Why are such different tissues referred to the tissues of the internal environment? What unites them?

They are united by a common position in the body (bordering on the external environment), the origin and common characteristic is a highly developed intercellular substance.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.