Erythropoiesis

Erythropoiesis is the process of the origin, development and maturation of erythrocytes. Hemopoiesis or hematopoiesis is the process of origin, development and maturation of all the blood cells.

SITE OF ERYTHROPOIESIS

IN FETAL LIFE

In fetal life, the erythropoiesis occurs in three stages:

1. Mesoblastic Stage

During the first two months of intrauterine life, the RBCs are produced from mesenchyme of yolk sac.

2. Hepatic Stage

From third month of intrauterine life, liver is the main organ that produces RBCs. Spleen and lymphoid

organs are also involved in erythropoiesis.

3. Myeloid Stage

During the last three months of intrauterine life, the RBCs are produced from red bone marrow and liver.

IN NEWBORN BABIES, CHILDREN AND ADULTS

In newborn babies, growing children and adults, RBCs are produced only from the red bone marrow.

1. Up to the age of 20 years: RBCs are produced from red bone marrow of all bones (long bones and all

the flat bones).

2. After the age of 20 years: RBCs are produced from membranous bones like vertebra, sternum,

ribs, scapula, iliac bones and skull bones and from the ends of long bones. After 20 years of age,

the shaft of the long bones becomes yellow bone marrow because of fat deposition and looses the

erythropoietic function. In adults, liver and spleen may produce the blood cells if the bone marrow is destroyed or fibrosed. Collectively bone marrow is almost equal to liver in size and weight. It is also as active as liver. Though bone marrow is the site of production of all blood cells, comparatively 75%

of the bone marrow is involved in the production of leukocytes and only 25% is involved in the production

of erythrocytes. But still, the leukocytes are less in number than the erythrocytes, the ratio being 1:500. This is mainly because of the lifespan of these cells. Lifespan of erythrocytes is 120 days whereas the lifespan of leukocytes is veryshort ranging from one to ten days. So the leukocytes need larger production than erythrocytes to maintain the required number.

PROCESS OF ERYTHROPOIESIS

STEM CELLS

Stem cells are the primary cells capable of self-renewal and differentiating into specialized cells.Hemopoietic stem cells are the primitive cells in the bone marrow, which give rise to the blood cells.

Hemopoietic stem cells in the bone marrow are called uncommitted pluripotent hemopoietic stem cells (PHSC). PHSC is defined as a cell that can give rise to all types of blood cells. In early stages, the PHSC are not designed to form a particular type of blood cell. And it is also not possible to determine the blood cell to be developed from these cells: hence, the name uncommitted PHSC. In adults, only a few number of these cells are present. But the best source of these cells is the umbilical cord blood. When the cells are designed to form a particular type of blood cell, the uncommitted PHSCs are called committed PHSCs. Committed PHSC is defined as a cell, which is restricted to give rise to one group of blood

cells. Committed PHSCs are of two types:

1. Lymphoid stem cells (LSC) which give rise to lymphocytes and natural killer (NK) cells

2. Colony forming blastocytes, which give rise to myeloid cells. Myeloid cells are the blood cells other

than lymphocytes. When grown in cultures, these cells form colonies hence the name colony forming

blastocytes. Different units of colony forming cells are:

i. Colony forming unit-erythrocytes (CFU-E) – Cells of this unit develop into erythrocytes

ii. Colony forming unit-granulocytes/monocytes (CFU-GM) – These cells give rise to granulocytes

(neutrophils, basophils and eosinophils) and monocytes

iii. Colony forming unit-megakaryocytes (CFU-M)

– Platelets are developed from these cells.

CHANGES DURING ERYTHROPOIESIS

Cells of CFU-E pass through different stages and finally become the matured RBCs. During these stages four important changes are noticed.

1. Reduction in size of the cell (from the diameter of 25 to 7.2 μ)

2. Disappearance of nucleoli and nucleus3. Appearance of hemoglobin

4. Change in the staining properties of the cytoplasm.