Question

Describe the origin and lifecycle of the red blood cells.

Answer 1

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, the erythropoiesis occurs in three stages:

• Mesoblastic Stage:

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

• Hepatic Stage:

From the third month of intrauterine life, the liver is the main organ that produces RBCs. Spleen and lymphoid organs are also involved in erythropoiesis.

• Medullary Stage:

During the last three months of intrauterine life, the RBCs are produced from red bone marrow and liver. The medullary hematopoiesis becomes more effective towards the end of the third trimester. After birth, the bone marrow becomes the sole site of erythropoiesis. In the adolescent period, the marrow cavities of all bones are involved in erythropoiesis, after which the regresses in the limb bones.

Steps of erythropoiesis:

Stem cells:

• Pluripotent stem cell:

They are the mother cell lines that form the stem cells for different cell lines. They have two unique properties: self-renewal and differentiation.

• Committed stem cells:

They develop from the pluripotent stem cells. There are two categories of stem cells:

The myeloid cells stem cells form the myeloid series, megakaryocyte series, granulocytic series, and monocytic series.

The erythroid stem cells give rise to progenitor cells for erythroid cell lines.

• Progenitor cells:

They are of two types:

BFU-E (Burst forming units erythroid progenitor cell): they give rise to a large number of colony-forming unit-erythroid cells.

CFU-E (Colony-forming-erythrocytes): Cells of this unit develop into erythrocytes. They give rise to moderate number of blast cells of erythrocytic series.

• Precursor cells:

The precursors of the erythrocytic series are the blast cells.

1. Proerythroblast (Megaloblast):

Proerythroblast or megaloblast is the first cell derived from CFU-E. It is very large in size with a diameter of about 20 µ. Its nucleus is large and occupies the cell almost completely. The nucleus has two or more nucleoli and a reticular network. Proerythroblast does not contain hemoglobin. The cytoplasm is basophilic. Proerythroblast multiplies several times and finally forms the cell of the next stage called early normoblast.

2. Early Normoblast:
The early normoblast is little smaller than proerythroblast with a diameter of about 15 µ. The nucleus occupies three-fourth of the cell area. It is composed of dark violet heterochromatin clumps interspersed with pink clumps of euchromatin. This often gives the nucleus the appearance of wheel spokes. The cytoplasm is basophilic. So, this cell is also called basophilic erythroblast. Hemoglobin appears the first time in these cells in erythropoiesis.

3. Intermediate Normoblast:
The cells are smaller than the early normoblast with a diameter of 10 to 12 µ. The nucleus is still present. The distribution of heterochromatic clumps in the nucleus gives the appearance of the checkerboard pattern. The nucleoli disappears. The chromatin network shows further condensation. The presence of RNA material makes the cytoplasm eosinophilic. Hemoglobin synthesis increases which makes the cell acidophilic. Thus, the mixture of acidophilic hemoglobin and eosinophilic RNA in the cytoplasm makes it polychromatic. So, this cell is called polychromophilic or polychromatic erythroblast. This cell develops into the next stage called the late normoblast.

4. Late Normoblast:
The diameter of the cell decreases further to about 8 to 10 µ. The nucleus becomes very small with very much condensed chromatin network and it is known as the ink-spot nucleus. The quantity of hemoglobin increases and the cytoplasm becomes deeply eosinophilic. So, the cell is now called orthochromatic erythroblast. In the final stage of late normoblast just before it passes to the next stage, the nucleus disintegrates and disappears. The process by which the nucleus disappears is called pyknosis.

5. Reticulocyte:
Reticulocyte is otherwise known as immature RBC. It is slightly larger than the mature RBC. The cytoplasm contains the reticular network or reticulum, which is formed by remnants of disintegrated organelles. Due to the reticular network, the cell is called reticulocyte. The reticulum of reticulocyte stains with supravital stain. The number of reticulocytes decreases during the first week after birth. Later, the reticulocyte count remains constant at or below 1% of RBCs.Reticulocyte is basophilic due to the presence of remnants of disintegrated Golgi apparatus, mitochondria, and other organelles of cytoplasm. During this stage, the cells enter the blood capillaries through the capillary membrane from the site of production by diapedesis.

6. Matured Erythrocyte:

Reticular network disappears and the cell becomes the matured RBC and attains the biconcave shape. The cell decreases in size to 7.2 µ diameter. The matured RBC is with hemoglobin but without nucleus.

Duration of erythropoiesis:

The total period for erythropoiesis occurs in 7 to 9 days. It takes 5 to 7 days for progenitor cells to become reticulocytes and another 2 days for reticulocytes to become red blood cells.

Red cells have a finite lifespan of about 120 days. The new cells are formed at a pace that replaces the cells destroyed.