The cell is an amazing entity which is capable of sustaining life. It is interesting that this integral entity has a form of life. On the other hand, if we synthesize the cell from its molecular components then there would be a structure of the cell without sign of life. In humans, the type of cells varies according to the function.
For example, muscle cells are specialized for the function of muscle contraction. They possess many nuclei inside them. However, they do not divide. Therefore, atrophy of muscle tissue cannot be compensated by hyperplasia. Hyperplasia signifies a muscle cells increase in number.
Cells of the blood system are generated by sequential differentiation from mother cells that are created in the bone marrow which differentiate finally to red blood cells. Red cells production depends on the supply of the mother cells that are called reticulocytes for their generation.
Reticulocytes become red blood cells by the process of getting rid of their nucleus. Thus red blood cells do not possess nucleus. Once dead they are compensated only by the process of reticulocytes differentiation into red blood cells and not by direct mitosis due to the lack of nucleus in their structure.
Other type of cells in the human body are the nervous system cells which are divided to neurons and glial cells. These two types of cells differ in function. On the one hand, neurons are the functional units of the nervous system. They process the communication between the parts of the nervous system by initiating an action potential which transmits electric signals from one neuron to another. This process is mediated by ion channels of sodium and potassium ions which are responsible for the generation of the action potential.
The other cell type in the nervous system is called glial cells. They form a supporting function mainly including building the blood-brain barrier which is a very selective barrier that protects the brain from noxious material. It is mainly permeable to lipid soluble compounds. Neurons di not divide while glial cells do divide. This difference has an important result which is related to malignancy of the nervous system. Cancer of the nervous system originates in the glial cells and not from neurons due to the mitotic activity of the glial cells.
Cells of the immune system such as macrophages are also differentiated from mother cells that are called monocytes. This type of cells has an immune function which is very important in preventing pathology in the body.
Hyperplasia is a process of increasing the number of cells in a given tissue such as occurs in the medical condition congenital adrenal hyperplasia in which cells of the adrenal cortex increase in number due to hormonal stimulus. Hyperplasia occurs only in cells which are capable of division.
Another process, which is called hypertrophy, is an increase in cellular size as occurs in hypertrophy of muscle tissue which usually occurs in tissue cells that are incapable of division or are unable of mitosis. An example is the hypertrohy of muscle tissue in the uterus during pregnancy, in which case a marked enlargement of the uterus occurs based on a stimulus from the hormone estrogen.
Other cells of neuroendocrine origin that specialize in secreting hormones are located in the hypothalamus and the pituitary gland. They are important for the proper functioning of the body tissue. Especially notable here are cells of the hypothalamus and the pituitary gland which have an important role in controlling the function of most tissues of the body.
Cells of the endocrine organs are also important for their secreting of important hormones which have vital role in maintaining homeostasis in the body. An example is cells of the thyroid gland, which secrete two hormones that are important for the proper metabolism of the body tissue and for calcium metabolism.