THE ENDOCRINE SYSTEM
SARATZIS THANASSIS
PART 1
INTRODUCTION
The endocrine system is a collection of ductless glands that secrete chemical messages, known as hormones.
The role of the endocrine system is to maintain homeostasis and long-term control of the human body using chemical signals (the hormones). Also, the endocrine system works in parallel with the nervous system to control growth and maturation along with body homeostasis.
The hormones produced by the glands of the endocrine system are passed through the blood circulation to arrive at a target organ, which possesses a series of cells that bear an appropriate hormone receptor. This receptor binds with the hormone molecule and triggers a series of chemical reactions inside the cell.
As mentioned before, the endocrine system is constituted by the endocrine glands – which secrete hormones.
The major human endocrine glands include:
§ the hypothalamus
§ the pituitary gland
§ the thyroid gland
§ the pancreas
§ the adrenal glands
§ the gonads or sex organs
The endocrine system in females and males
HORMONES
A hormone is a messenger molecule synthesized and secreted by a group of specialized cells that constitute an endocrine gland. These glands are ductless, which means that their secretions (hormones) are released directly into the bloodstream and travel elsewhere in the body to target organs, upon which they act. Note that this is in contrast to the exocrine glands, which have ducts for releasing the substances that they produce. Exocrine glands (not part of the endocrine system) secrete products that are passed outside the body. Sweat glands and salivary glands are examples of exocrine glands.
There are three general groups of hormones. These are classified as follows – according to their chemical structure:
· Steroid hormones including prostaglandins which function especially in a variety of female functions and the sex hormones all of which are lipids made from cholesterol.
· Amino acid derivatives (like epinephrine) which are derived from amino acids, especially tyrosine.
· Peptide hormones (like insulin) which are the most numerous/diverse group of hormones.
Mechanisms of Hormone Action
Hormones trigger actions in specific target cells, after binding to an appropriate receptor. Receptors are membrane proteins that bind to hormones. A certain hormone receptor located on a specific target cell can only bind to one type of hormone. More than fifty human hormones have been identified; all act by binding to receptor molecules. The binding hormone causes a change in the shape of the receptor. This alteration of the receptor’s molecule causes the cell to respond to the hormone.
There are two different mechanisms of hormone action on all target cells:
Steroid and non – steroid hormones
§ Nonsteroid Hormones
Nonsteroid hormones (water soluble) do not enter the cell but bind to plasma membrane receptors, generating a chemical signal (second messenger) inside the target cell. Five different second messenger chemicals, including cyclic AMP have been identified. Second messengers activate other intracellular chemicals to produce the target cell response.
§ Steroid Hormones
The second mechanism involves steroid hormones, which pass through the plasma membrane and act in a two step process. Steroid hormones bind, once inside the cell, to the nuclear membrane receptors, producing an activated hormone-receptor complex. The activated hormone-receptor complex binds to DNA and activates specific genes, increasing production of proteins.
Maintenance of the hormones homeostasis in the blood
When hormone levels reach a certain normal or necessary amount, further secretion is controlled by important body mechanisms to maintain that level of hormone in the blood. This regulation of hormone secretion may involve the hormone itself or another substance in the blood related to the hormone. For example, if the thyroid gland has secreted adequate amounts of thyroid hormones (such as thyroxin) into the blood, the pituitary gland senses the normal levels of thyroid hormone in the bloodstream and gears down its release of thyrotropin, the pituitary hormone that stimulates the thyroid gland to produce thyroid hormones. Parathyroid hormone increases the level of calcium in the blood. When the blood calcium level rises, the parathyroid glands sense the change and decrease their secretion of parathyroid hormone. This turn-off process is called "negative feedback" system.
Negative feedback in the thyroxin release reflex
GLANDS OF THE ENDOCRINE SYSTEM
1. HYPOTHALAMUS – PITUITARY GLAND
The pituitary gland (often called the master gland) is located in a bony cavity at the base of the brain. A stalk links the pituitary to the hypothalamus, which controls release of pituitary hormones. The pituitary gland has two lobes: the anterior and posterior lobes. The anterior pituitary is glandular.
The hypothalamus contains neurons that control the releases from the anterior pituitary, through seven hypothalamic hormones which are released into a portal system connecting the hypothalamus and pituitary. These hormones cause targets in the pituitary to release various hormones.
Growth hormone (GH) is a peptide anterior pituitary hormone essential for growth. The hypothalamus maintains homeostatic levels of GH, by the release of GH-releasing and GH-inhibiting hormones. GH-releasing hormone stimulates release of GH. GH-inhibiting hormone suppresses the release of GH. Cells under the action of GH increase in size (hypertrophy) and number (hyperplasia). GH also causes increase in bone length and thickness by deposition of cartilage at the ends of bones. During adolescence, sex hormones cause replacement of cartilage by bone, halting further bone growth even though GH is still present. Too little or two much GH can cause dwarfism or gigantism, respectively.
Gonadotropins and prolactin are also secreted by the anterior pituitary. Gonadotropins (which include follicle-stimulating hormone, FSH, and luteinizing hormone, LH) affect the gonads by stimulating gamete formation and production of sex hormones. Prolactin is secreted near the end of pregnancy and prepares the breasts for milk production.
The posterior pituitary stores and releases hormones into the blood. Antidiuretic hormone (ADH) and oxytocin are produced in the hypothalamus and transported by axons to the posterior pituitary where they are dumped into the blood. ADH controls water balance in the body and blood pressure. Oxytocin is a small peptide hormone that stimulates uterine contractions during childbirth
Hypothalamus receptors also monitor blood levels of thyroid hormones. Low blood levels of Thyroid-stimulating hormone (TSH) cause the release of TSH-releasing hormone from the hypothalamus, which in turn causes the release of TSH from the anterior pituitary. TSH travels to the thyroid where it promotes production of thyroid hormones, which in turn regulate metabolic rates and body temperatures.
.
2.
THYROID GLAND
Image from: F. Netter Atlas of Anatomy – Ciba Geigy
The thyroid gland is located in the front of the neck, below the larynx. The small, two-inch gland consists of two lobes, one on each side of the windpipe, connected by tissue called the isthmus.
The microscopic structure of the thyroid is quite distinctive. Thyroid epithelial cells - the cells responsible for synthesis of thyroid hormones - are arranged in spheres called thyroid follicles. Follicles are filled with colloid, a proteinaceous depot of thyroid hormone precursor. In the low (left) and high-magnification (right) images of thyroid below, follicles are cut in cross section at different levels, appearing as roughly circular forms of varying size.
In addition to thyroid epithelial cells, the thyroid gland houses one other important endocrine cell. Nestled in spaces between thyroid follicles are parafollicular or C cells, which secrete the hormone calcitonin.
Most of the thyroid tissue consists of the follicular cells, which secrete iodine-containing hormones called thyroxine (T4) and triiodothyronine (T3). The parafollicular cells secrete the hormone calcitonin. Iodine is essential in order for the thyroid to produce the hormones.
Thyroid hormones are poorly soluble in water, and more than 99% of the T3 and T4 circulating in blood is bound to carrier proteins. The principle carrier of thyroid hormones is thyroxine-binding globulin, a glycoprotein synthesized in the liver. Two other carriers of import are transthyrein and albumin. Carrier proteins allow maintenance of a stable pool of thyroid hormones from which the active, free hormones are released for uptake by target cells.
The thyroid hormones play an important role in regulating the body's metabolism and calcium balance. The T4 and T3 hormones stimulate every tissue in the body to produce proteins and increase the amount of oxygen used by cells. The calcitonin hormone works together with the parathyroid hormone to regulate calcium levels in the body. Levels of hormones secreted by the thyroid are controlled by the pituitary gland's thyroid-stimulating hormone, which in turn is controlled by the hypothalamus.
3. PANCREAS
The pancreas serves two principal functions. Firstly, it acts as a ducted gland, secreting digestive enzymes into the small intestine. It also serves as a ductless gland in that the islets of Langerhans secrete insulin and glucagon to regulate the blood glucose level. The islet cells secrete glucagon, which causes the liver to catabolise stored carbohydrates in order to raise a low glucose sugar level. The islet cells also secrete insulin which causes the liver to take excess glucose out of circulation to lower a high blood glucose level.
Insulin is required by almost all of the body's cells, but its major targets are liver cells, fat cells and muscle cells. Insulin serves the following actions:
· Stimulates liver and muscle cells to store glucose in glycogen
· Stimulates fat cells to form fats from fatty acids and glycerol
· Stimulates liver and muscle cells to make proteins from amino acids
· Inhibits the liver and kidney cells from making glucose from intermediate compounds of metabolic pathways (gluconeogenesis)
As shown in this diagram, insulin and glucagon have an opposite effect on liver and other tissues for controlling blood-glucose levels. Together, they regulate the amount of glucose that circulates in the blood.
Image from: http://www.howstuffworks.com/diabetes1.htm
4. ADRENAL GLANDS
The adrenal glands are situated on top of the kidneys. They consist of the outer cortex and the inner medulla. The medulla secretes epinephrine (also known as adrenaline) and other similar hormones in response to stressors such as fright, anger, caffeine, or low blood sugar. The cortex secretes several classes of steroid hormones (glucocorticoids and mineralocorticoids). Despite their organization into a single gland, the medulla and cortex are functionally different endocrine organs, and have different embryological origins.
Cells in the adrenal medulla synthesize and secrete norepinephrine and epinephrine. Following release into blood, these hormones bind adrenergic receptors on target cells, where they induce essentially the same effects as direct sympathetic nervous stimulation.
The adrenal cortex is a factory for steroid hormones. In total, at least two to three dozen different steroids are synthesized and secreted from this tissue, but two classes are of particular importance:Class of Steroid / Major Representative / Physiologic Effects
Mineralocorticoids / Aldosterone / Na+, K+ and water homeostasis
Glucocorticoids / Cortisol / Glucose homeostasis and many others
Additionally, the adrenal cortex produces some sex steroids, particularly androgens.
Like all steroids, adrenal "corticosteroids" are synthesized from cholesterol.
The adrenal cortex and medulla
Image from: http://a248.e.akamai.net
5. GONADS (SEX ORGANS)
The gonads include the female ovaries and the male testes. In addition to producing gametes, the female ovaries and male testes also secrete a number of hormones (sex hormones)
Testes
Male sex hormones, as a group, are called androgens. The principal androgen is testosterone, which is secreted by the testes. A small amount is also produced by the adrenal cortex. Production of testosterone begins during fetal development, continues for a short time after birth, nearly ceases during childhood, and then resumes at puberty. This steroid hormone is responsible for:· The growth and development of the male reproductive structures
· Increased skeletal and muscular growth
· Enlargement of the larynx accompanied by voice changes
· Growth and distribution of body hair
· Increased male sexual drive
Testosterone secretion is regulated by a negative feedback system that involves releasing hormones from the hypothalamus and gonadotropins from the anterior pituitary.
Ovaries
Two groups of female sex hormones are produced in the ovaries, the estrogens and progesterone. These steroid hormones contribute to the development and function of the female reproductive organs and sex characteristics. At the onset of puberty, estrogens promotes:· The development of the breasts
· Distribution of fat evidenced in the hips, legs, and breast
· Maturation of reproductive organs such as the uterus and vagina
Progesterone causes the uterine lining to thicken in preparation for pregnancy. Together, progesterone and estrogens are responsible for the changes that occur in the uterus during the female menstrual cycle.
INTERNET LINKS
GENERAL
· http://www.martindalecenter.com/Medical1_1_EaGa.html#Endo - general information on endocrinology
· http://training.seer.cancer.gov/module_anatomy/unit6_3_endo_glnds5_gonads.html - a tour of the endocrine system
· http://www.innerbody.com/image/endoov.html
· http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookENDOCR.html
THYROID GLAND
· http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/thyroid/ - general information and graphics concerning the thyroid gland
· http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Thyroid.html - thyroid hormones and diseases of the thyroid
· http://www.endocrineweb.com/thyroid.html - thyroid gland diseases
· http://www.bartleby.com/107/272.html - anatomy of the thyroid
· http://www.endocrineweb.com/tests.html - common tests to examine thyroid gland function
PANCREAS
· http://www.howstuffworks.com/diabetes1.htm - general information concerning the pancreas and its main functions
· http://w3.uniroma1.it/step/chir/h.html - anatomical views of the pancreas
· http://members.aol.com/OhLarry922/home.html - information on pancreas transplant
· http://www.endocrineweb.com/pancreas.html - pancreas hormones
ADRENAL GLANDS
· http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/adrenal/ - general information and illustrations on the adrenal glands