All the circulating blood cells derive from pluripotential stem cells in the marrow. They divide into three main types. The most numerous are red cells which are specialized for carriage of oxygen from the lungs to the tissues and of carbon dioxide in the reverse direction (Table 2.1). They have a 4‐ month lifespan, whereas the smallest cells, platelets involved in haemostasis, circulate for only 10 days. The white cells are made up of four types of phagocyte, neutrophils, eosinophils, basophils and monocytes, which protect against bacterial and fungal infections, and of lymphocytes, which include B cells, involved in antibody production, and T cells (CD4 helper and CD8 suppressor), concerned with the immune response and in protection against viruses and other foreign cells. White cells have a wide range of lifespan (Table 2.1).
The red cells and platelets are counted and their diameter and other parameters measured by an automated cell counter (Fig. 2.1). This also enumerates the different types of white cell by flow cytometry and detects abnormal cells.
We each make approximately 1012 new erythrocytes (red cells) each day by the complex and finely regulated process of erythropoiesis. Erythropoiesis passes from the stem cell through the progenitor cells, colony‐forming unit granulocyte, erythroid, monocyte and megakaryocyte (CFUGEMM), burst‐forming unit erythroid (BFUE) and erythroid CFU (CFUE) (Fig. 2.2), to the first recognizable erythrocyte precursor in the bone marrow, the pronormoblast. This process occurs in an erythroid niche in which about 30 erythroid cells at various stages of development surround a central macrophage.
The pronormoblast is a large cell with dark blue cytoplasm, a central nucleus with nucleoli and slightly clumped chromatin (Fig. 2.2). It gives rise to a series of progressively smaller normoblasts by a number of cell divisions. They also contain progressively more haemoglobin (which stains pink) in the cytoplasm; the cytoplasm stains paler blue as it loses its RNA and protein synthetic apparatus while nuclear chromatin becomes more condensed (Figs 2.2 and 2.3). The nucleus is finally extruded from the late normoblast within the marrow and a reticulocyte results, which still contains some ribosomal RNA and is still able to synthesize haemoglobin (Fig. 2.4). This cell is slightly larger than a mature red cell, and circulates in the peripheral blood for 1–2 days before maturing, when RNA is completely lost. A completely pink‐staining mature erythrocyte results which is a non‐nucleated biconcave disc. One pronormoblast usually gives rise to 16 mature red cells (Fig. 2.3). Nucleated red cells (normoblasts) are not present in normal human peripheral blood (Fig. 2.4). They appear in the blood if erythropoiesis is occurring outside the marrow (extrame ullary erythropoiesis) and also with some marrow diseases.