Cellular differentiation

Cellular differentiation is the process in which a stem cell changes from one type to a more specialized type. This happens many times during the development of a multicellular organism as it grows from a simple zygote into a complex system of tissues and different cell types. Even after we are born, differentiation continues as adult stem cells divide and create new cells to repair tissues and replace old ones.

This process changes a cell's size, shape, membrane potential, metabolic activity, and how it responds to signals. These changes are mostly due to controlled changes in gene expression, a field known as epigenetics. Importantly, these changes do not alter the DNA sequence itself, though the metabolic makeup of the cell changes greatly. Because of this, different cells can look and act very differently even though they all have the same genome.

There are different levels of how strongly a cell can change into other types, called cell potency. A totipotent cell can become any cell type, including placental tissue. In mammals, only the zygote and early blastomeres are totipotent. A pluripotent cell can become any cell type in the adult organism, such as embryonic stem cells in animals. Other cells can only become a few related types and are called multipotent, oligopotent cells, or unipotent depending on how many types they can become.

Mammalian cell types

See also: List of distinct cell types in the adult human body

Mammalian bodies are made up of three main types of cells: germ cells, somatic cells, and stem cells. Humans have about 37.2 trillion cells, each with its own copy of the genome, except for red blood cells which lack nuclei. Most cells are diploid, meaning they have two copies of each chromosome. These somatic cells form most of the body, like skin and muscle cells, and they specialize for different functions through a process called differentiation.

Germ cells produce gametes—eggs and sperm—and continue through generations. Stem cells can divide indefinitely and create specialized cells. During development, after a sperm fertilizes an egg, the resulting cell divides and eventually forms a blastocyst. Inside the blastocyst is a group of cells called the inner cell mass that can become almost any cell type in the body. These cells are pluripotent and later become multipotent progenitor cells, which then develop into functional cells like neurons, blood cells, and bone cells.

Dedifferentiation

Dedifferentiation, or integration, is a process where a specialized cell changes back to a simpler form. This happens in some animals like worms and amphibians, usually to help them heal or grow new body parts. It can also occur in plant cells and in cells grown in a lab, where they might change shape or lose certain features.

Scientists have found that a molecule called reversine can cause dedifferentiation in certain cells. These cells then act more like stem cells and can change into different types of cells again, such as bone-building cells known as osteoblasts or fat cells called adipocytes. Some think dedifferentiation might be linked to cancers, while others believe it is a natural part of how the immune system works.

Mechanisms

See also: Embryonic differentiation waves

Cells change from one type to another by turning certain genes on and off. This process is guided by special networks that control which genes are active in a cell. These networks help create the many different cell types in our bodies.

Cells also receive signals from other cells that tell them what to become. These signals can start a chain reaction inside the cell, leading to changes that help the cell specialize. Sometimes, when a cell divides, the two new cells can become different types right from the start, depending on what they inherit from the parent cell. This helps in forming patterns and structures as an organism grows.

Epigenetic control

Main article: Epigenetics in stem cell differentiation

All cells in your body have the same set of instructions, or genes. But different cells, like skin cells or brain cells, act in very different ways. This happens because of a process called epigenetic control. It helps decide which genes are turned on or off, guiding a cell to become a specific type.

Epigenetic control is very important for cells to know what they should become. For example, studies with special types of stem cells show that these cells have patterns of chemical tags—called DNA methylation—that are very similar to normal embryonic cells. These patterns help keep the cells flexible and able to develop into many different types. Scientists study these patterns to understand how cells decide their future roles during growth and development.

Evolutionary history

See also: Bangiomorpha

A very old, simple creature called Bicellum brasieri lived over a billion years ago and had two different kinds of cells. This shows that the ability for cells to change and specialize, which is important for creating many types of life, started at least a billion years ago. It likely happened in freshwater lakes rather than in the ocean.