Cellular differentiation

Cellular differentiation is a concept from developmental biology describing the process by which cells acquire a "type". The morphology of a cell may change dramatically during differentiation, but the genetic material remains the same, with few exceptions.

A cell that is able to differentiate into many cell types is known as pluripotent. These cells are called stem cells in animals and meristematic cells in higher plants. A cell that is able to differentiate into all cell types is known as totipotent. In mammals, only the zygote and early embryonic cells are totipotent, while in plants, many differentiated cells can become totipotent with simple laboratory techniques.

Overview
In most multicellular organisms, not all cells are alike. For example, cells that make up the human skin are different from cells that make up the inner organs. Yet, all of the different cell types in the human body are all derived from a single fertilized egg cell through differentiation. Differentiation is the process by which an unspecialized cell becomes specialized into one of the many cells that make up the body, such as a heart, liver, or muscle cell. During differentiation, certain genes are turned on, or become activated, while other genes are switched off, or inactivated. This process is intricately regulated. As a result, a differentiated cell will develop specific structures and perform certain functions.

Differentiation can involve changes in numerous aspects of cell physiology; size, shape, polarity, metabolic activity, responsiveness to signals, and gene expression profiles can all change during differentiation.

In cytopathology the level of cellular differentiation is used as a measure of cancer progression. "Grade" is a marker of how differentiated a cell in a tumor is.

Mammalian cell types
Three basic categories of cells make up the mammalian body: germ cells, somatic cells, and stem cells. Each of the approximately 100,000,000,000,000 (1014) cells in an adult human has its own copy, or copies, of the genome, with the only exception being certain cell types that lack nuclei in their fully differentiated state, such as red blood cells. The majority of the cells are diploid, meaning they have two copies of each chromosome. This category of cells, called somatic cells, includes most of the cells that make up the human body, such as skin and muscle cells.

Germ line cells are any line of cells that give rise to gametes&mdash;eggs and sperm&mdash;and are continuous through the generations. Stem cells, on the other hand, have the ability to divide for indefinite periods and to give rise to specialized cells. They are best described in the context of normal human development. Development begins when a sperm fertilizes an egg and creates a single cell that has the potential to form an entire organism. In the first hours after fertilization, this cell divides into identical cells. In humans, approximately four days after fertilization and after several cycles of cell division, these cells begin to specialize, forming a hollow sphere of cells, called a blastocyst. The blastocyst has an outer layer of cells, and inside this hollow sphere, there is a cluster of cells called the inner cell mass. The cells of the inner cell mass will go on to form virtually all of the tissues of the human body. Although the cells of the inner cell mass can form virtually every type of cell found in the human body, they cannot form an organism. These cells are referred to as pluripotent.

Pluripotent stem cells undergo further specialization into multipotent progenitor cells that then give rise to functional cells. Examples of stem and progenitor cells include:
 * Hematopoietic stem cells (adult stem cells) from the bone marrow that give rise to red blood cells, white blood cells, and platelets
 * Mesenchymal stem cells (adult stem cells) from the bone marrow that give rise to stromal cells, fat cells, and types of bone cells
 * Epithelial stem cells (progenitor cells) that give rise to the various types of skin cells
 * Muscle satellite cells (progenitor cells) that contribute to differentiated muscle tissue