Cytology · MU Sofia

25. Bone marrow.

Covers the red and yellow types of bone marrow, how the proportions change from newborns to adults and when yellow marrow can revert to red. It then describes the stroma, extracellular matrix, haemopoietic cords and sinusoidal capillaries of red marrow, the niche that supports stem cells and the growth factors involved, and how new blood cells enter the circulation. It ends with clinical uses such as aspiration, transplantation and regenerative medicine.

Inside: 7 sections · 3 labelled pictures · 13 flashcards · 4 min

Topic 64 of 97 in the MU Sofia syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 7 sections, about 499 words (4 min of reading), with 13 flashcards.

In this note

  1. Introduction
  2. Types and General Features of Bone Marrow
  3. Composition of Red Bone Marrow
  4. The Hemopoietic Niche
  5. Entry of Blood Cells into Circulation
  6. Medical Applications
  7. Key Points to Remember

Introduction

Bone marrow is a specialised tissue found within the cavities of bones. It plays a vital role in hemopoiesis, which is the process of forming all the different types of blood cells.

Types and General Features of Bone Marrow

  • Red Bone Marrow: Active in hemopoiesis and characterized by its red colour due to the abundance of blood and developing blood cells.
  • Yellow Bone Marrow: Primarily filled with adipocytes, which exclude most hemopoietic cells, giving it a yellow colour.
  • Distribution: In newborns, all bone marrow is red and active in blood cell production. As a child grows, most marrow gradually transitions to the yellow type.
  • Reversibility: Yellow marrow can revert back to red marrow under certain conditions, such as severe bleeding or hypoxia, to meet the increased demand for blood cells.

Composition of Red Bone Marrow

  • Stroma: Consists of a network of specialized fibroblastic cells called stromal cells (also called reticular or adventitial cells) and reticular fibres that support the developing blood cells and macrophages.
  • Extracellular Matrix (ECM): Contains collagen type I, proteoglycans, fibronectin, and laminin, which help bind cells to the matrix.
  • Hemopoietic Cords: Clusters or islands of developing blood cells supported by the stroma.
  • Sinusoidal Capillaries: Specialized blood vessels with a discontinuous endothelium, allowing newly formed blood cells to enter the circulation.
  • Macrophages: Phagocytose older, defective erythrocytes and recycle iron from heme for delivery to developing red blood cells.

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