What triggers puberty?

Puberty is defined as the transient phase between childhood and adulthood. During this stage the second growth spurt occurs, genital organs gradually mature, reproductive function is attained, and major physiological and psychological changes happen. Primates other than homo sapiens also experience the similar developmental phase. However we know little about the control and onset of puberty. For this reason, Science listed “What triggers puberty?” as one of the 125 questions (25 “Big Questions” along with 100 smaller ones) posed on its 125th Anniversary Issue. Existing research findings show that the timing of puberty is a complex phenomenon involving evolution, genetics, development, and many other fields. Phylogenesis and ontogenesis are thought to be the two major perspectives for understanding the phenomenon. From the phylogenetic point of view, puberty is the survival strategy chosen by species. The emergence of puberty indicates that individuals will spend a relatively long time on developing and preparing for reproduction. Such a developmental pattern promotes the physical, behavioral, and cognitive development of individuals. This is a long-term result of adaptation to the environment, which promises a higher survival rate of the offspring. Meanwhile the ontogenetic perspective focuses on neuroendocrine mechanisms of the puberty onset. Researches show that puberty starts from the activation of the hypothalamic-pituitary-gonadal (HPG) axis. Kisspeptin, which is secreted by hypothalamic arcuate nucleus neurons, is the first signal of the whole chain reaction. Kisspeptin initiates the pulsatile release of gonadotropin releasing hormone (GnRH) so that adenohypophysis synthesizes and releases follicle stimulating hormone (FSH) and luteinizing hormone (LH). Both FSH and LH can stimulate gonads to release sex hormones, which leads to the development of secondary sex characteristics and the maturation of sexual organs. Additionally, some neuroendocrine factors involving in reproductive regulation and metabolism are also shown to influence the onset of puberty by regulating the process of releasing GnRH in the upstream through the dual effects of excitation and inhibition. In brief, the pulsatile secreted GnRH in the HPG system together with other neurohormones and neurotransmitters trigger the onset of puberty, which regulate the reproductive functions in species. The timing when puberty occurs is also influenced by environmental resources. In general, the onset of puberty occurs earlier and earlier with the development of human society. Individuals living in impoverished areas reach puberty later than their counterparts living in developed areas. The body composition of individuals can also have an impact on the starting time of puberty. The body fat rate suggest the body has stored enough energy to support reproduction. In consistent with the evolutionary model, the adjustment of maturation time is also a reaction towards environmental stressors. In addition, studies on precocious or delayed puberty and the underlying genetic factors can also help us understand the reasons for the onset of puberty, especially the mechanisms related to neuroendocrine processes. In summary, the onset of puberty is a complex phenomenon regulated by genetic, environmental, endocrine and many other factors. Although accumulated data has explained the possible processes of puberty initiation on the physiological level, only some descriptions of the phenomenon are available when it comes to the psychological and environmental factors. Further research is needed to investigate how these various factors have independent and interactive effects on the onset of puberty.