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