Makindo Medical Notes

Anatomy of Bone Marrow

Bone marrow is a soft, spongy tissue found within the hollow interior of bones. It plays a crucial role in hematopoiesis (the production of blood cells). Bone marrow is classified into two types: red bone marrow (active in hematopoiesis) and yellow bone marrow (mainly fat storage).

Types of Bone Marrow

• Red Bone Marrow: Primarily responsible for the production of red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). Red marrow is abundant in young children and decreases with age, becoming confined to certain bones in adults.
  • In adults, red bone marrow is primarily found in flat bones such as the sternum, ribs, vertebrae, and pelvis, as well as in the epiphyses (ends) of long bones such as the femur and humerus.
• Yellow Bone Marrow: Composed mainly of fat cells, yellow marrow serves as an energy reserve. As an individual ages, some red marrow is gradually replaced by yellow marrow. Under certain conditions (e.g., severe blood loss), yellow marrow can revert back to red marrow to produce blood cells.

Bone Marrow Structure

• Stroma: The supportive tissue of bone marrow, consisting of reticular connective tissue, adipocytes (fat cells), fibroblasts, and endothelial cells. The stroma provides a microenvironment for haematopoiesis.
• Hematopoietic Tissue: Contains stem cells and progenitor cells that differentiate into various types of blood cells. The hematopoietic tissue is found in red bone marrow.

Hematopoiesis

Hematopoiesis is the process by which blood cells are formed and developed from hematopoietic stem cells (HSCs) in the red bone marrow. HSCs are multipotent stem cells capable of differentiating into all types of blood cells.

Hematopoietic Lineages

• Myeloid Lineage: Gives rise to red blood cells, platelets, and certain types of white blood cells (e.g., neutrophils, basophils, eosinophils, and monocytes).
  • Erythropoiesis: The production of red blood cells (erythrocytes), which transport oxygen and carbon dioxide in the blood.
  • Thrombopoiesis: The formation of platelets (thrombocytes), which are essential for blood clotting.
  • Granulopoiesis: The production of granulocytes (neutrophils, eosinophils, and basophils), which play important roles in the immune response.
  • Monocytopoiesis: The production of monocytes, which differentiate into macrophages and are involved in phagocytosis and immune regulation.
• Lymphoid Lineage: Produces lymphocytes (B cells, T cells, and natural killer cells), which are key players in the adaptive immune response.
  • B Lymphocytes: Produce antibodies and mediate humoral immunity.
  • T Lymphocytes: Involved in cell-mediated immunity. T cells can directly attack infected or cancerous cells.
  • Natural Killer Cells (NK Cells): Part of the innate immune system, these cells can destroy virus-infected and cancerous cells.

Regulation of Haematopoiesis

• Growth Factors and Cytokines: Hematopoiesis is tightly regulated by several growth factors and cytokines that influence the proliferation and differentiation of hematopoietic stem cells. Examples include:
  • Erythropoietin (EPO): Produced by the kidneys, EPO stimulates the production of red blood cells in response to hypoxia (low oxygen levels).
  • Thrombopoietin (TPO): Produced by the liver and kidneys, TPO regulates the production of platelets.
  • Granulocyte Colony-Stimulating Factor (G-CSF): Promotes the proliferation and differentiation of neutrophils.
  • Interleukins: A group of cytokines that regulate the immune system and support the development of both myeloid and lymphoid cells.

Bone Marrow Microenvironment

• The bone marrow microenvironment consists of the stromal cells, extracellular matrix, and soluble factors that support hematopoiesis. The microenvironment is crucial in maintaining the balance between self-renewal and differentiation of hematopoietic stem cells.

Functions of Bone Marrow

• Production of Blood Cells: The primary function of bone marrow is the production of red blood cells, white blood cells, and platelets through the process of hematopoiesis.
• Immune Response: Bone marrow plays a key role in the immune system, as it produces B and T lymphocytes that are essential for adaptive immunity.
• Stem Cell Reserve: Bone marrow contains hematopoietic stem cells (HSCs), which serve as a lifelong reserve for replenishing blood cells as needed.
• Fat Storage: Yellow bone marrow stores fat, which acts as an energy reserve and can be converted back into red bone marrow in cases of severe blood loss or anaemia.

Clinical Relevance

• Bone Marrow Aspiration/Biopsy: Bone marrow samples are taken to diagnose various hematological conditions, such as leukemia, anaemia, and myelodysplastic syndromes. Aspiration is used to obtain a liquid sample, while a biopsy involves removing a small core of solid bone marrow tissue.
• Leukemia: A cancer of the blood-forming tissues, including bone marrow. It leads to the overproduction of abnormal white blood cells, impairing normal blood cell production.
• Aplastic Anemia: A condition where the bone marrow fails to produce enough blood cells, leading to pancytopenia (a deficiency of red cells, white cells, and platelets).
• Bone Marrow Transplant: A treatment for certain cancers, bone marrow diseases, and immunodeficiency disorders. Hematopoietic stem cells from a donor are transplanted to replace the patient's damaged or destroyed bone marrow.
• Myelofibrosis: A bone marrow disorder where scar tissue forms in the bone marrow, impairing its ability to produce blood cells and leading to anaemia, splenomegaly, and fatigue.