Multicellular organisms begin life as a single cell that replicates, forming new cells that grow and differentiate to make up distinct tissues as the organism develops. Tissues are assemblies of cells working together and connected by an extracellular matrix, which adds structural support and assists in cell signaling. In plants, tissues are tough and flexible because of cellulose microfibrils, which direct the growth of cells. In animals, tissues resist stretching because of collagen, an extracellular matrix protein. Animal tissues resist compression because of polysaccharides called glycosaminoglycans (GAGs). There are four major types of animal tissues: connective, muscle, nervous, and epithelial. Epithelial tissues are sheets of side-by-side cells and form a barrier on all body surfaces. All body cells arise from stem cells. Stem cell therapy includes the use of induced pluripotent stem cells (iPS cells), which are body cells that have been grown as undifferentiated cells in a laboratory. Another form of stem cell therapy is cloning, in which an embryo is generated from a body cell and an egg cell that has had its nucleus removed.
At A Glance
Tissues are integrated, cooperative assemblies of cells working together, which are usually surrounded by an extracellular matrix, a collection of molecules outside the cells that provides structural support and biochemical signals.
Plant tissues are both tough and flexible, owing to cellulose microfibrils that align to allow the plant cell to expand in a particular direction.
Animal tissues have limited stretching ability because of collagen, which forms networks of ropelike fibers, and resist compression because of glycosaminoglycans (GAGs), which link to core proteins to form bottlebrush-like structures.
- Cells are often found in sheets called epithelia, in which cells seal one surface from the other via tight junctions, connect to each other via gap junctions, adhere to each other using desmosomes, and adhere to the basal lamina using hemidesmosomes.
Model organisms, such as the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the zebrafish Danio rerio, have given scientists many insights into the mechanisms that underlie embryological development.
Stem cells are undifferentiated cells, meaning they have not developed specializations; stem cells may be totipotent (giving rise to an entire organism from a single cell), pluripotent (giving rise to many different body cell types), or multipotent (giving rise to a limited range of body cells).
- Stem cells may be used therapeutically through the creation of induced pluripotent stem cells (iPS cells) or through cloning, in which an embryo is generated by injecting the nucleus of a body cell into an egg that has had its nucleus removed.