*6.4.1 Morphology and immunophenotype*

MCL consists of a homogeneous population of small lymphocytes with irregular to occasionally deeply clefted (cleaved) nuclear contours. In most cases the nuclear chromatin is condensed, nucleoli are inconspicuous, and the cytoplasm is scant. It usually has a diffuse growth pattern or surrounds reactive germinal centers in a mantle zone pattern. Extension of the lymphoma into the capsule and perinodal fat is common Occasionally, tumors composed of intermediate-sized cells with more open chromatin and a brisk mitotic rate are observed; immunophenotyping is necessary to distinguish these "blastoid" variants.

Mantle cell lymphomas express high levels of cyclin D1. Most tumors are also express CD19, CD20. It is usually CD5+, SOX11 +, FMC7+ (Flinders Medical Centre), BCL1+ and CD23−, CD10−, CD200−, LEF1−, which help to distinguish it from CLL/SLL. The IgH genes lack somatic hypermutation, supporting an origin from a naive B cell [25] (**Figure 4**).
