Cytology is a part of biology that studies living cells, their structure, functioning, cell cycle (reproduction, aging, and death), evolutionary variability, and interactions between cells, as well as between cells and non-cellular structures. In addition to this term (cytology), modern literature employs such synonyms as cell biology, and biology of the cell.
A cell is the smallest structural and functional unit of life with all the typical features of living bodies.
Prokaryotic cells have no nuclei while their chromosomes are not tightly packed in the cytoplasm; eukaryotic cells have nuclei.
Cells are independent units that have developed in the course of evolution (in unicellular organisms) or special body systems (in multicellular organisms). This is a level of structural organization in the body (cellular level). Cells are the principal elements of tissues.
Human body includes more than 250 cell types, which differ significantly by their shape, size, structural organization, and functional specialization.
Various cell types can form cell communities for cooperative functioning according to specific properties of tissues or organs.