Variation in shape is often very marked. Oval, pyriform, caudate, saddle-shaped, and club-shaped corpuscles are common. They are called poikilocytes, and their presence is spoken of as poikilocytosis.
Red corpuscles which vary from the normal in size and shape are present in most symptomatic anemias, and in the severer grades are often very numerous. Irregularities are particularly conspicuous in leukemia and pernicious anemia, where, in some instances, a normal erythrocyte is the exception. In pernicious anemia there is a decided tendency to large size and oval forms, and megalocytes are rarely found in any other condition.
(2) Variations in Staining Properties (See [Plate VIII]).—These include polychromatophilia, basophilic degeneration, and basophilic stippling. They are probably degenerative changes, although polychromatophilia is thought by many to be evidence of youth in a cell, and hence to indicate an attempt at blood regeneration.
(a) Polychromatophilia.—Some of the corpuscles partially lose their normal affinity for acid stains, and take the basic stain to greater or less degree. Wright's stain gives such cells a faint bluish tinge when the condition is mild, and a rather deep blue when severe. Sometimes only part of a cell is affected. A few polychromatophilic corpuscles can be found in marked symptomatic anemias. They occur most abundantly in malaria, leukemia, and pernicious anemia.
(b) Basophilic Granular Degeneration (Degeneration of Grawitz).—This is characterized by the presence, within the corpuscle, of small basophilic granules. They stain deep blue with Wright's stain; not at all, with Ehrlich's triple stain. The cell containing them may stain normally in other respects, or it may exhibit polychromatophilia.
Numerous cells showing this degeneration are commonly found in chronic lead-poisoning, of which they were at one time thought to be pathognomonic. Except in this disease, the degeneration indicates a serious blood condition. It occurs in well-marked cases of pernicious anemia and leukemia, and, much less commonly, in very severe symptomatic anemias.
(c) Basophilic Stippling.—This term has been applied to the finely granular appearance often seen in red corpuscles, which harbor malarial parasites ([Plate VI]). It is commonly classed with the degeneration just described, but is probably distinct. Not all stains will show it. With Wright's stain it can be brought out by staining longer and washing less than for the ordinary blood-stain. The minute granules stain reddish purple.
(3) Variations in Structure.—The most important is the presence of a nucleus (Plates [VI] and [VIII and Fig. 84]). Nucleated red corpuscles, or erythroblasts, are classed according to their size: microblasts, 5 µ or less in diameter; normoblasts, 5 to 10 µ; and megaloblasts, above 10 µ. Microblasts and normoblasts contain one, rarely two, small, round, sharply defined, deeply staining nuclei, often located eccentrically. Occasionally the nucleus is irregular in shape, "clover-leaf" forms being not infrequent. The megaloblast is probably a distinct cell, not merely a larger size of the normoblast. Its nucleus is large, stains rather palely, has a delicate chromatin network, and often shows evidences of degeneration (karyorrhexis, etc.). In ordinary work, however, it is safer to base the distinction upon size than upon structure. Any nucleated red cell, but especially the megaloblast, may exhibit polychromatophilia.
Normally, erythroblasts are present only in the blood of the fetus and of very young infants. Pathologically, normoblasts occur in severe symptomatic anemia, leukemia, and pernicious anemia. They are most abundant in myelogenous leukemia. While always present in pernicious anemia, they are often difficult to find. Megaloblasts are found in pernicious anemia, and with extreme rarity in any other condition. They almost invariably exceed the normoblasts in number, which is one of the distinctive features of the disease. Microblasts have much the same significance as normoblasts, but are less common.
2. The Leukocytes.—An estimation of the number or percentage of each variety of leukocyte in the blood is called a differential count.