Endocrine System

• Regulatory system involved in transmitting messages and correlating various body functions - by releasing chemical messengers called hormones into the bloodstream.
• Actions of hormones:

  • Help control internal environment by regulating its chemical composition and volume
  • Respond to marked changes in environmental conditions to help the body cope with emergency demands (infection, trauma, emotional stress, dehydration, starvation, haemorrhage and temperature extremes)
  • Assume a role in the smooth, sequential integration of growth and development
  • Contribute to the basic processes of reproduction, incl. gamete production, fertilisation, nourishment of embryo and foetus, delivery and nourishment of the newborn.
• Endocrine Glands make up the system.

  • Secrete their products (hormones) into the extracellular space around the secretory glands.
  • Secretions then pass into capillaries to be transported by the blood.
• Endocrine Glands incl:

  • Pituitary
  • Thyroid
  • Parathyroids
  • Adrenal
  • Pineal
  • Thymus
• Several organs contain endocrine tissue:

  • Pancreas
  • Stomach
  • Ovaries
  • Testes
  • Kidneys
  • Small intestine
  • Placenta

   
• Pituitary Gland

  Hormones regulate multiple body activities - "Master Gland"

  Relatively small structure measuring about 13mm in diameter

  Divided structurally and functionally into:

  • Anterior lobe
  • Posterior lobe

  Both lobes connected to hypothalamus.

   
• When anterior pituitary gland is stimulated it secretes:

  • Somatotroph cells: growth hormone - controls body growth
  • Lactotroph cells: prolactin -initiates milk production by mammary glands
  • Corticolipotroph cells: adrenocorticotrophic hormone -> stimulates adrenal cortex to secrete
  • Thyrotroph cells: thyroid stimulating hormone
  • Gonadotroph cells: follicle stimulating hormone (egg/sperm production)
  • Luteinising hormones -> stimulate other sexual and reproductive activities
  • Intermediate lobe secretes: melanocyte stimulating hormone -> related to skin pigmentation
• Posterior Pituitary

  Technically not an endocrine gland and does not produce hormones.

  Neurosecretory cells of the hypothalamus produce 2 hormones:

  • Oxytocin
  • Antidiuretic hormone

  Transported and stored in the posterior pituitary gland awaiting release.
• Location of the thyroid:

  Just below larynx

  Right and left lateral lobes lie on either side of the trachea connected by a mass of tissue called an isthmus
• Thyroid Histology:

  Composed of spherical sacs called thyroid follicles

  Wall of each follicle consists of 2 cell types:

  • Follicular cells
  • Parafollicular or C cells
• Thyroid Hormones:

  Follicular cells manufacture thyroxine or T4 and tri-iodothyronine or T3. together these hormones are referred to as the thyroid hormones.

  Parafollicular cells produce calcitonin.
• Parathyroid Location:

  Typically embedded on posterior surfaces of lateral lobes of thyroid.

  Small, round masses of tissue.

  Usually, 2 parathyroids, superior and inferior, are attached to each lateral thyroid lobe
• Parathyroid Histology:

  Parathyroids contain 2 types of epithelial cells:

  • Principal or Chief cells, these cells produce parathyroid hormone
  • Oxyphil cells are believed to produce a reserve capacity of same hormone.
• Adrenal Gland Location:

  2 adrenal glands, one located superior to each kidney.

  Structurally, and functionally differentiated into 2 sections.

  • Adrenal cortex
  • Adrenal medulla
• Adrenal Histology:

  1. Zona glomerulosa, the outer zone, directly underneath the connective tissue capsule. It comprises about 15% of the total cortical volume. Its primary secretions are a group of hormones called mineralocorticoids.
  2. Zona fasciculate: middle zone and widest of the 3 consisting of cells arranged in long, straight cords. Secretes mainly glucocorticoids.
  3. Zona reticularis: inner zone, contains cords of cells that branch freely. Produces small amounts of gonadocorticoids.
• Pancreas Function:

  Exhibits the dual functionality of being both an exocrine and endocrine gland.
• Pancreas Histology: endocrine portion of pancreas consists of clusters of cells called pancreatic islets or islets of Langerhans.

  3 types of cells found in islets:

  • Alpha cells - secrete glucagon
  • Beta cells - secrete insulin
  • Delta cells - secrete somatostatin

   
• Pineal Gland

  Location:

  Attached to roof of third ventricle.

  Covered by a capsule formed by the pia mater and consists of masses of neuroglial cells called pinealocytes.

  Around the cells are scattered postganglionic sympathetic fibres.

  Hormones:

  Main hormone produced by the pineal gland is melatonin.
• Thymus

  Location:

  Upper anterior part of the chest directly behind the sternum and between the lungs.

  Has 2 thymic lobes divided into cortical and medullary regions.

  Hormones:

  Produces a number of hormones involved in the maturation of T-lymphocytes

  Incl. thymosin, thymic humoral factor, thymic factor and thymopoietin