7. HAEMOPOESIS AND DISORDERS OF BLOOD COMPONENTS

Introduction:

Haemopoiesis (also spelled hemopoiesis) is the process of formation of blood cells in the body. It ensures a continuous supply of red blood cells (RBCs), white blood cells (WBCs), and platelets, since most blood cells live only for a few days or weeks. The body maintains their numbers through feedback mechanisms, while WBC levels change in response to infections or immune challenges.

Haemopoiesis: The Formation of Blood Cells:

Origin and Site of Formation:

During early embryonic life, haemopoiesis begins in the yolk sac.
Later, it occurs in the liver, spleen, thymus, and lymph nodes of the fetus.
In the last three months before birth, the red bone marrow becomes the major site of blood cell formation and remains active throughout life.

Red Bone Marrow and Stem Cells:

Red bone marrow is a soft, highly vascular tissue located in the spongy parts of bones such as the sternum, ribs, vertebrae, and pelvis.
About 0.05–0.1% of its cells are pluripotent stem cells (hemocytoblasts) — primitive cells capable of forming all types of blood cells.
These stem cells come from mesenchyme, a tissue from which most connective tissues develop.

In newborns, all bone marrow is red and actively produces blood cells.
With age, some of it is replaced by yellow bone marrow, made mostly of fat cells.
During severe blood loss, yellow marrow can revert back to red marrow to boost blood cell production.

Process of Cell Formation:

Pluripotent stem cells give rise to two main stem cell lines:
1. Myeloid stem cells → develop in red bone marrow to form RBCs, platelets, monocytes, neutrophils, eosinophils, basophils, and mast cells.
2. Lymphoid stem cells → begin in bone marrow but mature in lymphatic tissues to form lymphocytes (B cells, T cells) and natural killer (NK) cells.

Once formed, most blood cells do not divide after leaving the bone marrow, except for lymphocytes.

Progenitor and Precursor Cells:

Myeloid and lymphoid stem cells further develop into progenitor cells, which are committed to forming specific blood components.
Progenitor cells like CFU–E (for RBCs), CFU–Meg (for platelets), and CFU–GM (for granulocytes and monocytes) eventually form precursor cells (blasts).
These precursor cells undergo several divisions to become mature blood cells such as erythrocytes, monocytes, eosinophils, and others.

Hormonal Control of Haemopoiesis:

Several growth factors and hormones control haemopoiesis:

Thrombopoietin (TPO): Produced by the liver; stimulates platelet formation from megakaryocytes.
Cytokines: Small protein molecules (like colony-stimulating factors (CSFs) and interleukins) that promote the formation of WBCs and regulate immune responses.
These factors ensure a balanced production of different blood cells as needed by the body.

Disorders of Blood Components:

1. Anemia:

Anemia means a reduction in hemoglobin or RBC count, leading to decreased oxygen delivery to tissues.
Types include:

Pernicious anemia: Lack of intrinsic factor prevents vitamin B₁₂ absorption. RBCs become large (macrocytic).
Microcytic anemia: Due to iron deficiency; RBCs are smaller than normal.
Hemolytic anemia: Caused by shortened RBC lifespan due to hereditary defects or antibodies destroying RBCs.
Sickle cell anemia: Abnormal hemoglobin causes crescent-shaped RBCs with a short lifespan.

2. Polycythemia:

Polycythemia refers to excessive RBC production.

Primary polycythemia: Bone marrow becomes overactive, skin and mucous membranes appear red.
Secondary polycythemia: A compensatory response to low oxygen (e.g., at high altitudes).

3. Bleeding Diseases:

In these disorders, the normal clotting mechanism fails.

Diagnosed using bleeding time and coagulation time tests.
Hemophilia is a hereditary disorder characterized by prolonged bleeding even from minor cuts due to deficiency of clotting factors.

4. Leukemias:

Leukemia is a cancer of the bone marrow, where leukocyte-producing tissues proliferate abnormally.
This leads to a huge increase in WBC count, crowding out normal blood cells and causing anemia, infections, and bleeding.

5. Agranulocytosis:

A condition marked by loss of granulocytic WBCs from blood.

Often causes ulcers in the mouth, gums, and jaw bones due to infections.

6. Thalassemia:

A genetic disorder in which defective hemoglobin leads to shorter RBC lifespan.

RBCs are abnormally shaped, and many nucleated cells appear in blood.
Can result in severe anemia and organ enlargement.
Other causes of hemolytic anemia include drug reactions, toxins, malarial infection, or Rh incompatibility in newborns.

Detailed Notes:

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