a. Pituitary Gland
The pituitary gland is a small pea-sized lump of tissue connected to the inferior portion of the hypothalamus of the brain. Many blood vessels surround the pituitary gland to carry the hormones it releases throughout the body. Situated in a small depression in the sphenoid bone called the sella turcica, the pituitary gland is actually made of 2 completely separate structures- the posterior and anterior pituitary glands.
Posterior Pituitary: The posterior pituitary gland is actually not glandular tissue at all, but nervous tissue instead. The posterior pituitary is a small extension of the hypothalamus through which the axons of some of the neurosecretory cells of the hypothalamus extend. These neurosecretory cells create 2 hormones in the hypothalamus that are stored and released by the posterior pituitary.
* Oxytocin triggers uterine contractions during childbirth and the release of milk during breastfeeding.
* Antidiuretic hormone (ADH) prevents water loss in the body by increasing the re-uptake of water in the kidneys and reducing blood flow to sweat glands.
Anterior Pituitary: The anterior pituitary gland is the true glandular part of the pituitary gland. The function of the anterior pituitary gland is controlled by the releasing and inhibiting hormones of the hypothalamus. The anterior pituitary produces 6 important hormones.
* Somatotropic Hormone
Somatotropic Hormone is also called growth hormone. Peptide hormone secreted by the anterior lobe of the pituitary gland. It stimulates the growth of essentially all tissues of the body, including bone.
GH is synthesized and secreted by anterior pituitary cells called somatotrophs, which release between one and two milligrams of the hormone each day. GH is vital for normal physical growth in children; its levels rise progressively during childhood and peak during the growth spurt that occurs in puberty.
GH deficiency is one of the many causes of short stature and dwarfism. It results primarily from damage to the hypothalamus or to the pituitary gland during fetal development (congenital GH deficiency) or following birth (acquired GH deficiency). GH deficiency may also be caused by mutations in genes that regulate its synthesis and secretion.
Excess GH production is most often caused by a benign tumour (adenoma) of the somatotroph cells of the pituitary gland. In some cases, a tumour of the lung or of the pancreatic islets of Langerhans produces GHRH, which stimulates the somatotrophs to produce large amounts of GH. In rare cases, ectopic production of GH (production by tumour cells in tissues that do not ordinarily synthesize GH) causes an excess of the hormone. Somatotroph tumours in children are very rare and cause excessive growth that may lead to extreme height (gigantism) and features of acromegaly.
* Thyroid Stimulating Hormone (TSH):
TSH is produced when the hypothalamus releases a substance called thyrotropin-releasing hormone (TRH). TRH then triggers the pituitary gland to release TSH. TSH causes the thyroid gland to make two hormones:
* Triiodothyronine (T3)
* Thyroxine (T4). T3 and T4 help control your body's metabolism.
* Adrenocorticotrophic Hormone (ACTH):
Adrenocorticotropic hormone (ACTH or corticotropin) is a polypeptide hormone synthesised from pre-opiomelanocortin and secreted from corticotropes in the anterior lobe of the pituitary gland in response to the corticotropin-releasing hormone released by the hypothalamus. ACTH consists of 39 amino acids. It stimulates the cortex of the adrenal gland, resulting in the synthesis of corticosteroids- mainly glucocorticoids, but also mineralcorticoids and, to a lesser extent, androgens. ACTH secretion is also modulated by the circadian rhythm.
* Mammotrophic Hormone
Mammotrophic hormone is also called Prolactin or lactogenic hormone. Prolactin, a non-tropic hormone- the word lacto- means milk. Lactose is the sugar in milk.
If a woman is lactating, it means she’s producing milk. Prolactin is the hormone that stimulates the mammary glands to produce milk.
During pregnancy, prolactin stimulates growth of the breast, but high estrogen and progesterone secretion prevent milk production. After delivery, estrogen and progesterone levels drop and prolactin stimulates the secretion of milk by alveolar cells in the breast. Milk-let down or milk ejection is the name for milk squirting out a nipple.
* Follicle Stimulating Hormone (FSH)
Follicle Stimulating Hormone is also called Gametokinetic harmone. Follicle stimulating hormone is one of the hormones essential to pubertal development and the function of women’s ovaries and men’s testes. In women, this hormone stimulates the growth of ovarian follicles in the ovary before the release of an egg from one follicle at ovulation. It also increases oestradiol production. In men, follicle stimulating hormone acts on the Sertoli cells of the testes to stimulate sperm production (spermatogenesis).
The production and release of follicle stimulating hormone is regulated by the levels of a number of circulating hormones released by the ovaries and testes. This system is called the hypothalamic pituitary gonadal axis.
In women, when hormone levels fall towards the end of the menstrual cycle, this is sensed by nerve cells in the hypothalamus. These cells produce more gonadotrophin-releasing hormone which in turn stimulates the pituitary gland to produce more follicle stimulating hormone and luteinising hormone and release these into the bloodstream.
* Luteinizing Hormone (LH):
Luteinizing Hormone is a gonadotrophic hormone produced and released by cells in the anterior pituitary gland. It is crucial in regulating the function of the testes in men and ovaries in women.
In men, luteinising hormone stimulates Leydig cells in the testes to produce testosterone, which acts locally to support sperm production. Testosterone also exerts effects all around the body to generate male characteristics such as increased muscle mass, enlargement of the larynx to generate a deep voice and the growth of facial and body hair.
In women, luteinising hormone carries out different roles in the two halves of the menstrual cycle. In weeks one to two of the cycle, luteinising hormone is required to stimulate the ovarian follicles in the ovary to produce the female sex hormone, oestradiol.
Around day 14 of the cycle, a surge in luteinising hormone levels causes the ovarian follicle to tear and release a mature oocyte (egg) from the ovary, a process called ovulation. For the remainder of the cycle (weeks three to four), the remnants of the ovarian follicle form a corpus luteum. Luteinising hormone stimulates the corpus luteum to produce progesterone which is required to support the early stages of pregnancy, if fertilization occurs.
* Oxytocin:
Oxytocin is a mammalian neurohypophysial hormone. Produced in the supraoptic and paraventricular nuclei of the hypothalamus by nerve axons, and stored in the posterior pituitary gland, oxytocin acts primarily as a neuromodulator in the brain.
Oxytocin plays an important role in the neuroanatomy of intimacy, specifically in sexual reproduction of both sexes, in particular during and after childbirth. It is released in large amounts after distension of the cervix and uterus during labor, facilitating birth, maternal bonding, and, after stimulation of the nipples, lactation.
Oxytocin is a peptide of nine amino acids. Its systematic name is cysteinetyrosine- isoleucine-glutamine-asparagine-cysteine-proline-leucine-glycine-amide.
* Vasopressin:
Vasopressin is also called Anti Diuretic Harmone (ADH). Vasopressin is a peptide hormone formed in the hypothalamus, then transported via and released from, the posterior pituitary into the blood.
The primary function of AVP in the body is to regulate extracellular fluid volume by affecting renal handling of water, although it is also a vasoconstrictor and pressor agent (hence, the name "vasopressin"). AVP acts on renal collecting ducts via V2 receptors to increase water permeability, which leads to decreased urine formation (hence, the antidiuretic action of "antidiuretic hormone"). This increases blood volume, cardiac output and arterial pressure.
A secondary function of AVP is vasoconstriction. AVP binds to V1 receptors on vascular smooth muscle to cause vasoconstriction through the IP3 signal transduction pathway and Rhokinase pathway, which increases arterial pressure. However, the normal physiological concentrations of AVP are below its vasoactive range. Studies have shown, nevertheless, that in severe hypovolemic shock, when AVP release is very high, AVP does contribute to the compensatory increase in systemic vascular resistance.
b. Hypothalamus
Hypothalamus is the link between endocrine and nervous system. The portion of the brain that maintains the body’s internal balances (homeostasis). The hypothalamus produces releasing and inhibiting hormones, which stop and start the production of other hormones throughout the body.
The hypothalamus is a section of the brain responsible for the production of many of the body’s essential hormones, chemical substances that help control different cells and organs. The hormones from the hypothalamus govern physiologic functions such as temperature regulation, thirst, hunger, sleep, mood, sex drive, and the release of other hormones within the body. This area of the brain houses the pituitary gland and other glands in the body.
Although this portion of the brain is small in size, it is involved in many necessary processes of the body including behavioral, autonomic (involuntary or unconscious), and endocrine functions, such as metabolism and growth and development.
The hypothalamus' primary function is homeostasis, which is to maintain the body's status quo system-wide. Hypothalamic hormones include thyrotropin-releasing, gonadotropin-releasing, growth hormone-releasing, corticotrophin-releasing, somatostatin, and dopamine hormones. These hormones release into the blood via the capillaries and travel to the pituitary gland. Oxytocin and vasopressin are also hypothalamic hormones.
The hypothalamus uses a set-point to regulate the body's systems, including electrolyte and fluid balance, body temperature, blood pressure, and body weight. It receives inputs from the body then makes the proper changes if anything differentiates from this set-point. The set-point can temporarily change, but remains remarkably fixed from day-to-day.
Pineal Gland
The pineal gland (or pineal body) is an important endocrine gland. It is a small, oval structure descending from the roof of the diencephalon, a section of the brain that relays sensory information between the brain's different regions. Although it's very tinyonly about six millimeters long-the pineal gland produces several important hormones.The most significant of these is melatonin, a hormone which regulates the circadian rhythm, or sleep cycle.
In some of the lower vertebrates this gland grows into an eyelike structure; in others, although it isn't a fully developed eye, it is still able to act as a light receptor. Because of this, the pineal gland is also known as the third eye.
Along with secreting melatonin, the pineal gland also regulates other endocrine functions and converts signals from the nervous system into endocrine signals. Melatonin production (or the lack of it) can contribute to a person feeling awake or becoming sleepy, and the pineal gland's endocrine function regulation can also influence sexual development.
Functions of Pineal Gland are
* Secretion of the Hormone Melatonin
* Regulation of Endocrine Functions
* Conversion of Nervous System Signals to Endocrine Signals
* Causes Feeling of Sleepiness
* Influences Sexual Development
c. Thyroid Gland
The thyroid gland covers the windpipe from three sides. Two hormones of the thyroid gland, T3 (thyroxine) and T4 (triiodothyronine), help the body to produce and regulate the hormones adrenaline (also called epinephrine) and dopamine. Hormones are chemical substances that help control certain cells and organs. Adrenaline and dopamine are active in many physical and emotional responses, including fear, excitement, and pleasure. Other hormones from this gland also help regulate metabolism, which is the process by which calories and oxygen are converted into energy.
Without a functioning thyroid, the body would not be able to break down proteins and it would not be able to process carbohydrates and vitamins. For this reason, problems with this gland can lead to uncontrollable weight gain. For many people, these irregularities can be controlled through medications, as well as a modification of their diet.
However, there is one other controlling factor. The gland cannot produce hormones on its own. It needs the assistance of the pituitary gland, which creates thyroid stimulating hormone (TSH).
As a result, a nonfunctional pituitary gland will eventually lead to thyroid-gland-related issues. TSH will either trigger the production of thyroxine or triiodothyronine. If TSH is not present at the right levels, too much or too little of either hormone will be made.
d. Parathyroid Glands
Parathyroid glands are small glands of the endocrine system which are located in the neck behind the thyroid. Parathyroid glands control the calcium in our bodies how much calcium is in our bones, and how much calcium is in our blood. Calcium is the most important element in our bodies, so calcium is regulated very carefully. Parathyroid glands control the calcium.
Parathyroid glands (we all have 4 of them) are normally the size of a grain of rice. Occasionally they can be as large as a pea and still be normal. The parathyroid glands share a similar blood supply, venous drainage, and lymphatic drainage to the thyroid glands. The parathyroid glands are derived from the epithelial lining of the third and fourth branchial pouches, with the superior glands arising from the fourth pouch, and the inferior glands arising from the higher third pouch. The relative position of the inferior and superior glands, which are named according to their final location, changes because of the migration of embryological tissues.
The parathyroid glands are two pairs of glands usually positioned behind the left and right lobes of the thyroid. Each gland is a yellowish-brown flat ovoid that resembles a lentil seed, usually about 6 mm long and 3 to 4 mm wide, and 1 to 2 mm anteroposteriorly. There are typically four parathyroid glands. The two parathyroid glands on each side which are positioned higher are called the superior parathyroid glands, while the lower two are called the inferior parathyroid glands. Healthy parathyroid glands generally weigh about 30 mg in men and 35 mg in women. These glands are not visible or able to be felt during examination of the neck.
e. Adrenal Glands
The adrenal glands are two glands that sit on top of kidneys that are made up of two distinct parts. The adrenal glands are two, triangular shaped organs that measure about 1.5 inches in height and 3 inches in length. They are located on top of each kidney.
The adrenal cortex the outer part of the gland produces hormones that are vital to life, such as cortisol (which helps regulate metabolism and helps your body respond to stress) and aldosterone (which helps control blood pressure).
There is a third class of hormone released by the adrenal cortex, known as sex steroids or sex hormones. The adrenal cortex releases small amounts of male and female sex hormones. However, their impact is usually overshadowed by the greater amounts of hormones (such as estrogen and testosterone) released by the ovaries or testes.
The adrenal medulla the inner part of the gland produces nonessential (that is, you don’t need them to live) hormones, such as adrenaline (which helps your body react to stress).
The adrenal cortex and the adrenal medulla have very different functions. One of the main distinctions between them is that the hormones released by the adrenal cortex are necessary for life those secreted by the adrenal medulla are not.
Hormones secreted by adrenal medulla are-
Epinephrine: Most people know epinephrine by its other name adrenaline. This hormone rapidly responds to stress by increasing your heart rate and rushing blood to the muscles and brain. It also spikes your blood sugar level by helping convert glycogen to glucose in the liver. (Glycogen is the liver’s storage form of glucose.)
Norepinephrine: Also known as noradrenaline, this hormone works with epinephrine in responding to stress. However, it can cause vasoconstriction (the narrowing of blood vessels). This results in high blood pressure.
f. Pancreas Gland
The pancreas is really two glands that are intimately mixed together into one organ. The bulk of the pancreas is composed of “exocrine” cells that produce enzymes to help with the digestion of food. These exocrine cells release their enzymes into a series of progressively larger tubes (called ducts) that eventually join together to form the main pancreatic duct. The main pancreatic duct runs the length of the pancreas and drains the fluid produced by the exocrine cells into the duodenum, the first part of the small bowel.
The second functional component of the pancreas is the "endocrine" pancreas. The endocrine pancreas is composed of small islands of cells, called the islets of Langerhans.
These endocrine cells don’t release their secretions into the pancreatic ducts, instead they release hormones, such as insulin and glucagon, into the blood stream, and these hormones in turn help control blood sugar (glucose) levels.
First, as they grow large tumors of the pancreas will interfere with both of these important bodily functions. When tumors block the exocrine system, patients can develop pancreatitis and pain from the abnormal release of digestive enzymes into the substance of the pancreas instead of into the bowel, and they can develop digestive problems, such as diarrhea, from the incomplete digestion of food. When tumors destroy the endocrine function of the pancreas, patients can develop sugar diabetes.
The second reason that the two functional components of the pancreas are important to understand is that tumors can arise in either component. The vast majority of tumors of the pancreas arise in the exocrine part and these cancers look like pancreatic ducts under the microscope. These tumors are therefore called "ductal adenocarcinomas," or simply "adenocarcinoma," or even more simply "pancreatic cancer." Less commonly, tumors arise from the endocrine component of the pancreas and these endocrine tumors are called "pancreatic neuroendocrine tumors," or "islet cell tumors" for short.
g. Gonads
The gonads develop from three sources- the mesothelium (coelomic epithelium) lining the posterior abdominal wall, the underlying mesenchyme (intermediate mesoderm), and the primordial germ cells. The mesothelium proliferates to form the genital ridge, a bulge of tissue medial to the mesonephros. From this epithelum primary sex cords penetrate the mesenchyme. The indifferent gonad now consists of a medulla and cortex. In XX embryos the ovary will originate from the cortex and the medulla will decline. In the XY embryo the medulla will develop into the testes and the cortex regress.
Male Gonad:
Male gonad regulated by SRY protein. The primary sex cords enter the medulla and differentiate into the seminiferous cords. These are the precursors to the seminiferous tubules where sperm will be produced. The parts of the primary sex cords that extend deepest into the medulla form the rete testes, the first in a series of structures by which sperm leave the testes in adulthood. As the seminiferous tubules are forming, the PGC enter the gonad and associate with the tubules. The PGC will give rise to sperm (after puberty), the cords give rise to the “sustentacular cells” of the tubules, the Sertoli cells.
In the presence of Sertoli cells, the germ cells remain in meiotic arrest and are inactive spermatogonia until puberty. This phenomenon is due to the secretion by the Sertoli cells of antimullerian hormone.
Female gonad
The ovary develops more slowly than the testes. Although the primary sex cords enter the genitial ridge at the same time the ovary cannot be distinguished histologically until the 10 - 11th week. The primary sex cords degenerate and the secondary sex cords (also called cortical cords) extend from the surface epithelium (mesothelium). As these cords increase in size the PGC are incorporated into them.
At about 16 weeks these cords break up into isolated clusters called primordial follicles. Each follicle consists of an oogonium (from the PGC) and a single layer of flattened cells (follicular cells) derived from the cords. Oogonia undergo a period of rapid proliferation. In the absence of SRY and AMH, PGC undergo the first prophase of meiosis.
Chromosomal Sex:
The discovery of sex chromosomes was first discovered in 1923 in insects. The relationship of the human X and Y chromosomes to genital differentiation was not made until 1959. Karyotype analysis of Turner’s syndrome (XO = female) and Klinefelter’s syndrome (XXY = male) patients provided the necessary data on the role of the chromosomes in gonadal differentiation with the female being XX and the male having XY. These genetic factors initially exert their influence upon the bipotential gonad.
Male gonads (Testes) secrete Testosterone which maintains male characteristics. Female gonads (ovaries) secrete Estrogen which maintains female characteristics. Female gonads secrete progesterone which is important for plantation of embryonic in Uterus.
h. Thymus Gland
The thymus gland, despite containing glandular tissue and producing several hormones, is much more closely associated with the immune system than with the endocrine system. The thymus serves a vital role in the training and development of T-lymphocytes or T cells, an extremely important type of white blood cell. T cells defend the body from potentially deadly pathogens such as bacteria, viruses and fungi.
The thymus is a soft, roughly triangular organ located in the mediastinum of the thoracic cavity anterior and superior to the heart and posterior to the sternum. It has two distinct but identical lobes that are each surrounded by a tough, fibrous capsule. Within each lobe is a superficial region of tissue called the cortex and a histologically distinct deep region called medulla. Epithelial tissues and lymphatic tissues containing dendritic cells and macrophages make up the majority of both regions of the thymus.
The function of the thymus is to receive immature T cells that are produced in the red bone marrow and train them into functional, mature T cells that attack only foreign cells. T cells first reside within the cortex of the thymus where they come in contact with epithelial cells presenting various antigens. The immature T cells that respond to the antigens corresponding to foreign cells are selected to survive, mature, and migrate to the medulla while the rest die via apoptosis and are cleaned up by macrophages. This process is known as positive selection.
