Endocrine basis for puberty in heifers and ewes.

Maturation processes that culminate in puberty and sexual maturity are initiated before birth, continue through prepuberty (> 50 days before puberty) and peripuberty (50 days before puberty) and are completed early after puberty. The hypothalamus is the primary site of change during transition to sexual maturity. Maturation of the hypothalamus results from decreased negative feedback of oestradiol that leads to increased frequency of release of LH pulses. Increased tonic release of LH pulses during sexual maturation is the primary endocrine factor that regulates the onset of puberty in ewe lambs and heifers. Increased frequency of release of LH pulses enhances development of ovarian follicles which produce enough oestradiol to induce behavioural oestrus and a preovulatory surge of gonadotrophins. In later stages of peripuberty, ovulation or luteinization of follicles results in transient increases in progesterone for shorter periods than is typical for luteal phases of the oestrous cycle of mature ewes and cows. Transient increases in progesterone are not generally preceded by behavioural oestrus. After the demise of the transient luteal structures, puberty is attained with occurrence of the first behavioural oestrus that is accompanied by ovulation and development of a corpus luteum with a typical lifespan. At puberty, all components of the hypothalamic-pituitary-ovarian axis are in place for oestrous cycles to be expressed. Factors that can influence the pubertal rise in release of LH pulses are genotype, gender, season of year when pubertal age is attained, growth or nutritional intake, social cues or treatment with exogenous progestins. Sexual maturation continues after puberty with an enhanced probability of pregnancy occurring from actions of ovarian steroids at the uterus.