Endocrine System

• Typically epithelial in origin, that aggregated as cords or clusters and surrounded by extensive networks of capillaries
• Secretory cells release signaling products called hormones
• Endocrine glands referred to as ductless glands because hormones

  diffuse directly into the bloodstream into the neighboring vascularized compartment for uptake by capillaries and distribution throughout the body
• Hormones is a chemical substances that act like messenger molecules in the body. After being made in one part of the body,

  they travel to other parts of the body where they influence cell growth and metabolism
• Hormones maintains the constancy of the chemical composition of

  extracellular and intracellular fluid
• each hormones is produced by specific glands
• Hormones are directly released from the tissue into the bloodstream and carried to the site
• Each hormone act to a specific site or target cell to induce certain characteristic biochemical changes
• Dispersal in interstitial fluid or through short loops of blood vessels

  • Gastronomy made by pyloric G cells reaches target cells in the fundic glands


  Paracrine Secretion
• Signaling molecule remains on the secreting cells surface or adjacent extracellular matrix and affects target cells when the cells make contact

  • embryonic and regenerative tissue interactions

  Juxtacrine Secretion
• Cells may produce molecules that act on themselves

  • Insulin like growth factor produced by several cell types may act on the same cells that produced it


  Autocrine Secretion
• Pituitary gland also known as hypophysis lies below the brain in a small cavity on the sphenoid bone called sella turcica
• • Neural component: Neurohypophyseal Bud
  • Oral component: Hypophyseal Pouch
• Two major parts of pituitary glands:

  1. Posterior pituitary (neurohypophysis) - has a part called para nervosa develops as a down growth of the developing brain and is attached in the hypothalamus by the infundibulum
• Two major parts of pituitary glands:

  2. Anterior pituitary (Adenohypophysis) - includes large pars distal and pars tuberalis that surrounds the infundibulum and thin pars intermedia adjacent to pars nervosa
• Hypothalamic hypophyseal portal system - has a great importance because it carries neuropeptides to the adenohypophysis where they either stimulate or inhibit hormone release by the endocrine cells there
• Components of pituitary gland

  • Primary capillary plexus - in the infundibulum and lower hypothalamus
  • Secondary plexus - in the pars distal connected by portal veins and draining to the hypophyseal vein
• Hypothalamic hypophyseal tract

  1. Supraoptic nuclei - ADH (Antidiuretic hormone) or vasopressin
  2. Paraventricular nuclei - oxytocin
• ADH – is released in response to increase blood tonicity (hypertonic), sensed by osmoreceptor cells in the hypothalamus; increases the permeability of the renal collecting ducts to water so that more

  water is reabsorbed from the filtrate in these tubules and osmotic balance of body fluids is restored.
• Oxytocin – stimulates contraction of uterine smooth muscle during childbirth and the myoepithelial cells in the mammary gland
• Bundle of axons called the hypothalamic-hypophyseal

  tract courses into the neurohypophysis from two

  important hypothalamic nuclei.
• Endocrine cells of the anterior pituitary can be called

  acidophils, basophils, or chromophobes based on their

  general staining properties; the latter lack secretory

  granules and stain poorly.
• Acidophils and basophils can be identified as to which

  pituitary hormone they produce using

  immunohistochemistry and antibodies against specific

  hormones.
• Chromophils (acidophils and basophils)

  • Are secretory cells in which hormone is stored in cytoplasmic granules
• Chromophobes

  • They have few or totally lack secretory granules and stain poorly
• • Acidophils are primarily somatotrophs producing somatotropin (growth hormone) or lactotrophs (or mammotrophs) producing prolactin (PRL)
• • Basophils include gonadotrophs producing FSH and LH and thyrotrophs making TSH
• A third type of basophil is the corticotrophs, synthesizes

  protein POMC – pro-opiomelanocortin (POMC), that is

  cleaved by proteases into the polypeptide hormones adrenocortical trophic hormone (ACTH) and Beta- lipotropin hormone
• Pituitary is considered as the master gland
• Hypothalamic hormones controls the secretion of

  hormones from anterior pituitary. Carried directly to the anterior pituitary gland via the hypothalamic- hypophyseal system.
• The releasing hormone (start the production of hormones)

  TRH, PRH, GnRH, CRH, GHRH
• The releasing hormone

  • TRH – triggers the release of the thyroid-stimulating hormone (TSH)
  • Prolactin releasing hormone – produce prolactin
  • GnRH – release or produce FSH and LH
  • CRH – sends a message to the anterior pituitary gland to release ACTH
  • GHRH – from the anterior pituitary to release growth hormone
• • The inhibiting hormone – PIH, GIH (stop the production of other hormones)
• The inhibiting hormone

  • PIH – dopamine; inhibits prolactin; inhibits milk production
  • GIH – somatostatin; this hormone is the opposite of the GHRH; inhibits the release or production of growth hormones
• Thyroid gland

  • Located anterior and inferior to the larynx, consists of two (2) lobes united by an isthmus
  • Originates in early embryonic life from the foregut endoderm near the base of the developing tongue
  • Consists mainly of spherical follicles composed of simple epithelium
• T3, T4 and Calcitonin – help control the basal metabolic rate in cells throughout the body
• Calcitonin – triggered by elevated blood Ca2+ levels, and it inhibits osteoclast activity.
• Parathyroid glands

  • are four small ovoid masses. They are located on the back of the thyroid gland, usually embedded in the larger gland’s capsule
  • Endocrine cells of the parathyroid glands are called principal (chief) cells
• Parathyroid gland secrete parathyroid hormone (PTH), an important

  regulator of blood calcium levels
• Parathyroid gland is derive from the embryonic pharyngeal pouches – the superior glands from the fourth pouch and the inferior glands from the third pouch