(Transcribed from Dr. Kalliecharan’s lecture, 28 Mar 2000 by Brian Buschman)
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Hormones function to regulate:
1) Homeostasis
2) Metabolic activities
3) Growth and development
4) Reproduction
5) Immune system
6) Smooth and cardiac muscle
The hormone glands can be described as:
The pituitary gland functions in two independent systems.
1) The hypothalamo-hypophyseal system which involves the hypothalamus which secretes releasing hormones into the first capillary bed of a portal system and the pituitary which is the destination for those hormones via the second capillary bed. The releasing hormones stimulate the pituitary to release all sorts of hormones that act on the body is various ways.
2) The posterior pituitary is made of neuroendocrine cells that have vesicles called Herring bodies which contain and secrete hormones under nervous control.
The pituitary is broken into two major parts, the anterior (called the adenohypophysis) and the posterior (called the neurohypophysis). The adenohypophysis arises from ectoderm from the roof plate of the mouth and the neurohypophysis from neuroendoderm. The adenohypophysis is then divided into two main regions, the pars distalis anteriorly and the pars tuberalis posteriorly.
The pars distalis has two types of cells, chromophobes and chromophils based on if they absorb the stain or not. The chromophils are divided into basophils and acidophils on which stain they absorb. Chromophobes are probably undifferentiated cells. The basophile include:
|
Cell
Type |
Produces |
Stimulated
by |
|
Corticotrophs |
ACTH, MSH |
CRH |
|
Gonadotrophs |
LH, FSH |
GnRH |
The acidophils are:
|
Cell
Type |
Produces |
Stimulated
by |
Inhibited
by |
|
Somatotrophs |
GH |
GRH |
Somatostatin |
|
Lactotrophs |
Prolactin |
|
Prolactin inhibiting hormone |
|
Thyrotrophs |
TSH |
TRH |
Somatostatin |
The pars tuberalis has basophilic cells that secrete FSH and LH. Right behind it is the pars intermedia which has no function in humans.
The neurohypophysis has no secretory cells, per say, but does have unmyelinated axons that are secretory neurons. They contain Herring bodies which release oxytocin to cause the contraction of uterine smooth muscle and the let-down reflex. They also release vasopressin (ADH) which increases water permeability of the kidneys for extra filtration.
The whole secretory process of the pituitary is regulated by a negative feedback system. When the blood level of the desired products rises less of the releasing hormone is secreted.
One example is when the cellular levels of T3 and T4 rise the hypothalamus secretes less TRH. Lower levels of TRH lead to lower levels of TSH secretion and therefore less T3 and T4 are produced.
In the case of thyroid hormones dwarfism is caused by hyposecretion of TRH and gigantism by hypersecretion. The improper levels of TRH is most likely due to an improper feedback mechanism.
The thyroid gland is located just below the larynx and has multiple lobes. Histologically you find thyroid follicles made of two cell types:
1) Follicular cells that produce T3 and T4.
2) Parafollicular cells that secrete calcitonin. Calcitonin acts to inhibit osteoclast which leads to absorption of Ca2+ into bones.
The feedback system works so that a high level of blood Ca2+ will lead to inhibition of PTH secretion and increase of calcitonin secretion.
Follicular cells transport I2 into the cells and use peroxidase from their plasma membrane to oxidize it to a form that binds to tyrosine. The tyrosine is taken up into the cell by pinocytosis. It will bind to give T1, T2, T3 or T4. They will combine if small to give T3 (triiodothyrosine) or T4 (thyroxine). All parts of the thyroxine can be reused within the cell.
T3 and T4 are useful in the regulation of the basal metabolic rate, cellular metabolism and shortages can cause mental retardation due to early development.
The size of the thyroid gland is proportional to hormones secreted and the amount of tyrosine taken up. If there is a shortage of I2 then there will be a lack of production of T3 and T4. Since the levels of these are low more TRH will be released leading to more TSH. TSH leads to an increase of the tyrosine taken into the cells. If there is not the I2 to form the hormone the tyrosine builds up and gives a goiter.
The parathyroid glands are located posteriorly to the thyroid in two lobes. Functionally they have two cell types:
1) Oxyphil cells that have an unknown function.
2) Principle cells which secrete PTH. PTH stimulates osteoblasts to secrete osteoclast stimulating factor and they differentiate into osteoclasts. Osteoclasts function to
a. Reabsorb bone to increase blood Ca2+ and PO4 levels.
b. PO4 is removed by the kidneys.
c. Reabsorb more Mg2+ and Ca2+ in the gut and kidneys which is mediated in the gut by calcitriol (the active form of vitamin D).
Hyposecretion of PTH in fetal life leads to cretinism which is a dwarfism and mental retardation. This is because the extra osteoclast activity will break down too much bone during the developmental time period.
Hypersecretion if PTH is called Grave’s disease which is an autoimmune disease that is related to goiter (enlarged thyroid).
The adrenal glands are located around the superior pole of the kidneys. Exteriorly they have a capsule and functionally have a cortex and a medulla.
The blood supply is by three arteries, the superior suprarenal artery from the inferior phrenic, the middle suprarenal from the aorta, and the inferior suprarenal from the renal. Venous drainage is into the IVC on the right and into the left renal vein on the left. The arteries, once inside the adrenals, branch into capsular, cortical and medullary arteries. The capsular arteries supply the outer capsule. The cortical arteries drain into capillaries that run through the cortex and continue into the medulla. The medullary arteries pass through the cortex and do not branch into capillaries until reaching the medulla.
The cortex is composed of three cell layers that each have a unique arrangement and secretory product.
The most superficial is the zona glomerulosa which releases aldesterone in response to stimulation by angiotensin II. Aldesterone increases filtration and excretion in the kidney. The cells of the zona glomerulosa are arranged into circular bunches.
The next layer is the zona faculata where glucocortacoids (cortisol, cortisone and corticosterone) are released in response to stimulation by corticotrophin. The glucocortacoids regulate metabolism. The cells appear as columns of cells with capillaries on at least one side.
The deepest layer of the adrenal cortex is the zona reticularis which produces androgen in response to corticotrophin stimulation. The androgen, dehydroepiandrosterone, produces a neglible effect because it is very weak in comparison to testicular androgens. The cells are arranged in a random fashion in the zona reticularis.
The hormones are steroidal so they freely cross membranes. That means that they are not secreted into vesicles but are synthesized as needed and they just diffuse across the plasma membrane.
The medulla is composed of cells called chromaffin cells that can be found in two forms:
1) NE secreting cells are chromaffin cells that are supplied by capillaries from medullary arteries.
2) Epi secreting cells are chromaffin cells that are supplied by capillaries that just drained through the cortex from cortical arteries.
The medullary cells secrete amino acid derived hormones so they are put into vesicles and released by exocytosis as needed.
The pineal gland is a small gland attached to the roof of the third ventricle. It contains astrocytes and pinealocytes. The pinealocytes receive SSA input from the optic tract and secrete melatonin to regulate the body’s diurnal cycles.
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