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.

Sex Differences in Brain Thyroid Hormone Levels during Early Post-Hatching Development in Zebra Finch (Taeniopygia guttata)

“…Several studies on altricial birds have suggested the absence of a distinct perihatch peak for the plasma thyroid hormone [12–16]. Typically, the concentration of circulating thyroid hormone increases gradually to reach levels typically recorded in adult in approximately 3–4 weeks post-hatching [16].…”

“…Typically, the concentration of circulating thyroid hormone increases gradually to reach levels typically recorded in adult in approximately 3–4 weeks post-hatching [16]. In contrast to precocial birds, altricial birds hatch with their eyes and ears closed and show less locomotor activity [1].…”

Hampered Lung Maturation in Methimazole-Induced Hypothyroidism in Fetal Chicken: Morphological and Molecular Correlates to Human Fetal Development

“…Interestingly, the perihatching period in chickens is characterized by a thyroid hormone peak approaching a ninefold increase when internal pipping is initiated [2]. At pipping, the chicken fetus switches from only chorioallantoic to a combined pulmonary-chorioallantoic respiration that lasts through hatching.…”

“…Thyroid hormones are essential for normal development and play a crucial role in developmental processes in vertebrates, including humans and birds [1,2]. During the middle and last trimester, corticosteroids and thyroid hormones cooperate to promote sufficient lung maturation and surfactant production for survival of the newborn [3].…”

Iodothyronine Deiodinases: Key Enzymes Behind the Action of Thyroid Hormone

“…This marked decrease in D3 activity is accompanied by a plasma T 3 surge, which suggests that the decrease in hepatic D3 activity leads to the elevation of plasma T 3 level ( Van der Geyten et al, 1997). The increase in plasma T 3 has been found indispensable to advance a number of processes for normal hatching of chicken embryos (De Groef et al, 2013).…”