Cell Differentiation

Cell Differentiation Definition

Cellular differentiation, or simply cell differentiation, is the process through which a cell undergoes changes in gene expression to become a more specific type of cell. The process of cell differentiation allows multi-cellular organisms to create uniquely functional cell types and body plans. The process of cell differentiation is driven by genetics, and their interaction with the environment.

All organisms begin from a single cell. This single cell carries the DNA coding for all the proteins the adult organism will use. However, if this cell expressed all of these proteins at once it would not be functional. This cell must divide repeatedly, and the cells must begin the process of cell differentiation as they divide. The cell lines begin to emerge, and the cells get more and more specific. Eventually, an entire organism is formed with hundreds of different cell types from this process of cell differentiation.

The original mass of cells, which have not undergone differentiation, are known as stem cells. Unlike normal cell division, which creates two identical daughter cells, the division of stem cells is asymmetric cell division. In this case, one of the cells remains identical to the parent stem cell. In the other cell, chemical triggers activate the process of cell differentiation, and the cell will start to express the DNA of a specific cell type. Stem cells which can differentiate into entire organisms are known as embryonic stem cells and are said to be totipotent.

By contrast, the body also has many cells which are only pluripotent. These cells have already undergone some cell differentiation. These stem cells can only divide into a narrow range of cell types. Bone marrow, for instance, contains somatic stem cells which can only become red blood cells. These cells are necessary for the constant replenishment of blood cells, which are mostly inactive besides their oxygen-carrying ability.

Cell Differentiation Examples

In Animals

After the process of fertilization in animals, a single-celled organism called the zygote is formed. The zygote is totipotent, and will eventually become an entire organism. Even the largest animal on Earth, the blue whale, starts as a single cell. The complex tissues and organ systems, which are completely different in their form and function, all come from the zygote. The process of cell differentiation starts early within the organism. By the time the gastrula has formed, the cells have already started expressing various portions of the DNA.

These changes drive the first folding processes within the embryo. As the tissues continue to form, some cells begin releasing hormones, or chemical triggers which signal various cells to react. Hormone signals direct the expression of DNA in various body parts, which drives their cell differentiation further. In humans it only takes a little over a month for a rudimentary heart and circulatory system to form.

As the systems continue to form, many of the stem cells lose their totipotency, themselves undergoing cell differentiation. This allows for faster production of specialized cells, which the growing organism needs to sustain its growth and enter the world with success. Through cell differentiation, tissues as different as brain tissue and muscle are formed from the same single cell.

In Plants

While the plant lifecycle sometimes seems alien and complex, the process of cell differentiation is very similar. While there are different hormones involved, all plants also develop from a single cell. A seed is simply a protective housing for the zygote, which also provides a food supply. It is very similar to an egg in the animal world. The zygote inside undergoes cell division, and becomes a small embryo. Development is halted, as the seed is distributed into the world.

After winter, or anytime the environment is prime, the seed will soak up moisture and restart the process of development. The embryo will begin to form two meristems. A meristem is a unique portion of stem cells, which undergo cell differentiation as they grow outward. One will grow towards the surface, while the other will become the roots.

In the roots a layer of cells forms around the meristem, forming the root cap. This layer of cells sloughs off as the roots move through the soil, and are consistently replaced by the meristem. On the inside of the meristem, cell differentiation happens in a different direction. The hormones and environment here directs the cells to become vascular tissue and supporting cells. These will eventually carry water and nutrients to the top of the plant.

On the surface, the meristem acts in a similar way. As it divides upward, it creates both inward and outward cells. The inward cells undergo a differentiation similar to that of the roots, creating more vascular tissue. On the outside, the cells undergo cell differentiation into stems and leaves. These are equivalent to the different organs of animals, and are as different from the starting cells as animal cells. If you aren’t convinced, pick up an acorn and compare it to the massive tree it will become. Not only is it vastly smaller, it also contains completely different cell types. This can accounted for through the process of cell differentiation.

Cell Differentiation Process

One of the keys to the cell differentiation process is transcription factors. These hormones and chemicals direct the activities surrounding DNA, determining what gets transcribed and what is ignored. The factors present in cells from birth to death are determined by the body, and other cells in the vicinity.

For instance, the pancreas or thyroid may release a hormone calling for cellular growth. This transcription factor directly impacts the proteins which transcribe DNA, turning it eventually into functional proteins and more cells. However, when cells start to squeeze together, they will also signal to each other that there is no more room. Thus, the process of cell differentiation has a multitude of inputs and possible outcomes.

This complex process is still being studied. Scientist have made considerable advances in understanding cell differentiation, starting with the complete understanding of the nematode C. elegans. This tiny, worm-like creature has a total of 959 cells as an adult female. With such a small number, they are relatively easy to track from the zygote to the adult. Tracing their cell lineage, scientists have started to determine some of the complex and epigenetic forces working on cell differentiation. In other words, it matters not only what DNA a cell has, but where and how that DNA is expressed.

Quiz

1. Why is cell differentiation an important process?
A. It allows for multi-cellular life forms
B. It creates new species
C. We could do without it

Answer to Question #1
A is correct. Without the process of cell differentiation, multicellular organisms would not be possible. Some algae live in colonies, but this is nowhere near the level of complexity developed by insects or vertebrates. Cell differentiation allows the creation of tissues and organs, which can serve specific and useful functions for an organism.

2. What is the difference between cell differentiation and development?
A. Development does not include differentiation
B. Cell differentiation is part of development
C. There is no difference

Answer to Question #2
B is correct. Development is the entire process of creating a new organism from a single cell. It includes everything from forming the correct tissues to creating the neural connections to support a new body. Cell differentiation is simply the process through which cells begin to express only certain portions of DNA, thereby becoming specialized cell types.

3. If each stem cell divides into more specialized cells, where do you get more stem cells from?
A. You don’t
B. Stem cells divide asymmetrically
C. The zygote creates them

Answer to Question #3
B is correct. When most stem cells divide, one of them retains the original character of being a stem cell. The zygote divides without cell differentiation 3 times, creating 8 identical totipotent cells. These cells will continue to divide asymmetrically, and will eventually give rise to three general tissues, the ectoderm, mesoderm, and endoderm. These tissues will undergo further cell division to become specific tissues.

References

  • Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., . . . Matsudaira, P. (2008). Molecular Cell Biology (6th ed.). New York: W.H. Freeman and Company.
  • McMahon, M. J., Kofranek, A. M., & Rubatzky, V. E. (2011). Plant Science: Growth, Development, and Utilization of Cultivated Plants (5th ed.). Boston: Prentince Hall.
  • National Institutes of Health. (2018, March 11). Stem Cell Basics III. Retrieved from Stemcells.nih.gov: https://stemcells.nih.gov/info/basics/3.htm
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