Several major types of animal cells may be distinguished, including absorptive, secretor, nerve, sensory, muscle, and reproductive cells. All must arise during morphogenesis from cells that are less differentiated.
Absorptive cells often occur as continuous sheet on surfaces where material is transported to the cells. For example, the single layer of epithelial cells lining the surface of the small intestines selectively absorbs, food molecules from the gut into the blood stream. These cells have a free surface that is in contact with the capillaries. The free surface is covered with many projection called microvilli, which vastly increase the area available for molecular flow. In digestion, the products of the ingested food are transported through the microvilli into the cell. They are then pumped into the capillaries from the other side. Similar cells are found in the kidney. The microvilli are example of a cell structure precisely fitted to the function of the cell. Because of an absorptive cell needs maximum area for transport, the shape of the cell surface is altered to achieved the optimum transfer of molecules.
Secretor cells produce products that are subsequently deposited in either the bloodstream or a special duct to an organ, where they are used. The pancreas and pituitary are glands that have large number of secretor cells. Protein and other cell products are synthesized throughout the cytoplasm and transported to the Golgi apparatus, where they are packed into a membrane bonded vesicle that come to a cell's surface and discharge the secretion outside the cell.