Objection 1. It would seem that the hair and nails will not rise again in the human body. For just as hair and nails result from the surplus of food, so do urine, sweat and other superfluities or dregs. But these will not rise again with the body. Neither therefore will hair and nails.
Objection 2. Further, of all the superfluities that are produced from food, seed comes nearest to the truth of human nature, since though superfluous it is needed. Yet seed will not rise again in the human body. Much less therefore will hair and nails.
Objection 3. Further, nothing is perfected by a rational soul that is not perfected by a sensitive soul. But hair and nails are not perfected by a sensitive soul, for "we do not feel with them" (De Anima i, 5; iii, 13). Therefore since the human body rises not again except because it is perfected by a rational soul, it would seem that the hair and nails will not rise again.
On the contrary, It is written (Luke 21:18): "A hair of your head shall not perish."
Further, hair and nails were given to man as an ornament. Now the bodies of men, especially of the elect, ought to rise again with all their adornment. Therefore they ought to rise again with the hair.
I answer that, The soul is to the animated body, as art is to the work of art, and is to the parts of the body as art to its instruments: wherefore an animated body is called an organic body. Now art employs certain instruments for the accomplishment of the work intended, and these instruments belong to the primary intention of art: and it also uses other instruments for the safe-keeping of the principal instruments, and these belong to the secondary intention of art: thus the art of warfare employs a sword for fighting, and a sheath for the safe-keeping of the sword. And so among the parts of an animated body, some are directed to the accomplishment of the souls' operations, for instance the heart, liver, hand, foot; while others are directed to the safe-keeping of the other parts as leaves to cover fruit; and thus hair and nails are in man for the protection of other parts. Consequently, although they do not belong to the primary perfection of the human body, they belong to the secondary perfection: and since man will rise again with all the perfections of his nature, it follows that hair and nails will rise again in him.
Reply to Objection 1. Those superfluities are voided by nature, as being useful for nothing. Hence they do not belong to the perfection of the human body. It is not so with the superfluities which nature reserves for the production of hair and nails which she needs for the protection of the members.
Reply to Objection 2. Seed is not required for the perfection of the individual, as hair and nails are, but only for the protection of the species.
Reply to Objection 3. Hair and nails are nourished and grow, and so it is clear that they share in some operation, which would not be possible unless they were parts in some way perfected by the soul. And since in man there is but one soul, namely the rational soul, it is clear that they are perfected by the rational soul, although not so far as to share in the operation of sense, as neither do bones, and yet it is certain that these will rise again and that they belong to the integrity of the individual.
Source: Summa Theologica (New Advent)