Electron Microscopy
It is easy and inexpensive to fix small samples of lymph node routinely in glutaraldehyde at the time of initial processing and decide later if they should be embedded for electron microscopic processing or discarded. In most cases, the glutaraldehyde-fixed tissue will not be needed because electron microscopy contributes little to diagnosing typical lymph node lesions. Ultrastructural examination can enhance the diagnosis in less common disorders, such as Gaucher disease, mycosis fungoides, granulocytic sarcoma, lymphoblastic lymphoma with convoluted nuclei, amyloidosis, and others (1,2,3). Even more important, electron microscopic examination may decide the difficult diagnosis of some metastatic tumors—identifying, for example, melanosomes or premelanosomes in metastatic melanoma (4), tonofilaments and various specialized desmosome-like or other types of intercellular junctions in metastatic carcinomas, abundant glycogen deposits in metastatic Ewing sarcoma (5), and neurosecretory granules in metastatic neuroblastoma, islet cell tumors, and small-cell carcinomas (6).
Immunoelectron microscopy can be used for the study of cell membranes, including surface immunoglobulins (Ig) and various cell markers. This is achieved by applying peroxidase-labeled antibodies to suspensions of live, unfixed cells or to fixed sections, and visualizing them under the electron microscope (Fig. 4.1).
Non-Hodgkin B-Cell Lymphomas
Various B-cell malignant lymphomas have a unique ultrastructure. In chronic lymphocytic leukemia, Ig light chains are synthesized in excess over heavy chains. Ultrastructural immunoperoxidase study reveals Ig light chains in both perinuclear space lumina and rough endoplasmic reticulum, whereas heavy chains are limited to rough endoplasmic reticulum (7). After stimulation with pokeweed mitogen, the Ig heavy- and light-chain production appears balanced (7). Although a “signet ring” light microscopic appearance is most common in adenocarcinoma, several lymphoma variants occur; B-cell lymphomas of the lymphoplasmacytic type are characterized by an accumulation of Ig-containing rough endoplasmic reticulum cisternae (8), and the vacuolar type, seen in both B- and T-cell lymphomas, consists of a large, membrane-limited vacuole containing giant multivesicular bodies (9) (see Chapter 61). The ultrastructural features of mantle cell lymphoma with the translocation t(11;14)(q13;q32) include nuclear clefts or indentations and evenly dispersed heterochromatin, absent or inconspicuous nucleoli, abundant mitochondria, and a Golgi zone that can be distinguished from that of follicular lymphoma, prolymphocytic leukemia, and chronic lymphocytic leukemia (10).
Follicular lymphoma with abundant periodic acid–Schiff-positive extracellular material composed of membranous structures, membrane-bound vesicles, and electron-dense bodies probably accumulated by a process of deregulated and excessive cell membrane synthesis (11). Rarely, B-lymphoma cells engulfed by macrophages appear to be undergoing emperipolesis (12). Human herpesvirus 8 has been ultrastructurally demonstrated in a clinical sample of AIDS-related body cavity–based lymphoma (13). Ultrastructural morphometry of non-Hodgkin lymphomas has identified the interchromatic (matrix) nuclear contents as contributing the most to nuclear volume (14).
Diffuse large-cell B-cell lymphomas are characterized by large cells with a high nuclear–cytoplasmic ratio. The scarce cytoplasm contains few, poorly developed organelles, including a small number of mitochondria. The large nuclei have marginated blocks of chromatin (Fig. 4.2). The immunoblastic variant is characterized by the single, prominent nucleolus (Fig. 4.3).
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