Four basic tissue types are required to form an organ: epithelial tissue, connective tissue, muscle tissue, and nervous tissue.
Epithelial tissues either cover things (like organs or the body) or line things (like organs or glands). They are avascular, so they have no blood vessels within the tissue. They have a basement membrane from which the cells grow. With the epithelial cells on one side of the basement membrane, there is always connective tissue on the other side.
With epithelial tissues we must know two things: (1) how many layers of cells in the tissue so we can determine whether it is simple (1 layer), stratified (more than 1 layer), or pseudostratified (layered nuclei with all cells still attached to the basement membrane) and (2) the shape of the cells in the tissue so we can determine whether it is squamous (flat hexagonal), cuboidal (cube shaped), or columnar (rectangular).
Photo by Marian Rice, Micrograph Transparencies for the Life Sciences, 1990. The Benjamin/Cummings Publishing Company, Inc.
Ross, Michael H., et al., 1995. Histology: A Text and Atlas, Third Edition. Williams and Wilkins, Baltimore. pg 85.
The 1st photo shows a cross section of an alveolus in the lungs. We can see a single layer of very thin cells from the side forming a ring around a space. From the top the same cell would be shaped like a flat hexagon (2nd photo), attacted at each side to a neighbor cell to form a sheet. Our slides only show separated cells, but I thought these photos would be helpful.
Photo by Ed Reschke, Micrograph Transparencies for the Life Sciences, 1990. The Benjamin/Cummings Publishing Company, Inc.
This photo is different from the slides in class. The tissue of interest is the single layer of (kindof) cube shaped cells forming a ring around the "holes". This photo is of kidney tubules and has obvious cell membranes. The slide in class, from the thyroid gland, shows larger "holes" sometimes filled with pink hormone (which was secreted by the cuboidal cells). The tissue in class also has invisible cell membranes, so use your imagination!
Photo by Marian Rice, Micrograph Transparencies for the Life Sciences, 1990. The Benjamin/Cummings Publishing Company, Inc.
This shows parts of villi in the small intestine. The white space is the lumen. The closest single row of cells to the lumen are the simple columnar epithelium. We can see the shape of the cell is rectangular (look for the cell membranes). The nuclei are sitting at the bases of the cells, nearest the basement membrane. Note the connective tissue on the other side of the basement membrane. Microvilli on the lumen side of the cells look somewhat like cilia, so be careful not to mix this tissue with the next, pseudostratified ciliated columnar epithelium.
Photo by Ed Reschke, Micrograph Transparencies for the Life Sciences, 1990. The Benjamin/Cummings Publishing Company, Inc.
This is a higher power of the trachea than we can get in class, but we can see the stratified-looking nuclei and the cilia. The thing to remember about this tissue is that all the cells are attached to the basement membrane, unlike truely stratified tissues. Mature cells in this tissue are rectangular in shape.
Photo by Ed Reschke, Micrograph Transparencies for the Life Sciences, 1990. The Benjamin/Cummings Publishing Company, Inc.
This tissue lines the outside of our bodies and the insides of the vagina in females. It has a single layer of germinal cells (very dark purple) attached to the basement membrane (the wavy line between the very light pink and very dark purple). Above this layer are many layers of maturing cells. The closer we get to the top of these layers the flatter and more squamous-like they become. The tissue forms layers of sheets, with each sheet formed by squamous cells attached to each other. Beneath the basement membrane we can see dense connective tissue (very light pink).
Connective tissues fill the spaces in our bodies and perform many attachment functions. There are four major divisions of connective tissues: connective tissue proper, cartilage, bone, and blood tissues. We will only discuss the 1st three divisions. The important differences in these tissues are within the following three catagories: (1) cell type, (2) matrix type, and (3) fiber type(s). The cells secrete matrix (filler) and fibers (strengtheners). Matrix can be liquid, gel, or solid. Fibers are any combination of collagen (for strength) and elastin (for elasticity/bendability).
The following tissues (except adipose tissue) will have liquid matrix, both elastic and collagen fibers, and fibroblasts for cells.
Photo by Biophoto Associates/Photo Researchers Inc., Micrograph Transparencies for the Life Sciences, 1990. The Benjamin/Cummings Publishing Company, Inc.
The purple atrands are elastin and the pink (wider and kind of transparent) are collagen. The nuclei are of fibroblasts.
Photo by Ed Reschke, Micrograph Transparencies for the Life Sciences, 1990. The Benjamin/Cummings Publishing Company, Inc.
The cells here are adipocytes. There is very little matrix or fibers between the cells so don't worry about that. The white spaces are vacuoles (1 per cell) storing triglycerides. Only a small corner of the cells have the cytoplasm and nucleus.
Ross, Michael H., et al., 1995. Histology: A Text and Atlas, Third Edition. Williams and Wilkins, Baltimore. pg 97.
The nuclei are of fibroblasts which produce both collagen and elastin fibers and a liquid matrix. This tissue forms the perichomdrium and periosteum we will see in the cartilage and bone sections of connective tissue. In perichondrium, the fibroblasts differentiate into chondroblasts. In the periosteum, special fibroblasts called osteo-progenitor cells, differentiate into osteoblasts. This tissue also forms the wrapping layers of skeletal muscle: endomysium, perimysium, and epimysium; which together extend into tendons.
The following tissues will have gel matrix containing either predominantly elastic or predominantly collagen fibers secreted by two types of cells: chondroblasts (immature cartilage cells) and chondrocytes (mature cartilage cells within lacunae).
Photo by Ed Reschke, Micrograph Transparencies for the Life Sciences, 1990. The Benjamin/Cummings Publishing Company, Inc.
Our slides have a pink staining matrix. But note the "smoothness" of the gel matrix (between the chondrocytes) with predominately collagen fibers. We see chondrocytes and their lacunae. This cartilage has a perichondrium (dense connective tissue) which is not shown. It is found as articular cartilage on all bones, and as structural cartilage in the nose, between the ribs and sternum, and in the trachea.
Photo by Biophoto Associates/Photo Researchers Inc., Micrograph Transparencies for the Life Sciences, 1990. The Benjamin/Cummings Publishing Company, Inc.
This cartilage has predominantly elastin fibers in a gel matrix and also possesses a perichondrium. The matrix is much "hairier" than that of hyaline cartilage, due to the thinner, darker staining elastin fibers. It is found in the ear and the epiglottis.
Photo by Biophoto Associates/Photo Researchers Inc., Micrograph Transparencies for the Life Sciences, 1990. The Benjamin/Cummings Publishing Company, Inc.
This cartilage does not have a perichondrium. The chondrocytes secrete predominantly collagen fibers and gel matrix. It is found in the menisci of the knee, intervertebral discs, and in the pubic symphasis. Note chondrocytes in lacunae.
Bone tissue has a solid matrix and predominatly collagen fibers for strength. The cells which secrete the matrix and fibers are osteoblasts (immature) and osteocytes (mature with lacuna).
Compact bone is arranged into osteons or Haversian Systems. You can see three distinct osteons in the photo. The dark centers of the osteons are Haversian canals where blood vessels run. Concentric circles of bone matrix, called lamallae, are around the canal. The dark dots between the lamallae are lacuna where osteocytes would live. There are thread-like canals (too small to see here) connecting the lacuna to each other and the Haversian canal of the osteon. These are called canaliculi and are for diffusion of nutrients and waste to and from the osteocytes.
Muscle tissue is contractile, meaning it can shorten itself. We use the following three types of muscle tissue to move things around in our body. Three characteristics help us tell the types apart: (1) the cell shape, (2) the placement and number of nuclei, and (3) the level of oraganization of the contractile fibers, actin and myosin (ie. whether its striated or not).
Smooth muscle is an involuntary tissue (ie. it is controlled subconsciously). It is found in the walls of our blood vessels, digestive tract, and in our skin (attached to our hairs - goose bumps!).
The cells have a tapered shape and have low organization of actin and myosin, so they are not striated. A single nucleus is centrally located.
Cardiac muscle is also an involuntary tissue. It is found only in the walls of the heart.
The cells of this tissue are branched with a centrally located nucleus. This tissue is slightlly striated (not quite visible here), so the actin and myosin are much more organized than in smooth muscle tissue. A very identifyable characteristic of this tissue are the intercalated discs which connect cells together, which we can see as the horizontal lines in this photo.
Skeletal muscle is a voluntary tissue. We can control it by thinking about it. Attached to bones across joints, it allows us to move our skeleton.
Cells of this tissue are cylindrical and run the length of the muscle. Since the cells are so long, their many nuclei are spread the length of the cell. The actin and myosin are very highly organized so striations are prominant. The strands of actin and myosin are compacted into the center of the cell which causes the nuclei to be pushed to the periphery of the cell, just inside the cell membrane.
Skeletal muscle cells have a dense connective tissue layer around their cell membranes called the endomysium. The cells are grouped together into grous called fascicles which are surrounded by another layer of dense connective tissue called the perimysium. Many fascicles are grouped into a single muscle which is wrapped with a 3rd dense CT layer, the epimysium. These 3 dense CT layers extend past the muscle cells and combine to insert on the bone as a tendon.
Nervous tissue transmits information about senses and directions for movement from and to, respectively, all our body parts by electrochemical means.
Just be able to identify this as nerve tissue.
Please e-mail me with comments or suggestions. Last updated: June 11, 1996.
