Cell Differentiation

Introduction

The human body is made up of cells. A cell is the basic unit of life. Each cell is specialised to perform specific functions. Thus, there is a division of labor among the cells. For example, nerve cells are specialised to conduct nerve impulses, and similarly, muscle cells are specific for muscle movement. This ability of cells to perform specialised functions allows them to make different organs of the body and perform specific functions. This specialization occurs due to cell differentiation. 

Definition

Cell differentiation is defined as a biological process in which cells gain specialised functions in the body and change from one cell type to another. 

Everything in the body nails, hairs, organs, and even blood is made up of cells. It is by the process of cell differentiation, that embryonic cells are changed to specialized cells. 

Types of cells 

The embryonic cells, formed during the stages of fetus development and act as precursor cells that differentiated into specialized cells, in the body include: 

  1. Stem cells 
  2. Germ cells 

Stem cells 

A cell having the ability to differentiate into any type of cell is known as “totipotent” cells. In mammals, zygote and the products of the initial few cell divisions are the totipotent cells. Some other cells too have the ability to differentiate into different types of cells and are known as “pluripotent” cells or stem cells in animals. In higher plants, stem cells are known as meristemic cells. These undifferentiated cells are of two types: 

  • Embryonic stem cells 
  • Adult stem cells 

The cells formed during fetal development are known as embryonic stem cells and adult stem cells are present in the body at three main sites of the body: 

  • Adipose tissue 
  • Bone marrow 
  • Blood

These stem cells are further classified on the basis of their functions in the following types: 

  • Hematopoietic stem cells 
  • Mesenchymal stem cells 
  • Epithelial stem cells 
  • Muscle satellite cells 

Hematopoietic stem cells 

These are present in the bone marrow and are responsible for the production of blood cells i.e. red blood cells, white blood cells, and platelets. 

Mesenchymal stem cells 

These cells are present in the adipose tissue and originate from bone marrow. They are involved in the production of fat cells, stromal cells, and any type of bone cell. 

Epithelial cells 

These stem cells are responsible for the production of skin cells. 

Muscle satellite cells 

These stem cells differentiate into muscle tissue. 

These undifferentiated stem cells undergo division by mitosis and differentiate into diverse somatic cells (all the cells of the body other than the germ-line cells). 

Germ cells 

Germ cells are produced during embryonic development in the primitive streaks of the embryo and move through the gut to the maturing gonads (sex organs). In gonads, these undifferentiated germ cells are differentiated by meiosis to egg or sperm cells. 

Process/Mechanism of cell differentiation 

It all starts with the formation of a single-celled zygote. Later it is decided what specific cell will be formed by the cell. The human body consists of 210 different types of cells e.g. nerve cells, muscle cells, brain cells, etc. Each cell is formed as a result of gene expression of a particular gene that differentiates the cell to perform its specific functions. The process of cell differentiation starts very early in humans, during the fetus developmental stages, and continues in adulthood as well. Cell differentiation is thus a consequence of changes in the pattern of genes that makes cells specialized to perform their functions. So, changes in gene expression, as a result of mutation or environmental factors, can cause abnormalities in the process of cell differentiation. 

The process of cell differentiation begins with the fertilization of a female egg by male sperm resulting in the formation of a zygote. After fertilization, cell multiplication starts, and a clone of cells is formed that is known as a blastocyst. The blastocyst differentiates and specializes to form a zygote and then adheres to the womb for fulfilling nutritional requirements. The process of multiplication of blastocyst continues and it increases in size as well. Differentiation results in the formation of different organs of the body. 

Genetic material and cell differentiation 

After the process of fertilization, the cells formed as a result of cell divisions has genetic material (DNA) that is similar. The DNA is present on the chromosome and different regions of chromosomes are associated with different cell functions and production. Thus, only the regions of the chromosomes, that are expressed, are needed to perform a specialized function in a specific cell. 

These regions of chromosomes refer to the genes present in them. These genes determine the type of cell that will be formed after their expression. But not each and every gene is expressed during the process of cell differentiation. 

Many signal molecules known as growth factors are involved in the process of cell differentiation. There are two mechanisms that cause altered commitments in various regions of the embryo: 

  • Cytoplasmic localization 
  • Induction 

Cytoplasmic localization 

It occurs in the initial stages of embryonic development. In this condition, the embryo undergoes multiple divisions without growth that produces blastomeres (separate cells). All of these cells have a specific region of the cytoplasm of the parent cell that may have cytoplasmic determinants. 

At the stage of morula (a solid mass of blastomeres), the embryo contains two or more than two differently working cell populations. The cytoplasmic determinants, mRNA and protein, affect specific development. 

Induction 

During induction, a substance released by one specific group of cells results in developmental changes in some other group. The process is influenced by chemical signals and determinants. 

At the final phase of cell differentiation, different types of differentiated cells are formed from a single population of stem cells. The process of cell differentiation occurs in a controlled manner. After the production of the required number of differentiated cells, the process is inhibited by lateral inhibition. In this, the differentiating cells release signals that cause inhibition of similar differentiation by surrounding cells. 

Need of Cellular Differentiation

Cell differentiation is a natural process needed for the growth and development of an organism. Cells become specialized to perform specific functions. These specialized cells form tissues and similar tissues form organs that are specified to perform specialized functions. The most common reasons for cell differentiation are: 

  1. The continuous turnover of cells like blood cells in developed organisms. 
  2. The growth of a fetus into an adult organism. 
  3. For the repair of worn-out tissues; specialized cells are required in such a situation. 
  4. Under the influence of cytoplasmic changes. 
  5. Due to changes in the hormonal balance and activity. 

Factors affecting cell differentiation 

In multicellular organisms, the factors which affect cell differentiation include: 

Environmental factors 

Hormones are protein in nature and environmental factors such as temperature change, availability of oxygen, etc. affect the normal functioning of hormones. As a result, the process of cell differentiation and development is affected as well. 

Cell signaling 

Cell signaling is the process of sending chemical signals to the cells to tell them what type of cell or protein is required in body. The process of cell signaling includes: 

Diffusion: In diffusion, cells release chemical signals that spread to all tissues in the body. 

Direct contact: In this, cell membranes have special chemicals that act as signaling molecules. 

Gap Junctions: Chemical signaling molecules can migrate easily from one cell to another. 

Cells continuously send and receive chemical messages to coordinate their activities with their neighborhood in their specific tissues and organs. These signals are thus a major factor in cell specialization. 

Stage of development of an organism

In the embryonic stage, a greater number of cells are in an undifferentiated state. But as development occurs, the rate of cell differentiation is increased significantly. Thus, the stage of development directly correlates with the number of differentiated cells. 

Gene expression 

Gene contains crucial information for the functions of the cells. This information decides the physical traits as well. Any change in the gene can affect the information present on the gene. This will influence the cell differentiation process and organism development. 

Process of cell differentiation in Plant VS Animal cell

Cell differentiation in Plants 

No doubt, many hormones are included but cell differentiation in plants too occurs from a single cell. In plant cells, the seed protects and nourishes the zygote just like an egg in animal cells. After cell division, the zygote is divided into an embryo. 

The embryo then after cell differentiation is forms meristems. In roots girdling meristems, a root cap is formed which is removed when roots emerge through the soil and is replaced by meristems. Meristems on the surface are not alike and create both the inward and outward cells. The inward cells undergo further differentiation like the roots and produce more vascular tissues. The outer cells differentiate and form stem and leaves. This can be compared to the different organs in the animal cells where precursor cells are not similar. 

Cell differentiation in Animals 

In animals as well, the process of cell differentiation starts from a single cell and a multicellular organism is formed. Zygote is formed in animal cells as a result of the fertilization of egg and sperm. The zygote in animals is totipotent (Totipotent cells have the specialty to differentiate into any other cell type). All the complex tissues present in the advanced animals originate from the zygote. In animals, the process of cell differentiation starts early at embryonic stages. 

Errors in cell differentiation 

Errors in cell differentiation are possible and result in various abnormalities. These abnormalities are divided into three types: 

  • Anaplasia 
  • Dysplasia 
  • Metaplasia 

Anaplasia 

It is defined as the loss of the apparent cell differentiation that can happen in last stages of cancer. In the initial stages of cancer, the cancer cells copy the tissues from which they are formed. With the development in severity, variants of more variable features are generated which results in expanding malignancy. Consequently, a highly anaplastic growth occurs where the cancerous cells have no connection apparently with the parent tissue. 

Dysplasia 

It refers to the mutated cell arrangement resulting from the disturbance in the normal growth patterns of cells. Some of these mutations are extremely dangerous and lead to cancer while some are harmless and can be reversed immediately. 

Metaplasia 

It refers to the abnormal change of a differentiated cell to another type of differentiated cell. This change occurs as a result of a mutation that may occur due to some abnormal stimulus. This may possibly occur due to the change in the environment of the cell that the cell can’t withstand and change into some other differentiated cell type. Squamous metaplasia of bronchi is an example of this condition. 

Significance of cell differentiation 

Cell differentiation is an important process as it gives specialized functions to the cells. Most importantly, this process is responsible for the development of multicellular organisms. Firstly, cell differentiates and tissue is formed. Similar tissues combine and form organs which then develop a multicellular organism capable of performing diverse functions. 

Conclusion

The process of cell differentiation can both be reversible and irreversible. The reversible type of cell differentiation is described as “specification” and irreversible differentiation is referred to as “determination”. In the latest studies have shown that adult stem cells exhibit a reverse process of cellular differentiation. Such cells are known as IPS or induced pluripotent stem cells. 

Cell differentiation is a biological process occurring in all living organisms. By this process, the cells become specialized to perform specific functions of organisms by forming tissues (a collection of similar differentiated cells) that form organs. Stem cells have the ability to divide and differentiate into different types of cells. The stem cells are mainly present in bone marrow, blood, and adipose tissue. Cellular differentiation starts with the formation of a single-celled zygote which is then differentiated to perform different functions. This process is primarily controlled by gene expression, cell signaling that releases special chemical signals, and environmental factors. This process has great importance for the development of a multicellular organism by the differentiation of stem and germ cells.

References

Slack, J.M.W. (2013) Essential Developmental Biology. Wiley-Blackwell, Oxford.