Miniaturization
Miniaturization is the primary reason for hair loss. It is the process by which hair follicles are genetically susceptible to miniaturize overtime. Hair loss is not from the hair falling out per say. Depending on your genetic predisposition, a person can have early hair loss and/or steady hair loss over a number of years. The hair follicles over time decrease in diameter. When the miniaturization is at its end point, the hair does finally fall out. Pharmaceuticals such as finasteride and minoxidil help to stop miniaturization in its tracks and sometimes reverse it. Below are pictures of miniaturization under a follicle scope on a shaved head.  Below is normal unminiaturized area of the scalp. Here you see strong follicle units. Each follicle unit comes in groupings of 1’s, 2’s 3’s and 4 follicles per unit. These units are all course and healthy.
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Below is an example of hair miniaturization on the same individual taken in an area of hair loss. As one can see, there is a huge difference in the courseness in these follicle units. There are different levels of hair caliber within these follicle units.
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