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Endocrine System. The endocrine system is composed of several ductless glands , clusters of cells located within certain organs, & isolated endocrine cells [so-called diffuse neuroendocrine system cells (DNES)] in the epithelial lining of the gastrointestinal & respiratory systems.

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endocrine system
Endocrine System
  • The endocrine system is composed of several ductless glands, clusters of cells located within certain organs, & isolated endocrine cells [so-called diffuse neuroendocrine system cells (DNES)] in the epithelial lining of the gastrointestinal & respiratory systems.
  • Glands of the endocrine system include the pituitary, thyroid, parathyroid, adrenal, & pineal glands.
  • Function. The endocrine system secretes hormones into nearby capillaries & interacts with the nervous system to modulate & control the body’s metabolic activities.
  • Hormones (Gr. hormaein, to excite) are chemical messengers that are carried via the bloodstream to distant target cells. Its functions are essential in maintaining homeostasis and coordinating body growth & development.
  • Steroids, cholesterol-derived compounds
    • gonadal & adrenocortical steroids
  • Small peptides, polypeptides, & proteins
    • insulin, glucagon, GH, ACTH, FSH, LH, ADH, oxytocin, interleukins, various growth factors
  • Amino acids & arachidonic acid analogs & derivatives
    • catecholamines, prostaglandins, & leukotriens
    • thyroid hormones
hormone s receptors
Hormone’s Receptors
  • Cell surface receptors
    • peptide hormones or catecholamines
    • second messengers
      • adenylate cyclase/cAMP system
      • guanyly cyclase/cGMP system
      • tyrosine kinase system
      • phosphatidylinositol system
      • activation of ion channels
  • Intracellular receptors
    • steroids & thyroid hormones
    • influence gene expression directly
pituitary gland hypophysis
Pituitary Gland (Hypophysis)
  • The pituitary gland lies below the hypothalamus, to which it is structurally & functionally connected (“master organs” of the endocrine system).
  • It is divided into 2 major subdivisions, the adenohypophysis & the neurohypophysis.
  • Each subdivision is derived from a distinct embryonic analog, which is reflected in its unique cellular constituents & functions.
  • The adenohypophysis is also called the anterior pituitary gland.
  • It is derived from an evagination of the ectoderm of the oropharynx (Rathke’s pouch).
  • It is subdivided into the pars distalis, pars intermedia, & pars tuberalis.
pars distalis
Pars Distalis
  • The pars distalis is supported by a connective tissue capsule & framework.
  • It consists of irregular cords of parenchymal cells lying adjacent to fenestrated sinusoidal capillaries.
  • Chromophils are parenchymal cells that stain intensely due to their hormone-containing secretory granules. They synthesize, store, & release several hormones. They are regulated by specific stimulatory & inhibitory hormones produced by neurosecretory cells in the hypothalamus & are conveyed to the pars distalis via a system of portal blood vessels originating in the median eminence.
  • Chromophils are classified into 2 types, depending on the dyes they bind using special histologic stains. With hematoxylin-eosin stain, the distinction between the 2 cell types is much less obvious.
  • Acidophils bind acid dyes & often stain orange or red. They are small cells of 2 subtypes: somatotropes & lactotropes.
    • Somatotropes, which produce somatotropin (GH), are stimulated by GHRH or ghrelin & are inhibited by somatostatin.
    • Lactotropes (mammotropes) produce prolactin, which is stored in small secretory granules. They are stimulated by TRH or VIP & are inhibited by dopamine.
  • Basophils bind basic dyes, typically stain blue & react PAS reagent. They include 3 subtypes: corticotropes, gonadotropes & thyrotropes.
    • Corticotropes produce proopiomelanocortin (POMC) - ACTH, b-lipotropic hormone (b-LPH), melanocyte-stimulating hormone (MSH), b-endorphin, & enkephalin. They are stimulated by CRH.
  • Gonadotropes produce FSH & LH in both sexes, although in men the latter is sometimes referred to as interstitial cell-stimulating hormone (ICSH). Gonadotropes are stimulated by GnRH.
  • Thyrotropes produce TSH (thyrotropic hormone) & are stimulated by TRH.
  • Are parenchymal cells that do not stain intensely
  • Appear as small cells under the light microscope; the cells lack (or have only a few) secretory granules & are arranged close to one another in clusters
  • Sometimes resemble degranulated chromophils in the electron microscope, suggesting that they may represent different stages in the life cycle of various acidophil & basophil populations
  • May also represent undifferentiated cells that are capable of differentiating into various types of chromophils
folliculostellate cells
Folliculostellate Cells
  • Are numerous in the pars distalis & lie between the chromophils & chromophobes
  • Possess long processes that form gap junction with processes of other folliculostellate cells
  • Produce many peptides that are thought to regulate the production of pars distalis hormones via a paracrine effect
pars intermedia
Pars Intermedia
  • The pars intermedia lies between the pars distalis & pars nervosa.
    • It is characterized by the presence of many colloid-containing cysts (Rathke’s cysts) that are lined by cuboidal cells.
    • It possesses basophils, which sometimes extend into the pars nervosa, & chromophobes. The function of the pars intermedia cells in humans remains unclear.
pars tuberalis
Pars Tuberalis
  • The pars tuberalis surrounds the cranial part of the infundibulum (hypophyseal stalk).
    • It is composed of cuboidal basophilic cells, arranged in cords along an abundant capillary network.
    • Its cells often show immunoreactivity for ACTH, FSH & LH.
  • The neurohypophysis is also called the posterior pituitary gland.
  • It is derived from a downgrowth (the future infundibulum) of neuroectoderm of the floor of the 3rd ventricle (the diencephalon) of the developing brain.
  • It is divided into the infundibulum, which is continuous with the hypothalamus, & the pars nervosa or main body of the neurohypophysis.
hypothalamohypophyseal tract
Hypothalamohypophyseal Tract
  • Contains the unmyelinated axons of neurosecretory cells whose cell bodies are located in the supraoptic & paraventricular nuclei of the hypothalamus
  • They do not terminate on other neurons or target cells but end in close proximity to the fenestrated capillary network of the pars nervosa.
  • They contain secretory vesicles in all parts of the cells, i.e., the cell body, axon, & axon terminal.
pars nervosa
Pars Nervosa
  • Contains the distal ends of the hypothalamo-hypophyseal axons.
  • 3 sizes of membrane-bounded vesicles
    • neurosecretory vesicles (10~30 nm) in Herring bodies (oxytocin, ADH, ATP, & neurophysin)
    • 30-nm vesicles containing acetylcholine
    • 50~80 nm vesicles
  • Releases oxytocin & ADH into fenestrated capillaries in response to nerve stimulation
  • occupy approximately 25% of the volume of the pars nervosa.
  • are glial-like cells that support axons in this region.
  • possess numerous cytoplasmic processes & contain lipid droplets, intermediate filaments (GFAP), & pigment vesicles.
vascularization of the pituitary gland
Vascularization of thePituitary Gland
  • Arterial supply is from 2 pairs of blood vessels derived from the internal carotid artery.
    • The right & left superior hypophyseal arteries serve the pars tuberalis, infundibulum, & median eminence.
    • The right & left inferior hypophyseal arteries serve mostly the pars nervosa.
hypophyseal portal system
Hypophyseal Portal System
  • The primary capillary plexusconsists of fenestrated capillaries coming off the superior hypophyseal arteries.
    • This plexus is located in the median eminence where stored hypothalamic neurosecretory hormones enter the blood.
    • It is drained by hypophyseal portal veins, which descend through the infundibulum into the adenohypophysis.
hypophyseal portal system38
Hypophyseal Portal System
  • The secondary capillary plexus consists of fenestrated sinusoidal capillaries coming off the hypophyseal portal veins.
    • This plexus is located in the pars distalis where neurosecretory hormones leave the blood to stimulate or inhibit the parenchymal cells.
regulation of the pars distalis
Regulation of the Pars Distalis
  • Neurosecretory cells in the hypothalamus synthesize specific hormones that enter the hypophyseal portal system & stimulate or inhibit the parenchymal cells of the pars distalis.
  • The hypothalamic neurosecretory cells in turn are regulated by the level of hormones in the blood (negative feedback) or by other physiologic (or psychologic) factors.
  • Some hormones (e.g., thyroid hormones, cortisol) exert negative feedback on the pars distalis directly.