endo system

• The endocrine system works with the nervous system but independently - functional association

  
• Most important part of the endocrine system

  The brain
• Components of the endocrine system

  • Endocrine cells
  • Hormones
  • Target cells
• Endocrine cells


  • Produce hormones
• Hormones


  Chemical substances
• Target cells


  Contain appropriate receptors
• Arrangement of endocrine cells in the body


  • Embedded singly or in clusters
  • As distinct organs (called endocrine glands)
  • As component structures of organs
• Principal parts of the brain

  • Cerebrum
  • Diencephalon
  • Thalamus
  • Hypothalamus
  • Cerebellum
  • Brainstem
  • Medulla
  • Pons
  • Midbrain

  
• Hypothalamus


  Located at the base of the brain, a component of the diencephalon, controls functions related to thirst, hunger, satiety, temperature, sexual behavior, and circadian rhythms

  
• Secretory cells that produce ADH, oxytocin, and vasopressin are found in the hypothalamus

  
• Two groups of hormones produced by the hypothalamus

  • Posterior pituitary
  • Hypophysiotropic hormones

  
• Oxytocin (vasopressin)


  Target organs are the breast and the uterus, stimulates contraction of the myometrium of the uterus which aids in the delivery of the fetus (parturition), promotes ejection of milk during lactation by stimulating contraction of myoepithelial cells that surround the alveoli
• ADH


  Target organs are the kidneys, increases permeability of the DCT and collecting tubules thus forming more concentrated urine
• Hypophysiotropic hormones (neurohormones)


  • CRH - Corticotrophin RH
  • TRH - thyrotropin RH
  • GRH - growth hormone RH
  • GnRH - gonadotropin RH
  • LHRH - luteinizing hormone RH
  • GHIH - growth hormone inhibiting hormone
  • PIH - prolactin inhibiting hormone
• Pituitary gland (Hypophysis)


  Ovoid body, weighs 500mg about 12mm in transverse diameter, attached to the inferior surface of the hypothalamus, logged in hypophyseal fossa in the sella turcica of the sphenoid bone

  
• Two parts of the pituitary gland

  • Neurohypophysis - arises from the neural ectoderm
  • Adenohypophysis - arises from the oral ectoderm, derived from Rathke's pouch

  
• Histology of the pituitary gland

  • Neurohypophysis is more fibrous, pars nervosa is composed of unmyelinated, pars distalis looks like a cellular
  • Adenohypophysis has 3 regions: anterior lobe, pars tuberalis, and intermediate lobe

  
• Regions of the neurohypophysis

  • Median eminence
  • Pituitary stalk - infundibulum, infundibular stem, hypophyseal stalk
  • Posterior lobe - pars nervosa, infundibular process

  
• Herring bodies


  Basophilic structures in the neurohypophysis
• Pituicytes


  Cells in the neurohypophysis

  
• Regions of the adenohypophysis

  • Anterior lobe - pars distalis
  • Pars tuberalis - pars infundibularis
  • Intermediate lobe - pars intermedia

  
• Hormones secreted by the adenohypophysis


  • FSH
  • LH
  • ACTH
  • TSH
  • Prolactin
  • EH
  • GH
• GPA

  Growth hormone and prolactin are secreted by acidophils
• BFLAT

  Basophils secrete FSH, LH, ACTH, TSH
• Histology of the adenohypophysis

  • Anterior lobe is easily recognized because of the presence of follicles (Rathke's follicles) containing colloid material

  
• The thyroid gland is the largest endocrine organ

  
• Thyroid gland


  • Weighs 25-40g, bigger in women than in men, consists of 2 lateral lobes and an isthmus, arises as an epithelial invagination at the base of the tongue then migrates downward in the lower part of the neck

  
• Cell types in the thyroid gland

  • Parafollicular cells - clear cells
  • Principal cells - follicular cells
  • Colloid-Thyroglobulin

  
• Follicular/principal cells


  • Predominant cell, darker than the clear cell, have round nucleus that contains fine chromatin material, have one or two nucleoli with slightly basophilic cytoplasm, luminal surface is provided with numerous microvilli, produce thyroglobulin to synthesize T3 and T4 thyroid hormones using TPO

  
• Parafollicular/clear cells


  • Comprise 0.1% of the epithelial cell population, bigger than the principal cells, scattered singly or in small group in the epithelium, rest on the basal lamina of the epithelium but their apices do not touch the colloid, bigger and lighter staining than follicular cells, cytoplasm contains many secretory granules, produce calcitonin
• Thyroid hormones


  • Thyroxine (T4)
  • Triiodothyronine (T3)
  • Calcitonin (thyrocalcitonin)

  
• TSH


  Regulates the secretion of T3 and T4
• T3 and T4


  Regulate the metabolism of protein, vitamins, etc.
• Calcitonin


  Secreted by the clear cells, lowers the blood calcium level by suppressing the activity of the osteoclast
• The parathyroid gland is usually two pairs (superior and inferior) of tiny, ovoid, yellowish brown bodies attached to the posterior surface of the lateral lobes of the thyroid gland

  
• Parathyroid gland


  • Each gland weighs about 50 mg and about 6mm long x 3.4mm wide and 1-2mm thick, the inferior and superior gland develops in the embryo from the 3rd and 4th pharyngeal pouch respectively

  
• Cell types in the parathyroid gland

  • Chief cells (principal cells)
  • Oxyphil cells (acidophil cells)
• Chief cells


  • Compromise the majority, has prominent nucleus, cytoplasm is homogenous and faintly acidophilic, contains small secretory granules, secretes PTH
• Oxyphil cells


  • Bigger than a chief cell, occur singly or in clusters in the lobules, nucleus is smaller, cytoplasm is intensely eosinophilic because of the many acidophilic granules (actually mitochondria), non-secretory with unknown function
• Parathyroid hormone (PTH)


  Synthesized and secreted by the chief cells, important regulator of blood calcium levels, secretion is controlled by the blood level of Ionized Ca, indirectly increases the activity of osteoclasts, inhibits bone formation activity of osteoblasts, enhances Ca reabsorption in the renal tubules, increases conversion of vitamin D to its active form, increases excretion of phosphate by the kidneys, promotes Ca absorption in the GIT