Hatching the cleidoic egg: The role of thyroid hormones

Abstract

A major life stage transition in birds and other oviparous sauropsids is the hatching of the cleidoic egg. Not unlike amphibian metamorphosis, hatching in these species can be regarded as a transition from a relatively well-protected "aqueous" environment to a more hazardous and terrestrial life outside the egg, a transition in which thyroid hormones (THs) (often in concert with glucocorticoids) play an important role. In precocial birds such as the chicken, the perihatch period is characterized by peak values of THs. THs are implicated in the control of muscle development, lung maturation and the switch from chorioallantoic to pulmonary respiration, yolk sac retraction, gut development and induction of hepatic genes to accommodate the change in dietary energy source, initiation of thermoregulation, and the final stages of brain maturation as well as early post-hatch imprinting behavior. There is evidence that, at least for some of these processes, THs may have similar roles in non-avian sauropsids. In altricial birds such as passerines on the other hand, THs do not rise significantly until well after hatching and peak values coincide with the development of endothermy. It is not known how hatching-associated processes are regulated by hormones in these animals or how this developmental mode evolved from TH-dependent precocial hatching. Apart from being key metabolic hormones, thyroid hormones (THs) play an important role in development by controlling the growth and differentiation of almost every organ in the vertebrate body. The first clear evidence for the need of THs in vertebrate development came from frogs, where THs, in synergy with corticosteroids, control the transition from an aquatic larva to a terrestrial juvenile during metamorphosis. Superficially, hatching in birds and other oviparous sauropsids resembles anuran metamorphosis in that it marks a transition from an "aqueous" environment, to some extent protected against desiccation and predation by the eggshell, to a more exposed terrestrial life. The similarity is more striking when the endocrinology of hatching is considered, at least in precocial birds. In all precocial bird species studied to date, hatching is accompanied by and dependent on a rise in THs (and corticosteroids). We will first discuss these hormonal changes in precocial species and compare them to what is known in altricial birds and other oviparous sauropsids. Secondly, we will briefly review the role of THs in hatching and hatching-associated processes such as lung and gut maturation, the development of endothermy, and imprinting behavior. © 2013 De Groef, Grommen and Darras.