnancies, including 43 early pregnancy losses. In other primates, an abrupt increase in the progesterone metabolite occurs at the time of implantation. This pattern occurred in an estimated 45% of the pregnancies in the present study. In the remaining pregnancies, there was a delayed rise (18%), neither a rise or decline (22%), or a decline (15%) during the week after implantation. The estimated rate of early pregnancy loss increased across these categories (from 5% loss with an abrupt rise at implantation to 100% loss with progesterone metabolite decline). Low urinary hCG levels in early pregnancy were significant determinants of a decline in postimplantation progesterone metabolite. However, preimplantation steroid metabolite levels were not significant, suggesting no inherent problem with the corpus luteum. Examination of individual progesterone metabolite profiles in relation to hCG profiles also indicated that few losses were caused by corpus luteum failure. Delineating the functional importance of an abrupt progesterone rise at the time of implantation may provide new strategies for promoting successful implantation in assisted reproduction.     

Morphological characteristics of the bovine corpus luteum during the estrous cycle and pregnancy

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.     

The size of the corpus luteum during pseudopregnancy and pregnancy in the rat.

The corpus luteum in mature Sprague Dawley rats was weighted at the various stages of pseudopregnancy and pregancy. The average size of these corpora lutea was 1.0 +/- 0.10 mg, 1.61 +/- 0.69 mg, 1.90 +/- 0.25 mg, 3.69 +/- 0.36 mg, and 4.37 +/- 0.50 mg on day 2 of diestrus, on days 10-15 of psuedopregnancy, on days 9-10, 14, and 20 of pregnancy, respectively. The fact that the average size of the corpus luteum on days 10-15 of pseudopregnancy was larger than that on day 2 of diestrus is thought to drive from prolonged exposure of the corpus luteum to prolactin. The average size of the corpus luteum on days 9-10 of pregnancy had a tendency to be larger than that on days 10-15 of pseudopregnancy and this seems to demonstrate that the placenta secreted placental lactogen by this stage of pregnancy. The average size of the corpus luteum on day 14 of pregnancy was larger than that on days 9-10 of pregnancy. This phenomenon might be attributed to the presence of large amounts of placental lactogen secreted from the placenta between days 10 and 14 of pregnancy. Furthermore, it was noted that the size of the corpus luteum on day 20 of pregnancy was larger than that of day 14, which suggests that further secretion of placental lactogen continued after day 14 of pregnancy. As there was a remarkable decrease in the number of fetuses on day 20 of pregnancy when overiectomy was performed on day 14 of pregnancy, the ovary was considered indispensable in maintaining pregnancy in the rat.     

Ultrastructure of rat ovarian interstitial gland cells during pregnancy

Summary The fine structure of the interstitial gland of the rat ovary was studied at estrus and on Days 4, 6, 10, 14 and 18 of pregnancy. At estrus, ovarian interstitial cells have small nuclei with dense irregular clumps of heterochromatin. Mitochondria are small and rod-shaped and have predominantely lamellar cristae. Numerous osmiophilic lipid droplets are present. At Days 4 and 6, nuclear heterochromatin is reduced, and nucleoli are larger and complex. Mitochondria are enlarged and often bizarre-shaped and have tubular cristae. Golgi and smooth endoplasmic reticulum are more conspicuous. At Day 10, prominent ultrastructural features include nuclei with conspicuous heterochromatin, smaller mitochondria with both lamellar and tubular cristae, numerous ribosomes and lipid droplets with decreased osmiophilia. At Days 14 and 18, nuclei have increased heterochromatin, mitochondria are small and have lamellated cristae and an increase in the size and number of lipid droplets occurs. These observations suggest that steroidogenic activity of interstitial cells is highest during the first half of pregnancy and regresses during the last half. It is suggested that the interstitial gland is an important ovarian source of pregnancy hormone(s) during the first half of gestation and that LH may modulate steroidogenic activity in this ovarian component.     

Ultrastructural changes in granulosa lutein cells and progesterone levels during preimplantation,implantation, and early placentation in the western spotted skunk

Summary The ultrastructure of corpora lutea obtained during the preimplantation, implantation and early postimplantation periods has been studied in 20 western spotted skunks. Fine structure of granulosa lutein cells was correlated with progesterone levels. The corpus luteum of the prolonged (7 month) preimplantation period contained undifferentiated small granulosa cells and differentiated large granulosa lutein cells. The former ranged in size between 12 and 20 and the latter between 20 and 45 . The ratio of small and large cells was about equal in an animal 2 days prior to nidation whereas only few small cells and numerous large cells were observed in an animal estimated to be 8 to 12 hours from nidation. Occasionally small cells were observed amidst large ones during the 24 hour nidation period, i.e. adhesion of trophoblast with the luminal uterine epithelium, but small cells were absent in animals after this period. Small cells had some smooth and rough endoplasmic reticulum, rod-shaped mitochondria with platelike cristae, small Golgi complex, and relatively smooth plasma membranes. Large lutein cells had abundant smooth endoplasmic reticulum, membranous whorls of smooth endoplasmic reticulum, usually round mitochondria with tubular and lamellar cristae, a well developed Golgi complex, variable amounts of lipid droplets, and highly plicated and ruffled plasma membranes. Peripheral plasma progesterone levels during the prolonged preimplantation period ranged between 1.1 and 7.9 ng/ml, but during implantation it was between 8 and 16.6 ng/ml. It is suggested that plasma progesterone levels fluctuate during the time of implantation and should not be regarded as a basis to predict actual nidation in the western spotted skunk.This research was supported in part by Grant Number HD06556 from the National Institute of Child Health and Human Development.     

Ever-changing cell interactions during the life span of the corpus luteum: Relevance to luteal regression

The corpus luteum (CL) undergoes dramatic morphological and functional changes throughout its lifespan. It initially develops from cells that remain in the follicle following ovulation. Eventually the mature CL is composed of multiple, distinctive cell types including steroidogenic cells (small and large luteal cells) and other cell types (endothelial cells, pericytes, fibroblasts, and immune cells). Robust angiogenesis accompanies CL formation, establishing an elaborate blood vessel network at mid cycle. In the absence of embryonic signals, the CL will regress in a process triggered by prostaglandin F2α (PG). Luteal demise in the responsive gland is characterized by cessation of steroid production, angio-regression, and apoptotic cell death, brought about by leukocyte infiltration, inflammatory responses, and diminished angiogenic support. However, the young immature CL is resistant or refractory to the luteolytic actions of PG. Evidence based on functional genomics and other studies highlight the roles played by endothelial, immune, and steroidogenic luteal cells and their interactions in the PG-responsive vs. PG-refractory CL.     

Differential regulation of apoptosis in the corpus luteum of pregnancy and newly formed corpus luteum after parturition in rats

Apoptosis contributes to luteal regression in many species. In the postpartum rat, there are two different types of corpora lutea (CL) in the ovary: CL of pregnancy (CLP) and newly formed CL (NCL). To investigate the regulation of apoptosis in the two different types of CL during luteal regression, apoptosis and caspase-3 activity were examined in the CL obtained on Days 7, 15, and 21 of pregnancy and Days 0, 1, 3, 5, 7, and 9 postpartum. Furthermore, the effect of lactation on apoptosis in the CL was examined in two groups of postpartum rats: lactating rats that nurse more than 10 pups, and nonlactating rats that nurse no pups. Apoptotic cells were detected after Day 21 of pregnancy. In the CLP, remarkable increases in the number of apoptotic cells on Days 5 and 9 postpartum were observed in nonlactating rats (P < 0.01), but not in lactating rats. Changes in caspase-3 activity in the CLP were not consistent with those in number of apoptotic cells. In the NCL, an increase in apoptosis was found only on Day 5 postpartum in nonlactating rats (P < 0.01), but not in lactating rats. Changes in caspase-3 activity in the NCL were consistent with those in number of apoptotic cells. In conclusion, apoptosis is, at least in part, involved in luteal regression after parturition, and lactation appears to inhibit apoptosis. This study also suggests the presence of a caspase-3-independent mechanism for apoptosis in CLP regression in the rat.     

The fine structure of the corpus luteum in Hyrax

Summary Hyrax possess luteal cells with many ultrastructural features similar to those described in other mammalian species. The process of luteinization is gradual. Cells with increasing amounts of smooth endoplasmic reticulum become predominant in the developing gland. The luteal cells of Procavia and Heterohyrax show many ultrastructural similarities. In both species the corpus luteum persists throughout pregnancy.