There are many cell types in a complex multicellular organism. All are derived from a single cell, the fertilized ovum, that during the embryonic development produces millions of cells in numerous mitotic cycles.
During embryonic and fetal development, undifferentiated and totipotent cells gradually undergo a process called differentiation, during which some genes are silenced or repressed and others are activated.
Groups of cells that have undergone activation of sets of genes begin to produce new proteins or greatly increase the production of pre-existing ones. After these complex processes, the cells acquire characteristic structural and functional properties (a different morphology, type of organization, functions), resulting in the formation of “cell families”.
Families of cells with similar structure, function and behavior constitute the tissues of the body. They have been classified based on several criteria, such as their embryological origin, morphology, position in the body and functions.
Four basic tissues are customarily recognized: epithelial tissue, connective tissue, muscular tissue and nervous tissue.
This classification is useful for systematizing tissues and for teaching purposes. However, over time and as a result of advances in the knowledge of cells and the extracellular matrix it has been found that this classification is simplified and may not longer be applied indiscriminately.
An example is the term secretory cell, which was formerly applied to glandular cells of epithelial tissue, as it was thought that only they acted in the function of secretion. Today we know that many types of cells belonging to the epithelial, connective, muscular and nervous tissue are also secretory.