Endocrine organs are organs whose cells secrete their products, i.e., hormones, into the bloodstream whereas exocrine organs such as sweat glands, salivary glands and sebaceous glands secrete their products into a duct system.  These hormones travel via the blood circulation and when they reach their "target organ" they exert their specific effect.  Some organs are primarily exocrine while some are primarily endocrine and some contain elements of both.  The ovary and testes are both exocrine organs, "secreting" ova and spermatozoa, respectively yet they are endocrine organs as well secreting hormones such as estrogen, progesterone and testosterone.  In the pancreas, part of the organ is an exocrine gland (the acini) secreting and part is endocrine , i.e.,  the islets of Langerhans which secrete various hormones such as insulin and glucagon.  In the digestive system, some cells are endocrine cells, i.e., the enteroendocrine cells.  

Other endocrine organs include the thyroid, parathyroid, adrenal, and pituitary gland.


I.  Gross Anatomy

The thyroid gland is located dorsolateral to the trachea, close to the larynx.  It has two lobes that are connected by a narrow isthmus.  

II.  Histology

The thyroid gland is composed of follicles and interfollicular connective tissue.  The capsule, classified as loose areolar connective tissue, surrounds the mass of thyroid follicles and sends smaller pieces of connective tissue into the gland to surround the individual thyroid follicles. 

Near the thyroid gland and embedded in the same connective tissue capsule is the parathyroid gland.

Sometimes patches of lymphocytes can be observed in the thyroid/parathyroid glands.


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Micrographs of thyroid gland (Lab slide 95).

Thyroid follicles consist of a layer of simple epithelium surrounding a gel-like pinkish material called colloid

The principal cell is the most numerous cell present in the simple epithelial layer and is responsible for secreting the thyroid hormones as well as thyroglobulin, a glycoprotein.

Thyroid hormones are stored extracellularly as part of the thyroglobulin which is the main component of the colloid.

The size of follicles and the height of principal cells varies even within one section of the gland. Squamous principal cells indicate a relatively inactive gland whereas cuboidal to columnar cells indicate more activity in removing the hormone from the stored form.  

In addition to principal cells there is another type of functional cell in the thyroid gland.  This is the  parafollicular cell which may be found as single cells in the epithelial lining of the follicle or in groups in the connective tissue between follicles.  They usually appear as large, clear cells since they do not stain well with hematoxylin and eosin.  They are sometimes called parafollicular cells based on their location and clear cells (C cells) based on their appearance of their cytoplasm.

Parafollicular cells secrete calcitonin, a hormone that lowers the level of calcium in the blood.


III.  Function


IV. Mechanism of Secretion of Tand T4 (thyroxin).


V. Parafollicular Cells



I.  Gross Anatomy  The parathyroid gland is difficult to see at the gross level.  It is very close to and usually embedded within the capsule of the thryroid gland.

II.  Histology


There are three types of cells in the parathyroid gland: adipocytes, chief cells and oxyphil cells.  

A reticular connective tissue framwork surrounds and supports these cells.


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Micrograph of parathyroid gland (Lab slide 95).

The main secretory cell is the chief cell.  These cells secrete parathyroid hormone.  Unfortunately these cells have no distinguishing features.


Another cell type present is the oxyphilic cell which occur in the human, ox and horse. These are large cells that contain numerous mitochondria.  Their function is unknown.


III.  Function 

       The parathryoid gland secretes parathyroid hormone which is essential for regulating the levels of calcium and phosphate in the blood.  Parathyroid hormone acts on                     the following  target organs.


IV.  Summary of hormonal control of blood calcium levels through action on bone.

Calcitonin    As the level of calcium in the blood rises, the amount of calcitonin secreted by the C cells of the thyroid increases.  Calcitonin stimulates osteoblasts to form bone taking calcium out of the circulation.  At the same time, calcitonin inhibits the mobilization of bone (and calcium) by osteoclasts.  The end result is a decrease in the level of calcium in the blood thus helping to maintain proper blood calcium levels.

Parathyroid Hormone  A decrease in the normal levels of calcium in the blood causes the chief cells of the parathyroid gland to secrete more parathyroid hormone which stimulates osteoclasts to mobilize bone resulting in an increase in the level of calcium in the blood.  Parathyroid hormone also increases Ca ion reabsorption in the kidney and decreases the reabsorption of phosphate ions.

Note: Whereas calcitonin is important in regulating the level of caclium in the blood, parathyroid hormone is essential!!  


I.  Gross Anatomy  The adrenal glands are located at the cranial end of the kidneys.  They are flat organs embedded in fat. Each gland has an outer cortex that appears yellow in fresh tissue and an inner medulla that appears gray in fresh tissue.


II. Histology of the Adrenal Gland and Adrenal Cortex


The adrenal gland is surrounded on the surface by a connective tissue capsule.  This capsule has projections into the cortex and through the cortex down into the medulla in some species.

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Micrograph of dog adrenal gland (Lab slide 97).

In most species 4 cortical zones can be identified.  From the zone nearest the capsule these are:
  • Zona glomerulosa
  • In ruminants this zone consists of cells in clusters.
  • In carnivores, horses and pigs this zone consists of columnar cells in arches and is sometimes called a zona arcuata.
  • Zona intermedia: This zone is relatively thin and contains mostly undifferentiated cells.
  • Zona fasciculata   This is the largest zone containing large, round, foamy-appearing cells arranged in cords that radiate from the zona glomerulosa down toward the medulla.  
  • Sinusoidal capillaries are located between cords of cells that are one cell thick.
  • Cells in the zona fasiculata have a foamy appearance due to the presence of many lipid droplets prior to processing for microscopy. These lipid droplets represent the  precursors for steroid hormones.
  • Zona reticularis
  • Cells of the zona reticularis are arranged in a network of cords no longer arranged in parallel as in the zona fasiculata.


III. Function of the Adrenal Cortex

IV. Histology of the Adrenal Medulla
  • Cells in the medulla are arranged in groups or cords, clustered around capillaries and venules.
  • The cells have secretory granules which contain either epinephrine or norepinephrine.
  • When fixed in potassium bichromate, the medullary cells become brown. Therefore, they are called chromaffin cells. The color is the result of a reaction between chromate and epinephrine or norepinephrine. With the typical H&E stain, the cells appear as shown in the adjacent figure as somewhat stellate-shaped cells containing a rather prominent, round nucleus.
  • Chromaffin cells are derived from neural crest cells. They are innervated by preganglionic sympathetic fibers. They release hormone by exocytosis when stimulated by those fibers.


V. Function of the Adrenal Medulla

VI. Vasculature of the adrenal gland.


Blood is supplied to the adrenal gland via the suprarenal arteries.  These arteries penetrate the capsule and form a plexus just beneath it.  From this plexus, blood is further supplied via two different routes.  
  1. Cortical arteries of the subcapsular plexus branch into sinusoids in the cortex which intimately surround cells in the zona fasciculata and zona reticularis.  These sinusoids drain into venules that empty into the central vein of the medulla.  This blood supply provides a mechanism by which cells in the cortex can influence cells in the medulla (see below).  
  2. The second route of blood supply is more direct to the medulla.  In this case, cortical arteries run along trabecular branches of the connective tissue capsule directly into the medulla without forming capillaries or sinusoids in the cortex.  These arteries then branch into capillaries in the medulla supplying the chromaffin cells with oxygenated blood.  These capillaries join the same small medullary venules as the sinusoids of the cortex and all the blood flows into the central vein of the medulla.  Because of this peculiar blood supply to the adrenal gland, the cortex has no veins.





I. Gross Anatomy and Development


The pituitary gland or hypophysis is located at the base of the brain.  Its function reflects its development from two different types of ectoderm, i.e., oral ectoderm which forms Rathke's pouch and neural ectoderm from the base of the diencephalon which forms the infundibulum.  As developments proceeds, the ectoderm of the infundibulum grows downward and wraps around Rathke's pouch.  In the adult the infundibulum forms the infundibular stalk and the pars nervosa; the ectoderm of Rathke's pouch forms the pars distalis and the pars intermedia.  Some of the oral ectoderm remains associated with the infundibular stalk to form in the adult the pars tuberalis.   A vestige of Rathke's pouch often remains visible in the adults of many species and forms a cleft that serves to distinguish the anterior lobe of the pituitary from the posterior lobe.  The terms anterior lobe and posterior lobe are useful in humans but not in domestic species; in most domestic species such as the cow, ventral lobe and dorsal lobe are more appropriate terms, respectively.    



In the adult, the pituitary gland remains "connected" to the brain via both vascular and nervous routes (see below under Function).  These relationships with the hypothalamus of the brain provide a basis for understanding the different functions of the pituitary gland and form the basis of the integration of the nervous system with the endocrine system.  

Vascular Connection:  The vascular connection provides the mechanism by which factors secreted by neuroendocrine cells in the hypothalamus reach and affect secretory cells in the pars distalis.  These secretory cells respond to the "releasing hormones" brought to them through the portal blood vessels.


Neural Connection:  The neural connection provides the mechanism by which hormones secreted by neuroendocrine cells located in specific nuclei in the brain are actually released into the bloodstream at the level of the pars nervosa of the pituitary gland.







II. Histology


The pituitary gland can be divided into various regions based on structure and function.  

One way to describe the pituitary gland is by the type of tissue present in different regions.  If this system is used then regions are as follows:

Adenohypophysis - based on grouping of all regions composed of glandular tissue  
This includes the...

  • pars distalis

  • pars intermedia

  • pars tuberalis

Neurohypophysis - based on grouping of all regions composed of neural or neurosecretory tissue  
This includes the . . .

  • median eminence (not shown)
  • infundibular stalk
  • pars nervosa ( infundibular process)

Another way to describe the pituitary gland is using the terms anterior lobe and posterior lobe (in domestic species, ventral lobe and dorsal lobe) as follows . . .

Anterior or Ventral Lobe 
This includes the . . .

  • pars distalis

  • pars intermedia

Posterior or Dorsal Lobe
This includes the . . .

  • pars nervosa




The Adenohypophysis

Pars distalis:
  This region of the pituitary gland is organized as cords or clusters of cells supported by a reticular connective tissue.  With routine staining two types of cells can be observed: (1) chromophiles which stain readily and are either red (acidophiles), blue or purple (basophiles) depending on the type of secretory material present, and (2) chromophobes which do not take up the stain and thus appear unstained or rather clear.  Chromophobes may be chromophiles that have lost their secretory granules or chromophiles that have not accumulated large numbers of secretory granules. Use of specific antibodies against the protein secretory products has allowed the identification of the different cells. The cells of the par distalis are:

  • Somatotrophs secrete growth hormone which affects many cells

  • Mammotrophs secrete prolactin which controls milk production during lactation

  • Corticotrophs secrete ACTH which controls secretion of cortisol by cells in the adrenal cortex

  • Thyrotrophs secrete TSH which controls secretion of thyroid gland

  • Gonadotrophs secrete FSH and LH which control development of follicles and ovulation in the ovary.

Pars intermedia: With routine histological staining, the cells in the pars intermedia stain blue-purple and thus are basophilic.

  • Cells secrete ACTH, MSH, endorphins and lipotrophins.

Pars tuberalis: This region is an extension of the glandular pituitary gland and its cells resemble those of the pars intermedia and pars distalis.  The specific function of the cells in the pars tuberalis, however, is not clear.

The Neurohypophysis

Pars nervosa:  This region consists of unmyelinated nerve axons (cell bodies are in the hypothalamus) and supportive cells called pituicytes.  

  • Secretes ADH (antidiuretic hormone) which is synthesized by neurons in the supraoptic nucleus of the hypothalamus.

  • Secretes vasopressin which is synthesized by neurons in the paraventricular nucleus of the hypothalamus.

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Micrograph of ox pituitary (Lab slide 93).

III. Function - Control of Secretion 



The cells in the adenohypophysis secrete two classes of hormones: (1) direct acting and (2) trophic.  Direct acting hormones include growth hormone (GH) and prolactin from the pars distalis, and melanocyte stimulating hormone (MSH) from the pars intermedia.  Trophic hormones include adrenocorticotrophic hormone (ACTH), thyroid stimulating hormone (TSH), follicle stimulating hormone (FSH) and luteinizing hormone (LH). 

Secretion of these hormones is controlled by specific releasing hormones in the hypothalamus.  Most of the releasing hormones are stimulatory in their action except for the one for prolactin which is inhibitory and the one for growth hormone which has both inhibitory and stimulatory releasing hormones.  Releasing hormones are produced in the median eminence of the hypothalamus and reach the adenohypophysis via a portal system of veins known as the pituitary portal system.



The cells in the neurohypophysis secrete only direct acting hormones : (1) antidiuretic hormone (ADH) also known as vasopressin secreted by neurons in the supraoptic nucleus in the hypothalamus and (2) oxytocin secreted by neurons in the paraventricular nucleus in the hypothalamus.  After synthesis in the hypothalamus, these hormones move down the axons of the hypothalamohypophyseal tract through the infundibular stalk and terminate near blood vessels in the pars nervosa.  Accumulations of these hormones bound to specific glycoproteins can be observed along the axons of the hypothalamophypophyseal tract and in the pars nervosa.  These "accumulations" often called Herring bodies represent a storage form of the hormone.  Release of these hormone stores is determined by impulses in the axons of the hypothalamophypophyseal tract originating in the hypothalamus.  Such a mechanism of secretion controlled by nerve impulses is called "neurosecretion".





Copyright 2002 Charlotte L. Ownby
Histology Part 2 Index