Blood Cell Morphology in Health and Disease

Anisocytosis (ανισοζ, unequal) and poikilocytosis (ποικιλοζ, varied)

Anisocytosis and poikilocytosis are nonspecific features of almost any blood disorder. The terms imply more variation in size or shape than is normally present ( Figs. 5-5 and 5-6 ). Anisocytosis may be a result of the presence of cells larger than normal (macrocytosis), cells smaller than normal (microcytosis) or both; frequently both macrocytes and microcytes are present ( Fig. 5-5 ).

Poikilocytes are produced in many types of abnormal erythropoiesis, for example, megaloblastic anaemia ( Fig. 5-7 ), iron deficiency anaemia, thalassaemia, myelofibrosis (both primary and secondary) ( Fig. 5-8 ), congenital dyserythropoietic anaemia ( Fig. 5-9 ) and the myelodysplastic syndromes. Elliptocytes and ovalocytes are among the poikilocytes that may be present when there is dyserythropoiesis; they are often present in megaloblastic anaemia (macro-ovalocytes) and in iron deficiency anaemia (‘pencil cells’), but they may also be seen in myelodysplastic syndromes and in primary myelofibrosis ( Fig. 5-8 ). The number of elliptocytes and teardrop poikilocytes has been observed to correlate with the severity of iron deficiency anaemia. Poikilocytes are not only characteristic of disordered erythropoiesis but are also seen in various congenital haemolytic anaemias caused by membrane defects and in acquired conditions such as microangiopathic haemolytic anaemia and oxidant damage; in these disorders, the abnormality of shape results from damage to cells after formation and is described later in this chapter.

Macrocytes

Classically found in megaloblastic anaemias ( Fig. 5-10 ), macrocytes are also present in some cases of aplastic anaemia, myelodysplastic syndromes and other dyserythropoietic states. In patients being treated with hydroxycarbamide (previously known as hydroxyurea) the red cells are often macrocytic. A common cause of macrocytosis is excess alcohol intake, and it occurs in alcoholic and other types of chronic liver disease. In these conditions, the red cells tend to be fairly uniform in size and shape and there may also be stomatocytes ( Fig. 5-11 ). In the rare type III form of congenital dyserythropoietic anaemia, some of the macrocytes are exceptionally large. Another rare cause of macrocytosis is benign familial macrocytosis. Macrocytosis also occurs whenever there is an increased rate of erythropoiesis, because of the presence of reticulocytes. Their presence is suspected in routinely stained films because of the slight basophilia, giving rise to polychromasia (see p. 78 ), and is easily confirmed by special stains (e.g. New methylene blue, see p. 27 ). These polychromatic macrocytes should be distinguished from other macrocytes because the diagnostic significance is quite different.

Microcytes

The presence of microcytes usually results from a defect in haemoglobin formation. Microcytosis is characteristic of iron deficiency anaemia ( Fig. 5-12 ), various types of thalassaemia ( Fig. 5-13 ), and severe cases of anaemia of chronic disease. Causes that are rarer include congenital and acquired sideroblastic anaemias. Microcytosis related to a defect in haemoglobin synthesis should be distinguished from red cell fragmentation or schistocytosis (see p. 72 ). Both abnormalities can lead to a reduction of the mean cell volume (MCV). However, it should be noted that a low MCV is common in association with a defect in haemoglobin synthesis, whereas it is uncommon in fragmentation syndromes because the fragments usually comprise only a small percentage of erythrocytes.

Basophilic stippling

Basophilic stippling or punctate basophilia means the presence of numerous basophilic granules distributed throughout the cell ( Fig. 5-14 ); in contrast to Pappenheimer bodies (see below), they do not give a positive Perls reaction for ionised iron. Punctate basophilia has quite a different significance from diffuse cytoplasmic basophilia. It is indicative of disturbed rather than increased erythropoiesis. It occurs in many blood diseases: thalassaemia, megaloblastic anaemias, infections, liver disease, poisoning by lead and other heavy metals, unstable haemoglobins and pyrimidine-5′- nucleotidase deficiency.

Hypochromia (Hypochromasia) ( υπορ, under)

The term hypochromia (previously often designated hypochromasia ), refers to the presence of red cells that stain unusually palely. (In doubtful cases, it is wise to compare the staining of the suspect film with that of a normal film stained at the same time.) There are two possible causes: a lowered haemoglobin concentration and abnormal thinness of the red cells. A lowered haemoglobin concentration results from impaired haemoglobin synthesis. This may stem from failure of haem synthesis – iron deficiency is a very common cause ( Fig. 5-15 ) and sideroblastic anaemia ( Fig. 5-16 ) is a rare cause – or from failure of globin synthesis as in the thalassaemias ( Fig. 5-17 ). Haemoglobin synthesis may also be impaired in chronic infections and other inflammatory conditions. Abnormally thin red cells (leptocytes) (see p. 74 ) can be the result of a defect in haemoglobin synthesis (e.g. in thalassaemias and iron deficiency) but they also occur in liver disease. It cannot be too strongly stressed that a hypochromic blood picture does not necessarily mean iron deficiency, although this is the most common cause. In iron deficiency, the red cells are characteristically hypochromic and microcytic, but the extent of these abnormalities depends on the severity; hypochromia may be minor and may be overlooked if the haemoglobin concentration (Hb) exceeds 100 g/l. In heterozygous or homozygous α ⁺ thalassaemia, heterozygous α ⁰ thalassaemia or heterozygous β thalassaemia, hypochromia is often less marked, in relation to the degree of microcytosis, than in iron deficiency. The presence of target cells or basophilic stippling also favours a diagnosis of thalassaemia trait rather than iron deficiency. In homozygous or compound heterozygous β thalassaemia (β thalassaemia intermedia or major), the abnormalities are greater than in iron deficiency at the same Hb and nucleated red cells are present, whereas they are not a feature of iron deficiency. If the patient is being transfused regularly, normal donor cells will also be present, producing a dimorphic blood film ( Fig. 5-18 ).

Anisochromasia ( αυισοζ, unequal) and dimorphic red cell population

A distinction should be made between anisochromasia, in which there is abnormal variability in staining of red cells, and a dimorphic picture, in which there are two distinct populations. Anisochromasia, in which some but not all of the red cells stain palely, is characteristic of a changing situation. It can occur during the development or resolution of iron deficiency anaemia ( Fig. 5-19 ) or the anaemia of chronic disease. In thalassaemia trait, in contrast, anisochromasia is much less common. A dimorphic blood film can be seen in several circumstances. It can occur when an iron deficiency anaemia responds to iron therapy, when a patient being treated for megaloblastic anaemia develops iron deficiency, after the transfusion of normal blood to a patient with a hypochromic anaemia and in sideroblastic anaemia ( Fig. 5-20 ). In acquired sideroblastic anaemia as a feature of a myelodysplastic syndrome, the two populations of cells are usually hypochromic microcytic and normochromic macrocytic, respectively.

Hyperchromia (Hyperchromasia) ( υπερ, over)

Unusually deep staining of the red cells with a lack of central pallor may be seen in two circumstances: first, in the presence of macrocytes; and second, when cells are abnormally rounded. In macrocytosis, as in neonatal blood and megaloblastic anaemias, it is the increased red cell thickness that causes the hyperchromia, and the mean cell haemoglobin concentration is normal. When hyperchromia results from cells being of abnormal shape, the red cell thickness is greater than normal and the MCHC is increased. Abnormally rounded cells may be either spherocytes or irregularly contracted cells. The distinction between these two cell types is of diagnostic importance.

Spherocytosis ( σφαιρα, sphere)

Spherocytes are cells that are more spheroidal (i.e. less disc-like) than normal red cells but maintain a regular outline. Their diameter is less and their thickness is greater than normal. Only in extreme instances are they almost spherical in shape. It is useful to draw a distinction between spherocytes of normal size and microspherocytes; the latter result from red cell fragmentation or from removal of a considerable proportion of the red cell membrane by splenic or other macrophages. Spherocytes may result from genetic defects of the red cell membrane as in hereditary spherocytosis ( Fig. 5-21 ); from the interaction between immunoglobulin- or complement-coated red cells and phagocytic cells, as in delayed transfusion reactions, ABO haemolytic disease of the newborn ( Fig. 5-22 ) and autoimmune haemolytic anaemia ( Fig. 5-23 ); and from the action of bacterial toxins (e.g. Clostridium perfringens lecithinase; Fig. 5-24 ) or snake venoms.

Spherocytes usually appear perfectly round in contour in stained films; they have to be carefully distinguished from both irregularly contracted cells and ‘crenated spheres’ or sphero-echinocytes ( Fig. 5-25 ), which are the end result of crenation (see p. 74 ). Sphero-echinocytes develop as artefacts, especially in blood that has been allowed to stand before films are spread ( Fig. 5-26 ). The blood film of a patient who has been transfused with stored blood may show a proportion of sphero-echinocytes ( Fig. 5-27 ).

Irregularly contracted red cells

There are a number of causes of irregularly contracted cells. In oxidant drug- or chemical-induced haemolytic anaemias, a proportion of the red cells are smaller than normal and unusually densely stained (i.e. they appear contracted) and their margins are slightly or moderately irregular and may be partly concave ( Fig. 5-28 ). Similar cells may be seen in films of some unstable haemoglobinopathies before splenectomy (e.g. that caused by the presence of haemoglobin Köln or haemoglobin St Mary’s; Fig. 5-29 ), in haemoglobin E homozygosity ( Fig. 5-30 ) and, to a lesser extent, haemoglobin E heterozygosity. They can result from oxidant damage due to release of copper from liver cells in Wilson disease. An extreme degree of irregular contraction is characteristic of severe favism or any other very acute haemolytic episode in individuals who are glucose-6- phosphate (G6PD) deficient. It is typical to see cells in which the haemoglobin appears to have contracted away from the cell membrane, an appearance sometimes referred to as a blister cell or a hemi-ghost ( Fig. 5-28 ); there may also be ghost cells – cells with almost empty membranes containing negligible haemoglobin ( Fig. 5-31 ). Irregularly contracted cells can be seen in small numbers in β thalassaemia trait and in heterozygosity for haemoglobin C. There may be a considerable number in haemoglobin C homozygosity, and in this condition haemoglobin C crystals may also be seen ( Fig. 5-32 ).

Heinz bodies are not normally readily visible in Romanowsky-stained blood films, although they are sometimes seen in such films as pale pinkish-brown–staining bodies at the cell margin or even protruding from the erythrocytes. This observation may be made in severe unstable haemoglobin haemolytic anaemias after splenectomy and in acute oxidant-induced haemolytic anaemia, including that occurring in deficiency of G6PD.

A type of irregular contraction of unknown origin has been described by the term ‘pyknocytosis’. The pyknocytes closely resemble chemically damaged red cells but in addition are spiculated. As already mentioned (see p. 62 ), a small number of pyknocytes may be found in the blood of infants in the first few weeks of life, especially in premature infants. The term ‘infantile pyknocytosis’ refers to a transient haemolytic anaemia, related to glutathione peroxidase and selenium deficiency, affecting infants in whom many pyknocytes are present ( Fig. 5-33 ). ^,

Elliptocytosis and ovalocytosis

Elliptocytes are often present in large numbers in hereditary elliptocytosis ( Fig. 5-34 ). In hereditary pyropoikilocytosis, elliptocytes are only one of the many types of poikilocyte present ( Fig. 5-35 ). Southeast Asian ovalocytosis is characterised by the presence of a variable number of elliptocytes, ovalocytes, macro-ovalocytes and stomatocytes ( Fig. 5-36 ). In all these conditions, the reticulocytes are round in contour (i.e. the cell assumes an abnormal shape only in the late stages of maturation). Although the causative condition is inherited, the abnormalities of red cell shape only become apparent as the cells reach maturity.

Schistocytosis (Fragmentation) ( σχιστοζ, cleft)

Schistocytes or erythrocyte fragments are found in many blood diseases. They are smaller than normal red cells and of varying shape. Sometimes they have sharp angles or spines (spurs), and sometimes they are round in contour, usually staining deeply but occasionally palely as the result of loss of haemoglobin at the time of fragmentation. If they are both round and densely staining, they may be referred to as microspherocytes. They occur in the following situations:

• 1.

  In certain genetically determined disorders (e.g. thalassaemias, congenital dyserythropoietic anaemia and hereditary pyropoikilocytosis)

• 2.

  In acquired disorders of red cell formation when erythropoiesis is megaloblastic or dyserythropoietic

• 3.

  As the consequence of mechanical stresses (e.g. in the microangiopathic haemolytic anaemias [ Figs. 5-37 to 5-39 ] and in mechanical haemolytic anaemias such as those caused by a perivalvular leak accompanied by turbulence of left ventricular flow in a patient with a prosthetic cardiac valve [ Fig. 5-40 ])

  
  
  
  

• 4.

  As the result of direct thermal injury, as in severe burns ( Fig. 5-41 )

  

It is important to be aware of schistocytes as a feature of megaloblastic and dyserythropoietic anaemias so that misattribution to a thrombotic microangiopathy is avoided. In burns, schistocytes are often rounded, being either microspherocytes or very small disc-shaped fragments. In addition, erythrocytes may be seen to be budding off small rounded blebs of cytoplasm. Not infrequently, as, for instance in the haemolytic uraemic syndrome in children, the blood picture is made more bizarre by the superimposition of varying degrees of echinocytic change. Schistocytes are also a feature of thrombotic thrombocytopenia purpura. International Council for Standardisation in Haematology (ICSH) guidelines are available for the recognition and enumeration of schistocytes in suspected thrombotic thrombocytopenic purpura.