Haematopoiesis Overview and Types

Study Notes

Haematopoiesis Overview

Haematopoiesis is the formation and development of blood cells.
Blood is a highly regenerative tissue, with roughly one trillion cells arising daily in adult bone marrow.
The blood system is a well-understood developmental system, despite its complexity.
In 1909, Alexander Maximow proposed blood is organized in a hierarchical manner, starting with a single lymphocyte-like cell.
This hypothesis was proven through functional rescue experiments, demonstrating complete hematopoietic system reconstitution in irradiated mice following transplantation with healthy donor cells.

Types of Haematopoiesis

Embryonic Haematopoiesis: The initial blood cell production occurs in the yolk sac.
- It produces transitory hematopoietic cells, including primitive erythrocytes and some myeloid cells.
- Its main function is to support oxygenation of the developing embryo.
Adult Haematopoiesis: Bone marrow (BM) is the primary site for adult hematopoiesis, providing an environment for supporting the maintenance and differentiation of hematopoietic stem cells (HSCs).
- While BM is the main site, some blood cells mature elsewhere, such as T and B cells in the thymus and spleen, respectively.
- Current analysis using cell surface antibodies and flow cytometry has shown a developmental hierarchy with HSCs at the top and differentiated cells at the bottom.

Stem and Progenitor Cells

Haematopoietic Stem Cells (HSCs): The starting point of hematopoiesis, characterized by:
- Long-term self-renewal.
- Differentiation into diverse blood cell types.
- Rare cells, approximately one in 20 million nucleated cells in bone marrow (<0.5%).
- Can reconstitute entire bone marrow from lethally irradiated or chemically treated tissue.
- Morphologically indiscernible.
- Phenotypically, human HSCs are CD34+ CD38– and negative for lineage markers (Lin–).
Progenitor Cells: Derived from HSCs, they have:
- progressively restricted differentiation and proliferation capacity.
- demonstrated by in vitro culture techniques.

Composition of Whole Blood

Whole blood comprises plasma (46-63%) and formed elements (37-54%).
Formed elements include platelets (0.1%), white blood cells (99.9%).
White blood cell types include neutrophils (50-70%), eosinophils (2-4%), basophils (<1%), lymphocytes (20-30%), and monocytes (2-8%).

Plasma Composition

Plasma Proteins: Major components:
- Albumins (60%): Osmotic pressure regulation, lipid and hormone transport.
- Globulins (35%): Transport of ions, hormones, lipids, and immune function.
- Fibrinogen (4%): Essential for blood clotting.
Other solutes: Include water (92%), electrolytes, organic nutrients, and organic wastes.

Buffy Coat

Buffy coat is the thin layer between plasma and red blood cell layers containing thrombocytes and white blood cells (0.1%).

Regulation of Hematopoiesis

Growth Factors: Crucial glycoprotein hormones regulating proliferation and differentiation.
- Examples include erythropoietin (kidney) and thrombopoietin (liver).
Transcription Factors: These proteins bind DNA, influencing gene transcription.
- Mutations in transcription factors can lead to hematological malignancies.
Stromal Cells: Crucial source of growth factors essential to hematopoiesis; such as those found in bone marrow, for example

Sites of Haematopoiesis

Haematopoiesis sites shift throughout development.
Prenatal: Yolk sac, liver, spleen, and bone marrow.
Postnatal: Primarily bone marrow, with contributions from other sites like ribs, vertebral and pelvic bones, and less prominently lymph nodes and the spleen.

Description

This quiz covers the formation and development of blood cells, known as haematopoiesis. It explores both embryonic and adult haematopoiesis, detailing the sites of blood cell production and their functions. Test your understanding of the complex yet well-understood blood system and its hierarchical organization.