Reproductive and metabolic characteristics of dairy cattle supplemented with beta-carotene

Seventy-eight Holstein cows alternately were assigned at calving to receive beta-carotene supplementation or act as controls to determine effects of beta-carotene on reproduction and carotene, luteinizing hormone, progesterone, insulin, glucose, and glucagon concentrations in blood plasma. Cows were fed a corn silage-based complete ration. Biweekly jugular blood samples were collected beginning the week after parturition through 90 days. At day 30, supplemented cows received 600 mg synthetic beta-carotene daily for 60 days. Plasma carotene reached a peak of 2.45 micrograms/ml compared to 1.50 micrograms/ml in controls. Supplementation significantly increased plasma carotene but had no effect on luteinizing hormone, progesterone, insulin, glucose, glucagon, or reproductive measures. Days to first heat, days to first breeding, days open, and services per conception averaged 74, 74, 95, and 1.7 for supplemented cows and 64, 76, 102, and 1.9 for control cows. Progesterone increased as lactation progressed. Somatic cells were not different between supplemented and control cows. Supplementation of beta-carotene did not improve reproductive efficiency or alter luteinizing hormone, progesterone, insulin, glucose, or glucagon in blood plasma or affect somatic cells in milk.     

Effect of additional cobalt, copper, manganese, and zinc on reproduction and milk yield of lactating dairy cows receiving bovine somatotropin.

The objective of this study was to determine whether organically complexed Co, Cu, Mn, and Zn would improve the reproductive performance and milk and milk component production in lactating dairy cows that began receiving bovine somatotropin in the ninth week of lactation. Holstein (n = 50) and Jersey (n = 10) cows were blocked by breed, lactation number, and incidence of retained fetal membranes. Two diets assigned within blocks and fed from parturition until 154 d of lactation were control or control supplemented daily with 26 mg of Co as Co glucoheptonate, 125 mg of Cu as Cu-Lys, 199 mg of Mn as Mn-Met, and 359 mg of Zn as Zn-Met. Cows were fitted with electronic pressure-sensing devices in the second week of lactation for detection of estrus. Ovarian structures were determined via transrectal ultrasonography at 7-d intervals from parturition until observation of the first corpus luteum. Blood samples were taken at 7-d intervals and analyzed for plasma concentrations of progesterone, insulin, and urea nitrogen. Onset of luteal activity was identified by progesterone concentrations > or = 1 ng/ml. Retained fetal membranes increased days to first estrus (detected via electronic estrous detection), first luteal activity, and first corpus luteum in control cows but not in supplemented cows. Days to first observed estrus were greater for control cows than for supplemented cows. Days to first service, days open, days from first service to conception, services per conception, milk yield, milk components, and somatic cell counts were similar for control and supplemented cows. Supplementation with complexed trace minerals effectively reduced days to first estrus.     

Effects of adding beta-carotene to rations of lactating cows consuming different forages

Thirty-six Holstein cows fed a corn silage-based ration and 34 fed an alfalfa-grass silage-based ration were assigned according to calving date to receive either 300 mg/head per day of synthetic beta-carotene in a gelatin capsule or an empty gelatin capsule daily for the first 100 days postpartum. Supplemental vitamin A was provided at 3919 IU/kg of ration dry matter. Cervix diameters for cows supplemented with beta-carotene were smaller at 21 days and 28 days postpartum. Days from parturition to first observed estrus were less when beta-carotene was added and less when corn silage was fed. Means of other reproductive traits were more favorable for cows treated with beta-carotene (fewer services per conception and shorter intervals between parturition and conception). Milk progesterone at 21 and 28 days postpartum was not affected by either roughage or beta-carotene treatment. Milk from cows receiving alfalfa-grass silage contained lower somatic cell concentrations than that produced by cows fed corn silage. Milk somatic cell concentrations were lower in both roughage treatment groups for cows supplemented with beta-carotene. No effects of roughage or beta-carotene on milk production or milk fat percentages were significant.     

Effect of supplemental beta-carotene on incidence and responsiveness of ovarian cysts to hormone treatment

Sixty-two multiparous and 35 primiparous Holstein cows were assigned randomly at 10 days postpartum to receive a ration with or without 300 mg beta-carotene/cow per day. Multiparous and primiparous cows were grouped separately and group-fed complete rations once daily. Incidence of ovarian cysts (26% by rectal palpation) was not affected by beta-carotene fed. Multiparous cows had greater incidence (39%) of ovarian cysts than primiparous cows (11%). Fifty-seven percent of cysts were classified follicular by rectal palpation. Progesterone concentration of milk also was used for diagnosis of type of cyst. Cows with ovarian cysts and with progesterone concentrations in milk less than 1 ng/ml were classified follicular, and those having concentration greater than 1 ng/ml were classified luteal. As determined by milk progesterone, rectal palpation was more accurate for diagnosis of luteal cysts than for diagnosis of follicular cysts. Progesterone concentrations of milk for animals with luteal and follicular cysts were 10.66 +/- 1.29 and .37 +/- .07 ng/ml. beta-Carotene did not affect response or days to respond to treatment with human chorionic gonadotropin or gonadotropin-releasing hormone. Supplemental beta-carotene was not beneficial for reducing incidence of ovarian cysts in cows receiving an adequate supply of beta-carotene in their diet.     

Reproductive hormones associated with normal and abnormal changes in ovarian follicles in postpartum dairy cows.

Concentrations of reproductive hormones and ovarian changes were monitored every 4 days during the postpartum period in 35 dairy cows. The cows were classified as either control (22 cows) or cystic (13 cows) based on per rectum examination of the ovaries. Control cows were detected with a corpus luteum after the first postpartum ovulation while in cystic cows, ovarian follicles of at least 2.5 cm in diameter persisted in the absence of a corpus luteum for at least 14 days. The mean postpartum interval to the first follicle 1.5 to 2.0 cm in diameter was about 16 days for both groups. Ovulation was detected earlier postpartum for cows in the control group (18.4 +/- 1.7 days) than for cows in the cystic group (36.3 +/- 2.7 days). Luteinizing hormone in plasma increased during the early postpartum period (days 1 to 10) for both groups, but mean concentrations of luteinizing hormone, progesterone, and estradiol-17 beta in plasma were not different between groups then or on days -8, -4, or 0 (day of the first postpartum follicle 1.5 to 2.0 cm in diameter). During this period (days -8 to 0), estradiol-17 beta and luteinizing hormone in plasma were correlated positively, but regression coefficients differed between groups. These results suggest a partial hypothalamic and/or pituitary failure in releasing pituitary luteinizing hormone as a cause for postpartum ovarian cysts.     

Relationship between ovarian activity and energy status during the early postpartum period of high producing dairy cows

The effect of energy status upon ovarian activity during early lactation was assessed in 54 multiparous Holstein cows. Dry matter intake and milk yield were measured daily from parturition through 9 wk of lactation. Milk composition and body weight were measured weekly during this time. Energy balance corrected for body weight loss was calculated weekly. Blood was collected via tail vein three times weekly and plasma analyzed for progesterone and nonesterified fatty acids. Fifteen cows were anestrus for the 9 wk based on their plasma progesterone (less than 1 ng/ml). These cows were compared with two cycling groups, one group of 25 cows showing corpus luteum activity within 40 d after parturition and a second group of 14 cows showing corpus luteum activity between 40 and 60 d postpartum. Anestrus cows ate less feed, produced less milk, and lost more body weight, resulting in a more negative energy status than cycling cows. Differences in energy balance among cow groups were greatest the first 3 wk postpartum. Anestrus cows and cows showing corpus luteum activity between d 40 and 60 obtained more energy from body reserves for milk production the first 2 wk of lactation, than cows cycling prior to d 40.     

Genetic parameters for atypical reproductive patterns in dairy cows estimated from in-line milk progesterone profiles

Our aim was to estimate genetic parameters of atypical reproductive patterns and estimate their genetic correlation with milk production and classical fertility traits for commercial dairy cows. In contrast with classical fertility traits, atypical reproductive patterns based on in-line milk progesterone profiles might have higher heritability and lower genetic correlation with milk production. We had in-line milk progesterone profiles available for 12,046 cycles in 4,170 lactations of 2,589 primiparous and multiparous cows (mainly Holstein Friesian) from 14 herds. Based on progesterone profiles, 5 types of atypical reproductive patterns in a lactation were defined: delayed ovulation types I and II, persistent corpus luteum types I and II, and late embryo mortality. These atypical patterns were detected in 14% (persistent corpus luteum type II) to 21% (persistent corpus luteum type I) of lactations. In 47% of lactations, at least 1 atypical pattern was detected. Threshold model heritabilities for atypical reproduction patterns ranged between 0.03 and 0.14 and for most traits were slightly higher compared with classical fertility traits. The genetic correlation between milk yield and calving interval was 0.56, whereas genetic correlations between milk yield and atypical reproductive patterns ranged between ?0.02 and 0.33. Although most of these correlations between milk yield and atypical reproductive patterns are still unfavorable, they are lower compared with the correlations between classical fertility traits and milk yield. Therefore selection against atypical reproductive patterns may relax some constraints in current dairy breeding programs, to enhance genetic progress in both fertility and milk yield at a steady pace. However, as long as the target trait for fertility is calving interval, atypical reproductive patterns will not add additional value to the breeding goal in the near future due to the low number of available records.     

Effects of gonadotropin-releasing hormone and human chorionic gonadotropin on pregnancy rate in dairy cattle

This study was to determine if gonadotropin-releasing hormone, human chorionic gonadotropin, or their interaction would affect pregnancy rate or milk progesterone profiles in dairy cattle. Cows and heifers eligible for first, second, and third services were assigned to four treatments: 1) saline i.m. within 5 min after AI (d 0) and saline 15 d after AI (n = 222 AI); 2) gonadotropin-releasing hormone (100 micrograms) i.m. within 5 min after AI and saline on d 15 (n = 223); 3) saline after AI and human chorionic gonadotropin (3500 IU) i.m. on d 15 (n = 196); 4) gonadotropin-releasing hormone at AI and human chorionic gonadotropin on d 15 (n = 195). Progesterone was quantified in milk samples collected twice weekly for 5 wk from all cows. Neither gonadotropin-releasing hormone, human chorionic gonadotropin, gonadotropin-releasing hormone x human chorionic gonadotropin interaction, age, nor service number affected pregnancy rate, which averaged 46.2%. Pregnancy and human chorionic gonadotropin affected milk progesterone profiles. Pregnancy maintained progesterone concentrations and human chorionic gonadotropin on d 15 increased them. This study does not support the general use of gonadotropin-releasing hormone at AI or human chorionic gonadotropin 15 d after AI as methods for enhancing pregnancy rates of dairy cattle.     

Effect of supplemental beta-carotene on luteinizing hormone released in response to gonadotropin-releasing hormone challenge in ovariectomized Holstein cows

Thirteen lactating Holstein cows were assigned randomly to either a control or beta-carotene (600 mg/d) treatment group to determine the effect of long-term beta-carotene supplementation on releasable luteinizing hormone in response to gonadotropin-releasing hormone challenge. The experimental period was 28 wk. Milking was terminated at wk 16, ovariectomy performed at wk 20, and response to gonadotropin-releasing hormone determined at wk 27. Serum beta-carotene concentrations reflected dietary intake and were higher in the cows fed beta-carotene after 2 wk of supplementation and remained higher for the duration of the trial. Feeding supplemental beta-carotene had no effect on circulating progesterone concentration, corpora lutea size or corpora lutea progesterone concentration, or basal concentrations of luteinizing hormone, frequency, and amplitude of luteinizing hormone pulses, or the release of luteinizing hormone in response to gonadotropin-releasing hormone. Thus, pituitary responsiveness to exogenous gonadotropin-releasing hormone was not affected by feeding supplemental beta-carotene.     

Vitamin E supplementation of high fat diets for dairy cows

High fat diets +/- 1 g alpha-tocopheryl acetate per head per day were fed to 28 cows for 4 mo to determine whether dietary fat increases dietary requirement for vitamin E. Tocopherol supplementation did not affect milk production or composition. After 4 mo there was no difference between treatment groups for oxidative stability (thiobarbituric acid test) of milk stored for 5 days with .1 ppm added copper, but milk of supplemented cows was more stable when 1 ppm copper was added. Oxidative stability was normal in the unsupplemented group. Tocopherol concentration of blood plasma increased and selenium concentration decreased with tocopherol supplementation; activity of glutathione peroxidase was lower in plasma, whole blood, and erythrocytes of the supplemented group. Tissue integrity was unchanged by tocopherol supplementation, as estimated by erythrocyte hemolytic resistance and activities of lactic dehydrogenase and glutamic oxalacetic transaminase in blood serum. Clearance of bromosulphthalein from blood plasma was less for cows fed fat and supplemented with tocopherol than for control cows fed neither fat nor vitamin E. No indication of impaired vitamin E status was observed when 4% stabilized fat was added to the diet.     

Use of prostaglandin F2 alpha or gonadotropin releasing hormone in treating problem breeding cows

Ten veterinarians diagnosed and treated 296 cows for reproductive irregularities on 28 dairy farms. Average interval from treatment with 25 mg of prostaglandin F2 alpha (137 cows) to estrus for cows with a corpus luteum (14.6 days) was reduced compared to saline (52 cows) (21.8 days), and a greater percentage (41.6 versus 17.3%) showed estrus within 6 days of treatment. Cows with a cystic corpus luteum (28) were detected in estrus at comparable intervals after prostaglandin F2 alpha or saline (21.8 versus 21.2 days), and similar proportions of cows were detected in estrus within 6 days of treatment (16.7 versus 10.0%). Intervals to estrus among cows with cystic follicles (40 cows) or anestrous cows with follicular ovaries (39 cows) did not differ after treatment with saline or 100 or 200 micrograms of gonadotropin releasing hormone. Within 25 days after treatment, 47.5% of cows with cystic ovaries and 64.1% of anestrous cows showed estrus. Number of cows pregnant after as many as three artificial inseminations of the number examined was 46%, whereas average number of conceptions per service was .32. The long interval from parturition to examination (112 days), the low rate of estrous response after treatment, and the long interval (1.6 estrous cycles) between estruses are consonant with inadequate estrous detection.     

Rumen-protected B vitamin complex supplementation during the transition period and early lactation alters endometrium mRNA expression on day 14 of gestation in lactating dairy cows

Greater metabolic demands in high-producing dairy cows are believed to be a cause of sub-fertility in these animals. Previously, supplementation with vitamin B complex molecules has shown benefits in improving milk production, health, and reproductive efficiency of dairy cows. The primary aim of this project was to determine the effects of rumen-protected vitamin B complex supplementation of 100 g of Transition VB (Jefo, St. Hyacinthe, QC, Canada) and 4 g of Lactation VB (VB; Jefo), during the transition and early lactation periods, respectively, compared with a control diet containing no supplementation on d 14 endometrial outcomes of pregnancy. In the vitamin B supplemented cows, we expect to see a change in the mark-up of endometrial genes important for embryo survival before implantation. Multiparous Holstein cows were enrolled into the study 3 wk before parturition and were randomly assigned to either the VB or control treatment. Twice-a-week blood samples, weekly milk samples, and daily feed intake were collected. Cows were enrolled onto a double-ovsynch protocol at 33 ± 3 d postpartum and inseminated by timed artificial insemination. Milk production and components, concentrations of BHB, haptoglobin, and progesterone in serum, and ovarian dynamics were also measured, but no treatment effect was observed. The uterus was flushed on d 14 after artificial insemination (around 72 DIM) for conceptus collection, and endometrial samples were collected at the same time. Overall, 42 cows were flushed and 13 embryos were collected. Analysis of mRNA expression of genes related to embryo development, immune system, adhesion, and regulation of vitamin B molecules showed that OXTR, MUC5B, MUC1, IL1B, SPP, TRD, FZD8, and FOLR1 genes were significantly upregulated in the VB group. Vitamin B supplementation had no effect on the size of the embryo and ovulatory follicle or corpus luteum diameter at embryo collection. In conclusion, the benefits of strategic dietary VB supplementation during the transition and early lactation might be directly linked to endometrial functions required for embryo survival during the peri-implantation period.     

Genetic merit for fertility traits in Holstein cows: II. Ovarian follicular and corpus luteum dynamics, reproductive hormones, and estrus behavior

The objective of this study was to characterize the estrous cycle of cows with similar proportions of Holstein genetics, similar genetic merit for milk production traits, but with good (Fert+) or poor (Fert-) genetic merit for fertility traits. In total, 37 lactating cows were enrolled on a protocol to synchronize estrus. Nineteen Fert+ and 12 Fert- cows that successfully ovulated a dominant follicle and established a corpus luteum underwent daily transrectal ultrasonography. Blood sampling was carried out at 8-h intervals from d 0 to 6 and from d 15 to ovulation, and once daily from d 7 to 15. Blood samples were analyzed for progesterone, estradiol, follicle stimulating hormone, and luteinizing hormone. Estrus behavior was recorded using neck activity collars and mounting pads. The Fert+ cows tended to have fewer follicular waves (2.2 vs. 2.7) and had a shorter estrous cycle (21.0 vs. 25.1 d) than Fert- cows. We observed no effect of genotype on day of first-wave emergence or day of first-wave dominant follicle peak diameter, but the peak diameter of the first-wave dominant follicle tended to be larger in Fert- cows. During the first 13 d of the cycle, Fert+ cows developed a corpus luteum that was 16% larger than that in Fert- cows. Circulating progesterone concentrations were 34% greater in Fert+ than in Fert- cows (5.15 vs. 3.84ng/mL, respectively) from d 5 to 13. During the final follicular wave, the interval from preovulatory follicle emergence to ovulation and the interval from preovulatory follicle dominance to ovulation were similar in both genotypes. Maximum preovulatory follicle diameter was larger in Fert+ than Fert- cows (17.9 vs. 16.8mm, respectively); however, circulating concentrations of estradiol were not different between genotypes. A greater proportion of Fert- cows ovulated to a silent heat than Fert+ cows (22 vs. 2%, respectively). Of cows that showed behavioral estrus, Fert+ cows had 41% greater mean activity count; however, no difference was seen in mounting behavior between genotypes. These results demonstrate, for the first time, that genetic merit for fertility has pronounced effects on corpus luteum development, progesterone concentration, preovulatory follicle diameter, and behavioral estrus.     

Effects of biotin supplementation on performance and claw lesions on a commercial dairy farm

A controlled 14-mo field trial was conducted to evaluate the effect of biotin supplementation on hoof lesions, milk production, and reproductive performance in a commercial dairy herd. One hundred seventy cows were studied and supplemented with either 0 or 20 mg/d of biotin by computer feeder. All were housed in the same free-stall facility with the same environment, base diet, and management. The feet of 99 cows were trimmed three times at 6-mo intervals, and hoof health was evaluated. Milk production and fertility data were captured monthly by the Dairy Herd Improvement Association. At the final hoof trimming, sole hemorrhages were significantly higher in control (50%) vs. biotin-supplemented animals (24%). The incidents of cows affected with double soles, hoof wall grooves, and heel horn erosion did not differ between control and biotin-supplemented animals. Biotin supplementation of trimmed cows resulted in 878 kg more milk than control cows when compared with previous lactation yield (n = 46 biotin supplemented, n = 48 control cows). At the end of the study, for both trimmed and untrimmed animals, biotin supplemented cows (n = 81) produced 481 kg more milk and 25 kg more fat than the controls (n = 81). There was no interaction between biotin supplementation and hoof trimming on milk production. There were variations in the response of fertility to biotin between age groups. First lactation heifers fed supplemental biotin had significantly fewer days from calving to conception and required fewer inseminations per pregnancy than controls of the same parity.     

The influence of long-term supplementation with biotin on the prevention of lameness in pasture fed dairy cows

In a double-blind study, the influence of biotin supplementation on lameness in dairy cows was investigated over a 13-mo period. The experimental site was a tropical upland environment and involved over 2705 Holstein and Friesian cows on 20 participating farms. Cows on 10 farms received biotin at a rate of 20 mg/head per day in the concentrate, and cows on 10 other farms received feed without the biotin supplement. Premixes with or without biotin were incorporated into a grain concentrate that was fed at a constant rate to cows at milking. Farmers maintained accurate records of the nature of hoof problems and any treatment applied. Each herd was evaluated for locomotion scores at 8-wk intervals. Locomotion scores were significantly correlated with the number of days with measurable rainfall per month (r = 0.88). The biotin-supplemented herds exhibited better locomotion scores than the unsupplemented herds. In the wet summer period the number of lame cows, as observed by the farmer, were significantly fewer during the rainy period for the biotin-supplemented herds and required fewer antibiotic treatments than unsupplemented herds. Most hoof lesions were most commonly observed in the outer claws of the hind limb. Daily milk production (17.3 vs. 18.5 L) was not affected by biotin supplementation. Reduced milk fat percentage and somatic cell counts of bulk milk were recorded in the biotin supplemented herds during the wet, summer period.     

Delayed effect of low progesterone concentrations on bovine uterine PGF(2alpha) secretion in the subsequent oestrous cycle

Low progesterone concentrations during the bovine oestrous cycle induce enhanced responsiveness to oxytocin challenge late in the luteal phase of the same cycle. The delayed effect of low progesterone concentrations during one oestrous cycle on uterine PGF(2alpha) secretion after oxytocin challenge on day 15 or 16 of the subsequent cycle was studied by measuring the concentrations of the major PGF(2alpha) metabolite (13,14-dihydro-15-keto PGF(2alpha); PGFM) in plasma. Two experiments were conducted, differing in the type of progesterone treatment and in the shape of the low progesterone concentration curves. In Expt 1, progesterone supplementation with intravaginal progesterone inserts, with or without an active corpus luteum, was used to obtain high, or low and constant plasma progesterone concentrations, respectively. In Expt 2, untreated cows, representing high progesterone treatment, were compared with cows that had low but increasing plasma progesterone concentrations that were achieved by manipulating endogenous progesterone secretion of the corpus luteum. Neither experiment revealed any differences in plasma progesterone concentrations between the high and low progesterone groups in the subsequent oestrous cycle. In both experiments, both groups had similar basal concentrations of PGFM on day 15 (Expt 1) or 16 (Expt 2) of the subsequent oestrous cycle, 18 days after progesterone treatments had ended. In both experiments, the increases in PGFM concentrations in the low progesterone groups after an oxytocin challenge were markedly higher than in the high progesterone groups. These results indicate that low progesterone concentrations during an oestrous cycle have a delayed stimulatory effect on uterine responsiveness to oxytocin during the late luteal phase of the subsequent cycle. This resulting increase in PGF(2alpha) secretion may interfere with luteal maintenance during the early stages of pregnancy.     

Follicular dynamics and corpus luteum growth and function in pregnant versus nonpregnant cows

The effects of pregnancy on follicular dynamics and corpus luteum growth and function are somewhat contentious. In an effort to learn more about the effects of pregnancy, the reproductive tracts of 16 bred dairy cows were monitored using an ultrasound device with a rectal probe through the first 60 d of pregnancy or upon a return to estrus. Additionally, blood samples were collected for progesterone determination. Eleven of 16 cows were diagnosed pregnant by 25 d postbreeding. There were two embryonic mortalities between 28 and 32 d. A two-wave pattern of follicular growth and atresia, each wave resulting in a large dominant follicle, was seen in both pregnant and nonpregnant cows during the first 24 d postbreeding. Pregnant cows had more follicles than nonpregnant cows; however, there was no difference in the size of the largest follicle. A wave-like pattern of growth of large follicles continued throughout the study period in pregnant cows. Progesterone profiles, corpus luteum growth rate, and maximum size of the corpus luteum were similar in both pregnant and nonpregnant cows. It is concluded that growth of dominant follicles and of the corpus luteum is unaffected during the first 60 d of pregnancy.     

The human corpus luteum: which cells have progesterone receptors?

Studies comparing the regressing corpus luteum with the rescued corpus luteum have demonstrated that human chorionic gonadotrophin (hCG) has effects on cell types that do not express hCG receptors. As progesterone synthesis is hCG dependent and the corpus luteum has been shown to express genomic progesterone receptors, progesterone is a candidate molecule for these paracrine effects. This study aimed to define the cellular localisation of progesterone receptors in the human corpus luteum using dual-staining immunohistochemistry for genomic progesterone receptors and specific cellular markers. Well-characterised corpora lutea (n = 12) from different stages of the luteal phase were studied. The same distribution was observed in all corpora lutea examined. The steroidogenic cells (3beta-hydroxysteroid dehydrogenase positive) and both theca-lutein (17alpha-hydroxylase positive) and granulosa-lutein (aromatase positive) express progesterone receptors, as do stromal fibroblasts (vimentin positive, fibroblast antigen positive). Vascular endothelial cells (CD31 positive), pericytes (alpha-smooth muscle actin positive), macrophages (CD68 positive) and fibroblasts within the central clot do not express nuclear progesterone receptors. Progesterone is a candidate messenger molecule for the effects of hCG on the matrix metalloproteinase-producing stromal fibroblasts. Some of the effects of hCG on steroidogenic cells may be mediated by progesterone, but its effects on blood vessels and macrophages require alternate paracrine signalling mechanisms. In addition, there appears to be at least two fibroblast populations in the corpus luteum.