THE ENDOCRINE

• The endocrine system consists of all structures of

the secretory and hormonal systems.

• Secreted hormones provide the medium for the

exchange of information between systems.

• This is accomplished by ensuring the regulation of

physiologic functions in conjunction with the

•This system takes its origin from

glands that have lost their

These ductless glands secrete their

hormones directly into the

bloodstream or lymphatic system;

hence their being referred to as

This system is comprised of,

•organs

that function as internal secretors,

Other endocrine cell groups found in other organs include;

• The

chorionic epithelium

in the

placenta,

• The

enteroendocrine cells

of the

mucosa and submucosa of the

• Whether they originate from an organ or group of cells,

endocrine glands secrete special substances referred to as

hormones.

• Hormones secreted from endocrine glands enter directly into

the bloodstream or lymphatic system within which they are

transported to distant target cells where they exert their

effects by adhering to their surfaces.

Some tissue hormones of peptide or amine

structure (parathormone), in addition to

being secreted directly into the bloodstream,

can also be secreted into the tissue spaces

found in connective tissues where they

•Eg; gastrin, secretin, and somatostatin

are synthesized in special cells with

paracrine actions on adjacent local cells

by way of local diffusion.

•Somatostatin is synthesized by the D

cells of the pancreas. It has a

suppressive effect on the enzyme

Also, some cells contain receptors for

the hormones they produce.

This hormonal action known as

autocrine control is the means by which

these cells regulate their secretory

functions.

•There is also an INTERACRINE EFFECT.

This is the process where the hormone

produced exerts its effect within the cell

without having to be excreted. Eg;

various growth factors like the growth

hormone are also referred to as

•Hormones exert their effects by

interacting with the receptors found

within or on the target cells. The response

elicited depends upon the genetic

program. Accordingly, the same hormone

may have different effects on various

tissues.

•Epithelial cells form the functional units

of endocrine or secretory glands. These

cells have no direct or indirect

THE HYPOPHYSIS

(THE PITUITARY GLAND)

As a result of the effects of hormones on the sexual organs important events such as

• Sexual maturation,

• Cyclic genital changes,

• Pregnancy and lactation take place.

• The hypophysis consists of two distinct parts according to its structure and origin:

ADENOHYPOPHYSIS: Rich in cellularity and made up of heterochromatic cells

• Has three parts:

• pars anterior (p. distalis) • pars tuberalis

1- Pars anterior (pars distalis)

The capsule surrounding the organ sends prolongations into the internal parts.

• Cells of the pars anterior have varying types and their secretions are different properties.

Thyrotropic hormone

(thyroid stimulating

hormone-TSH) stimulates the synthesis and

secretion of thyroid hormones.

Thyroid hormone level up to a certain limit

when present in the blood leads to

suppression of the production of TSH from

the hypophysis.

•Adrenocorticotropic hormone

(ACTH) is

Follicle Stimulating Hormone

(FSH) is

responsible for the development of ovarian

follicles, and the seminiferous tubules in the

testis as well as the initiation of

•Luteinizing hormone

(LH) has as its target

organs;

The Leydig cells in the testis induce the secretion

of testosterone which controls the development

of secondary sexual characters (libido, deepening

of the voice, and the growth of horns, and growth

of the beard) as well as the maturation of the

sperms.

• Apart from its effect on other endocrine glands, the frontal lobe of the hypophysis also acts as an incretoric organ.

• By this, it secretes hormones that directly affect non-endocrine tissues.

• Example:

• Somatotropic hormone (STH) that influence the development and growth of the body,

• Cells of the frontal hypophyseal lobe are divided into two groups based on their staining properties:

a) Chromophobic cells:

• These, due to their

neutrophilic properties fail to absorb dyes and therefore appear as cells with clear cytoplasm under the light microscope.

• Most of these are

considered stem cells that will later differentiate into various cell types.

• A quarter of the stem cells being G cells secrete the adrenocorticotropic

b) Chromophilic cells:

• These;

Acidophilic cells far outnumber their basophilic counterparts.

The acidophilic and basophilic cells are in close contact with the

•The walls of capillaries are especially

fenestrated to allow for secreted

materials to easily overcome the

• Hormones secreted by the acidophilic cells are in the

protein structure.

• Growth hormoneproducing (somatotropic hormone

-STH) cells, prolactin or luteomammotropic hormone (LTH) producing cells, and basophilic cells secrete hormones

with glycoprotein structure.

2- Pars tuberalis :

• Occupies a very narrow area.

3- Pars intermedia :

Unlike the pars distalis, this has no acidophilic staining cells. However, it does have numerous basophilic staining cells.

•Between the pars intermedia and

pars anterior (distalis) lies a

B- Neurohypophysis: Part rich in nerve fibers

• Neurosecretions produced from the neurons of the hypothalamus are carried along the entire axons where they are stored, to be released when needed.

• This part consists of non-myelinated nerve fibers from the hypothalamus and modified glial cells (Pituicytes).

Pars infundibularis (infundibulum) the narrow part of the

components of the neurohypophysis and the broader pars proximalis

• The non-myelinated nerve fibers arise from nuclei of the giant cells of the hypothalamus (Nucleus supraopticus and Nucleus paraventricularis).

• Granules secreted from these nuclei (neurosecretory granules), binds to the transporter protein, where they get carried through the neuronal axoplasm (intraxonally) into the proximity of the fenestrated capillary vessels in the neurohypophysis

• Between the fenestrated capillaries and the axonal ends are cells found called pituicytes.

• They cover the axons filled with neurosecretions and bind in a special manner to the fenestrated capillaries

•The hormones,

oxytocin

, and

vasopressin

are the

hormones found in these neurosecretions.

Vasopressin (antidiuretic hormone-ADH)

is

responsible for the contraction of muscles within

the vessel wall, and together with aldosterone acts

especially in the reabsorption of water from the

•In situations in which the fluid content of

the blood is reduced such as in coma and

dehydration, secretion of ADH is

increased.

•In the deficiency of ADH (diabetes

•The hypothalamus is connected with

the neurohypohysis through the

non-myelinated nerve fibers and also with

the frontal lobe through the blood

vessels.

•The hypophysis together with the

hypothalamus are known together as

the

hypothalamo-hypophyseal system

due

to

their

functional

The required substances for the release of several releasing and inhibitory hormones from the frontal lobe comes from the

neurons of the hypothalamus:

• Releasing (releasing hormone-RH)

• RH and RIH are formed in the nucleus arcuatus, nucleus ventromedialis and nucleus infundibularis of the

hypothalamus.

• For every glandotrop, there is a special "releasing hormone (RH)" and

• An antagonistic effect eliciting "releasing inhibiting hormone (RIH)".

•Production and secretion of the

hypophyseal hormones is

THE EPIPHYSIS

•it is also called

corpus

pineale.

Its stalk referred to as the Habenula is the means by which the epiphysis is connected to the midbrain (diencephalon); myelinated nerve fibers (sympathetic fibers) enter the organs from here and thereafter lose their myelin sheaths and proceed into the parenchyma where they synapse with pinealocytes. By this, they control the release of melatonin.

• The pineal gland as an endocrine organ in mammals has two types of cells in the parenchyme.

1.Modified glial cells

2.Pineal cells or pinealocytes

• Modified glial cells are cells of fibrous astrocytic type. The pores that lie between the cellular extensions are filled by pinealocytes.

• Pineal cells or pinealocytes account for approximately 95% of the cellular content of the gland.

• They are irregularly shaped with peripheral processes, and lightly staining large round nuclei.

• _{Pineal cells make up 95%}

Pineal cells secrete melatonin which regulates

sexual behavior in humans and various animal

species. Melatonin is an important hormone of the

body. It is required for the functioning of all organs

of the endocrine system.

This organ functions as a biological clock

showing a circadian rhythm. Light and

darkness have specific implications on the

release of melatonin from the pineal gland. In

general, light suppresses the production of

melatonin while darkness stimulates it.

The amount of light reaching the eye

• The epiphysis which demonstrates an active metabolic function delays sexual development until puberty. In other words, melatonin synthesized in the epiphysis shows an anti-gonadotropic property.

• It regulates the seasonal changes observed in animals that

show seasonal reproductive activity.

• In fall and winter months the shorter days and longer

nights serve to increase melatonin production. This is

reversed in the spring and summer months. With the

decrease in the synthesis of melatonin comes a rise in the

gonadotropin-releasing factor secretion. This, in turn,

It also regulates some periodic states of the body; example it regulates sleep.

The Pineal gland is known to contain

hydroxyapatite crystals of glial and

stromal origin, which have been

demonstrated to accumulate in the

brain as it ages referred to as the

THE THYROID GLAND

GLANDULA THYROIDEA

Originates from the pharyngeal endoderm.

• Unlike other endocrine glands, the thyroid gland resembles exocrine glands in structure.

Thin connective tissue extensions from the capsule continues into the gland and divides it into incomplete lobules.

The luminal side of the follicle epithelial cells is microvilli.

• The parenchyma of the thyroid is composed of millions of rounded epithelial structures called thyroid follicles.

The thyroid is the only endocrine gland in which a large quantity of secretory product is stored.

Moreover, the accumulation is outside the cells, in the colloid of the follicles, which is also unusual.

There is sufficient hormone in follicles to supply the body for up to three months with no additional synthesis.

• Between the apical surface of the follicle epithelium and the colloid, vacuoles are seen on a light microscope.

Iodine found in the extracellular space is bound to the free thyroxin groups in the forms of tri-iodothyronine(T3) and

tetra-iodothyronine(T4) (thyroxine).

• Hormones released into the interstitium from the cell base by exocytosis are carried into the capillaries.

As seen here, thyroid follicles have different functions:

-Release of the secretion into the follicular space (extrusion). -Deposition of colloid or the secretion, and its maturation. -Reabsorption of the matured colloid, re-processing and release through the basal membrane into the blood.

All these are undertaken, to a large extent, under the effect of

Changes in the general conditions

associated with life (

temperature

changes, hunger, pregnancy, aging

, etc..)

• Parafollicular cells secrete the calcitonin hormone, which plays a role in calcium metabolism.

THE PARATHYROID GLAND

(GLANDULA PARATHYREOIDEA)

• This organ embedded within the thyroid capsule is made up of several parts.

• The parathyroid also originates from the pharyngeal endoderm.

• A thin layer of connective tissue separetes from capsule, and protrudes into the organ.

• Epithelial cells that form the parenchyme are in the form of heaps with the periphery surrounded by reticulum fibers as thin webs.

• The epithelial cells here are of two types:

•Both types of cells are polygonal.

•Principal (chief) cells

are much smaller

but larger in number; have small

mitochondria within their cytoplasm,

•Principal cells (Chief cells) have two

types based on staining properties of

cytoplasm as

pale

and

dark

staining.

•Pale cells

are rich in glycogen. These

Dark staining principal cells secrete the parathormone.

This hormone demonstrates a regulatory effect on calcium metabolism.

With an antagonistic action against Calcitonin, parathormone

• The larger but less number of oxyphyl cells have acidic staining cytoplasm; are seen in very old individuals.

ADRENAL GLAND

GLANDULA SUPRARENALIS

• The Adrenal gland is a gland formed by two

tissues of mesodermal and neuroectodermal origin.

• The Cortex develops from the coeliac epithelia

(mesodermal).

• The organ is surrounded by a fibroelastic capsule externally. • Connective tissue divisions that extend into the organ from the

The connective tissue containing large

amounts of reticulum fibers are seen as

wide lumen and fenestrated capillary

vessels in both the cortex and medulla.

•Cells that make up the parenchyma

constitute as cords or groups in the cortex

and medulla.

In the cortex immediately underneath the capsule is found a very thin area of young cells called the subcapsular blastema.

• More than fifty hormones from the Adrenal

cortex, all without a steroid structure, have

been analyzed.

• Cholesterol

is required by the cells for

hormone synthesis.

• This material is stored in large fat droplets.

• Enzymes required for synthesis are found in

smooth ER and mitochondria.

Hormones from the cortex are grouped

into three main groups as:

• Glucocorticoids

Glucocorticoids:

• Glucocorticoids are required for metabolism of carbohydrate, protein and fat.

• Stimulates gluconeogenesis (synthesis of glucose from non-carbohydrate substrates in the liver).

• Increases protein biosynthesis and burning of fats.

• Glucocorticoids thought to be secreted from all parts of the cortex include;

• cortisone

• hydrocortisone

Mineralocorticoids:

• The most prominent of the hormones in this group include

aldosterone and desoxycorticosterone.

• Aldosterone is a hormone required for water and electrolyte balance. Its principal effect is demonstrated on sodium. It induces the reabsorption of sodium from the renal tubules

while increasing the excretion of potassium and hydrogen ions in urine. With an increase in the sodium content, there is an increase in the osmotic pressure and consequent increase in water resorption. Thus, water and electrolyte balance is

• Desoxycorticosterone affects healing by increasing connective tissue growth in inflammations.

• In situations of inadequate secretion of aldosterone a decrease in sodium, water and chloride reabsorption ensues. A decrease in blood volume and heart beat follows and consequently shock

Androgenic Hormones

:

• According to the arrangement of the cords, the cortex is divided into three areas:

Zona arcuata:

• It is the narrow area immediately underneath the subcapsular blastema.

Zona fasiculata:

Is the widest area.

• Cells that form these columns are polygonal; are cells with the most abundant fat

deposits in their cytoplasm compared to all areas of the cortex and appear foamy or honeycombed.

This is the area with the highest secretion of the Glucocorticoid type of hormones.

However, the same type of hormone are thought to be secreted in other parts of the adrenal

cortex, albeit, in smaller amounts.

When the hormonal

synthesis is increased (as under stress and times of need) the zona

fasiculata, expands to the disadvantage of its adjacent layers

(progressive change).

In situations of

decreased function, however, the same region regresses (regressive change).

Zona reticularis:

The cellular arrangement has irregular and they network (reticulum).

The cytoplasm of the cells is stained darker than the

others.

• Cells here contain large amounts of lysosome and phagolysosome lipofuscin and several other pigment materials.

• Cells of the zona reticularis secrete the adrenal

androgen known as

dehydroepiandrosterone. • This substance, like

testosterone and estradiol, is metabolized to other sex hormones.

• The cortex and medulla border are not regular.

Adrenal medulla

• Chromaffin cells are the main cells here.

• They are of two types based on

the size of the chromaffin

granules they contain:

• Adrenaline cells

Adrenaline containing

cells account for 80% and noradrenaline containing cells account for the

remaining 20%.

Adrenaline containing granules are large and few in number.

Noradrenaline containing granules, on the other hand, are small but numerous.

Between these two types of chromaffin cells lies a single sympathetic

• Also, in the medulla, small dark appearing cell

populations that resemble lymphocytic cells under light microscopy are seen.

• These are the undifferentiated

sympatheticoblasts that can differentiate into

paraganglionic cells.

• Chromaffin cells are seen yellow-brown in reaction with chromium salts.

• There are enlarged capillaries in the

connective tissue between the

cords.

• These capillaries open to the large central vein inside the organ.

• In the intimal walls of these central veins are found

longitudinal muscle pads. • Blood flow is regulated by

this pads.

• Controlling of the blood flow is functionally very

important.

• Even in very low amounts

THE PARAGANGLION

• Paraganglions are the groups of cells that, originate from the crista neuralis during embryonic development and adopts a glandular character.

• Paraganglial cells turn into gland cells that lose their neural properties and lose their extension.

• These cells synthesize effective substances like

THE PARAGANGLION

• Since all paraganglia cells receive innervation from the

central nervous system, they are kept under control of the vegetative nervous system.

• Adrenal medulla is the largest and most important organ of the paraganglia and remains throughout life.

• Cells in paraganglia secrete substances that are either sympathetic or parasympathetic.

• For this reason, they are also referred to as the sympathetic

and parasympathetic paraganglia.

• Cells that secrete substances with adrenergic activity like adrenaline and noradrenaline when dyed after fixation with chrome salts can be demonstrated under light microscopy; hence they are referred to as chromaffin cells. These belong to the sympathetic group.

• In contrast, cells that do not show chromaffin properties can not be demonstrated by this method of dyeing; these cells containing acetylcholine belong to the parasympathetic group.

REFERENCES:

Tanyolaç, A. (1999): Özel Histoloji. Yorum Basın Yayın Sanayi Ltd. Şti. Ankara. Özer, A., Girgin, A., Alabay B., Liman, N., Özfiliz, N., Gülmez, N., Özcan, Z.,

Yörük, M., Erdost, H., Aslan, Ş., Ergün, L., Zık, B. (2008): Veteriner Özel Histoloji. Nobel Yayın Dağıtım Tic. Ltd. Şti. Ankara

Dellmann, H. D., & Eurell, J. A. (1998). Textbook of Veterinary Histology, 5th. Edn., Philadelphia, Lea and Febiger. P, 450.

Gartner, L.P. & Hiatt, J.L. (1997). Color textbook of Histology: W.B. Saunders Company. Philadelphia, Pensilvanya, USA.