Please choose the correct ending to complete the given sentence: Health: [header] How to study the structure and function of human skin [title] Learn about the histology of skin in humans. [step] The skin of humans is composed of several histologic layers that are different than other creatures in the animal kingdom. This histologic structure is important for various reasons.

Ending1: The histologic structure is important because it establishes the animal in the human body that can withstand its winter forms. [substeps] The human body of humans is composed of: an outer layer which is composed of v and, b, " body layer. Ending2: A characteristic of a human body is its structure, which is actually what we suffer from when humans are looking at the human bodies in our image. We understand because we can understand what others are like and our behavior and thoughts of ourselves. Ending3: Most people rely primarily on the underdeveloped bodies of humans. [substeps] Classifications of human skin are based on species, sizes, parts of the body and hormones that feed off of them. Ending4: One of them is its use as a selective physical barrier against foreign pathogens and harmful chemicals. At this level it serves as part of the nonspecific immune system in humans.

Answer format: ending1/ending2/ending3/ending4