THE PITUITARY GLAND
Study guide/Handout

Web address: http://www.cytochemistry.net/Endocrine_System/pituitary.htm

Gwen V. Childs, Ph.D.
Shorey 9/32
Email: childsgwenv@uams.edu

To help you with your study of the Endocrine System, read pp 301-309, Chapter 13, Gartner and Hiatt, W.B. Saunders, Philadelphia, PA follow the guide in the questions below. 

TEST YOURSELF.  How much do you know about the hypophysis?

A. INTRODUCTION AND DEFINITIONS

For illustrations, consult the brief slide presentation at: http://www.cytochemistry.net/Endocrine_System/Endocrine_System_Intro.htm
(note: these are best viewed with Explorer 4.0 or greater.)

1. Endocrine gland/cells: secrete product called hormone into the blood stream that affects other cells or systems.
2. Hormone: from root "horare" ="to arouse". Hormones are chemicals that may stimulate, inhibit, or maintain status quo of a system or group of cells. May be diverse--proteins, peptides, amines, steroids.
3. Neuroendocrine: A nerve cell produces a hormone and secretes it into the blood stream. The nerve cell itself may be affected by hormones from other nerve or endocrine cells.
4. Mechanisms of Regulation  
     a. Endocrine: Cell in one organ secretes a hormone that travels via blood stream to another organ and     regulates a target cell
     b. Paracrine: Cell in a given organ secretes a hormone that affects another cell type in same organ.
     c. Autocrine: Cell produces hormone that regulates cells in its own population, or family.
     d. Juxtacrine: Cell produces a hormone or growth factor that regulates its neighboring (adjacent) cell.  May be a tethered ligand, bound in the membrane of the signaling cell.

B. ANATOMY OF THE HYPOPHYSIS

For more illustrations, consult the slide presentation at:
http://www.cytochemistry.net/Endocrine_System/Pituitary_lecture_slides.htm
(note: these are best viewed with Explorer 4.0 or greater.)

1. Locate the hypophysis: This gland is in a depression at the base of the skull in the sphenoid bone called the "sella turcica". Connected by a stalk of neural tissue to the brain.  (Figure in text)
2. Major divisions of Adenohypophysis:

3. Divisions of Neurohypophysis:

For more illustrations, check out your text or http://www.cytochemistry.net/Endocrine_System/Pituitary_lecture_slides.htm
(note: these are best viewed with Explorer 4.0 or greater.)

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C. BLOOD SUPPLY TO THE HYPOPHYSIS.

Adenohypophysis- Superior hypophyseal arteries from internal carotids and the posterior communicating artery of the circle of Willis branch to form a capillary plexus in the pars tuberalis and median eminence region.

These capillary loops distribute throughout the stalk/median eminence region. What might be the significance of this distribution?  (Answer. This increases the surface area so that many axonal endings can lie against the capillaries). 

These capillaries then send long portal vessels down to the anterior lobe/pars distalis to communicate with a second capillary bed. This second capillary bed is a sinusoidal type (fenestrated with diaphragms) and provides the supply to the cells of the anterior lobe. Venous drainage is to Y-shaped veins that may be shunted into the posterior lobe. From there, drainage is to the cavernous sinus.

Key concept: Blood supply sets up an important neurovascular link between the hypothalamus and the pituitary gland. Axonal endings in the median eminence contain stores of regulatory hormones that control the anterior pituitary cells. At the appropriate time, these are secreted and carried down the portal vessels to the anterior lobe target cell. The short distance makes this an efficient delivery system for these hormones.

Neurohypophysis- Inferior hypophyseal arteries, also from internal carotid, penetrate the posterior lobe and branch to form a capillary bed. Drainage is to the cavernous sinus. A few inferior hypophyseal arteries also supply the outer part of the anterior lobe. For the most part, the arterial supply is separate from that of the adenohypophysis. However, the venous supply from the anterior lobe may connect with that from the posterior lobe. This vascular link may provide regulatory hormones from the anterior lobe that affect secretion from the posterior lobe.
 

D. REGULATION OF THE ANTERIOR LOBE CELLS

Neurovascular links: How do the anterior pituitary cells communicate with the outside world and regulate one another?

1) Communication from the brain: Nerve cells in the hypothalamus (in specific nuclei) produce hormones that regulate each of the anterior pituitary cells. Most of the above cell types have a releasing hormone (named after the cell it stimulates) that is produced in a set of neurons and sent via an axon to the median eminence. There it is released into the pituitary portal blood. Some of the cells have an inhibitory hormone that also regulates secretion. These hormones are stored in granules in the axons until they are released. This is the neurovascular link we talked about earlier. The capillary network in the pituitary stalk provides a broad surface area for the secretion of these hormones. Some hormones may stimulate more than one pituitary cell. Some cells have more than one stimulating hormone. In that case, the axons may store these hormones together so that the cells receive maximal stimulation.

2) Communication from the target cells.. Target cells in the periphery may secrete hormones into the blood stream that will affect the pituitary cells.  This is called "feedback".   If their products stimulate the pituitary cell, they are inducing positive feedback; if they inhibit the pituitary cell, they  induce negative feedback.  

Note: Removal of a target organ (such as the ovary, testes, adrenal, or thyroid) removes a source of feedback and has major effects on the pituitary cells. Often it stimulates both hypertrophy and hyperplasia of specific cell types because the pituitary "thinks" that it needs to give the target organ more stimulation.  Thus, that feedback is very valuable to let the pituitary know how much stimulation the body needs. 

F. ORGANIZATION OF THE PARS NERVOSA

1. How is the posterior lobe organized?  The posterior lobe consists of axons, and axonal endings and glial cells (called pituicytes). In other words, it is basically white matter. The nerve endings store two peptide hormones:

a) oxytocin which is important during uterine contractions (stimulates myometrium) and lactation (stimulates myoepithelial cells)  and

b) antidiuretic hormone (ADH) which stimulates water absorption by the collecting tubules. ADH is also called vasopressin because it is a vasoconstrictor. These hormones are stored in secretory granules in the nerve endings in the posterior lobe and released at the appropriate time into the capillaries.

2) Where are the cell bodies that belong to these axons? These cell bodies are in the paraventricular and supraoptic nuclei in the hypothalamus (which is a brain region on the floor of the third ventricle). Different cells produce oxytocin or vasopressin. Oxytocin and vasopressin are synthesized on the rough endoplasmic reticulum and packaged in granules by the Golgi complex. Then they are sent down the axon which travels to the median eminence. Some of the axons end there. Others go down to the posterior lobe and end on the capillaries in this region. Some endings are so large, they accumulate many storage granules. These endings are called Herring Bodies.

3) How are these cells controlled? Control of release of oxytocin can be at all points in the circuitry, at the site of the cell body in the hypothalamus, at the median eminence, and at the site of the ending in the pars nervosa/posterior lobe. Control can be either via blood supply or via direct nerve fiber communication up in the brain.

G. CYTOLOGY AND HISTOLOGY OF THE ANTERIOR LOBE 

How are the anterior pituitary cells designed to perform their function?  

All of the anterior lobe endocrine cells produce protein/peptide hormones.  What organelles would you expect to find?

All the hormone producing cells in the pituitary contain rough endoplasmic reticulum, a prominent Golgi Complex, and secretory granules in which the hormone is stored. When stimulated, the cells release the granules and  also begin to synthesize more of the hormone.  The cells may be polarized so that the secretory apparatus points towards the side adjacent to the blood vessel.  Many of the cell types produce distinctive granules so that they can be identified at the electron microscopic level by the size and shape of their granules. 

How are the cells organized to produce the different hormones? Based on staining characteristics at the light microscopic level, the cell are divided into 3 classes. Red or orange staining cells are called acidophils, blue or purple cells are basophils and cells with little stain are called chromophobes.   Consult your text for photographs.   

Clinical note: Why is it important to know this?  Pituitary tumors have the same classification and reference is often made to acidophilic adenomas, etc. It is important to recognize that this tinctorial stain helps only to narrow down a potential product from a tumor. A Tumor can actually be multihormonal in that it may be derived from a multipotential stem cell.   

Last update: 12/02/2002
Web address: http://www.cytochemistry.net/Endocrine_System/pituitary.htm
Gwen V. Childs, Ph.D.
University of Arkansas for Medical Sciences
Shorey 9/32; (501) 686-7020
For questions or comments, Email:  childsgwenv@uams.edu