Osteology

The Skeleton

Osteology

  • Osteology is defined as the study of bones that make up the body.

  • The skeletal framework of the body shows remarkable similarity across mammals.

  • Differences among species arise as adaptations to lifestyle and environmental conditions.

  • Homologous structures refer to anatomical features that originate from a common ancestor.

  • Analogous structures are features that have similar functions but do not share a common evolutionary origin.

Major Bones and Their Relationships

Bone Classification

  • Humerus

  • Radius

  • Ulna

  • Carpals

  • Metacarpals

  • Phalanges

  • Classification of skeletal elements across various species:

    • Human

    • Dog

    • Bird

    • Whale

Bones: Osseous Tissue

Characteristics of Bones

  • Bones are classified as living, dynamic tissues.

  • Structural components:

    • Blood vessels

    • Lymphatic vessels

    • Nerves

  • Bones undergo processes such as:

    • Growth

    • Fracture

    • Repair

    • Remodeling

    • Disease susceptibility

  • Functions of osseous tissue extend beyond merely providing body framework.

Functions of Bone/Skeleton

Structural Functions

  1. Support:

    • Provides a structural framework for the body.

    • Aids in the support of body weight.

  2. Protection:

    • Skull protects the brain (central nervous system).

    • Vertebrae protects the spinal cord (central nervous system).

    • Rib cage protects the heart and lungs.

    • Pelvic girdle protects the urogenital system.

Functional Framework

  1. Muscle Attachment:

    • A framework for muscle attachment is critical for locomotion, grasping, lifting, and various activities.

    • Human body contains approximately 650 muscles, which may not all attach to bones (total of 206 bones).

    • Nevertheless, virtually every lever involved in movement is a bone.

Nutrient Storage

  1. Mineral Storage:

    • Bones serve as a dynamic storage area for minerals.

    • Important minerals stored include:

      • Calcium (99% of total body calcium)

      • Phosphorus (85% of total body phosphorus)

      • Magnesium (60% of total body magnesium)

      • Fluoride (100% in bones and teeth)

  2. Energy Storage:

    • Yellow marrow acts as a fat reservoir.

    • Comparison between yellow and red marrow is significant.

Yellow vs. Red Marrow

Characteristics of Yellow Marrow

  • Contains adipocytes and fat tissue.

  • Comprises up to 10% of the fat mass of a lean healthy adult.

  • Previously viewed as merely inactive fat tissue occupying the medullary cavity; it is now recognized as an energy source that contributes to:

    • Bone and whole-body energy metabolism

    • Lipid synthesis, storage, and release of hematopoietic stem cells (HSCs)

Bone Marrow Adipose Tissue

  • Bone marrow adipose tissue (BMAT) is distinct from other adipose tissues due to:

    • Unique lipid composition containing cholesterol, triacylglycerol (TAG), and monoacylglycerol (MAG).

    • Its role in hematopoiesis reflects this distinction.

Hematopoiesis in Red Marrow

  • Red bone marrow comprises hematopoietic stem cells (HSCs) that give rise to:

    • Red blood cells (RBCs), white blood cells (WBCs), and platelets.

  • Red marrow is primarily found in flat bones and the proximal ends of long bones.

Bone Composition and Structure

Total Bone Count in Adults

  • The adult human skeleton consists of a total of 206 bones (excluding sesamoid bones).

Classification of Skeleton

  • Axial Skeleton: 74 bones, consisting of:

    • Skull

    • Vertebral column

    • Thoracic cage

  • Appendicular Skeleton: 126 bones, involving:

    • Pectoral girdle and upper limbs

    • Pelvic girdle and lower limbs

  • Auditory Ossicles: 6 bones:

    • Malleus

    • Incus

    • Stapes

Bone Types by Composition

  • The adult human skeleton is made up of 80% cortical (compact) bone and 20% trabecular (spongy) bone.

  • Characteristics:

    • Cortical bone: denser and less porous, with varied metabolic activity compared to trabecular bone.

  • Ratios of cortical to trabecular bone can differ based on skeletal location:

    • Vertebrae: 25:75

    • Femoral head: 50:50

    • Radial diaphysis: 95:5

Structural Units of Bone

  • Both types of bone (cortical and trabecular) are comprised of osteons:

    • Osteons are cylindrical structures that form the functional unit of compact bone, also referred to as the Haversian system.

    • Components include:

    • Central canal, which houses capillaries, nerves, and lymphatic vessels

    • Lamellae, which are layers of mineralized collagen

    • Osteocytes, which are mature bone cells located within the bone matrix

Trabecular Bone Structure

  • Trabecular bone features a honeycomb-like organization of plates and rods, forming a porous network.

  • This structure is particularly found in the bone marrow compartments and at the ends of long bones.

  • Trabecular osteons are referred to as packets.

Composition of Bone

  • Bones contain:

    • Mineralized material, primarily hydroxyapatite crystals, comprising 50-70% by weight

    • Osteoid material, which includes an organic (collagen) matrix, making up 30-40% by weight

    • Living cells (osteocytes, osteoblasts, and osteoclasts), which constitute less than 2% of the total bone tissue

Types of Bone

General Categories of Bone

  1. Long

  2. Short

  3. Flat

  4. Irregular

  5. Sesamoid (potential inclusion)

  6. Pneumatic (potential inclusion)

Examples of Bone Types

  • Long Bones:

    • Identification: Have a longitudinal axis and two expanded ends.

    • Examples: Clavicles, humeri, radii, ulnae, metacarpals, femurs, tibiae, fibulae, metatarsals, and phalanges.

  • Short Bones:

    • Description: About equal in length and width.

    • Examples: Carpals (bones of the wrist) and tarsals (bones of the ankle).

  • Flat Bones:

    • Characteristics: Plate-like with broad surfaces.

    • Examples: Skull, mandible, scapulae, sternum, and ribs.

  • Irregular Bones:

    • Definition: Do not fit the previous categories.

    • Examples: Vertebrae, sacrum, coccyx (tailbone), and hyoid bone (supports the tongue).

Specialized Bone Types

  • Sesamoid Bones:

    • Develop within tendons, enhancing leverage and protecting tendons from muscle forces.

    • Examples: Patellae (kneecap) and various sesamoid bones in the hands and feet.

    • Adults can have as many as 42 sesamoid bones.

  • Pneumatic Bones:

    • Contain air spaces or sinuses that connect to the atmosphere.

    • Examples: Frontal bones and maxillary bones in mammals; many avian bones.

Remodeling and Adaptation

  • Each bone undergoes continuous modeling to adapt to changing biomechanical forces as well as remodeling to eliminate old or damaged bone and replace it with new, stronger bone for structural integrity.

Anatomy of Long Bones

General Structure

  • Diaphysis: The cylindrical shaft located between the two epiphyses.

  • Epiphysis: The ends of long bones, responsible for joint formation with other bones;

    • The proximal epiphysis is the end closest to the body.

    • The distal epiphysis is the end further from the body.

  • Metaphysis: The flared area adjacent to the epiphysis in mature bones.

Periosteum and Articular Cartilage

  • Periosteum: A fibrous membrane covering the bone surface where articular cartilage is not present; involved in increasing bone diameter through osteoblasts and aiding in fracture healing using periosteal cells.

  • Articular Cartilage: A specialized layer of hyaline cartilage covering joint surfaces;

    • Appears white, shiny, and is smooth to the touch.

Bone Types in the Structure

  • Compact Bone:

    • Also known as dense, cortical, or cortex bone.

    • It forms the hard outer layer of most bones and comprises almost all of the diaphysis.

  • Spongy Bone:

    • Known as cancellous or trabecular bone, it is made of trabeculae forming a porous network spongy with spaces typically filled with marrow.

Medullary Cavity and Endosteum

  • Medullary Cavity:

    • The cavity inside the cortex of long bones, filled with bone marrow that is red in young animals, transitioning to yellow (fat tissue) as the animal matures.

  • Endosteum: A fibrous membrane lining the marrow cavity and osteonal canals, serving as a location for bone deposition and resorption.

Microscopic Anatomy of Bone

Structure of Spongy Bone

  • Spongy bone has a network of branching bony plates, called trabeculae, which contain pores; this provides strength against compression while allowing space for red marrow.

Structure of Compact Bone

  • Compact bone consists of functional units known as osteons.

    • Each osteon features a central canal surrounded by concentric circles of osteocytes located inside lacunae.

    • Canaliculi provide channels for nutrient exchange between osteocytes and the central canal.

Bone Growth, Modeling, and Remodeling

Growth Mechanisms

  • Longitudinal Growth: Occurs at the growth plate.

  • Radial Growth: Takes place at the periosteum.

Remodeling Processes

  • Modeling: Refers to the changes in overall shape of bones in response to physiological influences or mechanical forces.

  • Remodeling: The process of renewal to maintain bone strength and mineral homeostasis, involving resorption of old bone and formation of new bone, preventing the accumulation of microdamage.

Types of Bone Formation

Ossification Processes

  1. Endochondral Ossification:

    • Involves a cartilage template.

    • Primary for forming long bones and most other bones.

  2. Intramembranous Ossification:

    • Occurs in the absence of a cartilaginous template.

    • Typically responsible for forming flat bones of the skull, clavicle, and mandible.

Bone Cell Types

  1. Osteoblast:

    • Cells responsible for bone mineral matrix deposition.

  2. Osteoclast:

    • Cells responsible for bone mineral resorption.

  3. Osteocyte:

    • A mature bone cell that originates from embedded osteoblasts within osteons.

Questions So Far?

  • Discussion of topics covered and further questions regarding the anatomy and functions of bone.