Morphological and functional nature
The neurohypophysis is an outgrowth of the diencephalon (diencephalon) and a direct continuation of the caudal end of the hypothalamus.
- Tissue type: consists of nervous tissue.
- Cellular composition: contains glia and processes, but is completely devoid of neuronal cell bodies and its own endocrine secretory cells (adenocytes).
- Functional essence: is not a true endocrine gland. It serves as a site for the storage and release into the systemic circulation of hormones synthesized within the structures of the hypothalamus.
Mechanism of neurosecretion and hormone transport
The connection between the hypothalamus and the posterior pituitary is not humoral via the portal system (as in the case of the adenohypophysis), but rather via axons.
- Sources of secretion: the cell bodies of secretory neurons are located in the supraoptic and paraventricular nuclei of the hypothalamus.
- Transport: axons of these cells descend through the pituitary stalk, forming the supraopticohypophysial (hypothalamohypophysial) tract. Hormones are transported within the axons directly to the neurohypophysis.
- Storage and release: hormones are stored in swollen axon terminals as a protein complex. Release into the systemic circulation occurs via axovasate synapses.
- Blood supply: the inferior hypophysial artery supplies the neurohypophysis, and hormone drainage into the systemic circulation occurs via a vein.
Neurohypophyseal hormones and their targets
The posterior lobe secretes two main hormones into the blood, which act directly on peripheral target organs:
- Antidiuretic hormone (ADH, vasopressin): regulates water reabsorption. The primary target organ is the kidneys.
- Oxytocin: stimulates the contraction of uterine smooth muscle and milk ejection from the mammary glands. Synthesized during the final weeks of pregnancy.
Regulation of oxytocin release is reflex-mediated. Somatosensory stimulation (e.g., nipple touching or suckling) generates afferent impulses. These are conducted through the thalamus and cerebral cortex, after which they activate neurosecretory neurons of the hypothalamus. This system is linked to emotional state: fear or stress sharply decreases milk production.
Clinical significance and pathologies
Diseases of the neurohypophysis (posterior pituitary syndrome) are rarely diagnosed.
- Diabetes insipidus: occurs due to ADH deficiency. Manifests as polyuria (excretion of over 3 L of hypoosmolar urine per day), increased plasma osmolarity, thirst, and dryness of the skin and mucous membranes. Causes include tumors, trauma, infections (meningitis, encephalitis), and genetic factors.
- Hyperhydropexy syndrome (Parhon's syndrome): hypersecretion of ADH. Characterized by fluid retention, hypervolemia, decreased plasma osmolarity, hyponatremia, and oliguria. Can lead to water intoxication (headaches, nausea, vomiting).
- Panhypopituitarism (Simmonds' disease): a severe syndrome involving the impairment of all pituitary functions. When accompanied by neurohypophyseal damage, the general clinical picture (weight loss, decreased libido, bradycardia) is supplemented by symptoms of diabetes insipidus.
The components of DIDMOAD syndrome are easily remembered by their acronym: Diabetes insipidus, Diabetes mellitus, Optic atrophy, Deafness.
Class and exam questions
Does the neurohypophysis synthesize its own hormones?
No, the posterior lobe completely lacks secretory cells (adenocytes). It serves as a depot and release site for hormones produced in the hypothalamic nuclei.
How do hormones travel from the hypothalamus to the posterior pituitary?
Via neurosecretion: they are transported within the axons of neurons from the supraoptic and paraventricular nuclei along the hypothalamohypophysial tract.
What happens in case of neurohypophyseal hormone deficiency?
A shortage of antidiuretic hormone (ADH) leads to the development of diabetes insipidus. It manifests as thirst and the excretion of more than 3 liters of hypoosmolar urine per day.
What else to study on this topic
- Hypothalamus and its nuclei
- Adenohypophysis and tropic hormones
- Antidiuretic hormone (ADH)
- Oxytocin and the regulation of lactation
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