Exploring the Interthalamic Adhesion: A Comprehensive Review of Its Morphology, Neuroconnectivity and History of Research.

The interthalamic adhesion, or massa intermedia, is a midline bridge of neural tissue connecting the thalami across the third ventricle and usually containing the nucleus reuniens. It is important radiologically and neurosurgically: accessing the third ventricle or structures through the third ventricle, endoscopic surgery at third ventricle. We aim to consolidate current knowledge on the interthalamic adhesion, focusing on its morphology, nomenclature, development, histology, connections and anatomical variations to clarify longstanding inconsistencies. The interthalamic adhesion develops around 13-14 weeks of gestation and contains neurons and commissural fibers that continue maturing postnatally. Fibers were mentioned to occur in a subset of individuals; however, this feature is not observed constantly. When present, these fibers project to the contralateral hemisphere, potentially indicating a role analogous - albeit significantly reduced - to that of the corpus callosum, anterior commissure, or posterior commissure. As the cortical evolution progresses, it may lose its functional relevance. Recent studies estimate the prevalence of the interthalamic adhesion at 87.3%. It is more common in females, some authors suggest it is also larger. Its morphology varies widely - commonly single styloid or broad, but also bilobar, duplicated or even multiple. The interthalamic adhesion was most commonly located in the antero-superior quadrant of the third ventricle. Its morphology is associated with some psychiatric disorders such as schizophrenia, bipolar disease and depression. Histologically, the interthalamic adhesion is rich in glia, featuring a broad subependymal zone and contains various neuron types - oval, fusiform, triangular, and multipolar, some form unique circular arrangements.