What are 4 fxns that bones provide?
1. structural integrity (support)
2. movement of limbs
3. storage of calcium and phosphate
What are the 2 main compositions of bone? What cells are responsible for producing them?
1. type 1 collagen (organic matrix) (30%)
2. calcium phosphate (hydroxyapatite-inorganic matrix) (68%)
collectively referred to as matrix of bone. comprises 98% of bone
cells within bone are responsible for their production: osteocytes and osteoblasts (2%)
What 4 processes occur in bone? (give names and definitions)
1. growth: begins in fetus and ceases at end of pubery
2. modeling: altering structure of bone in response to mechanical loads placed on it
3. remodeling: repair of micro-damage
4. repair: occurs when trauma damages bone
modeling and remodelling occur continuously (even after growth phase)
True or false: Bone is a very metabolically active tissue.
True. Although it is composed of only 2% of cells, these cells are highly metabolically active. Bone receives more blood flow than resting skeletal muscle and kidney
Where to osteoblasts originate? Where are they located? What is their fxn?
1. mesenchymal stem cells
2. surface of bone (periosteum and endosteum)
3. formation of bone
Where to osteocytes originate? Where are they located? What is their fxn?
1. matured osteoblasts
2. located within bone (in mineralized portions)
3. mechanotransduction: sensing mechanical strain and sending signals to osteoblasts to take action. also fxn in clacium and phosphate transport
What are the origins of periosteal and endosteal cells? Where are they located? What is their fxn?
1. originate from osteoblasts
2. periosteum and endosteum
3. regulate movement of calcium and phosphate into and out of bone
What are the origins of osteoclasts? Where are they located? What is their fxn?
1. hematopoietic stem cells (specialized macrophages)
2. surfaces of bone: periosteum and endoteum.
3. resorption of bone. use H+ to break down mineralized portions and proteases to degrade organic portions
Mesenchymal stem cells (osteoprogenitor cells) develop into preosteoblasts which then develop into osteoblasts. What 2 hormones stimulate the production of osteoblasts from preosteoblasts?
Vitamin D and PTH. both fxn to increase blood Ca2+. since players in Ca2+ homeostasis, it makes sense that they would lead to development of more active cells that are responsible for Ca2+ homeostasis in bone
Describe the precursors that lead to formation of osteoclasts (starts with hematopoietic stem cells)
1. hematopoietic stem cells
2. osteoclast precursors
3. mononuclear osteoclasts
4. multinuclear osteoclasts
What 2 factors can osteoblasts release to stimulate osteoclast formation and where in the pathway do they act?
osteoblasts release M-CSF (macrophage colony stimulating factor) and stimulates formation of osteoclast precursors from hematopoietic stem cells. M-CSF acts in concert with RANKL (Receptor Activator of NF-KB Ligand) which is the ligand for RANK. RANK is a receptor on osteoclasts precursor membranes and induces osteoclastgenesis
see pg 12 of course notes (did not place here bc contains info that is in future cards)
What hormone is the primary endocrine regulator of bone remodeling in adults? How does it stimulate osteoblasts and osteoclasts?
1. PTH (parathyroid hormone)
2. stimulates preosteoblasts to turn into osteoblasts. only osteoblasts express receptors for PTH. Therefore, PTH directly stimulates osteoblast activity and indirectly stimulates osteoclast activity through release of M-CSF and RANKL from osteoblasts. M-CSF and RANKL stimulate osteoclast formation
How can PTH stimulate bone formation and resorption? What hormone/vitamin is needed for PTH to have its effects and what happens without it?
1. intermittent administration of low doses of PTH promotes osteoblast survivla and bone anabolic fxns, incrases bone density, and reduces risk of fx. sustained, elevated levels of PTH shift balance to a relative increase in osteoclast activity-increase in bone turnover and reduction in bone density. so, lower basal levels cause bone fomration and higher levels (such as levels seen when plasma Ca2+ drops, PTH causes bone resorption and mobilization of Ca2+ from bone.
2. vitamin D. without it, normal levels of Ca2+ are less effective at suppressing PTH secretion and increased bone resorption results
What do osteoblasts produce to form bone?
produce collagen (predominantly type 1). collagen type 1 forms fibrils and fibers. osteoblasts then mineralize collagen with calcium phosphate salt (hydroxyapatite). crystals lodge in the spaces in and around collagen molecules and are oriented along the length of fibers. mixture of fibers and mineral gives bone its great mechanical strength
What factor allows for mineralization of collagen type 1 (secreted into matrix by osteoblasts) through formation of Pi (inorganic phospate) from PPi (pyrophospate)? Why is Pi produced? What portion of the osteoblast is it found?
TNAP hydrolyzes PPi to form Pi. Pi is neded for formation of hydroxyapatite. it is located on the cell surface of osteoblasts
Osteoblasts mineralize bone matrix by promoting ______ crystal formation and growth in the interior membrane-limited matrix vesicles (MV's). and by precipitating the crystals onto the collagenout ECM.
hydroxyapatite. Do this through TNAP
please see pg 17 of self study to understand this
True or false: Although calcium phospate ions are present in most body fluids (including bone) in high enough concentrations to lead to precipitation of calcium salts, this does not occur because of the presence of agents that inhibit this reaction.
True. please see pg 17 of notes to understand this concept
What is the most important factor osteoblasts produce to promote deposition of salts in the oranic matrix of bone?
What role does PPi play in formation of hydroxyapatite? NPP1 (ectoplasmic PC-1/nucleotide pyrophosphatase/phosphodiesterase-1)? ANK (ankylosis protein?
1. PPi inhibits matrix mineralization. it is formed by NPP-1 located in ECM from ATP and ADP. ANK is a transporter on osteoblasts that passes PPi into the ECM-inhibits hydroxyapatite formation
please see pg 17 of notes
What role does osteopontin (OPN) play in mineralization? What is thought to control its expression and under what conditions does its expression increase?
1. OPN is a protein that when its high number of serine residues are phosphorylated generate a great deal of negative charge. negative charge allows it to strongly bind calcium atoms available at surface of hydroxyapatite and acts to inhibit further mineralization
2. expression is not well understood but is under control of vitamin D and expression is increase with hypocalcemia and hypophosphatemia (makes sense because cannot produce hydroxyapatite if do not have enough of the minerals that compose it)
see pg 17 of notes
What is bone sialo protein (BSP) and what role does it play in mineralization of bone?
is thought to serve as a part of the collagen matrix and sesrve as a nidus (place where something is formed or deposited) for crystalization of hydroxyapatite
What is hypophosphatasia? What enzyme is mutated in this disease? How severe is this disease? How is it diagnosed biochemically?
due to deficiency in TNAP enzyme that results in less bone mineralization. Severity is variable and is modulated by nauture of TNAP mutation. range from complete absence of bone mineralization and stillbirth to spontaneous fractures and loss of decidual teeth in adult life. diagnosed by low levels of serum TNAP and accumulation of phosphocompounds such as PPi
What is the fxn of osteoclasts? How do they attach do bone? What is the ruffled border? What does it secrete?
1. responsible for bone resorption by destroying the matrix and freeing up calcium phosphate salts
2. integrin receptors. form a tight seal to precent leakage of acidic contents to surrounding tissues
3. invaginated plasma membrane structure interfaced with bone. acts as a huge lysozyme by secreting HCl onto the surface. also secretes proteases
What is the source of acid that osteoclasts secrete? What pumps are used to secrete it?
1. pumps protons via protons produced from activity of carbonic anhydrase: CO2 + H2O = H2CO3 = H+ + HCO3-
2. pumps HCl using an ATP dependent proton pump and a chloride channel (similar mechanism used to produce stomach acid). HCO3- is pumped out of cell via Chloride carbonate exchanger
What protease is released from osteoclasts? What is done with the products?
1. capthesin K
2. products are taken into the cell and released in bone canaliculi that connect to the vascular compartment
What are the 2 fxns of osteocytes? Where are they located within bone? How do they communicate with other cells?
1. sensing mechanical load and contributing to Ca2+ and PO4 homeostasis by makig possible the movement of these electrolytes into and out of bone
2. within mineralized portions of bone. are matured osteoblasts
3. send cytoplasmic processes into canaliculi. form a functional syncetium via gap junctions btwn one syncytial process and the next cell body they are in contact with. osteocyte syncytial processes also extend to osteblasts on surface of bone (how they stimulate their activity). strain due to mechanical load on bone causes movement of bone fluid that is present in canaliculi that surrounds osteocytes. fluid movement is transduced into an electrical signal that alters fxn of osteocytes and to osteoblasts to which they are functionally connected. repeated strain on boan causes bone remodeling.
fxn in calcium homestasis through osteocytic osteolysis: when stimulated directly by PTH (during low Ca2+ concentrations) stimulated osteocytes free up Ca2+ from their local micro-environment and active pumps move calcium into plasma compartment. only limited amnt of calcium is freed but does serve to maintain a normal Ca2+ concentration in the face of disturbances that would reduce it
True or false: Long bones grow in both length and diameter.
True or false: Growth in length in long bones takes place as the cartilage ends of the bone (epipiphyses) continues growing and secondary centers of ossification appear.
In endochondral ossification of long bones, proliferation of chondrocytes occurs at the (epiphyseal/diaphyseal) side and replacement by bone occurs at the (epiphyseal/diaphyseal) side.
1. epiphyseal (in growth plate)
What 3 types of hormones is growth of bones stimulated by?
1. growth hormone (GH)
2. IGF-1 (insulin like growth factor)
3. sex hormones