QS Study

The thyroid is an endocrine gland, situated in the lower part the front and sides of the neck. Grossly, the gland appears brownish-red and the left and right lobes are connected by an isthmus.

Histological structure

The thyroid glands are made up of –

(i) Two types of secretory cells:

  1. Follicular cells.
  2. Parafollicular cells.

(ii) Interfollicular stroma.

(iii) Two connective tissue capsules.

The thyroid gland is a bi-lobular endocrine gland that is found in the neck, anterior and inferior to the larynx. Grossly, the gland appears brownish-red and the left and right lobes are connected by an isthmus. The thyroid is divided into lobules composed of about 20 to 40 evenly dispersed follicles. The follicles range from uniform to variable in size and are lined by cuboidal to low columnar epithelium which is filled with thyroglobulin. They secrete thyroxine (T4) and tri-iodothyronine (T3) in response to Tat secreted from the anterior pituitary. It regulates the basal metabolic rate, and it is regulated by the pituitary hormone TSH. It also secretes calcitonin – which regulates blood calcium levels. These hormones stimulate the basal metabolic rate and somatic and psychic growth of the individual.

Parafollicular cells lie in the interfollicular stroma in between follicles. They secret thyrocalcitonin which promotes deposition of calcium in skeletal and other tissues and causes hypocalcemia. Interfollicular stroma also contains numerous blood vessels.

The true capsule is the peripheral condensation of the interfollicular stroma and the false capsule derives from the cervical fascia.

Histological Structure of Thyroid Gland 1

Fig: Histological Structure of Thyroid Gland

Developments

The thyroid develops from a median endodermal thyroid diverticulum which grows down in front of the neck from the floor of the primitive pharynx, just caudal to the tuberculin imper. The lower end of the diverticulum enlarges to form the gland. The rest of the diverticulum is known as the thyroglossal duct.

Most of the duct disappears. The position of the upper end is marked by the foramen caecum of the tongue and the lower end often pursuits as the pyramidal lobe. At 1st, thyroid gland consists of a solid mass of cells. Later, due to the invasion by surrounding vascular mesenchymal tissue 20 to 40 follicles are formed. The neural crest cells but incorporated into the thyroid gland to form parafollicular cells.

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