Page 186 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

168 Veterinary Histology of Domestic Mammals and Birds

Calcitonin reduces plasma calcium levels by stimulat- and meshwork of reticular fibres, as well as a dense net-
VetBooks.ir ing osteoblasts, inhibiting the activity of osteoclasts and work of fenestrated capillaries (rete capillare) and lymph
increasing urinary calcium excretion through decreased vessels (rete lymphocapillare).
The follicles constitute the parenchyma of the thyroid
tubular reabsorption.
gland (Figures 9.7 to 9.10). The epithelial cells of the single-
Structure of the thyroid gland celled follicular wall exhibit polarity. The lumen of the follicle
The thyroid gland is unique among endocrine glands in is filled with colloid, a gelatinous material produced by the
that it stores inactive hormones within vesicular struc- follicle epithelium. Colloid may be viscous or thin, and is aci-
tures termed follicles. It therefore serves as a glandular dophilic and PAS positive. Depending upon the activity of the
hormone reservoir. gland, the colloid becomes thickened by removal of water.
The thyroid gland is enclosed within a thin connective Thyroxin and tri-iodothyronine are stored within the
tissue capsule. Delicate septa extend from the capsule into large glycoprotein molecule thyroglobulin. When release
the interior of the organ, dividing the gland into lobes and of the active hormone is required, the thyroglobulin
lobules. Division of the gland is particularly prominent in molecule is endocytosed and hydrolysed by lysosomes.
ruminants and pigs. The thyroid follicles represent a struc- According to the functional status of the gland, the follicles
tural and functional unit, bounded by a simple cuboidal vary in shape (round to oval), size (diameter 50–500 μm)
epithelium. These are surrounded by a basal membrane and height of the epithelium (Figures 9.7 to 9.9):

9.7 Thyroid gland (horse). During the
secretory phase, the thyroid gland
synthesises thyroglobulin which is
stored within follicles. The follicle
wall is composed of simple epithelium
that varies in height according to the
functional status of the gland (cuboidal
in this image). Loose connective tissue
surrounding the follicles contains vessels
and autonomic nerve fibres. Goldner’s
Masson trichrome stain (x480).

9.8 Thyroid gland (horse). In the storage
phase, the follicles are enlarged with
a flattened cuboidal epithelium. The
follicles are round to oval in shape
and therefore appear variably sized in
histological section. Haematoxylin and
eosin stain (x240).

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