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postembryonic development, two proteins have been identified as involved
in TH transport in the blood, the thyroxine-binding globin (TBG), and the
transthyretin (TTR). TBG level is high during pouch life and disappears af-
ter pouch leaving, whereas TTR level is low during pouch life and rise in
adulthood (
Richardson et al., 2002, 2005
). This switch of TH carrying pro-
tein is concomitant with the timing of the T3 peak. These data, gathered in a
marsupial, show that a switch in TH transport protein expression is concom-
itant with young's exit of the pouch. This provides indirect evidence that
THs are involved in the pouch exit event and that their levels must be finely
regulated during development.
5.6. A partial conclusion on mammals
There are large disparities among mammals, between metatherians and
eutherians but also within eutherians depending on their altricial or precocial
development strategies. Nevertheless, in the late development/early post-
embryonic life of mammals, it seems clear that a peak of TH is mandatory
for the normal development of organs such as intestine or brain. In euthe-
rians, this peak is concomitant with brain and intestine development
(
Fraichard et al., 1997; Plateroti et al., 2001
). Moreover, this peak is appar-
ently delayed after birth in the species, such as mouse, requiring a long pos-
tbirth development until autonomy. In contrast, this peak can occur very
early in species with long gestation time exhibiting precocial strategies: this
is the case of the guinea pig. In human, the peak is concomitant to birth. In
metatherians, it is apparently concomitant with the establishment of endo-
thermy, fur, intestine development, and the first leaving of the pouch, that is
to say, the beginning of autonomy (
Table 14.1
).
Table 14.1 Summary of the features observed at birth in the mammals described in this
review.
“
þ
”
indicates the presence of the feature when
“
”
indicate its absence
Feature
Species
Eyes open Self-feeding Self-heating Fur/feather TH peak
Mice
At weaning
Guinea
pig
þ
þ
þ
þ
Evidence before
birth
Human
þ
þ
þ
/
Right after birth
Possum
Right before the
first pouch exit
