Introduction to Bones

What Are Bones For?

Bones are essential components of the human body, providing structure, protection, and support for movement. They also play important roles in physiological processes such as blood cell formation and mineral storage.

• Support: Bones form the framework that supports the body and cradles soft organs.

• Protection: Bones protect vital organs (e.g., the skull protects the brain, the rib cage protects the heart and lungs).

• Movement: Bones act as levers for muscles to pull on, enabling movement.

• Mineral Storage: Bones store minerals, especially calcium and phosphorus, which can be released into the bloodstream as needed.

• Blood Cell Formation: Hematopoiesis occurs in the red marrow cavities of certain bones.

• Triglyceride Storage: Fat is stored in yellow bone marrow.

• Hormone Production: Bones produce osteocalcin, a hormone involved in bone formation and energy metabolism.

Science, Ethics, and History of Bone Study

Ethical Considerations in Bone Research

The study of bones intersects with ethical, scientific, and historical issues, especially regarding the treatment of human remains.

• Scientific Value: Human bones provide valuable information about health, disease, ancestry, and evolution.

• Ethical Responsibility: Scientists must balance research with respect for the deceased and their descendants.

• Indigenous Concerns: Many indigenous groups advocate for the respectful treatment and repatriation of ancestral remains.

Example: The Native American Graves Protection and Repatriation Act (NAGPRA) mandates the return of certain cultural items, including human remains, to Native American tribes.

Native American Graves Protection and Repatriation Act (NAGPRA)

• Enacted in 1990 to address the rights of Native American tribes regarding ancestral remains and cultural items.

• Requires institutions to return remains and artifacts to affiliated tribes.

• Promotes collaboration and respect between scientists and indigenous communities.

Example: In 2017, over 200 members of five Native American tribes gathered to rebury "The Ancient One" (Kennewick Man), illustrating the importance of cultural respect in scientific research.

Structure and Classification of Bones

Are Bones Organs?

Bones are considered organs because they contain multiple types of tissues, including bone tissue, nervous tissue, cartilage, fibrous connective tissue, muscle, and epithelial cells in blood vessels.

• Gross Anatomy: The macroscopic structure of bones.

• Microscopic Anatomy: The cellular and tissue-level structure.

• Chemical Composition: The molecular and mineral makeup of bone.

Classification of Bones

Bones are classified by their shape and structure:

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

• Short Bones: Cube-shaped (e.g., carpals, tarsals).

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

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

Gross Anatomy of Bone

Bone Textures

• Compact Bone: Dense outer layer; smooth and solid.

• Spongy (Cancellous) Bone: Honeycomb of trabeculae (small needle-like or flat pieces) deep to compact bone.

Structure of a Long Bone

• Diaphysis: Shaft of the bone, composed of compact bone surrounding a medullary cavity.

• Epiphyses: Ends of the bone, consisting of spongy bone covered by compact bone.

• Medullary Cavity: Central cavity containing yellow marrow in adults.

• Periosteum: Double-layered membrane covering the external surface of bone.

• Endosteum: Membrane lining the internal bone surfaces.

• Articular Cartilage: Covers joint surfaces to reduce friction.

Bone Markings

Bone markings are features on the surface of bones that serve as sites for muscle, ligament, and tendon attachment, as well as conduits for blood vessels and nerves.

• Projections: Sites of muscle and ligament attachment (e.g., tuberosity, crest, trochanter).

• Depressions: Openings or cavities (e.g., fossa, groove).

• Openings: Passageways for blood vessels and nerves (e.g., foramen, canal).

Microscopic Anatomy of Bone

Compact Bone

• Osteon (Haversian System): The structural unit of compact bone, consisting of concentric lamellae around a central canal.

• Lamellae: Layers of bone matrix.

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Perforating (Volkmann's) Canals: Connect blood vessels and nerves of the periosteum to those in the central canal.

• Lacunae: Small spaces containing osteocytes.

• Canaliculi: Tiny canals connecting lacunae to each other and the central canal.

Spongy Bone

• Appears poorly organized but is actually organized along lines of stress.

• Trabeculae contain irregularly arranged lamellae and osteocytes interconnected by canaliculi.

• No osteons are present in spongy bone.

Chemical Composition of Bone

Organic Components

• Cells: Osteogenic cells, osteoblasts, osteocytes, bone-lining cells, and osteoclasts.

• Osteoid: Organic part of the matrix, made of ground substance (proteoglycans and glycoproteins) and collagen fibers, which provide tensile strength and flexibility.

Inorganic Components

• Hydroxyapatites (Mineral Salts): Mainly calcium phosphate crystals, which account for the hardness and resistance to compression of bone.

Formula for Hydroxyapatite:

Bones last long after death due to their mineral composition.

Bone Development

Ossification (Osteogenesis)

Ossification is the process of bone tissue formation, which occurs during:

• Embryonic development (formation of bony skeleton)

• Postnatal bone growth (until early adulthood)

• Bone remodeling and repair throughout life

Types of Ossification

• Endochondral Ossification: Bone forms by replacing hyaline cartilage; most bones below the skull are formed this way.

• Intramembranous Ossification: Bone develops from a fibrous membrane; forms flat bones such as those of the skull and clavicle.

Cartilage in the Skeletal System

Skeletal Cartilages

Cartilage is a resilient, semi-rigid form of connective tissue found in various parts of the skeleton. All skeletal cartilages contain chondrocytes in lacunae and an abundant extracellular matrix.

• Types of Cartilage: Hyaline, elastic, and fibrocartilage (not all types shown in the slides, but relevant for context).

Growth of Cartilage

• Appositional Growth: Cells in the perichondrium secrete new matrix against the external face of existing cartilage.

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

• Calcification of Cartilage: Occurs during normal bone growth and old age; calcified cartilage is not bone.

Summary Table: Bone Classification

The following table summarizes the main types of bones and their examples:

Additional info:

• Some slides referenced specific images and diagrams (e.g., labeled skeletons, histological images of bone and cartilage) that are standard in anatomy and physiology textbooks.

• Tables in the slides compared bone types and provided examples, which have been summarized above.

• Ethical and legal considerations (NAGPRA) are included to provide context for the study of bones in both scientific and societal frameworks.