Here's a breakdown:

* Hair Shaft: The visible part of the hair that extends from the scalp.

* Cortex: The largest part of the hair shaft, located between the cuticle and the medulla. It's composed of tightly packed, elongated cells called cortical cells. These cells contain keratin, melanin (pigment), and other proteins that give hair its strength, color, and texture.

* Cuticle: The outermost layer of the hair shaft, composed of overlapping scales that protect the cortex.

* Medulla: The central core of the hair shaft, present in some but not all hair types.

So, to answer your question directly, the cortical cells are the building blocks of the cortex of the hair shaft.