Bones and Skeletal Tissues 6

Skeletal Cartilage Avascular & without nerves Vascularized perichondrium Three types Hyaline Elastic Fibrocartilage

2 types of growth Calcification Growth of Cartilage Appositional –perichondrium cells secrete matrix Interstitial – chondrocytes within cartilage secrete matrix Calcification bone growth geriatric

Classification of Bones by Skeletal Region Axial skeleton skull, vertebral column, ribs & sternum Appendicular skeleton limbs, pectoral girdle, & pelvic girdle

Classification of Bones: By Shape Long bones Figure 6.2a

Classification of Bones: By Shape Long bones Short bones Cube-shaped carpals, tarsals, & patella Figure 6.2b

Classification of Bones: By Shape Long bones Short bones Flat bones sternum, & most skull bones Figure 6.2c

Classification of Bones: By Shape Long bones Short bones Flat bones Irregular bones Vertebrae & pelvic bones Figure 6.2d

Gross Anatomical Bone Features Bulges, depressions, and holes that serve as: Attachment sites for ligaments & tendons (muscles) Joint surfaces Conduits for blood vessels and nerves Table 6.1 Know It

Gross Anatomy: Structure of Long Bone Figure 6.3

Periosteum – Endosteum Bone Membranes Outer – dense, regular connective tissue Inner (osteogenic) - osteoblasts and osteoclasts nerves, blood, & lymphatic vessels enter via nutrient foramina Secured to underlying bone by Sharpey’s fibers Endosteum Thin membrane covering internal surfaces of bone Endosteum

Structure of Short, Irregular, and Flat Bones Periosteum-covered compact bone on the outside Endosteum-covered spongy bone (diploë) on the inside no diaphysis or epiphyses Red marrow among trabeculae of diploë

Red Yellow Bone Marrow Hematopoietic stem cells childhood medullary cavity & all areas of spongy bone adults diploë of flat bones, head of the femur & humerus Yellow Adipose-like tissue Medullary cavity & epiphyseal spongy bone

Microscopic Structure of Bone: Compact Bone Osteoid Ossified minerals

Composition of Bone: Components Osteocytes – mature bone cells Osteoblasts – bone-forming cells Osteoclasts – resorb or break down bone matrix Osteoid – unmineralized ECM of proteoglycans & collagen Hydroxyapatite calcium phosphates 65% of bone by mass

Osteogenesis begins by 8th week of embryonic development Bone Development Chondrogenesis forms cartilaginous model of skeleton in embryos beginning in 6th week Osteogenesis begins by 8th week of embryonic development collagen matrix Ossification Intramembranous develops within a fibrous membrane Endochondral replacement of hyaline cartilage model

Intramembranous Ossification Formation of most of the flat bones of the skull and the clavicles Fibrous connective tissue membranes are formed by mesenchymal cells

Stages of Intramembranous Ossification

Stages of Intramembranous Ossification

Endochondral Ossification Formation of the long bones and many irregular bones (vertebrae) Chondrocytes 1st form cartilaginous model of the bone that is replaced by osteoblasts and then mineralized

Stages of Endochondral Ossification Hyaline cartilage Primary ossification center Bone collar 1

Stages of Endochondral Ossification Deteriorating cartilage matrix Hyaline cartilage Primary ossification center Bone collar 1 2

Stages of Endochondral Ossification Deteriorating cartilage matrix Hyaline cartilage Spongy bone formation Primary ossification center Bone collar Blood vessel of periosteal bud 1 2 3

Stages of Endochondral Ossification Secondary ossification center Epiphyseal blood vessel Deteriorating cartilage matrix Hyaline cartilage Spongy bone formation Primary ossification center Medullary cavity Bone collar Blood vessel of periosteal bud 1 2 3 4

Stages of Endochondral Ossification Secondary ossification center Articular cartilage Epiphyseal blood vessel Spongy bone Deteriorating cartilage matrix Hyaline cartilage Epiphyseal plate cartilage Spongy bone formation Primary ossification center Medullary cavity Bone collar Blood vessel of periosteal bud 1 2 3 4 5 Figure 6.8

Functional Zones in Long Bone Growth

Long Bone Growth and Remodeling Figure 6.10

Appositional Growth of Bone Central canal of osteon Periosteal ridge Periosteum Penetrating canal Artery 1 Osteoblasts beneath periosteum secrete bone matrix & form ridges following periosteal blood vessels 2 As the ridges meet, they form a tunnel containing the blood vessel 3 The periosteum lining the tunnel is transformed into an endosteum and the osteoblasts just deep to the tunnel endosteum secrete bone matrix, narrowing the canal. 4 As the osteoblasts beneath the endosteum form new lamellae, a new osteon is created. Meanwhile new circumferential lamellae are elaborated beneath the periosteum and the process is repeated, continuing to enlarge bone diameter. Figure 6.11

Hormonal Regulation of Bone Growth During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone During puberty, testosterone and estrogens:

Requires protein, vitamins C, D, A, Ca, P, Mg, & Mn Resorption Bone Remodeling Osteoblasts and osteoclasts deposit and resorb bone at periosteal and endosteal surfaces Requires protein, vitamins C, D, A, Ca, P, Mg, & Mn Resorption Osteoclasts in resorption bays secrete enzymes that digest matrix acids that dissolve Ca salts Deposition Osteoblasts Lay down fresh osteoid matrix and induce mineralization

Mechanical and gravitational forces Control of Remodeling Hormonal control loops regulate bone remodeling & maintain Ca homeostasis in the blood Mechanical and gravitational forces

Hormonal Mechanism Figure 6.12

Importance of Ionic Calcium in the Body Transmission of nerve impulses Muscle contraction Blood coagulation Secretion by glands and nerve cells Cell division

Response to Mechanical Stress Wolff’s law – A bone grows and/or remodels in response to mechanical stresses Bones are thickest where the stresses are maximal Long bones - midway along the shaft Curved bones where the curvature is greatest Trabeculae form along lines of stress Large, bony projections occur where heavy, active muscles attach

Response to Mechanical Stress Figure 6.13

Bone Fracture Classification Position of the bone ends nondisplaced v displaced Completeness of the break Orientation of the break to the long axis linear v transverse Whether or not the ends penetrate skin compound v simple

Common Types of Fractures

Common Types of Fractures

Common Types of Fractures

Stages in the Healing of a Bone Fracture Hematoma formation Hematoma 1 Hematoma formation Figure 6.14.1

Stages in the Healing of a Bone Fracture Fibrocartilaginous callus formation (soft callus) Granulation tissue (fibrocartilage) grows Capillaries grow and phagocytic cells clear debris External callus New blood vessels Internal callus (fibrous tissue and cartilage) Spongy bone trabeculae 2 Fibrocartilaginous callus formation Figure 6.14.2

Stages in the Healing of a Bone Fracture Bony callus formation fibrocartilage of fibrocartilaginous callus converts into spongy bone Bone callus begins 3-4 weeks after injury, and continues until firm union is formed 2-3 months later Bony callus of spongy bone 3 Bony callus formation Figure 6.14.3

Stages in the Healing of a Bone Fracture Bone remodeling Excess material on is removed Compact bone is laid down to reconstruct shaft walls Healing fracture 4 Bone remodeling Figure 6.14.4

Homeostatic Imbalances Ca deficiency conditions Dietary or hormonal (vit D) Inadequate mineralization causing softened, weakened bones Osteomalacia elderly Rickets children

Osteoporosis Pathology Preventive measures Treatments Condition when bone reabsorption outpaces bone deposit Spongy bone is most vulnerable (especially spine) Bones become very fragile Occurs most often in postmenopausal women Preventive measures Dietary Ca and vitamin D Increased weight-bearing exercise Treatments Hormone replacement therapy (HRT) – estrogens Statins

Excessive bone remodeling Paget’s Disease Excessive bone remodeling Initially, an excess of spongy to compact bone forms Later, osteoclast activity wanes, but osteoblast activity continues resulting in filling in spongy bone and loss of marrow Usually localized in the spine, pelvis, femur, and skull

Developmental Aspects of Bones Mesoderm gives rise to embryonic mesenchymal cells, which produce membranes and cartilages that form the embryonic skeleton The embryonic skeleton ossifies in a predictable timetable that allows fetal age to be easily determined from sonograms At birth, most long bones are well ossified (except for their epiphyses) By age 25, nearly all bones are completely ossified In old age, bone resorption predominates