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.
