THE MENINGES

 

The meninges covering the central nervous system are composed of three layers: the dura mater, the arachnoid mater and the pia mater. The arachnoid and pia jointly form the leptomeninges.

 

The dura mater (pachymeninx) is a tough, dense membrane which surrounds the brain. It has two extensions: the faix, between the two cerebral hemispheres,  and  the  tentorium  cerebelli, separating the contents of the posterior fossa from the rest of the brain. The cranial dura is closely attached to the skull; its two layers, the periosteal and meningeal dura, are fused and separate only to form the venous sinuses. In the spinal canal, however, the dura is separated from the vertebral periosteum by the epidural space, which contains fibro-fatty tissue and an epidural venous plexus.

 

The dura is formed by dense, interlacing bundles of collagen in which flattened fibroblasts are embedded. The central part contains more cells and occasional blood vessels. Its outer surface is covered by thin, overlapping cell processes and its inner border also has a covering of flattened cells. The subdural space is artefactual, since the dura and arachnoid are closely apposed in life with no appreciable gap between them.

 

The arachnoid mater has a variable thickness, in places being formed by several cell layers. Its outer, dural aspect is smoother than the inner, pial -aspect from which trabeculae emerge to bridge the subarachnoid space. The arachnoid cells are joined together by specialised contacts, including tight junctions, which ensure an effective physiological barrier impermeable to CSF.

 

The cells of the pia mater are similar to those of the arachnoid, but the pia itself is thinner than the arachnoid. Pial cells form a complete layer joined by desmosomes and gap junctions.156 The subpial space separates the pia from the glia limitans of the underlying neural tissue and the pia mater separates the subarachnoid space from the perivascular (Virchow-Robin) spaces of the brains.

 

Arachnoid villi are diverticula of the arachnoid mater and the subarachnoid space which extend into veins and venous sinuses of the dura. Arachnoid granulations are larger than villi and are visible to the naked eye, whereas villi are microscopical structures. Each villus or gran­ulation is coated on its venous aspect by endo­thelial cells and is bathed by venous blood. As the villus or granulation penetrates the dura, it forms a narrow neck which then expands to form a central core composed of channels and collagenous trabeculac. Towards the apex of the granulations there is a cap of arachnoid cells with wide channels running through to the coating endothelium.15~ These structures are a major pathway for the drainage of cerebrospinal fluid, which percolates through the cores of the villi or granulations and is transported across the endothelium into the blood.

 

 

THE SUBEPENDYMAL PLATE

 

The subependymal plate has long been recognised as a layer of primitive cells beneath the ependymal lining of the lateral ventricles in the adult human ~ It is the remnant of the embryonal matrix (the subventricular zone). Studies of the sub­ependymal plate in various animal species have revealed that in the fetus it gives rise to both neurons and glia, whilst after birth it is a source of glial cells oflly.161~162 The cells of the sub­ependymal plate display ultrastructural features common to primitive cells: high nuclear-cytoplasmic ratio, dominance of free ribosomes over membrane-bound ribosomes and a general scarcity of organelles. Mitotic activity persists into later adult life in various species, including primates.Unfortunately, information on the human subependyn extrapolation from may be misleading.

 

Human subependymal plate is limited, and an extrapolation from experimenta] animals to man may be misleading.

In addition to the subependymal plate there are other secondary germinal sites in the mammalian central nervous system, including the dentate gyrus of the hippocampus, the olfactory bulb and the external granular layer of the cerebellum. This latter zone, which has been more comprehensively studied than the other two, is formed in fetal life and postnatally continues to produce the neurons of the internal granular layer. The proliferative activity of these secondary germinal zones and the hormonal, nutritional and pharmacological factors which influence cellular turnover have been reviewed.'67

 

The presence of the subependymal plate with potential mitotic activity in the adult human brain raises the question of the replacement of glial cells and of their proliferative activity in the normal brain. The view that cells of the adult central nervous system do not divide cannot be main­tained any longeri68 as there is convincing evidence that astrocytes and cells of the subependymal plate maintain mitotic activity throughout adult life. Although oligodendrocytes undergo mitosis in pathologica] conditions,'69 their ability to divide in the normal brain has not been unequivocally demonstrated. Similarly, microglial cells do not appear to be mitotically active in the normal, adult central nervous system. Neurons, ependymal cells, choroid plexus epithelium and pericytes do xiot divide after they have become differentiated, whilst endothelial cells continue to undergo mitosis during adult life.'68 The low turnover of cells in the adult central nervous system, coupled with the difficulty of positively identifying dividing cells and the occasional cell which is not fully differentiated, makes precise assessment of the mitotic activity of a particular cell type difficult. Moreover, recent tissue culture studies of the developing rat optic nerve have revealed that glial precursors, depending on the composition of the culture medium, can differentiate into either astrocyte or oligodendrocyte even without the influence of other brain cells.170 If these cells persist into adult life thcy may retain their differentiation potential and mitotic activity.