Pituitary Gland

Introduction

• The hypophysis cerebri, or pituitary gland, is a small, ovoid neuroendocrine organ situated in the hypophyseal fossa of the sella turcica.
• It is traditionally called the master endocrine gland because its hormones regulate several other endocrine glands.
• Its activity is largely controlled by the hypothalamus.

Location

• The pituitary gland occupies the hypophyseal fossa on the superior surface of the body of the sphenoid bone.
• It is connected to the hypothalamus by a stalk called the infundibulum.
• It is enclosed by dura mater, which forms the diaphragma sellae over the gland.
• The diaphragma sellae has a central opening for the passage of the infundibulum.

Features

Measurements

• Shape: Ovoid.
• Colour: Reddish-grey.
• Transverse diameter: Approximately 12 mm.
• Anteroposterior diameter: Approximately 8 mm.
• Weight: Approximately 500 mg in adults.

Relations

Superiorly

1. Diaphragma sellae.
2. Optic chiasma.
3. Tuber cinereum of the hypothalamus.
4. Infundibular recess of the third ventricle.

Inferiorly

1. Venous channels around the hypophyseal fossa.
2. Floor of the hypophyseal fossa.
3. Sphenoidal air sinus.

Laterally

– Cavernous sinuses and their contents.

Anteriorly

– Tuberculum sellae.

Posteriorly

– Dorsum sellae.

Subdivisions and Parts of Pituitary Gland

• The pituitary gland consists of two major divisions:
  1. Adenohypophysis (glandular pituitary).
  2. Neurohypophysis (neural pituitary).
• These divisions differ in their embryological origin, histological structure, and functions.

Adenohypophysis

• It is the highly vascular, glandular portion of the pituitary gland.
• It consists of three parts:

1. Pars Distalis (Anterior Lobe)

• It forms the largest part of the adenohypophysis.
• It contains endocrine cells arranged in cords and clusters between fenestrated capillaries.

2. Pars Intermedia (Intermediate Lobe)

• It is a narrow zone between the pars distalis and pars nervosa.
• It may contain small colloid-filled cysts derived from remnants of Rathke’s pouch.
• It is poorly developed in adult humans.

3. Pars Tuberalis

– It is an upward extension of the adenohypophysis surrounding part of the infundibulum.

Development

• The adenohypophysis develops from Rathke’s pouch, an ectodermal diverticulum arising from the roof of the stomodeum.
• The residual cavity of Rathke’s pouch may persist as the intraglandular cleft.

Neurohypophysis

– It is the neural portion of the pituitary gland and consists of:

1. Pars nervosa (posterior lobe): It contains nerve fibres and supporting cells.
2. Infundibulum: It connects the pituitary gland with the hypothalamus.
3. Median eminence: It forms the proximal neurohaemal region at the base of the hypothalamus.

Development

– The neurohypophysis develops from a downward extension of the neuroectoderm of the diencephalon.

Blood Supply

Arterial supply

– The pituitary gland receives arterial branches mainly from the internal carotid arteries.

1. Superior Hypophyseal Arteries

• They arise from the internal carotid arteries and their branches.
• They supply the median eminence, upper infundibulum, and pars tuberalis.
• They contribute to the formation of the hypothalamo-hypophyseal portal system.

2. Inferior Hypophyseal Arteries

• They usually arise from the cavernous parts of the internal carotid arteries.
• They supply the pars nervosa and lower infundibulum.

Venous drainage

– Venous blood drains through hypophyseal veins into the adjoining cavernous and intercavernous venous sinuses.

Box 24.1: Hypothalamo-hypophyseal portal system

– The hypothalamo-hypophyseal portal system carries hypothalamic releasing and inhibitory hormones directly to the adenohypophysis.

Formation and Course

1. Branches of the superior hypophyseal arteries form a primary capillary plexus in the median eminence and upper infundibulum.
2. These capillaries unite to form hypophyseal portal veins.
3. The portal veins descend through the pituitary stalk towards the pars distalis.
4. They divide into a secondary capillary plexus within the pars distalis.
5. Hypothalamic hormones pass into the secondary plexus and regulate secretion from anterior pituitary cells.
6. Venous blood subsequently drains into the surrounding dural venous sinuses.

Functional Significance

• The portal system allows hypothalamic hormones to reach the anterior pituitary at relatively high concentrations without first entering the general circulation.
• It provides an efficient mechanism for regulating anterior pituitary hormone secretion.

Hypothalamo-Hypophyseal Tract

• Axons from the supraoptic and paraventricular nuclei of the hypothalamus descend through the infundibulum to the posterior pituitary.
• Supraoptic nucleus: It synthesises predominantly vasopressin (ADH).
• Paraventricular nucleus: It synthesises predominantly oxytocin.
• Both hormones are transported along axons, stored in the posterior pituitary, and released into the circulation when required.

Note: Both hypothalamic nuclei can synthesise ADH and oxytocin, although one hormone predominates in each nucleus.

Histology of Pituitary Gland

– The pituitary gland consists of two histologically distinct portions:

1. Adenohypophysis.
2. Neurohypophysis.

Adenohypophysis

• The adenohypophysis contains cords and clusters of endocrine cells separated by numerous fenestrated capillaries.
• Its cells are traditionally classified according to their staining characteristics.

1. Acidophils

• They contain eosinophilic (pink-staining) cytoplasmic granules.
• They include:
  • Somatotrophs: They constitute approximately 40–50% of anterior pituitary cells and secrete growth hormone (GH).
  • Lactotrophs: They constitute approximately 10–25% of cells and secrete prolactin.

2. Basophils

• They contain basophilic cytoplasmic granules.
• They include:
  • Corticotrophs: They constitute approximately 15–20% of cells and secrete adrenocorticotropic hormone (ACTH).
  • Gonadotrophs: They constitute approximately 10–15% of cells and secrete follicle-stimulating hormone (FSH) and luteinising hormone (LH).
  • Thyrotrophs: They constitute approximately 3–5% of cells and secrete thyroid-stimulating hormone (TSH).

3. Chromophobes

• These cells show poorly staining or inconspicuous cytoplasmic granules.
• They include degranulated endocrine cells and other poorly differentiated cell populations.

Pars Intermedia

• It is poorly developed in adult humans.
• It contains scattered endocrine cells and colloid-filled cysts representing remnants of Rathke’s pouch.
• Melanocyte-stimulating hormone (MSH) is derived from pro-opiomelanocortin (POMC), but the pars intermedia is not a major independent source of MSH in adult humans.

Neurohypophysis

– It consists mainly of:

1. Non-myelinated axons of hypothalamic neurosecretory neurons.
2. Pituicytes, which are specialised supporting glial cells.
3. Numerous fenestrated capillaries.
4. Herring bodies containing stored neurosecretory material.

• The neuronal cell bodies are located in the supraoptic and paraventricular nuclei of the hypothalamus.
• The neurohypophysis stores and releases ADH and oxytocin, but does not synthesise these hormones.

HERRING BODIES

• Herring bodies are localised dilatations of neurosecretory axons containing accumulated secretory granules.
• They are present in the pars nervosa and infundibulum.
• They contain stored ADH or oxytocin associated with neurophysins.
• On H&E staining, they appear as eosinophilic (pink) swellings.
• They may be demonstrated more prominently with specialised staining methods.

FUNCTIONS OF POSTERIOR PITUITARY HORMONES

Oxytocin

• It stimulates contraction of uterine smooth muscle during childbirth.
• It promotes contraction of myoepithelial cells surrounding mammary alveoli, causing milk ejection.
• It also participates in reproductive and neurobehavioural functions.

Vasopressin (Antidiuretic Hormone)

• It increases water reabsorption by the collecting ducts of the kidneys.
• It reduces urinary water loss and helps maintain blood volume and plasma osmolality.
• At higher concentrations, it produces vasoconstriction and may increase blood pressure.

Pituitary tumors and their effects on visual pathway

• Pituitary adenomas, now commonly termed pituitary neuroendocrine tumours (PitNETs), arise from endocrine cells of the adenohypophysis.
• They may produce symptoms due to excessive hormone secretion, reduced pituitary function, or compression of adjoining structures.

Compression of Adjacent Structures

1. Optic Chiasma

• Superior enlargement of a pituitary tumour may compress the central part of the optic chiasma.
• It damages crossing fibres from the nasal halves of the retina.
• This produces bitemporal hemianopia, characterised by loss of the temporal visual fields of both eyes.

2. Cavernous Sinus

• Lateral extension may involve the cavernous sinus and compress the ocular motor nerves.
• It may produce diplopia, ophthalmoplegia, and occasionally ptosis.

3. Hypophyseal Fossa

– Enlargement of the tumour may cause expansion or remodelling of the sella turcica.

Endocrine Disturbances

1. Hormonal Hypersecretion

• Excess GH: It produces gigantism in children or acromegaly in adults.
• Excess ACTH: It produces Cushing disease, a cause of Cushing syndrome.
• Excess prolactin: It may produce galactorrhoea, menstrual disturbances, infertility, and hypogonadism.
• Rarely, excessive secretion of TSH may produce hyperthyroidism.

2. Hormonal Deficiency

• Compression or destruction of normal pituitary tissue may produce hypopituitarism.
• GH deficiency in childhood: It may cause reduced linear growth.
• Gonadotropin deficiency: It may produce delayed puberty, infertility, or hypogonadism.
• TSH deficiency: It causes central hypothyroidism.
• ACTH deficiency: It causes secondary adrenal insufficiency.

Note: Diabetes insipidus is uncommon in ordinary pituitary adenomas. Its presence raises suspicion of involvement of the pituitary stalk, posterior pituitary, or another sellar lesion.

Development of Pituitary Gland

– The pituitary gland develops from two embryologically distinct sources.

1. Development of Adenohypophysis

• It develops from Rathke’s pouch, an ectodermal outgrowth from the roof of the stomodeum.
• The pouch grows upwards towards the developing diencephalon.
• It loses its connection with the oral cavity and differentiates into:
  1. Pars distalis.
  2. Pars intermedia.
  3. Pars tuberalis.
• The residual cavity of Rathke’s pouch forms the intraglandular cleft and may persist as small cystic spaces.

2. Development of Neurohypophysis

• It develops from a downward evagination of the neuroectoderm of the floor of the diencephalon.
• This neural outgrowth forms the infundibulum and pars nervosa.
• Hypothalamic neurosecretory axons subsequently grow into the posterior pituitary.

Developmental Significance

• The adenohypophysis is derived from oral ectoderm, whereas the neurohypophysis is derived from neuroectoderm.
• Their differing embryological origins explain the distinct histological appearance and functions of the two divisions.