Which structure acts as a barrier between the exterior environment and the underlying tissues of the skin?

The epidermis serves as the primary barrier between the exterior environment and the underlying tissues of the skin. It is the outermost layer of the skin and consists of several layers of cells that function to protect underlying tissues from environmental threats such as pathogens, UV radiation, and moisture loss. The epidermis is vital for the skin's protective properties and is continually renewed as the cells on the surface shed and are replaced by new cells from deeper layers.

While collagen is an essential protein that provides structural integrity and support to the skin, it is found in the dermis, which lies beneath the epidermis. The dermis supports the epidermis and houses blood vessels, nerves, and other structures but does not directly form the surface barrier.

The stratum corneum, although part of the epidermis, is just the outermost layer of it. It plays an important role in the barrier function by preventing water loss and protecting against external elements, but it is not the complete structure which encompasses the totality of the epidermis.

Thus, the epidermis is the correct answer because it represents the entire protective outer layer of the skin that interfaces with the environment.