Cell specialization is important during the growth and development of a multicellular organism. This process is most directly regulated by

Protein

Explanation:

A cell can be defined as the fundamental or basic functional, structural and smallest unit of life for all living organisms.

Cell specialization is also referred to as cell differentiation and it can be defined as a biological process through which generic cells transform into specific cell types, so as to enable them perform certain functions within the body of a living organism.

Simply stated, it's the special function or job in the body of a living organism that are associated with cells.

Generally, cell specialization plays a significant role in the development of embryos.

Some living organisms are unicellular while others are multicellular in nature.

A unicellular organism refers to a living organism that possess a single-cell while a multicellular organism has many (multiple) cells.

Cell specialization is fundamental and key for the growth and development of a multicellular organism.

Furthermore, cell specialization as a process is most directly regulated by protein; one super regulating protein encourages cell differentiation while the other tries to stop or prevent the process. These proteins are referred to as master regulating proteins or Helix-Loop-Helix proteins.