The endocrine system is made up of hormone-producing glands within the body which facilitate communication between cells. Glands that make up the endocrine system include the hypothalamus, pituitary gland, and pineal gland, all of which can be found within the brain; the thyroid and parathyroid glands which can be found in the neck; the thymus which is situated between the lungs; the adrenals, which sit on the kidneys; the pancreas, which is found behind the stomach; and the ovaries (women) or testes (men) which are in the pelvic region.
Within the endocrine system, an endocrine gland or tissue releases an amount of hormone, which amount is determined by the body’s need for that hormone. Through sensing and signalling systems, hormone-producing cells receive information and regulate hormone release amount and duration. Released hormones are carried by the blood to target cells, which contain receptors that bind the hormone, leading to the desired effect. This effect is then recognised by secretory cells through a feedback signal. Once the required hormonal effect is fully accomplished, the hormones are either removed by the liver or the kidneys, or else degraded by the target cells.
Hormonal secretion is regulated by negative feedback control so homeostasis within the body is maintained.
The Hypothalamus
The hypothalamus, which is located below the thalamus, acts as a link between the nervous system and the endocrine system. It receives inputs from various parts of the brain, and sensory signals from internal organs and the retina. Changes are triggered in the hypothalamic activity due to pain, stress, and other emotional factors. The hypothalamus controls the autonomic nervous system and regulates various bodily factors such as temperature, hunger and thirst, sexual behaviour, and defensive reactions.
Within the hypothalamus are clusters of specialised neurons – neurosecretory cells, which synthesise the hypothalamic hormones in their cell body. The hormones are transported inside vesicles by axonal transport.
Hypothalamus-Released Hormones
The hypothalamus is an important endocrine gland that produces hormones which, after being released into the blood, travel in the portal veins to a secondary capillary bed found in the anterior lobe of the pituitary, where their effects are produced. Hormones released in this way include:
- Thyrotropin-releasing hormone (TRH) – related to thyroid gland growth and function
- Gonadotropin-releasing hormone (GnRH) – related to the reproductive system
- Growth hormone-releasing hormone (GHRH) – related to growth
- Corticotropin-releasing hormone (CRH) – related to hormone secretion
- Somatostatin – related to the growth hormone
- Dopamine – acts as a neurotransmitter
Hormones which travel in the neurons to the posterior lobe of the pituitary before being released into circulation include:
- Antidiuretic Hormone (ADH) / Vasopressin – promotes regulation of the amount of water within the body
- Oxytocin – involved in childbirth and breastfeeding
The Pituitary Gland
The pituitary gland, which measures just about 1.3cm in diameter, is located in the cella turcica of the sphenoid bone. It is attached to the hypothalamus via the infundibulum – a stalklike structure. Pituitary gland hormones regulate body activities. The pituitary gland is divided into two lobes: the anterior lobe and the posterior lobe.
The pituitary gland anterior lobe accounts to around 80% of the pituitary gland. It is involved in growth regulation, metabolism, and reproduction, through its produced hormones. Hormone production happens through stimulation or inhibition by chemical messages originating from the hypothalamus. Thus, hypothalamic hormones act as a link between the nervous system and the endocrine system. They reach the anterior pituitary through the Hypophyseal Portal System.
The pituitary gland posterior lobe is involved in hormone transmission. Hormones originating from neurons within the region of the hypothalamus are secreted directly into peripheral circulation.
The lobes are divided by the pars intermedia – a relatively avascular zone.
The 5 Types of Glandular Cells
- Somatotroph Cells – produce GH (growth hormone) which is responsible for general body growth
- Lactotroph Cells – synthesise PRL (prolactin) which promotes milk production by the mammary glands
- Corticolipothroph Cells – synthesise ACTH (adrenocorticotropic hormone) which stimulates hormone secretion, and MSH (melanocyte-stimulating hormone) which is responsible for skin pigmentation
- Thyrothroph Cells – produce TSH (thyroid-stimulating hormone), which controls the thyroid gland
- Gonadotroph Cells – produce FSH (follicle-stimulating hormone), which stimulates egg and sperm production in the ovaries and testes, and LH (luteinizing hormone), which stimulates other sexual and reproductive activities.
Growth Hormone (GH)
- is released through two regulating factors from the hypothalamus, namely GHRF (growth hormone releasing factor) and GHIF (growth hormone inhibiting factor) or Somatostatin
- causes cells to grow and multiply by increasing the rate at which amino acids enter the cells to be built up into proteins
- acts on the skeleton and the skeletal muscles firstly by increasing their growth rate, and then maintaining their size when growth is attained
- increases the rate of protein synthesis a.k.a. protein anabolism
- promotes fat catabolism by causing cells to change from burning carbohydrates to burning fats to produce energy
- accelerates rate at which glycogen stored within the liver converts into glucose and releases itself into the blood
- converts other factors into growth-promoting substances – somatomedins and insulin-like growth factors (IGF), both of which are similar to insulin yet more potent than insulin
Growth Hormone Secretion Stimuli and Inhibition…
Prolactin (PRL)
- requires priming of the mammary glands through oestrogens, progesterone, corticosteroids, growth hormone, thyroxine, and insulin
- initiates and maintains milk secretion by the mammary glands (amount of milk is determined by oxytocin)
- has an inhibitory and an excitatory negative control system
- level rises during pregnancy, falls right after delivery, and rises again during breastfeeding, which is why in the 1st two days following birth, mothers do not produce milk but colostrum
NOTE: women on oral contraceptives may experience lack of milk production due to their hormonal effect.
Melanocyte-Stimulating Hormone (MSH)
- increases skin pigmentation through the stimulation of melanin granules dispersion in melanocytes
- secretion is stimulated by the melanocyte-stimulating hormone releasing factor (MRF), or inhibited by the melanocyte-stimulating hormone inhibiting factor (MIF)
- lack causes the skin to look pallid
- excess causes the skin to look dark
Thyroid-stimulating factor (TSH)
- stimulates the synthesis and secretion of hormonal production within the thyroid gland
- secretion is controlled by the thyrotropin releasing factor (TRF), which is released based on thyroxine blood level, metabolic rate of the body, and other factors through a negative feedback system
Adrenocorticotropic Hormone (ACTH)
- controls the production and secretion of some adrenal cortex hormones
- is secreted by the hypothalamic regulating factor called corticotropin releasing factor (CRF), which is released depending on stimuli and hormones through a negative feedback system
Follicle-Stimulating Hormone (FSH)
- in females initiates the development of an ova every month, and stimulates cells within the ovaries to secrete oestrogens
- in males stimulates the testes to produce sperm
- secretion depends on the hypothalamic regulating factor gonadotropin releasing factor (GnRF), which is released in response to oestrogens in females, and to testosterone in males through a negative feedback system
Luteinizing Hormone (LH)
- along with oestrogens, in females it stimulates the release of an ovum within the ovary, prepares the uterus for the implantation of the fertilised ovum, stimulates the formation of the corpus luteum in the ovary to secrete progesterone, and prepares the mammary glands for milk secretion
- in males it stimulates the interstitial endocrinocytes in the testes to develop and secrete testosterone
- secretion is controlled by GnRF, which works through a negative feedback system
Pituitary Gland Posterior Lobe
The posterior lobe of the pituitary gland a.k.a. neurohypophysis, does not synthesise hormones. It releases hormones to the circulation via the posterior hypophyseal veins to be distributed to target cells in other tissues. The cell bodies of the neurosecretory cells produce Oxytocin (OT) and Antidiuretic Hormone (ADH) / Vasopressin.
Oxytocin (OT)
- is released in high amounts just before birth
- stimulates contraction of smooth muscle cells in the pregnant uterus
- stimulates the contractile cells around the mammary gland ducts
- affects milk ejection
- works through a positive feedback cycle which is broken following birthing
- is inhibited by progesterone, but can work in conjunction to oestrogens
Antidiuretic hormone (ADH)
- affects urine volume; it causes the kidneys to excrete water from fresh urine and return it to the bloodstream, reducing urine volume
- absence causes an increase in urine output
- raises blood pressure by constricting arterioles
- secretion varies based on the body’s needs
- causes a decrease in sweat
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