Blood Cells
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.
