Cells, Tissues, and Organs -
Breast (Mammary Gland).
This milk-producing organ is full of pockets of secretory epithelium around milk-filled spaces and tubes, sometimes with smooth muscle visible in their linings, to carry the milk out. Strands of connective tissue should be visible, as well as bubbly-appearing fat tissue and outer skin.
Sperm are stored and nourished in this organ while they mature. The many open tubes of this organ contain sperm and are lined with epithelium and smooth muscles; between should be connective tissue, with epithelium around the whole structure. Bubbly-looking fat cells may be visible.
An eye focusses light though an epithelial lens, which may be visible in the middle of the section, on a screen of sense receptors, the retina. The eye is essentially hollow, so in some areas there is no tissue at all. Several layers of dotty-looking nerves preprocess the information before the large optic nerve carries that information off to the brain. Many of the "homemade" slides have folds on the section, which produce dark streaks - don't mistake these for tissue structures, and look for recognizable tissues in a nice flat area. Behind the retina is a black or dark-brown layer of epithelium that acts as a reflector, then the thick connective tissue layer, the sclera, seen outside as the white of the eye. Beyond that, some sections show outer epithelium, eye-steering muscles, and pads of fat tissue.
The femur is a thigh bone - this is a section through an thigh, minus the skin. Being from a young kitten, it is not completely mature, dense bone inside, but there is bone present, with blood-producing marrow in the center. The bone also contains fibrous connective tissue and has a surrounding epithelial sheathe. Around the bone are the muscles and connective tissue of the thigh. The underlayer of the skin, the dermis, may be visible, but the epithelial layer, the epidermis, is missing.
This is a lengthwise slice through almost an entire, slightly bent frog finger, whose form and function isn't all that different from a human finger. The outer epithelial skin also has many round glands. Under the skin is loose fibers of connective tissue, strands of tendons, and some cartilage around and between the finger bones, which are mostly hollow. There may be some skeletal muscle visible near the base of the finger.
The sections here are not great - some have folds in them (beware of dark streaks, or overlaps), and several have flaws where the cutting knife hit the hard dark blood clot in the chambers (the clot may be broken off and lying on the section, or the knife may have vibrated - "chattered" after hitting it, making light-dark parallel lines across the section). There are, however, good parts on every slide, where the cardiac muscle fibers can be seen well. It may take fine focus at high (40X) power to see the striations and the intercalated discs that zip the cells together. Other than lots of cardiac muscle, a heart also contains epithelial linings, connective tissues in the valves and reinforcements, and blood.
This digestive tube, where food made soupy in the stomach is broken down to small molecules and absorbed, is simply circular on the outside, with two layers of smooth muscle under the outer epithelium. The inside lining is highly folded and covered with fingerlike projections (villi, which have submicroscopic microvilli on them) of thick epithelium, with many mucus-secreting goblet epithelial cells over connective tissue; this arrangement of folds and villi increase the inner surface area across which nutrients are absorbed. Beneath the epithelium are many blood and lymph vessels to carry nutrients away, and occasional glands with a variety of functions.
Wastes from the cells are carried away in the blood; those wastes are removed from the blood in the kidneys. Under a microscope, this organ looks complex and confusing, but it's actually fairly simple. The outer functional layer, the cortex, has many filtering structures that look under the microscope like round blotches, almost like carnations in a thin white space. These filtering structures (glomeruli) are full of blood vessels with a leaky epithelial lining which lets anything under a certain molecular size pass - wastes, water, sugars, salts, etc. - through, leaving behind cells and larger molecules and the water that's stuck to them. All around the glomeruli are many many tubes, cut at many angles, some where the body absorbs back those things that filtered out that you don't want to lose (sugar, some salts, much of the water, etc.), and some where additional, large waste molecules are added to the fluid that will be urine. The tubes are all held together by thin strings of connective tissue.
This is a bit confusing - this was a mouse limb, but the angle that the limb was fed into the sectioning knife at made these more of a skin specimen. If you do this, don't do skin as another one of your specimens. The epithelial epidermis, with hair and hair follicles, should be visible. The dermis underneath, with its fibrous connective tissue, blood vessels, and sense receptors, should be visible, although all of these features just mentioned may not be. Often skeletal muscle can also be seen on these sections.
The thin, spongy epithelium that fills the liver all looks very much the same (the spaces faced by the epithelium in the spongy tissue may not be visible, but they are there). It's even more surprising considering all of the different jobs the liver does with these similar-appearing cells: materials from digestion are processed and detoxified, sugar is converted to starch and stored (or vice versa), bile is made, cell wastes are converted to urea, red blood cells are recycled, white blood cells are stored, and many more. The basic structures you should be able to see are the columns surrounding a loose opening called a central vein - the columns are usually roughly pentagons, contained by connective tissue, with a packet of blood and lymph vessels at each corner.
This section of lung is largely made up of alveoli, the bubbly, epithelial-lined air spaces where gases move in (mostly oxygen) and out (mostly carbon dioxide) of the blood vessels that surround them. Connective tissue allows the spaces to stay open. On most slide you can also see bronchioles, small breathing tubes. These tubes have an inner epithelium, a layer of smooth muscle, and many glands, along with reinforcing spirals (visible as round structures in a section) of cartilage and other connective tissues. There may be simpler-built blood vessels visible as well, thicker-walled, smooth muscled arteries and thinner-walled veins, both usually with some blood visible inside.
These are usually sections of what is known as teased muscle, where fibers have been separated out, sectioned, and stained. When the time comes to draw this tissue, you can't usually find more than one type, so the goal for Question 4 here will be to show the striations that run across the fibers (only visible on long fibers). These sections are very lightly stained, so creative focussing and adjustment of your iris diaphragm are very important to making the striations show up.
The "Mammal" sections are thinner, hairier, and less dense than the "Human" sections, and the "Limb" slide is also a skin section - you can't do this one and the limb specimen - but all of the same structures are visible in each, although to different extents. The denser-looking tissue here is the outer epithelium, and the fibrous or bubbly areas are connective tissue on the inside. The outer epidermis varies in thickness depending upon the level of wear & tear it's subjected to. This layer also dips to surround pit-like hair follicles, which may have glands and ducts attached to them. Under the epidermis, the dermis has fibrous, flexible connective tissue, sense receptors, and often bubbly-looking fat. Beneath the dermis you may be able to see skeletal muscle fibers, which could be cut at any angle.
Because of the way the organ was set up and cut, it can be difficult on these sections to figure out what's on the outside and what's on the inside; here, the layers will be described from the outside in. There is a thin, outer epithelial membrane, then three (they may not all be recognizably separate) layers of smooth muscle - the stomach churns as it breaks large chunks of food down to small chunks and begins to break down protein molecules. Below the muscle layers is a layer of connective tissue, then the thick inner epithelial lining with its long, thin, tightly-packed projections. Most of the inner epithelial cells are secreting materials, such as protective mucus, acid (inactive until beyond the mucus layer), and a protein-digesting enzyme (inactive until in the acid environment).
This male reproductive organ is full of sperm-producing tubes, cut at all angles, with a thin outer epithelium and several thick inner layers of sperm-producing cells and an inner space with free sperm. Between the tubes are often triangular-shaped strings of epithelial cells that produce testosterone, the male hormone. The outside of the organ has an inner and outer epithelium and may have strands of smooth muscle visible.
NOTE: Slides with a "+" mark include two other male structures: the epididymis, mentioned earlier in this handout, and the muscular vas deferens, which stores sperm and also helps push them out during ejaculation (this is the tube cut during a vasectomy). These structures also often have fat pads around them.
This is misspelled as "tounge" on many slides. It is a complicated structure, with a thick, bumpy epithelium - mice have rough tongues, so there may be pointed structures - above a complicated arrangement of skeletal muscles (look for striations) that run in many directions. Embedded in the epithelium but very difficult to see are tiny taste buds, and between the muscle fibers is connective tissue. Salivary gland epithelial tissue may be visible near the bottom of the tongue.
Original Version 1994; Web Version 2004, 2020. M. McDarby.
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