Haematopoiesis

The process responsible for the production of blood cells is known as haematopoiesis, which occurs in the bone marrow. In an adult human, the bone marrow weighs about 2.6 kg, which accounts for 4.5% of the total body weight. Although the bone marrow is dispersed throughout the body, it is nevertheless a larger organ than the liver, which weighs about 1.5 kg. About 55–60% of all cells produced by the bone marrow are neutrophils. The marrow is thus highly proliferative, with mitoses observed in 1–2.5% of all nucleated cells. The cellularity of the marrow varies considerably with age: for example, in the young about 75% of the marrow comprises cells, whereas in adults this figure is decreased to about 50% and in the elderly only 25% of the marrow is cellular.

The bone marrow

The bone marrow is comprised of the cells that divide and develop into mature blood cells and also of stromal cells consisting of fibroblasts, macro – phages and adipocytes. There are three major cellular types in the marrow (Fig. 2.1); these give rise, by the processes of division and differentiation, to the eight major blood cells.

1. i. Pluripotent or multipotent stem cells have the ability to divide and differentiate into blood cells of all lineages, but are also capable of selfrenewal. Thus, a pluripotent stem cell can differentiate to form two cells that are more mature (i.e. myeloid- or lymphocyte-like), or else it can divide to form two identical, uncommitted stem cells. Indeed, experiments with irradiated mice have shown that as few as 30 stem cells can replace all of the cells of the bone marrow.

2. […]