Bone and Overview of the Skeletal System

The Skeleton

• Includes bones and cartilages.
• Supports internal organs.
• Protects internal organs.
• Provides levers for movement by skeletal muscles.
• Stores lipids and minerals.
• Initial site of blood cell formation (hematopoiesis).

The Axial Skeleton

• Made up of bones that lie along the midline of the body.
• Includes the skull, hyoid, vertebrae, ribs, sternum, and sacrum.

The Appendicular Skeleton

• Made up of bones that support the limbs and attach them to the axial skeleton.
• Includes the pectoral girdles, upper limb, pelvic girdles, and lower limb.

Pectoral girdle

Upper limb

Pelvic girdle

Lower limb

The Appendicular Skeleton

• The pectoral girdles include the clavicles and the scapulae.
• The pelvic girdles consist of the innominate bones (os coxae).

Pectoral girdle

Upper limb

Pelvic girdle

Lower limb

Classification of Bones

• Most bones contain two different forms of osseous tissue, compact bone and spongy bone. More on this in a moment.
• Bones may be classified by shape.
• Long
• Short
• Flat
• Irregular
• Sesamoid
• Wormian (sutural)

Long Bones

• Long bones make up most of the bones of the appendicular skeleton.
• Ex:
• Femur
• Phalanges
• Metacarpals/metatarsals
• Humerus

Humerus

Short Bones

• Roughly cubical.
• Primarily composed of spongy bone.
• Ex:
• Carpals
• Tarsals

Trapezoid (a carpal)

Flat Bones

• Two thin layers of compact bone sandwiching a layer of spongy bone.
• Ex:
• Skull
• Ribs
• Scapula
• Sternum

Irregular Bones

• Irregular in shape and do not fit into any other category.
• Often made up of several components fused during development.
• Ex:
• Vertebrae
• Innominate bones

A vertebra

Sesamoid Bones

• Form within tendons.
• Patella is the only sesamoid bone that is consistently present in all people.
• Sometimes considered to be a subcategory of short bones.

Patella

Wormian (Sutural) Bones

• Form in the sutures of the skull.
• Not present in all people or in the same locations.
• Sometimes considered to be a subcategory of flat bones.

Wormian bones

Bone Markings

• Bone markings fall into four categories.
• Projections for attachment of muscles or ligaments.
• Projections that form joints.
• Cavities.
• Depressions or openings for the passage of blood vessels and nerves.
• If you familiarize yourself with the types of bone markings now, you will have an easier time remembering the specific bone markings listed on your structure list.

​

The Structure of Bones

• Individual bones are organs: a discrete structure composed of two or more tissue types that performs a specific function.
• Bones are composed of osseous connective tissue, dense irregular connective tissue, adipose connective tissue, and one or more types of cartilage.
• Most bones contain both compact and spongy bone:
• Compact bone
• provides strength along a single axis but is weak perpendicular to that axis.
• Spongy bone
• provides limited strength in many directions and helps to distribute forces.

​

Compact Bone

• Compact bone is built from units called osteons.
• An osteon (or Haversian system) is composed of a series of concentric rings called lamellae.
• In between the lamellae, osteocytes sit within spaces called lacunae.

Compact Bone

• Osteocytes send cytoplasmic projections into the surrounding lamellae via canaliculi.
• These projections allow osteocytes to take up and deposit material from the lamellae.
• They also allow the osteocytes to receive nutrients from the blood supply of the bone.

Compact Bone

Compact Bone

• Blood vessels and nerves travel through the central (or Haversian) canal of the osteon.
• Blood vessels and nerves also travel perpendicular to the osteons via perforating (or Volkman’s) canals.

Compact Bone

Spongy Bone

• Spongy bone is composed of bars of osseous tissue called trabeculae.
• Trabeculae are not arranged like osteons.
• Osteocytes sit within lacunae inside the trabeculae and communicate with each other and the surface of the bar via canaliculi.

Structure of Long Bones

• Two epiphyses separated by a diaphysis.
• composed of compact bone surrounding a central medullary cavity.
• Epiphyses are composed of spongy bone surrounded by a thin layer of compact bone.

Structure of Long Bones

• Entire bone is surrounded by a fibrous periosteum.
• The ends of the epiphyses are typically covered with hyaline cartilage as protection for the joint.

Structure of Long Bones

• The medullary cavity is filled with yellow marrow (adipose tissue).
• The spaces within the spongy bone of the epiphyses are filled with red marrow, which is the site of hematopoiesis.

Structure of Long Bones

• The epiphyseal line is the boundary between epiphysis and diaphysis (metaphysis).
• It marks the zone of growth in length of long bones.

Cartilage – This should be review

• All cartilage contains chondrocytes within lacunae.
• The matrix is composed of ground substance and different types of protein fibers, depending on the particular variety of cartilage.

​

• Cartilage is classified into three types:
• Hyaline cartilage has a uniform matrix without visible protein fibers.
• Elastic cartilage has thick branched elastin fibers.
• Fibrous cartilage has many parallel layers of collagen fibers.

Hyaline Cartilage

• Hyaline cartilage provides uniform structure but not much strength.
• It forms the embryonic skeleton, the articular surfaces of long bones, the costal cartilages, the nasal cartilage and the laryngeal and tracheal cartilages.

Elastic Cartilage

• Elastic cartilage provides flexibility.
• It forms the cartilage of the external ear and the structure of the epiglottis.

Fibrous Cartilage

• Fibrous cartilage provides great strength along a single axis.
• It allows for cushioning under heavy compression forces.
• It forms the intervertebral discs, the pubic symphysis, and the menisci of the knees.