Blood cells

Introduction

Blood has four major elements:

• Red blood cells (erythrocytes) transport oxygen from the lungs to the peripheral tissues.

• White blood cells (leukocytes) have a defensive role, destroying infecting organisms, such as bacteria and viruses, and assisting in the removal of dead or damaged tissues.

• Platelets (thrombocytes) are the first line of defence against damage to blood vessels, adhering to defects and participating in the blood clotting system.

• Plasma is the proteinaceous solution in which the previously mentioned cells circulate and carries nutrients, metabolites, antibodies, hormones, proteins of the blood clotting system and other molecules throughout the body.

In postnatal life, under normal circumstances, the formation of the cellular elements of the blood ( haematopoiesis ) occurs in the bone marrow in various bones (see p. 131). Most of the proteins in the plasma are made by the liver.

Bone marrow–derived stem cells

One population, called haematopoietic stem cells, can form all the types of blood cells in the body. These are the cells useful clinically for bone marrow transplants.

A second population is called bone marrow stromal stem cells or mesenchymal stem cells . These non-haematopoietic stem cells make up a small proportion of the stromal cell population in the bone marrow and can generate bone, cartilage, fat, cells that support the formation of blood and fibrous connective tissue (see Fig. 4.17 ). These cells are under study as sources of different cell types for regenerative medicine.