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Major endocrine glands

The body secretes chemical messengers called HORMONE that directly enters the blood stream without passing through ducts and goes on to acts on specific target organs, the secretion of these chemicals in the body has profound effects on many body processes, for instance, they regulate growth, secondary sexual characters, mood swing, sleep and even appetite.  The over production or under production of these substances can be detrimental to health, they can cause retarded growth and many other health condition.
 
Hormone

By definition, hormones are chemicals release by cells or glands in one part of the body that exert their effect in another part of the body.


Some of these synthetic substances exert their effects at the cellular level with the help of a second messenger or intermediate after activating the appropriate receptors called Prolein.


 Prolein are specific for each receptor, examples of receptors are vasopressin, Catecholamine, Glucagon, polypeptide, glycoprotein and hypothalamic releasing hormone found on the cell membrane, they acts on the enzyme Adenyl cyclase  to form Adenosine Monophosphate = Cyclic AMP or CAMP, the second messenger  from ATP.CAMP convert inactive protein Kinases   to active protein kinases which causes cell phosphorylation which finally leads to the expression of the desired hormonal action. Calcium ion (Ca2+) which regulates body function also acts as a third messenger.  


Hormones can also have receptor on the cytoplasm and nucleus, those with receptor on cytoplasm for example steroidal, glucocorticoid, mineralocorticoid, progestin, androgen and estrogen acts by activations of gene. 


Hormone having receptor in the nucleus interacts with DNA to bring about the synthesis of  MRNA leading to the production of protein that  brings about  hormonal action.


Properties of hormone

1.  They are controlled by gene

2.  Enters directly into the blood stream by diffusion and bind to cellular receptors which give out signals to change cellular activities.

3.  Acts on target organ.


What is endocrine gland?

Endocrine glands are ductless glands that produce hormone that are secreted directly into the blood stream without passing through ducts, they act on specific target organ and are distributed around the body by the circulatory system. In other words, the glands do not require duct (ductless) in other to exact their effect but needs nearness to blood vessels.


Characteristics of endocrine gland

1. Are specialized for secretion and their appearance depends on the nature of their secretary products and on the cellular mechanism needed to produce and store their end products.

2. They are secreted into interstitial fluid from where they diffuse into the blood stream.

3. Are arranged into cords or small clumps with each cell closely linked to adjacent vessels.

4. Have perforated endothelium to facilitate diffusion of hormone into blood vessels.

5.  Employs messengers called hormone.

6. Their secretions are directed towards specific organs known as “Target organ”.


  Endocrine includes the followings,

1.      Thyroid gland

2.      Pituitary gland

3.      Pancreases

4.      Gonad gland



Thyroid Gland

Thyroid gland is located in the cervical region anterior to the larynx and consist of  two lobe united by an isthmus,, its emerges from early embryonic life from the endoderm of initial portion of primitive gut.


Function of Thyroid Gland   

1. It synthesizes thyroxine hormone (T4) and triiodothyronine (T3) which are needed for growth, cell differentiation, control of oxygen consumption and basal body metabolism rate. T3, in particular affect almost all physiological processes in the body.



2.Carbohydrate metabolism: it enhances glucose absorption from the G.I.T and may cause hyerglucemia.



3. It enhances the action of catecholamine on the heart through beta adrenergic receptor causing increase in cardiac output and mental rate.


4. It facilitates mental process and enhance reflexes particularly stretch reflex.

      

5.  Lipid metabolism: hypothyroidism increases the level of serum cholesterol while hyperthyroidism decreases the level of cholesterol.

      

6. Caroligenic effect: speeds up the rate of metabolic processes and maintain body metabolism level leading to increase oxygen consumption and heat production with an added increase in Na+ - K- ATPase pump activity.

      

7. Growth and maturation: it is important in growth, deficiency causes critinism (Dwarfism), enlarge tongue and mental retardation, over secretion leads to gigantism. It is also known to cause hyperplastic goiter and mexoedema coma.



Thyroid gland secretes three hormones,

a. Triiodothyronine (T3): this is the active form of thyroxine hormone; thyroid gland secretes about 20% of this chemical substance directly into the blood stream while the remaining 80% is produced in the liver and kidney by conversions of thyroxine.



b. Calciton: this is a polypeptide hormone produce by the parafollicular cells of the thyroid in man but by the ultimopharyngeal in other animals.



c.  Thyroxine


Hyperthyroidism

Thyroid gland sometimes over produce it’s hormone resulting  in increase in body processes, usually, there is weight loss, increase heart rate, nervousness and mood swing, sometimes the condition remain asymptomatic. 


In essence, hyperthyroidism is the over production of thyroxine leading to increase in body mechanisms.


The condition is associated with Gravis disease, the immune system attack thyroid gland, while the gland retaliate by producing too much of the hormone.


Thyroid noodles (swollen thyroid) are another common cause of hyperthyroidism. 


Hyperthyroidism is a hereditary disease, it runs in the family.  It can be treated surgically by removal of affected part of the gland or with pills. Anti-thyroid drugs and iodine solution are also used to slow down the rate of the hormone production.


Parathyroid gland

Parathyroid glands are two pairs of glands positioned behind the thyroid gland, one on each end of the upper and lower pole usually in the capsule that covers the gland, sometimes, they occur embedded in the mother thyroid gland.


Parathyroid gland produces parathormone which is important in regulating calcium serum level. it decreases the concentration of phosphate in the blood, this leads to reduction in phosphate absorption  causing an increase in phosphate excretion in the urine.


 PITUITARY GLAND

This is a pear shape gland located at the base of the mid brain between the optic lobe. It is regarded as the master endocrine gland because it controls the activities of other endocrine gland secretions (hormone).  Pituitary gland controls other hormone functions such as thyroid activities, growth during childhood, testosterone production, and urine production.


It is made up of both posterior and anterior glands which secretes different type of hormones, each anterior pituitary gland is produced by a separate group of cells which according to their staining characteristics are either acidophilic or baseophilic. 


Anterior part of the gland produces 
        1-  Growth hormone somatotrophin (GH/STH): It is the only hormone that does not require target organ in other to produce its effect.  

          a- It stimulates growth in children and control metabolism in adult.

         b- It enhances lipolysis in adipose tissue causing liberation of free fatty acid (FTA) into the blood.


  c- It causes elevation of blood sugar level by antagonizing the actions of in female and spermatogenesis in male.


       d- It is the fountain of youthfulness in terms of positive feedback mind and energy.
     
Prolactin:  it stimulates mammary growth and milk production or lactation. 
-      Prevent ovulation in lactating women. 
-      Stimulates corpus liteum to produce progesterone.
-      Controls prolactin inhibitory hormone called      DOPAMINE.


      3.  Follicle stimulating hormones (FSH)

            - stimulates growth in female and spermatogenesis in male.

          - stimulates Estrogen production and induce the release of  ovary. 


       4. Luteinizing hormone (LH/SCH)

                 -  Induces ovulation

                 - Stimulates progesterone secretion by Corpus luteum

                 - Stimulates testosterone production.


      5. Thyroid stimulating hormone (TSH)

                 - Stimulates production of  thyroxine by thyroid gland.


      6. Adrenocorticotrophic hormone (ACTH)

               - Stimulates production of adrenocorticoid by the adrenal cortex.

               - It has MSH property responsible for ADDISON disease. 


        ADRENAL GLAND

Adrenal gland are  tiny structures located on top of each kidney, it js made up of two parts, the adrenal cortex which produces cortisol  and Aldosterone hormone and the adrenal medulla  which produces adrenalin and noradrenalin.


Functions of Cortisol and Aldosterone

      a - promotes gluconeogenesis and glycogenolysis

      b - Reduces gastric mucus predisposing to ulcer

      c - It helps to combat stress

    d - It performs anti immune and anti allergic function by reducing eosinophiles and lymphocyte circulation. It also reduces the size of lymphoid tissue.

      
 Adrenalin on the other hand, causes stress condition which results from harmful stimuli, for example, emotional or physical strain, pain and anger,  stressors stimulates the hypothalamus  to release Corticotropin- releasing hormone(CRH), which in turn causes the production of Adrenocorticotropic  hormone (ACTH) which stimulates cortisol production. Cortisol then effects its action on energy metabolism and vascular effects.


In addition to ACTH response, stress can also lead to an increase Sympatho - adrenal response resulting in increase heart rate , cardiac output,  increase blood pressure  and blood flow to muscles in response to danger or defence reaction.


Noradrenalin is a vasoconstrictor; it normalizes the body after stress thereby countering the effect of adrenalin.


Posterior part of the gland produces

  1. Antidiuretic hormone (ADH): also known as Pitressin or Vesopressin causes re-absorption of water (fluid) in the renal tubule thereby reducing urination, it also causes rise in blood pressure.



  2. Oxytocin: it stimulates uterine contraction, contraction of myoepithelial cells of mammary gland during milk ejection reflex.



PANCREAS

Pancreas is a whitish gland made up of many diffused lobe located between the lower part of the stomach and duodenum (small intestine).


It secretes pancreatic juice which contains three digestive enzymes, the Amylopesin, trypsin and lipase, about 1 to 1.5 litres of pancreatic juice is secreted daily by an average man and they are released into the duodenum through duct.


The enzyme Amylase converts   starch to maltose sugar; lipase converts fats and oil to fatty acid and glycerol, trypsin converts proteins to peptones and amino acid


The pancreas also secretes Insulin which is produced in the islets of Langerhans.  Insuline was discovered by Banting and Best who demonstrated the hypoglycemic action of Pancreas extract. 


Insulin is synthesized in the (β) of the Pancreas Islets Langerhans.


Function of Pancreas

     1.   Regulates the amount of glucose in the blood stream

     2.   Increase the amount of glucose entering into the cells

     3.   Converts glucose to glycogen

     4.  Increase synthesis of triglycerides in the liver

     5.  Reduce lipases in adipose tissue.

    6.  Enhance protein synthesis by increasing transport of amino acid into the cells.

     7.  Facilitate entry of potassium into the cell.


GONAD GLAND

Gonad gland produces sex hormone Gonadotrophi SEE DIAGRAM.


Physiological Importance Of  Gonadotrophin

       1 – Stimulation of follicular growth

       2 – Produces luteinizing hormone

    3 - Stimulates production of Progesterone, Testosterone, Estrogen and Androgen


PINEAL GLAND

Pineal gland  also known as conarium or  epiphysis cerebri is located in the  epithalamus near the center of the brain, it secrete melantonin  which regulate sleep circle.


HYPOTHALAMUS

Hypothalamus is not a gland but produces a hormone that controls the action of pituitary gland,  it’s found above the pituitary and optic chasma and below the hypothalamus sulcus, and is delineated into prominent nuclei and area SEE MORE.


It secretes the followings

-          Thyriotropin releasing hormone ( TRH)

-          Corticotrophin releasing hormone ( CRH)

-          Prolactin releasing hormone (PRH)

-          Growth releasing hormone (GRH)

-          Follicle stimulating hormone (FSH)

-          Luteinizing hormone( LH)


 Function of hypothalamus
  - Secretes many chemical substances that  controls different cells and organs 

 -  Regulate sleep, thirst, hunger, temperature  and sex drive 

 - Controls involuntary (unconscious actions) actions such as metabolism. 

-  Regulates fluid and electrolyte balance of the body. 
  
-  Maintains equilibrium or balance of the body (homeostasis), this is the primary function of hypothalamus.

DEFINITION OF TERMS

Prolein – Amino acid with alpha amino acid group, alpha carboxylic acid group and a pyrolydine side chain used in biosynthesis of protein.


Basophilic – used in histology to describe cell that is readily stained with basic dye.


Estrogen – primary female sex hormone and pill responsible for developments of secondary sex characters in female.


Gluconeogenolysis – is a metabolic pathway that leads to generation of glucose from non carbohydrate carbon substrate such as glucogenic amino acid, glycerol and lactate.


Duodenum – small intestine. 


Corticotropin- releasing hormone (CRH) – Peptide hormone involve in stress response. 


Hyperglucemia – this is the abnormal high level of glucose n the blood stream.






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