cells, growth, marrow

Functions of Bone Cells

• Bones are constantly remodelled through modelling (growth) and remodelling (replacement).

• Proper bone cell activity is essential for skeletal strength.

• Disorders such as osteoporosis result from imbalance in bone formation and resorption.

Bone Matrix

Components

• Organic (35%) – Osteoid
  

  • Type I collagen (≈90%)

  • Proteoglycans

• Inorganic (65%)
  

  • Hydroxyapatite crystals
    Ca₁₀(PO₄)₆(OH)₂ (calcium phosphate)

Cells of Bone

1. Osteogenic (Osteoprogenitor) Cells

• Stem cells, spindle-shaped with elongated nuclei

• Found in:
  

  • Inner layer of periosteum

  • Lining vascular canals of compact bone

• Function: Differentiate into osteoblasts

2. Osteoblasts

• Bone-forming cells

• Located on bone surfaces

• Cuboidal shape, large nucleus, prominent nucleolus

• Well-developed Golgi and mitochondria

• Cannot divide

• Functions:
  

  • Synthesize organic matrix (collagen)

  • Deposit inorganic minerals

3. Osteocytes

• Mature bone cells (former osteoblasts)

• Located in lacunae

• Connected by canaliculi

• Flattened cells with few RER and Golgi

• Cannot divide

• Function: Maintain bone matrix

4. Osteoclasts

• Large, multinucleated cells (5–50 nuclei)

• Irregular surface facing bone

• Rich in lysosomes and Golgi apparatus

• Derived from monocyte/macrophage lineage

• Function: Bone resorption (osteolysis)

• Create Howship’s lacunae (resorption bays)

Bone Growth

Endochondral Ossification

• Begins with a hyaline cartilage model

• Limb bones are cartilaginous at ~6 weeks

• Growth occurs by:
  

  • Interstitial growth → length

  • Appositional growth → width

• Length increase occurs at the epiphyseal plate

• Epiphysis and diaphysis are initially separated

Bone Marrow

Definition

• Soft, spongy tissue in:
  

  • Medullary cavity of long bones

  • Spaces of cancellous (spongy) bone

• Functions:
  

  • Blood cell production

  • Fat storage

Types of Bone Marrow

Red Bone Marrow

• Locations:
  

  • Ends of long bones

  • Flat bones, vertebrae, ribs, sternum, pelvis

• Contains hematopoietic cells

• Produces:
  

  • Red blood cells

  • White blood cells

  • Platelets

Yellow Bone Marrow

• Located in medullary cavity of long bones

• Rich in adipocytes

• Function: Energy storage (triglycerides)

Hematopoiesis

• Process of blood cell formation

• Occurs in bone marrow

• Driven by hematopoietic stem cells (HSCs)

• Highly regulated proliferation and differentiation

Cellular Components of Bone Marrow

Hematopoietic Cells

• Erythrocytes: Oxygen transport

• Leukocytes: Immune defense
  

  • Granulocytes: neutrophils, eosinophils, basophils

  • Lymphocytes: T cells, B cells, NK cells

  • Monocytes

• Platelets: Blood clotting

Stromal Cells

• Mesenchymal stem cells (MSCs):
  

  • Differentiate into osteoblasts, chondrocytes, adipocytes

• Osteoblasts, adipocytes, endothelial cells

• Provide structural and functional support

Clinical Relevance

• Bone marrow biopsy/aspiration: Diagnosis of blood disorders

• Disorders include:
  

  • Anemia

  • Leukemia

  • Multiple myeloma

• Bone marrow transplantation:
  

  • Autologous (patient’s own marrow)

  • Allogeneic (donor marrow)