Source of ovarian inhibin secretion during the oestrous cycle of the sheep

1989 ◽  
Vol 123 (2) ◽  
pp. 181-188 ◽  
Author(s):  
G. E. Mann ◽  
A. S. McNeilly ◽  
D. T. Baird
Keyword(s):  
Corpus Luteum ◽  
Luteal Phase ◽  
Venous Blood ◽  
Contralateral Side ◽  
Follicular Phase ◽  
Oestrous Cycle ◽  
Secretion Rate ◽  
Antral Follicles ◽  
Luteal Tissue

ABSTRACT The source of inhibin secretion by the ovary in the sheep at different stages of the oestrous cycle was investigated by in-vivo cannulation of the ovarian veins. Twenty-four Scottish Blackface ewes were allocated to four groups of six ewes, i.e. those operated on during the luteal phase (day 10), and those operated on during the follicular phase 24–30, 36 and 60 h following an injection of 125 μg cloprostenol on day 10 of the luteal phase. Samples of jugular and timed ovarian venous blood were collected under anaesthesia before and after enucleation of the corpus luteum. Ovaries were then removed and follicles dissected out. Following injection of cloprostenol, luteal regression occurred as indicated by a fall in the secretion of progesterone. The concentration of inhibin in jugular venous plasma and its ovarian secretion rate were similar at all stages of the follicular phase and during the luteal phase. In contrast, the secretion rate of oestradiol rose from 2·68 ±0·73 pmol/min during the luteal phase to 8·70± 2·24 pmol/min 24 h after injection of cloprostenol (P<0·05). Following enucleation of the corpus luteum the secretion rate of progesterone fell from 809 ± 270 pmol/min to 86 ± 30 pmol/min (P<0·001). There was also a smaller, artifactual fall in the secretion rate of oestradiol following enucleation of the corpus luteum, which was of similar size to a fall seen in the secretion rate of inhibin. This resulted in a significant (P<0·001) fall in the ratio of progesterone to inhibin, while the oestradiol to inhibin ratio remained unchanged. The secretion rate of inhibin from ovaries containing luteal tissue was similar to that from the contralateral side without luteal tissue (1·41±0·30 compared with 1·32±0·30 ng/min), while ovaries with large antral follicles secreted significantly (P< 0·001) more inhibin than those with no follicles ≥3 mm (2·28 ± 0·36 compared with 0·25 ±0·06 ng/min). From these results we conclude that, in the sheep, large antral follicles are responsible for most, if not all, the secretion of inhibin by the ovary at all stages of the oestrous cycle, and that the corpus luteum secretes little or no immunoactive or bioactive inhibin. Due to the fact that, unlike inhibin, the secretion rate of oestradiol rises during the follicular phase of the cycle, when the concentration of FSH is suppressed, it seems likely that oestradiol rather than inhibin is the major ovarian factor modulating the change in FSH secretion seen at this stage of the oestrous cycle. Journal of Endocrinology (1989) 123, 181–188

THE SHEEP CORPUS LUTEUM SECRETES INHIBIN

1988 ◽  
Vol 116 (3) ◽  
pp. R3-R5 ◽  
Author(s):  
C. G. Tsonis ◽  
D. T. Baird ◽  
B. K. Campbell ◽  
R. Leask ◽  
R. J. Scaramuzzi
Keyword(s):  
Corpus Luteum ◽  
Luteal Phase ◽  
Venous Blood ◽  
Follicular Phase ◽  
Oestrous Cycle ◽  
Luteal Tissue ◽  
Two Stages

ABSTRACT An experiment was performed in 20 Merino ewes in which ovarian venous blood was collected by venepuncture at surgery and at two stages of the oestrous cycle. The ovarian venous concentrations of inhibin, oestradiol-17β and progesterone were determined. The results demonstrate that during the luteal phase of the oestrous cycle the ovarian venous blood draining an ovary containing luteal tissue contains significantly more inhibin bioactivity than ovarian venous blood from an ovary not containing luteal tissue. During the follicular phase the concentration of inhibin bioactivity in ovarian venous blood was reduced compared with the luteal phase. From this data we conclude that the sheep corpus luteum secretes inhibin bioactivity into the ovarian venous blood.

Hormone production in vivo and in vitro from follicles at different stages of the oestrous cycle in the sheep

1992 ◽  
Vol 132 (2) ◽  
pp. 225-234 ◽  
Author(s):  
G. E. Mann ◽  
A. S. McNeilly ◽  
D. T. Baird
Keyword(s):  
Luteal Phase ◽  
Follicular Phase ◽  
Oestrous Cycle ◽  
Secretion Rate ◽  
Hormone Production ◽  
Luteal Regression ◽  
Follicle Diameter ◽  
Oestradiol Production

ABSTRACT This experiment was undertaken in order to investigate the production of inhibin, oestradiol and androstenedione by ovarian follicles at different stages of the oestrous cycle in sheep. Twenty-four Scottish Blackface ewes were allocated to four groups of six ewes, i.e. those operated on during the luteal phase (day 10), and those operated on during the follicular phase 24–30, 36 and 60 h after the induction of luteal regression by an injection of 125 μg cloprostenol on day 10 of the luteal phase. Samples of jugular and ovarian venous blood were collected under anaesthesia and ovaries were then removed and all follicles larger than 3 mm diameter dissected out and incubated in medium for 2 h. After injection of cloprostenol, luteal regression occurred as indicated by a fall in the secretion rate of progesterone. The ovarian secretion rate of inhibin was similar at all stages of the follicular phase and during the luteal phase while, in contrast, the secretion rate of oestradiol was significantly (P < 0·05) elevated in the group 24 h after injection of cloprostenol. There was good correlation between the in-vivo ovarian secretion rate and production rate during incubation in vitro for both inhibin (r = 0·57) and oestradiol (r = 0·60). When follicle diameter was compared with in-vitro hormone production there was good correlation for inhibin (r = 0·72) with larger follicles producing more inhibin, while the value for oestradiol was somewhat lower (r = 0·57) owing to the presence of large atretic follicles with low oestradiol production. Androstenedione production showed a lower correlation with follicle diameter (r = 0·39). When the four time periods were compared separately, there were significantly (P < 0·05) more follicles with high in-vitro oestradiol production (> 90 fmol/min) in the group at 36 h than in the other three groups, while inhibin release in relation to follicle size was similar in the four groups. Large oestrogenic follicles were responsible for 90% of the total oestradiol production during culture while only providing 55% of the total inhibin production, with large non-oestrogenic and small follicles contributing 33% and 12% of inhibin production respectively. From the results of this study we conclude that while oestradiol is mainly produced by the large oestrogenic follicles, a considerable amount of inhibin is also produced by large non-oestrogenic and small follicles. We also found that a lack of variation in inhibin secretion rate in the intact animal was paralleled by a lack of variation in the pattern of inhibin produced from individual follicles. Journal of Endocrinology (1992) 132, 225–234

Ovarian secretion rates and peripheral concentrations of inhibin in normal and androstenedione-immune ewes with an autotransplanted ovary

1990 ◽  
Vol 127 (2) ◽  
pp. 285-296 ◽  
Author(s):  
B. K. Campbell ◽  
D. T. Baird ◽  
A. S. McNeilly ◽  
R. J. Scaramuzzi
Keyword(s):  
Luteal Phase ◽  
Venous Blood ◽  
Plasma Concentrations ◽  
Active Immunization ◽  
Follicular Phase ◽  
Secretion Rate ◽  
Luteal Function ◽  
Prostaglandin F ◽  
Ovarian Inhibin ◽  
Secretion Rates

ABSTRACT Active immunization of sheep against androstenedione results in an increase in ovulation rate that is associated with increased plasma levels of LH and progesterone, but not FSH. Although immunized ewes have more activated follicles the secretion rate of oestradiol is not increased. An experiment was conducted to examine the effect of androstenedione immunity on the ovarian secretion and peripheral plasma concentrations of inhibin. Merino ewes in which the left ovary had been autotransplanted to a site in the neck were divided into control (n = 5) and androstenedione-immune (n = 6) groups. Ovarian and jugular venous blood was collected every 10 min at two stages of the follicular phase, 21–27 h and 38–42 h after a luteolytic dose of an analogue of prostaglandin F2α (PG), and every 15 min for 6 h on day 10 of the subsequent luteal phase. The ewes were monitored regularly for luteal function by measurement of the concentration of progesterone and preovulatory LH surges. The concentration of inhibin in jugular and ovarian venous plasma was determined by radioimmunoassay and ovarian secretion rates and peripheral concentrations are expressed as pg of 1–26 peptide fragment of the α chain. The ovarian secretion rate of inhibin tended to be greater in androstenedione-immune ewes at all stages of the oestrous cycle measured, with this difference being statistically significant (P <0·05) during the luteal phase (100±40 and 260±80 (s.e.m.) pg/min for control and immune groups respectively). The pattern of ovarian inhibin secretion exhibited pulsatile-like fluctuations which were not associated with LH pulses. Peripheral concentrations of inhibin were generally higher in immunized than in control ewes with this difference being significant (P < 0·01) from day 4 to 14 of the luteal phase (59±5 and 110±7 ng/1 for control and immune respectively). The ovarian secretion rate of immunoactive inhibin was greater (P <0·01) during the follicular phase than during the luteal phase in both groups of ewes, and peripheral concentrations of inhibin increased (P < 0·001) following injection of PG in ewes from both treatment groups. We concluded that androstenedione immunity results in an increase in ovarian inhibin secretion, an effect that can probably be attributed to the greater number of large oestrogenic follicles present in the ovaries of these ewes. Furthermore, this increase in the concentration of inhibin may override any decrease in the negative feedback effects of ovarian steroid produced by immunization and, hence, explain the paradoxical findings of normal concentrations of FSH and raised concentrations of LH in ewes which are immunized against androstenedione. Journal of Endocrinology (1990) 127, 285–296

STEROIDS IN THE OVARIAN VENOUS BLOOD OF EWES BEFORE AND AFTER GONADOTROPHIC STIMULATION

1963 ◽  
Vol 26 (1) ◽  
pp. 155-169 ◽  
Author(s):  
R. V. SHORT ◽  
M. F. McDONALD ◽  
L. E. A. ROWSON
Keyword(s):  
Luteinizing Hormone ◽  
Corpus Luteum ◽  
Luteal Phase ◽  
Venous Blood ◽  
Turnover Time ◽  
Chorionic Gonadotrophin ◽  
Secretion Rate ◽  
Ovarian Vein ◽  
Before And After ◽  
Order Of Magnitude

SUMMARY Ewes on the 9th and 15th days of the oestrous cycle were injected intravenously with follicle stimulating hormone, luteinizing hormone and prolactin of sheep pituitary origin, human chorionic gonadotrophin, pregnant mares' serum and an endometrial extract. The ovarian vein was cannulated, and progesterone measured chemically in the venous effluent. None of the above substances was capable of producing a pronounced or sustained alteration in the rate of progesterone secretion in the hour following injection. It was concluded that the ovine corpus luteum, if it responds at all to gonadotrophic stimulation, must do so in a manner very different from the testis, the response of which to luteinizing hormone is both immediate and sustained. The mean secretion rate of progesterone was approx. 150 μg./hr., thus being of the same order of magnitude as a previous indirect estimate of 113 μg./hr. There was no difference in the secretion rate between the 9th and 15th days of the cycle, apart from one animal at the 15th day in which the corpus luteum had ceased to secrete progesterone altogether. The turnover time of progesterone in the corpus luteum was 10 min., suggesting that the hormone is released from the gland as soon as it is formed. Pooled samples of ovarian vein plasma collected during the mid- and late luteal stages of the cycle contained, apart from progesterone, pregnenolone, 20α-hydroxypregn-4-en-3-one, oestrone and oestradiol-17β. In three samples of ovarian vein plasma collected from ewes in oestrus it was impossible to detect progesterone or 20α-hydroxypregn-4-en-3-one, even after chorionic gonadotrophin therapy. Oestradiol-17β and oestrone were present in concentrations similar to those found during the luteal phase of the cycle. Since the rate of ovarian vein blood flow is lower at oestrus than during the luteal phase, it follows that the oestrogen secretion rates are also lower at oestrus. The 24 hr. secretion rate of oestradiol-17β in two oestrous ewes was 3·3–7·4 μg. This is in close agreement with the median effective dose of 7 μg. of oestradiol-17β needed to induce oestrus in the spayed, progesterone-primed ewe.

Pulsatile secretion of inhibin, oestradiol and androstenedione by the ovary of the sheep during the oestrous cycle

1990 ◽  
Vol 126 (3) ◽  
pp. 385-393 ◽  
Author(s):  
B. K. Campbell ◽  
G. E. Mann ◽  
A. S. McNeilly ◽  
D. T. Baird
Keyword(s):  
Luteal Phase ◽  
Venous Blood ◽  
Pulse Amplitude ◽  
Pulse Frequency ◽  
Follicular Phase ◽  
Oestrous Cycle ◽  
Blood Samples ◽  
Early Follicular Phase ◽  
Lh Secretion ◽  
Late Follicular Phase

ABSTRACT The pattern of pulsatile secretion of inhibin, oestradiol and androstenedione by the ovary at different stages of the oestrous cycle in sheep was studied in five Finn–Merino ewes in which the left ovary had been autotransplanted to the neck. The ewes had jugular venous blood samples collected at 4-hourly intervals from 42 h before the induction of luteolysis by i.m. injection of cloprostenol (100 μg) on day 10 of the oestrous cycle, until day 3 of the following cycle. There were five periods of intensive blood sampling, when both ovarian and jugular venous blood samples were collected, as follows: (a) mid-luteal phase, before the second injection of cloprostenol on day 10 (15-min intervals for 6 h); (b) early follicular phase, 24 h after the second injection of cloprostenol (10-min intervals for 4 h); (c) late follicular phase, 48 h after the second injection of cloprostenol (10-min intervals for 4 h); (d) after the LH surge on day 1 of the cycle, 76 h after the second injection of cloprostenol (10-min intervals for 4 h); (e) early luteal phase on day 3 of the cycle, 120 h after the second injection of cloprostenol (10-min intervals for 3 h). Plasma was collected and the samples assayed for LH, FSH, progesterone, oestradiol, androstenedione and inhibin. The ovarian secretion rates for oestradiol, androstenedione and inhibin were calculated. All ewes responded normally to the luteolytic dose of cloprostenol with the preovulatory surge of LH occurring within 56·4±1·6 h (mean ± s.e.m.) followed by the establishment of a normal luteal phase. The pulse frequency of LH, oestradiol and androstenedione increased in the transition from the luteal to the follicular phase (P<0·01). On day 1 of the cycle LH secretion consisted of low-amplitude high-frequency pulses (1·0±0·1 pulse/h) to which androstenedione, but not oestradiol, responded. On day 3 of the cycle LH secretion was similar to that on day 1 but both androstenedione and oestradiol secretion were pulsatile in response to LH, indicating the presence of oestrogenic follicles. The stage of the cycle had no significant effects on LH pulse amplitude and nadir but the ovarian secretory response to LH stimulation did vary with the stage of the cycle. Prolactin pulse frequency, amplitude and nadir were higher (P<0·05) during the follicular phase than the luteal phase. Prolactin pulse frequency was depressed (P<0·05) on day 1 of the cycle but increased to follicular phase levels on day 3. Prolactin pulse frequency was significantly correlated to oestradiol pulse frequency (r = 0·54; P<0·01). During the luteal phase there were insufficient oestradiol pulses to obtain an estimate of pulse amplitude and nadir but both these parameters reached their highest level during the late follicular phase, fell to negligible levels on day 1 and increased to early follicular phase levels on day 3. Androstenedione pulse amplitude and nadir exhibited similar but less marked variation. Inhibin secretion was episodic at all stages of the cycle examined but did not exhibit significant variation with stage of cycle in any of the parameters of episodic secretion measured. Inhibin pulses were not related to either LH or prolactin at any stage of the cycle. FSH secretion was not detectably pulsatile but jugular venous concentrations of FSH at each stage of the oestrous cycle were negatively correlated with mean oestradiol (r= −0·52; P<0·01 but not inhibin secretion (r = 0·19). We conclude that (i) LH secretion is pulsatile at all stages of the oestrous cycle but the steroidogenic responses of the ovary varies with the stage of the cycle, reflecting changes in characteristics of the follicle population, (ii) ovarian inhibin secretion is episodic and displays little change with the stage of the oestrous cycle and (iii) episodic inhibin secretion is not related to either pulses of LH or prolactin. The aetiology of these inhibin pulses therefore remains unknown. Journal of Endocrinology (1990) 126, 385–393

DISTRIBUTION OF OESTRONE AND OESTRADIOL-17β IN SOW OVARIES

1963 ◽  
Vol 44 (4) ◽  
pp. 529-535 ◽  
Author(s):  
Torleiv Lunaas
Keyword(s):  
Follicular Fluid ◽  
Luteal Phase ◽  
Follicular Phase ◽  
Oestrous Cycle ◽  
Corpora Lutea ◽  
Luteal Tissue ◽  
Low Levels ◽  
The Absolute ◽  
Residual Tissue ◽  
Highly Correlated

ABSTRACT The contents of oestrone and oestradiol-17β were estimated in component parts of sow ovaries representative of the follicular and luteal phase of the oestrous cycle and also in whole juvenile ovaries. The values obtained of the sum total oestrone and oestradiol-17β in the follicular fluid and in the corresponding residual tissue were highly correlated. The proportions of oestradiol-17β tended to be larger in the follicular fluid than in the tissue (mean percentage: 84.3 and 69.6 respectively). In ovaries from the luteal phase of the oestrous cycle the oestrogen levels were generally lower than in ovaries from the follicular phase. Whereas no difference was found between the very low levels of oestrone in the corpora lutea and in the remainder of the ovary containing small follicles, the levels of oestradiol-17β differed significantly, being lower in the luteal tissue. Within each category of ovaries the absolute levels of the oestrogens were very variable. The results are discussed in relation to the pattern of urinary oestrone excretion in the sow during the oestrous cycle.

THE CORPUS LUTEUM OF THE SHEEP: RELATIONSHIPS BETWEEN MORPHOLOGY AND FUNCTION DURING THE OESTROUS CYCLE

1966 ◽  
Vol 51 (2) ◽  
pp. 245-263 ◽  
Author(s):  
Helen Wendler Deane ◽  
Mary F. Hay ◽  
R. M. Moor ◽  
L. E. A. Rowson ◽  
R. V. Short
Keyword(s):  
Corpus Luteum ◽  
Venous Blood ◽  
Secretory Activity ◽  
Oestrous Cycle ◽  
Rapid Decline ◽  
Electron Microscopic ◽  
Functional Aspects ◽  
Luteal Tissue ◽  
And Function ◽  
Regressive Changes

ABSTRACT Certain structural and functional aspects of the regression of the corpus luteum in the sheep were studied using biochemical, histological, electron microscopic and histochemical techniques. Alterations in the size and density of mitochondria in the lutein cells on Day 12 or 13 of the oestrous cycle were the first sign of luteal regression seen. This was followed by the appearance of cytoplasmic lipid droplets on Day 13 or 14, in the large, definitive lutein cells. The rapid decline in the secretory activity of the corpus luteum on Day 15 was associated with a reduction in Δ5-3β-hydroxysteroid dehydrogenase and diaphorase activities, as well as with shrinkage of the lutein cells and pyknosis of their nuclei. Despite a general correlation between all the above regressive changes, however, there was in a few instances a lack of complete concordance between the parameters studied. It is therefore not possible to state that any one criterion gives a consistently reliable indication of the functional state of the corpus luteum. A good correlation was found between the progesterone concentration in the ovarian venous blood and that in the luteal tissue during the regression of the corpus luteum. The occurrence of some regressive changes on Day 12 may be related to the fact that this is the time when the ovine corpus luteum must be stimulated by an embryo in the uterus to survive.

Developement, function and regression of the corpus luteum in the marsupial Trichosurus vulpecula

1973 ◽  
Vol 21 (4) ◽  
pp. 477 ◽  
Author(s):  
CD Shorey ◽  
RL Hughes
Keyword(s):  
Corpus Luteum ◽  
Luteal Phase ◽  
Secretory Activity ◽  
Oestrous Cycle ◽  
Secretion Rate ◽  
Trichosurus Vulpecula ◽  
Progesterone Concentration ◽  
Apparent Increase ◽  
Pregnant Females ◽  
Secretion Rates

Following ovulation, which usually takes place on day 1 of the 26-day oestrous cycle in T. culpecula, a single corpus luteum begins to develop. During the period before the uterine luteal phase begins on day 8 of the cycle, the granulosa lutein cells are active in the synthesis of what appears to be a cholesterol-like substance at a time when the ovarian secretion rates of progesterone are re!atively low. The uterine luteal phase extends from day 8 to day 15 of the cycle. During this phase the ovarian secretion rates of progesterone are relatively high, and reach maximal levels on days 12-1 3. This high secretory activity is manifested in the granulosa lutein cells by an apparent increase in the number of mitochondria, thought to be involved with the conversion of intracellularly stored cholesterol to pregnenolone, and the development of whorl-like configurations of agranular reticulum which are reported to be concerned with the conversion of pregnenolone to progesterone. A drop in progesterone concentration commences in pregnant and non-pregnant females on day 17 of the cycle, when early involution of the corpus luteunl is apparent. By day 20, the corpus luteum regresses to an early corpus albicans, and the ovarian secretion rate of progesterone falls progressively to levels similar to those in anoestrous females.

Seasonal and steroid-dependent effects on the modulation of LH secretion in the ewe by intracerebroventricularly administered β-endorphin or naloxone

1989 ◽  
Vol 122 (2) ◽  
pp. 509-517 ◽  
Author(s):  
R. J. E. Horton ◽  
H. Francis ◽  
I. J. Clarke
Keyword(s):  
Breeding Season ◽  
Luteal Phase ◽  
Pulse Frequency ◽  
Opioid Peptide ◽  
Follicular Phase ◽  
Oestrous Cycle ◽  
Opioid Antagonist ◽  
Endogenous Opioid ◽  
Endogenous Opioid Peptide ◽  
Lh Secretion

ABSTRACT The natural opioid ligand, β-endorphin, and the opioid antagonist, naloxone, were administered intracerebroventricularly (i.c.v.) to evaluate effects on LH secretion in ovariectomized ewes and in ovariectomized ewes treated with oestradiol-17β plus progesterone either during the breeding season or the anoestrous season. Ovary-intact ewes were also studied during the follicular phase of the oestrous cycle. Jugular blood samples were taken at 10-min intervals for 8 h and either saline (20–50 μl), 100 μg naloxone or 10 μg β-endorphin were injected i.c.v. after 4 h. In addition, luteal phase ewes were injected i.c.v. with 25 μg β-endorphin(1–27), a purported endogenous opioid antagonist. In ovariectomized ewes, irrespective of season, saline and naloxone did not affect LH secretion, but β-endorphin decreased the plasma LH concentrations, by reducing LH pulse frequency. The effect of β-endorphin was blocked by administering naloxone 30 min beforehand. Treating ovariectomized ewes with oestradiol-17β plus progesterone during the breeding season reduced plasma LH concentrations from 6–8 μg/l to less than 1 μg/l. In these ewes, saline did not alter LH secretion, but naloxone increased LH pulse frequency and the plasma concentrations of LH within 15–20 min. During anoestrus, the combination of oestradiol-17β plus progesterone to ovariectomized ewes reduced the plasma LH concentrations from 3–5 μg/l to undetectable levels, and neither saline nor naloxone affected LH secretion. During the follicular phase of the oestrous cycle, naloxone enhanced LH pulse frequency, which resulted in increased plasma LH concentrations; saline had no effect. In these sheep, β-endorphin decreased LH pulse frequency and the mean concentrations of LH, and this effect was prevented by the previous administration of naloxone. The i.c.v. administration of β-endorphin(1–27) to luteal phase ewes did not affect LH secretion. These data demonstrate the ability of a naturally occurring opioid peptide to inhibit LH secretion in ewes during the breeding and non-breeding seasons, irrespective of the gonadal steroid background. In contrast, whilst the gonadal steroids suppress LH secretion in ovariectomized ewes during both seasons, they only appear to activate endogenous opioid peptide (EOP)-mediated inhibition of LH secretion during the breeding season. Furthermore, these data support the notion that LH secretion in ovariectomized ewes is not normally under the control of EOP, so that naloxone has no effect. Journal of Endocrinology (1989) 122, 509–517

A comparative study of the corpus luteum

1995 ◽  
Vol 7 (3) ◽  
pp. 303 ◽  
Author(s):  
RT Gemmell
Keyword(s):  
Corpus Luteum ◽  
Luteal Phase ◽  
Secretory Granules ◽  
Hormonal Control ◽  
Egg Laying ◽  
Oestrous Cycle ◽  
Corpora Lutea ◽  
Embryonic Diapause ◽  
Alternative Reproductive Strategy ◽  
Uterine Development

The corpus luteum (CL) is a transitory organ which has a regulatory role in reproduction. Sharks, amphibians and reptiles have corpora lutea that produce progesterone which influences the rate of embryonic development. The egg-laying monotremes and the two major mammalian groups, eutherian and marsupial, have a CL that secretes progesterone. Most eutherians have allowed for the uterine development of their young by extending the length of the oestrous cycle and the CL or placenta actively secretes progesterone until birth. Gestation in the marsupial does not extend beyond the length of an oestrous cycle and the major part of fetal development takes place in the pouch. Where the extension of the post-luteal phase in the eutherian has allowed for the uterine development of young, the marsupial has extended the pre-luteal phase of the oestrous cycle and has evolved an alternative reproductive strategy, embryonic diapause. The mechanism for the secretion of hormones from the CL has been controversial for many years. Densely-staining secretory granules have been observed in the CL of sharks, marsupials and eutherians. These granules have been reported to contain relaxin, oxytocin or mesotocin, and progesterone. A hypothesis to suit all available data is that all hormones secreted by the CL are transported within such granules. In conclusion, although there are obvious differences in the mode of reproduction in the two main mammalian groups, it is apparent that there is a great deal of similarity in the hormonal control of regression of the CL and parturition.

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