Neurohypophysis: structural, physiological and clinical aspects

Abstract

Introduction. Much has been written regarding the ultrastructure of the human adenohypophysis, on the other hand, relatively little is available on the the neurohypophysis. Aim of the study. To apprehend the purpose of this unique tissue; to explore the embryology physiology and pathophysiology of the neurohypophysis, vasopressin and oxytocin; to highlight developments on the neural basis of osmo-sensing mechanism; to describe vasopressin and oxytocin regarding the molecular biology and to underline some of the disease processes in which both the structure and functions of the those two hormones are involved. Materials and methods. Analysis of literature and synthesis of scientific data from studies (microscopically evidence and clinical tryouts) Results. The posterior pituitary is derived from the forebrain/prosencefalon during development and is composed predominantly of neural tissue. It lies below the hypothalamus, with which it forms a structural and functional unit: the neurohypophysis. The supraoptic nucleus (SON) is situated along the proximal part of the optic tract. It consists of the cell bodies of discrete vasopressinergic (VP) and oxytotic(OX) magnocellular neurons projecting to the posterior pituitary along the supraoptico-hypophyseal tract. The paraventricular nucleus (PVN) also contains discrete vasopressinergic and oxytotic magnocellular neurons, also projecting to the posterior pituitary along the supraoptico-hypophyseal tract. The PVN contains additional, smaller parvocellular neurons that project to the median eminence and additional extrahypothalamic areas including forebrain, brain stem, and spinal cord. Some of these parvocellular neurons are vasopressinergic. The neurohypophysis is the structural foundation of a neuro-humoral system coordinating fluid balance and reproductive function through the action of two peptide hormones: vasopressin and oxytocin. Vasopressin is the main endocrine regulator of renal water excretion, facilitating adaptive physiological responses to maintain plasma volume and plasma osmolality. Oxytocin is important in parturition and lactation. Data support a wider role for both peptides in the neuro-regulation of complex behavior. Clinically, deficits in the production or action of vasopressin manifest as diabetes insipidus. An understanding of the physiology and pathophysiology of vasopressin is also critical in approaching the diagnosis and management of hyponatraemia, the most common electrolyte disturbance in clinical practice. Conclusions. The neurohypophysis represents a unique tissue having neural and endocrine characteristics and possessing ultrastructural features distinct from those of conventional endocrine organs such as the anterior pituitary, thyroid, pancreatic islets, etc. In contrast to these glands, the neurohypophysis consists from the processes of mature neurons, therefore, it is not capable of synthesizing hormones but only of their storage and release. On the other hand the hormones that it releases in the blood stream seems to have a higher impact on the physiological processes of human homeostasis and complex behavior, than it was originally thought back in the days.