A 48 year old women was admitted with a history of weakness for two months. On examination, cervical lymph nodes were found enlarged and spleen was palpable 2 cm below the costal margin. Her hemoglobin was 10.5 g/dl, platelet count 2.7 x 109/L and total leukocyte count 40 x 109/L, which included 80% mature lymphoid cells with coarse clumped chromatin. Bone marrow revealed nodular lymphoid infiltrate. The peripheral blood lymphoid cells were positive for CD 19, CD 5, CD20 and CD23 and were negative for CD 79 B and FMC - 7.The histopathological examination of the lymph node in this patient will most likely exhibit effacement of lymph node architecture by -