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Fig. 14.3
) on the other hand. One important difference between these spe-
cies concerns thermoregulation, as birds are homeotherm species, whereas
the other sauropsids are heterotherms. This is important to mention given
the well-known role of TH in thermoregulation. In the adult tortoise
(
Gopherus agassizii
), lizard (
Sceloporus occidentalis
), and alligator (
Alligator
mississippiensis
), TH levels in adult vary seasonally (
Boggs, Hamlin,
Lowers, & Guillette, 2011; John-Alder, 1984; Kohel, MacKenzie, Rostal,
Grumbles, & Lance, 2001
). These variations are problematic to evaluate
the changes in TH level in these species during development and especially
during the early postembryonic period. Indeed, the hierarchy of the param-
eters controlling TH level, among which metabolism, food intake, and tem-
perature, makes it much more difficult to interpret the levels observed in
these heterothermic sauropsids (
Brasfield, Talent, & Janz, 2008; Kohel
et al., 2001
).
In addition, in nonavian sauropsids, some data on TH level or TH trans-
porters exist. Investigation of TTR, a TH carrier protein, in the saltwater
crocodile (
Crocodylus porosus
) has shown that the protein is expressed only
before hatching but not after (
Richardson et al., 2005
) indicating a specific-
ity of TH transport in the embryo. It has been shown in this species that free
T3 and T4 rise at hatching compared to prehatching level (
Shepherdley,
Daniels, et al., 2002
) and are higher than juvenile TH level (unpublished
observation cited in
Shepherdley, Richardson, Evans, K¨hn, & Darras,
2002
). Consistent with the T3 increase, the T4 outer-ring deiodinase activ-
ity rises in the gut and brain of the crocodile prior hatching. Interestingly, at
the same period, the inner-ring deiodinase activity that controls TH degra-
dation is stopped. Taken together, the embryonic specific expression of
TTR and the rise of TH level at hatching indicate a fine regulation of
TH levels during development, particularly around hatching, suggesting
once again the existence of a peak during this critical period.
6.3. Effect of TH in bird
Japanese quail treated with T4 lay eggs with an enriched T4 yolk content
compared with nontreated hens. Nevertheless, the embryonic level of T4
is not directly influenced by the T4 level of the yolk indicating a fine-tuned
regulation of T4 level within the embryo (
Wilson & McNabb, 1997
). No
morphological or developmental difference is observed between chick of
adult Japanese quails treated or not with T4. But T4 treatment of the hens de-
laysthehatchingofthechickforseveralhours(
Wilson & McNabb, 1997
).
