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Histology of Endocrine system II
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Histology of Endocrine system II

Feb 24, 2016

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Histology of Endocrine system II. Adrenal gland. Paired organs that lie on top of the kidneys, embedded in adipose tissue.  General structure Covered by capsule of dense collagenous connective tissue. - PowerPoint PPT Presentation
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Page 1: Histology of Endocrine system II

Histology of Endocrine systemII

Page 2: Histology of Endocrine system II

Adrenal gland

• Paired organs that lie on top of the kidneys, embedded in adipose tissue.

• General structure – Covered by capsule of dense collagenous connective tissue.– Thin septa or trabeculae extend from the connective tissue

capsule into the interior of the gland.– Internally there are two major layers called the adrenal

cortex and adrenal medulla.– The supporting framework of cortex and the medulla are

collectively called the stroma. This framework contains many reticular fibers.

Page 3: Histology of Endocrine system II

Adrenal gland- Embryological development

• In a sense, the adrenal cortex and medulla may be considered as 2 morphologically distinct endocrine organs.

• Similar to what we saw in the hypophysis, the cortical and medullary layers of the adrenal glands have different embryonic origins. That is to say, they are derived from different basic tissue types in the embryo.

• As was the case for the hypophysis, we have a situation where neural and non-neural tissues become associated to form an organ.

• As an embryo develops, the cortex of the adrenal gland is derived from mesodermal cells in the region of the kidney.

• The cells of the adrenal medulla are derived from a specialized group of neural ectoderm cells that are called the neural crest. These cells are initially part of the larger group of ectodermal cells that will form the central nervous system (CNS).– The neural crest cells separate from the developing CNS and migrate through the body's

tissues giving rise to many different cell types, tissues, and organs.– Some of the neural crest cells migrate to the developing adrenal cortex, penetrate this

tissue and lodge themselves centrally within it to form the adrenal medulla.– So, the cells of the adrenal medulla are sometimes referred to as postganglionic

neurons that have lost their axons and dendrites and have become secretory cells.

Page 4: Histology of Endocrine system II

Cardiovascular circulation to the adrenal gland

• Blood is supplied to the adrenal glands by a number of arteries.• These vessels enter through the capsule tissue and then branch out into a

sub-capsular plexus of arterioles that give rise to capillaries that extend throughout the cortex.

• The capillaries supply blood to a network of sinusoids in the cortex.• Some of the arterial branches do not form capillaries in the cortex, but

rather run through it to the medulla.– These are the medullary arteries.– These arteries form a dense capillary network around the cells of the medulla.

• If we look at the fenestrated endothelium of the capillaries, we find pores that are occluded by a thin membrane. In the cortex, the basal lamina is not continuous. So again, as we might suspect, the circulatory components of this gland are set-up for the exchange of materials between the blood and surrounding cells or visa versa.

• Capillaries of medulla and cortex coalesce to form the adrenal veins that exit the adrenal glands.

Page 5: Histology of Endocrine system II

Adrenal cortex

• Composed of 3 layers– zona glomerulosa– zona fasciculata– zona reticularis

Page 6: Histology of Endocrine system II
Page 7: Histology of Endocrine system II

Zona glomerulosa - structure• Columnar to pyramidal cells that are arranged in folded cords

surrounded by capillaries and sinusoids.• Cells have spherical nucleus and basophilic granules in cytoplasm.• Adjacent to the endothelium of the capillaries and sinusoids there

is a space between the endothelium and surrounding cells.• The cell surface of the columnar or pyramidal secretory cells

adjacent to this space are thrown into folds in this area forming microvilli.

• Ultrastructure is typical for cells involved in lipid synthesis– Lots of smooth endoplasmic reticulum with a few short segments of

RER.– Well developed Golgi body.– Mitochondria are spherical or oval and have tubular cristae.– Small lipid droplets may be present in cytoplasm associated with

smooth endoplasmic reticulum; however, these are not particularly evident in histological sections for light microscopy.

Page 8: Histology of Endocrine system II

 Zona glomerulosa - function

• The zona glomerulosa cells secrete mineralcorticoids, mainly aldosterone which is important in maintaining water balance.

• These are steroid hormones that are lipid molecules derived from cholesterol.

Page 9: Histology of Endocrine system II

Zona fasciculata - structure• Cells are polyhedral and are arranged in straight cords

(columns) that are 1-2 cells thick with capillaries running between.

• Cells have central nucleus with basophilic cytoplasm. Microvilli are present within the sub-endothelial space (next to the capillaries).

• Many lipid droplets present in cytoplasm. These are extracted during most fixation and embedding procedures, so the cells often appear highly vacuolated in histological sections.

• Ultrastructure is typical for cells involved in lipid synthesis and secretion– However, these cells have more RER than those of the zona

glomerulosa– This is why the cytoplasm has an overall basophilic affinity.

Page 10: Histology of Endocrine system II

Zona fasciculata - function

• Zona fasciculata cells secrete glucocorticoids (important in lipid, protein and carbohydrate metabolism) that are another type of steroid.

Page 11: Histology of Endocrine system II

 Zona reticularis

• Cells are polyhedral and are arranged in irregular cords with capillaries and sinusoids between.

• Lipofuscin pigment granules in cells.• Cytoplasm acidophilic. Few lipid droplets.• Cells secrete glucocorticoids.

Page 12: Histology of Endocrine system II

• Secretion of steroids by adrenal cortex is good example of a feedback system between target organ and pituitary gland.– A psychological stimulus such as stress or exercise causes

neurosecretory neurons in the hypothalamus to secrete adrenocorticotropic hormone releasing factor into the capilaries in the median eminence.

– the releasing factor is carried by the hypophyseal portal system from the median eminence to the pars distalis.

– This causes secretion of adrenocorticotropic hormone (ACTH) by certain cells in the pars distalis.

– ACTH causes an increase in corticosteroid secretion (e.g. cortisol) by adrenal cortex cells.

– As levels of corticosteroids increase in blood, these inhibit the secretion of releasing factors by the hypothalamus and secretion of ACTH by pituitary.

Page 13: Histology of Endocrine system II

Adrenal medulla• Composed of one layer of cords of polyhedral, epithelioid, secretory cells that form

a compact irregular network surrounded by capillaries, venules and a few sympathetic ganglion cells.

• These epithelioid secretory cells are considered to be modified postganglionic neurons.

• Nerve fibers (axons) contact the epithelioid cells on the part of their surface that is adjacent to a capillary.– When stimulated , these nerve fibers release acetylcholine that causes the release of

the catecholamine secretory product accumulated in these cells.– The secretory product consists of epinephrine or norepinephrine.– Two different types of cells, one for each catecholamine.– The catecholamine secretory granules stain in a specific way when reacted with oxidizing

agents. This reaction is called the chromafin reaction. As a result, these cells are called chromafin cells.

• Adrenal medulla secretes continuously into blood stream.– The adrenal medulla cells secrete only small amounts of epinephrine and norepinephrine

unless stimulated by nervous activity related to emotional reactions.– Increased secretion of these substances prepares body to react to stressful situations. Blood

vessels constrict, blood pressure rises, etc.

Page 14: Histology of Endocrine system II

Thyroid gland• Located below the larynx, partially encircling the

esophagus.• Thyroid is covered by thin layer of loose connective tissue.• Associated with this loose connective tissue is a dense

network of blood and lymphatic capillaries.– Capillaries are fenestrated.

• Septa extend into organ from the connective tissue capsule.– These septa separate follicles of thyroid tissue from each other.– Septa composed mainly of reticular fibers.

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• The thyroid tissue within the connective tissue capsule is composed of follicles, each containing a lumen filled with a gelatinous substance called colloid.– Each follicle consists of a simple cuboidal epithelium surrounding the lumen• The epithelium changes to simple squamous if the follicle is inactive.

– Cells of follicles are responsible for synthesis of thyroid hormones, the most abundant of which is thyroxin.• These hormones are small molecules consisting of iodinated amino

acids that are all formed from the tyrosyl radicals of thyroglobulin. (i.e. side chains of thyroglobulin containing tyrosine are iodinated and separated from the thyroglobulin molecule and then processed to form the thyroid hormones.)

• Thyroxine stimulates mitochondrial respiration and oxadative phosphorylation. So, more ATP produced faster.

– In addition to follicle cells, parafollicular cells (C cells) are found between the follicles.• These cells secrete the polypeptide calcitonin that causes a reduction

of calcium in the blood by inhibiting the activities of osteoclasts.

Page 17: Histology of Endocrine system II

Control of thyroid hormone secretion• Neurogenic stimuli provided by axons from parasympathetic

and sympathetic ganglia can influence metabolism of thyroid cells.

• However, thyrotropin (thyroid stimulating hormone, TSH) is the major controlling factor.

• Thyrotropin is synthesized and secreted by cells in the adenohypophysis (pars distalis).

• Thyroid has feedback system with pituitary similar to adrenal cortex.– Releasing factors from the median eminence cause the

adenohypophysis to secrete thyrotropin.– This causes thyroid hormone production by the follicle cells of the

thyroid.– As thyroxin level rises in the blood, it causes an inhibition of the

secretion of releasing factors in the median eminence.

Page 18: Histology of Endocrine system II

• Thyroid hormones are synthesized from the colloid.– Colloid is composed of iodinated thyroglobulin.– When thyroid hormones are required, iodinated

thyroglobulin in colloid is endocytosed.– Endocytotic vesicles merge with lysosomes.– Thyroid hormones are synthesized as a result.– This liberates the 4 thyroid hormones into the

cytoplasm.– These diffuse across the cell membrane into

capillaries where blood carries them to target organs.

Page 19: Histology of Endocrine system II

Parathyroid glands• These are small, but important, organs that are

embedded in the wall of the thyroid.– History - complete removal of the thyroid gland was

noted to cause death because of spasms of the laryngeal and thoracic muscles that prevented breathing - called tetany

– In 1892, the French physiologist Gley showed that it was actually the removal of the parathyroid glands that "rode" along with the thyroid that was the cause of these titanic seizures. (Due to the lack of parathormone)

– Thus, when the thyroid is removed, it is critical that the parathyroid glands be separated from it and left in the body.

Page 20: Histology of Endocrine system II

• There are three cell types in the parathryroid glands– Chief cells• polygonal with vesicular nucleus• slightly acidiphilic, pale staining cytoplasm• secrete parathyroid hormone (parathormone).

– Oxyphil cells• Not present in thyroid at birth• start appearing in parathyroid tissues at about age 7 in

humans• no known function• Similar to, but larger than chief cells.

• Adipose cells - increase in number as one grows older.

Page 21: Histology of Endocrine system II

• Major function of parathyroid glands– Secretion of parathyroid hormone that causes an

increase in calcium in blood by promoting the activities of osteoclasts in the breakdown of calcified bone matrix.

• So the calcitonin of thyroid parafollicular cells and parathyroid hormone of chief cells balance and regulate calcium levels in the body.