Bone Tissue

Microscopic Anatomy of the Osteon

The osteon, also known as the Haversian system, is the fundamental structural unit of compact bone. Understanding its organization is essential for grasping how bone tissue supports and nourishes itself.

• Osteon: The cylindrical structure that runs parallel to the long axis of the bone, providing strength and support.

• Concentric Lamellae: Rings of calcified matrix arranged around the central canal. These layers give the bone its strength and resilience.

• Lacunae: Small spaces located between the lamellae, housing osteocytes (bone cells).

• Central Canal (Haversian Canal): The central channel within each osteon, containing blood vessels and nerves that supply the bone.

• Canaliculi: Tiny channels that radiate from the lacunae, allowing for communication and nutrient exchange between osteocytes.

• Perforating Canals (Volkmann's Canals): Canals that run perpendicular to the central canal, connecting blood vessels and nerves of the periosteum to those in the central canal.

• Circumferential Lamellae: Layers of bone matrix that encircle the entire bone, binding osteons together and providing additional strength.

Example: In a thin section of compact bone, the lamellae appear white, while the central canal, lacunae, and canaliculi appear black due to the presence of bone dust.

Additional info: Osteocytes maintain bone tissue and communicate via canaliculi, ensuring the health and repair of bone.

Bone Markings

Types and Functions of Bone Markings

Bones display various markings that serve as attachment points for muscles, passageways for nerves and blood vessels, and articulations with other bones.

• Foramen: A round or oval opening through a bone, allowing passage of nerves and blood vessels.

• Condyle: A rounded articular projection, often part of a joint.

• Process: Any bony prominence, typically serving as an attachment for muscles or ligaments.

• Fossa: A shallow depression or hollow in a bone.

• Groove: A furrow in the bone surface for tendons, nerves, or blood vessels.

Example: The foramen magnum in the occipital bone allows the spinal cord to pass through to the brain.

Anatomy of a Long Bone

Structural Features of Long Bones

Long bones, such as the femur and humerus, have a distinct anatomy that supports movement and weight-bearing functions.

• Diaphysis: The shaft of the bone, composed mainly of compact bone that surrounds the medullary (marrow) cavity.

• Epiphysis: The expanded ends of the bone, consisting of spongy (cancellous) bone covered by a thin layer of compact bone.

• Medullary Cavity: The central cavity within the diaphysis, containing bone marrow.

• Articular Cartilage: Hyaline cartilage covering the epiphysis at joint surfaces, reducing friction and absorbing shock.

• Periosteum: A dense connective tissue membrane covering the outer surface of the bone, serving as an attachment for tendons and ligaments.

• Endosteum: A thin membrane lining the medullary cavity.

Example: The humerus has a thick wall of compact bone in the diaphysis and spongy bone at the epiphyses, allowing for both strength and flexibility.

Axial Skeleton

Overview of the Axial Skeleton

The axial skeleton forms the central axis of the body and consists of the skull, vertebral column, and thoracic cage. It provides protection for vital organs and supports the body's posture.

• Skull: Composed of cranial and facial bones, protecting the brain and forming the structure of the face.

• Vertebral Column: A series of vertebrae extending from the skull to the pelvis, protecting the spinal cord and supporting the head and trunk.

• Thoracic Cage: Includes the ribs and sternum, protecting the heart and lungs and assisting in respiration.

Skull Anatomy

The skull is divided into cranial bones, which encase the brain, and facial bones, which form the structure of the face.

• Cranial Bones: Frontal, parietal, temporal, occipital, sphenoid, and ethmoid bones.

• Facial Bones: Maxilla, mandible, zygomatic, nasal, lacrimal, palatine, inferior nasal concha, and vomer.

• Sutures: Immovable joints between skull bones, such as the coronal, sagittal, lambdoid, and squamous sutures.

Example: The foramen magnum in the occipital bone allows passage of the spinal cord.

Vertebral Column Anatomy

The vertebral column consists of individual vertebrae classified by region, each with unique structural features.

• Cervical Vertebrae (C1-C7): Support the head and allow for its movement. The atlas (C1) and axis (C2) are specialized for head rotation.

• Thoracic Vertebrae (T1-T12): Articulate with the ribs and form the posterior part of the thoracic cage.

• Lumbar Vertebrae (L1-L5): Support the lower back and bear the most weight.

• Sacrum and Coccyx: Fused vertebrae forming the base of the spine.

• Intervertebral Discs: Pads of fibrocartilage between vertebrae, absorbing shock and allowing movement.

Example: The cervical vertebrae have transverse foramina for passage of arteries to the brain.

Table: Comparison of Vertebral Regions

Thoracic Cage Anatomy

The thoracic cage protects vital organs and supports breathing. It consists of the sternum, ribs, and thoracic vertebrae.

• Sternum: The flat bone at the center of the chest, composed of the manubrium, body, and xiphoid process.

• Ribs: Twelve pairs of curved bones attached posteriorly to the thoracic vertebrae and anteriorly to the sternum via costal cartilages.

• Costal Cartilage: Hyaline cartilage connecting ribs to the sternum, allowing flexibility during respiration.

• Articular Facets: Surfaces on the ribs and vertebrae for joint formation.

Example: The head of a rib articulates with the body of a thoracic vertebra at the costal facet.

Table: Rib Structure and Articulation

Additional info: The thoracic cage assists in breathing by expanding and contracting during respiration.