Types of Bones and Bone Tissue

Types of Bones and Bone Tissue

Introduction

  • Bones, like people, come in various shapes and sizes.

  • Purpose: Bones have specific functions that facilitate movement and support within the human skeleton.

Types of Bone Tissue

  • Compact Bone

    • Makes up 80% of the total weight of the human skeleton.

    • Comprises cylinder-shaped units known as osteons (also referred to as Haversian systems).

    • Osteons run lengthwise through the bone, providing structural integrity.

    • Bone cells in osteons are closely packed, allowing efficient nutrient delivery and waste removal during bone remodeling.

    • Appearance: Compact bone is smooth and forms the external layer of bones.

  • Cancellous Bone

    • Accounts for 20% of the human skeleton.

    • Does not contain osteons; instead, it has tiny branches called trabeculae crisscrossing each other.

    • Also known as trabecular bone.

    • Location: Found primarily in the epiphyses, which are the rounded ends of long bones.

Bone Marrow

  • Described as a soft connective tissue vital for bone function.

  • Types of Bone Marrow:

    • Red Bone Marrow

    • Site of hematopoiesis, the production of red blood cells.

    • Found in the diaphysis (straight section) and epiphysis (ends) of long bones in children.

    • Post-epiphyseal plate fusion, it is present only in the epiphysis and flat bones of adults.

    • Yellow Bone Marrow

    • Functions primarily for fat storage (adipose tissue).

    • Located within the medullary canal (marrow cavity) of long bones.

    • In infants, this cavity contains red marrow, which is gradually replaced by yellow marrow around the age of seven.

Classification of Bones

  • Bones are classified based on their shape into five categories:

1. Long Bones

  • Found in arms and legs.

  • Characteristics:

    • Longer than they are wide.

    • Composed of a diaphysis (straight shaft) and epiphyses (rounded ends).

    • The area between diaphysis and epiphyses is called the metaphysis.

  • Function: Designed to withstand significant weight and offer structural support.

2. Short Bones

  • Predominantly spongy and cube-shaped.

  • Locations: Found in the wrists (carpal bones) and ankles (tarsal bones).

  • Function: Allow for greater movement compared to long bones.

3. Flat Bones

  • Thin, often curved structures (like a peanut butter sandwich).

  • Examples: Skull, ribs, sternum, scapula.

  • Function: Protect delicate tissues, such as the brain and organs in the thorax.

4. Sesamoid Bones

  • Encased within a tendon that runs over an angled surface.

  • Example: Kneecap (patella).

  • Characteristics: Resemble sesame seeds; also present in hands and feet.

5. Irregular Bones

  • Exhibit a multitude of shapes and edges for muscle, tendon, and ligament attachment.

  • Definition: These do not fit into other categories and are also known as miscellaneous bones.

  • Examples: Jawbone (mandible), tailbone (coccyx), and vertebrae.

Bone Fractures

  • Definition: Fractures are breaks in bones that occur when they absorb more force than they can withstand.

  • Types of Fractures: Various, further research is encouraged on this category.

  • Healing Process: Involves a consistent series of events tied to remodeling:

    • Realignment: Bones must be realigned and immobilized using casts, slings, or bed rest.

Stages of Bone Healing

  • The healing process consists of four stages:

    1. Hematoma Formation:

    • Occurs immediately after a fracture, involving bleeding that forms a bruise known as a hematoma.

    • This stage lasts approximately 72 hours.

    1. Soft Callus Formation:

    • Begins around the end of the second week.

    • Nutrients and bone matrix rebuild around the fracture, resulting in a fibrocartilaginous callus consisting of cartilage, calcium, phosphorus, and osteoblasts.

    1. Hard Callus Formation:

    • Also known as ossification; occurs between three weeks and six months post-fracture.

    • Involves the decrease of distance between broken bone pieces.

    • Patients are advised to keep the fractured area immobilized until x-ray confirms healing.

    1. Remodeling:

    • The final phase where excess bone tissue is reabsorbed by osteoclasts.

    • Restores normal bone strength and shape over time.

Aging and Bone Health

  • Change in bone matrix with age:

    • As age increases, matrix layers become brittle and less capable of regeneration.

    • This results in reduced bone density and strength.

  • Cellular Impact:

    • Osteoclasts (bone breakdown cells) continue to function normally, but osteoblasts (bone-building cells) become less active.

    • Outcome: More bone is lost than built; this condition is known as osteoporosis.

  • Demographics:

    • Osteoporosis can affect males but is most common in post-menopausal females due to decreased estrogen production, which hampers osteoblast activity.

  • Complications of Osteoporosis:

    • Increased fragility of bones, leading to fractures from minor impacts or falls.