The cellular composition of CL from 6 cows on approximately Day 12 of the oestrous cycle, after synchronization with cloprostenol, was studied by ultrastructural morphometry. Point-count measurements of volume density (mean +/- s.d.) showed that large luteal cells occupied 40.2 +/- 7.0% of the luteal tissue, and small luteal cells 27.7 +/- 6.3%. Of the total of 393.4 +/- 52.0 x 10(3) cells per mm3 of luteal tissue, large luteal cells made up only 3.5% and small luteal cells 26.7%, a ratio of 1:7.6. Endothelial cells/pericytes, at 52.3%, were the most numerous cell type. The mean volume per large luteal cell was 29.6 +/- 6.3 x 10(3) microns 3, while that of small luteal cells was 2.7 +/- 0.4 x 10(3) microns 3. In spherical form, these volumes would represent mean diameters of 38.4 microns and 17.2 microns respectively, and are consistent with published measurements on dispersed luteal cells. However, the values for cell numbers are much higher than published values based on luteal tissue dispersion, suggesting that dispersion may result in substantial and possibly selective losses of luteal cells.

The effect of bull exposure on the resumption of estrous activity following parturition was studied in an experiment using mature Hereford and Hereford X Angus beef cows. In the spring of 1981 and 1982, cows were assigned by breed and calving date to one of two treatment groups. Cows were exposed to bulls either from 3 to 85 d postpartum (BE; n = 45, 1981; n = 35, 1982) or from 53 to 85 d postpartum (NE; n = 39, 1981, n = 36, 1982). Blood samples were collected from all cows once weekly from calving until 85 d postpartum to determine progesterone concentrations. The first increase in progesterone, which indicated onset of estrous cycles occurred at 43 +/- 2 vs 63 +/- 2 d (P less than .01) in 1981 and at 39 +/- 2 vs 61 +/- 3 d (P less than .01) postpartum in 1982 in BE cows and NE cows, respectively. Early postpartum exposure of cows to bulls reduced the postpartum anestrous interval.

Mature Merino rams were made hypothyroid by daily oral drenching with methylthiouracil or hyperthyroid by daily subcutaneous injections of thyroxine for 8 weeks. Neither hypothyroidism nor hyperthyroidism had any apparent effect on either spermatogenesis or daily sperm production, but motility of ejaculated spermatozoa and circulating testosterone concentrations were reduced in both conditions. The ratio of testosterone concentrations in plasma from the internal spermatic vein to those in peripheral blood plasma was higher in hyperthyroid (21.2 +/- 3.5) than in control (11.1 +/- 4.4) and hypothyroid (7.6 +/- 1.4) rams. The basal secretion rate for testosterone was slightly lower in hypothyroid rams and testosterone responses to human chorionic gonadotrophin and after LH-releasing hormone (LHRH) were very much reduced. Basal serum LH levels were low in both hypothyroid and hyperthyroid rams compared with controls whereas there were no differences in FSH levels. The LH response to exogenous LHRH was reduced in hypothyroid rams but not in hyperthyroid rams. Serum prolactin levels on the other hand were higher than control in both hypothyroid and hyperthyroid rams. Reduced testosterone secretion in hypothyroid rams indicates that the normal function of Leydig cells depends on an adequate level of thyroid hormones. The decrease in circulating testosterone concentrations in hyperthyroid rams with normal secretion rates suggests an increased testosterone clearance rate in these animals. The decreased spermatozoal motility in hypo- and hyperthyroid rams suggests that the lowered testosterone level in these animals has altered the androgen-dependent maturation of spermatozoa in the epididymis.(ABSTRACT TRUNCATED AT 250 WORDS)

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To determine whether effects of light pulses on the photoperiodic time measuring system involve changes in pineal gland function, melatonin profiles were determined in groups of ewes maintained under 10-h light, 14-h dark (10L:14D) or 10L:10D:1L:3D. Ewes exposed to 10L:14D had a significantly (P less than 0.01) longer duration of melatonin secretion (15.0 +/- 0.4 h, mean +/- SE) than ewes under 10L:10D:1L:3D (9.0 +/- 0.4 h). The 1-h pulse of light therefore acted as a dawn signal in the latter group. During a period of extended darkness imposed to study endogenous control of melatonin release, there was no change in the duration of elevated melatonin in control ewes (16.1 +/- 0.5 h), but a significant (P less than 0.05) lengthening occurred in pulsed ewes (13.2 +/- 1.4 h). PRL responses to a bolus iv injection of TRH (50 ng/kg BW) were significantly (P less than 0.01) smaller in control ewes (478 +/- 134 ng/ml) compared with pulsed ewes (1578 +/- 175 ng/ml), with responses in the latter group resembling those observed in ewes on long days. A 1-h pulse of light late in the dark phase, therefore, resulted in a melatonin pattern normally observed under long days in ewes, and this was associated with other endocrine functions also characteristic of sheep on long days. It is concluded that pulses of light modify activity of the pineal gland which in turn interacts with the photoperiodic time-measuring system via melatonin. The increase in duration of melatonin secretion observed in pulsed ewes under extended darkness suggests that the melatonin rhythm is under the control of two oscillators coupled to dusk and dawn, and that these oscillators interact more strongly when compressed by an interrupted dark phase.