Which two types of connective tissue are responsible for holding muscle fibers together?

Collagen and elastin are indeed the connective tissues responsible for holding muscle fibers together. Collagen provides strength and structural support to the muscle tissue, making it vital in maintaining the integrity and functionality of muscles. It is a major component of the tendons and ligaments that attach muscles to bones and other structures.

Elastin, on the other hand, contributes to the flexibility and elasticity of the muscle fibers. This elasticity is crucial for allowing muscles to stretch and contract effectively during movement. Together, collagen and elastin ensure that muscle fibers remain cohesive while also being able to withstand the forces exerted during contraction and movement.

The other options do not accurately reflect the primary types of connective tissue involved in this function. Keratin is more commonly found in the skin, hair, and nails and does not play a significant role in muscle fiber structure. Reticulin is a type of collagen, but it is primarily associated with the structural framework in various organs rather than in directly holding muscle fibers together. Muscle fibers themselves are not a connective tissue, but rather the contractile units of the muscle.