Pituitary (Hypophysis)

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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