Molecular biology of growth
The growth of vertebrate animals from neonatal to adult size depends upon the proper functioning of the hypothalamic/pituitary/end organ system known as the growth hormone axis. In Laron-type dwarves, in whom growth hormone (GH) insensitivity results from the absence of GHR signalling, increased growth velocity can be stimulated with the injection of insulin-like growth factor-I, although hypoglycaemia is a common side effect. The complexity of the hypothalamic/pituitary growth axis can be further appreciated by consideration of the variety of growth dynamics among species. At one extreme are animals such as rats and pigs, which continue somatic growth indefinitely into adulthood. In addition, an ever increasing number of animal model systems, both naturally occurring and transgenic, have facilitated the in vivo study of animal growth. The most severe form of growth deficiency, called isolated GH deficiency, type IA, is caused by mutation or deletion of GH-N.