CELL DIFFERENTIATION: IN ANIMALS AND PLANTS - BIOLOGY - 2025

The cell differentiation is the gradual phenomenon whereby multipotential cells of organisms achieve certain specific characteristics. It occurs during the development process and physical and functional changes are evident. Conceptually, differentiation occurs in three stages: determination, differentiation itself, and maturation.

These three mentioned processes occur continuously in organisms. In the first stage of determination, the multipotential cells in the embryo are assigned to a defined cell type; for example, a nerve cell or a muscle cell. In differentiation, cells begin to express the characteristics of the lineage.

Finally, maturation occurs in the last stages of the process, where new properties are acquired that result in the appearance of characteristics in mature organisms.

Cell differentiation is a process that is very strictly and precisely regulated by a series of signals that include hormones, vitamins, specific factors, and even ions. These molecules indicate the initiation of signaling pathways within the cell.

Conflicts may occur between the processes of cell division and differentiation; therefore, development reaches a point where proliferation must cease to allow for differentiation.

General characteristics

The process of cell differentiation involves the change in shape, structure, and function of a cell in a given lineage. In addition, it implies the reduction of all the potential functions that a cell can have.

The change is governed by key molecules, between these proteins and specific messenger RNAs. Cellular differentiation is the product of the controlled and differential expression of certain genes.

The differentiation process does not involve the loss of initial genes; what happens is a repression in particular places of the genetic machinery in the cell that is undergoing the process of development. A cell contains about 30,000 genes, but it only expresses about 8,000 to 10,000.

To exemplify the previous statement, the following experiment was proposed: the nucleus of a cell already differentiated from the body of an amphibian -for example, a cell from the intestinal mucosa- is taken and implanted in the ovule of a frog whose nucleus was previously extracted.

The new nucleus has all the information necessary to create a new organism in perfect condition; that is, the cells of the intestinal mucosa had not lost any genes when undergoing the differentiation process.

Cell differentiation in animals

Development begins with fertilization. When morula formation occurs in the development processes of the embryo, the cells are considered totipotent, which indicates that they are capable of forming the whole of an organism.

Over time, the morula becomes a blastula, and the cells are now called pluripotent because they can form the body's tissues. They cannot form the complete organism because they are not capable of giving rise to the extra-embryonic tissues.

Histologically, the fundamental tissues of an organism are the epithelial, the connective, the muscular and the nervous.

As cells progress further they are multipotent, because they differentiate into mature and functional cells.

In animals - specifically in metazoans - there is a common pathway of genetic development that unifies the ontogeny of the group thanks to a series of genes that define the specific pattern of body structures, controlling the identity of the segments in the anteroposterior axis. of the animal.

These genes code for particular proteins that share a DNA-binding amino acid sequence (homeobox in the gene, homodomain in the protein).

Turning genes on and off

DNA can be modified by chemical agents or by cellular mechanisms that affect - induce or repress - the expression of genes.

There are two types of chromatin, classified according to their expression or not: euchromatin and heterochromatin. The former is loosely organized and its genes are expressed, the latter has a compact organization and prevents access to the transcription machinery.

It has been proposed that, in the processes of cellular differentiation, the genes that are not required for that specific lineage are silenced in the form of domains made up of heterochromatin.

Mechanisms that produce different cell types

In multicellular organisms there are a series of mechanisms that produce different types of cells in development processes, such as the secretion of cytoplasmic factors and cell communication.

The segregation of cytoplasmic factors involves the unequal separation of elements such as proteins or messenger RNA in the processes of cell division.

On the other hand, cell communication between neighboring cells can stimulate the differentiation of various cell types.

This process occurs in the formation of ophthalmic vesicles when they meet the ectoderm of the cephalic region and cause the thickening that forms the lens plates. These fold to the inner region and form the lens.

Cell differentiation model: muscle tissue

One of the best described models in the literature is the development of muscle tissue. This tissue is complex and is composed of cells with multiple nuclei whose function is to contract.

Mesenchymal cells give rise to myogenic cells, which in turn give rise to mature skeletal muscle tissue.

For this differentiation process to begin, certain differentiation factors must be present that prevent the S phase of the cell cycle and that act as stimulants of genes that cause the change.

When these cells receive the signal, they start the transformation towards myoblasts that cannot undergo cell division processes. Myoblasts express genes related to muscle contraction, such as those encoding the actin and myosin proteins.

Myoblasts can fuse with each other and form a myotube with more than one nucleus. In this stage, the production of other proteins related to contraction occurs, such as troponin and tropomyosin.

When the nuclei move towards the peripheral portion of these structures, they are considered a muscle fiber.

As described, these cells have proteins related to muscle contraction, but lack other proteins such as keratin or hemoglobin.

Master genes

Differential expression in genes is under the control of "master genes." These are found in the nucleus and activate the transcription of other genes. As their name indicates, they are key factors that are responsible for controlling other genes by directing their functions.

In the case of muscle differentiation, the specific genes are those that code for each of the proteins involved in muscle contraction, and the master genes are MyoD and Myf5.

When regulatory master genes are missing, subaltern genes are not expressed. In contrast, when the master gene is present the expression of the target genes is forced.

There are master genes that direct the differentiation of neurons, epithelial, cardiac, among others.

Cell differentiation in plants

As in animals, plant development begins with the formation of a zygote inside the seed. When the first cell division occurs, two different cells originate.

One of the characteristics of plant development is the continuous growth of the organism thanks to the continuous presence of cells that have an embryonic character. These regions are known as meristems and are organs of perpetual growth.

The differentiation pathways give rise to the three tissue systems present in plants: the protoderm that includes the dermal tissues, the fundamental meristems and the prochange.

Prochange is responsible for creating the vascular tissue in the plant, formed by xylem (transporter of water and dissolved salts) and phloem (transporter of sugars and other molecules such as amino acids).

Meristems

Meristems are located at the tips of the stems and roots. Thus, these cells differentiate and give rise to the different structures that make up plants (leaves, flowers, among others).

Cellular differentiation of flora structures occurs at a certain point in development and the meristem becomes “inflorescence” which, in turn, forms the floral meristems. From here arise the floral pieces consisting of sepals, petals, stamens and carpels.

These cells are characterized by having a small size, cuboidal shape, a thin but flexible cell wall, and a cytoplasm with high density and numerous ribosomes.

Role of auxins

Phytohormones have a role in cellular differentiation phenomena, particularly auxins.

This hormone influences the differentiation of vascular tissue in the stem. Experiments have shown that application of auxins to a wound leads to the formation of vascular tissue.

Similarly, auxins are related to stimulating the development of vascular cambium cells.

Differences between animals and plants

The process of cell differentiation and development in plants and animals does not occur identically.

Cell and tissue movements must occur in animals for organisms to acquire a three-dimensional conformation that characterizes them. Furthermore, cell diversity is much higher in animals.

In contrast, plants do not have growth periods only in the early stages of the individual's life; they can increase in size throughout the life of the plant.

References

1. Campbell, NA, & ​​Reece, JB (2007). Biology. Panamerican Medical Ed.
2. Cediel, JF, Cárdenas, MH, & García, A. (2009). Histology Manual: Fundamental Tissues. Rosario University.
3. Hall, JE (2015). Guyton and Hall textbook of medical physiology e-Book. Elsevier Health Sciences.
4. Palomero, G. (2000). Lessons in embryology. Oviedo University.
5. Wolpert, L. (2009). Principles of development. Panamerican Medical Ed.