AS 440 - bone cells, bone formation, and fracture repair

what are the 4 major bone cells?

• osteoblasts

• osteocytes

• osteoclasts

• osteoprogenitor cells

what are osteoblasts?

main cells involved in ossification process

what are osteocytes?

osteoblasts trapped in the bone matrix (in spaces called lacunae), they become less active but serve important roles in bone remodeling, canaliculi (tunnels) connect adjacent lacunae

what are osteoclasts?

multinucleated cells present at bone surfaces actively undergoing resorption and are derived from immune cells and may be described as bone tissue macrophage (mobile)

what are osteoprogenitor cells?

stem cells that can become new osteoblasts if required such as when fracture repair is needed

what are haversian canals?

tiny, interconnecting longitudinal channels in bone tissue through which blood vessels, nerve fibers, and lymphatic vessels pass

where are haversian canals located?

at the center of the osteons, which run parallel with the long shaft bones within the compact bone

what canals are also connected to the Haversian canals and also contain blood vessels and nerves?

lateral canals called volkmann’s canals

what do bones consist of?

inorganic salts deposited within an organic matrix of collagen fibrils and glycoproteins

what is a general structure for the bone mineral consisting of hydroxyapatite crystals?

Ca₁₀ (PO₄)₆ (OH)₂

_________ can replace hydroxyl groups and in moderation will strengthen the bone but will weaken the structure with large amounts

fluoride

what type of collagen does bone contain?

type 1 collagen

what type of collagen does cartilage contain?

type 2 collagen

type 1 collagen in the bone is produced by?

osteoblasts and osteocytes

type 1 collagen contains the unique amino acid called?

hydroxyproline

what are the two types of marrow?

red and yellow

what is red marrow?

responsible for hematopoiesis (blood cell production) contains mature and immature blood cells, including RBC, WBC, and stem cells (pelvis, rib, spongy bone), these cells enter the circulation through osteonal and lateral canals (Haversian and Volkmann)

what is yellow marrow?

primarily adipocytes but can become red marrow if anemic conditions arise (long bone medullary space)

boen develops on a _____________

pre-existing connective tissue (transformation)

what is intramembranous ossification?

the process when bone formation occurs within connective tissue; this is generally how flat bones are formed and bone repair occurs

what is endochondral ossification?

when pre-existing cartilage is converted to bone; this occurs at growth plates of long bones

what is ectopic bone formation?

where under pathological conditions, connective tissues not normally ossified can be converted to bone

bones are formed from a _________ tissue matrix that undergoes ossification (flat bones of the skull)

connective

what are the steps to intramembranous ossification?

1. osteoprogenitor cells are differentiated into osteoblasts

2. osteoblasts synthesize and release glycoproteins and collagen fibrils (forming extracellular type 1 collagen)

3. the longer and thicker type 1 collagen form osteoid and trabeculae, the collagen remains randomly interwoven and is often referred to as “woven bone”

4. collagen fibrils thicken and mineralize (calcify), osteoblasts that have become surrounded by the calcified bone develop into osteocytes

5. trapped osteocytes remain anatomically connected to osteoblasts and other osteocytes by canaliculi

6. the matrix mineral content increases (minerealization/calcification) and bone is formed

7. peripheral osteoblasts continue to synthesize bone, resulting in an increase in bone thickness, and formation of dense or compact bone

in endochondral ossification, it is the bone formation process where ______________ is replaced by bone

hyaline cartilage

chondrocytes become tremendously enlarged (hypertrophy) and secrete what?

type X (ten) collagen

what grows in length through endochondral ossification if the cartilage of the epiphyseal plate continues to grow?

long bones

what elongates through growth of epiphyseal cartilage, followed by replacement with bone on the diaphyseal side of the growth plate?

bone shaft

bone growth can only occur at what two specialized sites?

• on the surface, or periosteal and endosteal layers

• or at specialized epiphyseal plates

where are epiphyseal growth plates located?

between the head (epiphysis) and shaft (diaphysis) of long bones such as the femur

in endochondral bone formation, what occurs in the zone of growth?

• germinal

• proliferating

• palisading

in endochonral bone formation, what occurs in the zone of cartilage transformation?

• hypertrophy

• calcification

• degeneration

in endochonral bone formation, what occurs in the zone of ossification?

• vascular entry

• osteogenesis

what is the first step in endochondral ossification?

a hyaline cartilage model is produced by chondrocytes. it increases in length by interstitial growth, but also grows by appositional growth

what is the second step in endochondral ossification?

the connective tissue around the middle of the cartilage changes to periosteum, which is osteogenic and contains osteoblasts

what is the third step in endochondral ossification?

chondrocytes in mid region hypertrophy forming irregular cartilage plates; cells synthesize alkaline phosphatase; surrounding matrix calcifies

what is the fourth step in endochondral ossification?

chondrocytes die, matrix begins breaking down. blood vessels grow into region. fibroblasts/osteoprogenitor migrate with blood vessels, differentiate into osteoblasts; lay down osteoid on remaining spicules

what is the fifth step in endochondral ossification?

chondrocytes die; calcified matrix begins breaking down. other primitive cells enter via a new vasculature and give rise to marrow. endochondral bone formed on remaining spicules

what is the sixth step in endochondral ossification?

a secondary ossification center is established in the upper epiphysis.

__________ are instrumental in the bone resorption process

osteoclasts

as a new bone is formed at the periosteum, some of the deeper bone is resorbed to increase the diameter of?

the medullary, or marrow cavity

what are the types of bone fractures?

• displaced

• non-displaced

• complete

• incomplete

• linear

• transverse

• simple/closed

• compound/open

• greenstick

what are the four key steps to fracture repair?

1. hematoma formation

2. fibrocartilaginous callus formation

3. bony callus formation

4. bone remodeling

what results in the formation of a mass clotted blood (hematoma)?

the tearing of small and large blood vessels during a fracture

what happens when bone cells begin to die?

the site undergoes the inflammatory response (pain, swelling, redness, and loss of function/weakness)

what happens during fibrocartilaginous callus formation?

• capillaries infiltrate the hematoma and phagocytic cells remove debris

• fibroblasts and osteoblasts from the periosteum and endosteum migrate to the fracture site

• the fibroblasts release collagen to span the break, the osteoblasts begin spongy bone formation (intramembranous)

• fibrocartilaginous mass forms that fills and spans the fracture

what happens during bony callus formation?

• bone trabeculae begin to appear as osteoblasts convert the fibrocartilaginous callus into spongy bone or woven bone (intramembranous bone formation)

• this continues until the spongy bone spans the break and re-connect the two ends of the fractured bone

what happens in bone remodeling?

• excess materials comprising the bony callus are resorbed and replaced from the periosteal and the endosteal spaces

• dense compact bone is re-formed, inclusing alignment of the Haversian canals