The pituitary endocrine mechanisms involved in mammalian maturation: maternal and photoperiodic influences.

This review is designed to describe some pituitary mechanisms indispensable for growth and sexual maturation during the neuroendocrine adaptation of the female mammal to the extrauterine environment. We define the phases of postnatal development on the basis of secretory patterns of hormones. The infantile period is characterized by accelerated growth, and elevated secretion of growth hormone (GH) and follicle-stimulating hormone (FSH) in contrast to the diminished secretion of luteinizing hormone (LH). The transition from infancy to prepuberty generates the attenuation of somatic growth in non-primate mammals and the beginning of sexual maturation. The mechanisms of this transition involve the effects of weaning, which is associated with a rupture of the young-mother bond and, if abrupt, results in the stress of maternal deprivation. Maternal deprivation involves the stress-like endocrine response of pituitary and influences the mechanisms underlying the secretion of GH and FSH. An acute decrease in the secretion of GH and FSH at the initiation of prepuberty and an increase in the storage and pulsatile release of LH according to progressive prepubertal stages are pituitary endocrine features of post-infantile maturation. There are two factors important for timing of puberty, the maturity of gonadotroph population manifested by the adequate size of LH-containing cell subpopulation and the circumstances of an external environment optimal for reproductive functions in adults. Thus, the intrapituitary endocrine mechanisms of maturation have a psychosomatic nature during weaning and histomorphological nature during the postinfantile transition to puberty. In seasonal breeders, the endocrine timing of puberty has a circumannual seasonal nature.