The pituitary gland Flashcards
The pituitary gland
The ‘master gland’ of the endocrine system, responsible for the secretion of trophic hormones, which act on other glands, and hormones which act directly on target organs and tissues.
Structure of the pituitary
A pea-sized gland, divided into two lobes, the anterior pituitary and the posterior pituitary. Connected to the hypothalamus via the infundibulum (pituitary stalk), which contains both nerves and blood vessels.
Location of the pituitary
Sits in the sella tursica (pituitary fossa) of the sphenoid bone, located in the middle cranial fossa. Inferior to the hypothalamus and optic chiasm, posterior to the sphenoid sinus, and lateral to the cavernous sinus.
Hypothalamic-pituitary axis
The secretion of hormones by the anterior pituitary gland is regulated by hormones secreted from the hypothalamus, and feedback from circulating hormones and metabolites.
Hypothalamus
A region of the diencephalon comprised of many nuclei, regulated by hormone mediated signals and neural inputs. It carries signals to the anterior pituitary and parts of the nervous system, and is also involved in non-endocrine functions. Located inferior to the thalamus.
Anterior pituitary
Composed of epithelial tissue, which secretes trophic hormones that act on other endocrine glands. Linked to the hypothalamus via a portal vascular system. Derived from the ectoderm of the developing oral cavity (Rathke’s pouch).
Posterior pituitary
Composed of neuroendocrine tissue, which secretes hormones that act directly on target organs and tissues. Linked to the hypothalamus via the axons of neurones whose cell bodies are in the supraoptic and paraventricular nuclei, which pass through the infundibulum. Derived from the neuroectoderm of the future diencephalon.
Blood supply of the pituitary
The anterior pituitary is supplied by the superior hypophoseal artery, while the posterior pituitary is supplied by the inferior hypophoseal artery, both of which branch from the internal carotid artery. The hypothalamus and anterior pituitary are connected via a portal vascular system, enabling the transport of hormones between them.
Posterior pituitary hormones
- anti-diuretic hormone (ADH)
- oxytocin
Anterior pituitary hormones
- growth hormone (GH)
- thyroid stimulating hormone (TSH)
- adrenocorticotrophic hormone (ACTH)
- follicle stimulating hormone (FSH)
- luteinising hormone (LH)
- prolactin
Growth hormone (GH)
Responsible for linear growth in children, acquisition of bone mass, stimulation of protein synthesis, fat breakdown (lipolysis) and glucose metabolism, regulation of body composition and psychological wellbeing. Secretion is controlled by GHRH and somatostatin from the hypothalamus. Released throughout life in a pulsatile pattern.
Follicle stimulating hormone (FSH) and luteinising hormone (LH)
Responsible for testosterone/oestrogen production, development of secondary sex characteristics, onset of puberty, sperm production in males and follicular genesis in females. Secretion is controlled by gonadotropin releasing hormone (GnRH) from the hypothalamus. In females, levels fluctuate during the menstrual cycle, rising closer to ovulation.
Prolactin
Essential for lactation. Inhibits gonadal activity by suppressing GnRH, decreasing FSH/LH. Inhibited by dopamine. Levels increase during pregnancy and lactation, and it can cause disease when present in excess.
Adrenocorticotrophic hormone (ACTH)
Acts on the adrenal cortex to promote secretion of glucocorticoids (cortisol), which are important for the stress response, immune response, and metabolism. Secretion is controlled by corticotrophin releasing hormone (CRH) from the hypothalamus.
Thyroid stimulating hormone (TSH)
Acts on the thyroid gland to promote secretion of thyroid hormones (T3 and T4), which are important for the regulation of metabolism. Secretion is controlled by thyrotropin releasing hormone (TRH) from the hypothalamus.
Anti-diuretic hormone (ADH)
A polypeptide hormone which determines the rate of free water excretion by opening aquaporins in the luminal membranes of the collecting tubules of the kidneys, thus increasing permeability and water reabsorption. Secreted in response to hyperosmolarity or decreased blood volume.
Oxytocin
Stimulates contraction of the smooth muscle of the breast and uterus during lactation and parturition (birth). Controlled by a positive feedback mechanism.
Hypopituitarism
Defective secretion of pituitary hormones, resulting in hormone deficiency diseases. May be caused by tumours, radiotherapy, infarction, trauma etc.
Cavernous sinus
A space between the layers of the dura mater, filled with venous blood. Contains the internal carotid arteries, and many of the cranial nerves. Positioned laterally to the pituitary gland and sella tursica.
Optic chiasm
Where the optic nerves cross over one another, located superior to the pituitary gland.
Pituitary adenomas
Benign tumours of the pituitary gland, which can cause mass effects (visual/neurological) or hypersecretion of pituitary hormones. The symptoms may be treated with medication, or the tumour may be removed using surgery or radiotherapy.
Acromegaly
Caused by hypersecretion of GH, resulting in enlargement of the hands and feet, visceromegaly, hypertension and congestive heart failure, among other symptoms.
Cushing’s disease
An ACTH-dependent disease of excess cortisol, resulting in weight gain, fatigue, slow healing, an increased risk of infection, and a moon-shaped face, among other symptoms.
Visual effects of a pituitary adenoma
A large tumour (macroadenoma, >1cm) may exert pressure on the optic nerves or optic chiasm, causing visual symptoms. A characteristic pattern of vision loss is the loss of peripheral vision in one or both eyes (bitemporal hemaniopia). It may also push on the nerves supplying the extraocular muscles, resulting in weakness and misalignment of the eyes, which causes double vision (diplopia).
