Newly formed cells in hair are primarily filled with which protein?

Newly formed cells in hair are primarily filled with keratin, which is a fibrous structural protein. Keratin is essential for the strength and resilience of hair, as it makes up the majority of the hair shaft. It provides the necessary properties that allow hair to withstand wear and tear from environmental factors and styling.

Other proteins mentioned, such as elastin and collagen, play different roles in the body. Elastin is primarily responsible for providing elasticity to tissues, while collagen serves as a structural component in connective tissues. Actin is a protein involved in muscle contraction and cellular movement, rather than being a major component of hair structure. Therefore, keratin is the correct choice, as it is specifically associated with the formation and integrity of hair.