The Endocrine Glands II

 

Table of Content

Pancreas Testis Ovary Mechanism of Hormone Action Difference between Nervous and Endocrine Coordination Difference between Hormone and Enzymes Number of Hormones Secreted by Different Endocrine Glands Anterior Pituitary Hormones Posterior Pituitary Hormones Hormones, Their Chemical Nature and Functions Disease Caused by Hormonal Irregularities

The endocrine' pancreas consists of 'Islets of Langerhans'. There are about 1 to 2 million Islets of Langerhans in a normal human pancreas representing only 1 to 2 per cent of the pancreatic tissue. The two main types of cells in the Islet of Langerhans are called a-cells and b-cells. The a-­cells secrete a hormone called glucagon, while the b-cells secrete insulin.

Glucagon is a peptide hormone, and plays an important role in maintaining the normal blood glucose levels. Glucagon acts mainly on the liver cells (hepatocytes) and stimulates glycogenolysis resulting ill an increased blood sugar (hyperglycemia).

Glucagon reduces the cellular glucose uptake and utilisation, Thus, glucagon is a hyperglycemic hormone.

Insulin is a peptide hormone, which plays a major role in the regulation of glucose homeostasis. Insulin acts mainly on hepatocytes and adipocytes (cells of adipose tissue), and enhances cellular glucose uptake and utilisation.

Insulin also stimulates conversion of glucose to glycogen (glycogenesis) in the target cells.

Prolonged hyperglycemia leads to a complex disorder called diabetes mellitus which is associated with loss of glucose through urine and formation of harmful compounds known as ketone bodies.

Testis

A pair of testis is present in the scrotal sac (outside abdomen) of male individuals.

Testis performs dual functions as a primary sex organ as well as an endocrine gland.

Testis is composed of seminiferous tubules and stromal or interstitial tissue. The Leydig cells or interstitial cells, which are present in the intertubular spaces produce a group of hormones called androgens mainly testosterone. 

Androgens regulate the development, maturatior and functions of the male accessory sex organs like epididymis, vas deferens, seminal vesicles, prostate gland, urethra etc. These hormones stimulate muscular growth, growth of facial and axillary hair, aggressiveness, low pitch of voice etc. Androgeru playa major stimulatory role in the process of spermatogenesis (formation of spermatozoa), Androgens act on the central neural system and influence the male sexual behaviour (libido). These hormones produce anabolic (synthetic) effects , on protein and carbohydrate metabolism. 
 

Ovary

Females have a pair of ovaries located in the abdomen.

Ovary is the primary female sex organ which produces one ovum during each menstrual cycle. In addition, ovary also produces two groups of steroid hormones called estrogen and progesterone. 

Ovary is composed of ovarian folliclos and stromal tissues.

The estrogen is synthesised and secreted mainly by the growing ovarian follicles, After ovulation, the ruptured follicle is converted to a structure called corpus luteum, which secretes mainly progesterone.

Estrogens produce wide ranging actions such as stimulation of growth and activities of female secondary 'sex organs, development of growing ovarian follicles, appearance of female secondary sex characters (e.g., high pitch of voice, etc.), mammary gland development.

Progesterone supports pregnancy. Progesterone also acts on the mammary glands and stimulates the formation of alveoli (sac-like structures which store milk) and milk secretion.
 

Mechanism of Hormone Action

Hormones produce their effects on target tissues by binding to specific proteins called hormone receptors located, in the target tissues only.

Each receptor is specific to one hormone only and hence receptors are specific.

Hormone-Receptor complex formation leads to, certain biochemical changes in the target tissue, 'Target tissue metabolism and hence physiological functions are regulated by hormones.

The hormone receptor complex may acts in one or the two ways –

Formation of cAMP: Mechanism of formation of cAMP was discovered by E.W. Sutherland in 1950. The hormone receptor complex causes the release of an enzyme adenyl cyclase, from the receptor site. This enzyme hydrolyses the A. TP into c-AMP. The c-AMP activates the existing enzvmo system of the cell. This accelerates the metabolu­reactions in cell. The hormone is called firs: messenger and the c-AMP is termed the second messenger. e.g., Adrenaline causes the secrertion of glucose from the liver cell from this mechanism. 

(ii) Change in membrane permeability: The receptor proteins of some hormones are large transmembrane intrinsic protein acting as ion channels for facilitated diffusion of Na⁺, K⁺, Ca²⁺ etc. On binding with specific hormone these receptor proteins undergo conformational changes, so that the membrane permeability for ions is altered, resulting into important changes in metabolism.

For example, insulin promotes the entry of glucose from blood into the muscles cells by increasing the permeability of- sarcolemma to glucose.

The steroid hormones act within the cell. Their small, lipid soluble molecules pass through the cell membrane and bind to specific receptor molecules present in the cytoplasm. The receptor molecules carry them into the nucleus. Here, the receptor hormone complex binds to a specific receptor site on the chromosome and activates certain genes that were previously repressed. The activated gene transcribe m-RNA which directs the synthesis of enzyme (protein molecule) in the cytoplasm. The enzyme molecule promote the metabolic reactions in the cell. 

Note:

The atrial wall of our heart secretes a very important peptide hormone called atrial natriuretic factor (ANF), which decreases blood pressure. When blood pressure is increased, ANF is secreted which causes dilation of the blood vessels. This reduces the blood pressure.

The juxtaglomerular cells of kidney produce a peptide hormone called erythropoietin which stimulates erythropoiesis (formation of RBC).

Endocrine cells present in different parts of the gastro-intestinal tract secrete four major peptide hormones, namely gastrin, secretin, cholecystokinin (CCK) and gastric inhibitory peptide (GIP).

Gastrin acts on the gastric glands and stimulates the secretion of hydrochloric acid and pepsinogen. Secretin acts on the exocrine pancreas and stimulates secretion of water and bicarbonate ions.

CCK acts on both pancreas and. gall bladder and stimulates the secretion of pancreatic enzymes and bile juice, respectively. GIP inhibits gastric secretion and motility.

Hormones which interact with intracellular receptors (e.g., steroid hormones, iodothyronines, etc.) mostly regulate gene expression or chromosome function by the interaction of hormone-receptor complex with the genome.

Heterocrine gland: These are those endocrine glands which are involved in hormone secretion as well as some other function eg. pancrease, gonads, placenta, GI mucosa and kidneys.

In females prolactin induce maternalism i.e. strong emotional attachment.

In male prolactin promotes paternalism i.e. protective attitudes towards family members and intensive food gathering for the family.

Contrary to thyroid dwarf (cretins), the pituitary dwarf have a normal mental development and proportionate body.

Sporadic cases of simple goitre (sporadic goitre) in a population are normally due to genetic defect.

Muller organ and subneural gland is homologous to pituitary gland.

Tropic hormone: A hormone which stimulates another endocrine gland to secrete its hormone is called trophic hormone.

Simmond's disease: This condition is due to atrophy of the anterior lobe of pituitary gland.

Pheochromocytoma: It is due to hypersecretion of adrenaline causes, high blood pressure, high level of sugar in blood and urine, high metabolic rate, nervousness and sweating.

Sexual pseudoprecocity results from adrenal cortex, testes, ovary or from other sources, including extragonadial tumours.

Eunuchoidism: Failure of testosterone secretion in male-causes eunuchoidism.

eunuch has a undeveloped secondary sex organs like prostrate, seminal vesicle and penis

does not produce sperm.

Growth hormone: Stimulate the liver to form "Somatomedins" ("Insulin like growth factors"). This somatomedins potent effect to bone growth.

In heart cGMP has antagonistic effect to cAMP, cAMP mediate muscle contraction in response to adrenaline, while cGMP slow down muscle contraction in response to acetylcholine.

cGMP is used as second messenger in atrial natriuretic peptide and nitric oxide.
 

Difference between Nervous and Endocrine Coordination

	Nervous Co-ordination		Endocrine coordination
1	Information passes as electrical impulses along nerve fibres.	1	Information passes as a chemical substance through the blood and lymph.
2	There is rapid transmission of information.	2	There is slow transmission of information.
3	Response is immediate.	3	Response is usually slow,
4	Response is very exact.	4	Response is usually widespread.
5	Response is short lived.	5	Response is long-lasting.

 

Thyroid gland is the largest endocrine gland in the body.

Endostyle of lower vertebrates like Herdmania, Amphioxus is homologous of thyroid gland.

Thyroid is the only endocrine gland in the body which stores its hormone in its inactive state.

The oxidation of iodine is promoted by the enzyme peroxidase.

21 October is Iodine deficiency day

Thyroxine stimulates the metamorphosis of tadpole larva in amphibians.
 

Difference between Hormone and Enzymes

S.No.	Characters	Enzymes	Hormones
1.	Chemistry	Always proteinaceous	May be proteinaceous, or amine or steroids.
2.	Molecular weight	Macromolecules with high molecular weights.	Have low molecular weights.
3.	Diffusibility	Non-diffusible through cell membrane.	Diffusible through cell membrane.
4.	Site of action	Either act intracellalarly or carried by some duct to another site.	Generally carried by blood to a target organ.
5.	Mode of action	Always act as biocatalysts and increase the rate of metabolic physiological process.	May be excitatory or inhibitatory in their physiological action.
6.	Reversibility	These catalyze reversible reactions.	Hormone controlled reactions are not reversible.
7.	Effect of concentration	Reaction rate increase with increase in their concentration upto a limit.	Deficiency or excess of hormone causes metabolic disorders and diseases,
8.	Speed	Act quickly	Some are quick acting, while some are slow acting with a lag period.
9.	Consumption	Not used in metabolic functions.	Used up in metabolic functions.

Number of Hormones Secreted by Different Endocrine Glands

Endocine-glands	Number of secreted hormones
Pituitary - Anterior	-	7
Hypothalamus	-	2
Pineal body	-	2
Thymus	-	3
Thyroid	-	2
Parathyroid	-	1
Islets of Langerhans	-	3
Adrenal cortex	-	46
Adrenal medulla	-	2
Testes	-	1
Ovary	-	3
Placenta	-	2
Kidneys	-	2
Stomach	-	1
Duodenum	-	5
Ileum	-	2

Anterior Pituitary Hormones

Hormone and Target Tissues	Principal Action	Hormone and Target Tissues	Principal Action
Human growth hormone (hGH) or somatotropin	Stimulates liver, muscle, cartilage, bone, and other tissues to synthesize and secrete insulin like growth factor of body cells, protein synthesis tissue repair, glucose concentration.	Prolactin (PRL)	Together with other hormones, promotes milk secretion by the mammary glands.
Thyroid -stimulating hormone (TSH) or thyrotropin	Stimulates synthesis and secretion of thyroid hormones by thyroid gland.	Adrenocorticotropic hormone (ACTH) or corticotropin	Stimulates secretion of glucocorticoids (mainly cortisol) by adrenal cortex.
Follicie- stimulating hormone (FSH)	In females, initiates development of occytes and induces ovarian secretion of estrogens. In males stimulates testes to produce sperm,	Melanocyte-stimulating hormone (MSH)	Exact role in humans is unknown but may influence brain activity, when present in excess, can cause darkening of ski

Posterior Pituitary Hormones

Hormone and Target Tissues	Control of Secretion	Principal Actions
Oxytocin (OT)	Neurosecretory cells of hypothalamus secrete OT in response to uterine distention and stimulation of nipples.	Stimulates contraction of smooth muscle cells of uterus to cause child birth; stimulates contraction of myoepithelial cells in mammary glands to cause milk ejection.
Antidiuretic hormone (ADH) or vasopressin	Neurosecretory cells of hypothalamus secrete ADH in response to elevated blood osmotic pressure, dehydration loss of blood volume. Pain or stress; low blood osmotic pressure, high blood volume, and alcohol to inhibit ADH secretion.	Conserves body water by, decreasing urine volume; decreases water loss through perspiration: raises blood pressure by constricting arterioles

Hormones, Their Chemical Nature and Functions

S.No.	Name of endocrine gland	Name of hormone and its chemical nature	Functions
1.	Neurosecretory     cells of Hypothalamus     (Supraoptic Nucleus and Paraventricular Nucleus)	Oxytocin and vasopressin monopeptide Gonadotropin releasing hormones Other releasing hormones e.g. TSHRH, MSHRH, ACTHRH, GHRH etc. Proteinaceous	Milk ejection and parturition (oxytocic effect). Vasoconstriction and antidiuretic (vasotocin) effects. Stimulates FSH and LH synthesis Stimulate TSH, MSH, ACTH GH secretions from pituitary.
2	Pituitary (a) Neurohypophysis (Pars Nervosa) (b) Adenohypophysis contains diverse cell types)	Store and release Oxytocin and Vasopressin. Proteinaceous or glycoprotein	Hormone release is related to physiological state and requirements. Affect growth, development differential pubertal changes and other metabolic mechanism
3.	Pineal	Melatonin-derived from the amino acid tyrosine	Antagonist to FSH / LH   Regulates biological/circadian rhythms.
4.	Thyroid gland (amine hormone) having - NH2 group)	(a) Thyroxine, iodinated amino acid called tyrosine (T2, T3, T4). (b) Thyrocalciton in (Peptide)	(a) Controls basal metabolic rate (BMR). All organ / system of body responds to thyroxine. (b) Facilitates Ca+2 absorption
5.	Parathyroid gland	Parathormone, Peptide	Ca+2 and PO-4 metabolism.
6.	Thymus	Thymosine (polyneotide)	Anti-FSH and LH; dela; delays puberty
7.	Islets of lengerhans		(i) Gluconeogenesis / Glycogenolysis (ii) G lycogenesis (iii) Gastric functions
8.	(= Endocrine pancrease) (i) a-cells (ii) b-cells (iii) d-cells	(a) Catecholamines (epinephrine = adrenaline, and norepinephrine = noradrenaline (derived from tyrosine) (b) Mineralcorticoids and glucocorticoids and traces of androgen and estrogen steroids derived from cholesterol	Stresses = emergency = Fright, Fight and Flight Hormone (3F) acclerates cardiac functions muscle activity etc. (b) Electrolyte and carbohydrate metabolism.
9.	Adrenal gland (a) Adrenal medulla (Amine hormone have – NH2) (b) Adrenal cortex	Estrogen (Steroid) Estrone, estradiol Estrogen and Progesterone (Steroid) (a)Steroid secreted are estrogen and progesterone (b) Relaxin-Polypeptide	(a) Secondary sex character primary action on uterine endometrium mitogenic. (a) Secreted during -luetal phase of menstrual cycle in human female and oestrous cycle of other mammals. Prepares uterine endometrium for receiving blastocytes for implantation, Progesterone is also called pregnancy hormone and is anti-FSH and anti-LH/anti-LTH. (a) Maintenance of pregnant state, prevents lactogenesis folliculogenesis, and Ovulation. (b) Act on pubic symphysis and enlarges the birth canal to facilitate birth. Acts synergestically with oxytocin during this process (parturition)
10.	Ovary (a) Granulosa cells steroid fat soluble have sterol group derived from cholesterol (b) Corpus luteum Placenta temporary endocrine gland formed during pregnancy	Inhibin - Polypeptide (ii) Estradiol-Steroid Androgens (e.g Testosterone) Steroid androstenedione)	Inhibits FHS action and attenuates spermatogenesis decrementally -do- (i) Pubertal changes in male, (ii) Seco. Sex characters in male, (iii) Sex drives. (iv) Spermatogenesis
11.	Testis (i) Sertoli cells (= sustentacular cells) (ii) Leydig cells (=Interstitial cells)	Gastrin (i) Secretin (ii) Cholecystokinin (CCK) (iii) Enterogastrone (iv) Duocrinin, (v) Enterokinin     (vi) Villikrinin	Stimulates gastric juices secretion from gastric gland, movement of sphincters of stomach and increased movement of stomach (i) Stimulates secretion of succus entericus, (ii) Bile released from gall bladder, (iii) Inhibits gastric secretin, (iv)  Stimulates secretion of mucous from Brunner's gland, (v) Stimulate intestinal gland, (vi) Stimulate villi movement.

Disease Caused by Hormonal Irregularities

Disease	Hormone	Quantity	Gland
Dwarfism	GH	Deficiency	Pituitary
Gigantism	GH	Excess	Pituitary
Acromegaly	GH	Excess	Pituitary
Simmond's disease	GH	Deficiency	Pituitary
Diabetes insipidus	ADH	Deficiency	Pituitary
Cretinism	Thyroxine	Deficiency	Thyroid
Simple goitre	Thyroxine	Deficiency	Thyroid
Myxaedema	Thyroxine	Deficiency	Thyroid
Exophthalamic goitre	Thyroxine	Excess	Thyroid
Tetani	Parathyroid	Deficiency	Parathyroid
Plummer's disease	Thyroxine	Excess	Thyroid
Addison's disease	Mineralocorticoids (Aldosterone)     and Glucocorticoids (cortisol)	Deficiency	Adrenal cortex
Conn's 'disease	Mineralocorticoids	Excess	Adrenal cortex
Cushing's disease	Corticosteroid	Excess	Adrenal cortex

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