Gross Anatomy of Bones

Bone Types

Bones are classified into five main types based on their shape and structure. Each type serves specific functions in the body.

• Long Bones: Longer than they are wide. Examples include the humerus, ulna, radius, and femur.

• Short Bones: Small and boxy; length and width are close to equal. Examples: carpal bones of the wrist and tarsal bones of the ankle.

• Flat Bones: Have thin, parallel surfaces. Examples: cranial bones, ribs, and sternum. Sutural bones are flat bones found in the suture lines of the skull; their number, size, shape, and location vary from person to person.

• Irregular Bones: Complex shapes. Examples: vertebrae, bones of the pelvis, sesamoid bones (small and flat, develop inside tendons), and patella (kneecap).

Skeletal System Overview

Tissue Types in Bone

Bones are composed of several types of tissues, including:

• Osseous (bone) tissue

• Cartilage

• Dense connective tissue

• Epithelium

• Adipose tissue

• Nervous tissue

Seven Primary Functions of Bone

• Support: Provides structural support for the entire body.

• Protection: Surrounds vital tissues (e.g., ribs protect the heart and lungs; skull protects the brain).

• Movement: Skeletal muscles attach to bones and pull on them to create movement.

• Mineral Homeostasis: Stores minerals such as calcium and phosphorus; minerals are released into the blood when needed.

• Blood Cell Production: Red bone marrow produces red blood cells, white blood cells, and platelets.

• Fat Storage: Yellow marrow stores fat.

• Hormone Production: Produces hormones involved in insulin secretion and glucose balance.

Bone Structure

Long Bone Anatomy

• Diaphysis: Shaft or central part of the bone; thick layer of compact bone surrounds the marrow cavity.

• Epiphysis: Ends of the bone; usually wider than the diaphysis and made up of spongy (cancellous) bone. Ends are covered with articular cartilage for shock absorption.

• Epiphyseal Plate: Growth plate; hyaline cartilage that covers the end of a bone and is a shock absorber.

• Periosteum: Dense connective tissue that encloses bone; provides protection, nourishment, and serves as an attachment point for tendons and ligaments.

• Medullary (marrow) cavity: Space in the middle of the bone containing yellow marrow in adults and red marrow in children.

• Endosteum: Thin layer of connective tissue lining the inside of the medullary cavity.

Short, Flat, and Irregular Bones

• Thin layer of compact bone covers spongy bone on the inside.

• Contain bone marrow but do not have a marrow cavity.

Bone Tissue Composition

Matrix

The bone matrix is very dense and made up of mineral salts, water, and collagen.

• About ⅔ of matrix is composed of calcium phosphate, oxygen, and hydrogen, deposited around a framework of collagen fibers, which crystallizes and hardens.

• Makes bone very resistant to compression, but also very brittle and inflexible.

• About ⅓ is made up of collagen, which helps resist tension (pulling) and gives bone flexibility.

Cells in Bone Tissue

• Osteogenic Cells: Stem cells that develop into osteoblasts; found on the inside of the periosteum and endosteum and in canals containing blood vessels.

• Osteoblasts: Immature bone cells that make and secrete collagen fibers and ingredients of matrix; once surrounded by matrix, they develop into osteocytes.

• Osteocytes: Mature bone cells found inside lacunae, surrounded by layers of matrix called lamellae. They maintain the bone matrix and receive nutrients from blood through canaliculi (small channels connecting lacunae).

• Osteoclasts: Cells that break down and remove bone; involved in resorption, maintenance of calcium and phosphate levels in the blood, and bone repair.

Compact Bone Structure

Osteon (Haversian System)

The functional unit of compact bone, resembling a cross-section of a tree.

• Central (Haversian) Canal: Middle of the osteon; contains blood vessels, nerves, and lymphatic vessels; runs parallel to the surface of the bone.

• Perforating (Volkmann's) Canals: Travel perpendicular to the central canals, providing blood to osteons and bone marrow tissue.

• Lamellae: Layers of matrix containing collagen fibers that run at angles to add strength and resilience.

• Lacunae: Small cavities that contain osteocytes.

• Canaliculi: Small channels connecting lacunae and the Haversian canal, allowing for nutrient and waste exchange.

Spongy Bone Structure

• No osteons; matrix is arranged in thin columns of bone called trabeculae.

• Found in areas of lower stress; can resist forces from many directions.

• Contains red bone marrow, which is important for red blood cell formation.

Blood and Nerve Supply to Bone

• Nutrient artery and vein: Main entry and exit for blood during development.

• Metaphyseal and epiphyseal vessels: Supply blood to the developing bone and epiphyseal cartilage.

• Periosteal vessels: Provide blood to the superficial osteons.

• Nerve supply is extensive; sensory nerves enter the bone with the nutrient artery and vein.

Bone Formation and Growth

Intramembranous Ossification

Bone forms from a fibrous membrane; occurs in bones of the skull, mandible, and clavicle.

1. Development of ossification center: Stem cells differentiate into osteogenic cells and osteoblasts.

2. Calcification: Osteoblasts secrete matrix, which hardens as calcium salts are deposited.

3. Formation of trabeculae: Trabeculae fuse to form spongy bone; red marrow develops.

4. Development of periosteum: Embryonic tissue develops into periosteum; osteoblasts lay down a layer of compact bone.

Endochondral Ossification

Bone replaces existing cartilage; most bones form this way.

1. Cartilage model forms; perichondrium forms around model.

2. Bone collar forms around diaphysis.

3. Osteoblasts lay down bone against cartilage.

4. Growth of cartilage and bone; chondrocytes die, matrix calcifies, and osteoblasts replace cartilage with bone.

5. Medullary cavity forms as bone is broken down in the center.

6. Secondary ossification centers form in the epiphyses.

Epiphyseal Plate Growth

• Cartilage cells reproduce in the epiphyseal plate, increasing the length of the bone.

• Osteoblasts "chase" the cartilage, replacing it with bone.

• Eventually, osteoblasts catch up to the cartilage and growth stops.

• Epiphyseal line is visible on x-rays in adults.

Appositional Growth

• Increase in the diameter of bone.

• Osteoblasts near the periosteum secrete matrix; osteoblasts are surrounded and become osteocytes.

Bone Homeostasis

Remodeling

• Bones are constantly being recycled and renewed.

• Balance is maintained by activity of osteocytes, osteoblasts, and osteoclasts.

• About ⅕ of spongy bone is replaced each year; ⅓ of compact bone is replaced 2-3 times a year.

Exercise and Bone Health

• Ability to remodel allows bones to respond to changes in the environment.

• Bumps and ridges on bones mark where tendons are attached.

• Regular weight-bearing exercise is important for maintaining strong skeleton.

• Up to ⅓ of bone mass can be lost after just a few weeks of inactivity.

Hormones and Nutrition

• Calcium and phosphate must come from diet (milk, cheese, nuts, legumes, cereals, grains).

• Calcitriol (vitamin D) is needed for absorption of calcium and phosphate; made by the kidneys.

• Vitamin C is needed for collagen synthesis and osteoblast differentiation.

• Vitamins A, K, and B12 are needed to make bone.

• Growth hormone and thyroxine (thyroid hormone) stimulate bone growth and osteoblast activity.

• Sex hormones (estrogen and testosterone) stimulate osteoblast activity and cause growth to stop after increased activity.

Calcium Homeostasis

• Calcium is the most abundant mineral in the body; involved in many body processes.

• Body must monitor calcium levels closely; decrease of 35% causes convulsions, decrease of 50% causes death.

• Hormones (calcitriol and parathyroid hormone, PTH) coordinate storage, absorption, and excretion.

• PTH stimulates osteoclasts to remove bone, releasing calcium into the blood; also increases rate of calcium absorption from the kidneys.

• Calcitonin (thyroid gland) inhibits osteoclasts, increases rate of excretion from kidneys.

Fractures and Bone Repair

Types of Fractures

Fracture Treatment and Repair

• Reduction: Setting a bone; can be closed (done with hands) or open (done with pins or rods).

• Repair (4 steps):

  1. Blood vessels around the break site become swollen and hematoma (clump of clotted blood) forms.

  2. Fibrocartilaginous callus forms; fibroblasts, chondroblasts, and osteoblasts start forming new collagen, cartilage, and bone.

  3. New trabeculae begin to show up and the fibrocartilaginous callus is replaced by bone.

  4. Bony callus is remodeled and refined until the bone looks similar to the way it did before the fracture.

Key Terms and Concepts

• Osteon: Functional unit of compact bone.

• Trabeculae: Network of spongy bone.

• Epiphyseal Plate: Growth plate in long bones.

• Periosteum: Membrane covering bone.

• Endosteum: Membrane lining the medullary cavity.

• Osteoblasts: Bone-forming cells.

• Osteoclasts: Bone-resorbing cells.

• Osteocytes: Mature bone cells maintaining bone tissue.

Important Equations

• Calcium Homeostasis Equation:

• Bone Mass Replacement:

Summary Table: Bone Cell Types

Additional info:

• Some context and explanations have been expanded for clarity and completeness.

• Tables have been recreated and summarized for study purposes.