The Skeletal System: Overview, Anatomy, and Growth Bone Functions Classification of Bones Long Bone Structure Bone Markings Microscopic Anatomy Bone Composition Role of Cartilage on Bone Bone Formation and Growth

Functions of Bones of the Human Body Support For the body and soft organs Protection For brain, spinal cord, and vital organs Movement Levers for muscle action Storage Minerals (calcium and phosphorus) and growth factors Triglyceride (energy) storage in bone cavities Blood cell formation (hematopoiesis) in marrow cavities

Respiratory tube cartilages in neck and thorax 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 Figure 6.1

Classification of Bones on the Basis of Type or Shape

The Skeletal System: Overview, Anatomy, and Growth Bone Functions Classification of Bones Long Bone Structure Bone Markings Microscopic Anatomy Bone Composition Role of Cartilage on Bone Bone Formation and Growth

Gross Anatomy of a Long Bone (Sharpey’s) fibers

Depressions and openings Bone Markings Projections for muscle or ligament attachment Tuberosity Crest Trochanter Line Tubercle Epicondyle Spine Process Depressions and openings Meatus Sinus Fossa Groove Fissure Foramen Projections for joints Head Facet Condyle Ramus

The Skeletal System: Overview, Anatomy, and Growth Bone Functions Classification of Bones Long Bone Structure Bone Markings Microscopic Anatomy Bone Composition Role of Cartilage on Bone Bone Formation and Growth

Four Types of Bone Cells Osteogenic cell Osteoblast Osteocyte Osteoclast Mature bone cell that maintains the bone matrix within lacunae Stem cell Matrix-synthesizing cell responsible for bone growth, found at edge of bone Bone-resorbing cell Bone remodeling is a process mediated by both osteoblasts and osteoclasts Figure 6.4a-b Figure 6.4c-d

Microscopic Anatomy of Bone

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) Figure 6.7a-c

The Skeletal System: Overview, Anatomy, and Growth Bone Functions Classification of Bones Long Bone Structure Bone Markings Microscopic Anatomy Bone Composition Role of Cartilage on Bone Bone Formation and Growth

Chemical Composition of Bone: Organic Osteogenic cells, osteoblasts, osteocytes, osteoclasts Osteoid—organic bone matrix secreted by osteoblasts Ground substance (proteoglycans, glycoproteins) Collagen fibers Provide tensile strength and flexibility

Chemical Composition of Bone: Inorganic Hydroxyapatites (mineral salts) 65% of bone by mass Mainly calcium phosphate crystals Responsible for hardness and resistance to compression Ca10(PO4)6(OH)2

Collagen provides shear and stretching strength Bone is Contains Both Organic and Inorganic Components Artery with capillaries Structures in the central canal Vein Nerve fiber Lamellae Collagen fibers run in different directions Collagen provides shear and stretching strength Hydroxyapatite (CaPO4) provides compression strength and hardness Twisting force

Location of Hematopoietic Tissue (Red Marrow) Red marrow cavities of adults Trabecular cavities of the heads of the femur and humerus Trabecular cavities of the diploë of flat bones Red marrow of newborn infants Medullary cavities and all spaces in spongy bone

The Skeletal System: Overview, Anatomy, and Growth Bone Functions Classification of Bones Long Bone Structure Bone Markings Microscopic Anatomy Bone Composition Role of Cartilage on Bone Bone Formation and Growth

Skeletal Cartilages Hyaline cartilages Elastic cartilages Provide support, flexibility, and resilience Most abundant type Elastic cartilages Similar to hyaline cartilages, but contain elastic fibers Fibrocartilages Collagen fibers—have great tensile strength

Skeletal Cartilages Contain no blood vessels or nerves Dense connective tissue girdle of perichondrium contains blood vessels for nutrient delivery to cartilage

Growth of Cartilage Appositional Interstitial Cells secrete matrix against the external face of existing cartilage Interstitial Chondrocytes divide and secrete new matrix, expanding cartilage from within Calcification of cartilage occurs during Normal bone growth Old age

The Skeletal System: Overview, Anatomy, and Growth Bone Functions Classification of Bones Long Bone Structure Bone Markings Microscopic Anatomy Bone Composition Role of Cartilage on Bone Bone Formation and Growth Intramembranous Growth Endochondral Growth Interstitial Growth and the Epiphyseal Plate

Osteogenesis (ossification)—bone tissue formation Bone Development Osteogenesis (ossification)—bone tissue formation Stages Bone formation—begins in the 2nd month of development Postnatal bone growth—until early adulthood Bone remodeling and repair—lifelong

Two Types of Ossification Intramembranous ossification Membrane bone develops from fibrous membrane Forms flat bones, e.g. clavicles and cranial bones Endochondral ossification Cartilage (endochondral) bone forms by replacing hyaline cartilage Forms most of the rest of the skeleton

Intramembranous Ossification I Mesenchymal cell Collagen fiber Ossification center Osteoid Osteoblast 1 Ossification centers appear in the fibrous connective tissue membrane. • Some of the centrally located mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center. Figure 6.8, (1 of 4)

Intramembranous Ossification II Osteoblast Osteoid Osteocyte Newly calcified bone matrix Bone matrix (osteoid) is secreted within the fibrous membrane and calcifies. • Osteoblasts begin to secrete osteoid, which is calcified within a few days. • Trapped osteoblasts become osteocytes. 2 Figure 6.8, (2 of 4)

Intramembranous Ossification III Mesenchyme condensing to form the periosteum Trabeculae of woven bone Blood vessel 3 Woven bone and periosteum form. • Accumulating osteoid is laid down between embryonic blood vessels in a random manner. The result is a network (instead of lamellae) of trabeculae called woven bone. • Vascularized mesenchyme condenses on the external face of the woven bone and becomes the periosteum. Figure 6.8, (3 of 4)

Intramembranous Ossification IV Fibrous periosteum Osteoblast Plate of compact bone Diploë (spongy bone) cavities contain red marrow Lamellar bone replaces woven bone, just deep to the periosteum. Red marrow appears. • Trabeculae just deep to the periosteum thicken, and are later replaced with mature lamellar bone, forming compact bone plates. • Spongy bone (diploë), consisting of distinct trabeculae, per- sists internally and its vascular tissue becomes red marrow. 4

Endochondral Ossification Uses hyaline cartilage models (a model is a structure made in the proper proportions of shape and size that can be used in forming another similar structure. Bone is formed as it destroys the model from which it’s made) Requires breakdown of hyaline cartilage prior to ossification

Childhood to adolescence Week 9 Month 3 Birth Childhood to adolescence Articular cartilage Secondary ossification center Spongy bone Epiphyseal blood vessel Area of deteriorating cartilage matrix Epiphyseal plate cartilage Hyaline cartilage Medullary cavity Spongy bone formation Bone collar Blood vessel of periosteal bud Primary ossification center 1 Bone collar forms around hyaline cartilage model. Cartilage in the center of the diaphysis calcifies and then develops cavities. 2 The periosteal bud inavades the internal cavities and spongy bone begins to form. 3 The diaphysis elongates and a medullary cavity forms as ossification continues. Secondary ossification centers appear in the epiphyses in preparation for stage 5. 4 The epiphyses ossify. When completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages. 5 Figure 6.9

Primary ossification center Week 9 Hyaline cartilage Bone collar Primary ossification center Bone collar forms around hyaline cartilage model. 1 Figure 6.9, step 1

Area of deteriorating cartilage matrix 2 Cartilage in the center of the diaphysis calcifies and then develops cavities. Figure 6.9, step 2

Blood vessel of periosteal bud Month 3 Spongy bone formation Blood vessel of periosteal bud The periosteal bud invades the internal cavities and spongy bone begins to form. 3 Figure 6.9, step 3

Epiphyseal blood vessel Secondary ossification center Birth Epiphyseal blood vessel Secondary ossification center Medullary cavity The diaphysis elongates and a medullary cavity forms as ossification continues. Secondary ossification centers appear in the epiphyses in preparation for stage 5. 4 Figure 6.9, step 4

Childhood to adolescence Articular cartilage Spongy bone Epiphyseal plate cartilage The epiphyses ossify. When completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages. 5 Figure 6.9, step 5

Childhood to adolescence Week 9 Month 3 Birth Childhood to adolescence Articular cartilage Secondary ossification center Spongy bone Epiphyseal blood vessel Area of deteriorating cartilage matrix Epiphyseal plate cartilage Hyaline cartilage Medullary cavity Spongy bone formation Bone collar Blood vessel of periosteal bud Primary ossification center 1 Bone collar forms around hyaline cartilage model. Cartilage in the center of the diaphysis calcifies and then develops cavities. 2 The periosteal bud inavades the internal cavities and spongy bone begins to form. 3 The diaphysis elongates and a medullary cavity forms as ossification continues. Secondary ossification centers appear in the epiphyses in preparation for stage 5. 4 The epiphyses ossify. When completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages. 5 Figure 6.9

The Skeletal System: Overview, Anatomy, and Growth Bone Functions Classification of Bones Long Bone Structure Bone Markings Microscopic Anatomy Bone Composition Role of Cartilage on Bone Bone Formation and Growth Intramembranous Growth Endochondral Growth Interstitial Growth and the Epiphyseal Plate

Postnatal Bone Growth: Growth of Long Bones Articular cartilage Epiphyseal plate Interstitial growth:  length of long bones Appositional growth:  thickness and remodeling of all bones by osteoblasts and osteoclasts on bone surfaces Cartilage grows here. Cartilage is replaced by bone here. Cartilage grows here. Cartilage is replaced by bone here.

Composition of the Epihyseal Plate Epiphyseal plate cartilage organizes into four important functional zones: Proliferation/Growth Zone (cartilage cells divide) Hypertrophic zone (cells grow bigger) Calcification zone (hydroxyapatite deposited) Ossification (osteogenic) zone (where new bone is actively being formed)

Zones of Epiphyseal Plate Cartilage Resting zone Proliferation zone Cartilage cells undergo mitosis. 1 Hypertrophic zone Older cartilage cells enlarge. 2 Calcification zone Matrix becomes calcified; cartilage cells die; matrix begins deteriorating. 3 Calcified cartilage spicule Osteoblast depositing bone matrix Ossification zone New bone formation is occurring. Osseous tissue (bone) covering cartilage spicules 4

Hormonal Regulation of Bone Growth Growth hormone stimulates epiphyseal plate activity Thyroid hormone modulates activity of growth hormone Testosterone and estrogens (at puberty) Promote adolescent growth spurts End growth by inducing epiphyseal plate closure

The Skeletal System: Overview, Anatomy, and Growth Bone Functions Classification of Bones Long Bone Structure Bone Markings Microscopic Anatomy Bone Composition Role of Cartilage on Bone Bone Formation and Growth Intramembranous Growth Endochondral Growth Interstitial Growth and the Epiphyseal Plate