Cellular differentiation

Cellular differentiation is the biological process where a stem cell transforms into a specialized cell type. This process allows a simple organism to develop into a complex system of diverse tissues. As cells differentiate, they undergo dramatic changes in size, shape, and metabolic activity. They also change their membrane potential and how they respond to external signals. While a cell's physical characteristics change, its DNA sequence usually remains exactly the same. Instead, differentiation is driven by highly controlled modifications in gene expression, a field known as epigenetics.

The mechanism of differentiation relies on how a cell uses its genetic instructions. Every cell in a multicellular organism contains the same genome, but each specialized cell expresses only a specific subset of genes. This transition is often managed by a gene regulatory network. In this network, regulatory genes and their cis-regulatory modules act as nodes that receive and send signals. Some scientists view this as a structured network, while others suggest it results from stochastic gene expression. This theory proposes that differentiation is a Darwinian selective process occurring among different cells.

Cell signaling often controls how these differentiation pathways are activated. Many of these signals are proteins called growth factors. The process usually begins when a ligand, or signal molecule, binds to a receptor on the outside of a target cell. This binding causes a conformational change, which is a change in the receptor's shape. This shape change activates the receptor's enzymatic activity on the inside of the cell. The receptor then catalyzes reactions that phosphorylate other proteins in a chemical cascade. This cascade eventually activates a transcription factor or a cytoskeletal protein to change the cell's function.

Cells also differentiate through asymmetric cell division. During this process, a single parent cell divides to produce two daughter cells with different fates. This can happen because of an uneven distribution of maternal cytoplasmic determinants. In this case, the daughter cells inherit different regulatory molecules during cytokinesis. Differentiation can also occur through inductive signaling. This is when one tissue sends signals to influence the developmental fate of another tissue. For example, researchers found that a lens vesicle can induce other parts of the eye to develop in fish.

Scientists categorize cells by their potency, which is their ability to differentiate into other types. A totipotent cell has the highest potency and can become any cell type, including placental tissue. In mammals, only the zygote and early blastomeres are totipotent. Pluripotent cells can form all the cell types of the adult organism but cannot form an entire organism. In animals, these are often embryonic stem cells, while plants use meristematic cells. Multipotent cells can become several related cell types, such as hematopoietic stem cells in bone marrow. Oligopotent cells are more restricted, and unipotent cells can only produce one specific cell type.

In human development, differentiation follows a specific sequence of stages. It begins with a single fertilized zygote that divides into identical cells. About four days after fertilization, these cells form a hollow sphere called a blastocyst. Inside the blastocyst is the inner cell mass, which contains pluripotent cells. These cells eventually become multipotent progenitor cells that create functional tissues. In mammals, these cells organize into three primary germ layers. The ectoderm forms the skin and nervous system, the mesoderm forms bones and muscles, and the endoderm forms internal organs.

Some cells undergo terminal differentiation, which is a permanent change. During this stage, a precursor cell leaves the cell cycle and can no longer divide. It dismantles its cell cycle machinery and expresses genes for its final function, such as myosin in muscle cells. This is vital in the vertebrate nervous system and the gut. Conversely, some organisms can undergo dedifferentiation. This is when a specialized cell reverts to an earlier, less specialized stage. This process is seen in worms and amphibians to help with regeneration.

Understanding differentiation is crucial for fields like medicine and cytopathology. In cancer research, the level of differentiation is used to measure disease progression. This is often referred to as the "grade" of a tumor. Scientists can even use specific transcription factors, like the Yamanaka factors, to turn adult cells back into pluripotent cells. This process creates induced pluripotent stem cells, or iPS cells. By studying these pathways, researchers gain insight into how the 37.2 trillion cells in an adult human maintain complex life.