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Endocrine system structure, function, disorders, Endocrine Glands and Hormones types

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The endocrine glands are the sites of synthesis and secretion of hormones, in certain amounts, which are disseminated throughout the body by the bloodstream where they act on target organs. Endocrine glands do not have a duct system, so they are also called “ductless glands”.

Endocrine system

The endocrine system and the nervous system are the two main coordinating and integrating systems of the body. The two systems are linked, however, through the hypothalamus which controls the secretion of many of the endocrine glands.

Endocrine glands, in general, are composed of a group of secretory cells of epithelial origin supported by a highly vascular connective tissue. The secretory cells discharge their hormones in the interstitial spaces from which they are rapidly absorbed into the circulatory system via fenestrated capillaries to reach the target organs and cells.

Histologically the endocrine system consists of:

  1. Individual endocrine glands e.g. the pituitary gland, the thyroid gland, the parathyroid gland, the suprarenals, and the pineal body.
  2. Group of endocrine cells in certain organs e.g. pancreas (islets of Langerhans), testis (interstitial cells of Leydig), ovary (ovarian follicles and corpus luteum), placenta.
  3. Dispersed endocrine cells e.g. APUD cells.

Hormones

A hormone can be defined as a chemical substance that is synthesized and secreted by a specific cell. It is transported by circulation and at very low concentrations. It elicits a specific response in distant target tissues.

Types of hormones

  • Endocrine hormones: They are synthesized by endocrine glands and transported by the blood to their target cells, e.g. Catecholamines and insulin.
  • Paracrine hormones: They are synthesized near their targets of action, i.e. cell to cell action, e.g. a and β-cells of the pancreas.
  • Autocrine hormones: They exert their actions on the cells of their own synthesis, e.g. Prostaglandin. 
Target cell is defined by its ability to bind selectively with a given hormone via a receptor. Receptors are proteins. All receptors have at least two functional domains. A recognition domain binds the hormone, and a second domain generates a signal to some intracellular function (what is called signal transduction).

The Principal Endocrine Glands are:

  1. Pituitary gland (anterior & posterior).
  2. Thyroid gland.
  3. Parathyroid glands.
  4. Adrenal or supra renal gland (cortex & medulla).
  5. Islets of Langerhans (in the pancreas).
  6. Male gonads (testes) & female gonads (ovaries).
  7. Placenta.

Other Endocrine Organs

In addition to the endocrine glands, the following organs also secrete hormones:
  1. The kidneys: The kidneys secrete: Renin, Erythropoietin, 1.25dihydroxycholecalciferol, and Certain prostaglandins. 
  2. The heart: Its atria secrete the ANP (atrial natriuretic peptide) which causes natriuretic and hypotensive effects.
  3. Adipose tissue: It secretes a number of hormones known as adipokines such as leptin, adiponectin, and resistin.

Chemistry of Hormones and General structure of cells secreting it:

The histological structure of the hormone-secreting cells depends on the type of the secreted hormone. Chemically, the hormones are classified into three types:

1- Steroid hormones

These are derived from cholesterol. Steroid hormones are secreted by:
  • The adrenal cortex “cortisol & aldosterone”.
  • The ovaries “estrogen and progesterone”.
  • The testes “testosterone”.
  • The placenta “estrogen and progesterone”.

General structure of cells secreting steroid and fatty acid derivatives:

These cells are characterized by:
  • Eosinophilic and vacuolated cytoplasm.
  • Well-developed smooth endoplasmic reticulum.
  • Mitochondria with tubular cristae.
  • Lipid droplets.

2- Derivatives of the amino acid tyrosine

Two groups of hormones are derivatives of the ammo acid tyrosine.
  • Thyroid hormones (Thyroxine & triiodothyronine) are iodinated forms of tyrosine derivatives.
  • Adrenal medullary hormones (catecholamine) are derived from tyrosine.

3- Proteins or Peptides

The remaining important endocrine hormones are proteins, peptides, or immediate derivatives of these. The anterior pituitary hormones, the posterior pituitary hormones, parathyroid hormones and hormones of the pancreas.

General structure of cells secreting protein hormones, polypeptides, and amino acids derivatives

These cells are characterized by:
  • Euchromatic nucleus with a prominent nucleolus.
  • Basophilic cytoplasm.
  • Well-developed rough endoplasmic reticulum.
  • Well-developed Golgi apparatus.
  • Mitochondria with lamellar cristae.
  • Secretory granules.

Hormone Transport and Inactivation

Once released into the bloodstream, water-insoluble hormones (e.g thyroid and steroid hormones) are bound to specific plasma binding proteins. The carrier proteins are secreted by the liver to prevent the loss of this hormone from the kidney and to act as physiological regulators of the free levels of the hormone.
Near the target cell, the free hormone can diffuse into the cell. Inactivation of hormones may occur in the blood, in the liver, in the kidney, or in other target tissues. Hormones may be inactivated by degradation, oxidation, reduction, methylation, or conjugation to glucuronic acid then excreted in the urine or bile.

Hormone Receptors and Their Activation:

The endocrine hormones almost never act directly, they first combine with hormone receptors on the surfaces of the cells or inside the cell. Almost all hormonal receptors are very large proteins. Each receptor is usually highly specific for a single hormone. The receptors in their unbound state usually are inactive Activation of a receptor occurs in different ways for different types of receptors. The receptors for the different types of hormones are generally located in the following:
  1. In the membrane: for the protein, peptide & catecholamine hormones.
  2. In the cytoplasm; for the different steroid hormones.
  3. In the nucleus; for the metabolic thyroid hormones.

Classification of hormones

Hormones can be classified according to:
  1. Chemical composition.
  2. Solubility properties.
  3. Site of the receptor.
  4. Nature of signal transmitted by them.

Hormones of group I

They include steroid Hormones, T3 & T4, calcitriol (1,25 dihydroxycholecalciferol), and retinoic acid. They are lipophylic (can penetrate the lipid bilayer of the cell membrane). They combine with a transport protein after their secretion. They have cytoplasmic receptors, and after binding to their receptors, they form hormone receptors, complex which is the intracellular messenger of the group.
Hormones of Group II
They include polypeptide and protein hormone, glycoproteins and catecholamines. They are hydrophilic (cannot penetrate the membrane bilipid layer). They combine with membrane receptors and send second messengers to induce the metabolic process. According to the nature of the second messenger, this group is sub-classified into:
  • Group Ila: Includes hormones that use cAMP as 2nd messenger, e.g. epinephrine and glucagon.
  • Group Ilb: Use cGMP as secondary messenger e.g. Atrial Natnuretic peptide and nitric oxide (NO).
  • Group llc: Use calcium or phosphatidyl inositol or both as a second messenger e.g. vasopressin and α-adrenergic catecholamines.
  • Group Ild: The secondary messenger is tyrosine kinase phosphorylation cascade e.g. Insulin.

Hormones of groups I and II

Group I

  • Types: Steroids, T3, T4, Calcitriol
  • Solubility: Lipophilic
  • Carrier protein: Present
  • Plasma life span: Long (hours-days)
    Receptor: Intracellular
    Mediator: Receptor-Hormone Complex

Group II

  • Types: Polypeptides, proteins, catecholamines
  • Solubility: Hydrophilic
  • Carrier protein: Absent
  • Plasma life span: Short (minutes)
  • Receptor: Plasma membrane
  • Mediator: cAMP, Ca+, Phosphatidyl inositol or others

Hormonal interactions: permissiveness, synergism, and antagonism

A given hormone‘s effects are influenced not only by the concentration of the hormone itself but also by the concentrations of other hormones that interact with it. Because hormones are widely distributed through the blood, target cells may be exposed simultaneously to many different hormones, giving rise to multiple hormonal interactions on target cells. A hormone can influence the activity of another hormone at a certain target cell in one of three ways: permissiveness, synergism, and antagonism.

With permissiveness, one hormone must be present in adequate amounts for the full exertion of another hormone‘s effect (the first hormone, by enhancing a target cell’s responsiveness to another hormone, “permits this other hormone to exert its full effect). For example, thyroid hormone increases the number of receptors for epinephrine in epinephrine’s target cells, increasing the effectiveness of epinephrine. In the absence of thyroid hormone, epinephrine is only marginally effective.

Synergism occurs when the actions of several hormones are complementary and their combined effect is greater than the sum of their separate effects. An example is the synergistic action of follicle-stimulating hormone and testosterone both of which are required for maintaining the normal rate of sperm production. Synergism results from each hormone‘s influence on the number or affinity of receptors for the other hormone.

Antagonism occurs when one hormone causes the loss of another hormone’s receptors, reducing the effectiveness of the second hormone. An example of antagonistic action is the action of cortisol against the action of insulin on carbohydrate metabolism.

Endocrine disorders

Endocrine disorders commonly result from abnormal plasma concentrations of a hormone due to inappropriate rates of secretion, either too little hormone secreted (hyposecretion) or too much hormone secreted (hypersecretion) Endocrine disorder, however, may arises because target-cell responsiveness to the hormone is abnormally low, even with the normal plasma level of the hormone.

Hypersecretion, hypersecretion by a certain endocrine gland is either primary or secondary depending on whether the defect lies in that gland (as tumor) or results from excessive stimulation of the gland from the outside (as increase secretion of stimulating hormone for such gland).

Hyposecretion: hyposecretion could be also primary or secondary. Primary hyposecretion occurs when the endocrine gland is secreting too little of its hormone because of an abnormality within that gland while secondary hyposecretion occurs when the endocrine gland is (normal) but is secreting too little hormone because of a deficiency of its tropic (stimulating) hormone.

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