Cells: The Functional Units of Organisms
Robert Hooke first described cells in cork in Micrographia, pp 112-116 (1665). Curator of Instruments for Royal Society, viewed thin slices of cork under 30x lens, showed network of tiny compartments, he called cellulae (little rooms)
Later, the unified cell theory developed from the combined works of a botanist, Mattias Schleiden who published a study of the cellular nature of plant tissues in 1838, and a zoologist, Theodor Schwann, who studied animal tissues and illustrated cells he found in cartilage:
Robert Brown 1833, English botanist, using improved lenses, noted that every orchid epidermis cell had round structure he termed nucleus
Matthias Schleiden 1838, German botanist, concluded all plant tissues composed of cells, and embryonic plant arose from a single cell.
Theodor Schwann 1839, German zoologist, examined cartilage tissue (better cellular definition in cartilage), concluded all animal tissues are composed of cells
Rudolf Virchow 1855, Ger Physiol, concluded cells arose only by division of preexisting cells: Omnis cellula e cellula.
The unified cell theory, developed from their work:
UNIFIED CELL THEORY (three tenets):
1) all organisms made of cells
2) cell is the basic unit of structure
3) cells arise only from preexisting cells.
We will repeat some of these classical studies by preparing and examining a variety of plant and animal tissues to search for evidence of their cellular composition. Remember to strictly follow correct protocol for microscope use.
(Wash all tools immediately after each use. Do not let material dry out on them.)
single edge sharp razor blades
dropper bottle of dH 2 O
0.3% methylene blue stain
clean slides and cover slips
forceps and fine scissors
cartilage from breast bone of a young chicken
hyaline cartilage, trachea, H&E, H 680
1. CORK: Repeat the experiment of Robert Hooke: First slice off a “good clear piece of cork” with a single-edged razor blade to produce a smooth, clean surface. Then shave a wedge of cork as thin as possible from the clean surface. Examine the thinnest edge of the slice under the microscope, and illustrate its structure seen at 400x. What exactly are you seeing?
2. ONION EPITHELIUM: Prepare a slide with three spots: 1) dry, 2) drop of water, 3) drop of methylene blue. Slice a wedge from a fresh onion and discard the outer layer or two. Separate a fresh layer and break it back to the membranous outer layer of epithelium. Peel off epithelium with forceps, cut into three small pieces, apply one to each of the three prepared spots on the slide (keep flat). Cover with cover slips. Write out your observations of each: 1) dry mount, 2) wet mount with water, 3) wet mount with methylene blue. Which mounting technique resolved the most detail? Illustrate any evidence of stained cells seen at 100x and 400x (on the same page). Label: plasma membrane, nucleus, nucleolus, and cytoplasm. Search for stomata, especially in outer epithelium. Here is a stoma stained.
Onion Epithelium stained. Note the nuclei apparent in many of the cells.
3. CHICKEN CARTILAGE: Slice an ultra-thin section of the hyaline cartilage from a chicken breast bone as you did for the cork in exercise 1. Prepare a wet mount in water and a wet mount in methylene blue. Compare the two views. Illustrate any evidence of stained cells seen at 400x. Why was this material a fortunate choice for Schwann to first study animal cells? (cells = chondrocytes. The space between = matrix). (Wash tools immed.)
Chicken Cartilage stained. Chondrocytes are embedded in a matrix which does not take the stain as well as the cells.
HYALINE CARTILAGE, PREPARED SLIDE: Examine hyaline cartilage as seen in the prepared slide (H 680) from a cross section of the trachea. Compare with the section you prepared in 3, illustrate at 400x. Label: matrix, lacunae and chondrocytes.
5. CHICKEN MUSCLE: For a contrasting tissue, cut a very thin section of muscle, suspend in stain, squash under a cover slip. Note muscle fibers at 100x, striations at 400x. The greenish oblong features running along and above the muscle fiber are mitochondria. Here is a prepared slide of skeletal muscle labeled.