intro- vs. intra-

The prefix extro- means ‘outwards’. The Oxford English Dictionary has entries for both extrovert and extravert, with no difference in meaning. The only other word in the OED beginning with extro- is ‘extrospective’. Its Latin origin is the preposition extra which means ‘outside’. The ‘o’ appears to have replaced the ‘a’ of ‘extra’ by analogy with the Latin adverb intro.

Intro itself means ‘inwards’, ‘to the inside’, and is used in that sense as a prefix in English verbs and adjectives. Intra is a Latin preposition meaning ‘inside’ and is the origin of the prefix in English adjectives, typically those found in biological contexts.