Biology Reference
In-Depth Information
In Japanese quail, goitrogen treatment (methimazole) does not significantly de-
crease plasma T4 level nor the T4 and T3 content of yolk of the laid eggs
highlighting a high inertia of goitrogen response in the hen probably because
of the storage capacity of the egg. However, this treatment increases the weight
of the thyroid gland and disrupts egg laying (
Wilson &McNabb, 1997
). Treat-
ment of young chicks started at 3 days posthatching with methimazole decrease
T4 thyroid content and increase thyroid weight, but T4 plasma content and
bodyweightareaffectedonlybyheavyandlongtreatment(
McNabb, Jang,
& Larsen, 2004
) as in adult. Similar results were found in bobwhite quail chick
(
Colinus virginianus
); blood T4 content and body weight are little or not affected
by goitrogen treatment (
McNabb, Larsen, & Pooler, 2004
)asinadult.Taken
together, these data suggest that the quail chick has a TH system functional
enough to bear a decrease in T4 synthesis, indicating that in the precocial
quails, TH gland seems to mature early.
As goitrogen treatment in the Japanese quail (
Wilson & McNabb, 1997
)
has shown that the hens are resistant to this form of TH disruption, it is dif-
ficult to infer the role of TH in the hatching quail. Goitrogen (ammonium
perchlorate) treatment for 4 weeks in Bobwhite quail of 4 days-old hatchling
has little effect or does not disturb the growth of the young (
McNabb, Jang,
& Larsen, 2004; McNabb, Larsen, & Pooler, 2004
). Two nonexclusive ex-
planations can be provided: (i) The quail hatchling has sufficient TH and
iodide stock in the yolk derived from the hen to handle goitrogen treatment.
(ii) TH is not required for a few days after hatching in the hatchling. Taking
into account the TH peak at hatching in the closely related Japanese quail
(
Wilson & McNabb, 1997
), maybe after hatching,
the highly TH-
dependent development period is over in quail.
6.4. A
in sauropsids?
Most of the data on TH level around hatching come from studies performed
in birds. Taken together, these data suggest a different TH regulation be-
tween precocial and altricial birds with a near-hatch peak for the former
and a smoother late one during posthatch development for the later. This
fits with the notion of a TH peak related to autonomy of the hatchling,
an idea that was proposed by McNabb several years ago (
McNabb et al.,
1984
; see also a more recent review in
McNabb, 2006
). Indeed, precocial
birds tend to have thermoregulatory independence and feed themselves,
whereas altricial birds rely on their parent for heating and feeding. More-
over, precocial hatchling such as quail appears to be resistant to thyroid
metamorphosis
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