Introduction to the Skeleton

Bones and Skeletal Tissue The human skeleton is made of both cartilage and bone tissue. An infant’s skeleton is primarily cartilage but as the infants grows it is replaced by bone.

Figure 6.17 Fetal primary ossification centers at 12 weeks. Parietal bone Occipital bone Frontal bone of skull Mandible Clavicle Scapula Radius Ulna Ribs Humerus Vertebra Ilium Tibia Femur

Skeletal Cartilage Cartilage consists primarily of water. This provides resilience and compressibility that protects the bones. There are three types:

Skeletal Cartilage Cartilage consists primarily of water. This provides resilience and compressibility that protects the bones. There are three types: Hyaline Elastic Fibrocartilage

Hyaline Cartilage Most abundant Chondrocytes appear spherical Collagen is the only fiber Locations: Articular cartilage Costal cartilage Respiratory cartilage Nasal cartilage

Fibro elastic Cartilage Very compressible, chondrocytes are found in parallel rows Found only in vertebral discs and meniscus of the knee.

Elastic Cartilage Similar to hyaline but contains elastic fibers Found in areas where there is repeated bending Two locations: External ear Epiglottis

Figure 6.1 The bones and cartilages of the human skeleton. Epiglottis Larynx Thyroid cartilage Cartilage in external ear Cartilages in nose Trachea Cricoid cartilage Lung Articular Cartilage of a joint Cartilage in Intervertebral disc Costal cartilage Respiratory tube cartilages in neck and thorax Pubic symphysis Bones of skeleton Axial skeleton Meniscus (padlike cartilage in knee joint) Appendicular skeleton Cartilages Articular cartilage of a joint Hyaline cartilages Elastic cartilages Fibrocartilages

Perichondrium This supplies nutrients via diffusion to the avascular cartilage. This limits the thickness of the cartilage.

Chondrocytes The chondrocyte is the cell responsible for the maintenance and growth of the cartilage. They reside in lacunae

Cartilage can grow two ways: Cartilage Growth Cartilage can grow two ways: Appositional growth Interstitial growth

Appositional Growth Appositional growth (from the edges) occurs where cells from the perichondrium lay down new a cartilage matrix.

Interstitial Growth Growth from inside the cartilage. The chondrocytes divide in the lacunae laying down more matrix.

Skeletal Classification There are 206 bones in the adult skeleton.

Skeletal Classification The skeleton is divided into the: Axial Skeleton Appendicular Skeleton

Axial Skeleton Forms the long axis of the body. Its primary roll is support and protection of the organs.

Appendicular Skeleton Bones of the upper and lower limbs including the shoulder and hips. It is involved in locomotion.

Bones are further classified by shape. There are 4 types: Bone Classification Bones are further classified by shape. There are 4 types: Long Bones Short Bones Flat Bones Irregular Bones

Long Bones Are longer than wide. It consists of a shaft plus two ends.

Long Bones Are longer than wide. It consists of a shaft plus two ends. Found on the Appendicular Skeleton.

Long Bones Are longer than wide. It consists of a shaft plus two ends. Found on the Appendicular Skeleton. Size doesn’t matter Femur vs phalanges

Short Bones Typically cube shape. These are found on the wrist (carpals) and ankle (tarsals)

Flat Bones Are flat and thin in 1 dimension. Often have a curve edge Examples include ribs, scapula and skull

Irregular Shape No specific shape, examples include the pelvic bones and vertebrae.

Figure 6.2 Classification of bones on the basis of shape. Flat bone (sternum) (a) Long bone (humerus) (b) Irregular bone (vertebra), right lateral view (d) Short bone (talus)

Function of the Skeletal System Support- respiration Protection-skull Movement-Appendicular skeleton Mineral Storage-calcium phosphate Blood Cell Formation- Hematopoiesis Fat Storage

Gross anatomy of Bone The surface of the bone is covered by projections and ridges that serve as attachment points for ligaments and tendons. Examples include: Trochanters Spines & Grooves

Gross anatomy of Bone Bone is can be classified as compact or spongy bone.

Gross anatomy of Bone Compact bone is dense and has a smooth outer surface.

Gross anatomy of Bone Spongy bone has a honey comb structure made up of small projections called trabeculae.

Structure of the long bone Diaphysis is the long central shaft of a long bone

Structure of the long bone Diaphysis is the long central shaft of a long bone Epiphyses are the ends of the bone

Structure of the long bone Diaphysis is the long central shaft of a long bone Epiphyses are the ends of the bone Membranes cover the exterior and interior of the bones

Structure of the long bone Diaphysis is the long central shaft of a long bone Epiphyses are the ends of the bone Membranes cover the exterior and interior of the bones Composed both of compact and spongy bone

Structure of the long bone The diaphysis has a large central cavity and is filled with fat (yellow marrow).

Structure of the long bone The epiphyses are filled with hematopoietic tissue (red marrow) which gives rise to the blood cells.

Structure of the long bone The periosteum covers the exterior of the bone. It contains bone forming cells and is involved with bone growing wider.

Articular cartilage Proximal epiphysis Spongy bone Epiphyseal line Periosteum Compact bone Medullary cavity (lined by endosteum) Diaphysis Distal epiphysis (a)

Figure 6.3c The structure of a long bone (humerus of arm). Endosteum Yellow bone marrow Compact bone Periosteum Perforating (Sharpey’s) fibers Nutrient arteries (c)

Microscopic Anatomy of Bone Four cell types populate the bone tissues. These are: Osteogenic cells Osteoblasts Osteocytes Osteoclasts

Microscopic Anatomy of Bone Osteogenic cells are stem cells that give rise to other bone forming cells

Microscopic Anatomy of Bone Osteoblasts are bone forming cells Osteocytes maintains the bone matrix Osteoclasts destroy bone

Figure 6.4 Comparison of different types of bone cells. (a) Osteogenic cell (b) Osteoblast (c) Osteocyte (d) Osteoclast Stem cell Matrix-synthesizing cell responsible for bone growth Mature bone cell that maintains the bone matrix Bone-resorbing cell

Microscopic Anatomy Compact Bone The structural unit of the compact bone is the osteon or Haversian System. It consists of a central canal that contains blood vessels and nerves.

Microscopic Anatomy Compact Bone Branches from the nerves and blood vessels go transversely through Volkmann's canals.

Figure 6.7 Microscopic anatomy of compact bone. Spongy bone Central (Haversian) canal Perforating (Volkmann’s) canal Endosteum lining bony canals and covering trabeculae Osteon (Haversian system) Circumferential lamellae (a) Perforating (Sharpey’s) fibers Lamellae Periosteal blood vessel Periosteum Nerve Vein Artery Lamellae Central canal Canaliculi Lacuna (with osteocyte) Osteocyte in a lacuna Lacunae Interstitial lamellae (b) (c)

Microscopic Anatomy Compact Bone Osteocytes are found in the lacunae.

Figure 6.7c Microscopic anatomy of compact bone. Lacuna (with osteocyte) Lamellae Central canal Lacunae Interstitial lamellae (c)

Bone Formation (Remodeling) Bone has an inorganic and organic component. The inorganic component is calcium phosphate. The organic component is made up of collagen and proteoglycans.

Bone Formation (Remodeling) Bone is unique because is can repair itself and adjust to mechanical stresses.

Bone Formation (Remodeling) Bone is unique because is can repair itself and adjust to mechanical stresses. This is a complex process involving a tug of war between Osteoclasts and Osteoblasts.

Bone Formation (Remodeling) Hormones such as Calcitonin (PHT), sex and growth hormones and vitamins such as Vitamin D are involved in this process.

Figure 6.12 Parathyroid hormone (PTH) control of blood calcium levels. Calcium homeostasis of blood: 9–11 mg/100 ml BALANCE BALANCE Stimulus Falling blood Ca2+ levels Thyroid gland Osteoclasts degrade bone matrix and release Ca2+ into blood. Parathyroid glands Parathyroid glands release parathyroid hormone (PTH). PTH

Bone Formation (Remodeling) Wolf’s law governs how a bone remodels. Compression and tension work together to shape a bone. For example, a person’s dominate arm has a thicker bone structure.

Figure 6.13 Bone anatomy and bending stress. Load here (body weight) Head of femur Tension here Compression here Point of no stress

(a) Cross- sectional dimension of the humerus Added bone matrix Figure 6.14 Vigorous exercise can lead to large increases in bone strength. (a) Cross- sectional dimension of the humerus Added bone matrix counteracts added stress (b) Serving arm Nonserving arm

Bone Formation (Remodeling) Bones can grow two ways: 1) Longitudinal growth occurs in long bones between the diaphysis and epiphysis, along a region called the epiphyseal plate. Cartilage expands and is replaced by bone.

Bone Formation (Remodeling) Bones can grow two ways: 2) Appositional growth (width) occurs between the periosteum and endosteum.

Figure 6.11 Long bone growth and remodeling during youth. Bone remodeling Articular cartilage Cartilage grows here. Epiphyseal plate Cartilage is replaced by bone here. Bone is resorbed here. Cartilage grows here. Bone is added by appositional growth here. Cartilage is replaced by bone here. Bone is resorbed here.

Diseases & Injuries of the Bone Fractures are the most common injury to bone. Most fractures will heal with time.

Diseases & Injuries of the Bone Fractures are the most common injury to bone. Most fractures will heal with time.

Diseases & Injuries of the Bone

Diseases & Injuries of the Bone

Table 6.2 Common Types of Fractures (1 of 3)

Table 6.2 Common Types of Fractures (2 of 3)

Table 6.2 Common Types of Fractures (3 of 3)

Diseases & Injuries of the Bone Osteoporosis is a disease of old age and is a major cause of hip fractures. The bone mass decreases. It is typically diagnosed with a bone density test.

Figure 6.16 The contrasting architecture of normal versus osteporotic bone.

Diseases & Injuries of the Bone

Diseases & Injuries of the Bone Rickets and Osteomalacia results in soft bones. Ricketts is a child hood disease while Osteomalacia is in adults. Rickets is often due to a Vitamin D deficiency.

Diseases & Injuries of the Bone