The process of keratinization begins in what layer of skin?
This question could be interpreted in several ways, but I would say that Keratinization occurs in the Stratum Granulosum
The keratinocytes (skin cells) are produced in the Basale layer, then are pushed up to the Spinosum where they are linked by filaments before reaching the Granulosum.
At this point, the cells secrete lipids and lose their organelles which are replaced with keratin. It is these granules of keratin that give it it's granular appearance.
Hope this helps, let me know if I can help with anything else:)