Leukodiapedesis and leukocyte migration in the leptomeninges and in the subarachnoid space.

Pleocytosis in cerebrospinal fluid and infiltration of the leptomeningeal tissue have been studied after injection of ferritin into the subarachnoid space (SAS) of cats. The most important source of granulocytes in the leptomeninges are the relatively large veins of the pia mater, which have very thin walls. Passing between the lining cells of the pia mater the granulocytes leave the connective tissue space of the pia mater and reach the SAS. Leukodiapedesis has also been observed in veins crossing the SAS. During this process, gaps between the lining cells of the perivascular leptomeningeal sheath may develop. There are two possible ways for the granulocytes to pass from the vascular pia mater to the avascular arachnoidea: either they migrate actively on the surface of the leptomeningeal trabeculae or they reach the arachnoidea passively by the circulation of the cerebrospinal fluid. Leukodiapedesis in the vessels of the dura mater occurs relatively seldom and would not be able to cause the occasionally massive infiltration of the arachnoidea.