Endocrine Mechanisms Governing Transition into Adulthood: A Marked Decrease in Inhibitory Feedback Action of Estradiol on Tonic Secretion of Luteinizing Hormone in the Lamb during Puberty*

Retardation of pubertal development by prenatal long days in goat kids born in autumn

“…This is in marked contrast to observations in sheep. Autumn-born female lambs raised in natural photoperiods delay onset of puberty to the breeding season almost a year after birth (Hammond, 1944), but respond to an annually-reversed photoperiod with puberty at the usual age of 35 ± 0-5 weeks (Foster, 1981;Foster & Ryan, 1981). Experiments by Foster and co-workers have examined the photoperiodic sequence necessary for the occurrence of puberty in female sheep Foster et al, 1988aFoster et al, , b, c, 1989Ebling étal.…”

“…Goats born at the natural time of year (spring) reach puberty during the autumn after birth, at about 30 weeks old (Ricordeau et al, 1984). Puberty onset in sheep (also short-day breeders) may occur by an interaction between physical maturity and photo¬ period (Foster & Ryan, 1981); photoperiod can induce puberty onset only if the reproductive axis is sufficiently developed (Foster, 1981;Foster, 1983;Nowak & Rodway, 1985;Foster et al, 1986Foster et al, , 1989. However, the regulation of sexual development, so that onset of puberty occurs at the appropriate time of year in short-day breeding mammals, is complex, involving numerous environmental and developmental factors (Shelton, 1978).…”

Effect of the introduction of rams during the anoestrous season on the pulsatile secretion of LH in ovariectomized ewes

“…Increases in the tonic secretion of LH in untreated, gonadectomized rams and ewes have also been observed after the transition from anoestrus to the breeding season (Lincoln & Short, 1980;Goodman & Karsch, 1981) and in ovariectomized ewe lambs around the expected time of puberty (Foster & Ryan, 1979). There are two possible explanations for all of these effects: first, there may be sufficient negative feedback from adrenal steroids for the ram stimulus to operate through this system; second, the ram stimulus may have a stimulatory effect independent of steroid feedback on the hypothalamic centres controlling the secretion of LH.…”

Endocrine changes from birth to puberty in the heifer

“…The hypothesis is based on the observation that gonado¬ trophin secretion escapes from steroid hormone suppression at the pubertal age in ovariectomized animals given oestradiol implants (rat: Steel & Weisz, 1974; sheep: Foster & Ryan, 1979;heifer: Day et al, 1984). These observations are believed to indicate that the hypothalamic mechanism controlling gonadotrophin secretion in the immature animal is highly sensitive to the inhibitory effects of oestradiol.…”