In a multicellular organism, there are many different types of cells, each with a specific function. But what makes them so different? Do they have other instructions inside that tell them what type to become? Have you heard of cell differentiation? Do you know its purpose? We'll learn all about the cell differentiation process in this article, including some examples and the difference with cell division.
The Definition of Cell differentiation
Differentiation is the natural process through which a less specialised cell, i.e., a stem cell, matures and becomes more distinct in function and shape.
All cells within an organism contain the same set of genetic instructions called the genome. What drives the unique characteristics of different cells, is reading only certain sections of these instructions. The areas of the genome which are needed are silenced in the differentiation process.
Single-celled organisms perform all of their basic functions within a single cell. For maximum efficiency in each process, a unique cellular structure and machinery are needed. No one cell can provide optimal circumstances for all functions.
In single-celled organisms, the relatively inefficient operations performed by a single cell may be adequate, but this falls short in multicellular organisms. Each cell in a multicellular organism, from a mushroom to a human being, becomes specialised in several ways to fulfill a specific role. And the adaptations these acquire guarantee that they are as effective as possible in performing their functions.
These activities can be contracting in a muscle cell or conducting electrical impulses in a neuron.
Stem Cells
The specialised cells result from the differentiation of stem cells.
Stem cells are the raw materials of the body, the cells that have the potential to give rise to all other cell types with specific shapes and functions.
All cells in most multicellular organisms, including humans and most plants, are generated from the fertilisation of two gametes from opposite biological sexes: an egg cell with a sperm cell.
Gametes contain only half the genetic information of the organism they are from. Therefore, the cell formed by their fusion (the zygote) has the same amount of DNA as other organisms of the same species.
A zygote is the first stem cell in an organism.
Some stem cells are also present in small numbers in most tissues, such as the bone marrow, skin, and gastrointestinal tract. They are called adult stem cells and can turn into a narrower range of specialised cells depending on what tissue they are located in. The primary role of adult stem cells is to regenerate damaged or old cells in tissues.
Fig. 1 - Stem cells differentiate into specialized cells that perform specific roles.
Cell Differentiation and Specialisation
Cell specialisation is the process by which cells differentiate and specialise in performing their role in a tissue, organ, and, eventually, the body. Specialised cells have distinct shapes and subcellular structures that aid in their roles.
Multicellular organisms can contain hundreds of different types of cells.
Humans, for instance, possess more than 200 different types of specialised cells in their bodies.
Specialisation is an essential process in the growth and maturation of embryos. During the early stages of development, the zygote goes through several mitotic divisions, resulting in a group of cells commonly referred to as embryonic stem cells. These stem cells mature and differentiate, turning into specialised cells.
The Process of Cell Differentiation
Stem cells and specialised cells have identical genetic content. While stem cells retain the ability to express every one of their genes, specialised cells lose this ability. They can only express genes that are essential for viability and function.
For example, the gene encoding haemoglobin is active in reticulocytes (precursors of red blood cells), but this gene is silenced and not expressed in neurons.
Regulation of gene expression drives cell differentiation. When cells express certain genes that define a specific type of cell, we say the cell has differentiated. Once a cell has differentiated, it only expresses the genes that code for the proteins that are unique to that kind of cell. Factors involved in transcription and translation determine which genes remain active and which are silenced.
Epigenetic modifications also regulate gene expression by modifying either the genes directly or the proteins associated with the genes, altering the accessibility of enzymes involved in transcription to DNA.
The Difference Between Cell Differentiation and Cell Division
Cell differentiation is the process through which cells specialise to perform their roles. A cell will express particular genes to differentiate. Once a cell is determined and has become specialised, it loses the ability to divide via mitosis. New cells generated by mitosis of stem cells may transform into specialised cells.
Mitosis is a type of cell division that occurs when cells divide to generate new cells identical to their parent cell.
Living organisms are constantly in need of developing new cells to replace old, damaged, or dead cells.
Cell differentiation and cell division are entirely different terms, even though they sound similar.
| Cell differentiation | Cell division (mitosis) |
| The process of turning undifferentiated stem cells into specialised cells. | The splitting of parent cells to produce new but identical daughter cells. |
| No new cell created. | New cells created. |
Table 1: Main differences between cell differentiation and cell division.
Examples of Cell Differentiation
There are many different cells within the body which can be used as examples for cell differentiation. Below are some, in both animals and plants, that we will take a closer look at.
Red Blood Cells
Red blood cells (erythrocytes) are derived from adult stem cells in the red bone marrow. These stem cells, called haemopoietic stem cells, are the precursor to all blood cells, including lymphocytes, neutrophils, basophils, and platelets.
Erythrocytes are oxygen carriers in the body. They contain large quantities of haemoglobin, a protein that picks up oxygen in the lungs and delivers it to all tissues around the body. During their differentiation, erythrocytes lose almost all organelles, including the nucleus and mitochondria, making more room for haemoglobin to maximise their oxygen-carrying capacity.
Red blood cells also adopt a biconcave structure, increasing their surface area for gas exchange and flexibility for going through narrow blood vessels.
Muscle Cells
Muscles are essential tissues in animals that enable movement. Three main types of muscles are found: cardiac, skeletal, and smooth.
Cardiac muscle cells are located in the heart and, by autonomous contracting, pump blood around the body.
Skeletal muscles are attached to bones via tendons and move the limbs and other skeletal structures under voluntary control.
Smooth muscles line the walls of blood vessels and the gastrointestinal (GI) tract and contract under the autonomous nervous system to regulate blood pressure and the flow of food in the GI tract.
Cells from these three types of muscles share several adaptions for their roles. These are:
The ability to contract and forcefully shorten. This contracting ability is enabled by the protein filaments called actin and myosin that slide over each other, contracting the cell.
Responding to signals from the nervous system and neurons.
Extensibility, which is the ability to stretch or extend.
The elastic ability to return to its resting length following extension or contraction.
Containing a large number of mitochondria, the cell's powerhouse, to provide the energy needed for contraction.
Root Hair Cells
Root hair cells, located in plant roots, are special cells that absorb water and minerals from the soil. They possess large numbers of mitochondria and many cellular extensions that give them a large surface area. These adaptations enable root hair cells to absorb nutrients efficiently, even against their concentration gradient.
Fig. 2 - Root hair cells have long extensions and many mitochondria. These adaptations enable these cells to efficiently absorb water and minerals from the soil.
Xylem and Phloem Cells
Xylem cells are specialised dead cells in plants that transport water up from the roots through the stem and deliver it to the leaves. These cells are hollow and have an elongated shape, forming tubes called xylem. Their lack of organelles or cytoplasm allows water to flow through them freely.
Xylem cells are lined with lignin, an impregnable polymer that keeps the water inside the tubes. Along the xylem are specific points called pits, where lignin is absent or very thin. Water flows through these pits, travelling to the surrounding tissues.
In contrast to xylem cells, phloem cells are living cells that transport the sugars made in photosynthesis from the leaves to all parts of the plant. Phloem cells consist of connecting sieve cells stacked on top of each other. These sieve cells share a highly perforated sieve plate to aid the movement of material from cell to cell. These living cells have limited cytoplasm and no nucleus to maximise their transporting ability.
Because of this, they rely on their neighbouring cells, called companion cells, to generate the energy and proteins needed for their survival and function.
Fig. 3 - Xylem and phloem cells are specialized transporting cells in plants. Dead xylem cells transport water up from the root, while phloem cells move sugars from leaves to all parts of the plant.
Cell Differentiation - Key takeaways
Differentiation is the natural process through which a less specialised cell, i.e., a stem cell, matures and becomes more distinct in function and shape.
All cells within an organism contain the same set of genetic instructions called the genome. What drives cell differentiation is the control of gene expression.
Specialised cells are formed from the differentiation of stem cells.
Stem cells have the potential to give rise to all other cell types with specific shapes and functions.
Some examples of specialised cells are red blood cells, muscle cells, root hair cells, xylem and phloem cells.
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