Histology of Endocrine system - PowerPoint Presentation

Slide1

Histology of Endocrine systemII

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.

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.

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.

Adrenal cortex

 Composed of 3 layers

zona

glomerulosa

zona

fasciculata

zona

reticularis

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.

 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.

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.

Zona fasciculata - function

Zona

fasciculata cells secrete 

glucocorticoids

 (important in lipid, protein and carbohydrate metabolism) that are another type of steroid.

 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

.

 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.

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.

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.

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

.

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.

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.

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.

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.

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

.