Hematopoiesis: Blood Cell Origin and Tissue Characteristics

Histogenesis of Blood Tissue

Histogenesis refers to the origin and development of cells.

Development of Granulocytes (Myelopoiesis)

Granulocytes are cells with granular cytoplasm, including neutrophils, eosinophils, basophils (cells ending in -phil), and mast cells.

Stages of Granulocyte Development:

• Myeloblast
• Promyelocyte
• Myelocyte (stage when granules appear)
• Metamyelocyte

Origin of Blood Cells

All blood cells originate from the Hematopoietic Stem Cell. These cells differentiate after being stimulated by hormones or interleukins. For example, B-lymphocytes become plasma cells after being stimulated by IL-2 in order to fight infections.

Characteristics and Composition of Blood

General Characteristics of Blood

• Consists of blood plasma and matured blood cells (mesenchyme origin).
• Blood is regarded as a modified connective tissue because blood cells are separated by a large amount of intercellular substance, such as blood plasma.
• Represents approximately 8% of total body weight.
• Average volume is 5 liters in the body.
• The Romanovsky-Giemsa method is used for staining blood smears.

Components of Blood

1. Red Blood Cells (Erythrocytes) and Platelets: Serve as the transport mechanism for oxygen, nutrients, waste removal, hormones, and inorganic salts.
2. Leukocytes (White Blood Cells): Essential cells of the immune system.
3. Blood Plasma:
   • Constitutes 55% of total blood volume.
   • Consists primarily of water and dissolved colloids and crystalloids.
   • Colloids: Proteins such as immunoglobulins, globulins, albumins, fibrinogens, and hormones.
   • Crystalloids: Ions (Na, Cl, K, Ca, Mg, P) and other substances like amino acids and glucose.

Classification of Blood Cells

Blood cells are classified based on their structure, function, and origin.

Classification by Origin

All blood cells originate from hematopoietic stem cells and then differentiate into:

• Myeloid Progenitor Cells: Such as erythrocytes, monocytes, myelocytes, and megakaryocytes.
• Lymphoid Progenitor Cells: Such as T-lymphocytes and B-lymphocytes.

Classification by Structure

• Agranular: Lymphocytes and monocytes (lacking visible granules).
• Granular: Basophils, neutrophils, and eosinophils.

Classification by Function

• Fibrocytes/Ground Substances
• Defending: Mast cells, macrophages, and plasmocytes.
• Storing: Pigment cells and adipocytes.

Erythrocytes (Red Blood Cells)

Erythrocyte Structure

• Hemoglobin: The main functional unit. It consists of a central iron ion (resulting in a pale center) surrounded by four heme groups (dark red surroundings).
• The erythrocyte loses its nucleus to accommodate hemoglobin.
• Oxidation of iron occurs in the lungs, and its reduction occurs in the respiring tissue.
• Biconcave disk shape.
• Lacks a nucleus and organelles.
• Diameter: 6–7 µm.
• Membrane Layers: Outer (contains pumps), middle, and inner (contains proteins).

Erythrocyte Function

• Oxygen transport: Oxygen + Heme = Oxyhemoglobin (life span of 120 days).
• Carbon Dioxide transport: CO₂ + Hemoglobin = Carbaminohemoglobin (transfer of CO₂ from respiring tissues to the lungs).

Normal Erythrocyte Count (per liter)

Male: 4.1–6.1 x 10¹²; Female: 3.8–5.5 x 10¹².

(Note from original text: This is unfair—women are equal, they should have the same;P)

Histogenesis of Erythropoiesis (5 Stages)

Erythropoiesis occurs in the bone marrow. (Drink milk!!!)

1. Proerythroblast
2. Basophilic erythroblast
3. Polychromatic erythroblast (Ribosome synthesis)
4. Orthochromatic erythroblast (Production of hemoglobin)
5. Normoblast (Moves into blood/loses nucleus) → Reticulocyte (0.5–1% of total RBC count)

Erythrocyte Recycling

Once an RBC dies, it is engulfed by macrophages in the spleen or liver. The globulin component is dismantled into amino acids and recycled. Fe²⁺ is converted to Fe³⁺ before being transported in the blood by transferase.