compact bone

Examples of compact bone in the following topics:

• Bone

  • Bones are made of a combination of compact bone tissue for strength and spongy bone tissue for compression in response to stresses.
  • There are two types of bone tissue: compact and spongy.
  • Compact bone (or cortical bone), forming the hard external layer of all bones, surrounds the medullary cavity (innermost part or bone marrow).
  • Compact bone tissue consists of units called osteons or Haversian systems.
  • Compact bone tissue forms the outer layer of all bones while spongy or cancellous bone forms the inner layer of all bones.

• Gross Anatomy

  • All the bones in the body can be described as long bones or flat bones.
  • Cortical bone is compact bone, while cancellous bone is trabecular and spongy bone.
  • The outer shell of the long bone is compact bone, below which lies a deeper layer of cancellous bone (spongy bone), as shown in the following figure.
  • They are composed of two thin layers of compact bone surrounding a layer of cancellous (spongy) bone.
  • The hard outer layer of bones is composed of compact bone tissue, so-called due to its minimal gaps and spaces.

• Microscopic Anatomy of Bone

  • The basic microscopic unit of bone is an osteon, which can be arranged into woven bone or lamellar bone.
  • Each osteon consists of a lamellae of compact bone tissue that surround a central canal (Haversian canal).
  • Osteons can be arranged into woven bone or lamellar bone.
  • Lamellar bone makes up the compact or cortical bone in the skeleton, such as the long bones of the legs and arms.
  • A photo taken through a microscope that shows the anatomy of compact bone with a detailed view of an osteon.

• Connective Tissues: Bone, Adipose, and Blood

  • Bone can be divided into two types: compact and spongy.
  • Compact bone is organized into subunits called osteons.
  • (a) Compact bone is a dense matrix on the outer surface of bone.
  • Spongy bone, inside the compact bone, is porous with web-like trabeculae.
  • (b) Compact bone is organized into rings called osteons.

• Bone Repair

  • The next phase is the replacement of the hyaline cartilage and woven bone with lamellar bone.
  • Substitution of the woven bone with lamellar bone precedes the substitution of the hyaline cartilage with lamellar bone.
  • This new lamellar bone is in the form of trabecular bone.
  • The remodeling process continues with substitution of the trabecular bone with compact bone.
  • The trabecular bone is first resorbed by osteoclasts, creating a shallow resorption pit known as Howship's lacuna, and then osteoblasts deposit compact bone within the resorption pit.

• Embryonic and Fetal Bone Formation

  • Eventually, woven bone is replaced by lamellar bone.
  • The primary center of ossification is the area where bone growth occurs between the periosteum and the bone.
  • Osteons are units or principal structures of compact bone.
  • When replacement with compact bone occurs, this blood vessel becomes the central canal of the osteon.
  • It is also an essential process during the rudimentary formation of long bones, the growth of the length of long bones, and the natural healing of bone fractures.

• Bone Remodeling and Repair

  • Bone is remodeled through the continual replacement of old bone tissue, as well as repaired when fractured.
  • Bone remodeling is the replacement of old bone tissue by new bone tissue.
  • It involves the processes of bone deposition or bone production done by osteoblasts and bone resorption done by osteoclasts, which break down old bone.
  • Bone turnover rates, the rates at which old bone is replaced by new bone, are quite high, with five to seven percent of bone mass being recycled every week.
  • Compact bone is added to create bone tissue that is similar to the original, unbroken bone.

• Capacity of Different Tissues for Repair

  • The next phase is the replacement of the hyaline cartilage and woven bone with lamellar bone.
  • Substitution of the woven bone with lamellar bone precedes the substitution of the hyaline cartilage with lamellar bone.
  • This new lamellar bone is in the form of trabecular bone.
  • The remodeling process substitutes the trabecular bone with compact bone.
  • The trabecular bone is first resorbed by osteoclasts, creating a shallow resorption pit known as a "Howship's lacuna. " Then osteoblasts deposit compact bone within the resorption pit.

• Bone Development

  • Intramembranous ossification stems from fibrous membranes in flat bones, while endochondral ossification stems from long bone cartilage.
  • Bone growth continues until approximately age 25.
  • Bones can grow in thickness throughout life, but after age 25, ossification functions primarily in bone remodeling and repair.
  • The non-mineralized portion of the bone or osteoid continues to form around blood vessels, forming spongy bone.
  • The spongy bone is remodeled into a thin layer of compact bone on the surface of the spongy bone.

• Supply of Blood and Nerves to Bone

  • The blood and nerve supply to bones are carried in Haversian canals that run along the long axis of bones.
  • Blood is supplied to mature compact bone through the Haversian canal.
  • The Haversian canals also surround nerve cells throughout the bone and communicate with osteocytes in lacunae (spaces within the dense bone matrix that contain the living bone cells) through canaliculi.
  • The vascular supply of long bones depends on several points of inflow, which feed complex sinusoidal networks within the bone.
  • The Haversian canals surround blood vessels and nerve cells throughout the bone.