A tissue
A tissue is a group of similar cells that are modified (or adapted) to carry out the same function(s)
Tissue culture
Tissue culture is the growth of cells in or on a sterile nutrient medium outside an organism
An organ
An organ is a structure composed of a number of tissues that work together to carry out one or more functions
An organ system
An organ system consists of a number of organs working together to carry out one or more functions