How does differentiation occur in cells

For instance, whereas the nerve cells play a crucial role in the transmission of signals to different parts of the body, blood cells play an important role carrying oxygen to different parts of the body. The differences in structure and functions between the cells mean that they are specialized cells.

To be able to perform different functions, cells have to become specialized. This becomes possible through the process referred to as cell specialization. A cell capable of differentiating into any type of cell is known as "totipotent". For mammals, totipotent includes the zygote and products of the first few cell divisions. There are also certain types of cells that can differentiate into many types of cells.

These cells are known as "pluripotent" or stem cells in animals meristemic cells in higher plants. While this type of cell can divide to produce new differentiated generations, they retain the ability to divide and maintain the stem cell population making them some of the most important cells.

Examples of stem and progenitor cells include:. Hematopoietic Stem Cells - These are from the bone marrow and are involved in the production of red and white blood cells as well as the platelets. Mesenchymal Stem Cells - Also from the bone marrow, these cells are involved in the production of fat cells, stromal cells as well as a given type of bone cell.

Epithelial Stem Cells - These are progenitor cells and are involved in the production of certain skin cells. Muscle Satellite Cells - These are progenitor cells that contribute to differentiated muscle tissue. The process of cell differentiation starts with the fertilization of the female egg. As soon as the egg is fertilized, cell multiplication is initiated resulting in the formation of a sphere of cells known as the blastocyst. It's this sphere of cells that attach to the uterine wall and continues to differentiate.

As the blastocyst differentiates, it divides and specializes to form a zygote that attaches to the womb for nutrients. As it continues to multiply and increase in size, the differentiation process results in the formation of different organs.

Once the female egg has been fertilized, the cells formed after cell division contain DNA that is identical. That is, the DNA in all the cells will be identical. However, different regions of a chromosome DNA is wound in to a chromosome code for different functions and cell type. Here, it's only the regions that are required to perform a given function that are expressed in each cell.

The regions genes that are expressed determine the type of cell that will be created. While the different types of cells that are formed contain the same DNA, it's the expression of different genes that results in different types of cells. This is to say that not all genes are expressed during differentiation. During the differentiation process, cells gradually become committed towards developing into a given cell type. Here, the state of commitment may be described as "specification" representing a reversible type of commitment or "determination" representing irreversible commitment.

Although the two represent differential gene activity, the properties of cells in this stage is not completely similar to that of fully differentiated cells.

For instance, in the specification state, cells are not stable over a long period of time. There are two mechanisms that bring about altered commitments in the different regions of the early embryo. These include:. Cytoplasmic Localization - This occurs during the earliest stage of embryo development.

Here, the embryo divides without growth and undergoes cleavage divisions that produce blastomeres separate cells. Each of these cells inherit a given region of the cytoplasm of the original cell that may contain cytoplasmic determinants reuratory substances.

Once the embryo becomes a morula solid mass of blastomeres it is composed of two or more differently committed cell populations. The cytoplasmic determinants may contain mRNA or protein a given state of activation that influence specific development. Induction - In induction, a substance secreted by one group of cells causes changes in the development of another group. During early development, induction tends to be instructive in that tissue assumes a given state of commitment in the presence of the signal.

In induction, inductive signals also evoke various responses at varying concentrations which results in the formation of a sequence of groups of cells, each being in a different state of specification. During the final phase of cell differentiation, there is formation of several types of differentiated cells from one population of stem cells of the precursor. Here, terminal differentiation occurs both in embryonic development as well as in tissues during postnatal life.

Control of the process largely depends on a system of lateral inhibition. That is, cells differentiating along a given pathway send out signals which repress similar differentiation by the neighboring cells. A good example of this is with the developing CNS of vertebrates central nervous system.

In this system, neurons cells from the tube of neuropithelium possess a surface receptor known as Notch and a cell surface molecule known as Delta that can bind to the Notch of adjacent cells and activate them. This activation results in a cascade of intracellular events that ultimately result in the suppression of Delta production as well as the suppression of neuronal differentiation.

As a result, the neuropithelium ends up only generating a few cells with high expression of Delta surrounded by a larger number of cells with low expression of Delta. As previously mentioned cell differentiation is a process through which a generic cell evolves into a given type of cell and ultimately allowing the zygote to gradually evolve in to a multicellular adult organism.

Cell differentiation is an important process through which a single cell gradually evolves allowing for development that not only results in various organs and tissues being formed, but also a fully functional animal.

While it plays a significant role in embryonic development, the process of cell differentiation is also very important when it comes to complex organisms throughout their lives. This is because of the fact that it causes changes in size, shape, metabolic activities as well as signal responsiveness of cells. For example, in adult mammal brains, neurons rarely divide.

However, glial cells in the brain continue to divide throughout a mammal's adult life. Mammalian epithelial cells also turn over regularly, typically every few days.

Neurons are not the only cells that lose their ability to divide as they mature. In fact, many differentiated cells lose this ability. To help counteract this loss, tissues maintain stem cells to serve as a reservoir of undifferentiated cells.

Stem cells typically have the capacity to mature into many different cell types. Transcription factors — proteins that regulate which genes are transcribed in a cell — appear to be essential to determining the pathway particular stem cells take as they differentiate.

For example, both intestinal absorptive cells and goblet cells arise from the same stem cell population, but divergent transcriptional programs cause them to mature into dramatically different cells Figure 1. Whenever stem cells are called upon to generate a particular type of cell, they undergo an asymmetric cell division.

With asymmetric division, each of the two resulting daughter cells has its own unique life course. In this case, one of the daughter cells has a finite capacity for cell division and begins to differentiate, whereas the other daughter cell remains a stem cell with unlimited proliferative ability. Figure 1: Transcriptional regulators can act at different stages, and in different combinations, through the path of cell development and differentiation.

Transcription factors can turn on at different times during cell differentiation. As cells mature and go through different stages arrows , transcription factors colored balls can act on gene expression and change the cell in different ways. This change affects the next generation of cells derived from that cell. In subsequent generations, it is the combination of different transcription factors that can ultimately determine cell type. Although most of the tissues in adult organisms maintain a constant size, the cells that make up these tissues are constantly turning over.

Therefore, in order for a particular tissue to stay the same size, its rates of cell death and cell division must remain in balance. A variety of factors can trigger cell death in a tissue. For example, the process of apoptosis, or programmed cell death, selectively removes damaged cells — including those with DNA damage or defective mitochondria. During apoptosis, cellular proteases and nucleases are activated, and cells self-destruct. Cells also monitor the survival factors and negative signals they receive from other cells before initiating programmed cell death.

Once apoptosis begins, it proceeds quickly, leaving behind small fragments with recognizable bits of the nuclear material. Specialized cells then rapidly ingest and degrade these fragments, making evidence of apoptosis difficult to detect. Figure 2 : Different cell types in the mammalian gut The gut contains a mixture of differentiated cells and stem cells.

The a intestine, b esophagus, and c stomach are shown. Through asymmetric division, quiescent stem cells d probably give rise to more rapidly dividing active stem cells, which then produce progenitor cells while losing their multipotency and ability to proliferate.

All these progeny cells have defined positions in the different organs. To maintain its function and continue to produce new stem cells, a stem cell can also divide into and produce more stem cells at the same position symmetric division. Stem cells in gastroenterology and hepatology.

All rights reserved. Figure Detail Tissue function depends on more than cell type and proper rates of death and division: It is also a function of cellular arrangement. Both cell junctions and cytoskeletal networks help stabilize tissue architecture. For instance, the cells that make up human epithelial tissue attach to one another through several types of adhesive junctions.

Characteristic transmembrane proteins provide the basis for each of the different types of junctions. At these junctions, transmembrane proteins on one cell interact with similar transmembrane proteins on adjacent cells. Special adaptor proteins then connect the resulting assembly to the cytoskeleton of each cell. The many connections formed between junctions and cytoskeletal proteins effectively produces a network that extends over many cells, providing mechanical strength to the epithelium.

The gut endothelium — actually an epithelium that lines the inner surface of the digestive tract — is an excellent example of these structures at work. Here, tight junctions between cells form a seal that prevents even small molecules and ions from moving across the endothelium. As a result, the endothelial cells themselves are responsible for determining which molecules pass from the gut lumen into the surrounding tissues. Meanwhile, adherens junctions based on transmembrane cadherin proteins provide mechanical support to the endothelium.

These junctions are reinforced by attachment to an extensive array of actin filaments that underlie the apical — or lumen-facing — membrane. These organized collections of actin filaments also extend into the microvilli , which are the tiny fingerlike projections that protrude from the apical membrane into the gut lumen and increase the surface area available for nutrient absorption.

Additional mechanical support comes from desmosomes , which appear as plaque-like structures under the cell membrane, attached to intermediate filaments. In fact, desmosome-intermediate filament networks extend across multiple cells, giving the endothelium sheetlike properties.

In addition, within the gut there are stem cells that guarantee a steady supply of new cells that contribute to the multiple cell types necessary for this complex structure to function properly Figure 2. The extracellular matrix ECM is also critical to tissue structure, because it provides attachment sites for cells and relays information about the spatial position of a cell. The ECM consists of a mixture of proteins and polysaccharides produced by the endoplasmic reticula and Golgi apparatuses of nearby cells.

Once synthesized, these molecules move to the appropriate side of the cell — such as the basal or apical face — where they are secreted. Final organization of the ECM then takes place outside the cell. To understand how the ECM works, consider the two very different sides of the gut endothelium.

One side of this tissue faces the lumen, where it comes in contact with digested food.