The nucleus, nucleoli, and mitochondria can be readily observed in a single 3T3 Swiss mouse embryo fibroblast. The organelles are located centrally, in the plasmasol, a portion of the cytoplasm that is more liquid than the plasmagel, which is transparent and distributed around the periphery of the cell. Plasmasol and plasmagel are interchangeable, and the form the cytoplasm assumes apparently depends upon its location in the cell. Plasmasol that streams into the tip of a growing pseudopodium becomes gel-like plasmagel, and the plasmagel of resorbed surface extensions liquefies into plasmasol.
Before the 3T3 cell appearing in the field of view at the beginning of this video divides all of its lamellipodia and other surface extensions are retracted, forming a sphere. At this time, all of the contents of the cell are replicated so that when the cell divides identical nuclear and cytoplasmic materials can be bestowed to each daughter cell. During high speed playback of time-lapse sequences mitosis and cytokinesis appear to occur very rapidly without any specific instigation, but in actuality the progression of the cell cycle is tightly controlled by a considerable series of biochemical reactions.
One of the new 3T3 daughter cells quickly flattens following division by extending large lamellipodia over the culture substratum, a process that is soon mimicked by its partner. The lamellipodia and thin filopodia that are continually expanded and contracted by the cells enable them to crawl in opposite directions while several other cells begin to enter the field of view. As the surface extensions extend out over the substratum, they form contacts with the surface that are used by each cell as anchorage points for contraction, which pulls the fibroblasts along their individual paths. Sites of adhesion near the rear of the cells are released as each fibroblast continues to progress further away from the other. The multi-step process of fibroblast locomotion may seem swift in the video, but each cell only moves about 40 micrometers per hour, or about 1 millimeter per day.
