The beginning of puberty and mammary gland development in girls is becoming increasingly common at an early or even very early stage (before the age of 8). But despite this precocious onset of puberty, the age at which menstruation occurs for the first time remains relatively unchanged or only moderately so. In boys, it has been observed that the final stages of puberty can occur later. These modifications show that, overall, the pubertal process is getting longer. Why take an interest in the age at which sexual maturity occurs and in variations in its duration? We should do so because, among other reasons, these events tell us a lot about the impact of the environment and certain pollutants on our organisms. Should we be worried?

Faced with some different ways in which puberty is developing and bearing in mind that it is such an essential process for human beings, researchers from Liège have reread and rectified some interpretations regarding this phenomenon that can now be classed as obsolete. They have also revealed exciting new avenues of investigation in two recently-published studies(1).

What were their objectives? They wanted to find an explanation for changes in the pubertal process and to decipher the underlying mechanisms involved in these changes. But among other things, they also wanted to interpret “the incredible and enormous individual variations in the times at which puberty occurs, with differences of up to five years between different individuals being observed", explains Jean-Pierre Bourguignon, pediatric endocrinologist at the University of Liege and head of the associated unit at the Liege University Hospital Centre (CHU) and a member of Giga Neurosciences (University of Liege) and who participated in the two studies.

Sexuality is in the mind

The body of the child grows into that of an adult and this takes place over a (relatively long) period of his or her life. This is explained in the study that appeared in the journal Frontiers in Neuroendocrinology. Work carried out on rats made it possible to illustrate that the pubertal process in humans is “exceptional”: in the context of a two-year life-span, the pubertal timing of rats varies from 4 to 5 days or 0.55% of its life. In practice, this period is eleven times shorter than in humans and primates… One of the possible implications of this observation is that the great variability in the age of onset of puberty and its long latent period between birth and this event are characteristic of the end of evolution of species.

In practice, during this period the sexual organs and the other parts of the body also undergo changes including through a peak in growth because the growth hormone is produced in large quantities during this period. Fundamentally, this entire process is triggered by the brain, or more precisely, by the hypothalamus.  

Several factors determine the start of the pubertal process and also the great variability of the age of puberty and related phenomena in countries where life conditions are relatively equivalent. Among these factors genetics certainly play a role. But nutrition and environment, particularly in relation to the impact of pollutants and toxic substances as well as psychosocial stress, can influence or compete with this factor. For example, we now know that certain endocrine disruptors can directly stimulate the hypothalamic machinery earlier than the average anticipated age, independently of the genetic control and therefore trigger precocious puberty.

The diversity of the causal factors, their development and their impact with regard to puberty, clearly increase the complexity of research into the phenomenon. But researchers must also take another element of vital importance into account: time. Certain periods of life are more receptive to the factors that lead to puberty.

Already in the fœtus...

In practice, the mechanism of pubertal development is triggered by hormonal changes in the brain.  "We observe the redundancy of different systems that control the production of GnRH (gonadotropin-releasing hormone), a neuro-hormone that controls puberty and reproduction. A little like in a funnel, this small peptide which is produced in the hypothalamus by the neurones, is the ultimate activator of the pituitary, with a message produced at a certain frequency. This frequency makes it possible to determine the response by the pituitary. The pituitary releases the gonadotropins, which stimulate the development of the sexual glands. The activation of the pituitary gonadotropic cells occurs in three successive waves: once at the fetus stage, a second time shortly after birth (the “mini-puberty”) and a third time at puberty."

At a certain age the endocrine glands (ovaries and testicles) become stimulated when they receive messages from the brain through the pituitary and they produce sex hormones which cause the body to undergo physical changes. The body changes, the bones get longer and muscles develop in the boys while adipose tissue develops in girls.

 (1) Jean-Pierre Bourguignon, Françoise Domine, Fabienne Glowacz, Marie-Christine Lebrethon, Anne-Simone Parent. "Changes in Pubertal Timing: Past Views, Recast Issues" (Springer International Publishing Switzerland, 2015)

Anne-Simone Parent, Delphine Franssen, Julie Fudvoye, Arlette Gérard, Jean-Pierre Bourguignon. "Developmental variations in environnemental influences including endocrine disruptors on pubertal timing and neuroendocrine control : Revision of human observations and mechanistic insight from rodents" (Frontiers in Neuroendocrinology, 2015).