A&P Lecture 8-9 (Ch. 6)

Skeletal system

Comprised of bone and cartilage

Bone function

Forms rigid scaffolding for protection support and movement. Important for mineral storage, triglyceride storage, and homatopoiesis. Classified as organs

Cartilage function

Forms joint structures, growth plates, and templates for future bones during embryonic development

Hematopoiesis

Red blood cell and platelet synthesis

Bone as an organ

Includes bone, cartilage, nerves, vessels, connective tissue, and marrow

Hyaline cartilage

Lines articular surfaces of joints, form growth plates near the ends of long bones, form cartilage models that many bones develop from

Fibrocartilage

Present in areas subject to both stretch and compression (ex: intervertebral discs, menisci in knee, pubic symphysis between pelvic bones)

Elastic cartilage

Forms the external ear and epiglottis

Cartilage growth

Includes interstitial growth and appositional growth

Interstitial growth

Cartilage grows from within- chondrocytes in lacunae divide and secrete new matrix

Appositional growth

Cartilage is surrounded by a layer of dense irregular connective tissue called perichondrium. Chondroblasts differentiate from cells in perichondrium and secrete new matrix on outer surface of cartilage

Perichondrium

Layer of dense irregular connective tissue surrounding cartilage

Axial bone classification

Vertebral column, rib cage, sternum, skull

Appendicular skeleton

Limbs and limb girdles (scapula/clavicle and pelvic bones)

Long bones

Bones made up of a shaft with distinct ends (most limb bones)

Short bones

Cube or round shaped bones (wrist, ankle, sesamoid bones)

Sesamoid bones

Bones that form in a tendon (ex: patella)

Flat bones

Thin, flat, and usually curved bones (ex: cranial bones of skull, ribs, sternum)

Irregular bones

Bones with shapes more complex than other categories (ex: vertebra, pelvic bones)

Gross bone structure

Macroscopic, includes cortical bone, trabecular bone, skeletal features, and marrow cavity

Compact (cortical) bone

Dense bone tissue lining the bone’s outer surface

Spongy (trabecular) bone

Made up of many trabeculae (small beams/spines of bone tissue that form a network on the interior of a bone). Arranged along lines of stress from mechanical forces

Microscopic bone structure

Includes osteons, canals, and lamellae

Chemical organization of bone structure

Includes organic (osteoid, bone cells) and inorganic (hydroxyapatite) components

Bone cell function

Create, maintain, and recycle bone

Osteoid

Unmineralized extracellular matrix made of collagen fibers, calcium-binding proteins, and ground substance

Collagen fibers function in organic component of bone

Give bone strength under tension and allow it to bend slightly without breaking

Hydroxyapatite crystals

Inorganic component of bone: mineral salts, mainly calcium phosphate. Give bone its rigidity and strength under compression

Mineralized bone

Composite of collagen encased in hydroxyapatite crystals

2 categories of bone tissue organization

1. Compact (cortical) bone

2. Spongy (trabecular) bone

Trabeculae

Small beams or spines of bone tissue that form a network in spongy bone. Arranged along lines of stress from mechanical forces

Periosteum

Connective tissue membrane that lines the outer surface of bone. Two layers- outer: fibrous; inner: osteogenic

Fibrous layer of periosteum

Outer layer, made of dense irregular connective tissue

Osteogenic layer of periosteum

Inner layer, contains osteoprogenitor cells

Endosteum

Connective tissue membrane that lines all interior surfaces of bone, including surface of trabecular bone and canals within cortical bone. Single layer, also contains osetoprogenitor cells

Organization of short, flat, and irregular bone

Outer compact bone (on both sides) surrounds inner spongy bone (called diploe in flat bones)

Diploe

Spongy bone inner layer found in flat bones

Organization of long bones

Diaphysis: shaft; epiphysis: ends

Diaphysis

Shaft of a long bone. Cylindrical arrangement of compact bone, surrounds medullary cavity

Epiphyses

Ends of long bone. Outer layer of compact bones, inner layer of spongy bone. Has epiphyseal line

Epiphyseal line

Remnant of the growth plate that forms the boundary between the diaphysis and epiphysis

Red marrow

Hematopoietic tissue that forms blood cells. Makes up marrow cavity and spaces between spongy bone in infants. In adults, limited to axial skeleton and proximal ends of humerus and femur

Yellow marrow

Mainly adipose tissue that replaces red marrow in the marrow cavity during growth. Can revert to red in certain conditions (sever, chronic anemia)

Osteoprogenitor cells

Mitotic stem cells found on bone-facing surfaces of the periosteum and endosteum (some can become osteoblasts

Osteoblasts

Mitotic bone-forming cells that secrete osteoid. Some become flat, inactive bone lining cells that line and maintain bone surface. Some become osteocytes

Osteocytes

Mature osteoblasts that maintain bone matrix and sense mechanical strain. Situated in lacunae

Osteoclasts

Large multinucleated cells that resorb or degrade bone extracellular matrix

Osteon

Structural unit of compact bone. Tiny pillars of bone made up of concentric bone layers arranged parallel to ling axis of bone

Osteon structure

Central (haversian) canal, lamellae, lacunae, canaliculi

Central (Haversian) canal

Neurovascular canal located at the center of an osteon

Perforating (Volkmann) canal

Perpendicular connecting channel between adjacent central canals

Lamellae

Concentric layers of bone surrounding the central canal

Lacunae in osteons

Spaces between lamellae that contain osteocytes

Canaliculi

Small channels radiating from lacunae that allow osteocyte-osteocyte communication

Spongy bone microanatomy

Has no proper osteons and no central canals, irregularly arranged lamellae, with osteocytes connected by canaliculi. Nutrients accessed and wastes removed through canaliculi that open into marrow space

Lamellar arrangement

Different directions of collagen fibers differs between neighboring lamellae. Resists torsional (twisting) forces as one lamella reinforces its neighbor

Bone formation

All bones are formed by either endochondral or intramembranous ossification

Endochondral ossification

Bone develops from a hyaline cartilage model (majority of bones form this way).

Intramembranous ossification

Bone develops directly from mesenchyme. Flat bones of skull and parts of clavicle form this way

Mesenchyme

Embryonic tissue composed of gel-like ground substance fibers and undifferentiated stem cells

Endochondral ossification steps

1. Bone collar forms around diaphyseal margins of cartilage model

2. Hypertrophied chondrocytes in center of shaft calcify surrounding cartilage matrix, cavities develop

3. Periosteal bud invades internal cavities, forming early spongy bone

4. Medullary cavity formed, diaphysis elongate, and secondary ossification centers form at epiphyses

5. Epiphyses ossify, articular cartilage and growth plate are remaining cartilage

Endochondral ossification step 1

Bone collar forms around the diaphyseal margins of the cartilage model. Perichondrium turns into periosteum, osteoblasts start forming bone on outside of shaft. Chondrocytes in center of shaft hypertrophy (grow bigger)

Bone collar

Layer of bone that forms around the outside of the developing diaphysis

Endochondral ossification step 2

Hypertrophied chondrocytes in the center of the shaft calcify the surrounding cartilage matrix, and cavities develop. Calcified matrix prevents diffusion of nutrients, chondrocytes die and matrix begins to deteriorate, creating cavities

Endochondral ossification step 3

The periosteal bud invades internal cavities and forms early spongy bone. Osteoclasts break down calcified cartilage, and osteoprogentior cells create osteoblasts, which secrete osteoid around remaining cartilage fragments. Early spongy bone is formed as the primary ossification center

Periosteal bud

Starter kit containing nerves, vessels, red marrow elements, osteoprogenitor cells, and osteoclasts

Endochondral ossification step 4

Medullary cavity forms, the diaphysis elongates, and secondary ossification centers form at the epiphyses. Epiphyseal cartilage grows at its ends, lengthening model. Primary ossification center expands with growth of overall model. Osteoclasts resorb early trabecular bone at center of diaphysis, forming marrow cavity. Secondary ossification centers established in epiphyses by periosteal buds

Endochondral ossification step 5

Epiphyses ossify, articular cartilage and growth plate are remaining cartilage. Secondary ossification centers follow same sequence as primary, except trabecular bone is maintained (no medullary cavity in epiphyses). Remaining hyaline cartilage makes up growth plate and articular cartilage on epiphyses

Growth plate

Hyaline cartilage located between the epiphysis and diaphysis

Intramembranous ossification steps

1. Mesenchymal stem cells become osteoblasts and form an ossification center

2. Osteoid deposited and calcified around blood vessels, forming early spongy bone

3. Mesenchyme condenses on outer face of bone, forms periosteum

4. Compact bone replaces early spongy bone deep to periosteum

5. Red marrow fills spaces between trabeculae of spongy bone

Bone growth in length

Occurs at the epiphyseal growth plate. Involves 5 zones: resting, proliferation, hypertrophic, calcification, and ossification

Resting zone of bone growth

Cartilage is inactive on the side closest to the epiphysis

Proliferation zone of bone growth

Chondrocytes line up in columns and divide, pushing the epiphysis away from the diaphysis

Hypertrophic zone of bone growth

Chondrocytes closer to the diaphysis hypertrophy and enlarge their lacunae

Calcification zone of bone growth

Cartilage calcifies ,chondrocytes die, and the matrix deteriorates. Former lacunae provide spaces that are invaded by blood vessels

Ossification zone of bone growth

Remnants of calcified matrix are eroded by osteoclasts, and new bone replaces them through osteoblast activity, and marrow elements are introduced

Bone growth in width

Osteoblasts from osteogenic layer of periosteum secrete osteoid on outer surface of bone, while osteoclasts resorb bone from the inner surface

Bone growth in width during early development

More bone is deposited than lost

Bone growth in width with aging

More bone is lost than gained

Bone remodeling

Bone tissue becomes more mineralized with time (more brittle) and accumulates microdamage. Remodeling isthe balanced resorption of older bone and replacement by new bone

Bone remodeling process

Osteoclasts move along the bone surface and create depressions as acids and enzymes break down the matrix. Osteoblasts follow osteoclasts, depositing osteoid. Osteoid calcification occurs

Osteoid calcification

1. Deposited proteins bind Ca 2+

2. Increase in local Ca 2+ concentration causes osteoblasts to release alkaline phosphatase (ALP)

3. ALP cleaves phosphate ions off osteoid proteins

4. Ca 2+ and phosphate forms hydroxyapatite crystal formation

Bone remodeling and calcium

Ca 2+ is constantly deposited into and withdrawn from bone to maintain extracellular calcium homeostasis

PTH negative feedback loop

1. Low Ca 2+ causes PTH to be released

2. PTH stimulates release of signaling protein RANK-L from osteoblasts, which increases osteoclast formation and resorption activity

3. Resorption release Ca 2+ from bone

4. As Ca 2+ levels rise, PTH release ends and resorption returns to normal

RANK-L

Protein from osteoblasts which increases osetoclast formation and resorption activity

PTH

Parathyroid hormone, released by low Ca 2+ levels and stimulate RANK-L to increase osteoclast activity and resorption

Remodeling and physical activity process

1. Mechanical stress causes bone to bend slightly

2. Causes ion-containing fluid to move through canaliculi

3. Osteocytes sense fluid movement and signal to osteoblasts/clasts to remodel accordingly

Increased mechanical stress

Leads to net formation remodeling

Decreased mechanical stress

Leads to net resorption remodeling

Aging and bone remodeling

Aging shifts remodeling toward net resorption, mainly due to hormonal decline, particularly estrogen

Role of estrogen in bone

Has protective effects on endosteal bone and keeps osteoclasts in check

Estrogen and bone loss

Estrogen decline leads to increased resorption, causing lower bone density

Bone density loss determined by sex

Post-menopausal decline in estrogen results in increased rate of bone density loss in females compared to males.

Osteoporosis

Condition in which bone density drops below a certain threshold

Low bone density management

Weight-bearing exercise, hormone replacement therapy, and RANK-L inhibitors

Bone fracture repair process

1. Inflammation: hematoma forms from torn vessels and clots, bone cells die and tissue is inflamed

2. Fibrocartilaginous (soft) callus forms from nearby fibroblasts and chondroblasts

3. Fibrocartilaginous callus replaced by bony (hard) callus (disorganized spongy bone) from osteoblast activity

4. Bony callus is continually remodeled until fracture completely repaired

Fibrocartilaginous callus

Soft callus formed after a bone fracture from nearby fibroblasts and chondroblasts

Bony callus

Hard callus formed after bone fracture made of disorganized spongy bone, formed through osteoblast activity