2. Physiology of Blood (Part 2).pdf

Erythropoiesis

- Erythropoiesis is the production of mature red blood cells

(erythrocytes)

- In the first two months of embryonic life, RBCs are

produced in the

yolk sac

.

- From third month of intrauterine life, liver is the main

organ that produces RBCs but reasonable numbers are

also produced in the

spleen and lymph nodes

.

Erythropoiesis

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.

- The bone marrow is the

site of production of all blood cells, comparatively 75%

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 nucleus.

3. Appearance of hemoglobin

4. Change in the staining properties of the cytoplasm

STAGES OF ERYTHROPOIESIS

1. Proerythroblast

2. Early normoblast

2. Intermediate normoblast.

STAGES OF ERYTHROPOIESIS

1. Proerythroblast

2. Early normoblast

2. Intermediate normoblast.

4. Late normoblast

5. Reticulocyte

STAGES OF ERYTHROPOIESIS

1.Proerythroblast (Megaloblast): It 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 in nature. Proerythroblast

multiplies several times and finally forms the cell of next stage called

early normoblast. Synthesis of hemoglobin starts in this stage. However, appearance of hemoglobin occurs only in intermediate normoblast.

2.Early Normoblast: The early normoblast is little smaller than

proerythroblast with a diameter of about 15 µ. In the nucleus, the nucleoli disappear. Condensation of chromatin network occurs. The

cytoplasm is basophilic in nature. So, this cell is also called basophilic erythroblast. This cell develops into next stage called intermediate

STAGES OF ERYTHROPOIESIS

3.Intermediate Normoblast: Cell is smaller than the early normoblast with a diameter of 10 to 12 µ. The nucleus is still present. But, the chromatin network shows further condensation. The hemoglobin

starts appearing. Cytoplasm is already basophilic. Now, because of the presence of hemoglobin, it stains with both acidic as well as basic stains. So this cell is called polychromophilic or polychromatic erythroblast. This cell develops into next stage called late normoblast.

4.Late Normoblast

Diameter of the cell decreases further to about 8 to 10 µ. Nucleus

becomes very small with very much condensed chromatin network and it is known as ink-spot nucleus. Quantity of hemoglobin increases. And the cytoplasm becomes almost acidophilic. So, the cell is now called

STAGES OF ERYTHROPOIESIS

5.Reticulocyte

Reticulocyte is otherwise known as immature RBC. It is slightly larger than matured 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. In newborn babies, the reticulocyte count is 2% to 6% of RBCs, i.e. 2 to 6 reticulocytes are present for every 100 RBCs. The number of

reticulocytes decreases during the first week after birth. Later, the reticulocyte count remains constant at or below 1% of RBCs. The

number increases whenever production and release of RBCs increase. Reticulocyte is basophilic due to the presence of remnants of

disintegrated Golgi apparatus, mitochondria and other organelles of

STAGES OF ERYTHROPOIESIS

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. It requires 7 days for the development and maturation of RBC from

Regulation of Red Blood Cell Production

- The total mass of RBCs in the circulatory system is regulated within narrow limits, so

(1) adequate red cells are always available to provide sufficient transport of oxygen from the lungs to the tissues

(2) the cells do not become so numerous that they impede blood flow

FACTORS NECESSARY FOR ERYTHROPOIESIS

Development and maturation of erythrocytes require variety of factors, which are classified into three categories:

1. General factors

FACTORS NECESSARY FOR ERYTHROPOIESIS

General factors :

- i.

Erythropoietin

- ii. Thyroxine

- iii. Hemopoietic growth factors

i- Erythropoietin

- Most important general factor for erythropoiesis is the hormone called erythropoietin. It is also called hemopoietin or erythrocyte stimulating factor.

- Erythropoietin Stimulates Red Cell Production, and Its Formation Increases in Response to Hypoxia.

- The principal stimulus for RBC production in low oxygen states is a circulating hormone called erythropoietin, a glycoprotein with a

molecular weight of about 34,000.

Erythropoietin

Erythropoietin

Role of the Kidneys in Formation of Erythropoietin.

- Normally, about 90 percent of all erythropoietin is formed in the kidneys; the remainder is formed mainly in the liver.

- Some studies suggest that erythropoietin is secreted mainly by fibroblast-like interstitial cells surrounding the tubules in the cortex and it is likely that other cells, including the renal epithelial cells also secrete the erythropoietin in response to hypoxia.

- Renal tissue hypoxia, hypoxia in other parts of the body, but not in the kidneys, stimulates kidney erythropoietin secretion, which suggests that there might be some non-renal sensor that sends an additional signal to the kidneys to produce this hormone. In

Erythropoietin

Effect of Erythropoietin in Erythrogenesis

Erythropoietin causes formation and release of new RBCs into circulation. After secretion, it takes 4 to 5 days to show the action.

Erythropoietin promotes the following processes:

a. Production of proerythroblasts from CFU-E of the bone marrow

b. Development of proerythroblasts into matured RBCs through the several stages – early normoblast, intermediate normoblast, late normoblast and reticulocyte

Other General Factors:

ii-Thyroxine

Being a general metabolic hormone, thyroxine accelerates the process of erythropoiesis at many levels. So, hyperthyroidism and polycythemia are common.

iii- Hemopoietic Growth Factors

Hemopoietic growth factors or growth inducers are the interleukins and stem cell factor. Generally these factors induce the proliferation of PHSCs. Interleukins (IL) are glycoproteins, which belong to the

cytokines family.

Interleukins involved in erythropoiesis: a. Interleukin-3 (IL-3) secreted by T-cells

b. Interleukin-6 (IL-6) secreted by T-cells, endothelial cells and macrophages

c. Interleukin-11 (IL-11) secreted by osteoblast.

iv. Vitamins

MATURATION FACTORS

Vitamin B12 and folic acid are necessary for the maturation of RBCs.

- Essential for the synthesis of DNA via the formation of thymidine triphosphate, one of the essential building blocks of DNA

- Lack of either vitamin B12 or folic acid causes abnormal and

diminished DNA and, consequently, failure of nuclear maturation and cell division.

- The erythroblastic cells of the bone marrow in absence of either

Maturation Failure Caused by Poor Absorption of

Vitamin B12from the Gastrointestinal Tract—Pernicious Anemia.

- A common cause of red blood cell maturation failure is failure to

absorb vitamin B12from the gastrointestinal tract. This often occurs in the disease pernicious anemia, in which the basic abnormality is an atrophic gastric mucosa that fails to produce normal gastric

secretions.

- The parietal cells of the gastric glands secrete a glycoprotein called intrinsic factor, which combines with vitamin B12 in food and makes the B12 available for absorption by the gut. It does this in the following way:

- (1) Intrinsic factor binds tightly with the vitamin B12.

- (2) Still in the bound state, intrinsic factor binds to specific receptor sites on the brush border membranes of the mucosal cells in the ileum.

- (3) Then, vitamin B12 is transported into the blood during the next - few hours, carrying intrinsic factor and the vitamin together through

Maturation Failure Caused by Poor Absorption of

Vitamin B12from the Gastrointestinal Tract—Pernicious Anemia.

- Once vitamin B12 has been absorbed from the gastrointestinal tract, it is first stored in large quantities in the liver and then released slowly as needed by the bone marrow.

Failure of Maturation Caused by Deficiency of Folic Acid.

- Folic acid is a normal constituent of green vegetables, some fruits, and meats (especially liver). However, it is easily destroyed during cooking.

Hemoglobin, The Respiratory Pigment

in Red Blood Cells

•

Found in the red blood cells in the cytosol

• Haemoglobin 33% of cell weight

• Carries oxygen (O

) and carbon dioxide (CO

)

• It also acts as a buffer.

• Molecular weight of hemoglobin is 68,000

- The value varies depending upon the age

At birth : 25 g/dl

After 3rd month : 20 g/dl

After 1 year : 17 g/dl

From puberty onwards: 12 to 16 g/dl

Each gram of pure hemoglobin is capable of combining with

1.34 ml

of oxygen

Male: 20 ml of oxygen/Hb/100 ml of blood

Female: 19 ml of oxygen/Hb/100 ml of blood

Quantity of Hemoglobin in the Cells

Hemoglobin

Female:

12-16 g/dl

Formation of Hemoglobin (Hb)

- Synthesis of Hb begins in the proerythroblasts and continues even into the reticulocyte stage of the RBCs.

- Synthesis of hemoglobin actually starts in proerythroblastic stage. However, hemoglobin appears in the intermediate normoblastic stage only.

- Production of hemoglobin is continued until the stage of reticulocyte.

Formation of Hemoglobin (Hb)

- Succinyl-CoA, formed in the Krebs metabolic cycle, binds with glycine to form a pyrrole molecule. - In turn, four pyrroles combine

to form protoporphyrin IX, which then combines with iron to form the heme molecule. - Finally, each heme molecule

combines with a long polypeptide chain, a globin synthesized by ribosomes,

forming a subunit of Hb called a hemoglobin chain.

- Each chain has a molecular

Types of Haemoglobin throughout development

Stage Type of Hb Globin chains

Embryo Hb Gower1 ζ₂ε₂

Hb Gower2 α₂ε₂

Hb Portland ζ₂γ₂

Foetus HbF α₂γ₂

The main Hb from 12 weeks to newborn. Higher

oxygen affinity than HbA

Adult HbA α₂β₂

Main Hb in adult (97%)

HbA₂ α₂δ₂

Haemoglobin (Hb) – Globin and haem

- Globin consists of four polypeptides, two alpha and two beta

chains

Heme Groups

Each hemoglobin chain has a heme prosthetic group containing an atom of iron, and because there are four Hb chains in each Hb molecule, one finds four iron atoms in each Hb molecule; each of

these can bind loosely with one molecule of oxygen, making a total of four molecules of oxygen that can be transported by each Hb

molecule

carry four molecules of oxygen