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Pituitary (Hypophysis)

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Topic updated on 05/18/17 12:32pm

Overview
  • Overview
    • pituitary, or hypophysis, rests in sella turcica, a depression of the sphenoid bone
    • primary function of pituitary is secretion of hormones
    • anatomically, pituitary is divided into anterior and posterior pituitary
Anterior Pituitary: Anatomy and Embryology
  • Anterior pituitary 
    • anterior pituitary, or adenohypophysis, is the glandular component of the pituitary
      • a collection of endocrine cells
        • responsible for synthesis and secretion of anterior pituitary hormones
    • consists of pars distalis, pars intermedia,and  pars tuberalis
    • embryological origin
      • oral ectoderm (Rathke's pouch)
        • upgrowth of oral ectoderm
        • primitive oral cavity
  • Anterior pituitary cell types
    • various cell types are responsible for synthesis and secretion of various hormones
    • gonadotrophs
      • FSH and LH
    • corticotrophs
      • ACTH
    • thyrotrophs
      • TSH
    • lactotrophs
      • prolactin
    • somatotrophs
      • growth hormone (somatotropin)
  • Histology
    • chromophils
      • anterior pituitary cells that contain granules that react with acidophilic/basophilic stains
        • acidophils
          • anterior pituitary cells (pars distalis) that contain granules that react with acidophilic stains
          • responsible for synthesis and secretion of PiG hormones
            • prolactin and growth hormone
        • basophils 
          • anterior pituitary cells (pars distalis) that contain granules that react with basophilic stains
          • responsible for synthesis and secretion of FLAT hormones
            • FSH, LH, ACTH, TSH
              • granules that contain FLAT hormones are PAS+
    • chromophobes
      • anterior pituitary cells that lack granules and that do not react with acidophilic/basophilic stains
        • e.g., stromal cells and degranulated chromophils
Anterior Pituitary: Hormones
  • Anterior pituitary hormones
    • FLAT PiG
      • FSH (follicle-stimulating hormone)
      • LH (luteinizing hormone)
      • ACTH (adrenocorticotropic hormone)
      • TSH (thyroid-stimulating hormone)
      • Prolactin
      • Growth hormone (somatotropin)
    • categories of hormones
      • corticolipotropins
        • ACTH and MSH (melanocyte-stimulating hormone)
      • glycoprotein hormones
        • FSH, LH, TSH
      • somatomammotropins
        • prolactin and growth hormone
  • Cortiolipotropins
    • synthesis
      • overview
        • corticolipotropins are derived from a single precursor, POMC
          • POMC = pro-opiomelanocortin
      • pathway details
      • MSH
        • corticolipotropin synthesis products (aka fragments) contain MSH
        • increased MSH levels skin pigmentation
        • e.g., Addison's disease
          • ↑ ACTH ↑ MSH skin pigmentation
  • Glycoprotein hormones
    • subunits of peptide hormones
      • glycoprotein hormones contain 2 subunits: α and β subunit
        • α subunits identical, β subunits non-identical
          • hormone specificity determined by β subunit
      • human chorionic gonadotropin (hCG) structurally related to glycoprotein hormone
        • hCG contains identical α subunit
  • Somatomammotropins
    • prolactin
    • growth hormone
      • secretion
        • pulsatile secretory pattern
          • secretory bursts approximately every 2 hours
            • ↑ in secretory bursts during exercise and sleep
      • functions
        • ↑ linear growth and muscle mass
          • growth mediated by production of somatomedins
            • aka insulin-like growth factors (IGFs)
        • diabetogenic effect
          • insulin resistance
            • decreases glucose uptake and utilization
          • "diabetogenic"
            • growth hormone produces increases in blood glucose
      • pathophysiology
        • GH deficiency
          • before puberty
            • symptoms
              • failure to grow
              • short stature
              • mild obesity
              • delayed puberty
          • treatment
            • growth hormone replacement
        • GH excess
          • growth-hormone secreting pituitary adenoma causes acromegaly
          • symptoms differ from pre-puberty vs post-puberty
            • before puberty
              • gigantism
                • aka increased linear growth
            • after puberty
              • increased organ size
              • increased hand and foot size
              • enlargement of tongue
              • coarsening of facial features
              • insulin resistance and glucose intolerance
          • treatment
            • somatostatin analog (e.g., octreotide)
Posterior Pituitary: Anatomy and Embryology
  • posterior pituitary
    • posterior pituitary, or neurohypophysis, is the neural portion of the pituitary
      • a collection of unmyelinated axons
        • axons extend from cell bodies in hypothalamus
    • consists of pars nervosa, infundibular stalk, and median eminence
    • neurophysins carry hormones made in the hypothalamus (ADH and oxytocin) from the hypothalamus to the posterior pituitary
    • embryological origin
      • neural ectoderm
        • downgrowth of neural ectoderm (diencephalon)
Posterior Pituitary: Hormones
  • Antidiuretic hormone (ADH; vasopressin) 
    • synthesis
      • hypothalamic supraoptic nucleus neuronal cell bodies synthesize ADH pro-hormone
        • ADH pro-hormone contains ADH and neurophysin II
      • ADH pro-hormones are packaged in secretory vesicles
        • secretory vesicles are transported via axonal transport to nerve terminals
          • nerve terminals in pars nervosa of posterior pituitary
      • ADH pro-hormone processing occurs in secretory vesicles during axonal transport
        • cleavage of neurophysin II and release of ADH hormone
    • secretion
      • action potential depolarizes nerve terminals
        • neurosecretory vesicles fuse with plasma membrane
          • releases ADH and neurophysin II into perivascular space of highly fenestrated capillaries by which ADH enters systemic circulation
  • Oxytocin
    • synthesis
      • neuronal cell bodies of the hypothalamic paraventricular nucleus synthesize oxytocin pro-hormone
        • oxytocin pro-hormone contains oxytocin and neurophysin I
      • oxytocin pro-hormones are packaged in secretory vesicles
        • secretory vesicles are transported via axonal transport to nerve terminals
          • nerve terminals in pars nervosa of posterior pituitary
      • oxytocin pro-hormone processing occurs in secretory vesicles during axonal transport
        • cleavage of neurophysin I and release of oxytocin hormone
    • secretion
      • action potential depolarizes nerve terminals
        • neurosecretory vesicles fuse with plasma membrane
          • releases oxytocin and neurophysin I into perivascular space of highly fenestrated capillaries by which oxytocin enters systemic circulation


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