Bones and Skeletal Tissues: Part B 6 Bones and Skeletal Tissues: Part B

Osteogenesis or ossification —bone tissue formation *Bone Development Osteogenesis or 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

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

Newly calcified bone matrix Osteoblast Osteoid Osteocyte Newly calcified bone matrix 2 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. Figure 6.8, (2 of 4)

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)

Diploë (spongy bone) cavities contain red marrow Fibrous periosteum Osteoblast Plate of compact bone Diploë (spongy bone) cavities contain red marrow 4 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. Figure 6.8, (4 of 4)

*Endochondral Ossification Uses hyaline cartilage models 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 3 The periosteal bud inavades the internal cavities and spongy bone begins to form. 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

*Postnatal Bone Growth Interstitial growth:  length of long bones Appositional growth:  thickness and remodeling of all bones by osteoblasts and osteoclasts on bone surfaces

Growth in Length of Long Bones Epiphyseal plate cartilage organizes into four important functional zones: Proliferation (growth) Hypertrophic Calcification Ossification (osteogenic)

Cartilage cells undergo mitosis. 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 Figure 6.10

*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

Bone growth 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. Figure 6.11

*Bone Deposit Occurs where bone is injured or added strength is needed Requires a diet rich in protein; vitamins C, D, and A; calcium; phosphorus; magnesium; and manganese

Sites of new matrix deposit are revealed by the Bone Deposit Sites of new matrix deposit are revealed by the Osteoid seam Unmineralized band of matrix Calcification front The abrupt transition zone between the osteoid seam and the older mineralized bone

*Bone Resorption Osteoclasts secrete Lysosomal enzymes (digest organic matrix) Acids (convert calcium salts into soluble forms) Dissolved matrix is moved across osteoclast, enters interstitial fluid and then blood

*Control of Remodeling What controls continual remodeling of bone? Hormonal mechanisms that maintain calcium homeostasis in the blood Mechanical and gravitational forces

*Hormonal Control of Blood Ca2+ Calcium is necessary for Transmission of nerve impulses Muscle contraction Blood coagulation Secretion by glands and nerve cells Cell division

*Hormonal Control of Blood Ca2+ Primarily controlled by parathyroid hormone (PTH) Blood Ca2+ levels drop  Parathyroid glands release PTH PTH stimulates osteoclasts to degrade bone matrix and release Ca2+ into the blood increases Blood Ca2+ levels

Stimulus Thyroid gland Osteoclasts Parathyroid degrade bone glands 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 Figure 6.12

*Hormonal Control of Blood Ca2+ May be affected to a lesser extent by calcitonin Blood Ca2+ levels are high  Parafollicular cells of thyroid release calcitonin Osteoblasts deposit calcium salts from blood into bone decreases Blood Ca2+ levels Leptin has also been shown to influence bone density by inhibiting osteoblasts

Response to Mechanical Stress Wolff’s law: A bone grows or remodels in response to forces or demands placed upon it Observations supporting Wolff’s law: Handedness (right or left handed) results in bone of one upper limb being thicker and stronger Curved bones are thickest where they are most likely to buckle Trabeculae form along lines of stress Large, bony projections occur where heavy, active muscles attach

Load here (body weight) Head of femur Tension here Compression here Point of no stress Figure 6.13

*Classification of Bone Fractures Bone fractures may be classified by four “either/or” classifications: Position of bone ends after fracture: Nondisplaced—ends retain normal position Displaced—ends out of normal alignment Completeness of the break Complete—broken all the way through Incomplete—not broken all the way through

*Classification of Bone Fractures Orientation of the break to the long axis of the bone: Linear—parallel to long axis of the bone Transverse—perpendicular to long axis of the bone Whether or not the bone ends penetrate the skin: Compound (open)—bone ends penetrate the skin Simple (closed)—bone ends do not penetrate the skin

*Common Types of Fractures All fractures can be described in terms of Location External appearance Nature of the break Bone fractures are treated by 2 basic steps Reduction - Realignment of the bone Immobilization – Holds it in position until healed

Table 6.2

Table 6.2

Table 6.2

*Stages in Fracture Healing Four Basic Steps 1. Hematoma (blood-filled swelling) is formed 2. Break is splinted by fibrocartilage to form a callus Osteoblasts begin forming spongy bone within 1 week

*Fracture Healing 3. Fibrocartilage callus is replaced by a bony callus. Bony callus formation continues until firm union is formed in ~2 months 4. Bone Remodeling Bony callus is remodeled to form a permanent patch in response to mechanical stressors over several months

Hematoma 1 A hematoma forms. Figure 6.15, step 1

2 External callus Internal callus (fibrous tissue and cartilage) New blood vessels Spongy bone trabecula Fibrocartilaginous callus forms. 2 Figure 6.15, step 2

Bony callus of spongy bone 3 Bony callus forms. Figure 6.15, step 3

Healed fracture 4 Bone remodeling occurs. Figure 6.15, step 4

Bony callus of spongy bone Hematoma External callus Bony callus of spongy bone Internal callus (fibrous tissue and cartilage) New blood vessels Healed fracture Spongy bone trabecula 1 A hematoma forms. 2 Fibrocartilaginous callus forms. 3 Bony callus forms. 4 Bone remodeling occurs. Figure 6.15

*Vertebral Column Transmits weight of trunk to lower limbs Surrounds and protects spinal cord Flexible curved structure containing 26 irregular bones (vertebrae) Cervical vertebrae (7)—vertebrae of the neck Thoracic vertebrae (12)—vertebrae of the thoracic cage Lumbar vertebrae (5)—vertebra of the lower back Sacrum—bone inferior to the lumbar vertebrae Coccyx—terminus of vertebral column

*Vertebral Column: Curvatures Increase the resilience and flexibility of the spine Two posteriorly concave curvatures Cervical and lumbar Two posteriorly convex curvatures Thoracic and sacral Abnormal spine curvatures Scoliosis (abnormal lateral curve) Kyphosis (hunchback) Lordosis (swayback)

Figure 7.16 C1 Cervical curvature (concave) 7 vertebrae, C1–C7 Spinous process Transverse processes Thoracic curvature (convex) 12 vertebrae, T1–T12 Intervertebral discs Intervertebral foramen Lumbar curvature (concave) 5 vertebrae, L1–L5 Sacral curvature (convex) 5 fused vertebrae sacrum Coccyx 4 fused vertebrae Anterior view Right lateral view Figure 7.16

*Intervertebral Discs Cushionlike pad composed of two parts Nucleus pulposus Inner gelatinous nucleus that gives the disc its elasticity and compressibility Anulus fibrosus Outer collar composed of collagen and fibrocartilage

Supraspinous ligament Intervertebral disc Supraspinous ligament Transverse process Anterior longitudinal ligament Sectioned spinous process Intervertebral foramen Ligamentum flavum Posterior longitudinal ligament Interspinous ligament Anulus fibrosus Nucleus pulposus Inferior articular process Sectioned body of vertebra Median section of three vertebrae, illustrating the composition of the discs and the ligaments Figure 7.17a

Vertebral spinous process (posterior aspect of vertebra) Spinal cord Spinal nerve root Transverse process Herniated portion of disc Anulus fibrosus of disc Nucleus pulposus of disc (c) Superior view of a herniated intervertebral disc Figure 7.17c

General Structure of Vertebrae Body or centrum Anterior weight-bearing region Vertebral arch Composed of pedicles and laminae that, along with centrum, enclose vertebral foramen Vertebral foramina Together make up vertebral canal for spinal cord Intervertebral foramina Lateral openings between adjacent vertebrae for spinal nerves

General Structure of Vertebrae Seven processes per vertebra: Spinous process—projects posteriorly Transverse processes (2)—project laterally Superior articular processes (2)—protrude superiorly inferiorly Inferior articular processes (2)—protrude inferiorly PLAY Animation: Rotatable Spine (horizontal) PLAY Animation: Rotatable Spine (vertical)

Posterior Vertebral Lamina arch Spinous Transverse process process Superior articular process and facet Vertebral foramen Pedicle Body (centrum) Anterior Figure 7.18

C1 to C7: smallest, lightest vertebrae Cervical Vertebrae C1 to C7: smallest, lightest vertebrae C3 to C7 share the following features Oval body Spinous processes are bifid (except C7) Large, triangular vertebral foramen Transverse foramen in each transverse process

Table 7.2

(a) Cervical vertebrae Dens of axis Transverse ligament of atlas C1 (atlas) C2 (axis) C3 Inferior articular process Bifid spinous process Transverse processes C7 (vertebra prominens) (a) Cervical vertebrae Figure 7.20a

C1 (atlas) and C2 (axis) have unique features Atlas (C1) Cervical Vertebrae C1 (atlas) and C2 (axis) have unique features Atlas (C1) No body or spinous process Consists of anterior and posterior arches, and two lateral masses Superior surfaces of lateral masses articulate with the occipital condyles

*Disorders associated with Homeostatic Imbalances Osteomalacia and rickets Calcium salts not deposited Rickets (childhood disease) causes bowed legs and other bone deformities Cause: vitamin D deficiency or insufficient dietary calcium

Rickets

*Disorders associated with Homeostatic Imbalances Osteoporosis Loss of bone mass—bone resorption outpaces deposit Spongy bone of spine and neck of femur become most susceptible to fracture Risk factors Lack of estrogen, calcium or vitamin D; petite body form; immobility; low levels of TSH; diabetes mellitus

Figure 6.16

*Osteoporosis: Treatment and Prevention Calcium, vitamin D, and fluoride supplements  Weight-bearing exercise throughout life Hormone (estrogen) replacement therapy (HRT) slows bone loss Some drugs (Fosamax, SERMs, statins) increase bone mineral density

*Paget’s Disease Excessive and haphazard bone formation and breakdown, usually in spine, pelvis, femur, or skull Pagetic bone has very high ratio of spongy to compact bone and reduced mineralization Unknown cause (possibly viral) Treatment includes calcitonin and biphosphonates

Paget’s Disease

Paget’s Disease

Paget’s Disease – 67 year old man’s tibia

Paget’s Disease

Developmental Aspects of Bones Embryonic skeleton ossifies predictably so fetal age easily determined from X rays or sonograms At birth, most long bones are well ossified (except epiphyses)

Parietal bone Occipital bone Frontal bone of skull Mandible Clavicle Scapula Radius Ulna Ribs Humerus Vertebra Ilium Tibia Femur Figure 6.17

*Developmental Aspects of Bones Nearly all bones completely ossified by age 25 Bone mass decreases with age beginning in 4th decade Rate of loss determined by genetics and environmental factors In old age, bone resorption predominates

Bones are joined by sutures The Skull Two sets of bones Cranium Facial bones Bones are joined by sutures Only the mandible is attached by a freely movable joint

The Skull

The Bones of the Skull

Human Skull, Superior View Figure 5.8 Slide 5.23 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Human Skull, Inferior View

Fetal Skull vs. Adult Skull The infant’s face is small compared to the size of the cranium The adult skull represents 1/8 of the body length, where the newborn infants represents 1/4 of the body length

Fetal Skull vs. Adult Skull *Newborn has fontanels (soft spots) where the skull has not calcified To allow for compression during birth To allow for brain growth