Functional and morphological nature
Histologically and functionally, the neurohypophysis is radically different from the anterior pituitary. Its main characteristic is that it lacks its own hormone synthesis. The tissue of the posterior lobe does not contain a single endocrine cell (adenocyte) capable of producing biologically active substances.
Morphologically, the neurohypophysis represents the neural part—an outgrowth of the diencephalon (diencephalon). Therefore, its tissue classification is classic nervous tissue specialized in neurosecretion.
The functional essence of the posterior lobe boils down to two global tasks:
- Storage — accumulation of hormones produced elsewhere.
- Secretion — release of accumulated hormones directly into the systemic bloodstream.
The hormones of the posterior lobe are antidiuretic hormone (ADH, or vasopressin) and oxytocin. In fact, they are synthesized in the supraoptic and paraventricular nuclei of the hypothalamus. After synthesis, these substances are transported along the axons of hypothalamic cells that descend directly into the neurohypophysis.
Microscopic structure: components of the posterior lobe
When studying a histological specimen of the posterior pituitary, three main structural components can be distinguished. It is important to remember that the neurohypophysis is devoid of neuronal cell bodies—only their processes and supporting elements are present here.
I. Pituicytes (Glial component) These are small glial cells that are essentially a type of protoplasmic astrocyte.
- They form the stroma—a kind of supporting framework for the entire posterior lobe.
- Pituicytes perform not only a mechanical function but also actively participate in regulating the release of neurohormones (ADH and oxytocin) into the bloodstream.
II. Axons (Neural component) The entire tissue of the posterior lobe is penetrated by numerous axon bundles. These processes belong to the neurosecretory cells of the hypothalamus. A key morphological structure here is the Herring bodies (storage bodies). These are large terminal expansions of axons. They are located in the area of synapses and serve as reservoirs where the neurosecretion accumulates before being released.
III. Blood vessels (Vascular component) The neurohypophysis is richly vascularized. The vascular network here is extensive, with capillaries predominating. Capillaries play a crucial role because the axons of hypothalamic neurons terminate directly on their walls. In these places, specialized contacts are formed—axovascular synapses, through which hormones from Herring bodies enter the bloodstream directly.
Briefly: answers to topical questions
From which embryological source does the posterior pituitary develop?
The posterior pituitary develops from the neural primordium, being an outgrowth of the diencephalon (diencephalon).
The process is initiated at weeks 4–5 of embryogenesis and includes the following stages:
- The hypothalamus is laid down from the base of the diencephalic vesicle, after which the pouch grows toward Rathke's pouch.
- This outgrowth forms the infundibulum of the 3rd ventricle.
- The distal part of the infundibulum transforms into the posterior lobe of the pituitary gland.
- The proximal part of the infundibulum becomes the pituitary stalk.
When the parts of the pituitary fuse, their differentiation and reciprocal inductive influence begin.
The neurohypophysis is a Warehouse, and the hypothalamus is a Factory. The warehouse (posterior lobe) lacks production equipment (neuronal bodies and secretory cells); it only stores products (ADH and oxytocin) manufactured at the factory in "containers" (Herring bodies) and ships them into the blood.
Class and exam questions
Are hormones synthesized in the posterior pituitary gland?
No, endogenous hormone synthesis is completely absent in the neurohypophysis. There are no endocrine cells (adenocytes) here; the lobe functions solely as a depot.
What are Herring bodies and what is their purpose?
These are large expansions of hypothalamic neuronal axons. They are located near capillaries and serve as a storage site for neurosecretion (ADH and oxytocin) before its release into the blood.
Are there neuronal cell bodies in the neurohypophysis?
No. The posterior lobe contains only processes (axons) of hypothalamic cells and glial cells (pituicytes). The cell bodies of the neurons themselves are located in the nuclei of the diencephalon.
What else to study on this topic
- Localization and morphology of the supraoptic and paraventricular nuclei of the hypothalamus
- Ultrastructure of axovascular synapses
- Mechanisms of pituicyte involvement in the regulation of hormone secretion
- Histological differences between the anterior and posterior lobes of the pituitary gland on a specimen
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