Lymphocytes undergo two phases of maturation: first in the bone marrow or thymus and then in the lymph nodes or other lymph tissue. Many hematology texts describe the maturation series as lymphoblast, prolymphocyte, and lymphocyte. This lymphocyte is the one seen in peripheral blood smears and is considered "naive" because it has not yet been stimulated by antigen. Also, mature lymphocytes are normally present in the bone marrow and, when clustered in a lymphoid follicle, can be very prominent.
Lymphocytes can be found scattered throughout the bone marrow and must be distinguished from early erythroid precursors, which they can closely resemble. Lymphocytes are frequently found in and around early NRBC clusters.
In the top image on the right, notice the medium-sized lymphocyte (red arrow) next to the two basophilic normoblasts (blue arrow). The color and texture of the scant lymphoid cytoplasm are almost identical to the NRBC, which can be a bit confusing. However, observe the differences in the nuclei between the two cell types. The lymphocyte has a less distinct chromatin clumping pattern than the basophilic normoblasts, and the lymphocyte does not have any "nuclear pores." Also, the lymphocyte has an irregularly-shaped nucleus that is hugging the cytoplasmic border, while the NRBC has a round and regular, centrally-placed nucleus.
Identify the three lymphocytes circling the NRBCs in the second image (see red arrows). Notice the chromatin of the lymphocytes; the lymphoid smudgy/clumpy pattern is certainly not as dense and clumped as what is noted in the NRBCs. This nuclear difference becomes more pronounced as the erythroids mature. The cytoplasmic differences should be more apparent as well since lymphocytes will never produce hemoglobin.
