Sentence:  Its synthesis has not been demonstrated in cell-free systems , nor has its synthesis by systems with intact thyroid <WSD>cells</WSD> in vitro been unequivocally proven . 
Meaning of the word marked with <WSD> tags:  (biology) the basic structural and functional unit of all organisms; they may exist as independent units of life (as in monads) or may form colonies or tissues as in higher plants and animals