WordNet records no synonym and no antonym for parenchyma. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
animal tissue that constitutes the essential part of an organ as contrasted with e.g. connective tissue and blood vessels
the primary tissue of higher plants composed of thin-walled cells that remain capable of cell division even when mature; constitutes the greater part of leaves, roots, the pulp of fruits, and the pith of stems
Synonyms, antonyms and the broader and narrower relations for parenchyma come from Princeton University’s WordNet. Full sources and licences.