HISTOLOGY BIOL 4000 - LECTURE NOTES 10
ENDOCRINE ORGANS
ENDOCRINE SYSTEM TEXT - POWERPOINT
I. General functions of endocrine organs
A. Endocrine glands are a group of organs that synthesize and release hormones that affect the function of other target organs or tissues in the body.
B. In some cases there are interactions between the hormone and the nervous system.
1. So we can say that the nervous system and the endocrine system are each affected by and modulate the activities of the other.
2. Thus, the nervous and endocrine elements of control are often lumped together under the heading neuroendocrine system.
II. Pituitary gland = Hypophysis - an endocrine organ with obvious neural and non-neural endocrine components
A. Embryological development
1. Non-neural hypophyseal tissue.
a. In mammals and birds it's called Rathke's pocket.
b. Composed of non-neural ectoderm.
c. Cells either
* separate from the ectodermal external surface and migrate through the head mesenchyme to the infundibulum of the diencephalon. These cells come to lie adjacent to floor of infundibulum, or
* the cells of the external ectoderm invaginate to form a tube that extends back to the infundibulum (mammals, birds). The tube eventually separates from the external epithelium.
2. Neural hypophyseal tissue
a. Infundibulum forms as outpocketing of floor of diencephalon that associates with Rathke's pocket.
3. Together these tissues form the hypothesis.
a. Infundibulum forms neurohypophysis
* pars nervosa = posterior lobe
* Continuous with hypothalamus of brain.
* Neurosecretory cells in hypothalamus extend axons through the median eminence and into the pars nervosa where neurosecretory products are released.
b. non-neural hypophyseal tissue (lining of Rathke's pocket in mammals and birds) forms the adenohypophysis or anterior lobe of the pituitary
* This is composed of 3 major parts.
** pars distalis (anterior lobe) - largest component of adenohypophysis
** pars intermedia - lies between neurohypophysis and pars distalis. Separated from pars distalis by "primitive fissure" which is the remainder of the cavity within the invagination that formed Rathke’s pocket.
** pars tuberalis - adenohypophyseal tissues that surround the stalk of the pars nervosa.
B. Caridovascular circulation to the pituitary gland
1. Since these tissues secrete hormonal substances, we would expect a well developed vascular system that would provide a means of carrying these hormones to other parts of the body.
2. Five arteries provide blood to the pituitary gland. these are,
a. 2 superior hypophyseal arteries
b. 2 inferior hypophyseal arteries.
c. the trabecular artery
3. These arteries enter the pars nervosa where they form a capillary mesh in its tissues. Some of the arterial branches entering the pars nervosa form a capillary mesh in the median eminence region.
4. The capillaries merge to form venules and veins
5. In the median eminence region, the venules that are formed extend into the pars tuberalis and then the pars distalis. Venules and veins originating from other capillaries exit the pars nervosa.
6. Those veins that enter the pars distalis form a second capillary mesh (a portal system) within its tissue = hypophyseal portal system
7. These capillaries coalesces to form venules and veins that leave the pars distalis
8. Why might this be? The hypophyseal portal system carries hormones from the median eminence region to the pars distalis where they have an effect on the secretion of hormones by cells in that part of the pituitary gland.
III. Adenohypophysis -Tissue structure and cell types of the pars distalis
A. Has typical appearance of endocrine tissue, that is groups of cells organized into cords or follicles
B. Through this tissue run fenestrated capillaries that are part of hypophyseal portal system.
DO YOU RECALL WHAT THE CHARACTERISTICS OF FENESTRATED CAPILLARIES ARE?
C. Two major classes of cells are present in the pars distalis. Classification is based on staining characteristics.
1. Chromophobes
a. don't pick up stain
b. cytoplasm appears white or clear in stained sectioned material
c. This group of cells includes the
* Follicular cells that form a supporting meshwork for the tissues of the adenohypophysis.
* Secretory cells that are thought to secrete hormones, but that are not well understood at present.
** One type of chromophobe in rats secretes adrenocorticotropic hormone that stimulates the adrenal gland. (However, in humans this hormone is produced by a basophilic chromophil - see below.)
2. Chromophils - these cells pick up acidic or basic stains
a. Basophils
* Gonadotropic cells
** Secrete hormones that affect reproductive organs (follicle stimulating hormone, FSH. leuteinizing hormone, LH. interstitial cell stimulating hormone, ICSH).
*** FSH - glycoprotein that stimulates and supports early growth of follicles in ovary and gametogenesis in testis.
*** LH - glycoprotein that reaches peak during menstrual cycle. 24 hr after peak, ovulation occurs. Can now buy a kit using monoclonal antibodies that allows a women to determine exactly when during her menstrual cycle she ovulates – expensive. In males leuteinizing hormone is called intertitial cell stimulating hormone (ICSH) - stimulates interstitial cells of Leydig in testes to secrete testosterone
** Small round cells with dense, basophilic, secretory granules
** Distributed throughout pars distalis.
** FSH and LH secreting cells test positive with the periodic acid schiff (PAS) stain because the hormones are glycoproteins.
* Thyrotropic cells
** Secrete thyroid stimulating hormone =thyrotropin (TSH)- stimulates synthesis of thyroid hormones.
** Positive PAS because hormone is a glycoprotein.
b. Acidophils
* Somatotropic cells - secrete growth hormone
** Growth hormone = somatotropin (STH)- protein.
*** Most marked affect is on epiphyseal cartilage of bone, hormone acts on liver to cause production of a peptide called somatomedin. This peptide stimulates growth of epiphyseal plate. Lack of this hormone results in hypopituitary dwarfism which can now be treated in some cases with hormone injections. Overproduction of STH can cause gigantism and may eventually result in acromegaly which is expressed as extra growth of bones and extremities (nose, fingers, jaw, etc.) causing deformity and disability.
* mammotropic cells - secrete prolactin
** Protein hormone that triggers secretion of milk by mammary glands.
D. The cells of the pars distalis are caused to secrete various hormones by the action of hormonal factors synthesized in the hypothalamus and secreted in the median eminence. These are called releasing factors, e.g. growth hormone releasing factor (GHRF), gonadotrophic releasing hormone (GnRH).
E. In many cases the hypothalamus, hypophysis, and affected organ work in concert with positive or negative feedback occurring between them.
F. READ ABOUT OTHER PARTS OF THE ADENOHYPOPHYSIS (pars intermedia, pars tuberalis) IN TEXT.
IV. Neurohypophysis -Tissue structure and cell types of the pars nervosa
A. This is the posterior lobe of the hypophysis that is formed from the tissues of the diencephalic infundibulum. These tissues remain attached to the part of the brain that becomes the hypothalamus.
B. The pars nervosa and the stem of the posterior lobe are mainly composed of unmyelinated axonal processes.
1. These axons arise from neuron perikarya that are located in the supraoptic and paraventricular nuclei of the hypothalamus.
2. These hypothalamic cells are neurosecretory neurons that synthesize neurosecretory products such as oxytocin and vasopressin, and the various releasing factors mentioned above. The oxytocin and vasopressin accumulate in the dialated blind endings of these axons that are located in the pars nervosa.
a. Major effect of vasopressin is to increase permiability to water of the tubules of the kidney. This causes a higher rate of re-absorption of water by these tubules and thus concentrates the urine.
b. Oxytocin promotes contraction of the smooth muscle of the uterine wall during copulation and parturition. Also, causes contraction of myoepithelial cells that surround ducts of the mammary glands helping to express the milk as a baby feeds.
* In the case of child birth and intercourse, distension of the vagina excites stretch receptors in the vaginal wall. These cause action potentials to be sent to the CNS where the appropriate neurosecretory cells of the hypothalamus receive nervous stimulation and release oxytocin in the pars nervosa. This hormone enters the circulatory system and is carried to the muscles of the uterine wall where it causes these muscles to contract.
3. So the the hypothalamus is actually a component of the posterior pituitary's endocrine function since the actual hormones are synthsized there and then stored in the blind endings of axons in the pars nervosa.
C. You will also find the cells called pituicytes in the tissues of the pars nervosa. Generally, only the stained nuclei of these cells can be seen in histological sections.
1. These cells are of irregular shape and often have numerous cytoplasmic processes.
2. They are considered to be a type of glial cell, but their function is not well understood.
D. Accumulations of oxytocin or vasopressin in the blind endings of axons in the pars nervosa form the Herring bodiers (corpuscles) seen in this tissue with the light microscope.
E. When appropriate neural stimulation of the neurosecretory cells occurs, the blind endings of the axons release their secretory products into capillaries in the pars nervosa. The hormones are then carried to their point of action in other parts of the body (e.g. oxytocin).
V. Adrenal glands -Paired organs that lie on top of the kidneys, embedded in adipose tissue. The tissues of this organ are sometimes found in other areas of body.
A. General structure – Also see text
1. Covered by capsule of dense collagenous connective tissue.
2. Thin septa or trabeculae extend from the connective tissue capsule into the interior of the gland.
3. Internally there are two major layers called the adrenal cortex and adrenal medulla.
4. The supporting framework of cortex and the medulla are collectively called the stroma. This framework contains many reticular fibers.
B. Embryological development
1. In a sense, the adrenal cortex and medulla may be considered as 2 morphologically distinct endocrine organs.
2. 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.
3. As was the case for the hypophysis, we have a situation where neural and non-neural tissues become associated to form an organ.
a. As an embryo develops, the cortex of the adrenal gland is derived from mesodermal cells in the region of the kidney.
b. 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.
C. Cardiovascular circulation to the adrenal gland
1. Blood is supplied to the adrenal glands by a number of arteries.
2. 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.
3. The capillaries supply blood to a network of sinusoids in the cortex.
4. Some of the arterial branches do not form capillaries in the cortex, but rather run through it to the medulla.
a. These are the medullary arteries.
b. These arteries form a dense capillary network around the cells of the medulla.
5. 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.
6. Capillaries of medulla and cortex coalesce to form the adrenal veins that exit the adrenal glands.
D. Adrenal cortex
1. Composed of 3 layers
a. zona glomerulosa
b. zona fasciculata
c. zona reticularis
2a. Zona glomerulosa - structure
a. Columnar to pyramidal cells that are arranged in folded cords surrounded by capillaries and sinusoids.
b. Cells have spherical nucleus and basophilic granules in cytoplasm.
c. Adjacent to the endothelium of the capillaries and sinusoids there is a space between the endothelium and surrounding cells.
d. The cell surface of the columnar or pyramidal secretory cells adjacent to this space are thrown into folds in this area forming microvilli.
e. Ultrastructure is typical for cells involved in lipid synthesis (see 3. below).
* 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.
2b. Zona glomerulosa - function
a. The zona glomerulosa cells secrete mineralcorticoids, mainly aldosterone which is important in maintaining water balance.
b. These are steroid hormones that are lipid molecules derived from cholesterol.
3a. Zona fasciculata - structure
a. Cells are polyhedral and are arranged in straight cords (columns) that are 1-2 cells thick with capillaries running between.
b. Cells have central nucleus with basophilic cytoplasm. Microvilli are present within the sub-endothelial space (next to the capillaries).
c. 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.
b. Ultrastructure is typical for cells involved in lipid synthesis and secretion (see 2a.-e. above)
* However, these cells have more RER than those of the zona glomerulosa
* This is why the cytoplasm has an overall basophilic affinity.
3b. Zona fasciculata - function
a. Zona fasciculata cells secrete glucocorticoids (important in lipid, protein and carbohydrate metabolism) that are another type of steroid.
4a. Zona reticularis
a. Cells are polyhedral and are arranged in irregular cords with capillaries and sinusoids between.
b. Lipofuscin pigment granules in cells.
c. Cytoplasm acidophilic. Few lipid droplets.
d. Cells secrete glucocorticoids.
5. Secretion of steroids by adrenal cortex is good example of a feedback system between target organ and pituitary gland.
a. 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.
b. the releasing factor is carried by the hypophyseal portal system from the median eminence to the pars distalis.
c. This causes secretion of adrenocorticotropic hormone (ACTH) by certain cells in the pars distalis.
d. ACTH causes an increase in corticosteroid secretion (e.g. cortisol) by adrenal cortex cells.
e. As levels of corticosteroids increase in blood, these inhibit the secretion of releasing factors by the hypothalamus and secretion of ACTH by pituitary.
E. Adrenal medulla
1. 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.
2. These epithelioid secretory cells are considered to be modified postganglionic neurons.
3. Nerve fibers (axons) contact (synapse on?) the epithelioid cells on the part of their surface that is adjacent to a capillary.
a. When stimulated , these nerve fibers release acetylcholine that causes the release of the catecholamine secretory product accumulated in these cells.
b. The secretory product consists of epinephrine or norepinephrine.
c. Two different types of cells, one for each catecholamine.
d. 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.
4. Adrenal medulla secretes continuously into blood stream.
a. The adrenal medulla cells secrete only small amounts of epinephrine and norepinephrine unless stimulated by nervous activity related to emotional reactions.
b. Increased secretion of these substances prepares body to react to stressful situations. Blood vessels constrict, blood pressure rises, etc.
VI. Thyroid
A. Located below the larynx, partially encircling the esophagus.
B. Thyroid is covered by thin layer of loose connective tissue.
1. Associated with this loose connective tissue is a dense network of blood and lymphatic capillaries.
a. Capillaries are fenestrated.
2. Septa extend into organ from the connective tissue capsule.
a. These septa separate follicles (see below) of thyroid tissue from each other.
b. Septa composed mainly of reticular fibers.
C. The thyroid tissue within the connective tissue capsule is composed of follicles, each containing a lumen filled with a gelatinous substance called colloid.
1. Each follicle consists of a simple cuboidal epithelium surrounding the lumen
a. The epithelium changes to simple squamous if the follicle is inactive.
2. Cells of follicles are responsible for synthesis of thyroid hormones, the most abundant of which is thyroxin.
a. 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.)
b. Thyroxine stimulates mitochondrial respiration and oxadative phosphorylation. So, more ATP produced faster.
4. In addition to follicle cells, parafollicular cells (C cells) are found between the follicles.
a. These cells secrete the polypeptide calcitonin that causes a reduction of calcium in the blood by inhibiting the activities of osteoclasts.
3. Control of thyroid hormone secretion:
a. Neurogenic stimuli provided by axons from parasympathetic and sympathetic ganglia can influence metabolism of thyroid cells.
b. However, thyrotropin (thyroid stimulating hormone, TSH) is the major controlling factor.
c. Thyrotropin is synthesized and secreted by cells in the adenohypophysis (pars distalis).
d. 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.
5. Thyroid hormones are synthesized from the colloid.
a. Colloid is composed of iodinated thyroglobulin.
b. When thyroid hormones are required, iodinated thyroglobulin in colloid is endocytosed.
c. Endocytotic vesicles merge with lysosomes.
d. Thyroid hormones are synthesized as a result.
e. This liberates the 4 thyroid hormones into the cytoplasm.
f. These diffuse across the cell membrane into capillaries where blood carries them to target organs.
VII. Parathyroid glands
A. These are small, but important, organs that are embedded in the wall of the thyroid.
1. 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
2. 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)
3. Thus, when the thyroid is removed, it is critical that the parathyroid glands be separated from it and left in the body.
B. There are three cell types in the parathryroid glands
1. Chief cells
a. polygonal with vesicular nucleus
b. slightly acidiphilic, pale staining cytoplasm
c. secrete parathyroid hormone (parathormone).
2. Oxyphil cells
a. Not present in thyroid at birth
b. start appearing in parathyroid tissues at about age 7 in humans
c. no known function
d. Similar to, but larger than chief cells.
3. Adipose cells - increase in number as one grows older.
C. Major function of parathyroid glands
1. 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.
D. So the calcitonin of thyroid parafollicular cells and parathyroid hormone of chief cells balance and regulate calcium levels in the body.