Functional and Morphological Nature
Histologically and functionally, the neurohypophysis differs radically from the anterior pituitary. Its defining characteristic is the complete absence of intrinsic hormone synthesis. The tissue of the posterior lobe lacks any endocrine cells (adenocytes) capable of producing biologically active substances.
Morphologically, the neurohypophysis represents the neural component — an outgrowth of the diencephalon. Therefore, its tissue classification is classical nervous tissue specialized in neurosecretion.
The functional role of the posterior lobe is summarized in two main tasks:
- Storage — retention of hormones produced elsewhere.
- Secretion — releasing the stored hormones directly into the systemic circulation.
The hormones of the posterior lobe are antidiuretic hormone (ADH, or vasopressin) and oxytocin. In reality, they are synthesized in the supraoptic and paraventricular nuclei of the hypothalamus. Following synthesis, these substances are transported along the axons of hypothalamic neurons, which descend directly into the neurohypophysis.
Microscopic Structure: Components of the Posterior Lobe
Examination of a histological slide of the posterior pituitary reveals three main structural components. Crucially, the neurohypophysis lacks neuronal cell bodies — it contains only their processes and supporting elements.
I. Pituicytes (Glial Component) These are small glial cells that are essentially a specialized type of protoplasmic astrocyte.
- They form the stroma, creating a supportive 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) All posterior lobe tissue is permeated by numerous bundles of axons belonging to neurosecretory hypothalamic cells. A key morphological feature here is the presence of Herring bodies (storage bodies). These are large terminal axonal dilations located near synapses, serving as reservoirs where neurosecretion accumulates prior to release.
III. Blood Vessels (Vascular Component) _The neurohypophysis is richly vascularized. The vascular network here is extensive and dominated by capillaries. Capillaries play a vital role, as the axons of hypothalamic neurons terminate directly on their walls. These specialized contacts are known as axovascular synapses, through which hormones from Herring bodies pass directly into the bloodstream.