endocrine gland| function| types| structure|

  Endocrine Gland   

Master of endocrine gland is Pituitary gland, which is even the smallest gland of body. Pituitary gland is regulated by hypothalamus and is located at base of hypothalamus. Pituitary gland is made up of covering called durometer. It is divided into 3 regions called Pars distalis, Pars intermedia and Pars neurosa . Depression in pituitary gland is called as Sella turcica which is made up of sphenoid bone. Pituitary gland can also be divided on basis of development into 2 regions Adenohypophysis, which develops from the roof of mouth cavity. It consists of 3 components called Pars distalis, Pars intermedia and Pars tuberalis . Other region is called neuro hypophysis and it consist of 2 components called Pars neurosa and Infundibulum (region that joins pituitary to hypothalamus). Matured pituitary gland consist of anterior lobe. Pars distalis make up 75% of anterior lobe of pituitary gland. It consists of two types of cells called chromophils and chromophobes. Chromophils stain with both acidic and basic stains while chromophobes do not stain at all.

Chromophils are further divided into; -

i)Acidophils

ii)Basophils

Acidophil consist of somatotrophs that secrete somatotrophic growth hormones and lactotrophs that secrete lactogenic hormone prolactin. 

Basophil consists of corticotropes that secrete adrenocorticoids hormones, thymotrophs that secrete thyroid stimulating hormone and gonadotrophs that secrete gonadotrophic hormones such as luteinizing stimulating hormone, follicle stimulating hormone. These hormones produced are tropic hormones, which target another endocrine hormone production. Pars intermedia, present between Pars distalis and Pars neurosa, is a narrow strip of tissues which is separated by Pars distalis by interglandular deft. In case of humans, it is poorly developed but in lower eukaryotes it is well developed. It secretes melanocyte stimulating hormone. Pars tuberalis is a tube-like structure surrounding infundibulum and it produces no hormone. Infundibulum is composed of neuroglial cells. Pars neurosa consist of pituicytes, herring bodies, most cells and non-myelinated fibers. Pituicytes are large branching spindle shaped neuroglial cells also called supporting cells. Herring bodies contain an accumulation of neurosecretory cells. They produce neurosecretory hormone on activation of nerve fibers, which secrete them and store them in Pars neurosa. Neurosecretory hormones are vasopressin and oxytocin. Secretion of neurosecretory hormone occurs by nerve fibers present in hypothalamus cells such as supra optic cells which Secrete vasopressin and other one paraventricular cell which secrete oxytocin.

Hormones secreted by pituitary gland:- 

i )Growth Hormones: -It is a protein of 191amino acids. It increases rate of metabolism and cell division. Its production is regulated by hypothalamus, which produces growth hormone-releasing hormone (GHRH) and growth hormone inhibitory factor (GHIF). On low glucose level of body, GHRH is activated that leads to activation of somatotrophs to Trelease growth hormone. In high glucose levels, GHIH/F is activated that leads to low production of growth hormone. 

Abnormalities: -

In children, growth hormone hyposecretion leads to dwarfism and its hypersecretion leads to a giant. In adults, growth Hormone hyposecretion lead to Simon's disease and its hypersecretion led to Acromegaly 

• Prolactin is a protein of 198 amino acids. It is produced on activation through prolactin releasing factor secreted by hypothalamus. Low level of estrogen and progesterone levels inhibit prolactin production and vice versa.
• Adrenocortico tropic hormones, are proteinaceous in nature with 39 amino acids. Its product is even regulated by corticotropic releasing hormone secreted by hypothalamus. Adrencortico tropic hormones are secreted on detecting. Low levels of cortical and indirectly affect adrenal glands.
• Thyroid stimulating hormone is glycoprotein in nature and is also under control of thyroxine releasing factor secreted by hypothalamus. It affects thyroid gland cells to secrete thyroxin.
• Follicle stimulating hormone (glycoprotein in nature) is also. Under control of hypothalamus - mediated secretion of follicle stimulating releasing hormone, which gets activated in low levels of progesterone and estrogen. It plays role in gametogenesis.
• Luteinizing hormone is glycoprotein in nature and is even under control of luteinizing hormone releasing factor secreted by hypothalamus and is secreted when there are low levels of progesterone and estrogen in females and low level of testosterone in male.
• Melanocytes stimulate hormones consist of a form of alpha and beta form of 13 amino acid and 22 amino acid and is regulated by Melanocyte stimulating hormone releasing factor release by hypothalamus and in case of fishes are activated in low intensity of light environment leading to pigmentation.
• Vasopressin which is also called antidiuretic hormone (ADH) is a protein of 10 amino acids sequence. It leads to water reabsorption from distal convoluted tube and collecting tube of nephrons. ADH enter kidney and form aquaporins through which water reabsorption occurs. ADH also inhibits sweat glands. Vasopressin production is inhibited when blood content rise, blood pressure rises and even on stretching of heart muscles that leads to production of atrial natriuretic hormone ANP that inhibit vasopressin production.
• Oxytocin is proteinaceous of 10 aa sequences, and it function in contraction of smooth muscles of uterus during time of parturition.
• High prolactin levels in blood are responsible for the initiation of maternal behavior in the first-time pregnant rats after parturition.

Thyroid gland It is largest gland of body. It is only gland which stores its own secretion. It is located close to larynx anteroventral side of trachea. It is bilobed structure. Its size depends on age, sex and physiological conditions. Fack lobe consists of thyroid follicles. Thyroid follicles are lined by simple cuboidal follicular epithelium and inside its follicle cavity, hormone is stored.

Formation of Thyroxine: -

• Iodine trapping Active transportation of iodide in epithelium occurs from blood circulating in thyroid gland.

• Formation of thyroglobulin Rough endoplasmic reticulum of follicular epithelium cells which are rich in tyrosine secretes thyroglobulin through exocytosis and reach the lumen of thyroid follicles. 

• Oxidation of iodide: - I + I = I2 molecule.

• Iodination of thyroglobulin: - If 1 molecule of iodine (l,) is added it forms monoiodotyrosine (T1) and if 2 molecules of iodine (I) is added it forms diiodotyrosine (T2). 
•  Coupling of T and T2: - Two molecules of T, combine to form mono-iodotyrosine which is formed but not active. One abundantly T1 and one T2 molecule combine to form T3 which is biological active form.
• Pinocytosis: - is mechanism through which T4 and T3 enter follicular epithelium cells and gets digested by lysozyme and then enter blood.

Function of Thyroxine: -

It increases metabolic rate, increase use of glucose and oxygen for ATP synthesis, increase body temperature stimulating protein synthesis, accelerate tissue growth and decrease body weight.

• Calcitonin: - is a protein hormone that is synthesized and secreted by humans and other mammals primarily by the parafollicular cells (C cells) in the thyroid gland. The overall effect of calcitonin is to lower the concentration of calcium in the blood when it rises above the normal value. It also lowers the concentration of phosphorus in the blood when levels exceed normal. 
•  Parathyroid Gland: - It is present at the back of thyroid gland. It is small oval yellowish brown and is present in 4 pairs. Upper pair is known as superior parathyroid and lower pair is known as inferior parathyroid. Parathyroid glands consist of 2 type of cells called principal cells which are smaller in size with large nucleus and no granules. Parathyroid hormone is also called collapse's hormone which deals with regulation of calcium and phosphorus balance. It increases blood calcium due to dissociation of calcium from bones.
  
  
  
  

ABNORMALITIES: - 

1. Hypersecretion 

2.Parathyroid tetany i.e., Ca+ increases 

3.Hypersecretion 

4.Ca++ dissolve in irregular manner, bone porous and osteoporosis.