Bone Marrow - Infiltration Cellular, Mast Cell
comment:Mast cell infiltration is a reactive change defined as increased numbers of loosely scattered, morphologically normal mast cells compared with concurrent controls. In rats, mast cells are present in relatively high numbers in the bone marrow, comprising approximately 1-3% of all nucleated bone marrow cells, in contrast to dogs, where they are rarely observed in bone marrow cytology, and humans, where they comprise less than 0.1% of all nucleated bone marrow cells. Less is known about total mast cell numbers in the bone marrow of mice. Typical mast cells are round to oval with a central to slightly eccentric, round nucleus and a pale granular cytoplasm as seen on hematoxylin and eosin–stained histologic slides ( Figure 1 and Figure 2 ). With most Romanowsky-type stains (e.g., Giemsa), mast cells are easily identified by their metachromatic cytoplasmic granules ( Figure 3 and Figure 4 ).
The causes of increased bone marrow mast cells in rodents are not well defined. In humans and dogs, mast cell infiltration is secondary to such conditions as inflammation, parasitism, uremia (renal failure), aplastic anemia, osteoporosis, and hematologic diseases (e.g., preleukemias, lymphoproliferative diseases). The increased mast cells may be accompanied by increases in lymphocytes and plasma cells.
Because of the variability of normal mast cell numbers in rodent bone marrow, comparison with concurrent controls is especially important when a perceived increase in marrow mast cells is observed in treated animals. Increases in numbers of mast cells should always be recorded and given a severity grade.
The term “mastocytosis” should not be used in lieu of the diagnosis of mast cell infiltration. In humans, mastocytosis refers to a heterogeneous group of disorders (most of which have been shown to be clonal and includes the human equivalent of mast cell tumors) characterized by abnormal growth and accumulation of mast cells in one or more organ systems. The diagnosis of a bone marrow mast cell tumor in rodents is appropriate when mast cells are found in multifocal or diffuse, dense compact clusters or sheets. However, the presence of increased numbers of loosely scattered mast cells in and of itself warrants a diagnosis of infiltration, unless other criteria (e.g., atypical morphology, mast cell tumor in another location) are indicative of neoplasia.
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