314 UNITY AND DIVERSITY IN BIOCHEMISTRY 



The mesenchymatic cells are not only transformed into fibrocytes, 

 cartilaginous cells, osteoblasts and osteoclasts. They also give rise to the 

 various types of blood cell. An erythrocyte begins as a reticulated cell in 

 the bone marrow, and, from a biochemical point of view, we can recognize 

 three phases in its differentiation. In the first stage, the cell contains a few 

 mitochondria and a nucleus with a very distinct nucleolus. The cytoplasm, 

 rich in RNA, is highly basophilic. The cell multiplies rapidly and an active 

 synthesis of proteins and nucleic acids is going on. In the second stage the 

 nucleus loses its nucleolus and the basophilia of the cytoplasm diminishes. 

 During the third stage, that of the polychromatic erythoblast, we see a fur- 

 ther diminution of RNA and at the same time the cell becomes acidophilic 

 and is coloured by acid stains. This is the result of the biosynthesis of a 

 new relatively basic protein, globin. At the same period, molecules of haem 

 and haemoglobin are rapidly formed. 



In the orthochromatic erythroblast, or normoblast, cell division does not 

 occur. The mitochondria have disappeared and only traces of RNA remain in 

 the c\i:oplasm. The nucleus is small. On the other hand, the haemoglobin con- 

 centration has increased from 1-2 parts per 100 to 20 parts per 100. The 

 nucleus having disappeared, the cell flattens and becomes biconcave so that 

 the red blood cell is nothing more than a bag of haemoglobin, only the cyto- 

 plasmic enzyme systems having been retained : those for glycolysis and the 

 hexosemonophosphate shunt, reducing systems able to maintain the 

 haemoglobin in the reduced state. The red blood corpuscle contains 

 relatively large amounts of catalase. It protects the haemoglobin by 

 decomposing the hydrogen peroxide which would otherwise form in the 

 corpuscle. 



The study of leucocytes is still in the hands of histologists. But a few 

 results of a biochemical nature have been obtained and they show some 

 aspects of differentiation. Neutrophil leucocytes, for example, show a high 

 aerobic glycolysis and they contain esters of hyaluronic acid. The granules 

 of the eosinophils are surrounded by an envelope of phospholipides and 

 among their constituents there is an antihistaminic substance. From the 

 megakaryocytes of the bone marrow are derived the blood platelets which 

 are highly specialized biochemically. They contain, and liberate into the 

 blood on clotting, a number of platelet factors, one activating thrombo- 

 plastin, another accelerating the formation of thrombin from prothrombin, 

 yet another accelerating the conversion of fibrinogen to fibrin, etc. 



The most remarkable example of differentiation in cells of mesodermic 

 origin is the metabolic series which continues the process of biosynthesis 

 of the steroid ring (see p. 236). We have already described one of these 

 continuations, the synthesis of bile acids by the liver cell. In the cells of 

 the adrenal cortex and the gonads, other systems continue the biosynthesis 

 of the steroid ring with the formation of steriod hormones. 



