Bones: Shape, Structure and Classification

• Bones acts as organs in the skeletal system
• Bone tissues consists of bone, cartilage, dense connective, blood and nervous tissue
• Bones are alive and multifunctional in the body
• Bones support and protect tissue, helps with movement, blood cell formation, and mineral storage
• The skeletal system has two parts: axial and appendicular

Bone Shape and Structure

• Bones vary in size, shape, but are similar in structure, development, and function

Bone Classification by Shape

• Long bones are long and narrow, and have expanded ends
• Short bones are cube-like where the length is nearly equal to the width
• Short bones include sesamoid bones embedded in tendons
• Flat bones are plate-like with broad surfaces
• Irregular bones have a variety of shapes and connect to other bones

Parts of a Long Bone

• Epiphysis: Expanded end
• Diaphysis: Bone shaft
• Metaphysis: Widening part between diaphysis and epiphysis
• Articular cartilage: Covers epiphysis
• Periosteum: Encloses bone, made of dense connective tissue
• Compact Bone: Diaphysis wall
• Spongy Bone: Makes up epiphyses
• Trabeculae: Branching bony plates that make up spongy bone
• Medullary Cavity: Hollow chamber in diaphysis that contains marrow
• Endosteum: Lines the spaces and cavities in bones
• Bone marrow can be red or yellow
• Red marrow lines the medullary cavity, spongy bone spaces, and is responsible for blood cell production
• Yellow marrow stores fat

Microscopic Structure of Bone

• Osteocytes are mature bone cells
• Lacunae are chambers occupied by osteocytes
• Canaliculi are tiny passageways for nutrient and waste exchange by osteocytes
• Extracellular matrix of bone is collagen fibers and inorganic salts
• Collagen gives bones resilience
• Inorganic salts make bone hard
• Compact bone consists of osteons, cylindrical units
• Osteons and lamellae form around the central canal in an osteon
• Compact bone is strong, solid, weight-bearing, and resists compression
• Spongy bone consists of branching plates called trabeculae, and is somewhat flexible
• Spongy bone has spaces between trabeculae to reduce weight

Bone Function

• Bones provide shape, support structures, protect structures, aid in movements, and produce blood cells
• Bones store inorganic salts

Support, Protection, and Movement

• Bones provide shape for the head, face, thorax, and limbs
• Bones support body weight on the lower limbs, pelvis, and vertebral column
• Skull bones protect the brain, ears, and eyes
• The rib cage and shoulder girdle protects the heart and lungs
• Pelvic Girdle protect organs and lower abdominal organs
• Bones and muscles provide movement

Blood Cell Formation

• Hematopoiesis is blood cell formation
• Blood cell production occurs in red bone marrow
• Over time red bone marrow is replaced by yellow bone marrow and stores fat and doesn't produce blood cells
• Red Marrow remains in adult skull, ribs, sternum, clavicles, vertebrae, and hip bones
• Bone marrow transplants treats a variety of conditions

Inorganic Salt Storage

• Bone matrix consists of 70% of inorganic mineral salts
• Hydroxyapatite, or calcium phosphate, is the most abundant salt
• Other salts include magnesium, sodium, potassium, and carbonate ions
• Osteoporosis results from loss of bone mineralization
• Calcium is vital in nerve impulse conduction and muscle contraction
• Blood calcium level is regulated by parathyroid hormone and calcitonin

Bone Development, Growth, and Repair

• Skeletal System begin develops in first few weeks of prenatal development
• The bones continue to grow and develop into adulthood
• Bones form when bone tissues replaces existing connective tissue two ways
• Intramembranous bones
• Endochondral bones

Bone Growth and Development

• Intramembranous Ossification: flat skull bones, clavicles, sternum and some facial bones
• Bones form between sheets of primitive connective tissue
• Endochondral Ossification: long bones and most of skeleton
• Bones form from Hyaline cartilage models

Intramembranous Bones

• Intramembranous bones originate within sheet-like layers of connective tissue
• Broad, flat bones include skull bones, clavicles, sternum, and some facial bones
• Intramembranous Ossification is the process of replacing embryonic connective tissue to form bone

Intramembranous Ossification Process

1. Mesenchymal cells in primitive tissue differentiate into osteoblasts
2. Osteoblasts or bone-forming cells, deposit bone matrix around themselves
3. When osteoblasts are completely surrounded by matrix, they are now osteocytes in lacunae
4. Mesenchyme on the outside forms periosteum

Endochondral Bones

• Start as masses of hyaline cartilage
• Include most bones of the skeleton, such as femur, humerus, radius, tibia, phalanges, vertebrae.
• Endochondral Ossification is when replacing hyaline cartilage forms endochondral bone

Endochondral Ossification Process

1. Begin as hyaline cartilage models
2. Chondrocytes, or cartilage cells, enlarge and lacunae grow
3. Matrix breaks down, chondrocytes die
4. Osteoblasts invade area, deposit bone matrix
5. Osteoblasts form spongy and then compact bone
6. Once encased by matrix, osteoblasts are now osteocytes

Development of Endochondral Long Bones

• Bone start as a hyaline cartilage model
• Primary ossification center is the area in center of diaphysis, where bone tissue first replaces cartilage
• Replacement with bone tissue proceeds toward ends of bone
• Osteoblasts from periosteum deposit compact bone around primary center
• Secondary ossification centers are in the epiphyses, where spongy bone forms later in development
• Epiphyseal plate is the band of cartilage that persists between the ossification centers

Growth at the Epiphyseal Plate

• A growing long bone where the diaphysis is separated from epiphysis by the epiphyseal plate
• Region where the bone grows in length
• Cartilaginous cells of epiphyseal plate form 4 layers: zone of resting cartilage, zone of proliferating cartilage, zone of hypertrophic cartilage and zone of calcified cartilage

Zone of Resting Cartilage

• Layer closest to end of epiphysis with resting cells
• Anchors epiphyseal plate to epiphysis

Zone of Proliferating Cartilage

• Rows of young cells
• Undergoing mitosis

Zone of Hypertrophic Cartilage

• Rows of older cells left behind when new cells appear
• This thickens epiphyseal plate, lengthening the bone
• Matrix calcifies and cartilage cells (chondrocytes) die

Zone of Calcified Cartilage

• Thin layer of dead cartilage cells and calcified matrix
• Osteoclasts break down calcified matrix
• Osteoblasts then invade, replacing cartilage with bone tissue
• Bone can continue to grow in length for as long as cartilage cells of epiphyseal plate remain active
• When ossification centers meet, and epiphyseal plate ossifies, bone can no longer grow in length
• Bone can thicken by depositing compact bone on outside, under periosteum

Bone Development and Growth Timeline

• Third month of prenatal development: Ossification in long bones begins
• Fourth month of prenatal development: Most primary ossification centers show in diaphyses of long bones
• Birth to 5 Years: Secondary ossification centers appear in the epiphyses of long bones
• 5 to 12 years (females): Ossification rapidly spreads from the ossification centers.
• 5 to 14 years (males): Ossification rapidly spreads from the ossification centers
• 15 to 18 years (females): Bones of the upper limbs and scapulae completely ossify.
• 17 to 20 years (males): Bones of the upper limbs and scapulae completely ossify.
• 16 to 21 years (females): Bones of the lower limbs and hip bones completely ossify.
• 18 to 23 years (males): Bones of the lower limbs and hip bones completely ossify.
• 21 to 23 years (females): Bones of the sternum, clavicles, and vertebrae completely ossify.
• 23 to 25 years (males): Bones of the sternum, clavicles, and vertebrae completely ossify.
• By 23 years (females): Nearly all bones completely ossify.
• By 25 years (males): Nearly all bones completely ossify.

Homeostasis of Bone Tissue

• During development of intramembranous and endochondral bones, osteoblasts and osteoclasts remodel them
• Bone remodeling occurs throughout life
• Opposing processes of deposition and resorption occur on surfaces of endosteum and periosteum
• Bone Resorption occurs during removal by osteoclasts
• Bone Deposition- Happens through formation of bone, by osteoblasts
• 10% to 20% of skeleton is replaced each year

Factors that affect Bone Development, Growth and Repair

• Nutrition, sunlight exposure, hormone levels, and physical exercise effect bones
• Vitamin D helps with calcium absorption, and deficiencies cause rickets
• Vitamin A affects osteoblast and osteoclast activity, and deficiencies retard bone development
• Vitamin C helps with collagen synthesis and deficiencies result in slender, fragile bones
• Growth Hormone stimulates cartilage cell division
• Insufficiency in a child can result in pituitary dwarfism
• Excess causes gigantism in child, and acromegaly in adult
• Thyroid Hormone causes replacement of cartilage with bone in epiphyseal plate and osteoblast activity
• Parathyroid Hormone (PTH) stimulates osteoclasts and bone breakdown
• Sex hormones such as estrogen and testosterone promote bone formation, and stimulate ossification of epiphyseal plates
• Physical Stress stimulates bone growth

Clinical Applications- Fractures

• Fractures are classified by its cause and break
• Traumatic fractures are caused by injury
• Spontaneous (pathologic) fractures are caused by disease
• Simple of closed fractures are covered by uninjured skin or mucous membranes
• Compound (open) fractures expose the bone to the outside and an opening of the skin or membrane

Steps to Fracture Repair

• Hematoma forms: Large blood clot forms right after fracture
• Cartilaginous (soft) Callus: Osteoblasts invade and produce spongy bone, phagocytes remove debris, and fibrocartilage is produced
• Bony (hard) callus: Cartilaginous callus breaks down, osteoblasts fill the space
• Remodeling: Restore bones to original shape and osteoclasts remove bones

Fragility Fractures

• Occur after a fall from less than standing height is a sign of low bone density
• Bone remodeling occurs though-out life
• Over time osteoclasts remove more bone than osteoblasts
• It can result in osteopenia(bone loss) and progress to osteoporosis( severe bone loss that leaves in spaces and canals of bones, which can weaken them)
• It is estimated that 50% women, and 25% men over 50 has one of the bone loss conditions if not treated
• Common in postmenopausal women, because of hormone changes