# DEVELOPMENT OF THYROID GLAND :
* Thyroid gland starts developing in the third week of embryonic life in the base of tongue from area called foramen caecum.
* The development is completed by third month .
* Then gland descends along the trachea to reach the correct place .
# BLOOD SUPPLY OF THYROID GLAND :
* Arterial supply of thyroid gland :
* Thyroid gland is supplied by superior and inferior thyroid artery , which are branches of external carotid artery.
* Venous drainage of thyroid gland:
* There are three sets of veins
Superior thyroid veins , middle thyroid veins and inferior thyroid veins.
* These veins drain into internal jugular vein and brachio cephalic vein.
# NERVE SUPPLY OF THYROID GLAND :
* Parasympathetic supply :
* Thyroid gland is supplied by recurrent laryngeal nerve which is branch of vagus nerve
* Sympathetic innervation :
* It is by the superior , middle and inferior sympathetic cervical ganglia.
* Mainly by middle cervical sympathetic chain .
# HORMONES SECRETED BY THYROID GLAND :
* It synthesises thyroxine ( T4 ) and tri- iodothyronine ( T3 ) and calcitonin.
* T3 and T4 are secreted by thyroid follicular cells and calcitonin is secreted by chief cells
# COLLOID :
It is the glycoprotein rich area , which is surrounded by follicular cells and which is filled with thyroglobulin with iodine (nothing but T3 and T 4 hormones ).
#PROCESS OF SYNTHESIS OF THYROID HORMONES :
* This mainly involved six steps
1) Uptake of Iodine ( in the form of iodides ) by follicular cells also known as " iodide trapping ":
* Iodide from the diet is carried by blood vessels .
* From the blood , follicular cells take iodine with the help of special transporters called iodide pumps.
* Iodine is polar molecule and large molecule so it can't diffuse through cell membrane.
* These pumps utilise energy in the form of ATP for iodide uptake.
2 ) Synthesis of Iodine :
* Iodide pump uptake iodine in the form of iodide .
* In the folliclular cells there are enzymes called thyroid peroxidases which converts iodides to Iodine.
* Conversion of iodides to iodine is called organification of Iodine.
3 ) Synthesis of thyroglobulin :
* Thyroglobulin is the glycoprotein molecule .
* It is synthesized in the follicular cells in endoplasmic reticulum and modified by golgi apparatus.
* After synthesis of thyroglobulin , it is moved into colloid , where iodine molecules get attached.
4 ) Binding of thyroglobulin with the iodine :
* Binding of thyroglobulin with the iodine molecules is facilitated by thyroid peroxidases.
* Thyroglobulin molecule contains many tyrosine residues .
* Iodine gets attached to these tyrosine residues.
5 ) Coupling reaction :
* Coupling reaction is facilitated by thyroid peroxidase enzyme .
* Iodine present on tyrosine Residue gets coupled with the neighbouring iodine .
* When mono iodo tyronine couples with di -iodo tyronine then T3 will be formed
* When two adjacent Diiodo tyronine molecules gets coupled then T4 will be formed.
* T3 and T4 are still attached to the thyroglobulin molecule .
5 ) Proteolysis :
* The thyroglobulin molecules undergo proteolysis releasing free T3 and T4 hormomes .
* The thyroglobulin molecules with T3 and T4 are are taken up by thyroid follicular cells process called endocytosis .
* Uptake of these thyroglobulin is facilitated when there is low concentration of thyroid hormones in circulation .
* In follicular cells , lysosomal enzymes are released into the vesicle containing thyroglobulin .
* These lysosomal enzymes break the thyroglobulin and releases free T3 and T4 .
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