Something

var tbl = document.getElementById(tableName);
tbl.tBodies[0].appendChild(document.createElement('tr'));		  
var rowsNum = theTable.tBodies[0].rows.length;

// Alt Name
var td = document.createElement('td');
td.class = 'Cell_Def';
td.width = '30%';
td.appendChild(document.createTextNode('ALT NAME'));
tbl.tBodies[0].rows[rowsNum].appendChild(td);