Ultrastructure of the arachnoid membrane in man

Summary

The arachnoid membrane in man is formed of two layers of cells.

Those of the outermost layer are very electron dense and have long interweaving processes containing numerous vacuoles. Spaces which may appear empty or contain granular material may separate these cells and their processes. Beneath this outer layer is an inner layer consisting of groups of less electron dense cells, their interweaving processes and collections of collagen fibres and fibrillar material. Desmosomes are frequent between the cells. Occasional electron dense cells occur amongst the clear cells and desmosomes occur between them. The internal aspect of the layer of dark cells is covered by an interrupted layer of basement membrane and a space may occur between it and basement membrane overlying the outer aspect of the clear cells. In places, however, only a single layer of basement membrane separates the dark and clear cells or they may be in close contact, desmosomes being present between them. Basement membrane separates the superficial dark cells from the collagen fibres of the inner layer of the arachnoid membrane and amongst the collagen occur elongated electron dense cells similar to fibroblasts. Macrophages occur within the subarachnoid space.

Sometimes the arachnoid and the pia mater are in contiguity and the subarachnoid space is not apparent. Evidence of bulk flow of cerebrospinal fluid into the subdural space via the arachnoid membrane was not found.