Professor Dave here, let’s talk tissue.
When learning about the human body, there is so much to talk about that it can be difficult
to know where to start.
But the best place to begin will be an overview of the types of tissues that can be found.
Tissues are collections of cells that are similar in structure and perform a common
or related function.
This is a very important feature in any multicellular organism, because something like a human being
contains trillions of cells.
If they were all the same, we could not be so complex, so it is the fact that cells can
become specialized that allows for the sophisticated appearance and behavior of a human.
It will be extremely important to recognize different types of tissues and know their
features, because every part of the body we will be looking at will be made of different
combinations of these types of tissues, so let’s go through a survey of these now.
We will be looking at four types of tissue all together, and those are epithelial tissue,
connective tissue, nervous tissue, and muscle tissue.
Let’s start with epithelial tissue.
In short, this is tissue that covers things, including our entire body.
The outer layer of our skin is made of epithelial tissue.
This tissue also dips into body cavities, covering the digestive and respiratory systems,
and it even lines blood vessels and various organs.
All of this can be considered covering and lining epithelium, which act as boundaries,
and any substance that enters or exits the body must pass through some kind of epithelium.
There is also glandular epithelium, which makes up all the glands in the body, and we
will discuss those in more detail a little later.
For now, let’s just describe the key features of epithelial tissue.
First, since we are talking about cells that line the exterior of the body or a cavity,
then we should distinguish between the apical surface, or the surface exposed to the exterior,
and the basal surface, which is attached to some connective tissue within, and the rest
of the organism.
The apical surface is sometimes smooth, but it is often lined with microvilli, which are
tiny projections of the plasma membrane meant to maximize the exposed surface area.
Because the two opposite sides of the cell have different features, we say that epithelial
cells exhibit polarity.
These features also exhibit different functionality.
Microvilli enhance certain cells’ ability to either secrete or absorb substances, while
other cells may have cilia lining the apical surface, which are hair-like structures that
propel substances along.
On the other side, the basal surface of these cells are attached to something called the
basal lamina, which is not made of cells, but rather glycoproteins and collagen fibers,
which act as a kind of scaffolding.
This is adjacent to the reticular lamina, and these two components together form the
basement membrane.
Supported by this basement membrane, epithelial cells will typically be found in sheets, right
up next to each other.
They are often bound together by desmosomes, which connect adjacent cells with filaments.
Depending on the specific function of the tissue, there can also be tight junctions,
which block the flow of fluids between cells, and gap junctions, which are pores that connect
adjacent cells, allowing for communication via the transmission of ions that can carry
electric current.
Next, we should be aware that epithelial tissues are avascular, which means there are no blood
vessels, but they are innervated, which means they are supplied with nerves.
We will learn much more about blood vessels and nerves later in the series.
And lastly, epithelial cells regenerate very quickly, by necessity, because given their
position at the boundary between some component of the body and the exterior, there is the
potential for significant friction, or even injury.
So these are the features of epithelial tissue, which allow it to perform a variety of functions,
including protection, absorption, filtration, excretion, secretion, and sensory reception.
Now let’s discuss further classification of epithelial cells, as they vary in the number
of layers present in a tissue, as well as the specific shape of each cell.
First, there can be simple epithelia, which means a single layer of cells, or there can
be stratified epithelia, which means there are two or more layers of cells stacked on
top of each other.
Then in terms of shape, each cell typically has a hexagonal base, which allows for close
packing, but the height of each cell can differ.
Squamous cells are very flat, kind of like scales.
Cuboidal cells are boxy, of medium height.
And columnar cells are very tall, like columns.
Putting these two characteristics together, we will need one word to indicate the number
of layers in the epithelium, and another word to indicate the shape of each cell in the
epithelium.
So for example, we can have a simple squamous epithelium, a single layer of flat cells,
with disk-shaped nuclei.
This looks kind of like a tiled floor, and this structure is ideal when rapid exchange
of materials by diffusion is important.
Two types of simple squamous epithelia are endothelium and mesothelium, which differ
in their location in the body, and we will talk more about these later.
Next we have simple cuboidal epithelium, a single sheet with boxy cells, perfect for
secretion and absorption.
Rounding out the simple epithelia, we have simple columnar epithelium, a single layer
of very tall cells.
These are also ideal for secretion and absorption, particularly in the context of digestion.
Now before moving on to stratified epithelia, we should mention pseudostratified columnar
epithelium, which is like simple columnar epithelium, but the nuclei are staggered in
their positioning, and sometimes the cells are of varying heights, with some not reaching
the apical surface.
This gives it the appearance of being stratified, hence the name.
Now let’s move on to stratified squamous epithelium, which is several layers of thin cells.
This is extremely common, and perfect for protection, which is why the outermost part
of our skin has this composition.
The cells at the surface are squamous, able to quickly regenerate if rubbed away, but
as we go deeper, the cells actually become cuboidal or even columnar.
Much more rare are stratified cuboidal and stratified columnar epithelia, where all the
layers have cells of these other shapes, but they are found in a few locations, like certain
types of glands.
Lastly, there is also transitional epithelium, which is able to stretch and change shape
to accommodate a cavity being filled with liquid, and this will be discussed when we
look at the urinary system.
Before moving on, we must mention glandular epithelia.
These are different from the types we have just mentioned, and they are found in glands,
which are collections of one or more cells whose purpose is to generate and secrete specific
substances, often signaling molecules like hormones.
These glands can be exocrine glands, which produce and secrete substances through a duct
onto an epithelial surface, or endocrine glands, which secrete their products directly into
the blood.
We will talk about the endocrine system later in the series, as well as a variety of exocrine
glands as they become relevant.
For now, that wraps up our introduction to epithelial tissue, so let’s move on to the
next type, connective tissue.