Outer mantle zone of the follicle in the human spleen.

The follicular circumferential connective fibre reticulum, which is a continuation of the circumferencial reticulum (CR) of the periarterial lymphocyte sheaths, was visualized at the periphery of lymphoid follicles in the human spleen by means of anticartilaginous antiserum. If this CR is considered to be the borderline between the follicular and extrafollicular structures, then at the inner aspect of the CR and at the periphery of the fully developed follicle two mantle zones can be distinguished: an inner zone consisting of small lymphocytes, and an outer consisting of medium-sized lymphocytes. Both all groups are B-lymphocytes displaying positive surface IgM. The reticulum is the same in both mantle zones and it is completely different from the reticulum of the marginal zone. In some instances it is so sparse as to appear to be absent completely. The newly delimitated outer mantle zone also has a specific cellular composition differing from the marginal zone. Most importantly, it does not possess that mixture of lymphocytes and other blood cells which is considered to be the defining feature of the marginal zone. The characteristic cellular components of the outer mantle zone are the densely accumulated medium-sized lymphocytes, with other cells interspersed very sporadically.