POSTNATAL DEVELOPMENT OF THYROID GLAND IN KUTTANAD DUCK (Anas platyrhynchos domesticus)
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Date
2011
Authors
FIRDOUS AHMAD DAR
Journal Title
Journal ISSN
Volume Title
Publisher
COLLEGE OF VETERINARY AND ANIMAL SCIENCES-MANNUTHY,THRISSUR
Abstract
Postnatal development of thyroid gland in the Kuttanad duck was studied
using 104 female birds from day-old to 24 weeks of age. The material was collected
from eight birds in each group at fortnightly intervals. The paired thyroid glands in
the day-old Kuttanad duckling were placed at the base of the neck between the
common carotid artery and the jugular vein. The right thyroid was cranially situated
than the left one. Out of the 104 ducks studied, the left thyroid glands were absent in
two birds. The thyroid glands were round to oval, flat bodies and coloured yellow to
reddish brown. Maximum contribution of thyroid gland to the body weight was
noticed in 24 weeks old birds. Upto four weeks of age, the weight of thyroid
increased gradually but a spurt in growth was noticed between four and six weeks
and from there onwards, it increased slowly upto 20 weeks of age. From 22 weeks
although the body weight showed a slight decrease, thyroid weight remained the
same until 24 weeks. The result showed that there were three phases of activity for
the thyroid gland. First phase was at the day of hatch itself. The second phase
coincided with the fast growth rate of the birds between four to six weeks. The third
phase coincided with the beginning of egg laying. Length, breadth and thickness of
the thyroid increased about 3.5, 3.6 and 4.5 times, respectively from day-old birds to
24th week of age. Size of the thyroid gland showed more correlation with the age
than the weight of the gland. Weight of the thyroid was more correlated with the
body weight than with the age.
Histologically, the thyroid glands were enclosed by a thin capsule consisting
of well developed collagen, reticular and a very few elastic fibres. The capsule was
made of three layers which were evident from the day-old bird itself. The external
layer was a mesothelial layer, the middle layer was rich in fat cells, blood vessels and
nerves and the inner layer was closely adherent to the gland. In adult birds, the
subcapsular areas of thyroid gland disclosed a non-follicular type of parenchymal
90
cell. Substance of the gland consisted of roughly spherical thyroid follicles. In dayold birds follicles were small and dispersed towards the periphery of the gland with a
few larger follicles seen towards the centre. A distinct basement membrane was
evident. As age advanced, small and medium sized follicles were found throughout
the gland while larger follicles were restricted more to the periphery. Extra large
follicles could be identified from eighth week onwards. The blood vessels were
present around the follicle in a basket-like manner.
Each follicle was lined by a single layer of cells. Based on the type of
epithelium and nature of colloid, the follicles were categorized as active and inactive
follicles. The active follicles were lined by simple cuboidal epithelium with a
basophilic colloid, while the inactive follicles were lined by simple squamous
epithelium with acidophilic colloid. Most of the large follicles were inactive from
fourth week onwards. Mean epithelial height of the active follicles increased upto
sixth week. From eight weeks onwards there was a gradual decrease in the height
upto 24th week. Colloid cells of Langendorff were seen from 4th week onwards.
From the TAI it was inferred that the thyroid was moderately active from day-old to
four weeks old birds. From four weeks onwards the glands became very strongly
active upto eight weeks, which coincided with the rapid growth phase of the bird.
From eight weeks onwards the glands were categorized as strongly active upto 20
week but the activity was comparatively less than the previous group. This can be
correlated with the fact that thyroid hormones, grouped under morphogenetic
hormones having direct effect on the growth of reproductive system. By 20 weeks,
development of the oviduct is complete and all the birds started laying by this age.
A homogenous translucent colloidal mass filled the thyroid follicles. Vacuoles of
variable sizes and numbers were seen in the periphery and occasionally in the central
part of the colloid. In the follicles lined with simple cuboidal or columnar epithelium,
the colloid was predominantly basophilic, while in the follicles lined with simple
squamous epithelium it was acidophilic. The quantity of colloid varied according to
the activity of the gland. In the inactive follicles, it was more and thick due to
accumulation of large amount of colloid, whereas in active follicles it was lesser and
thinner. The colloid was amphoteric also.
Clusters of polyhedral cells, the parafollicular cells were present in the
interfollicular zone of thyroid and were seen from day-old birds itself. Groups of
blood cells, including RBCs and lymphocytes could be seen in the interfollicular area
at different ages. No thymic tissue could be detected within the thyroid gland of
Kuttanad ducks during postnatal period. Histological parameters of thyroid gland
such as thickness of capsule and number of total, large, small, active and inactive
follicles showed greater correlation with age than with the body weight.
Histochemically, the colloid showed a strong PAS positive reaction. Glycogen was
present in the follicular epithelium. Lipid accumulation was observed in the capsule
and interfollicular area. Acid and alkaline phosphatases’ activity was shown by the
lining epithelium of the thyroid follicles. Alkaline phosphatase activity was
maximum in six weeks old birds.
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