Endocrine Glands Endocrine glands are ductless organs that produce substances, termed hormones, which are released into circulatory system and transported to distant receptor organs.

Most endocrine glands release their hormones into postcapillary veins that do not drain into the portal vein, but circulate around the whole organism before reaching the liver.

Hormones are produced by parenchymal cells, which can be found singly,as aggregate or organized in Endocrine organs.

The function of endocrine tissue is regulated by simple or complex feedback mechanisms, many of which involve pituitary gland.

Glands of Endocrine system:

Endocrine system includes following glands.

Pituitary Gland Pineal Gland Thyroid Gland Parathyroid Glands Adrenal Gland Pancreas Gonads

Pituitary Gland: The pituitary gland plays a major regulatory role in the entire endocrine system.It is also reffered as the “master gland” of the body.

Location:

The pituitary gland is situated at the base of the skull.It is suspended below the diencephalon, in the hypophyseal fossa of the sphenoid bone between the optic

chiasm and the mamillary body.Pituitary gland is connected to hypothalamus by “pituitary stalk”

Embryological development of pituitary gland: Parts of pituitary glands are derived from different embryological origin.The Posterior pituitary is formed from neural tissue in floor of the third ventricle,at the site of development of hypothalamus.

The anterior pituitary and the intermediate pituitary develop from epithelial tissue,in roof of oral cavity.

Hormones production,in pituitary gland,starts after approximately one third of fetal development,has been completed.

Morphology of pituitary gland:

Pituitary gland is an un-paired organ and has size of only pea.

Anatomy of pituitary gland:

Anatomically the pituitary gland is made up of three parts.

Adenohypophysis (anterior pituitary) Neurohypophysis (posterior pituitary) Intermediate lobe

Adenohypophysis: Adenohypophysis or anterior pituitary consists of endocrine cells,numerous blood vessels and very few nerve fibers. The pituitary stalk contains a special portal venous system,organized in same manner as portal venous system in the liver,with two capillary networks following one another.

Blood supply to adenohypophysis: Vessels from first capillary network convey blood to

anterior lobe where second capillary network surrounds the hormone producing cells.

Nerve supply to adenohypophysis:Neurons in hypothalamus send their axon directly to the first capillary network.The capillary walls contain pores that permit rapid transport directly into blood of relatively larger molecules from ends of axon terminals.

Hormones of Adenohypophysis:Adenohypophysis secretes following hormones.

Hormone Effect Somatotropin Growth via

secretion of IGF-I

Thyroid stimulatin hormone (TSH)

Secretion of thyroid hormone

Adenocorticotopic Secretion of

Hormone (ACTH) glucocorticoids and androgens

Prolactin (PRL) Production of milk

Gonadotropin:- Follicle stimulating

hormone (FSH) Luteinzing

hormone (LH)

Production and maturation of spermatozoa and oocyte.

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 the adenohypophysis and is a neural outgrowth of hypothalamus.

Nerve supply to neurohypophysis: The neurons present in neurohypophysis have their cell bodies in the hypothalamus.

Anatomy of neurohypophysis: It consists of stalk which connects the pituitary gland to the tuber cinereum of hypothalamusand the distal, major portion of neurohypophysis.The third ventricle extends into the neurohypophysis through the cylindrical stalk as the neurohypophysial recess. Neurohypophysis stores and releases hormones produced by the neurosecretory cells of supraoptic and paraventricular nuclei of hypothalamus.

Mode of secretion: The posterior pituitary contains no hormone producing cells , instead , hormones of posterior pituitary are released from neurons that have their cell bodies in the hypothalamus. Because the posterior pituitary hormones are produced in the hypothalamus , it is not entirely accurate to call the posterior as endocrine

gland.Rather it serves as hormone storage store.

Hormones of neurohypophysis: Following hormones are produced by neurohypothalamus.

Hormone Effect Oxytocin Elicit milk let

down Increase uterine

contractions Antideuratic

Hormone Reabsorption of

water in kidneys

Intermediate lobe: The portion of adenohypophysis, in direct contact with distal part of neurohypophysis is termed as intermediate lobe.

Location:

Intermediate lobe is present between two major parts of pituitary gland, which thing gives it its name.

Anatomy of intermediate lobe: Intermediate lobe extends around neurohypophysis and the anterior lobe is separated from the intermediate lobe by the hypophyseal cleft which is also known as cavum hypophysis.

Embryonic development of intermediate lobe: During embryonic development intermediate lobe develops from epithelial tissue.

Hormones produced by intermediate lobe: The intermediate lobe of pituitary gland produces melanocyte

stimulating hormone that regulates the skin colour. This hormone regulates the skin colour only in lower vertebrates but the function of this hormone is not known in mammals and birds, although it is secreted.

Pineal Gland:Location: Pineal gland is an organ which is

unpaired and located in epithalamus.Morphology of pineal gland: The size of pineal gland varies greatly among species and between individuals. It structurally resembles a pin cone.

Anatomy of pineal gland: Pineal gland is a part of diencephalon. It is unpaired. It is connected to roof of the diencephalon by habenulae and the short peduncle.

The pineal gland is innervated by postganlionic sympathetic fibers from the cranial cervical ganglion, that extends to the organ with in the adventitia of small blood vessels.

Nerve supply to pineal gland: The pineal gland cells have a chain of neurons which come to pineal gland by passing from the retina, via the hypothalamus, thoracic spinal cord and the cranial cervical ganglia.

Hormones of pineal gland and their function: Pineal gland produces melatonin which has gonadotropic effects which are important in seasonality of reproductive cycles in certain species. Pineal gland also works as biological clock regulating seasonal and diurnal variation in gonadal activity.

In horse, where melatonin has antigonadotropic effects, melatonin production is inhibited by light stimulus, so that as day length increases in spring, melatonin production decreases and its inhibitory effect on gonad activity is reduced.