The timing of puberty (oocyte quality and management)

This review aims at giving an overview on the physiological events leading to puberty onset in mammals and more specifically in cattle. Puberty is an important developmental milestone in mammals involving numerous changes in various physiological regulations and behaviors. It is a physiological unique event integrating several important central regulations at the crossroad of adaptation to environment: reproductive axis, feeding behavior and nutritional controls, growth, seasonal rhythm and stress. Puberty onset is also an important economic parameter in replacement heifer program and in genomic selection (genomic bulls). The quest for advanced puberty onset should be carefully balanced by its impact on physiological parameters of the animal and its offspring. Thus one has to carefully consider each step leading to puberty onset and set up a strategy that will lead to early puberty without being detrimental in the long term. In this review, major contributions in the understanding of puberty process obtained in rodents, primates and farm animals such as sheep and cattle are discussed. In the first part we will detail the endocrine events leading to puberty onset with a special focus on the regulation of GnRH secretion. In the second part we will describe the neural mechanisms involved in silencing and reactivating the GnRH neuronal network. These central mechanisms are at the crossroad of the integration of environmental factors such as the nutritional status, the stress and the photoperiod that will be discussed in the third part. In the fourth part, we will discuss the genetic determinants of puberty onset and more particularly in humans, where several pathologies are associated with puberty delay or advance and in cattle where several groups have now identified genomic regions or gene networks associated with puberty traits. Last but not least, in the last part we will focus on the embryologist point of view, how to get good oocytes for in vitro fertilization and embryo development from younger animals.