Chapter 7

Osteogenic Cell

Adult stem cells responsible for generating osteoblasts, which give rise to osteocytes. Separate stem cells give rise to osteoclasts.

Hematoma Formation

As a bone is broken and blood vessels are severed, blood at the site of the damage starts to form a clot called a hematoma. Then acts as scaffolding for further repair of the tissue.

Osteoid Deposition

As osteoblasts release the clumps of bone matrix called osteoids into the surrounding mesenchyme, they become trapped in their own matrix and differentiate into osteocytes. Osteoblasts are responsible for the production of osteoids during intramembranous ossification.

Intramembranous Ossification

Bone tissue is formed directly from the mesenchyme. UNlike endochondral ossification, it does not have an intermediate step involving cartilage. Mesenchyme develops into bone. In flat bones of the skull and clavicles.

Osteoclast

Bone-dissolving cells found on bone surface -Osteoclasts develop from same bone marrow stem cells that give rise to blood cells (different origin from other bone cells) -Very large cells formed from fusion of several stem cells -Have multiple nuclei in each cell -Ruffled border (large surface area) faces bone -Cells often reside in resorption bays (pits in bone surface) -Dissolving bone is part of bone remodeling

Bony Collar (Endo Ossif)

Bony collar forms around the diaphysis of the bone (the location of the primary ossification center) shortly after formation of the primary ossification center. Provides stability during the early stages of ossification.

Matrix of osseous tissue

By dry weight, about one-third organic and two-thirds inorganic matter. Is a composite material—a combination of a ceramic and a polymer -Hydroxyapatite and other minerals are the ceramic and collagen (protein) is the polymer -Ceramic portion allows the bone to support body weight without sagging -Rickets is a disease caused by mineral deficiency and resulting in soft, deformed bones -Polymer (protein) gives some flexibility -Osteogenesis imperfecta (brittle bone disease) results from a defect in collagen deposition

Perforating Canal

Channels running perpendicular to central canals within an osteon. They connect the nutrient foramina to central canals.

Concentric Vs Circumferential Lamellae

Concentric lamellae surround the central atom of an osteon. The circumferential lamellae surround the entire diaphysis of a bone, running all the way around its periphery. Circumferential surround multiple osteon.

Spongy Bone

Consists of: Lattice of bone covered with endosteum --Slivers of bone called spicules --Thin plates of bone called trabeculae Spaces filled with red bone marrow. Few osteons and no central canals --All osteocytes close to bone marrow Provides strength with minimal weight --Trabeculae develop along bone's lines of stress

Hypocalcemia

Deficient calcium in blood. Results in hyperexcitability in nervous tissue and muscle tissue. Caused by vitamin D deficiency, diarrhea, thyroid tumors, underactive parathyroid glands.

Calicium Homeostasis

Depends on a balance between dietary intake, urinary and fecal losses, and exchanges between osseous tissue. Calcitriol, most active form of Vitamin D. Mainly increases calcium absorption (concentration) through intestinal absorption- -From the diet to the blood to bone. UV radiation or directly from Milk. Calcitonin, secreted by paraphylicular of thyroid gland when blood calcium levels rise too high. Lowers blood calcium concentration in two ways: -Inhibits osteoclasts thereby reducing bone resorption -Stimulates osteoblasts to deposit calcium into bone Allows pregnant women to retain calcium.

Long Bone

Diaphysis- Shaft of a long bone. Epiphysis- Enlarged ends of a long bone. Strengthens joint and anchor ligaments and tendons. Distal and proximal ends. Medullary cavity (marrow cavity)—space in the diaphysis of a long bone that contains bone marrow

Bone Elongation

Epiphyseal plate—cartilage transitions to bone Functions as growth zone where bone elongates Has typical hyaline cartilage in the middle with transition zones on each side where cartilage is replaced by bone Metaphysis is zone of transition facing the marrow cavity This is interstitial growth—growth from within. Bone elongation is a result of cartilage growth within the epiphyseal plate Epiphyses close when cartilage is gone—epiphyseal line of spongy bone marks site of former epiphyseal plate --Lengthwise growth is finished --Occurs at different ages in different bones

Hypercalcemia

Excessive calcium levels. Makes ion channels less responsive and thus nerves and muscles are less excitable. Can cause emotional disturbance, muscle weakness, lethargy, and cardiac arrest. Rarely occurs

Osteocyte

Former osteoblasts that have become trapped in the matrix they deposited Lacunae- cavities where osteocytes reside. Canaliculi- channels that connect lacunae. processes of osteocytes reach into canaliculi and contact processes of neighboring cells Gap junctions allow for passage of nutrients, wastes, signals Some osteocytes reabsorb bone matrix while others deposit it Act as strain sensors—when stressed, produce biochemical signals that regulate bone remodeling (shape and density changes that are adaptive)

Chondrocyte

Forms when chondroblasts become trapped within the matrix and differentiate into chondrocytes. Located along the periphery of hyaline cartilage.

Osteoblasts

Responsible for producing clumps of bone matrix called Osteoids into the surrounding mesenchyme. They are located along the periphery of the tissue, and lay down matrix from the outside inward. Once trapped within the matrix, they become osteocytes.

Epiphyseal Plate

Growth plate- Area of hyaline cartilage that separates epiphyses and diaphyses of children's bones -Enables growth in length -Epiphyseal line- in adults, a bony scar that marks where growth plate used to be

Lamella

Layers of the bone matrix. Produced in series to give distinct layered appearance.

Parathyroid Gland

Located on the dorsal side of the thyroid gland. Often confused with the follicles of the thyroid gland and parafollicular cells. Parafollicular cells regulate blood calcium levels by releasing calcitonin.

Central Canal

Located within the center of each osteon and contains arteries, veins, and lymphatic vessels.

Healing of Fractures

Mostly intramembranous ossification Hematoma- is converted to granulation tissue. by invasion of cells and blood capillaries. Soft callus formation

Types of Bone fractures

Nondisplaced- bone stays in place. Displaced- bones separate and need more attention to heal. Comminuted- several pieces of bone broken Closed- bone is broken but not broken skin. Open- broken skin. Oblique- sideways

Endochondral ossification.

Occurs within a framework of hyaline cartilage. Mesenchyme first develops into hyaline cartilage. The cartilage is then systematically replaced by expanding bone matrix. Perpetually expanding cartilage persists well past childhood, allowing for expansion and growth of the skeleton. In all other bones beside flat bones of the skull and clavicles.

Matrix Cont'd Organic Vs Inorganic Components

Organic components -Synthesized by osteoblasts. Collagen is approximately 1/3 of of the mass of the tissue by weight. Inorganic Components -Crystallized calcium-phosphate salt (hydroxyapatite) makes up 85% -Calcium carbonate makes up 10% -Remaining include other minerals (fluoride, sodium, potassium, and magnesium)

Periosteum

Outer fibrous layer of collagen -Some fibers continuous with tendons -Perforating fibers—penetrate into bone matrix Inner osteogenic layer of bone-forming cells -Important to bone growth and healing of fractures

Parathyroid Hormone

Released from the parathyroid gland in response to low blood calcium levels.

Role of Calcitonin

Releases in response to elevated blood calcium levels (hypercalcemia). Functions to inhibit osteoblasts - which decreases bone resorption. Also for the stimulation of osteoblasts - which causes the formation of new bone matrix.

Interstitial Lamellae

Remains of older osteons located between new osteons. Located outside of the concentric lamellae of newer osteons.

Parathyroid Hormone

Secreted by parathyroid glands on posterior surface of thyroid -PTH released when calcium levels low in blood -PTH raises calcium blood level by four mechanisms Stimulates osteoblasts to secrete RANKL, thereby increasing osteoclast population and bone resorption. Promotes calcium reabsorption by kidneys, so less lost in urine Promotes the final step of calcitriol synthesis in the kidneys, enhancing calcium-raising effect of calcitriol Inhibits collagen synthesis by osteoblasts, inhibiting bone deposition

Canaliculus

Small tunnels through the matrix of bone tissue, in between individual lacunae.

Bone Marrow

Soft tissue occupying marrow cavities of long bones and small spaces of spongy bone. Red marrow (myeloid tissue) --Contains hemopoietic tissue—produces blood cells. --In nearly every bone in a child. --In adults, found in skull, vertebrae, ribs, sternum, part of pelvic girdle, and proximal heads of humerus and femur. Yellow marrow found in adults --Fatty marrow that does not produce blood. --Can transform back to red marrow in the event of chronic anemia.

Fractures and their Repairs

Stress fractures- break caused by abnormal trauma to a bone; a fall. Pathological Fracture- break in a bone weakened by disease. (such as bone cancer or osteoporosis) Usually caused by a stress that would not break a healthy bone. Fractures classified by structural characteristic. --Direction of fracture. --multiple pieces.

Endosteum

The internal surface of the medullary cavities and the surfaces of the spongy bone are lined with a layer of reticular connective tissue. -Has cells that dissolve osseous tissue and others that deposit it.

Ossification

The process by which a tissue, other than bone, is converted to bone tissue. Ex. Osteoblasts differentiate within embryonic tissue, as they produce matrix, they begin to convert the tissue to bone from the inside out.

MIneral Resorption

The release of minerals from bone tissue into the blood. Osteoclasts are responsible for the resorption of bone tissue, resulting in a decrease of bone density and an increase in blood calcium and phosphate levels. Occurs after damage and subsequent repairs, remodeling of bone during growth and development, and release of calcium from bone tissue in response to low levels of blood calcium.

Metaphysis

The transition between the epiphysis and the diaphysis of a long bone. Epiphyseal (growth) plates are located within these structures.