Download MICROSCOPIC STRUCTURE OF THYROID GLAND

MICROSCOPIC STRUCTURE OF
THYROID GLAND
THYROID GLAND
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It is located in the cervical region anterior to the larynx
consists of 2 lobes united by an isthmus.
It originates in early embryonic life from the endoderm
of the initial portion of the primitive gut.
 It is an extremely vascularized organ with an extensive
blood and lymphatic network
EMBRYOGENESIS
 Derived from endoderm of cephalic portion of the
alimentary canal.
 Develops as an epithelial proliferation in the floor of
tongue and descends to its level infront of tracheal
rings in the 7th week.
EMBRYOGENESIS
 Ultimobranchial body (5th pharyngeal pouch) later
incorporated with the thyroid and give rises to
parafollicular or C-cells, secretes calcitonin.
THYROID GLAND
(Under L/M)
STROMA
 Thyroid gland is covered by 2 capsules:
 1. False --- Pre-tracheal layer
 2. True -- Thin loose CT (connective tissue).
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Stroma is highly vascular and consists of reticular CT, containing
lymphocytes and macrophages.
PARENCHYMA
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Consists of follicles (0.02 to o.9 mm in diameter).
SECTION OF A THYROID
Showing the follicles formed
by a simple epithelium,
containing colloid.
THYROID LOBULES
THYROID FOLLICLES
 The thyroid is composed of spherical follicles that
selectively absorb iodine (as iodide ions, I-) from the
blood for production of thyroid hormones, but also for
storage of iodine in thyroglobulin,
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Iodine is necessary for other important iodineconcentrating organs as breast, stomach, salivary
glands, thymus etc.
THYROID EPITHELIAL CELLS OR FOLLICULAR
CELLS
 The follicles are surrounded by a single layer of thyroid
epithelial cells, which secrete T3 and T4.
 When the gland is not secreting T3/T4 (inactive), the
epithelial cells range from low columnar to cuboidal
cells.
 When active, the epithelial cells become tall columnar
cells.
FOLLICULAR CELLS
Follicular cells or Principal cells
 Generally cuboidal in shape and rest on
extremely thin basal lamina.
 Cytoplasm basophilic, contains a large
spherical nucleus.
FOLLICULAR CELLS
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Cytoplasm contains
Extensive RER
Large number of free ribosomes
Abundant mitochondria.
In apical region well-developed Golgi apparatus &
membrane bound vesicles called “colloid resorption
droplets.”
 Apical surface has microvilli.
 They secrete thyroid hormones.
THYROID GLAND
COLLOID
 Amount varies according
to the functional activity
of gland, being lesser in
an active gland.
 Fresh colloid is
homogenous and
uniformly distributed in
follicular cavity.
 During processing, its
coagulation & shrinkage,
creates space b/w colloid
& lining epithelium.
PARAFOLLICLULAR CELLS
 SCATTERED AMONG FOLLICULAR CELLS AND IN SPACES
BETWEEN THE SPHERICAL FOLLICLES ARE ANOTHER
TYPE OF THYROID CELL, PARAFOLLICULAR CELLS,
WHICH SECRETE CALCITONIN.
PARAFOLLICULAR CELLS (C-CELLS)
 They may be found in epithelium or scattered in CT
between the follicles.
 When present in follicles, they may be wedged b/w
principal cells & basal lamina, and do not extend to
the lumen.
 They are somewhat larger than follicular cells.
 Cytoplasm poorly stained with routine methods.
PARAFOLLICULAR CELLS ( C- CELLS) E/M
 Most striking feature Presence of numerous
dense membrane-bound secretory granules
(100 to 180 nm in diameter).
CALCITONIN SECRETION
 They secrete calcitonin, a hormone which
lowers the blood calcium level by inhibiting
bone resorption.
FUNCTIONS of THYROID GLAND
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To synthesize the hormones (T4) and (T3), which are
important for :
- growth,
- cell differentiatiation,
- control of oxygen consumption,
- basal metabolic rate in the body.
Thyroid hormones affect the metabolism of proteins,
lipids and carbohydrates.
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