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Pituitary and Pineal Gland Lecture Notes

Pituitary Gland (Hypophysis) & Pineal Gland

Introduction to Endocrine Secretion

  • Secretory cells release hormones into vascular compartments.
  • Hormones are distributed via capillaries.
  • No ducts are involved in this process.
  • Endocrine cells exist in the heart, thymus, gut, kidneys, testis, and ovaries.

Hormone Distribution Mechanisms

  • Endocrine: Hormones travel via the blood to distant target cells.
  • Paracrine: Hormones affect nearby cells.
  • Juxtacrine: Hormones act via direct cell-to-cell contact.
  • Autocrine: Hormones affect the cells that produce them.

Hormone Types

  • Hydrophilic:
    • Proteins
    • Peptides
    • Amino acid derivatives
  • Hydrophobic:
    • Steroids
    • Thyroid hormones (require transport proteins)

The Pituitary Gland (Hypophysis)

  • Weighs approximately 0.5 g and is located in the sella turcica.
  • Known as the 'master gland' because it links the brain and the endocrine system.
  • Has a dual origin, developing from both the brain and oral ectoderm.
  • The pituitary gland is composed of an anterior part and a posterior part that is directly attached to the hypothalamus region of the brain by an infundibular stalk.
  • The gland occupies a fossa of the sphenoid bone called the sella turcica.

Pituitary Development

  • Neurohypophyseal bud: Originates from the diencephalon.
  • Hypophyseal (Rathke) pouch: Originates from the oral cavity.
  • Forms:
    • Posterior: Neurohypophysis
    • Anterior: Adenohypophysis
  • The pituitary gland forms from two separate embryonic structures.

Pituitary Structure

  • Neurohypophysis:
    • Pars nervosa
    • Infundibulum
  • Adenohypophysis:
    • Pars distalis (75% of the pituitary)
    • Pars tuberalis
    • Pars intermedia

Hypothalamic-Hypophyseal Tract & Blood Supply

  • Neural connection occurs via the hypothalamic-hypophyseal tract.
  • Blood supply comes from the superior and inferior hypophyseal arteries.
  • A portal system carries regulatory peptides from the hypothalamus to the anterior pituitary.

Adenohypophysis (Anterior Pituitary)

  • Pars distalis constitutes 75% of the anterior pituitary and is responsible for hormone production.
  • Cell types present include:
    • Chromophils (Basophils and Acidophils)
    • Chromophobes
  • Regulates metabolism, growth, and reproduction.

Pars Tuberalis and Pars Intermedia

  • Pars tuberalis: Contains gonadotrophs.
  • Pars intermedia: Contains corticotrophs, chromophobes, and colloid cysts.
  • Produces MSH (melanocyte-stimulating hormone), β-endorphin, and γ-LPH (gamma-lipotropin).

Anterior Pituitary Hormone Regulation

  • Controlled by hypothalamic hormones.
  • Feedback loops maintain hormone levels.
  • CNS (central nervous system) stimuli influence pituitary function.

Neurohypophysis (Posterior Pituitary)

  • Composed of neural tissue, including unmyelinated axons.
  • Contains pituicytes (glial-like cells).
  • Hormones:
    • ADH (antidiuretic hormone, vasopressin): Promotes kidney water reabsorption.
    • Oxytocin: Stimulates uterine contraction and milk ejection.

Hormone Release Mechanism (Posterior Pituitary)

  • Hormones are stored in Herring bodies.
  • Released into fenestrated capillaries.
  • Stimuli:
    • Increased blood osmolality leads to ADH release.
    • Nursing triggers the oxytocin reflex.

Pars Nervosa

  • Contains neurosecretory (Herring) bodies (NB), pituicytes (P), and capillaries (C).

Clinical Relevance (Pituitary Gland)

  • Feedback mechanisms maintain balance.
  • Disruption can affect growth, metabolism, and reproduction.
  • Understanding the pituitary is key to managing endocrine disorders.
  • Posterior pituitary function can be adversely affected by heritable mutations in the gene for vasopressin (ADH)-neurophysin, by compression from a tumor in adjacent tissues, and by head trauma.
  • By lowering levels of vasopressin, such conditions can produce diabetes insipidus, a disorder characterized by inability to concentrate urine, which leads to frequent urination (polyuria) and increased thirst (polydipsia).

The Pineal Gland

  • Also known as epiphysis cerebri.
  • Regulates daily bodily rhythms.
  • Small, pine cone-shaped organ.
  • Size: 5–8 mm by 3–5 mm.
  • Develops from neuroectoderm.
  • Located in the posterior wall of the third ventricle.

Structure of the Pineal Gland

  • Covered by pia mater connective tissue.
  • Divided into lobules by septa containing blood vessels.
  • Attached to the brain by a short stalk.

Pinealocytes

  • Prominent, abundant secretory cells.
  • Features:
    • Slightly basophilic cytoplasm
    • Irregular, euchromatic nuclei
    • Contain secretory vesicles, mitochondria, and long processes
    • Extend to vascularized septa, ending near capillaries
  • Produce melatonin (a tryptophan derivative).

Interstitial Glial Cells

  • The pineal gland also has interstitial glial cells that are modified astrocytes, staining positively for glial fibrillary acidic protein, which represent about 5% of the cells.
  • These have elongated nuclei more heavily stained than those of pinealocytes and are usually found in perivascular areas and between the groups of pinealocytes.

Melatonin Secretion

  • Controlled by light and darkness.
  • Promoted by darkness; inhibited by daylight.
  • Causes diurnal blood melatonin fluctuations.
  • Affects hypothalamus, pituitary, and other endocrine tissues.
  • Governs the circadian (24-hour) rhythm. The pineal gland acts, therefore, as a neuroendocrine transducer, converting sensory input regarding light and darkness into variations in many hormonal functions.

Neural Pathways (Pineal Gland)

  • Unmyelinated sympathetic nerve fibers end among pinealocytes.
  • Light/dark signals transmitted via:
    • Retinas
    • Retinohypothalamic tract
    • Suprachiasmatic nucleus
    • Sympathetic fibers

Interstitial Glial Cells (Pineal Gland)

  • Modified astrocytes.
  • Stain positive for glial fibrillary acidic protein (GFAP).
  • About 5% of pineal gland cells.
  • Located in perivascular areas and between pinealocyte groups.

Corpora Arenacea (Brain Sand)

  • Calcium and magnesium salt concretions.
  • Formed by mineralized extracellular protein deposits.
  • Appear in childhood and increase with age.
  • No effect on gland function but are useful as radiological landmarks.

Pineal Tumors

  • Tumors from pinealocytes are very rare.
  • Can be benign or highly malignant.