A variety of morphologic alterations in red blood cells (RBCs) can be induced by changes in the cell membrane. Three of the most common variants, namely spiculated cells (echinocytes and acanthocytes) and target cells, will be discussed here. Other variants such as stomatocytes (hydrocytes) (picture 1) and xerocytes (dessicocytes) (picture 2) are discussed separately. (See "Stomatocytosis" and "Xerocytosis".)
ECHINOCYTES AND ACANTHOCYTES
Definitions — Echinocytes and acanthocytes are two types of spiculated red blood cells. A freshly prepared peripheral blood smear is essential for distinguishing between these cell types ; wet preparations or scanning electron microscopy may also be useful.
- Echinocytes (also called burr cells) have serrated edges over the entire surface of the cell and often appear crenated in a blood smear (picture 3). Although often confused with acanthocytes, the projections of the red cell membrane are smaller and much more uniform in shape and distribution in echinocytes.
- Acanthocytes (from the Greek acantha "thorn") have only a few spicules of varying size that project from the cell surface at irregular intervals (picture 4). They appear contracted and dense on stained peripheral blood smears. Spur cells appear to be the extreme form of acanthocytes and are seen in patients with severe liver disease (see 'Liver disease' below). Spur cells are felt to be acanthocytes remodeled by the spleen, whereby the spicules become more blunt and the associated membrane loss makes the cell more spherocytic (spheroacanthocyte).
Pathophysiology — The shape of these cells is thought to be related to changes in the organization of cell membrane components. In acanthocytes, this is often due to a defect in the lipid or protein composition of red cell red membrane. In a study of patients with chorea-acanthosis and McLeod syndrome, for example, acanthocytes had a greater tendency to form spikes than normal RBCs in an incubation system, indicating their weakness against membrane tension . Focal changes in the membrane skeleton appeared to predispose to the acanthocyte phenotype. In one patient echinocytes were present in association with a massive splenic hemangioma and disappeared following splenectomy .
A hypothesis has been presented that implicates changes in the conformation in Band 3, a major component of the membrane skeleton, as the basis for acanthocyte formation in both congenital and acquired disorders in which these cells are found . In comparison, echinocytes are easily produced in vitro and are often produced by a material in the patient's plasma that causes normal red cells to become echinocytic.