Which structures in the human body are similar in function to the cell wall of a plant?

The integumentary system.

The integumentary system is the skin system where the epithelial cells function as the barrier between the external and internal part of the body.
Further, there are at least three primary functions that are performed by the epithelial tissue.
Firstly it provides a great protective covering for all the tissues lying beneath it from radiation, toxins, physical trauma, desiccation and invasion of pathogens.
Secondly it helps to regulate the exchange of chemicals between the body cavity and the tissues that lie underneath the epithelial tissues.
It also helps in the secretion of the hormones in to the bloods. These are all major but primary functions of the epithelial tissues. From this we can easily understand the importance of the epithelial tissues.

Oblate