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Biomedical subjects

R B Land

Publications and source records attributed to R B Land.

At least 37 records · Page 2Linked to original sources
PubMed1978

Reproduction in young sheep: some genetic and environmental sources of variation.

"The extent of genetic and environmental variation" in the development of reproduction in sheep is illustrated by examples with particular reference to variation among breeds and to the effects of photoperiod. The interactions between genetic and environmental effects are introduced; these may be so great that genetic groups may reverse their ranking for rate of development in different environments. The "physiology of puberty" is then discussed. The difficulty of separating puberty from seasonal variation is stressed, and a possible contrast is drawn between the physiological characteristics of genetic variation and those of environmental variation in reproductive development. Finally the physiological factors associated with sterility in young females are discussed; most studies, however, have been conducted during the time of year when adult females would also be expected to be sterile, so that conclusions are difficult and a "missing link" cannot be identified.

Animals↗
PubMed1978

Oestradiol levels in sheep plasma during the oestrous cycle.

The concentration of oestradiol was measured by radioimmunoassay in ovarian venous plasma collected from ewes of three breeds (Finnish Landrace, Scottish Blackface and Tasmanian Merino) on Day 9 of the oestrous cycle and in jugular venous plasma collected daily around oestrus in two of these breeds. The mean +/- s.e.m. concentration in the ovarian venous plasma of the Merino (44.1 +/- 7.6 pg/ml) was lower than that in Blackface (72.2 +/- 10.2 pg/ml) and Finn ewes (66.8 +/- 10.2 pg/ml). The overall fitted mean concentration in peripheral venous plasma was 1.7 pg oestradiol-17beta/ml, with no difference between the Finn and Blackface ewes, in which the highest preovulatory values were 3.9 +/- 0.5 and 3.4 +/- 0.5 pg/ml respectively.

Animals↗
PubMed1978

Selection among males for the genetic improvement of female fertility.

The quantitative physiological study of genetic variation in reproductive performance has shown that differences in the litter size and incidence of lambing in sheep are associated with variation in the release of LH. This variation is detectable in young animals of both sexes, and the injection of LHRH may aid its recognition. In the male it is associated with variation in mating behavior and testis growth. The ovarian activity of both sheep and mice has been shown to change following selection for testis growth. The use of measures of testis growth as criteria for selection to improve the reproductive performance of females is discussed.

Animals↗
PubMed1977

Seasonal variation in oestrus, the secretion of oestrogen and progesterone and LH levels before ovulation in the ewe.

Ewes with cervical ovarian autotransplants were studied after PGF-2alpha-induced luteal regression in November and February (mid- and late-breeding season) and June (anoestrum). Progesteron and oestradiol-17beta secretion rates and LH concentrations were determined in serial ovarian venous blood samples and the ewes were frequently tested for oestrus. All 4 ewes in November and 3 of the 4 ewes in February exhibited oestrus and endocrine changes indicative of ovulation. The remaining ewe in February and the three ewes in June failed to show elevated oestradiol-17beta secretion rates after luteal regression, indicating the absence of follicles in the final stages of maturation. The preovulatory rise in the oestradiol-17beta secretion rate, the LH surge and the display of oestrus all occurred earlier, with respect to the PGF-2alpha infusion, in November than in February, suggesting a greater stimulation of folliculogenesis, and therefore a greater availability of maturing follicles, in November than in February.

Animals↗
PubMed1976

Dynamic studies of prostaglandin F-2 alpha in the utero-ovarian circulation of the sheep.

The concentration of prostaglandin (PG) F-2alpha in ovarian arterial blood was compared to that in aortic and carotid blood at different stages of the ovine oestrous cycle, and its concentration measured in the ovarian artery following the infusion of radioactive PGF-2alpha to the uterine vein. In all cases the concentration of PGF-2alpha increased during passage through the ovarian artery, and the increase was proportional to the log of the concentration in the uterine vein. No such change was observed in the concentration of progesterone in the ovarian artery or of PGF in the uterine artery. It was concluded that PGF-2alpha can be and is normally transferred from the uterine vein to the ovarian artery. These results are compatible with the hypothesis that PGF-2alpha of uterine origin is the normal luteolytic agent in the sheep.

Animals↗
PubMed1976

Endocrine changes associated with luteal regression in the ewe; the secretion of ovarian oestradiol, progesterone and androstenedione and uterine prostaglandin F2alpha throughout the oestrous cycle.

The concentrations of oestradiol, androstenedione, progesterone and prostaglandin F2alpha (PGF2alpha) were measured in utero-ovarian autotransplants. The secretion of oestradiol was closely correlated with that of androstenedione (r = 0-67, P less than 0-001) indicating a common origin from the Graafian follicle. The concentration of these two steroids fluctuated at random throughout the luteal phase with the maximum secretion occurring about 2 days before the onset of oestrus. Functional regression of the corpus luteum, as indicated by a fall in the secretion of progesterone, began on day 12 or day 13, i.e. about 4 days before the onset of oestrus. In five of the six cycles the first significant rise in the secretion of PGF2alpha occurred on days 12-14 at the time of decline of progesterone secretion, although the release of PGF2alpha was maximal on the day before the onset of oestrus. There was very little release of PGF2alpha from the uterus before day 12. The temporal relationship of these events suggests that the uterus will only release PGF2alpha after it has been primed for 7-10 days with progesterone. The initiation of luteal regression is independent of secretion of oestradiol by the pre-ovulatory follicle which may, however, stimulate the further release of PGF2alpha responsible for irreversible structural luteolysis on the day of pro-oestrus.

Androstenedione↗
PubMed1975

Plasma luteinizing hormone levels and testis diameters of ram lambs of different breeds.

Plasma LH levels were examined at 1 1/2-hr intervals in ram lambs of the Finnish Landrace, Border Leicester and Cheviot breeds on three occasions at 3-week intervals by radioimmunoassay. On each occasion, the mean plasma LH concentration was found to be significantly higher in the Finnish Landrace than the other two breeds, although big variations occurred between individual samples in the same animal. This form of hormonal pulsatile release was most marked in the Finnish Landrace lambs. Testis growth was also measured and was found to be correlated, independently of body weight, with LH concentration on the final sampling occasion.

Animals↗
PubMed1975

The effects of nutrition on the reproductive performance of Finn x Dorset ewes. I. Plasma progesterone and LH concentrations during late pregnancy.

Progesterone and LH concentrations were measured in the plasma of blood samples taken from forty-eight pregnant ewes on Days 100, 120 and 134 of gestation. The ewes, in two groups of twenty-four were maintained from Day 100 until parturition on two planes of nutrition which supplied daily energy and protein intakes of about 4-1 or 2-3 Mcal metabolizable energy and either 192 or 111 g digestible crude protein per ewe. Within the groups, the ewes carried one, two or three fetuses and the feed intake was adjusted according to litter size to produce a uniform nutritional state within the group. On Day 100, litter size affected the concentration of plasma progesterone (P less than 0-001), but had no effect on Days 120 or 134 when the ewes were fed according to litter size. The low feed intake however caused a significant increase in plasma progesterone concentrations. The LH concentrations showed no major changes during late pregnancy and no effect of nutrition or little size on the plasma hormone concentration was observed. It was concluded that the effect of litter size on plasma progesterone concentration recorded on Day 100 or gestation was partly mediated by level of nutrition.

Animal Nutritional Physiological Phenomena↗
PubMed1975

The effects of nutrition on the reproductive performance of Finn x Dorset ewes. II. Post-partum ovarian activity, conception and the plasma concentration of progesterone and LH.

After lambing forty-five ewes were allocated to three groups, two of sixteen and one of thirteen ewes. The lambs of the two groups of sixteen ewes were weaned on Day 1 after lambing and the ewes were fed a diet of 100% (Group H) or 50% (Group R) of maintenance energy requirements. The thirteen ewes in the third group (Group L) suckled twin lambs and were fed freely. During the first 3 weeks after lambing, oestrus was observed for 11/16 (Group H) and 8/16 (Group R) ewes; of the ewes which had shown oestrus in the two groups, ovulation occurred in 5/8 and 5/7 respectively. Only 1/13 Group-L ewes showed oestrus and ovulated during the same period. The mean plasma concentrations of progesterone and LH were unaffected by the treatments and were around 0-4 and 1-5 ng/ml, respectively. Restricted feeding had no effect on oestrus, ovulation or the hormone levels during the oestrus cycle following synchronization. The onset of oestrus and the start of the preovulatory discharge of LH were 3 and 6 hr later, respectively, in the lactating ewes (Group L) than in those in Groups H and R. Ewes in Group L also had a higher ovulation rate, 2-8 +/- 0-2 versus 2-1 +/- 0-2 (P less than 0-05). Restricted feeding reduced the number of ewes lambing; only 1/11 ewes in Group R, considered to have conceived because of the presence of high progesterone levels 17 days after mating, subsequently lambed compared with 6/12 in Group H and 5/9 in Group L.

Animal Nutritional Physiological Phenomena↗
PubMed1975

Testis growth and plasma LH concentration following hemicastration and its relation with female prolificacy in sheep.

The mean testis diameter of 20- to 25-week-old Blackface, Finn and Merino rams was ranked in the same order as the ovulation rates of females of their breeds. The removal of one testis at 12 or 16 weeks of age resulted in hypertrophy of the remaining testis. The relative increase in testis growth following hemicastration was greatest in the Merino rams (72%), least in the Finns (42%) and intermediate in the Blackfaces (57%), so that it was inversely related to their breed ovulation rates. This hypertrophy was associated with increases in the concentration of plasma LH in all breed types. The results indicate that differences in testis growth rate are associated with differences in gonadotrophic stimulation rather than in intrinsic growth potential, and it is postulated that these may arise from breed differences in sensitivity to negative feed-back from the testes.

Age Factors↗