One element often neglected by the Martial Artist is the difference between striking in combat sport competition and striking in an actual defense situation.In almost every combat sport,some type of glove or padding is worn on the hands(sometimes feet)as well as taping/wrapping of the wrist & fist for added stability of the wrist and to support the relatively small bones of the hands to prevent damage or even breaks.
Boxers,Kickboxers & even MMA fighters will frequently tape their hands/wrists during work-outs & heavy bag & pad drills.This is a good idea as it will help you develop & deliver power since it removes the "weak link"(bending of wrist,hand collapsing), but you also have to learn to deliver force & receive impact without such aids since you will not have them to rely on during an actual confrontation. I'm not a fan of traditional "hand conditioning"as I feel most of these practices actually do more harm than good and will turn your hand into a useless "claw"that isn't good for anything except fist fighting, not a good idea since you still want to be able to pull a trigger,manipulate other weapons as well perform everyday tasks that require manual dexterity . The idea of building up of callouses is something else that is unnecessary and does virtually nothing to help protect your hands . Your goal should be to strengthen the small muscles of the hands & the forearm muscles to the point where they will essentially do the same job as the tape/wraps,but as natural support. Traditional weight training exercises for the forearms(especially rolling the weight with the fingers) will accomplish much of this and weight training itself builds stronger,denser bones & connective tissue.Isometric fist clinching exercises(roll up tight fist,then series of exhales while squeezing tighter & holding for as long as possible) will train you to contract your fist into a formidable weapon.Heavy bag & hitting the pads will also strengthen your wrists & fist ,but don't go overboard and cause compression injuries to the wrist joint or impact injuries to the hand. Alternate wrapped & natural training for a good balanced training mix.
However, no matter how much you train and strengthen your fist,there is still a good chance you could break your hand should you hit a hard object(like someones head) full force.Therefore I recommend giving strong consideration to using open hand techniques for self-defense. It's virtually impossible to break you hand using any of the various palm heel strikes.You do lose a little range as well as snap & penetration, but it is a safer to apply & more solid weapon than a fist strike.The hammer fist or bottom fist is somewhere in-between the two.
.............."Hard Weapons for Soft Targets & Soft Weapons for Hard Targets"........ is an old saying that is often heard repeated in Martial Arts & Combatives systems. I agree with it for the most part,but you must also consider that although you might be aiming to strike a "soft target"(such as the nose) & end up hitting a "hard target"(the forehead) ,because the opponent moved at the last moment. With an open hand, it wouldn't make much difference, but a fist would be a different matter. Even so, my recommendations are to develop all of your natural weapons equally in a safe & controlled fashion to give yourself as many options as possible.