Morphological and Functional Nature
The neurohypophysis represents an outpocketing of the diencephalon and the direct continuation of the caudal end of the hypothalamus.
- Tissue type: Composed of neural tissue (pars nervosa).
- Cellular composition: Contains neuroglia and axonal processes, but is completely devoid of neuronal cell bodies and primary endocrine secretory cells (adenocytes).
- Functional essence: It is not a true endocrine gland. Instead, it serves as a site for the storage and release into the systemic circulation of hormones synthesized within hypothalamic structures.
Mechanism of Neurosecretion and Hormone Transport
The connection between the hypothalamus and the posterior pituitary is not humoral via a portal system (unlike the adenohypophysis), but rather axonal.
- Sources of secretion: The cell bodies of the neurosecretory neurons are located in the supraoptic and paraventricular nuclei of the hypothalamus.
- Transport: The axons of these cells descend through the pituitary stalk, forming the supraopticohypophysial (hypothalamo-hypophysial) tract. Hormones are transported within the axons directly to the neurohypophysis.
- Storage and release: Hormones are stored in swollen axonal terminals as part of carrier protein complexes (neurophysins). Release into the systemic capillary bed occurs via axovasal synapses.
- Blood supply: The neurohypophysis is supplied by the inferior hypophysial artery, and hormone outflow into the systemic circulation occurs via venous drainage.
Neurohypophysial Hormones and Targets
The posterior lobe releases two main hormones into the blood that act directly on peripheral target organs:
- Antidiuretic hormone (ADH, vasopressin): Regulates water reabsorption. The primary target organ is the kidney.
- Oxytocin: Stimulates smooth muscle contraction of the uterus and milk ejection from the mammary glands.
Regulation of oxytocin release is reflex-mediated. Somatosensory stimulation (e.g., nipple touch or suckling) generates afferent impulses. These travel through the thalamus and cerebral cortex before activating the neurosecretory neurons of the hypothalamus. This system is linked to emotional state: fear or stress can acutely inhibit milk ejection.
Clinical Significance and Pathologies
Disorders of the neurohypophysis (posterior pituitary syndromes) are clinically distinct.
- Diabetes insipidus: Caused by a deficiency of ADH. Manifests as polyuria (excretion of >3 liters of hypoosmolar urine per day), increased plasma osmolarity, polydipsia, and dry skin and mucous membranes. Causes include tumors, trauma, infections (meningitis, encephalitis), and genetic factors.
- Syndrome of inappropriate antidiuretic hormone secretion (SIADH): Hypersecretion of ADH (Schwartz-Bartter syndrome). Characterized by fluid retention, hypervolemia, decreased plasma osmolarity, hyponatremia, and oliguria. Can lead to water intoxication (headaches, nausea, vomiting).
- Panhypopituitarism: A severe syndrome involving the loss of all anterior and posterior pituitary functions. When the neurohypophysis is concurrently affected, the general clinical picture (weight loss, loss of libido, bradycardia) is supplemented by symptoms of diabetes insipidus.