In this lesson on multicellular organisms, you'll take a look at what it actually means to be multicellular and how cells are organized into tissues, organs, and organ systems. This lesson also covers one of the four main tissue types: epithelial tissue.
Have you ever thought about what it means to be a multicellular organism? Probably not, but maybe that's because it seems like an obvious question. I mean, if an organism has more than one cell, it's multicellular. Case closed, right? Well, let's look at an example and see how easy it really is. If we have a colony of bacteria growing in a petri dish, do we define the colony as one multicellular organism, or billions of organisms living all piled on top of each other?
Let's start by defining an organism. An organism is defined as 'an individual living thing that is capable of responding to stimuli, growing, reproducing, and maintaining homeostasis.' Because each cell in the bacterial colony can perform all of these essential functions by itself, we define each cell as its own organism.
A key characteristic of multicellular organisms is specialization
By contrast, a multicellular organism is made up of many cells that are dependent on each other to perform these essential functions as a group. The cells have divided up the labor and each cell has become specialized to perform a specific task. This specialization is a key characteristic of multicellular organisms.
But why specialize? If a cell cannot perform all essential functions on its own, doesn't that limit what it can do? The answer to this question really depends on the level of organization that you're looking at, and there are several levels of organization within an organism. Starting at the cellular level, each cell has its own set of functions and requirements that it must carry out.
Organization of Organisms
Most cells form groups called tissues which are 'groups of cells that share a common structure and function within an organism.' Tissues are the next level of organization in an organism and are often very specialized to perform only one or maybe a few very specific tasks. For instance, muscle tissue is specialized to contract, and that's about all that it does.
The next level of organization above tissues is organs. An organ is 'a structure composed of several different tissues that work together to perform a certain task.' The tissues within an organ work together to perform more complicated tasks than any one tissue is capable of. An example of an organ would be the heart. The heart is mostly muscle tissue, but it is also composed of connective tissue which provides structure for the heart, an epithelial layer that serves as a barrier to keep the blood contained in the chambers, and nervous tissue which regulates the heart rate. Together, these tissues perform the more complicated task of pumping blood.
But we're not done yet, there's another level of organization above organs and that is the organ system, which is 'a group of organs and tissues that work together to perform a major function of the organism.' Let's go back to our heart example. The heart is one organ within the circulatory system which is itself composed of the heart, blood vessels, and blood. The heart pumps blood, but the whole circulatory system working together circulates blood throughout the body.
And finally, the last level of organization is the organism itself, which as a whole, is capable of all essential functions. So back to the question of whether or not specialized cells are more limited than single-celled organisms; at the cellular level, the specialized cell is limited to performing its specific task and is more limited than a single-celled organism. However, at the organismal level, specialization and division of labor opens up almost limitless possibilities for variation of structures, and it frees multicellular organisms from the limits of single-celled organisms in terms of size and complexity. In this set of lessons, we're going to talk about the anatomy and functions of animals, with a particular emphasis on humans.
Let's start by talking about tissues. In a human being, there are dozens of very specific tissues, but they can be classified into four main tissue types: connective tissue, epithelial tissue, muscle tissue, and nervous tissue. In this lesson, we'll take a look at the characteristics and functions of epithelial tissue. We'll talk about connective, muscle, and nervous tissue when we talk about those systems.
Epithelial tissue is 'tissue that is organized into tightly packed sheets of cells that line organs and body cavities.' In epithelial tissues, one or more layers of cells completely cover a basement membrane, which is a 'well-defined layer of extracellular matrix that serves as a base for epithelial tissues.'
Epithelial tissues can take many forms. They can be as simple as a single layer of very flat cells like the epithelial cells right here that line this blood vessel, or they can be a very complex mix of cells of different shapes and composition like all of these epithelial cells of the human skin in this picture here.
Epithelial tissues can also serve several functions. One major function of epithelial tissue, and especially skin, is to serve as a barrier to protect against injury, invading organisms, and loss of fluids. But epithelial tissues aren't just limited to serving as barriers. Some epithelial tissues are specialized to absorb nutrients like the epithelial tissues of the small intestine. You may notice in this picture of the small intestine, that we have a well-defined layer of epithelial cells here that sit on a very thin basement membrane that you can barely see here, and underneath, you have connective tissue right about here.
Some cells in respiratory epithelial tissue secrete mucus; others move mucus through the lungs
The epithelial cells come in contact with the contents of the small intestine and are organized into these finger-like structures here. These structures are called villi and are 'finger-like projections of epithelial tissue which increase surface area.' The increased surface area created by the villi allow a greater number of epithelial cells to have access to nutrients in the small intestine, and therefore, increases absorption efficiency. Other epithelial tissues like prostate gland epithelium are specialized for secretion of proteins. In this section through the prostate gland, we can see the round, blue nuclei of the prostate epithelial cells that line the ducts of the gland. In addition, one of the secretory products of the prostate has been stained brown in this particular section to show which cells are actively secreting proteins into the ducts, and as you can see based on the brown color, pretty much every cell lining the ducts is actively secreting proteins.
And finally, some epithelia like the lining of our respiratory tract, have some cells that secrete mucus, and other cells that have lots of small cilia on their cell surface that move the mucus and trapped particles out of the lungs. Here is a picture which shows a section through a ciliated epithelium. In this picture, you can see the basement membrane here, with several layers of cells on top, and then in the very top layer you can see some cells which secrete mucus here, here, and here, and other cells that have visible cilia located here, here, and here. The mucous cells secrete mucus, which is then moved by the cilia cells out of the lung.
So let's review. Multicellular organisms were created when cells started to specialize and divided up the labor among themselves. These groups of specialized cells that share a common structure and function are called tissues. In animals, there are four main tissue types: connective tissue, epithelial tissue, muscle tissue, and nervous tissue. Epithelial tissue is 'tissue that is organized into tightly packed sheets of cells that line organs and body cavities.' Epithelial tissues can serve several purposes with the three major functions being protection, secretion, and absorption. In epithelial tissues, one or more layers of cells completely cover a basement membrane, which is a 'well-defined layer of extracellular matrix that serves as a base for epithelial tissues.' Epithelial tissues can take many forms; they can be as simple as a single layer of very flat cells like the epithelial tissue that lines blood vessels, or they can be a complex mix of cells of different shapes and composition like the epithelial tissue of the human skin.