Bones and Skeletal Tissues

Overview of the Skeletal System

The skeletal system is the body's primary support structure, providing shape, enabling movement, and protecting vital organs. It also serves as a reservoir for minerals, produces blood cells, and acts 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 such as 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 Human Skeleton

Structure and Function 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: Made of chondrocytes encased in lacunae within a jelly-like extracellular matrix.

• Perichondrium: Dense connective tissue surrounding cartilage, providing nutrients and resisting outward expansion.

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.

  • Locations: Articular surfaces of joints, costal cartilage (ribs), respiratory tract, nasal cartilage.

• Elastic Cartilage:

  • Similar to hyaline but contains elastic fibers for added flexibility.

  • Locations: External ear, epiglottis.

• Fibrocartilage:

  • Contains thick collagen fibers for tensile strength.

  • Locations: Menisci of knee, pubic symphysis, 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: Long axis of the body, including the skull, vertebral column, and rib cage.

• Appendicular Skeleton: Bones of the limbs and girdles attaching limbs to the axial skeleton.

Classification by Shape

Bones are classified based on their shapes:

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

• Short Bones: Cube-shaped (e.g., wrist, ankle bones); sesamoid bones form within tendons.

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

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

Bone Structure

Levels of Bone Structure

Bones are organs containing various tissues, including osseous tissue, nervous tissue, cartilage, dense connective tissue, muscle cells, and epithelial cells in blood vessels. Bone structure is studied at three levels:

• Gross Anatomy

• Microscopic Anatomy

• Chemical Composition

Gross Anatomy: Compact and Spongy Bone

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

• Spongy Bone: Honeycomb of small, needle-like or flat pieces called trabeculae; spaces are filled with red or yellow bone marrow.

Structure of Short, Irregular, and Flat Bones

• Consist of thin plates of spongy bone (diploë) covered by compact bone.

• Compact bone is sandwiched between connective tissue membranes (periosteum externally, endosteum internally).

• Bone marrow is scattered throughout spongy bone; hyaline cartilage covers areas involved in movable joints.

Structure of a Typical Long Bone

• 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 occurred.

Membranes of Long Bones

• Periosteum: White, double-layered membrane covering external surfaces except joints.

  • Fibrous layer: Dense irregular connective tissue with Sharpey's fibers anchoring to bone matrix.

  • Osteogenic layer: Contains stem cells that give rise to bone cells.

  • Rich in nerve fibers and blood vessels; provides 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: Found within trabecular cavities of spongy bone in flat bones (e.g., sternum) and some irregular bones (e.g., hip bone).

• In newborns, medullary cavities and all spongy bone contain red marrow.

• In adults, red marrow is located in heads of femur and humerus, but most active in flat bone diploë and some irregular bones.

• Yellow bone marrow can convert to red marrow if the person becomes anemic.

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 serving as attachment points for muscles and ligaments.

  • Examples: Tuberosity, crest, trochanter, line, tubercle, epicondyle, spine, process.

• Surfaces: Form joints.

  • Examples: Head, facet, condyle, ramus.

• Depressions and Openings: Allow passage for blood vessels and nerves.

  • Examples: 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

• Cranial Vault (Calvaria): Forms the superior, lateral, and posterior aspects of the skull.

• Cranial Base: Forms the skull's inferior aspect, divided into anterior, middle, and posterior fossae where the brain sits.

• Cranial Cavity: Encloses the brain.

• Other Cavities: Middle and internal ear cavities, nasal cavity, orbits (eye sockets), and sinuses.

• Openings: Foramina, canals, and fissures provide passageways for the spinal cord, blood vessels, and cranial nerves.

Cranial Bones

• Eight cranial bones: frontal, parietal (left and right), occipital, temporal (left and right), sphenoid, and ethmoid.

Sphenoid Bone

• Complex, bat-shaped bone; keystone bone that articulates with all other cranial bones.

• Contains sphenoidal sinuses and the sella turcica (hypophyseal fossa for the pituitary gland).

• Processes: Greater wings, lesser wings, pterygoid processes.

• Foramina: Optic canal, superior orbital fissure, foramen rotundum, foramen ovale, foramen spinosum.

Ethmoid Bone

• Deepest skull bone; forms the roof of the nasal cavity and floor of the anterior cranial fossa.

• Features: Cribriform plates, crista galli (attachment for dura mater), perpendicular plate (nasal septum), ethmoidal labyrinths (contain ethmoidal air cells), superior and middle nasal conchae, orbital plates.

Facial Bones: Mandible

• Largest, strongest bone of the face; U-shaped lower jawbone.

• Features: Body (chin), rami, mandibular angle, coronoid process (muscle attachment), condylar process (temporomandibular joint), mandibular notch, alveolar process (teeth sockets), mandibular and mental foramina (nerves and blood vessels).

Summary Table: Types of Cartilage and Their Locations

Summary Table: Bone Markings

Key Equations

• Bone Growth (Generalized): Additional info: This equation is a simplified representation; actual bone growth involves complex cellular processes.

Additional Info

• Bone remodeling and development involve osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells).

• Bone tissue is dynamic and responds to mechanical stress and hormonal signals.