ENDOCRINE SYSTEM

• endocrine system and nervous system

  • major integration and control systems of the body
  • receive and respond to stimuli
  • nervous
  • responds rapidly and precisely to stimuli
  • consciously generated
  • endocrine - responds more slowly and more diffusely
  • responses usually longer lasting
  • done unconsciously
  • work parallel but independently
• endocrine system

  • composed of all the endocrine cells - produce substance called hormones
  • endocrine cells in the body occur:
  • singly or clusters (JG cells in the kidneys, enteroendocrine cells in the gastrointestinal tract, and interstitial cells (of Leydig) in the testes)
  • distinct organs called endocrine glands, i.e., pituitary, thyroid, parathyroid, adrenal, and pineal;
  • endocrine portion of the hypothalamus and the islets of Langerhans of the pancreas

  
• hormones

  • chemical messenger
  • secreted into capillaries and carried by blood to target organ/s or tissue/s that contain/s the cells (target cells) that possess appropriate receptors for them
• Hypothalamus

  • is part of the brain
  • component of the diencephalon,' which together with the telencephalon comprises the forebrain or prosencephalon
  • located at the base of the brain, behind the optic chiasm
  • forms the floor and part of the wall of the third ventricle
  • 11 major nuclei, nuclear areas and tracts
  • controls thirst, hunger and satiety, temperature, sexual behavior, and circadian rhythms
  • produces two groups of hormones:
  • posterior pituitary
  • hypophysiotropic
• Posterior Pituitary Hormones

  • are oxytocin and antidiuretic hormone (ADH; vasopressin)
  • synthesized by neuron (magnocellular secretory neurons) that are in the supraoptic and paraventricular nuclei of the hypothalamus
  • stored in the posterior lobe of the pituitary gland
  • supraoptic nucleus is located above and lateral to the optic chiasm
  • paraventricular nucleus is in the lateral wall of the third ventricle
• Oxytocin

  • target organs are the uterus and breasts
  • uterus
  • stimulates contraction of the smoot muscles in the myometrium
  • aids in parturition (i.e., delivery of the fetus)
  • mammary glands
  • promotes ejection of milk during lactation by stimulating contraction of the myoepithelial cells that surround the alveoli
  • males
  • facilitates sperm transport within the male reproductive system
• antidiuretic hormone (ADH; vasopressin)

  • target organs are the kidneys
  • increases permeability of the distal and collecting tubule of the kidneys, leading to the formation of more concentrated urine
• Hypophysiotropic Hormones (neurohormones)

  • include:
  • corticotrophin releasing hormone (CRH)
  • thyrotropin releasing hormone (TRH)
  • growth hormone-releasing hormone (GRH)
  • gonadotropin-releasing hormone (GnRH; luteinizing hormone-releasing hormone, LHRH)
  • growth hormone-inhibiting hormone (GHIH; somatostatin, SS)
  • prolactin-inhibiting hormone (PIH)
• Hypophysiotropic Hormones (neurohormones)

  • synthesized by secretory neurons (parvocellular secretory neurons)
  • smaller than the neurons that produce the posterior pituitary hormone
  • widely distributed within the hypothalamus
  • mostly in the •arcuate, paraventricular and periventricular nuclei
  • regulate the activity of the secretory cells of the pituitary gland
• PITUITARY GLAND (HYPOPHYSIS CEREBRI)

  • a small but very important endocrine gland
  • ovoid body
  • 500 mg
  • 12 mm in transverse
  • 8mm in AP diameters
  • attached to the inferior surface of the hypothalamus
  • lodged in a concavity (hypophyseal fossa)in the sella turcica of the sphenoid bone
  • 2 distinct parts:
  • neurohypophysis
  • adenohypophysis

  
• neurohypophysis

  • arises from neural ectoderm
  • develops as a downgrowth of the diencephalon
• adenohypophysis

  • arises from oral ectoderm
  • is derived from Rathke's pouch, an upgrowth of the oral mucosa
  • embryonic development
• pituitary gland

  
• Neurohypophysis

  • 3 regions:
  • median eminence
  • pituitary stalk (infundibulum, infundibular stem: infundibular stalk, hypophyseal stalk)
  • posterior lobe (pars nervosa, infundibular process)
  • downward extension of the hypothalamus
  • cell bodies
  • are located in the nuclei and nuclear area
  • unmyelinated axon (parvocelullar secretory neurons)
  • travel downwards
  • leave the hypothalamus and terminate in the neurohypophysis
  • short
  • terminate in the median eminence
• Neurohypophysis

  • unmyelinated axon (magnocellular secretory neurons)
  • longer
  • originate from the nuclei (i.e., supraoptic and paraventricular) where the cell bodies are located
  • pass through the pituitary stalk and end in the posterior lobe of the pituitary gland
  • axons of the secretory neurons are surrounded by numerous non-secretory supporting cells called pituicytes
  • only other cellular elements are the endothelial cells and the blood cells in the sinusoids
  • more fibrous than cellular
• median eminence

  • axons, together with the numerous capillaries and supporting cells that are associated
• pituitary stalk

  • comprise the hypothalamohypophyseal tract, , the main component
  • axons (magnocellular secretory neurons)
  • dilatations - storage sites for the secretory granules that are synthesized in the cell bodies ofthe neuron
• Herring bodies

  • Aggregations of secretory granules in the axonal dilatations
  • deeply-staining, basophilic structures
• Pituicytes

  • are stellate cells whose slender cytoplasmic processes are interconnected with those of other pituicytes by gap junction
  • morphologically similar to astrocytes
• Pituitary Gland, Posterior Lobe

  
• Adenohypophysis

  • part of the pituitary gland that produces hormones
  • more cellular than fibrous
  • 3 regions:
  • anterior lobe (pars ditalis)
  • pars tuberalis (pars infundibularis)
  • intermediate lobe (pars intermedia)
• pars tuberalis

  • forms a thin and incomplete sleeve around the pituitary stalk of the neurohypophysis
  • inferiorly continuous with the anterior and intermediate lobes
• Anterior Lobe (Pars Distalis)

  • about 70% of the pituitary gland
  • nearly all the hormones produced by the pituitary gland come from this lobe
  • parenchyma consists of epithelial cells (secretory)
  • parenchymal cells
  • form irregularly-arranged anastomosing cords and clusters
  • surrounded by fenestrated sinusoid
  • minimal amount of reticular tissue
  • serves as the supporting structure (stroma) for the cell cords and sinusoid

  
• Parenchymal Cells of the Auterior Lobe of the Pituitary Gland

  • LM (acid dyes) parenchymal cells in the anterior lobe of the pituitary gland can be classified into two general types:
  • chromophils
  • chromophobes
• chromophils

  • cytoplasm is intensely colored
  • classified into:
  • acidophils (alpha cells)
  • basophils (beta cells)
  • 5 types of chromophils:
  • somatotrophs (STH cells)
  • mammotrophs (lactotrophs)
  • thyrotrophs
  • corticotrophs
  • gonadotrophs
• acidophils

  • stains red with acid dyes
  • somatotrophs
  • mammotrophs
• basophils

  • stains blue or purple
  • nucleus is less dense, thus, lighter staining
  • less numerous but are larger
  • thyrotrophs
  • corticotrophs
  • gonadotrophs
• Acidophils

  • possess a reddish yellow-staining cytoplasm
  • a dense nucleus
  • distinct cell outline
• Somatotrophs

  • comprise the majority (about 50%) of the chromophils in the anterior lobe
  • somatotropin (growth hormone) - do not have specific target cell
• Mammotrophs

  • comprise about 20% of the chromophils in the anterior lobe
  • scattered singly
  • secrete the hormone prolactin (mammotropin)
  • pregnancy
  • increase in size
  • stimulates growth and activity ofthe mammary glands
  • male
  • regulating testicular function
• somatomammotrophs

  • secrete both somatotropin and prolactin
• Thyrotrophs

  • comprise about 5% of the chromophils in the anterior lobe
  • polygonal cell
  • long cytoplasmic processes
  • secretory granules (smallest)
  • produce thyrotropin (thyroid-stimulating hormone, TSH)
• thyrotropin (thyroid-stimulating hormone, TSH)

  • stimulates the cells in the thyroid gland that are responsible for producing thyroid hormones
• Corticotrophs

  • ovoid cells
  • constitute 20% of the chromophils in the anterior lobe
  • secretory granules - few but slightly larger
  • secrete corticotropin (adrenocorticotropin, adrenocorticotropic hormone, ACTH)
• corticotropin (adrenocorticotropin, adrenocorticotropic hormone,ACTH)

  • stimulates certain cells in the adrenal cortex to produce hormones
• Gonadotrophs

  • large, round cells
  • constitute around 5% of the chromophils in the anterior lobe
  • secretory granules - moderate
  • produce gonadotropins, ~..e., luteinizing hormone (LH) and follicle stimulating hormone (FSH)
• Chromophobes

  • Electron microscopy and histologic studies - are actually chromophils (mostly corticotrophs)
  • have pale cytoplasm
  • resting, or have just recently released most of their secretory granules, or are still in the process of producing new ones
  • 2 kinds:
  • folliculostellate cell (FS cell)
  • undifferentiated stem cell
• folliculostellate cell (FS cell)

  • long, branching processes
  • non-secretory and performs some supportive role for the other cells
• Pars Tuberalis (Pars Infundibularis)

  • tubular sleeve that the pars tuberalis forms around the pituitary stalk
  • not uniform in thickness
  • 2 to 60 um and is thickest anteriorly
  • separated from the pituitary stalk by connective tissue that is continuous with the pia-arachnoid membrane of the brain
  • more vascular
  • blood vessels - hypophyseoportal system
  • parenchymal cells - gonadotrophs and thyrotrophs
• Intermediate Lobe (Pars Intermedia)

  • is the thin and poorly developed region of the gland
  • in between the posterior lobe and the anterior lobe
  • better developed in the fetus than in adults, where it is rudimentary
  • presence of follicles (Rathke's cysts) that contain some eosinophilic colloidal material
  • parenchymal cells
  • form irregular clusters
  • contain some fine secretory granules
  • synthesize melanocyte-stimulating hormone (MSH)
  • also produce B endorphins.