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neurons in the ileum, the intestine during weaning is physiologically close to
the adult intestine. This indicates that in the guinea pig, the main steps of the
intestine development occur before weaning ( Abalo, Vera, Rivera, Moro-
Rodr´guez, & Mart´n-Fontelles, 2009 ). These results may suggest that the
main maturation of the intestine takes place before birth in this species, con-
sistent with the fact that the peak of TH also occurs before birth. A study on
brain development in guinea pig has shown high TR
mRNA ex-
pression in the guinea pig brain, particularly in the hippocampus ( Chan et al.,
2005 ) at 52 days of gestation. At the same time, deiodinases D2 and D3 are
found throughout the brain ( Chan et al., 2005 ). Interestingly, detailed obser-
vations of brain development in this species indicate that the most important
part of brain development occurs from 50 days postgestation until birth in
guinea pig ( Matthews, 1998 ). Taken together, these data suggest that brain
maturation is already advanced at birth in guinea pig, on the contrary tomouse
and rat. As for intestine maturation, this provides indirect evidence indicating
that THs are involved in this prebirth maturation.
a
and TR
b
5.4. The young mammal as a larva?
As discussed above, mouse, rat, and, to a lesser extent, humans are the only
mammals for which we have sufficient data to deduce that TH is controlling
a postembryonic remodeling process that can be compared to metamorphosis,
as described in the canonical amphibian model. But in fact, many mammalian
species exhibit pronounced differences in their postembryonic life history
strategies. Strong variations in the length of the gestation period among eu-
therian mammals can be observed. Gestation length may be linked to behav-
ioral factors as long gestations give rise to precocial neonates that are
immediately able to be active (e.g., horse), whereas species displaying a long
period of maternal dependency (altricial, e.g., cat) have much shorter gesta-
tions. Such differences can occur even between quite closely related species,
for example, between mouse and guinea pig. The length of the gestation is
extremely different between the two species: 21 days for the mouse and more
than 63-68 days for the guinea pig. The aspect of the neonate is also different:
the guinea pig is a precocial species with newborn harboring fur and teeth,
open eyes able to move, to fully regulate their temperature and eat solid food
just 2 days after birth ( Dawes & Mesty´n, 1963 ). In contrast, mouse pups are
altricial: they are totally maternally dependent, naked, and incapable of mov-
ing around. One cannot avoid concluding that mouse pups are “larvae,”
whereas guinea pigs neonates are already “juveniles.”
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