The Endocrine Glands | Histology | Anjani Mishra

 What is Endocrine System ?

The organ system is composed of the endocrine or ductless gland which elaborate internal secretion or hormones. The word endocrine derived from (Greek word, endon,-within; krino,-to separate). It refers principally to ductless gland whose secretions or hormones are absorbed directly into the blood stream rather than being extruded into a system of ducts.

Endocrine Glands

Pure endocrine glands of the body are as follows;

1. Pituitary gland

2. Adrenal gland

3. Thyroid gland

4. Parathyroid gland

5. pineal gland

Mixed glands;

1. Testis

2. Ovary

3. Uterus

4. Kidney

5. Pancreas

6. Liver

7. Stomach

8. Intestine

Temporary endocrine glands;

1. Thymus gland

2. Corpus luteum

3. placenta

Pituitary gland (Hypophysis cerebri)

This is a cylindrical reddish grey coloured body about 1cm in length and 3cm in diameter (adult cattle) situated in the sella turcica of sphenoid bone, connected at the base of the brain with a stalk and enclosed by duramater.

Fig: Median section of the brain of a horse

Fig: Hypophysis of bovine, midsagittal section

a. Infundibulum with adjacent pars tuberalis,  b. Infundibular cavity,  c. Hypophyseal cavity,  d. Pars tuberalis and intermedia,  e. Pars distalis

Fig: Pituitary gland

The gland is composed of two distinct types of tissue and accounts for the gross appearance of the two major lobes of the hypophysis, namely;

A.   A.   Anterior lobe (Adenohypophysis)

B.    B.   Posterior lobe (Neurohypophysis)

A. Anterior lobe

 Anterior lobe sub-divided into two parts, namely;

a) Pars distalis or pars anterior

b) Pars tuberalis

c) Pars intermedia

 

Fig: Pituitary gland, Horse

5. Follicle,  8. Pars distalis,  9. Pars intermedia,  10. Pars nervosa 

a) Pars distalis or pars anterior

v  Pars distalis composes the major part of the adenohypophysis, around 75% and is covered by thin fibrous capsule.

v  Few fibroblasts are present which produce reticular fibers that support the cords of hormone secreting cells.

v  The hormones produced by these cells are stored as secretory granules.

v  It consists of cells arranged in cords or clumps which are separated by blood vessels around sinusoids.

There are two types of cells, namely;

 i)  Chromophil cells

 ii) Chromophobe cells

i) Chromophil cells

The cells which have affinity towards stain known as chromophil cells. They are larger containing cytoplasmic eosinophilic granules.

These cells are also of two types, namely;

·         Acidophil cells

·         Basophil cells

Fig: Pituitary gland, Horse

1.Acidophil,  2. Basophil,  4. Chromophobes,  12. Sinusoid

Acidophil cells

•   The cells staining with acid are called acidophil cells.
•   They are round or ovoid in shape with centrally located nucleus.
•   They take acidic stain and appear pink in H & E stain preparation.
•   Cytoplasmic granules are larger than basophil.
•   They constitute about 35% of the total cell population in pars anterior.

These cells are also sub-divided into two types;

·         Erythrophil cells, &

·         Aurantiphil cells

Erythrophil cells

They are responsible for the production of prolactin or lactogenic or mammotropic or luteotropic (LTH) hormone. These cells are called mammotropes/lactotropes/luteotropes.

Aurantiphil cells

They are responsible for the production of growth or somatotropic hormone (STH).

It acts on epiphyseal plate of cartilage of the long bone. These cells are called somatotropes.

Basophil cells

•  The cells staining with basic stain is called basophil cells.
•  These are the largest variety of cells in pars distalis.
•  They are round or ovoid in shape with centrally located nucleus.
•  They take basic stain and appear dark with H & E stain preparation.
•   Cytoplasmic granules are smaller than acidophil.
•   They constitute about 15% of the total cell population in pars anterior.

Fig: Pituitary gland

 These cells are also of two types;

·         Cyanophil

·         Amphophil  

                Red

                Purple

Cyanophil cells

They are responsible for the production of thyroid stimulating hormone(TSH) which stimulates thyroid gland to secrete thyroxin. These cells are called thyrotropes.

 Amphophil cells

They are responsible for the production of follicular stimulating hormone(FSH) and interstitial cell stimulating hormone(ICTH). These cells are called gonadotropes. 

Amphophil cells are of two types, namely;

Red amphophil

They are responsible for the production of follicular stimulating hormone (FSH). It is also called folliculotropes.

Purple amphophil

They are responsible for the production of interstitial cell stimulating hormone (ICSH)

ii) Chromophobe cells

•  The cells which have no affinity towards stain known as chromophobe cells.
•  They are smaller in size than chromophil.
•  They are small rounded or polygonal cells with centrally located nucleus.
•   The cytoplasm is agranular and takes light stain or do not stain intensely.
•   They constitute about 50% of the total cell population in pars anterior.

     They are responsible for the production of adreno-corticotropic hormone (ACTH), which stimulates adrenal cortex of kidney. These cells are called corticotropes.

b) Pars tuberalis

• The pars distalis extends dorsally for some distance forming a thin epithelial cell layer around the infundibulum called the pars infundibularis adenohypophysis.
• The cells are stratified cuboidal in shape and arranged in clusters along the columns with many  sinusoidal blood vessels and cellular tubes.
• Function of pars tuberalis is not known.

c) Pars intermedia

• The pars distalis is separated from the neurohypophysis by an interglandular cleft or the residual  lumen(Rathke's pouch). The caudal wall of the cleft is known as pars intermedia adenohypophysis.
• The cells are like pars distalis where the islands of well differentiated tissue occur regularly.
• It consists of a connective tissue framework occupied by basophilic cells. Glandlike follicles also occur.
• They are responsible for the production of melanocyte stimulating hormone (MHS)

B) Posterior lobe (Pars nervosa)

• This part of pituitary gland is lobulated having distinct lobule.
• Thick connective tissue septa presents outside the lobule, containing blood vessels and nerves.
• The parenchyma consists of neuroglia like cells with interwoven process and the cellular elements, the pituicytes showing fine granules and droplets (indicative of secretory activity).
• The axon of the cells of supra-optic nucleus and paraventricular nucleus of hypothalamus, passes along the pituitary stalk(infundibulum) and terminate at the pars nervosa as bulbar expansion.
• The hormone produced in these cells pass through the axon in the form of secretory granules and stored in the bulbar expansions called Herring bodies.
• The infundibular cavity and canal are lined by ependymal cells.

                                                    Fig: Pituitary gland, Chicken

 9. Pars distalis(caudal portion),  12. Pituicytes

Neuro secretion:

The secretion presents in the pars nervosa is called neurosecretion, since the secretion is coming from the nerve cells in the hypothalamus of the brain. The secretion produce from supra optic nucleus and paraventricular nucleus and come down via hypothalamo-hypophyseal tract to pars nervosa. Two types of hormones are present in the neurosecretion.

i) Antidiuretic hormone/Vasopressin/Pitressin, &

ii) Oxytocin hormone

Adrenal(suprarenal) gland

The adrenal glands are paired organs that lie close or near to the cranial pole/ends of the corresponding kidneys. The gland is covered by a thick fibro-elastic capsule containing few smooth muscle cells. The gland is richly supplied with blood vessels. Numerous fine arterioles originate from the subcapsular capillary plexus and enter the gland through septa.

Histology

The gland consists of two concentric layers, that are distinct structurally, developmentally, and functionally.

A.    A.  Adrenal cortex- peripheral yellow layer, mesodermal in origin.

B.     B.  Adrenal medulla- central reddish brown layer, ectodermal in origin.

 

Fig: Adrenal gland

A.   Cortex

It consists of three histological zones;

a)      Zona glomerulosa

b)      Zona fasciculata

c)      Zona reticularis

a)      Zona glomerulosa

• It is a thin outer zone situated below the capsule constituting 15% of the total adrenal volume.
• It is composed of columnar or pyramidal cells, that are arranged in closely packed, rounded or arched cords surrounded by capillaries.
• Each cell contains deeply stained nucleus and basophilic cytoplasm.
• This zone secretes mineralocorticoid hormones(aldosterone and deoxycorticosterone).

b)     Zona fasciculata

• It is a thick middle zone and is the largest layer of cortex, constituting 65% of the total adrenal volume.
• It is composed of one or two cell thick that are arranged in straight cords or parallel rows and have a straight capillaries and connective tissue elements in between them.
• The cells are polyhedral in shape and their cytoplasm contains a great number of lipid droplets. So the cells appear vacuolated in common histological preparation.
• This zone secretes glucocorticoids(cortisol and cortisone or corticosterone).

Fig: Adrenal gland

c)      Zona reticularis

• It is the innermost zone constituting 7% of the total adrenal volume.
• It is composed of polyhedral cells (smaller than those of other two zones) that are arranged in irregular cords of an anastomosing network around the sinusoids.
• Both dark and light staining cells are present. Many large lipofuscin pigment granules are common in these cells.
• This zone secretes sex hormone (androgens and some androstenedione).

B.    Medulla

• The adrenal medulla is completely surrounded by cortex except at the hilus.
• It is composed of large polyhedral cells, called chromaffin cells, and a few parasympathetic ganglion cells, arranged in cords of an anastomosing network around wide sinusoids.
• These cells contain variable amount of secretory granules in their cytoplasm.
• This zone secretes adrenalin(epinephrine) and nor-adrenalin(nor-epinephrine). 

Thyroid gland

These are two flattened triangular dark brown lobes situated at the ventrolateral aspect of the junction of larynx and trachea.

Fig: Thyroid gland 

Histology

• It is surrounded by a connective tissue capsule which gives off septa of varying thickness. These divide the organ into flat or round interconnected lobules.
•  The lobules consists of vesicles of varying sizes- the thyroid follicles.
• The follicles are completely closed and are surrounded by a network of delicate reticular fibers.
• They are usually spherical or ovoid in shape, but some are tubular or sacculated.
• The follicles contain colloid, a viscous fluid rich in iodine secreted by the cells. There are free epithelial cells in the colloid.
• The cells lining the follicles are cuboidal or low columnar in shape with pale or clear cytoplasm and have a round or oval shape large nucleus located apically.
• These cells secretes thyroxin which is responsible for normal growth and development of the body.

Fig: Thyroid gland, Dog

2. Colloid,  5. Follicle,  6. Follicular cell,  8. Parafollicular cell

•  Para-follicular cells or 'C' cells are a second variety of cells remain in scattered form either singly or in clusters.
• The cells are polyhedral or oval in shape with pale or clear cytoplasm containing secretory granules.
• The nuclei are typically larger than those of surrounding follicular epithelial cells. They exhibit an enlarged, elongated, oval or irregular shape located basally.
• These cells secretes thyrocalcitonin, which maintain blood calcium level of the body. 

 

 Fig: Thyroid gland, Dog

2. Colloid,  6. Follicular cell,  7. Parafollicular cell

Parathyroid gland

These are four(two cranial and two caudal) small glandular mass of yellow brown colour. These glands are situated at variable position, close to the thyroid gland. Generally cranial glands are located cranial to the thyroid glands and the caudal glands are located at the caudal end of the corresponding thyroid glands.

Fig: Parathyroid gland

Histology

• The connective tissue framework is similar to that of thyroid gland but the lobules do not contain any follicle.
• The parenchyma is formed by the epithelial cells in the form of cords or clusters, placed around sinusoids.

Two types of cells are found in these glands;

(a) Chief cells/principal cells and

(b) Oxyphil cells

(a) Chief cells/principal cells

• The chief cells are more in number.
• They contain clear cytoplasm  and vesicular nucleus.
• These cells secrete parathormone which is responsible for calcium and phosphate metabolism of the body.

(b) Oxyphil cells

•  The oxyphil cells are large and their cytoplasm contains acidophilic granules.
•   Function of these cells are not known.

 

Pineal gland(Epiphysis cerebri)

This is cone or nodule like small body attached to the roof of third ventricle with a stalk. It is located on the midline and directed upward and caudally at a site caudal to the thalamus, dorsal to the rostral colliculi and ventral to the cerebral hemisphere.

 

Fig: Median section of brain of horse

Histology

• The connective tissue capsule of the epiphysis send septa inside the gland.which are richly vascularised.
• The parenchyma of the gland is arranged into lobules separated by thin connective tissue septa. The lobules contain cells, predominantly pinealocytes.
• The  pinealocytes contain large, light nuclei and the cytoplasm often contain droplet-like homogeneous inclusions.
• The cells are characterized by long, slender processes, which have club-shaped endings terminating in the connective tissue septa.
• Some supporting neuroglial cells are also present.
• A third variety of tall cells which generally lines the lumen of the lobules particularly in birds are called Ependymocytes.

Fig: Pineal gland

A.    Astrocytes,  B. Blood vessel,  P. Pinealocytes

v  Sometimes a fourth variety of cells, structurally somewhat different from the pinealocytes and ependymocytes are called Hypendymocytes.

v  Pineal gland secretes melatonin and serotonin.

v  The exact function of the pineal body is still clearly unknown. However, it helps to receive information about the state of the light-dark cycle from the environment and convey this information to produce and secrete the hormone melatonin. Melatonin production is stimulated by darkness and inhibited by light.

v  The pineal gland was commonly dubbed the “third eye” for many reasons, including its location deep in the center of the brain and its connection to light. Mystic and esoteric spiritual traditions suggest it serves as a metaphysical connection between the physical and spiritual worlds.

 

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