Date post: | 01-Jun-2015 |
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THYROID GLAND
THYROID GLAND
• Located in the anterior neck region adjacent to the larynx and trachea
• Consists of two lateral lobes united by an isthmus
• Develops from the endodermal lining of the floor of the primitive pharynx
• Composed of connective tissue septa and follicles
MAIN FUNCTION
• Synthesize the thyroid hormonesThyroxine (Tetra – iodothyronine or T4)Tri – iodothyronine (T3)Calcitonin
LOCATION
Located in the anterior neck region adjacent to the larynx and trachea
STROMA
• Thin loose areolar connective tissue capsuleForming connective tissue septa
• Coarse and fine collagenous fibers• Presence of fibroblasts and blood vessels
STROMA
A low-power micrograph of thyroid gland shows the thin capsule (C), from which septa (S) with the larger blood vessels, lymphatics, and nerves enter the gland
FOLLICULAR EPITHELIUM
• Surrounds each follicle• Composed of reticular fibers and network
of capillaries• Can be simple squamous, cuboidal, or low
columnar epithelium
THYROID FOLLICLE
• Structural unit• Roughly spherical cyst-like compartment• Wall formed by follicular epithelium• Central cavity contains a gelatinous
substance called colloidlarge glycoprotein thyroglobulin, precursor
for the active thyroid hormones
THYROID FOLLICLE
• Surrounded by 2 types of cellsFollicular cellsParafollicular cells
THYROID FOLLICLE
The lumen (L) of each follicle is surrounded by a simple epithelium in which the cell height ranges from squamous to low columnar. Also present are large pale-staining parafollicular or C cells (C).
FOLLICULAR CELLS
• Principal cells• Most numerous cell type• Cuboidal in shape with spherical nucleus• Periods of increased activity – cells are
more columnar and colloid is less abundant
FOLLICULAR CELLS
• Periods of decreased activity – cells are squamous and colloid content increases
• Exhibit organelles active protein synthesis and secretion,
phagocytosis and digestion
• Synthesize, release, and store their product outside their cytoplasm
• Secretion and synthesis of T3 and T4
FOLLICULAR CELLS
RED – Follicle with colloid
BLUE – Follicular cells
PINK - Connective tissue septa
PARAFOLLICULAR CELLS
• C (clear) cells• Larger than follicular cells• Contain secretory granules• Can occur either alone or in clumps• Synthesize and secrete calcitonin
PARAFOLLICULAR CELLS
RED – Follicle with colloid
BLUE – Parafollicular cells
PINK – Retracted or distorted colloid
FOLLICULAR AND C CELLS
C cells may be part of the follicular epithelium or present singly or in groups outside of follicles. Follicular cells (F) can usually be distinguished from C cells (C) by the smaller size and darker staining properties. Unlike follicular cells, C cells seldom vary in their size or pale staining characteristics.
FOLLICULAR AND C CELLS
FOLLICULAR AND C CELLS
ULTRASTRUCTURE (Follicular and C cells)
ULTRASTRUCTURE (Follicular and C cells)
HORMONES
• Thyroxine (Tetra – iodothyronine or T4)• Tri – iodothyronine (T3)• Calcitonin
THYROXINE (T4) and TRI - IODOTHYRONINE (T3)
• Growth• Cell differentiation• Control of the basal metabolic rate and
oxygen consumption in cells• Affect protein, lipid, and carbohydrate
metabolism
CALCITONIN
• Lower blood calcium levels through inhibition of bone resorption
• Secretion is triggered by elevation of blood calcium concentration
HORMONES
Hormone Composition Source Major Functions
T3 and T4 Iodinated tyrosine derivatives
Follicular cells(principal cells)
• Regulates tissue basal metabolism (↑ rate of carbohydrate use, protein & fat synthesis and degradation)• Regulates heat production• Influences body and tissue growth and development of the nervous system in the fetus and young child•↑ absorption of carbohydrates from the intestine
Calcitonin Polypeptide containing 32 amino acids
Parafollicular cells ( C cells)
•↓ blood Ca levels by inhibiting bone resorption and stimulating absorption of Ca by the bones