Endocrine GlandsEndocrine glands are ductless organs that producesubstances, termed hormones, which are released intocirculatory system and transported to distant receptororgans.Most endocrine glands release their hormones intopostcapillary veins that do not drain into the portal vein,but circulate around the whole organism beforereaching the liver. Hormones are produced byparenchymal cells, which can befound singly,asaggregate or organized in Endocrine organs. The function of endocrine tissue isregulated by simple or complex feedback mechanisms,many of which involve pituitary gland.Glands of Endocrine system: Endocrine system includesfollowing glands. Pituitary Gland
Pineal Gland Thyroid Gland Parathyroid Glands Adrenal Gland Pancreas Gonads Pituitary Gland:The pituitary gland plays a major regulatory role in theentire endocrine system.It is also reffered as the “mastergland” of the body.Location:The pituitary gland is situated at the base of the skull.Itis suspended below the diencephalon, in thehypophyseal fossa of the sphenoid bone between theoptic chiasm and the mamillarybody.Pituitary gland isconnected to hypothalamus by “pituitary stalk”Embryological development of pituitarygland: Parts of
pituitary glands are derived from differentembryological origin.The Posterior pituitary is formedfrom neural tissue in floor of the third ventricle,at thesite of development of hypothalamus. The anterior pituitary and the intermediatepituitary develop from epithelial tissue,in roof of oralcavity.Hormones production,in pituitary gland,starts afterapproximately one third of fetal development,has beencompleted.Morphology of pituitary gland:Pituitary gland is an un-paired organ and has size ofonly pea.Anatomy of pituitary gland: Anatomically the pituitary gland is made upof three parts. Adenohypophysis (anterior pituitary) Neurohypophysis (posterior pituitary) Intermediate lobe
Adenohypophysis:Adenohypophysis or anterior pituitary consists ofendocrine cells,numerous blood vessels and veryfew nerve fibers. The pituitary stalk contains a special portal venoussystem,organized in same manner as portal venoussystem in the liver,with two capillary networksfollowing one another.Blood supply to adenohypophysis:Vessels from first capillary network convey bloodto anterior lobe where second capillary networksurrounds the hormone producing cells.Nerve supply to adenohypophysis:Neurons in hypothalamus send their axon directly tothe first capillary network.The capillary wallscontain pores that permit rapid transport directlyinto blood of relatively larger molecules from endsof axon terminals.Hormones of Adenohypophysis:
Adenohypophysis secretes following hormones.Hormone Effect Somatotropin Growth via secretion of IGF-I Thyroid stimulatin Secretion of thyroid hormone (TSH) hormone Adenocorticotopic Secretion of Hormone (ACTH) glucocorticoids and androgens Prolactin (PRL) Production of milk Gonadotropin:- Production and Follicle stimulating maturation of hormone (FSH) spermatozoa and Luteinzing hormone oocyte. (LH) Secretion of sex hormone Neurohypophysis: The neurohypophysis consists of nerve fibers, supporting cells, glial cells, blood vessels and it is a part of central nervous system. Location:
Neurohypophysis is located caudal to theadenohypophysis and is a neural outgrowth ofhypothalamus.Nerve supply to neurohypophysis:The neurons present in neurohypophysis have theircell bodies in the hypothalamus.Anatomy of neurohypophysis:It consists of stalkwhich connects the pituitarygland to the tuber cinereum of hypothalamusand thedistal, major portion of neurohypophysis.The thirdventricle extends into the neurohypophysis throughthe cylindrical stalk as the neurohypophysial recess.Neurohypophysis stores and releases hormonesproduced by the neurosecretory cells of supraopticand paraventricular nuclei of hypothalamus.Mode of secretion:The posterior pituitary contains no hormoneproducing cells , instead , hormones of posteriorpituitary are released from neurons that have theircell bodies in the hypothalamus. Because theposterior pituitary hormones are produced in thehypothalamus , it is not entirely accurate to call the
posterior as endocrine gland.Rather it serves ashormone storage store.Hormones of neurohypophysis:Following hormones are produced byneurohypothalamus.Hormone Effect Oxytocin Elicit milk let down Increase uterine contractions Antideuratic Reabsorption of Hormone water in kidneysIntermediate lobe:The portion of adenohypophysis, in direct contactwith distal part of neurohypophysis is termed asintermediate lobe.Location:Intermediate lobe is present between two majorparts of pituitary gland, which thing gives it itsname.
Anatomy of intermediate lobe:Intermediate lobe extends around neurohypophysisand the anterior lobe is separated from theintermediate lobe by the hypophyseal cleft which isalso known as cavumhypophysis.Embryonic development of intermediatelobe:During embryonic development intermediate lobedevelops from epithelial tissue.Hormones produced by intermediatelobe:The intermediate lobe of pituitary gland producesmelanocyte stimulating hormonethat regulates theskin colour. This hormone regulates the skin colouronly in lower vertebrates but the function of thishormone is not known in mammals and birds,although it is secreted.
Pineal Gland:Location:Pineal gland is an organ which is unpaired andlocated in epithalamus.Morphology of pineal gland:The size of pineal gland varies greatly amongspecies and between individuals. It structurallyresembles a pin cone.Anatomy of pineal gland:Pineal gland is a part of diencephalon. It isunpaired. It is connected to roof of the diencephalonby habenulae and the short peduncle. The pineal gland is innervated bypostganlionic sympathetic fibers from the cranialcervical ganglion, that extends to the organ with inthe adventitia of small blood vessels.Nerve supply to pineal gland:The pineal gland cells have a chain of neuronswhich come to pineal gland by passing from theretina, via the hypothalamus, thoracic spinal cordand the cranial cervical ganglia.
Hormones of pineal gland and theirfunction:Pineal gland produces melatonin which hasgonadotropic effects which are important inseasonality of reproductive cycles in certain species.Pineal gland also works as biological clockregulating seasonal and diurnal variation in gonadalactivity.In horse, where melatonin has antigonadotropiceffects, melatonin production is inhibited by lightstimulus, so that as day length increases in spring,melatonin production decreases and its inhibitoryeffect on gonad activity is reduced.