Everything You Need To Know About Multicellular Organisms 

Cells make up every living organism on the planet. Some organisms have only one cell – these are unicellular or single celled organisms.

Any organism that is made up of more than one cell is called a multicellular organism.

There is a wide variety of multicellular organisms including lots of different types of plants, animals and fungi.. 

Multicellular organisms are a lot more complicated than single celled organisms.

They are made up of different types of cells that have specialized functions. The cells vary in shape, size, structure and composition. 

We have put together this guide to tell you everything you need to know about multicellular organisms – how they develop, theur cellular processes, their interactions, and their microscopy.

Keep reading to find out more. 

What Are Multicellular Organisms? 

Multicellular organisms are any organisms that are made up of more than one cell.

This is not to be confused with colonial organisms that are made up of a group of identical cells that are also capable of surviving on their own. 

Colonial organisms such as staphylococci bacteria are single celled organisms that can carry out all of the necessary functions to keep themselves alive within one cell.

However, they often group together and spread. They are all genetically identical, they are the same size, and they contain the same organelles.

Single celled organisms have a short lifespan because that one cell has to carry out a lot of work. 

Multicellular organisms are made up of multiple cells with different functions that work together to keep one organism alive – the individual cells cannot survive on their own.

Each type of cell within a multicellular organism has a different structure and is made up of different organelles which allow it to carry out its purpose.

They vary in size depending on which area of the organism they are for – heart, liver, skin etc.

The cells differentiate to become the particular type of cell that the organism needs.

The cells divide and multiply so that when the older cells die there are new cells to replace them.

Multicellular organisms are always repairing and regenerating, which gives them a longer lifespan than single celled organisms. 

The Development Of Multicellular Organisms 

All multicellular organisms begin as just one cell.

This is surprising when you think about how large and complex some multicellular organisms are once they are fully developed!

The single cell grows and divides to create more cells, and the cells differentiate to form different parts of the organism.

The process is quite complicated, but the end result is a fully developed multicellular organism that came from just one cell. 

The first cell will be a fertilized cell – such as a human egg that has been fertilized by a sperm cell to become one cell.

The single fertilized cell contains all of the genetic information needed to create the multicellular organism.

As the cell grows and divides, selective gene expression caused by the genome will cause the cells to differentiate and specialize. 

Processes Of Development

There are different stages to the process of the development of multicellular organisms. 

 Step One – Cell Proliferation

The first step in the process of development is called cell proliferation. This is when the cells grow and divide.

The growth takes place during interphase, the first part of the life cycle of the cell.

At the end of interphase, the cell will then go into mitosis which is the process of somatic cell division.

At the end of Mitosis, the parent cell will have split into two daughter cells which are genetically identical to the parent cell.   

Cell proliferation is how one single fertilized cell can turn into a large, complex multicellular organism.

Proliferation of cells is a carefully regulated process which ensures that the organism is well balanced and functioning. 

Step Two – Cell Specialization

The next step in the process of development is cell specialization.

This is when the cells develop different characteristics which will differentiate them into specialized cells with particular functions within the multicellular organism.

The simple embryonic cells change in structure and begin to look different from each other. 

Cell specialization happens due to a process called gene expression.

As mentioned earlier, the fertilized cell that was the beginning of the multicellular organism contains all genetic information (DNA) to create all of the different types of cells that the organism needs.

Selective gene expression turns certain genes on and off as needed, so that the daughter cells will express particular genetic traits.

This is how cells differentiate and become specialized.

The gene expression enables the cells to develop differently, creating the necessary structure and composition for the cell to carry its intended function.

Signals from within the organism and from outside the organism will trigger the expression of certain genes. 

Cell specialization is incredibly important as it sets multicellular organisms apart from single celled organisms.

It does not just happen at the beginning of the organism’s lifespan – it continues.

For example, a simple zygote is transformed into a fetus and then a fully developed baby through the process of cell specialization – different cells are made for all of the different organs and systems that make up the human body and allow it to survive.

As that baby grows up, it will encounter new antigens. The body will respond by differentiating the stem cells in order to counteract the antigen that could be potentially harmful.

This is how humans develop an immune system. 

As the body continues to grow, new cells are needed all the time.

These cells need to differentiate to become different parts of the body.

Even after the body has finished growing, new blood cells and organ cells will need to be made as the body repairs and regenerates itself. 

Differentiation/specialization enables the cells to carry out their functions and it keeps the multicellular organism alive. 

Step Three – Interactions Between Cells 

A multicellular organism is made up of lots of different cells that have various functions.

The cells work together to keep the organism alive and healthy, which requires communication.

There are lots of physiological processes that take place within the organism, and various conditions that the organism has to be able to survive in.

This requires an element of flexibility and adaptability, which means the cells need to be able to communicate with each other to figure out what the organism needs and how to maintain the ideal conditions. 

There are different types of cell to cell interactions.

The interaction can take place between the surfaces of directly neighboring cells. It can also happen in the matrix – the liquid material in between the cells.

Different molecules act as signals that are transferred between the cells to send messages, triggering responses in the form of biochemical and biological processes.

These molecules include proteins, amino acids, enzymes, steroids, peptides and many more.

Specialized cells are able to pick up on certain signals and ignore others, which allows multiple different processes to happen at once without them disrupting each other.

We will look at the different types of cell interactions in more detail in the next section. 

Different Types Of Cell To Cell Interaction 

The various cells that make up a multicellular organism can interact with each other in different ways. Here are some of the methods that they use. 

Direct Contact Interaction 

The surface of the cells within multicellular organisms have different molecules on them such as carbohydrates, proteins and lipids.

The molecules on the cell surfaces can come into contact with each other, sending signals between the cells.

Increased or decreased levels of certain molecules can trigger a reaction in the cells. 

Direct contact interaction between cells is very common in the early developmental stages of the multicellular organism when more complex communications pathways have not yet been established. 

Paracrine Signaling

Molecules cannot travel very far through the matrix (extracellular fluids) without being destroyed by particular types of enzymes or by being removed.

Molecules need to be transported long distances around the organism by particular cells, often inside a vesicle. 

However, molecules can use the matrix to travel between local cells if they are close by.

This is called paracrine signaling. This is another form of signaling that takes place during the early development of the multicellular organism.

It is quite basic and short lived but it still allows signals to be sent successfully, if not always reliably.  

Endocrine Signaling

Endocrine signaling is when the molecule that is carrying the signal travels through the multicellular organism via a specific pathway such as the circulatory system.

Hormones are often the signal molecule for endocrine signaling.

Hormones have a longer lifespan than a lot of other molecules which allows them to reach their target area unharmed, even if it is quite far away in the body.

The hormones are released from glands such as the thyroid gland. 

Synaptic Signaling

Synaptic signaling is how cells within the nervous system interact with each other.

In this scenario, the signal molecule is a neurotransmitter and they can travel all over the body very quickly via the nervous system to deliver their signal.

The neurotransmitters travel through the nerve cell extensions until they reach the target cells where the signal is released at the border of the target cell- the cell membrane.

The target cell will then respond to the signal accordingly. 

Step Four – Cell Movement

Cell movement is the final stage of the development of multicellular organisms.

The cells move and arrange themselves in order to form tissues and organs rather than just being groups of specialized cells.

This happens at a certain stage of embryonic development when the cells receive a signal to coordinate and get into the right position. 

When cell movement goes wrong it can result in deformities in the multicellular organism- for example, babies born with bodily deformities.

This is because the cells have not moved into the right position and the fetus has not formed correctly.

Some deformities are fatal and others are just cosmetic.

There are also some deformities that cause some issues for the person, but they can be managed with treatment such as medication or surgery. 

Microscopy Of Multicellular Organisms

As mentioned earlier, there are lots of different types of multicellular organisms.

A lot of them are large enough to see without a microscope, such as plants and animals.

If you want to study individual cells and their structure then you will need a microscope, but the organism as whole can be viewed without one. 

There are some multicellular organisms, such as mites, that are too small to be seen with the naked eye.

These organisms will need to be viewed with a microscope. 

Microscopy is also used to study various cellular processes in multicellular organisms, and to research how cancer cells or different viruses interact with the body.

This type of research and study is often called pathology. 

What Is A Stereo Microscope? 

A stereomicroscope, also known as a dissecting microscope, is a binocular microscope.

It is used for looking at large specimens, focusing on the finer details. This is why it is ideal for looking at multicellular organisms. 

Stereo microscopes allow students to look at multicellular organism specimens like ants or flies whilst they are still alive.

This means that you can see what external structures like filaments look like without needing to stain the sample.

You can also use a stereo microscope to look at organisms like mites if you stain them first. 

How To Look At Mites Under A Stereo Microscope

If you are interested in using a stereo microscope then keep reading to find out what you will need and the steps you need to follow. 

Equipment and Requirements

• Stereo microscope
• Crystal violet 
• Detergent 
• Sodium chloride solution 
• Mesh Sieve (45um) 
• House dust (or dust from head pillows) 

Method

• Step One – First you will need to measure out 0.05 grams of your dust sample and add it to 30ml of sodium chloride solution
  Step Two – Next, add five drops of detergent to the sodium chloride solution with your dust sample in it
  Step Three – The sample will now need to be subjected to ultrasonic treatment for a total of 20 minutes 
  Step Four –  After 20 minutes has gone by, rinse the suspended sample using the mesh sieve 
  Step Five –  Now you will need to stain the sample using crystal violet to make sure it is visible through the microscope 
  Step Six – Now you can look at the sample through the stereomicroscope 

If you want an easier method you can place a small sample of the dust onto water and use 20x magnification to view the sample.

If you want to look at a larger multicellular organism, such as an insect or a leaf, then you can place it under the stereomicroscope and adjust the magnification until the surface of the organism comes into view. 

What Is A Compound Microscope?

A compound microscope is able to provide a much higher magnification than a stereo microscope which is why it is ideal for studying samples at a cellular level, including samples from multicellular organisms.

You can take a sample from a particular part of a multicellular organism and use the microscope to look at the structure of the specialized cell and the organelles within it. 

You could use it to view epithelial tissue from the lining of the mouth, or to study the various stages of mitosis of onion cells. 

How To Look At Cheek Cells Using A Compound Microscope 

If you want to know how to look at cheek cells using a compound microscope then this is what you need to know. 

Equipment and Requirements

• Compound microscope
• Glass Slide and cover slip 
• Cotton swab 
• Saline water 
• Methylene blue 

Method

• Step One – You will need to start by placing a drop of saline water on the center of the glass slide
• Step Two –  Next, use the cotton swab and swipe it around the inner lining of the cheek (yours or someone else’s) to take a sample. Gently dip the swab into the saline water drop on the glass slide to transfer part of the sample onto the slide
• Step Three – In order to make the sample visible you will need to stain it by adding a drop of methylene blue 
• Step Four –  Use the cover slip to cover the glass slide and sample. Begin with the microscope on low power then gradually increase the magnification. You can then sketch the cells as they look at different magnifications to look at different levels of the cell. 

What Is Histology? 

Histology is the anatomical study of biological tissue.

Microscopy is used to look at the biological tissue on a cellular level.

Histology is used by pathologists to diagnose certain conditions, but it is also used for scientific research and study. 

When a sample is prepared for a histological study, it is a lot more complicated than the previous methods described above.

Extremely thin slices of the biological tissue must be prepared, and they need to be fresh.

This is very challenging, as fresh tissue is usually delicate. The samples need to be supported in order to get thin sections without damaging the sample.

There are two methods that can be used to support the sample as it is being taken to make sure the sample is thin:

Freezing

If you freeze the sample before you cut it then you should be able to have more control over the thickness of the sample that you take.

The sample will then be referred to as a frozen section. Even though the sample is frozen, it will still be of good quality. 

Paraffin Wax

You can also use paraffin wax to fix the sample.

This is called embedding tissue, as the biological tissue is embedded into a paraffin wax block and can then be cut into thin sections.

The cutting is done with a sharp blade called a microtome which is used at an angle. 

Other fixing substances include aldehydes, picrates and alcohols.

Once you have been able to obtain a thin sample of good quality, the sample can be stained in preparation for viewing it under the microscope. 

How Do You Stain Tissue Culture? 

Before you can stain your sample of biological tissue, you need to remove the fixing agent that you used.

For example, the removal of paraffin wax is called deparaffinization and it involves using alcohol, water and xylenes to get rid of all of the paraffin wax.

First the sample is run through xylenes, then alcohol, then water. It will be washed and hydrated and ready for staining. 

The stain that you use will depend on the sample you have taken and what you are looking for.

Some examples of stains include hematoxylin and Eosin (also called H&E stain), congo red, silver salts, safranin, oil red O, and fast green. 

Summary 

A multicellular organism is any organism that is made up of more than one cell.

The cells cannot function on their own, but they work together to keep the organism alive.

This is one of the things that makes multicellular organisms different from single celled organisms or colonial organisms. 

Multicellular organisms develop through a process that takes place over four stages.

First, the cell grows and divides which is called proliferation. Next, the cells differentiate and become specialized.

This means that they take on different structures and compositions in order to carry out different functions within the multicellular organism.

These specialized cells can then interact with each other using different methods in order to react to the environment and conditions both within the multicellular organism and outside of it.

Finally you have cell movement, when the specialized cells group together and arrange themselves to form organs and tissue, creating distinct parts of the organism. 

The microscopy of multicellular organisms is very interesting.

You can use different kinds of microscopes to view the organisms in different ways.

For example, a stereo microscope can be used to look at the surface of the organism, whereas a compound microscope can be used to study the organism on a cellular level. 

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