TARGET CELLS: CHARACTERISTICS AND EXAMPLE - BIOLOGY - 2025

A target cell or target cell is any cell in which a hormone recognizes its receptor. In other words, a target cell has specific receptors where hormones can bind and exert their effect.

We can use the analogy of a conversation with another person. When we want to communicate with someone, our goal is to deliver a message effectively. The same can be extrapolated to cells.

Source: Arturo González Laguna, from Wikimedia Commons

When a hormone is circulating in the bloodstream, it encounters several cells during its journey. However, only the target cells can "hear" the message and interpret it. Thanks to its specific receptors, the target cell can respond to the message

Definition of target cells

In the branch of endocrinology, a target cell is defined as any cell type that has specific receptors to recognize and interpret the message of hormones.

Hormones are chemical messages that are synthesized by the glands, are released into the bloodstream and produce some specific response. Hormones are extremely important molecules, as they play a crucial role in the regulation of metabolic reactions.

Depending on the nature of the hormone, the way to deliver the message is different. Those of a protein nature are not capable of penetrating the cell, therefore they bind to specific receptors on the membrane of the target cell.

In contrast, hormones of the lipid type if they can cross the membrane and exert their action inside the cell, on the genetic material.

Interaction characteristics

The molecule that is acting as a chemical messenger attaches itself to its receptor in the same way that an enzyme does to its substrate, following the pattern of the key and the lock.

The signal molecule resembles a ligand in that it binds to another molecule, which is generally larger.

In most cases, the binding of the ligand causes some conformational change in the receptor protein that directly activates the receptor. In turn, this change allows interaction with other molecules. In other scenarios, the response is immediate.

Most of the signal receptors are located at the level of the plasma membrane of the target cell, although there are others that are found inside the cells.

Cell signaling

Target cells are a key element in cell signaling processes, as they are in charge of detecting the messenger molecule. This process was elucidated by Earl Sutherland, and his research was awarded the Nobel Prize in 1971.

This group of researchers managed to pinpoint the three stages involved in cellular communication: reception, transduction and response.

Reception

During the first stage, the detection of the target cell of the signal molecule occurs, which comes from outside the cell. Thus, the chemical signal is detected when the binding of the chemical messenger to the receptor protein occurs, either on the surface of the cell or inside it.

Transduction

The binding of the messenger and the receptor protein alters the configuration of the latter, initiating the transduction process. At this stage, the signal is converted into a form that is capable of eliciting a response.

It may contain a single step, or encompass a sequence of reactions called the signal transduction pathway. Similarly, the molecules that are involved in the pathway are known as transmitter molecules.

Reply

The last stage of cell signaling consists of the origin of the response, thanks to the transduced signal. The answer can be of any kind, including enzymatic catalysis, organization of the cytoskeleton, or activation of certain genes.

Factors that affect the response of cells

There are several factors that affect the response of cells to the presence of the hormone. Logically, one of the aspects is related to the hormone per se.

The secretion of the hormone, the amount in which it is secreted and how close it is to the target cell, are factors that modulate the response.

Furthermore, the number, saturation level, and activity of the receptors also affect the response.

Example

In general, the signal molecule exerts its action by binding to a receptor protein and inducing it to change its shape. To exemplify the role of target cells, we will use the example of research by Sutherland and his colleagues at Vanderbilt University.

Epinephrine and glycogen breakdown

These researchers sought to understand the mechanism by which the animal hormone epinephrine promotes the breakdown of glycogen (a polysaccharide whose function is storage) within liver cells and cells of skeletal muscle tissues.

In this context, the breakdown of glycogen releases glucose 1-phosphate, which is then converted by the cell to another metabolite, glucose 6-phosphate. Subsequently, some cell (say, one in the liver) is able to use the compound, which is an intermediate in the glycolytic pathway.

In addition, phosphate can be removed from the compound, and glucose can fulfill its role as cellular fuel. One of the effects of epinephrine is the mobilization of fuel reserves, when it is secreted from the adrenal gland during physical or mental exertions of the body.

Epinephrine manages to activate the breakdown of glycogen, since it activates in the target cell an enzyme found in the cytosolic compartment: glycogen phosphorylase.

Mechanism of action

Sutherland's experiments reached two very important conclusions about the process mentioned above. First, epinephrine does not interact only with the enzyme responsible for degradation, there are other mechanisms or intermediary steps involved within the cell.

Second, the plasma membrane plays a role in signal transmission. Thus, the process is carried out in the three steps of signaling: reception, transduction and response.

Binding of epinephrine to a receptor protein on the plasma membrane of the liver cell leads to activation of the enzyme.

References

1. Alberts, B., & Bray, D. (2006). Introduction to cell biology. Panamerican Medical Ed.
2. Campbell, NA (2001). Biology: Concepts and relationships. Pearson Education.
3. Parham, P. (2006). Immunology. Panamerican Medical Ed.
4. Sadava, D., & Purves, WH (2009). Life: The Science of Biology. Panamerican Medical Ed.
5. Voet, D., Voet, JG, & Pratt, CW (2002). Fundamentals of Biochemistry. John Wiley & Sons.