Endocrine System of Mammals and their Function---- Hormones?

 

Mammals have more complex endocrine system than other animals. Which consist of endocrine gland, organs, hormones and other target tissue.

A brief overview of mammalian endocrinology follows.

Pituitary Gland

The pituitary gland also called as the hypophysis which is directly present below the hypothalamus (see figure). The pituitary has two seperate lobes: the anterior lobe that is called adenohypophysis and the posterior lobe is called neurohypophysis.

 The two lobes differ in some ways:

(1) The adenohypophysis is larger than the neurohypophysis.

 (2)  Pituicytes is the secretery cells which are present in the adenohypophysis, but not present in the neurohypophysis.

(3) The neurohypophysis much more supply of nerve endings. Pituicytes also produce and secrete hormones from the adenohypophysis, whereas the neurohypophysis obtains their hormones from the neurosecretory cells that is present in the hypothalamus and storing and releasing them when they are needed. These hypothalamic nerve cells inter their axons to nerve cells and than blood vessels, into the pituitary gland, that directly linking the nervous and endocrine systems called the infundibulum.

The pituitary of many vertebrates instead of the humans, birds, and cetaceans also has a functional intermediate lobe (pars intermedia) that is mostly glandular tissue. It secrete hormone such as melanophore-stimulating hormone that have function to changes in the coloration of the body surface of many animals.

Hormones of the Neurohypophysis

The neurohypophysis obtain hormone from neurosecretory cells of the hypothalamus that synthesize and secrete two hormones, antidiuretic hormone and oxytocin. Where they are stored in the axon terminals until released.
Oxytocin have the function to mammalian reproduction by its effect on smooth muscle. It  also stimulates contraction of the uterus during pregnancy.

Hormones of the Adenohypophysis

The major endocrine portion of the pituitary is the adenohypophysis, which synthesizes six different hormones. All of these hormones are
polypeptides. The two hormones are nontropic which are growth hormone and prolactin.
Growth hormone (GH), or somatotropin (STH), does notinfluence a particular target tissue and affects all parts of the
body.
Prolactin (PRL) has the range of actions of the adenohypophyseal hormones. It plays an essential role in reproduction.
Thyrotropin, or thyroid-stimulating hormone (TSH), stimulates the thyroid gland’s synthesis and secretion of thyroxine, the main thyroid hormone.

Adrenocorticotropic hormone (ACTH) stimulates the adrenal gland to produce and secrete steroid hormones called glucocorticoids.

Thyroid Gland

The thyroid gland present in the neck, that is anterior to the trachea. These gland release thyroxine and triiodothyronine, both of which influence  growth, development, and metabolic rates. Thyroid gland also secerate, calcitonin hormone, that maintance levels of calcium ions (Ca2) by promoting the deposition of these ions into bone tissue when their concentrations rise. When calcium ions returns to its homeostatic concentration than thyroid cells decrease the secretion of calcitonin hormone.

Parathyroid Glands

The parathyroid glands are tiny, pea-sized glands that embedded in the thyroid lobes, each lobes have two glands. The parathyroids secrete parathormone (PTH), which regulates the concentrations of calcium (Ca2) and phosphate (HPO24) ions in the blood.
When the calcium concentration in the blood is low, PTH secretion increases and has the following effects: It stimulates bone cells to break down bone tissue and release calcium ions into the blood. It also enhances calcium absorption from the small intestine into the blood. Finally, PTH promotes calcium reabsorption by the kidney tubules to decrease the amount of calcium excreted in the urine.

Pancreas

The pancreas is an elongated, fleshy organ that is present posterior to the stomach. It functions both as an exocrine gland to secrete digestive enzymes and as an endocrine (ductless) gland.

 The endocrine portion of the pancreas contain about 1% of the gland. This endocrine portion synthesize, stores, and secretes hormones, in the form of cells cluster called pancreatic islets.
The pancreas contains 200,000 to 2,000,000 pancreatic islets. Each islet contains four special groups of cells, called alpha (), beta (), delta (), and F cells. The alpha cells produce the hormone glucagon, and beta cells produce insulin. The somatostatin hormone, hypothalamic growth secrete from the delta the hypothalamic growth-hormone inhibiting factor that also inhibits glucagon and insulin secretion. F cells release a pancreatic polypeptide that is released into the bloodstream when meal and inhibits somatostatin secretion, gallbladder contraction, and the secretion of pancreatic digestive enzymes.

When glucose concentrations in the blood is increase, such as after a meal, beta cells secrete insulin. Insulin increase the level of glucose into the body’s cells, including liver cells, where excess glucose can be converted to glycogen. Insulin and glucagon are crucial to the regulation blood glucose concentrations. When the blood glucose concentration is low, alpha cells secrete glucagon. Glucagon breakdown of glycogen into glucose units, which are than released into the bloodstream which raise the blood glucose concentration to the homeostatic level.

Gonads

The gonads contain ovaries and testes that secrete hormones that help regulate reproductive functions.  Male have the testes that secrete testosterone, which secrete luteinizing and follicle-stimulating hormones that produce by the adenohypophysis to stimulate spermatogenesis. Testosterone perform following function that growth and maintenance of the male sex organs, promotes the development and maintenance the level of sexual behavior, and in male, stimulates the growth of facial and pubic hair, and  enlargement of the larynx, which deepens the voice. The testes also produce inhibin, which inhibits the secretion of FSH.

Female produce thee following hormones that is necessary for female reproductive functions.

 Estrogens regulate the menstrual and estrus cycles and the development of the mammary glands and other female secondary sexual characteristics.

 The progestins also regulate the menstrual and estrus cycles, and the development of the mammary glands, that help in placenta formation during pregnancy.

 Relaxin, which is produced in small quantities, softens the opening of the uterus (cervix) at the time of delivery.

 The ovaries also produce inhibin, which inhibits the secretion of FSH.
Thymus

The thymus gland is present near the heart. It is large in young birds and mammals, but diminishes in size throughout adulthood.  Thymus glands produces peptide hormones, it contain thymopoietin (TP) and alpha1 and beta4 thymosin, that is essential for the normal development of the immune system.

Adrenal Glands
In mammals, two adrenal glands are present in the top of kidneys. Each gland contain two separate glandular tissues. The inner portion is the medulla, and the outer portion, that is surrounding the medulla, is the cortex.
Adrenal Cortex  

The adrenal cortex secretes three steroid hormones: glucocorticoids, mineralocorticoids (aldosterone), and sex hormones such as,androgens and estrogens. The glucocorticoids, such as cortisol, regulate metabolism and the concentration of blood sugar.

 Aldosterone increase the rate of sodium reabsorption in the kidneys and, thus, water reabsorption, so, it plays a role to maintain the homeostasis of extracellular fluid.

 The sex hormones have a slight effect on male and female gonads. These sex hormones consist mainly of weak male hormones called androgens and lesser amounts of female hormones called estrogens.

Adrenal Medulla

 The adrenal medulla is under neural control. It contains neurosecretory cells that secrete  two hormone epinephrine and norepinephrine, which control heart rate and carbohydrate metabolism.
During times of excitement, emergency, or stress, the adrenal medulla contributes to the overall mobilization of the body through the sympathetic nervous system.