Basic Science of Bone Cells

Osteoblasts

• Osteoblasts are never found individually; they form clusters of cells on the bone surface

• Osteoblasts are active cells with an eccentric nucleus that is on the opposite side of the bone surface

• A prominent and well-developed endoplasmic reticulum and Golgi apparatus are present in the cell

• After the osteoblast finishes secreting the extracellular matrix, the osteoblast becomes either a resting lining cell or an osteocyte

• Osteoblasts are formed from pluripotential mesenchymal stem cells.

• Osteoblasts are bone lining cells. • Osteoblasts produce matrix proteins (osteoid). • Osteoblasts produce alkaline phosphatase

(marker for bone formation). • Osteoblasts have receptors for a number of

hormones including:     Estrogen (nucleus)     Vitamin D3 (nucleus)     Parathyroid hormone (PTH) (cell surface)

Osteocytes

• Osteocytes are formed from osteoblasts that have become imbedded in the bone matrix.

• Osteocytes have fine cytoplasmic connections with one another.

• Osteocytes have receptors for PTH.

• Osteocytes respond to mechanical signals and send signals for bone remodeling

Osteoclasts

• Osteoclasts resorb bone and reside in the Howship lacunae (usually 1 to 5 osteoclasts are found

• There is a ruffled border where the osteoclast is in contact with the bone surface (sealing zone)

• The area in contact with the bone is called the subosteoclastic bone-resorbing compartment

• Osteoclasts contain abundant Golgi apparatuses, mitochondria, and lysosomal enzymes

• The osteoclast acidifies the bone-resorbing compartment by pumping protons across the ruffled border via enzyme carbonic anhydrase

• As a result of pumping of the protons, there is an acidic environment, lysosomal enzymes, and bone matrix to be resorbed

• The acidic environment dissolves the hydroxyapatite crystals, and the lysosomal enzymes degrade the matrix. Collagen is degraded by collagenase or cathepsins

• As the hydroxyapatite dissolves and the collagen is broken down, one can measure the collagen byproducts (hydroxyproline, pyridoxiline, and N-telopeptide).

• Osteoclasts are giant multinucleated cells. • Osteoclasts are formed from hematopoietic

monocyte stem cell precursor. • Osteoclasts resorb bone to form Howship

lacunae or resorption pits. • Osteoclasts are responsive to osteoblasts

through receptor activator of nuclear factor -kB ligand (RANKL).

• Osteoclasts have receptors for calcitonin (calcitonin causes osteoclasts to shrink)

Osteocytic osteolysis

• Osteocytes have receptors for PTH. When PTH levels increase, the osteocytes are signaled to mobilize calcium. The osteocytes mobilize poorly crystallized calcium salts without damaging the organic matrix of the bone. Osteocytes pump the calcium into the extracellular fluid. Simply stated, the osteocytes mobilize calcium quickly in response to signaling by PTH without degrading the bone structure

Osteoclast activation regulation

• Osteoclast activation is a complex process. Osteoclasts have a hematopoietic monocyte stem cell lineage. Osteoblasts signal osteoclasts to form and resorb bone. The osteoblast produces a molecule called RANKL that sits on the cell surface of the osteoblast. On the surface of the osteoclast precursor cell, there is a receptor called receptor activator of nuclear factor -kB (RANK).

• RANKL binds to RANK, and in the presence of macrophage-colony stimulating factor (M-CSF), there is transformation of the osteoclast precursors into active osteoclasts. Interleukin-6 (IL-6), which is produced by osteoblasts, signals osteoclasts to resorb bone. Osteoprotegerin (OPG) acts as a soluble decoy inhibitor, binds to RANKL, and does not allow RANKL to bind to RANK on the surface of the osteoclast precursor cell. The osteoclast precursor cell does not become an active osteoclast.

Factors that increase osteoclast activation include

• RANKL

• M-CSF

A factor that decrease osteoclast activation includes

OPG

Osteoblast activation and function

• Osteoblasts secrete bone matrix, but they also play an important role in regulation. In response to low calcium levels, the C cells of the parathyroid gland secrete PTH that binds to the osteoblast and directs the osteoblast to produce bone. In addition, the osteoblasts produce RANKL and IL-6. The RANKL binds to the RANK receptor on the osteoclast precursor cells, and the precursor cells form active osteoclasts. IL-6 directs the osteoclasts to resorb bone.

• The osteoblasts line the bone surface and protect the bone from the osteoclasts. Once stimulated by PTH, the osteoblasts contract and withdraw from the bone surface to allow the osteoclasts to attach to the bone. The osteoblasts secrete neutral proteases that degrade the surface osteoid. The osteoclasts then attach to the bone and resorb the bone matrix and mineral

Vitamin D

• Fat-soluble steroid hormone • Sources

Diet (fatty fish, fortified cereals, bread, and milk) Skin production; ultraviolet light converts 7-dehydrocholesterol to vitamin D3

• Liver hydroxylation to 25 hydroxyvitamin D3 • Kidney hydroxylation to 1,25 dihydroxyvitamin

D3 (rate limiting step in vitamin D3 production) • Active form is 1,25 dihydroxyvitamin D3 • Recommended daily intake is 400 IU to 800 IU

Functions

1. Increases resorption of phosphate in the kidney proximal tubule

2. Maintains normal extracellular concentrations of calcium and phosphorus

3. Regulates production of calcium binding protein needed for calcium absorption

4. Enhances mobilization of calcium stores in bone5. 1,25 dihydroxyvitamin D3 induces monoctye stem cells

to form osteoclasts6. 1,25 dihydroxyvitamin D3 tells osteoblasts to produce

RANKL thereby mobilizing osteoclasts

24,25 dihydroxyvitamin D3 influences growth plate chondrocyte maturation and potent mitogen in the growth plate proliferative zone

Parathyroid hormone (PTH)

• Low calcium levels stimulate PTH release from parathyroid glands through cAMP pathway.

• Increased calcium levels inhibit PTH secretion. • PTH binds to osteoblast receptors to increase bone

formation. • PTH signals osteoblasts to produce IL-6 that directs

osteoclast to resorb bone. • PTH decreases phosphorus resorption in the proximal

tubule. • PTH increases distal tubule reabsorption of calcium. • PTH stimulates 1 alpha hydroxylase activity (increase

1,25 dihydroxyvitamin D3).

Calcitonin

• Calcitonin is a hormone secreted by parafollicular cells of the thyroid gland.

• Calcitonin function is not understood in humans. • Functions

     Causes rapid dissolution of osteoclastic ruffled border     Inhibits osteoclastic bone resorption     Causes shrinkage of osteoclasts

• Calcitonin secretion increases as calcium levels rise. • Calcitonin causes decreased reabsorption of calcium

and phosphorus in the kidney