Chapter 6 Osseous Tissue and Bone Structure

Function of Skeletal System

- Support - Protection - Leverage - Storage - Blood Cell Production

Appendicular skeleton

(126 bones) includes bones of the limbs and the pectoral and pelvic girdles that attach the limbs to the axial skeleton.

Axial skeleton

(80 bones) consists of the bones of the skull, hyoid, sternum, rib cage, vertebral column, sacrum, and coccyx.

Metaphysis

a narrow zone that connects the diaphysis to the epiphyses. The epiphyseal plate, a thin layer of hyaline cartilage more commonly called the growth plate, is important for growth in the length of bones.

Medullary cavity

within the shaft of a long bone is a cavity where bone marrow is located. In childhood, the medullary cavity is filled with red bone marrow but as we age, fat accumulates within the red marrow transforming it to yellow bone marrow.

Chemical composition of bone

1. Organic Osteoid = roughly 1/3 of the weight of bone is contributed by collagen fibers. Collagen fibers are strong and flexible, but if they are compressed, they bend. 2. Inorganic Hydroxyapatites = mineral salts account for almost 2/3 of the weight of bone. Calcium phosphate interacts with calcium hydroxide to form crystals of hydroxyapatite. As they form, these crystals incorporate other calcium salts, such as calcium carbonate, and ions such as sodium, magnesium, and fluoride. By combining the hydroxyapatite with the collagen fibers, a strong, somewhat flexible, material is produced. Furthermore, this protein-crystal combination is highly resistant to shattering. In fact, bone is far superior to concrete and is more in par with steel-reinforced concrete.

More info on microscopic anatomy of compact bone.

1. Osteon = the basic structural and functional unit of bone consisting of bone cells organized around a central canal and separated by concentric lamellae. Osteons form cylindrical-shaped rods that run parallel to the axis of the bone to protect against compression forces. 2. Central canal = also known as the Haversian canal, runs parallel to the axis of bone and are located in the middle of each osteon. Each central canal possesses an artery and vein, lymph vessel, and nerve. 3. Perforating canals = also known as Volkmann's canals, are passageways that extend perpendicular to the axis of the bone and connect the central canals of adjacent osteons. 4. Lamellae = nested, concentric rings of matrix surrounding the central canal. 5. Lacunae = mature bones cells, called osteocytes, are trapped within an open space called a lacuna or "little lake". Osteocytes cannot divide and therefore each lacuna contains only one osteocyte. 6. Canaliculi = processes of the osteocytes extend into narrow crevices, called canaliculi, that penetrate the lamellae and connect the lacunae to the central canal

Microscopic anatomy of spongy bone

1. Spongy bone is also called cancellous bone or trabecular bone. 2. Spongy bone consists of an open network of struts and plates called trabeculae that resemble a latticework with red bone marrow filling in the spaces between. The spongy bone is then covered by a thin layer of compact bone and articular cartilage. 3. Within flat bones, spongy bone is sandwiched between two layers of compact bone forming diploe.

Fractures and Bone Repair

A. A fracture is a crack or a break in a bone. 1. Fractures are classified on the basis of: a. Whether the bone penetrates the skin. i. Simple (closed) =bone breaks cleanly, but does not penetrate the skin. ii. Compound (open) =broken ends of bone protrude through the tissue and skin. b. Orientation of the break. i. Transverse=break occurs perpendicular to the long axis of a bone. ii. Linear=breaks parallel to the long axis of the bone. c. Position of the bone ends after the fracture. i. Non-displaced=the bone ends retain their position. ii. Displaced=the bone end are out of normal alignment.

Calcium Homeostasis

A. Bone is constantly undergoing deposition and resorption in a process known as remodeling. B. Coordinated activity by osteoblasts and osteoclasts regulates both processes. 1. Bone deposition occurs where bone is injured or added bone strength is needed and is accomplished by osteoblasts. Bands of new matrix deposited in the area are referred to as an osteoid seam. 2. Bone resorption (also known as osteolysis) is accomplished by osteoclasts. Osteoclasts secrete lysosomal enzymes that digest the organic matrix and then secrete metabolic acids that convert calcium salts into soluble forms. C. Remodeling is under negative feedback hormonal control.

Formation and Growth Bone

A. FORMATION OF BONE: The formation of bone, called osteogenesis or ossification, begins during embryonic development. Two types of osteogenesis occur in the embryo: 1. Intramembraneous Ossification 2. Endochondral Ossification

Anatomy of Bones

A. Long bones are designed to transmit forces along the shaft and have a rich blood supply. 1. Diaphysis 2. Epiphyses 3. Metaphysis 4. Medullary Cavity 5. Membranes associated with bone 6. Nutrient Foramen 7. Metaphyseal artery and vein 8. Articular cartilage

GROWTH OF BONE

The growth of bone occurs by two primary processes as well 1. Longitudinal Growth 2. Appositional Growth

HYPOCALCEMIA

When blood calcium levels are too low (below 8.5 mg/dL), the chief cells of the parathyroid gland secretes parathyroid hormone, or PTH. PTH has three effects all leading to a increases in blood calcium levels: a. Bone response - stimulates osteoclasts so they accelerate the erosion of bone matrix which leads to the release of stored calcium ions into the blood. b. Intestinal response - PTH enhances the calcium-absorbing effects of calcitriol on the intestine. As a result the rate of intestinal calcium absorption increases. c. Kidney response - PTH increases the production of the hormone calcitriol, which is continuously secreted by the kidneys at low levels. This hormone in turn stimulates calcium reabsorption at the kidney tubules.

Bone markings

a. Depressions and openings allowing blood vessels and nerves to pass i. Fossa = a shallow depression or recess in the surface of a bone ii. Fissure = a narrow, slit-like opening or an elongated cleft or gap iii. Foramen = round or oval opening through the bone iv. Canal or meatus = a large passageway through the a bone v. Sulcus or groove = a furrow or narrow trough in a bone vi. Sinus = a chamber within a bone filled with air and lined with a mucous membrane. b. Projections that are sites for muscle and ligament attachment i. Tuberosity= large, round or rough projection that may cover a broad area ii. Crest=narrow ridge of bone; usually prominent iii. Trochanter=very large, irregularly shaped projection iv. Line=narrow ridges of bone; less prominent than a crest v. Tubercle=small, rounded projection vi. Epicondyle=raised area above a condyle vii. Spine=sharp, slender, and often pointed process c. Projections that form joints i. Head=expanded proximal end of a bone carried on a narrow neck ii. Facet=smooth, flat articular surface iii. Condyle=smooth, rounded articular surface iv. Ramus=arm-like bar of a bone

Microscopic anatomy of compact bone.

- Osteon - Central canal - Performing canals - Lamellae - Lacunae - Canaliculi

HYPERCALCEMIA

- When blood calcium levels are too high (above 11 mg/dL), the clear cells of the thyroid gland secrete calcitonin. Calcitonin has three effects all leading to a drop in blood calcium levels: a. Bone response = calcitonin inhibits osteoclasts but does not affect osteoblasts so that they continue to deposit calcium ions into the matrix of bone. b. Intestinal response = calcitonin decreases the rate of calcium absorption from foods in the digestive tract. c. Kidney response = calcitonin inhibits the absorption of calcium in urine so that more calcium is excreted from the body.

Metaphyseal artery and metaphyseal vein

carry blood to and from the area of the metaphysis and to the epiphysis.

Articular cartilage

covers portions of the epiphysis that articulate with other bones. The cartilage is avascular, hyaline cartilage. It relies primarily on diffusion from the synovial fluid to obtain oxygen and nutrients and to eliminate wastes.

Epiphyses

ends of the bones composed primarily of spongy bone surrounded by a thin layer of compact bone. a. Proximal epiphyses=end closest to the origin of attachment. b. Distal epiphyses=end furthers from the origin of attachment.

Irregular bones

have complex shapes with short, flat, notched, or ridged surfaces. The spinal vertebrae, the bones of the pelvis, and facial bones (mandible or zygomatic for example) are irregular bones.

Osteoblasts

immature bone cells located on the surface of bone; build new bone matrix in a process called osteogenesis, or ossification. Osteoblasts make and release the proteins and other organic components of the matrix. Before calcium salts are deposited, this organic matrix is called osteoid. When osteoblasts have become completely surrounded by bone matrix and trapped within a lacuna, they transform into osteocytes.

Flat bones

thin, roughly parallel surfaces. Flat bones form the roof of the skull, sternum, the ribs, and the scapulae. They provide protection from underlying soft tissues and offer an extensive surface for the attachment of skeletal muscles. Among the flat bones are also the sutural bones (known as Wormian bones). These are small, flat, irregularly shaped bones between the flat bones of the skull.

Intramembranous Ossification

a. Formation of bones without a cartilage model. Typical in flat bones of the skull, mandible, clavicles, and patella. Begins approximately 8 weeks after fertilization. b. Mesenchyme cells differentiate into osteoblasts within fibrous connective tissues. This type of ossification normally occurs in the deeper layers of the dermis or in the connective tissues of tendons. c. Steps of intramembranous ossification: i. Formation of bone matrix within fibrous membrane: 1) Mesenchymal cells cluster and secrete organic components of the matrix. The location of this activity is the ossification center. 2) The resulting osteoid mineralizes and the mesenchymal cells differentiate into osteoblasts. 3) As ossification proceeds, the osteoblasts get trapped within lacunae and differentiate into osteocytes. ii. Formation of woven bone and periosteum: 1) Osteoid accumulates, fuses together forming struts called trabeculae, or spicules, around blood vessels. 2) The overall structure is similar to spongy bone. iii. Formation of compact bone plate: 1) Initially, the intramembranous bone consists of spongy bone only. 2) Subsequent remodeling around trapped blood vessels can produce osteons typical of compact bone. 3) As the rate of growth slows at the surface, the connective tissue around the bone becomes organized into the fibrous layer of the periosteum.

Endochondral Ossification

a. Formation of most bones using a hyaline cartilage model. Begins approximately 6 weeks after fertilization. b. Hyaline cartilage does not turn into bone instead it is broken down as ossification occurs. c. Steps of endochondral ossification: i. Cavitation of hyaline shaft: (picture #1 and #2 in the diagram) 1) Chondrocytes within the shaft hypertrophy (enlarge) as the surrounding matrix begins to calcify. 2) The impermeable matrix causes chondrocytes to die from lack of nutrients leaving the matrix that starts to deteriorate (cavitate). 3) Blood vessels grow around the edges of the cartilage. 4) The cells of the perichondrium convert to osteoblasts producing a superficial layer of bone sometimes called the bony collar. ii. Invasion of internal cavities: (picture #3 in the diagram) 1) Blood vessels penetrate the cartilage and invade the central region. This area within the shaft of hyaline cartilage is called the primary ossification center. 2) Migrating with the blood vessels are fibroblasts (which differentiate into osteoblasts), lymph vessels, nerve fibers, red marrow elements. Collectively, these are called the periosteal bud. 3) The osteoblasts secrete osteoid around remaining fragments of hyaline, forming trabeculae, or spongy bone. iii. Formation of the Medullary cavity: (picture #4 in the diagram) 1) As the primary ossification center enlarges, osteoclasts break down newly formed spongy bone and opens up a medullary cavity in the center of the diaphysis. 2) The osseous tissue of the outer shaft becomes thicker forming compact bone. iv. Formation of epiphyses: (picture #5 in the diagram) 1) Secondary ossification centers appear in the area at the opposite ends of the bone. The cartilage in the epiphyses calcifies and deteriorates, forming cavities that allow entry of a periosteal bud. 2) Soon the epiphyses are filled with spongy bone. The spongy bone is NOT broken down during the remodeling process

Longitudinal Growth (length)

a. Hyaline cartilage cells form tall columns at the epiphyseal plate (or growth plate) and within the articular cartilage. b. The cells at the top of the stack divide quickly (forming a zone of proliferation) increasing the thickness of the epiphyseal plates and causing the entire long bone to lengthen. c. Older chondrocytes closer to the shaft enlarge. This area is called the zone of maturation and hypertrophy. d. The matrix surrounding the chondrocytes becomes calcified resulting in the death of the chondrocytes and deterioration of the cartilage called the zone of calcification. e. Osteoblasts in the medullary cavity then ossify the cartilage spicules, forming spongy bone within the zone of ossification. f. The hyaline cartilage at the epiphyseal plate is eventually replaced entirely by bone. Once completely replaced with bone, the epiphyseal plate is now called the epiphyseal line. This typically occurs in the person's early twenties and as a result the person stops growing in height.

Appositional Growth (width or diameter)

a. Osteoprogenitor cells beneath the periosteum differentiate into osteoblasts and form new osteons on the external bone surface. b. While bone is being added to the outer surface through appositional growth, osteoclasts are removing and recycling lamellae at the inner surface. As a result, the medullary cavity gradually enlarges as the bone increases in diameter. c. Appositional growth is important in increasing the diameter of existing bones but it does not form the original bone.

Membranes associated with bone

a. Periosteum=outermost covering of bone made primarily of dense irregular tissue and held on by Sharpey's fibers (collagen). b. Endosteum=internal membrane of bone made of connective tissue. Also lines the many canals that pass through bone to supply blood and nerves to the bone.

Sesamoid bones

are generally small, flat, and shaped somewhat like a sesame seed. They develop inside of tendons and are most commonly located near joints at the knees, the hands, and the feet. Everyone has sesamoid patellae, or kneecaps, but individuals vary in the location and abundance of other sesamoid bones. This variation, among others, accounts for disparities in the total number of bones in the skeleton.

Long bones

are relatively long and slender. They are located in the arm, forearm, thigh, lower leg, palms, soles, fingers and toes. The femur, the long bone of the thigh, is the largest and heaviest bone in the body.

Nutrient foramen

in order for bones to grow and be maintained, they require an extensive blood supply. The nutrient foramen is a tunnel that penetrates the diaphysis and provides access for the blood vessels into the shaft of the bone. a. Nutrient artery = transports oxygenated, nutrient-rich blood to the bone. b. Nutrient vein = transports deoxygenated, waste-laden blood from the bone.

Diaphysis

long tubular shaft that forms the axis of a typical long bone; the walls of the shaft are made primarily of compact bone.

Osteocytes

mature bone cells that maintain the protein and mineral content of the surrounding matrix through the turnover of matrix components.

Short bones

small and boxy. Examples of short bones include bones of the wrist (carpals) and bones of the ankles (tarsals).