Bone Flashcards
Bone
a type of connective tissue, with an ossified (by deposition of calcium) extracellular matrix (ECM), that supports the body, protects internal organs, forms blood cells in bone marrow, and acts as the body’s Ca2+ and PO43– ion reservoir.
Three major types of bone cells
Osteoblasts, Osteocytes, Osteoclasts
Osteoblasts function
Secrete the matrix which then hardens by calcification, trapping the
differentiating cells now called osteocytes in individual lacunae.
Secrete osteoid rich in Type 1 collagen
Osteocytes function
Maintain the calcified matrix and receive nutrients from vessels in the central canals of the osteons via tiny channels called canaliculi that connect the lacunae.
Osteoclasts function
digestion and reabsorption of bone matrix in large resorption cavities (Howship lacunae) on the matrix surface.
Two types of connective tissue covering bone that contain osteogenic cells
Periosteum and Endosteum
Periosteum
A type of connective tissue covering bone that contains osteogenic cells.
A dense connective tissue. Bone is vascularized by vessels from the periosteum.
In the outer layer, bundles of collagen called perforating (or Sharpey)
fibers bind the periosteum to the bone.
The periosteum’s inner layer is more cellular and includes osteoblasts, and mesenchymal stem cells referred to as osteoprogenitor cells.
Endosteum
A type of connective tissue covering bone that contains osteogenic cells.
Covers all trabeculae around the marrow cavities.
Contains osteoprogenitor cells and osteoblasts.
Osteoblasts misc.
Osteoblasts originate from mesenchymal osteoprogenitor cells and
produce the organic components of bone matrix, including type I collagen fibers.
Osteoblasts are located exclusively at the surfaces of bone matrix as a
single layer of cuboidal cells. They produce a layer of collagen-rich, uncalcified material called osteoid between the osteoblast layer and the bone surface.
Important components of osteoid
Produced from osteoblasts
Type I collagen, the protein
osteocalcin, and chondroitin sulfate, a GAG that forms part of proteoglycans. Chondroitin sulfate provides most of the resistance to compression in cartilage.
Osteoclasts misc.
Large multinucleated cells which are derived by the fusion of several blood-derived monocytes.
Inorganic matter
One of the two components of bone matrix.
About 50% of the dry weight of bone matrix is inorganic.
Calcium hydroxyapatite is the most abundant
inorganic compound.
The surface of hydroxyapatite crystals is hydrated, facilitating the exchange of ions within body fluids.
Organic matter
One of the two components of bone matrix.
90% of calcified matrix is Type I collagen, but also includes small proteoglycans and multiadhesive glycoproteins.
Calcium-binding proteins, notably osteocalcin, promote calcification of the matrix.
Types of bone
Woven bone, Lamellar bone, Compact (Cortical) bone, and Cancellous (Trabecular) bone
Woven bone
Newly calcified
Locations: Developing and growing bones; hard callus of bone fractures
Synonyms: Immature bone; primary bone; bundle bone
Lamellar bone
Remodeled from woven bone
Locations: All normal regions of adult bone
Synonyms: Mature bone; secondary bone, ossified
Compact (cortical) bone
~80% of all lamellar bone
Locations: Thick, outer region (beneath periosteum) of bones
Synonyms: Cortical bone
Cancellous (trabecular) bone
~20% of all lamellar bone
Locations: Inner region of bones, adjacent to marrow cavities
Synonyms: Spongy bone; trabecular bone; medullary bone
Two types of organization for compact bone and cancellous bone based on the stage of development
Woven bone: newly formed, first bone tissue to appear in embryonic development and in fracture repair, weaker than lamellar bone because of lower mineral content.
Lamellar bone: Most bone in adults, whether compact or cancellous, is organized
as lamellar bone, characterized by multiple layers or lamellae of calcified matrix; like the orientation of wood fibers in plywood; this type of bone is fully ossified
Osteons
constitute most of the compact bone. An
osteon (or Haversian system) refers to the
complex of concentric lamellae
What is the epiphyses (the bulbous ends) in long bones composed of?
Composed of cancellous bone covered by a thin layer of compact (cortical) bone
What is the diaphysis (the cylindrical part) in long bones composed of?
Almost totally dense compact (cortical) bone with a thin region of cancellous bone on the inner surface around the central marrow cavity
Two Processes of Osteogenesis
Intramembranous ossification and Endochondral ossification
Intramembranous ossification
Osteoblasts
differentiate directly from mesenchyme and begin
secreting osteoid. There is no cartilage precursor.
Endochondral ossification
A preexisting matrix of hyaline cartilage is eroded and invaded by
osteoblasts, which then begin osteoid production.
Longitudinal growth at epiphyseal plate
Appositional growth around diaphysis
Two Patterns of Osteogenesis
Longitudinal growth and Appositional growth
Longitudinal growth
in long bones is
regulated by the epiphyseal plate, which
separates the epiphysis from the diaphysis
(shaft).
Appositional growth
Occurs by formation of
a periosteal bone collar around the diaphysis.
Growth in the circumference of long bones does not involve endochondral
ossification. It occurs through osteoblasts developing from osteoprogenitor cells in
the periosteum by a process of appositional growth which begins with formation of the bone collar on the cartilaginous diaphysis.
Fracture Repair
Repair of a fractured bone uses the cells and mechanisms already in place for bone growth and
remodeling.
(1) Blood vessels torn within the fracture release blood that clots to produce a large fracture hematoma.
(2) The hematoma is removed by macrophages; a matrix of damaged, cell-free bone is resorbed by osteoclasts; the periosteum and the endosteum at the fracture site respond with intense proliferation and produce by a soft fibrocartilage-like mass called procallus tissue.
(3) The procallus is invaded by regenerating blood vessels and proliferating osteoblasts. In the next
few weeks the fibrocartilage is gradually replaced by woven bone which forms a hard callus throughout the original area of fracture.
(4) The woven bone is then remodeled as compact and cancellous bone and fully functional vasculature is reestablished. Stresses imposed on the bone during repair and gradual return to
activity help to remodel the bone callus.
Primary bone tumor
Cancer originating directly from bone cells (a primary
bone tumor) is fairly uncommon (0.5% of all cancer deaths), although a cancer called osteosarcoma can arise in osteoprogenitor cells.
Secondary metastatic tumors
The skeleton is often the site of secondary,
metastatic tumors, when cancer cells move into
bones via small blood or lymphatic vessels from
malignancies in other organs, such as the breast, lung, prostate gland, kidney, or thyroid gland.
What causes rickets
Calcium deficiency in children can lead to rickets, a disease in which the bone matrix does not calcify normally and the epiphyseal
plate can become distorted by the normal strains of body weight and muscular activity.
Osteogenesis imperfecta
“brittle bone disease,” refers to a group of related congenital disorders in which the osteoblasts
produce deficient amounts of Type I collagen or defective Type I collagen due to genetic mutations. Such defects lead to a
spectrum of disorders, all characterized by significant fragility of the bones.
Bone is a composite of
It is a composite of proteinaceous fibers and mineral.
Howship lacunae are formed by
Osteoclasts
What part of a long bone, such as the femur, is composed of cancellous bone covered by a thin layer of compact cortical bone?
Epiphysis
Hydroxyapatite is the mineral component of bone. It is composed primarily of which ions?
Calcium and phosphate
Endochondral ossification is not typical of these bones.
Flat bones of the skull, such as the frontal.
What is a part of the mature haversian system?
Nerves, Central canal, Osteocytes
Acromegaly is caused by
An increase in levels of growth hormone in adults.
