Pallium (neuroanatomy)

In neuroanatomy, pallium refers to the layers of gray and white matter that cover the upper surface of the cerebrum in vertebrates. The non-pallial part of the telencephalon builds the subpallium. In basal vertebrates the pallium is a relatively simple three-layered structure, encompassing 3-4 histogenetically distinct domains, plus the olfactory bulb. It used to be thought that pallium equals cortex and subpallium equals telencephalic nuclei, but it has turned out, according to comparative evidence provided by molecular markers, that the pallium develops both cortical structures (allocortex and isocortex) and pallial nuclei (claustroamygdaloid complex), whereas the subpallium develops striatal, pallidal, diagonal-innominate and preoptic nuclei, plus the corticoid structure of the olfactory tuberculum. In mammals, the cortical part of the pallium registers a definite evolutionary step-up in complexity, forming the cerebral cortex, most of which consists of a progressively expanded six-layered portion isocortex, with simpler three-layered cortical regions allocortex at the margins. The allocortex subdivides into hippocampal allocortex, medially, and olfactory allocortex, laterally (including rostrally the olfactory bulb and anterior olfactory areas).

The general layout or body plan of the pallium is already clearly defined in animals with relatively simple brains, including lampreys, sharks and amphibians. In teleost fish, reptiles, birds, and mammals, the pallial architecture is greatly modified (sharply divergently in fish), with differential growth and specialization of diverse sectors of the conserved pallial Bauplan. In all vertebrate brains, the telencephalic forebrain consists of two hemispheres, joined at the midline by a region called the septum. The septum is continuous with the preoptic area across the plane defined by the anterior commissure; it is largely subpallial, but also contains a small pallial portion, where the hippocampal commissure forms, which is contiguous to the medial pallium. The telencephalic part of the rostral choroidal tela (roof plate continuous caudally with a diencephalic part) is inserted at the back of this commissure at a locus where mammals show the subfornical circumventricular organ, and extends laterally over the interventricular foramen into a wing-shaped medial telencephalic territory, the so-called choroidal fissure. Here the choroidal tissue is attached to the fimbria of the hippocampus (also known as the cortical hem area), bordering lengthwise the medial pallium. At its rostral and caudal ends, the medial pallium contacts the ventral pallium, which builds the pallial portion that contacts the subpallium across the pallio subpallial boundary, observed at the lateral telencephalic wall. Inside the ring formed thus by the medial and ventral pallium there is a sort of island that contains the dorsal and lateral pallial portions. In older literature the pallium used to be subdivided only into three zones, called the medial pallium, dorsal (or dorsolateral) pallium, and lateral pallium. The old lateral pallium encompassed the modern lateral and ventral parts of the pallium. The medial pallium is the progenitor of the mammalian hippocampus, and is thought to be involved in spatial cognitive mapping and memory formation across a broad range of species. The lateral and ventral pallium is the progenitor of the mammalian piriform cortex, and has an olfactory function in every species in which it has been studied. The evolutionary diversifications and specialization in functions of the dorsal pallium have been more difficult to decipher. It is widely believed to be the progenitor of the bulk of the mammalian cerebral cortex, although the evidence for this is considered by some anatomists not yet to be conclusive.

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