Unsolved Problems October 28, 2016 Topics for class discussion:

Any particle that adheres to plasma membranes: iron, carbon black, plastic beads, gold beads.
Discovered by Francis H. C. Crick (1950): using microscopic iron particles and magnets. Exp. Cell Res. 1: 37-80

Crick assumed the moving particles must have been phagocytized, & inside the cytoplasm. He used magnetic fields to map forces inside cells.

Fluorescein labeled actin, injected into crawling cells gets assembled along the leading edges of crawling cells, and is pulled continually rearward, in the same directions and speeds as particles get transported on the outside surface of the plasma membrane.
The Fluid-Mosaic structure of biological membranes allows tangential traction forces to be exerted tangentially from actin on the inside to particles and collagen on the membrane's outside surface. .

Tissue culture cells crawling on clotted blood plasma produces convergent compression of gels.
Paul Weiss (1930s) interpreted this compression as being caused by shrinkage of the plasma gel.
However the same convergent distortion patterns are produced in thin sheets of silicone rubber.
The real cause of gel distortion is the same as retrograde transport of particles.
Collagen gels get distorted at longer distances (> 5 centimeters) than blood plasma clots (~1 millimeter).
But Weiss was probably correct that gel distortion serves to build anatomical patterns. .

Flexibility of elastic substrata can control which cell type embryonic stem cells differentiate into! (which was a surprise).
Directions of traction forces sometimes differ between cell types. For example, fish and amphibian skin epithelial cells ("keratinocytes") pull at 90 degrees to the direction the cells crawl! (which was not expected) .

Epithelial cells have apical surfaces and baso-lateral surfaces. Ciliated epithelia have their cilia or flagella only on their apical surfaces. Keratinizing epithelia have their germinal cells only on their basal surfaces. Pigmented epithelia concentrate their melanosomes just inside their apical plasma membranes. .

When any kind of epithelial tissue is cultured on a glass or plastic surface, their basal surfaces will (somehow!?) move to the side facing the glass, plastic, etc. For example, the cilia or melanosomes will get moved to the side of the epithelium away from the glass. If you put an epithelium with its apical surface oriented away from the glass, it will stay that way.
But if you plate out an epithelium with its apical surface "down", against the glass, then all the cells will (mysteriously) rotate end-for-end, so that their basal surfaces turn themselves toward the glass, and their apical surfaces will become reorganized /rotated/redirected to the side away from the glass. .

Even more remarkable is what happens when epithelial sheets of cells are cultured between two flat sheets of a permeable plastic (millipore filters): The epithelial cells rearrange to form two sheets with their apical surfaces facing each other, and each with a basal surface touching the plastic sheet!
(This was discovered by Prof. Mary Tyler, U. of Maine, when she was a graduate student here at UNC.) .

Later, I plated chicken embryo skin epithelia as a Moebius strip, with its basal surface down at one end and its apical surface down at the other end, and made a time lapse movie of how this epithelium behaved. The part with the basal surface juxtaposed to the plastic substrata remained in that orientation. The part with the apical surface facing toward the plastic inverted the polarity of its cells, so that what had been the apical surface became a new basal surface. The middle part of the strip of cells writhed and separated, part of it joining the area that had kept its basal surface facing the plastic and the other part merging into the part of the epithelium whose apical and basal surfaces had inverted. My expectation had been that the middle part might form two apical surfaces, one on each side of a cell sheet. It's difficult to out-think how experiments will turn out! This Moebius strip epithelium experiment deserves to be repeated with ciliated epithelia and at higher magnification. .

Heart muscle cells concentrate their adhesions into narrow bands perpendicular to the axis of acto-myosin fibers. These adhesions are named costameres, and line up along the Z-bands of sarcomeres. When heart muscle cells are cultured on thin sheets of silicone rubber, one compression wrinkle forms between each sequential costamere. (This was discovered by Barbara Danowski and Joseph and Jean Sanger at Penn.) .