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Biomedical subjects

R A Mead

Publications and source records attributed to R A Mead.

At least 55 records · Page 3Linked to original sources
PubMed1984

Plasma estrogen levels during pregnancy in the western spotted skunk.

The present study was undertaken to obtain an estrogen profile throughout gestation in the western spotted skunk with special emphasis on relating changes in estrogen levels to blastocyst development. Blood samples were collected from 130 pregnant animals by cardiac puncture and plasma estrogen levels were determined by radioimmunoassay (RIA). Estrogen levels varied throughout the period of embryonic diapause, with mean levels ranging between 3 to 18 pg/ml. There was a tendency for estrogen levels to be less variable and somewhat lower during the 15 days immediately prior to blastocyst implantation than during the preceding period. Plasma estrogen levels in skunks with delayed implanting blastocysts (diameters of 1.1 mm or less) were significantly higher (P less than 0.05) than those in females with activating blastocysts (i.e., diameters of 1.2 mm or greater), again suggesting that estrogen levels were reduced during the period immediately preceding implantation. However, the exact physiological significance, if any, of this modest reduction in estrogen levels remains to be determined.

Animals↗
PubMed1984

Steroid metabolism in corpora lutea of the western spotted skunk (Spilogale putorius latifrons).

The present study reports steroid metabolism by corpora lutea (CL) obtained from skunks with diapausing embryos ('delay' CL) and with activated embryos (activated CL). CL from both reproductive periods were incubated with various radioactive precursors. Control incubations without any tissue or with 50 microliter of packed skunk blood cells were also conducted simultaneously. Incubation of skunk CL with [3H]-pregnenolone for 3 h resulted in 36% of the precursor accumulating as progesterone. Metabolism of [3H]dehydroepiandrosterone (DHEA) to androstenedione proceeded with approximately the same amount of product accumulating (34-46%) as was observed in the conversion of pregnenolone to progesterone. These results suggest that delta 5 isomerase, 3 beta-hydroxysteroid dehydrogenase, is the most prominent enzyme in skunk CL. Metabolism of [3H]pregnenolone to 17 alpha-hydroxypregnenolone and [3H]progesterone to 17 alpha-hydroxyprogesterone occurred at low rates (1-7%), suggesting the presence of C21 steroid 17 alpha-hydroxylase in skunk CL. Aromatase activity, as estimated by measuring accumulation of oestradiol-17 beta from [3H]testosterone, was demonstrated in activated CL. These results suggest that skunk CL appear to metabolize steroids in a manner similar to CL of other mustelids such as the ferret and American badger.

17-alpha-Hydroxypregnenolone↗
PubMed1983

Luteal contribution to the termination of preimplantation delay in mink.

Five groups of mink were mated once between March 17-19 (Day O), Group 1, designated intact controls, received no further treatment. The remaining four groups of mink underwent unilateral ovariectomy on the day following mating. At that time, preovulatory follicles were transplanted to the ipsilateral kidney capsule of two groups (3 and 5) to become ectopic corpora lutea (CL). The second ovary was removed from all of the animals in Groups 2-5 on Day 8 after mating. At that time animals in Groups 4 (ovariectomized) and 5 (ovariectomized + ectopic CL) received 1-g Silastic implants releasing progesterone. Similar Silastic implants without progesterone were administered to Groups 2 (ovariectomized only) and 3 (ovariectomized + ectopic CL). Blood samples were taken for progesterone analysis and laparotomies performed on all mink through Day 44 of the experiment. Embryos implanted in all (7/7) of the animals in Group 1 (intact controls) at an average of 23.7 days after mating. In Group 5 (ectopic CL + progesterone implant) 6/8 mink were found to have embryos which were calculated to have implanted at an average of 36.3 days after mating. No embryo implantation occurred in Groups 2, 3 and 4 although some unattached blastocysts were recovered from the uteri of the latter two groups. Progesterone was elevated by implants to levels typical of mink gestation. Ectopic corpora lutea further increased progesterone levels in the presence of a progesterone-releasing Silastic implant. The results demonstrate the absolute necessity of the ovary for embryo implantation in mink. Further, the hormonal requirements for implantation consist of progesterone as well as some other factor or factors of luteal origin.

Animals↗
PubMed1983

Steroid metabolism in the corpus luteum of the ferret.

Implantation in the ferret is believed to be induced by a luteal substance which acts in concert with progesterone (P4) and which is secreted sometime between Days 6 and 8 of pregnancy. This experiment was designed to identify the steroid products synthesized by ferret corpora lutea (CL) on these 2 days of pregnancy. CL were dissected from ferrets on Day 6 or 8 of pregnancy and incubated with [3H] pregnenolone (P3), [3H] P4, or [3H] dehydroepiandrosterone (DHEA). Controls with no tissue or with 50 microliters packed blood cells were incubated at the same time. After incubation of Day 6 CL with [3H] P3 for 180 min, 39% of the added label was found incorporated into P4, 3% into 17 alpha-hydroxyprogesterone (17 alpha-OHP4) and 1% into androstenedione (A). Incubation of Day 8 CL with the same precursor resulted in 35%, 1% and 0.65% of the label being incorporated into the previously mentioned products, respectively. Incubations of Days 6 and 8 ferret CL with [3H] P4 or [3H] DHEA confirmed these results, demonstrating activity of C21-steroid, 17 alpha-hydroxylase and delta 5-isomerase, 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD). These results suggest that ferret CL primarily accumulate steroids of the delta4 pathway on both Days 6 and 8 of pregnancy, with P4, 17 alpha-OHP4, A and testosterone (T) being the most abundant products after in vitro incubation. Thus, ferret CL appear to metabolize steroids in a manner similar to that observed in rats, sows and mares.

17-Hydroxysteroid Dehydrogenases↗
PubMed1982

Is estrogen required for implantation in the ferret?

A series of experiments was designed to further test the hypothesis that ferret corpora lutea synthesize and secrete estrogen between Days 6 and 8 of pregnancy, and that this estrogen is required to initiate implantation of blastocysts on Day 12. Corpora lutea, removed on Day 8 of pregnancy contained significant quantities of testosterone. Incubation of aliquots of the same luteal tissue for 4 h significantly elevated estrogen levels above those of nonincubated controls. Peripheral plasma estrogen levels exhibited a slight increase on Day 8 over those observed on Day 6 of pregnancy (7.9 +/- 3.4 vs. 4.1 +/- 1.1 pg/ml). However, continuous release of estradiol from Days 6-8 from two different sizes of Silastic capsules failed to induce implantation in ovariectomized-progesterone treated ferrets, whereas this same treatment was compatible with nidation in intact ferrets. Administration of the aromatase inhibitor, androsta-1,4,6-triene-3,17-dione (ATD) on Days 5-8 of pregnancy prevented implantation of blastocysts on Day 13. Simultaneous administration of estradiol and ATD failed to reverse the inhibitory effect of ATD. Results of this study confirm that luteal tissue of ferrets possesses at least one aromatizable androgen which is converted to estrogen under physiological conditions. However the data do not support the hypothesis that estrogen is required for implantation in the ferret.

Androstatrienes↗
PubMed1981

Delayed implantation in mustelids, with special emphasis on the spotted skunk.

Three distinct reproductive patterns are exhibited by mustelids. Some species (ferret and least weasel) breed during spring and summer and do not exhibit delayed implantation. Others (mink and striped skunk) exhibit variable gestation periods with brief periods of delayed implantation occurring only if the females are mated early in the season. Most mustelids (western spotted skunk, badgers, marten, wolverine, etc.) always exhibit a prolonged period of delayed implantation lasting several months. In such species, the luteal cells fail to undergo complete differentiation during the prolonged period of embryonic diapause. The process of luteal differentiation, which involves doubling of luteal cell size, extensive production of smooth endoplasmic reticulum, and mitochondrial changes, and which is accompanied by increased progesterone secretion, occurs a few days before implantation and coincides with renewed embryonic development. Attempts to induce implantation in mustelids by injecting progesterone have failed, suggesting that other ovarian hormones hormones are required to induce nidation. Changes in photoperiod will hasten or delay luteal development, progesterone secretion and implantation. Hypophysectomy before the time of increased luteal function prevents implantation and increased progesterone secretion, whereas hysterectomy has no effect on progesterone secretion. This indicates that only pituitary hormones are required to induce the final steps in luteal cell differentiation and progesterone secretion. Attempts to induce increased luteal function and implantation by administration of exogenous gonadotrophic hormones have been inconclusive. Prolactin may be important in regulating luteal function in mink and ferrets but may be less significant in other mustelids such as the spotted skunk and European badger.

Animals↗
PubMed1979

The source of progesterone in preimplantation rabbit blastocysts.

In vitro and in vivo synthesis of progesterone, sequestration of progesterone from the surrounding medium, and its subsequent conversion to metabolites was investigated in 146 hr post coitus preimplantation rabbit blastocysts. No significant conversion of 3H-pregnenolone to 3H-progesterone was observed throughout the 8 hr incubation. Progesterone content in blastocysts and culture medium did not change during the course of an 8 hr incubation. This suggests that the failure to detect incorporation of label into progesterone was not due to the presence of a large endogenous pool of pregnenolone. Significant uptake (p less than 0.05) of 3H-progesterone from the incubation medium was observed as was significant conversion of the 3H-progesterone to unidentified metabolites. Therefore it would appear that the preimplantation rabbit blastocyst is not capable of de novo synthesis of progesterone from pregnenolone prior to implantation but sequesters progesterone from the surrounding medium and converts it to progesterone metabolites.

Animals↗
PubMed1976

Morphological changes in the trophoblast, uterus and corpus luteum during delayed implantation and implantation in the western spotted skunk.

Morphological interactions of tropholbast and uterus from stages of preimplantation and implantation were studied in 14 western spotted skunks. In addition, the granulosa lutein cells and plasma progesterone levels were studied. In animals several days from implantation the height of epithelial cells decreased, but began to increase in animals approaching implantation. During the preimplantation period a few leucocytes infiltrated the epithelium, but in animals just prior to implantation many leucocytes infiltrated the epithelium.

Animals↗
PubMed1975

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

The ultrastructure of corpora lutea obtained during the preimplantation implantation and early post-implantation 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) preimplantaion period contained undifferentiated small granulosa cells and differentiated large granulosa lutein cells. The former ranged in size between 12 and 20 mu and the latter between 20 and 45 mu. 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 plate-like 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.

Animals↗