Yeah! You're starting NEW anatomy and physiology material! Congratulations on making it through all the review material! Epithelial tissue is described in chapter 5, as are the other types of tissues. Also, since epithelial tissue is one of the main tissues in skin, we will be discussing skin this week as well.
This page contains:
For another web site that goes in depth into epithelial tissue, check out this one from Vanderbilt or the HistoWeb (where you'll have to go to the epithelial page; note that the first simple squamous photo is a look from on top, not a cross section).
Tissues are groups of cells that lie together to accomplish a common function. There are four main types of tissues: epithelial, connective, muscular, and nervous. All of these tissues are found in our bodies, but epithelial tissue has a special function-- it must cover all the surfaces of the body. Therefore, it is found in our skin, and it is also found covering all the surfaces of the openings (each one is called a lumen) within our bodies.
Take a look at this photograph... It was taken of a section through a real human cadaver (male) off the NPAC Visisble Human Viewer website:
Do you see the lumen of the stomach? Do you see the lumen of the aorta? Do you see the lumina (spaces within blood vessels) in the liver? How about the lumina within the arms (blood vessels)?...
...Each of those spaces has to be lined with epithelial tissue! Got it? I hope so! (note: you can also see the skin on the outside of the entire body)
Epithelial tissue lies on a basement membrane. That means that underlying the cells that are the cellular component of the epithelial tissue, there is a layer of acellular ("a-" means not, so "acellular" means not cellular) material. This basement membrane can be thought of as a sticky layer to keep the epithelial cells attached to whatever underlies them. The bottom edge of the epithelial tissue abuts the basement membrane; this bottom edge is called the basal surface. The edge of the epithelial tissue that faces the lumen (or the outside world) is called the apical surface.
Here's a drawing of all of these characteristics:
In this drawing, the cells (all dark blue with visible nuclei) are tightly opposed to one another, held together with tight junctions (yellow). The cells are stuck onto a basement membrane (pink). The side of the tissue facing the lumen is the apical surface of the epithelial tissue; the side against the basement membrane is the basal surface.
Since epithelial tissue marks the border between our bodies and some kind of space, the epithelial tissue must be able to handle whatever it encounters within that space. If the space is our external world (that the skin has to come into contact with), the epithelial tissue must be pretty hearty. If the space is a tiny little air passage deep within your lung, not much besides air would ever get in there and the epithelial tissue does not have to offer protection.
Also, as a bordering tissue, it may also have to allow materials to pass through it. Think again about epithelial tissue in the deep air passageways within the lungs-- oxygen has to be able to freely cross this epithelial tissue to enter our bodies.
In list format, the functions of epithelial tissues (as taken from Table 5.1 on page 136) are:
Most of the types of epithelial tissue are shown in this figure... click on the type to see a short description! (note that the top edge, toward the label, is the apical edge, while the bottom, toward the pink basement membrane, is the basal edge)
Simple epithelium offers little protection... it has only one row of cells that make it up. But if that row is made up of columnar cells, the protection is greater.
Stratified epithelium is layers of cells. This always offers protection. Pseudostratified epithelium looks stratified at a first glance (because the nuclei are not all at one level), but upon further inspection, one notices that pseudostratified (columnar) epithelium is made up of cells which all reach both the basal and the apical edges of the tissue.
Other types of epithelium: stratified columnar (not very common) and transitional epithelium (found lining the bladder & seen in these 3 images). These will be discussed in lab.
At this point in time, only concern yourself with exocrine glands. Exocrine glands are those that are directly connected to the epithelial apical surface via a pore; for this reason, they secrete material directly onto the apical surface of the epithelium. Therefore, an exocrine gland dips down below the surface, but it is still lined with epithelial tissue since it faces a space. One example of an exocrine gland is a sweat gland.
Shape of a gland: There is specific terminology about the appearance of an exocrine gland
You'll notice in Figure 5.10 (page 142) and in these links that an exocrine gland can have a simple shape or a compound shape-- the difference between these two shapes is that the major duct to the surface is unbranched in a simple gland, whereas it is branched in the compound gland.
You should also notice in Figure 5.10 that a gland can be branched. A branched gland just means that if you look at the deepest end of a duct, its tip is branched into several secretory endings.
Finally, the shape of the secretory regions of the gland can be rounded, called alveolar, or elongate, called tubular. If it is a tubular gland, and the tubular end bends around a bit, it is called a coiled tubular gland.
Secretions of a gland: There is specific terminology related to exactly what a gland secretes
merocrine glands: only dissolved materials are secreted (via exocytosis)
apocrine glands: pieces/chunks of cells get pinched off in the secretion
holocrine glands: entire cells are secreted
Just get familiar with these terms... they'll come back soon as you start to learn about some specific glands. OK?
© 2011 STCC Foundation Press