Such changes may be the result of exposure of {HEAD ~ the cells} to {TAIL ~ environmental toxins} 
to
Such changes may be the result of exposure of { HEAD ~ the cells } to { TAIL ~ environmental toxins }
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL