Bones and Skeletal Tissues

Overview of the Skeletal System

The skeletal system is the body's primary support structure, providing shape, enabling movement, protecting vital organs, storing minerals, producing blood cells, and acting as a framework for proper body function.

• Support: Bones support the body and soft organs.

• Protection: The skeleton protects the brain, spinal cord, and vital organs.

• Movement: Bones act as levers for muscle action.

• Mineral Storage: Bones store minerals, especially calcium and phosphorus.

• Blood Cell Formation: Hematopoiesis occurs in red bone marrow.

• Triglyceride Storage: Fat is stored in bone cavities as an energy source.

• Hormone Production: Osteocalcin produced by bones regulates insulin secretion, glucose homeostasis, and energy expenditure.

Cartilages in the Skeletal System

Role and Structure of Cartilage

The human skeleton initially consists of cartilage, which is later replaced by bone except in areas requiring flexibility. Cartilage is a resilient, molded tissue primarily composed of water and contains no blood vessels or nerves.

• Skeletal Cartilage: Highly resilient, molded tissue; consists mainly of water.

• Perichondrium: Dense connective tissue surrounding cartilage, providing resistance to outward expansion and containing blood vessels for nutrient delivery.

• Chondrocytes: Cartilage cells encased in lacunae within a jelly-like extracellular matrix.

Types of Cartilage

There are three main types of cartilage in the human skeleton, each with distinct properties and locations.

• Hyaline Cartilage: Provides support, flexibility, and resilience; most abundant type. Found in articular, costal, respiratory, and nasal cartilage.

• Elastic Cartilage: Similar to hyaline but contains elastic fibers for greater flexibility. Located in the external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers for tensile strength. Found in menisci of the knee, pubic symphysis, and intervertebral discs.

Growth of Cartilage

Cartilage grows by two mechanisms:

• Appositional Growth: New matrix is laid down on the surface by cells in the perichondrium.

• Interstitial Growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

Cartilage growth ends during adolescence.

Classification of Bones

Axial and Appendicular Skeleton

Bones are grouped into two main divisions:

• Axial Skeleton: Forms the long axis of the body; includes the skull, vertebral column, and rib cage.

• Appendicular Skeleton: Includes bones of the upper and lower limbs and girdles attaching limbs to the axial skeleton.

Bone Shapes

Bones are classified by shape:

• Long Bones: Longer than they are wide; e.g., femur, humerus.

• Short Bones: Cube-shaped; e.g., wrist and ankle bones.

• Flat Bones: Thin, flat, and slightly curved; e.g., sternum, skull bones.

• Irregular Bones: Complicated shapes; e.g., vertebrae, hip bones.

• Sesamoid Bones: Form within tendons; e.g., patella.

Bone Structure

Levels of Bone Structure

Bones are organs containing various tissues. The structure is studied at three levels:

• Gross Anatomy: Overall structure visible to the naked eye.

• Microscopic Anatomy: Cellular and tissue-level organization.

• Chemical Composition: Molecular and mineral content.

Compact and Spongy Bone

Bones consist of two types of tissue:

• Compact Bone: Dense outer layer that appears smooth and solid.

• Spongy Bone: Honeycomb-like structure of trabeculae; spaces filled with red or yellow bone marrow.

Structure of Short, Irregular, and Flat Bones

These bones consist of thin plates of spongy bone (diploë) covered by compact bone. Compact bone is sandwiched between connective tissue membranes:

• Periosteum: Covers outside of compact bone.

• Endosteum: Covers inside portion of compact bone.

Bone marrow is scattered throughout spongy bone; hyaline cartilage covers areas of bone that are part of movable joints.

Structure of a Typical Long Bone

Long bones have a shaft (diaphysis), ends (epiphyses), and membranes.

• Diaphysis: Tubular shaft forming the long axis; consists of compact bone surrounding a central medullary cavity filled with yellow bone marrow in adults.

• Epiphyses: Ends of long bones; compact bone externally, spongy bone internally. Articular cartilage covers joint surfaces.

• Epiphyseal Line: Remnant of childhood epiphyseal plate where bone growth occurs.

Membranes of Long Bones

• Periosteum: White, double-layered membrane covering external surfaces except joints. Contains fibrous (outer) and osteogenic (inner) layers, nerve fibers, blood vessels, and serves as anchoring points for tendons and ligaments.

• Endosteum: Delicate connective tissue membrane covering internal bone surfaces, including trabeculae of spongy bone and canals through compact bone. Contains osteogenic cells.

Hematopoietic Tissue in Bones

Red bone marrow is found within trabecular cavities of spongy bone in flat bones (e.g., sternum). In newborns, all spongy bone and medullary cavities contain red marrow. In adults, red marrow is mainly in flat bone diploë and some irregular bones (e.g., hip bone). Yellow marrow can convert to red if needed (e.g., in anemia).

Bone Markings

Categories of Bone Markings

Bone markings are features on bones that serve as sites for muscle, ligament, and tendon attachment, joint formation, or conduits for blood vessels and nerves. They are classified into three categories:

• Projections: Raised areas for muscle and ligament attachment (e.g., tuberosity, crest, trochanter, line, tubercle, epicondyle, spine, process).

• Surfaces: Form joints (e.g., head, facet, condyle, ramus).

• Depressions and Openings: Allow passage for blood vessels and nerves (e.g., fissure, foramen, notch, meatus, sinus, fossa).

The Axial Skeleton

Overview

The axial skeleton consists of 80 bones divided into three major regions: the skull, vertebral column, and thoracic cage. Its functions include forming the longitudinal axis of the body, supporting the head, neck, and trunk, and protecting the brain, spinal cord, and thoracic organs.

Skull Anatomy

The skull is divided into the cranial vault (calvaria) and cranial base. The base is further divided into anterior, middle, and posterior fossae, which house the brain. The skull also contains cavities for the middle and internal ear, nasal cavity, orbits (eye sockets), and sinuses. There are 85 named openings (foramina, canals, fissures) for the passage of nerves and blood vessels.

Cranial Bones

• Frontal Bone

• Parietal Bones (left and right)

• Occipital Bone

• Temporal Bones (left and right)

• Sphenoid Bone: Keystone bone that articulates with all other cranial bones; contains sphenoidal sinuses and several foramina for nerves and arteries.

• Ethmoid Bone: Deepest skull bone; forms roof of nasal cavity and part of the anterior cranial fossa; contains cribriform plates, crista galli, perpendicular plate, ethmoidal labyrinths, and orbital plates.

Facial Bones: Mandible

The mandible is the largest, strongest bone of the face, forming the lower jaw. It consists of a body (chin) and two rami. Key features include:

• Mandibular angle: Where rami and chin meet.

• Coronoid process: Insertion point for temporalis muscle.

• Condylar process: Forms part of the temporomandibular joint.

• Alveolar process: Contains tooth sockets.

• Mandibular and mental foramina: Passageways for nerves and blood vessels.

Summary Table: Types of Cartilage and Their Locations

Summary Table: Bone Markings

Key Equations

• Bone Growth (Generalized):

Additional info: For more detailed study, students should review the microscopic anatomy of bone (osteon structure, bone cells), chemical composition (organic and inorganic components), and processes of bone development and remodeling.