09 Control and Coordination - part 16 - Major endocrine glands

 

09 Control and Coordination - part 16 - Major endocrine glands

Major endocrine glands :
A. Hypothalamus :

• It is ectodermal in origin. 
• Located at the floor of diencephalon. 
• Major function : Maintain homeostasis.
• Controls the secretory activity of pituitary gland by the release and inhibiting hormones. 
• All hormones of hypothalamus are peptide hormones. 
• They are secreted by the neurosecretory cells so they are called neurohormones. 
• Hormones secreted by hypothalamus are : ADH, Oxytocin.

1. Adrenocorticotropin Releasing Hormone : 

• It stimulates the release of ACTH by the anterior pituitary gland.

2. Thyrotropin Releasing Hormone : 

• Stimulates the release of TSH by anterior pituitary gland.

3. Gonadotropin Releasing Hormone (GnRH) : 

• It stimulates pituitary to secrete gonadotropins.

4. Prolactin Inhibiting Hormone (Prolactostatin) : 

• It inhibits prolactin release by anterior piturary gland.

5. Somatostatin : 

• It inhibits the release of growth hormone.

6. Somatotropin : 

• stimulates release of growth hormone.

7. Gastrin Releasing Peptide (GRP) and
8. Gastric Inhibitory Polypeptide (GIP).

B. Pituitary gland or hypophysis gland :

• Smallest gland.
• Pea sized reddish-grey coloured gland. 
• Controls almost all other endocrine glands, hence earlier it was called the master endocrine gland. 
• Located just below the hypothalamus and is attached to it by a stalk called infundibulum or hypophyseal stalk. 
• Remains lodged in a bony depression called sella turcica of the sphenoid bone. 
• Consists of two lobes -

1. Anterior lobe (Adenohypophysis) and
2. Posterior lobe (Neurohypophysis). 

• Both the lobes develop from different parts of embryo. Hence it has of dual origin.
• Adenohypophysis  is an outgrowth from the roof of buccal cavity. This outgrowth is called Rathke’s pouch. 

• Grows upward towards the brain. 
• Neurohypophysis grows as a downward extension of hypothalamus. 
• Two outgrowths together form the pituitary gland. 
• Connection of Rathke’s pouch with pituitray gland is lost in embryo. 
• Intermediate lobe (Pars intermedia) is a small reduced part lying in the cleft between the anterior and posterior lob.
• Neurohypophysis is connected directly to the hypothalamus by axon fibres.
• Adenohypophysis and intermediate lobes are connected to the hypothalamus through hypophyseal portal system.

Hypophyseal portal system :

• Various hormones secreted by hypothalamus reach the pituitary gland through the hypophyseal portal system. 
• Portal vein collects blood from various parts of hypothalamus and opens into anterior lobe of pituitary. 
• From pituitary, the vein finally carries the blood into the superior vena cava.

Adenohypophysis: 

• Larger lobe of pituitary gland. 
• Highly cellular and vascular part of pituitary gland. 
• Contains various types of epitheloid secretory cells, acidophils, basophils, chromatophores. 
• Differentiated into three parts – 

1. Pars distalis
2. Pars intermedia and 
3. Pars tuberalis. 

Hormones of adenohypophysis :

1. Somatotropin /Somatotropic Hormone / STH / Growth hormone / GH :

• Stimulates growth and development of all tissues by accelerating protein synthesis and cell division. 
• Highest secretion the GH is seen till puberty and then its secretion of becomes low. 
• Continuously secreted through out life for repair and replacement of body tissue or cells.
• Improper secretion of GH produces various disorders. 
• Hyposecretion  : in childhood results in stunted physical growth [pituitary dwarfism.]
• Hypersecretion : in childhood causes Gigantism [a condition of overgrowth ]. 
• The individual attains abnormal height. 
• Hypersecretion :in middle aged adults, it results in  Acromegaly. [disproportionate growth causing disfigurement and enlargement of bones of nose, lower jaw, hands, fingers and feet.]

2. Thyrotropin / Thyroid stimulating Hormone / TSH : 

• Its primary action is to stimulate the thyroid gland secretion of the hormone thyroxine.

3. Adreno corticotropic hormone / ACTH / Adrenocorticotropin : 

• Stimulates adrenal cortex to produce and secrete its hormones. 
• Maintains functioning of adrenal cortex.

4. Prolactin / Luteotropin / Mammotropin:

• Unique among pituitary hormones as it is under predominant inhibitory control from hypothalamus.
• Activates growth of breasts during pregnancy (mammotropin) and stimulates the milk production and secretion of milk (lactogenic) by mammary gland after child birth.

5. Gonadotropin :
a. Follicle stimulating hormone/ FSH :

• Stimulates growth of ovarian follicles in the females.
• In males, it is concerned with the development of seminiferous tubules.

b. Leutinizing Hormone/ LH :

• In female, It helps in ovulation (discharge of ovum from graafian follicle). 
• FSH and LH are responsible for stimulation of ovaries to produce oestrogen while LH induces the ruptured follicles to develop into corpus luteum and for production of progesterone.

c. Interstitial cell stimulating hormone / ICSH :

• In males, it stimulates the testes to produce the androgen called testosterone. 
• Testosterone is responsible for development of secondary sexual characters.

Neurohypophysis: 

• Differentiated into three parts

1. Pars nervosa/ neural lobe 
2. Infundibulum 
3. Median eminence.

1. Pars nervosa :

• Acts as storage area for the secretions of hypothalamus. 
• Stores and releases oxytocin and vasopressin.

1. Oxytocin : 

• Stimulates contraction of uterus during parturition. 
• Stimulates the contraction of mammary glands to initiate ejection or release of milk. So is called birth hormone or milk ejecting hormone.

2. Antiduretic Hormone (ADH)/ Vasopressin:

• Stimulates the re-absorption of water in distal convoluted tubule and collecting ducts of uriniferous tubules of the kidneys. 
• Decreases loss of water by reducing the urine quantity.
• Increases blood pressure by causing vaso constriction.
• Deficiency of ADH reduces water reabsorption and increases urine output. This condition is called diabetes insipidus 
• No glucose is lost in the urine. 
• Excessive micturition causes excessive thirst. This condition is called polydipsia.

2. Pars intermedia : 

• Poorly developed in human beings. 
• Secretes Melanocyte Stimulating Hormone (MSH) in some lower vertebrates.
• MSH stimulates the dispersion of melanin granules in melanocytes and is responsible for skin pigmentation.

Pineal gland : 

• Given off from the roof of diencephalon.
• Located between the two cerebral hemispheres. 
• Sensitive to the biochemical signals of light. 
• Secretes a hormone called melatonin also known as sleep hormone.
• Melatonin is derived from tryptophan and plays a very important role in the regulation of Biological Clock (e.g. 24 hour diurnal rhythm) of our body. 
• Helps in maintaining the normal rhythm of sleep-wake cycle .
• Also influences body temperature, metabolism and reproductive cycles.

Thyroid gland :

• Largest endocrine gland. 
• Location : in front of the trachea just below the larynx. 
• It is richly supplied with blood vessels. 
• Two lobes of thyroid gland are connected a non-secretory band called isthmus. 
• Thyroid lobes are composed of rounded follicles held together by interfollicular connective tissue called stroma. 
• Stroma contains blood capillaries and small group of parafollicular cell or ‘C’ cells (clear cells).
• Thyroid follicles are composed of cuboidal epithelium resting on a basement membrane and is filled with a gelatinous colloid.
• Thyroid gland is stimulated to secrete its hormones by thyroid stimulating hormone (TSH). 
• Two hormones secreted by the follicular cells are - 
  

1. Thyroxine/tetra iodothyronine/ T4 (four atoms of iodine) and
2. Triiodothyronine or T3 (three atoms of iodine).

• Thyroxine is synthesized by attaching iodine to amino acid tyrosine by enzymatic action.
• The amino acid tyrosine molecule binds to iodine to produce Monoiodotyronine (T1) or 2 atoms of iodine to produce Diiodothyronine (T2). 
• T1 and T2 molecules bind end to end to make colloidal mass inside the follicle.
• They are further metabolised to prepare T3 and T4. 
• Triidothyronine or T3 is also secreted in small quantity. 
• It is physiologically more active. 
• Thyroid gland is the only gland that stores its hormones. 
• T3 and T4 hormones are stored before secretion and are regulated by thyrotropin of pituitary gland by negative feed back mechanism.
• Thyroxine regulates the basal metabolic rate of body. 
• Regulates metabolism by stimulating protein synthesis and promotes growth of body tissues. 
• Helps in thermoregulation by increasing heat production. 
• Increases action of neuro transmitters- adrenaline and nor adrenaline. 
• Supports the process of RBC production and maintenance of water and electrolyte balance. 
• Regulates reproductive cycles in females.
• Parafollicular cells or ‘C’ cells produce calcitonin hormone, which regulates calcium metabolism.

Disorders related to thyroid gland:
a. Hyperthyroidism :

• Caused by increase in the levels of thyroid hormones. 
• This increases metabolic rate, sensitivity, sweating, flushing, rapid respiration, bulging of eye balls, and affects various physiological activities.

Grave’s disease (Exopthalmic goitre) :

• Hyperthyrodism in adults, is characterised by protruding eyeballs, increased BMR and weight loss. 
• Increased BMR produces a range of effect like increased heart beat, increased BP, higher body temperature, nervousness, irritability, tremor of fingers and bulging eyeballs.

b. Hypothyroidism : 

• Caused by deficiency of thyroid hormones or removal of thyroid gland (Thyroidectomy).

Cretinism : 

• Hyposecretion in infants leads to cretinism. 
• A cretin has reduced BMR and low oxidation. 
• They are short statured because the skeleton fails to grow. 
• They are mentally retarded. 
• Show dry skin, thick tongue, prolonged neonatal jaundice, lethargy and constipation. 
• Can be treated by early administration of thyroid hormones. 
• The cretin shows stunted physical growth, delayed puberity and mental retardation.

Myxoedema :

• It is the deficiency of thyroid hormones in adults. 
• It is characterised by a peculiar thickening and puffiness of skin and subcutaneous tissue particularly of the face and extremities. 
• Patient lacks alertness, intelligence.
• Suffers from slow heart rate, low B.P., always feeling cold, low body temperature and retarded sexual development.

Simple goitre (Iodine deficiency goitre)  :

• Iodine is needed for synthesis of thyroid hormone. 
• If there is deficiency of iodine in the diet, it causes enlargement of thyroid gland leading to simple goitre. 
• This disease is common in hilly areas.
• Addition of iodine to table salt prevents this disease. 
• Size of the thyroid gland is increased but total output of thyroxine is decreased.

Calcitonin : 

• Secreted by the ‘C’ cells. 
• Regulates the concentration of calcium and phosphorus in the blood. 
• It is under feedback control of plasma calcium concentration in plasma. 
• Secreted when concentration of calcium rises in the blood. 
• It lowers concentration of calcium and phosphorus in the plasma by decreasing their release from the bones and accelerating the uptake of calcium and phosphorous by the bones.

Parathyroid gland :

• Situated on the posterior surface of the lobes of thyroid gland.
• Parathyroids are four in number and named as superior and inferior parathyroid glands. 
• The cells of parathyroid glands are arranged in a compact mass.
• Secrete a peptide hormone called parathormone (PTH). It is also called Collip’s hormone. 
• It regulates calcium and phosphate balance between blood and other tissues. 
• Release of parathormone increases blood calcium level. 
• It draws calcium from bones increases calcium absorption in the digestive tract and reduces loss of calcium in the urine. 
• Secretion of parathormone is under feedback control of blood calcium level.
• Concentration of calcium and phosphate is maintained by parathormone and calcitonin.
• These two hormones form an antagonistic pair like insulin and glucagon.
• Hyposecretion of parathormone lowers concentration of calcium in the blood. 
• This increases excitability of nerves and muscles causing muscle twitch and spasm. This is called parathyroid tetany or hypocalcaemic tetany.
• Hypersecretion of parathormone is responsible for more absorption of calcium from bones i.e., demineralization of bones resulting in softening, bending and fracture of bone. This is called osteoporosis. 
• It is common in women who have reached menopause.

Thymus gland :

• Located in the upper part of thorax on the dorsal side of the heart.
• Is a Soft, pinkish, bilobed mass of lymphoid tissue. 
• It is Prominent gland at birth but gets gradually atrophied in the adult, so it is called temporary gland
• Secretes the hormone thymosin. 
• Important role: development of immune system by maturation of T-lymphocytes.
• Also promotes production of antibodies by providing humoral immunity.

G. Adrenal gland/Suprarenal gland :

• Have dual origin from mesoderm and ectoderm. 
• Located on the upper border of each kidney. 
• Are small, conical yellowish glands and show two distinct regions- 

1. Outer cortex and 
2. Inner medulla.

1. Adrenal cortex :

• Derived from embryonic mesoderm. 
• Secretes many hormones together called corticoids. 
• It is differentiated into three concentric regions - 

1. Outer thin zona glomerulosa
2. Middle thick zona fasciculata
3. Inner thin zona reticularis

1. Outer thin zona glomerulosa : 

• Secretes Mineralocorticods. 
• Released for regulating sodium and potassium ion concentration. 
• Regulate - 

1. salt-water balance
2. blood volume and
3. blood pressure.

• Aldosterone (salt retaining hormone) is the main mineralocorticoid. 
• It balances Na-K levels.

2. Middle thick zona fasciculata : 

• Responsible for secretion of Glucocorticoids like cortisol. 
• Regulates metabolism of carbohydrates, proteins and lipids. 
• Cortisol is an important glucocorticoid. 
• Responsible for increase in blood glucose level. 
• Also immuno suppressive. 
• It suppresses synthesis of antibodies. So it is used in treatment of allergy.
• It prepares animals to face emergencies in nature.

3. Inner thin zona reticularis : 

• Responsible for production of sex corticoids (Gonadocorticoids). 
• In males, they have a role in development and maintenance of external sex characters. 
• Excess sex corticoids in female causes adrenal virilism and hirsutism (excess hair on face) while in males it causes gynaecomastia i.e. enlarged breast. 
• Androgens and estradiols are the produced by the adrenal cortex.

Disorders related to Adrenal cortex :
a. Hyposecretion : [mineralocorticoids and glucocorticoids ]

• Responsible for Addison’s disease. 
• Characteristic features :low blood sugar, low Na+ and high K+ concentration in plasma, increased loss of Na+ and water in urine. 
• It leads to weight loss, weakness, nausea, vomiting and diarrhoea.

b. Hyper secretion of glucocorticoids :

• produces Cushing’s disease. 
• It leads to high blood sugar level, excretion of glucose in urine, rise Na+ in blood volume, high blood pressure, obesity and wasting of limb muscles.

2. Adrenal medulla : 

• Develops from ectoderm. 
• Secretes two hormones adrenaline (epinephrine) and nor adrenaline (norepinephrine). 
• Adrenaline is also known as emergency hormone, also called 3F hormone – (fight, flight and fright). 
• Noradrenaline regulates the blood pressure under normal condition. It also acts as vasoconstrictor.

Pancreas :

• Develops from endoderm. 
• It is both exocrine and endocrine gland. 
• Endocrine cells of pancreas form groups of cells called Islets of Langehans.
• There are four kinds of cells in islets of Langerhans which secrete hormones.

I. Alpha (a) cells (20%)  :

• Secrete glucagon.
• Stimulates liver for glucogenolysis to increase blood glucose level.

II. Beta (b) cells (70%) :

• Secrete insulin.
• Stimulates liver and muscles for glycogenesis. 
• This lowers blood glucose level.

III. Delta (d) cell (5%)  :

• Secrete somatostatin which inhibits the secretion of glucagon and insulin.
• Also decreases the gastric secretions, motility and absorption in digestive tract.

IV. PP cells or F cells (5%) :

• Secrete pancreatic polypeptide (PP). 
• Inhibits the release of pancreatic juice.

Disorder related to pancreas :
Diabetes mellitus (Hyperglycemia) :

• Most common metabolic endocrine disorder of pancreas. 
• It leads to increase in blood glucose level. 
• This is due to under activity of Beta cells, which results in reduced secretion of insulin. 
• In children, such a condition is called insulin dependent diabetes mellitus/ Type I (IDDM) 
• The other form of diabetes is Non insulin dependent diabetes mellitus/ Type II (NIDDM). 
• It is caused due to failure of insulin to facilitate the movement of glucose into cells. 
• Reduced sensitivity to insulin is called insulin resistance.
• In both disorders, blood glucose level increases. 
• Some of the glucose is excreted in urine. It also causes excessive urination and dehydration of body tissues. 
• Degradation of fats increases formation of ketone bodies (ketosis). 
• Administration of insulin lowers blood glucose level.

I. Gonads :
Gonads are sex organs (the testes and the ovaries) :

Ovaries : 
1. Estrogen : 

• Secreted by developing follicle. 
• Estradiol is the main oestrogen. 
• Responsible for secondary sexual characters in female.

2. Progesterone : 

• Secreted by corpus luteum of the ovary after ovulation. 
• Essential for thickening of uterine endometrium, thus preparing the uterus for implantation of fertilized ovum. 
• Responsible for development of mammary glands during pregnancy.
• Inhibits uterine contractions during pregnancy.

3. Relaxin : 

• Secreted by the corpus luteum of the ovary at the end of gestation period.
• Relaxes the cervix of the pregnant female and ligaments of pelvic girdle for easy birth of young one.

4. Inhibin : 

• Secreted by the corpus luteum. 
• Inhibin inhibits the FSH and GnRH production.

Testes :Testes secrete male sex hormones called androgens such as testosterone.
Testosterone : 

• Secreted from interstitial cells or Leydig cells by the influence of luteinising hormone (LH). 
• Rise in testosterone level in blood above normal inhibits LH secretion.
• Responsible for appearance of secondary sexual characters such as facial and pubic hair, deepening of voice, broadening of shoulders, male aggressiveness, etc. 
• Also helps in maintenance of testes.

Diffuse endocrine glands
Placenta :

• It is the intimate connection between foetus and uterine wall of the mother for physiological exchange of the material. 
• Temporary endocrine gland.
• During pregnancy, placenta secretes hormones such as - 

1. Estrogen
2. Progesterone
3. hCG (Human Chorionic Gonadotropin) and 
4. human placental progesterone. 

• These hormones check the contraction of uterine muscles.
• Also maintain the thickness of uterine endometrium thus they help to maintain pregnancy.

Gastro intestinal tract :

• In the gastrointestinal mucosa, certain cells are endocrine in function. 
• Produce hormones which play vital role in digestive processes and flow of digestive juices.

1. Gastrin : 

• Stimulates gastric glands to produce gastric juice.

2. Secretin : 

• Responsible for secretion of pancreatic juice and bile from presence and liver.

3. Cholecystokinin CCK/ Pancreozymin PZ : 

• Stimulates the pancreas to release its enzymes and also stimulates gall bladder to release bile.

4. Entero-gastrone / Gastric inhibitory peptide (GIP) : 

• It slows gastric contractions and inhibits the secretion of gastric juice.

Kidney : 

• It produces renin, erythropoietin and calcitriol (calcitriol is the active form of vitamin cholecalciferol (D3)).

Heart : 

• Atrial natriuretic hormone /ANF.
• Increases sodium excretion by kidneys and reduces blood pressure.

Hormone therapy/ HT : 

• Hormone therapy is the use of hormones in medical treatment. 
• HT is applied in Pregnancy, Menopause, Osteoprosis, Growth hormone deficiency, Insulin Resistance, Cancer, etc.