Histology 2016-2017Department of Anatomy &Histology: Dr.Rajaa Ali***********************************************************

Endocrine system part II

Hypothalamus:

The hypothalamus regulates pituitary gland activity.

The hypothalamus is located in the middle of the base of the brain, and it encapsulates

the ventral portion of the third ventricle.

It coordinates most endocrine functions of the body and serves as one of the major

controlling centers of the autonomic nervous system.

Some of the functions that it regulates include blood pressure, body temperature, fluid

and electrolyte balance, body weight, and appetite.

The hypothalamus produces numerous neurosecretory products. In addition to oxytocin

and ADH, hypothalamic neurons secrete polypeptides that promote and inhibit the

secretion and release of hormones from the anterior lobe of the pituitary gland.

Pineal Gland:

The pineal gland (pineal body, epiphysis cerebri) is an endocrine or neuroendocrine

gland that regulates daily body rhythm.

It develops from neuroectoderm of the posterior portion of the roof of the diencephalon

and remains attached to the brain by a short stalk, it is located at the posterior wall of

the third ventricle near the center of the brain.

The pineal gland is a flattened, pine cone–shaped structure, hence its name.

The pineal gland contains two types of parenchymal cells:

Pinealocytes and interstitial (glial) cells.

Pinealocytes are the chief cells of the pineal gland. They are arranged in clumps or

cords within lobules formed by connective tissue septa that extend into the gland from

the pia mater that covers its surface.

The interstitial (glial) cells constitute about 5% of the cells in the gland. They have

staining and ultrastructural features that closely resemble those of astrocytes. In

addition to the two cell types, the human pineal gland is characterized by the presence

of calcified concretions, called corpora arenacea or brain sand (Fig. 1).

The human pineal gland relates light intensity and duration to endocrine

activity.The pineal gland is a photosensitive organ and an important timekeeper and

regulator of the day/night cycle (circadian rhythm).

Fig.(1) • Photomicrograph of human pineal gland. This higher-magnification photo micrograph shows the characteristic concretions called brain sand or corpora arenacea.

Thyroid Gland:

The thyroid gland, located in the cervical region anterior to the larynx, consists of

two lobes united by an isthmus (fig.3).

It originates in early embryonic life from the foregut endoderm near the base of the

future tongue.

Its function is to synthesize the thyroid hormones: thynoxine (tetra-iodothyronin. or

T4)and tri-iodothyronine (T3), which are important for growth, for cell

differentiation, and for the control of the basal metabolic rate and oxygen

consumption in cells throughout the body.

Thyroid hormones affect protein, lipid, and carbohydrate metabolism .

The thyroid is composed of epithelial structures called thyroid follicles (fig.2)..

Each follicle consists of a simple epithelium and a central lumen filled with a

gelatinous substance called colloid .

Thyroid is the only endocrine gland in which a large quantity of secretory product is stored . Moreover the accumulations is outside the cells, in the colloid of the follicles.

The thyroid gland is covered by a fibrous capsule from which septa extend into the

parenchyma , dividing it into lobules and carrying blood vessels, nerves, and

lymphatics.

Follicles are densely packed together, separated from one another only by sparse

reticular connective tissue (fig.2).

Follicular cells range in shape from squamous to low columnar and the follicles are quite variable in diameter (fig.2).

The size and cellular features of thyroid follicles vary with their functional activity. Active glands have more follicles of low columnar epithelium ; glands with mostly squamous follicular cells are considered hypoactive.

Follicular epithelium contains two types of cells: follicular and parafollicular cells.

The parenchyma of the thyroid gland is composed of epithelium containing two types of cells:

Follicular cells (principal cells) are responsible for production of the thyroid hormones T4 and T3, exhibit a slightly basophilic basal cytoplasm.

Parafollicular cells (C cells) are located in the periphery of the follicular epithelium and lie within the follicle basal lamina. These cells have no exposure to the follicle lumen. They secrete calcitonin, pale staining and occur as solitary cells or small clusters of cells.

Figure (2). Thyroid gland .( a): 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 . ( b):T he lumen( L) of each follicle is filled with colloid. (c,d,e) parafollicular cells (C) which secrete calcitonin , may be part of the follicular epithelium or present in singly or in groups outside of follicles follicular cells (F).

Parathyroid Glands: The parathyroid glands are four small oval masses.

They are located on the back of the thyroid gland (fig.3), one at each end of the

upper and lower poles, usually embedded in the larger glandʾs capsule.

The parathyroid glands are derived from the pharyngeal pouches: the superior

glands from the fourth pouch and the inferior glands from the third pouch.

Each parathyroid gland is contained within a capsule which sends septa into the

gland, where they merge with reticular fibers that support elongated cordlike

clusters of secretory cells.

With increasing age many secretory cells are replaced with adipocytes, Two

types of cells are present in parathyroid glands: chief (or principal) cells and

oxyphil cells (fig.4).

The chief cells are small polygonal cells with round nuclei and pale-

staining ,slightly acidophilic cytoplasm filled with irregularly shaped. These are

secretory granules containing the polypeptide parathyroid hormone (PTH).

Much smaller often clustered populations of oxyphil cells are sometime

present more commonly in older individuals (fig.4).

These are much larger than the principal cells and are characterized by

acidophilic cytoplasm filled with abnormally shaped mitochondria Some

oxyphilc ells show low levels of PTH synthesis ,suggesting these cells are

transitional derivatives from chief cells.

Parathyroid hormone targets osteoblasts.

Figure (3). Parathyroid glands

Figure (4). Parathyroid principal cells. (a): A small lobe of parathyroid gland surrounded by connective tissue septa (s), shows mainly densely packed cords of small principal cells (P), also called chief cells, oxyphil cells (O), ( b): The micrograph shows that principal cells are slightly eosinophilic present in cords separated by capillaries( C)