Hair follicles forms a cluster of cells in the upper layer of the skin.
These cluster of cells are called the primitive hair germ, which needs nourishment to grow into a developed hair follicle.
To get nourishment it works all the way down into the lower layer of the skin.
As it does, the cell cluster pulls the upper layer down with it creating a follicle called the root Sheath, out of which the hair will grow.
The shape of this follicle then determines the shape of the hair shaft as it grows from the follicle.
The hair shaft grows out from follicle so the diameter of the hair fiber will be the same as the diameter of the inside of the follicle.
In straight hair the hair are less vertical to the surface of the scalp and are slightly tilted.
The follicle in straight or wavy hair are are round or oval.
In kinky or curly hair the follicle grow stronger from the scalp at a good angle.