Epithelial migration in organ culture. Effect of incubation in unsupplemented balanced salt solution

A serum-free primary culture system for studying cell-substrate interactions during newt epidermal cell migration

To study the interaction of migrating newt epidermal cells with purified extracellular matrix (ECM) molecules we have developed an in vitro migration assay using pieces of newt skin explanted onto culture dishes coated with various ECM molecules and cultured for 18 h in defined serum-free medium. Newt epidermal cells migrate out from explants placed on dishes coated with either collagen, vitronectin, fibronectin, or fibrinogen but not on albumin-coated or uncoated dishes. Explant outgrowth on collagen was best in CEM 2000 medium diluted to 60% of mammalian osmolarity. Other media such as RPMI 1640 or Ex-Cell 300, diluted similarly, may also be used although in our hands CEM 2000 always allowed more migration. We found no migration on collagen when skin explants were incubated in Holtfreter's solution (an amphibian saline solution that we have previously shown allows reepithelialization on amputated newt limbs). Supplementation of Holtfreter's solution with glucose did not improve its ability to support migration. By testing various supplement combinations in conjunction with CEM 2000 and RPMI 1640 we found that neither serum, insulin, selenium, transferrin, nor L-glutamine is required for explant outgrowth. Of the additives tested, outgrowth was stimulated only by insulin. Epidermal cell outgrowth on collagen was inhibited by both puromycin and cycloheximide, indicating the necessity for protein synthesis in this system. Whether the effects of these protein synthesis inhibitors are specifically on migration-related events or on general metabolic requirements is not clear. Inasmuch as there was no correlation (r = -0.227) between DNA synthesis (measured by incorporation of tritiated thymidine) and the amount of outgrowth, we believe that our assay is a measure of cell migration alone rather than a combination of mitosis and migration. This explant outgrowth system represents a new and relatively simple assay that can be used in the study of cell-substrate interactions during newt epidermal cell migration over extracellular matrix molecules in a defined serum-free environment.

Epibolin: a protein of human plasma that supports epithelial cell movement

Earlier studies suggested that there is a specific activity in mammalian serum and plasma that supports epidermal (epithelial) cell movement. This activity was shown to be nondialyzable and heat labile. In the studies reported here, using standard biochemical procedures--i.e., ammonium sulfate fractionation, ion-exchange and gel filtration chromatography, isoelectric precipitation, and preparative polyacrylamide gel electrophoresis--we have purified a factor from human plasma that supports epidermal cell movement. The factor travels as an apparent single band on disc gel electrophoresis and corresponds to a glycosylated single-chain protein of approximately 65,000 +/- 3,000 daltons. The purified fraction is necessary and sufficient for dissociated epidermal cells, (ii) outgrowth of epithelial sheets from skin explants, and (iii) epiboly, epithelial sheet movement over a floating skin explant. The purified fraction is active at a concentration of 1-2 micrograms/ml of growth medium. It is destroyed by trypsin and its activity is augmented more than 10-fold by a second, as yet unpurified, fraction of plasma. These studies support the notion that a single protein of plasma supports epidermal cell movement and that this protein may play an important role in wound closure. Because it supports epiboly, the most biologically relevant of the assays, it has been named epibolin.

Effect of cAMP and related compounds on newt epidermal cell migration both in vivo and in vitro

The effect of cyclic AMP and related compounds on both in vivo and in vitro epidermal cell migration during wound closure in the adult newt was examined. Cyclic AMP (cAMP) and N6,O2'-dibutyryl cyclic AMP (db-cAMP) inhibited migration both in vivo and in vitro when used with equimolar concentrations of theophylline, an inhibitor of 3',5'-cyclic nucleotide phosphodiesterase. Neither db-cAMP nor theophylline alone inhibited migration in vivo. Adenosine 5' monophosphate (AMP), cyclic guanosine 3',5' monophosphate (cGMP) and imidazole, a potentiator of phosphodiesterase were tested in vivo and had no effect on migration. Isoproterenol and epinephrine, which are known to stimulate adenylate cyclase, inhibited migration in vitro. Experiments using the protein synthesis inhibitor, cycloheximide, suggest that cAMP could be acting partially through regulation of protein synthesis but that other factors are involved. Dibutyryl cyclic AMP and theophylline had no effect on the incorporation of 3H-leucine into protein. The inhibition of migration both in vivo and in vitro provides further evidence for a role of cAMP in the regulation of cell motility.

Development in vitro of epithelial-cell monolayers derived from fetal rat pancreas

Purified epithelial-cell monolayers were generated in vitro from explants of fetal rat pancreas. The extent of the development of the epithelial monolayer, as determined by planimetric analysis, was enhanced by the application of two methodological procedures: (a) preincubation of fetal pancreas in situ at 27 degrees C for 5 hr prior to dissection and explantation; and (b) incubation of the explants in medium containing a high concentration (50% to 70%) of fetal bovine serum. By utilizing such culture conditions, sheets of contiguous epithelial cells, with little or no peripheral fibroblastic contamination, were maintained for 9 days. Whereas the majority of cells within the monolayer had morphological characteristics of pancreatic ductal cells, endocrine cells were identified by the specific immunocytochemical localization of insulin and glucagon. In addition, insulin could be detected in the incubation medium throughout the course of experiment. The simplicity of this preparation offers some advantages over other techniques including reduced chance of contamination and reduced cellular damage or death. It provides a model for future studies directed toward developing individual cell strains derived from pancreatic epithelial cells.

Human serum and epithelial spread in tissue culture

The dependence of epithelial spread on human serum in culture has been studied. Using measured pieces of mouse ear skin epithelial outgrowth about floating explants (epiboly) and from adherent explants was studied. Where compared directly the two systems show similar results. Because of its ease of quantitation, the adherent explant culture was studied in greater detail. In this system in the presence of serum appreciable spread was found only after 48 h but spread continued for at least the next 4 days. In the absence of serum only minimal epithelial spread occurred. Adding serum to deficient media enhanced spreading and removing serum from media depressed spreading. The extent of spread appeared independent of mouse age for the first 4-10 weeks though 2-week-old mouse skin showed quantitatively greater spreading. The activity in human serum responsible for epithelial spread acts under conditions of minimal DNA synthesis and is not reproduced by bovine serum albumin, fetuin, or serum that had been exposed to 100 degrees C for 5 min. The activity is not dialyzable and it is resistant to the protease inhibitors DFP and PMSF. These studies suggest that a specific serum component(s) serves to support epithelial spread in vitro.

Epithelial migration in organ culture. A morphological and time lapse cinematographic analysis of migrating stratified squamous epithelium

The migration of stratified squamous epithelium in organ cultures of rat palatal explants has been studied using scanning and transmission electron microscopy. The scanning microscope revealed plate-like folds at the margins, and microvilli on the bodies of cells. These structures were most highly developed on those cells nearest the leading edge of the sheet of cells and are interpreted as an index of cells that are migrating. The cells at the leading edge have broad flat pseudopodia in direct contact with the collagen bundles. A time lapse cinemicrographic study showed that the net forward movement of cells (nuclei) remote from the leading edge was at least as great as that at the leading edge immediately in front of them and the distance they travelled was greater than that of the leading edge. In transmission electron micrographs of these migrating epithelial cells from in vivo wounds, profiles that could correspond to the microvilli and plate-like folds could be found on the surface of the migrating cells. The results of this study suggest a simple model for the particular type of movement that occurs in stratified squamous epithelium in healing wounds where a mass of cells is produced that can both migrate into the wound and undergo stratification and cornification. A tracked vehicle shedding a broken track is used as an analogy of the model proposed.

The role of serum in the epithelial outgrowth of mouse skin explants

An experimental system is described which facilitates the measurement of epithelial outgrowth in vitro. Using this sytem the properties of the serum component supporting epithelial spread have been studied. It has been found that appreciable epithelial outgrowth occurs in concentrations of cow serum greater than 0.5% and that maximal outgrowth occurs at concentrations of 1% or greater. Only minimal outgrowth is apparent during the first day of culture. The lack of epithelial outgrowth in the absence of serum is not due to reduced cell viability since replacing serum after 3 days leads to cellular outgrowth. Though this activity is found in several sera studied (cow, fetal calf, human, horse, mouse and chicken), there are considerable quantitative differences between species. Epithelial cell outgrowth does not occur in the presence of medium plus bovine fibrin, serum albumin or thrombin. The serum component is heat labile and non-dialysable. It appears to be needed continuously and does not require DNA synthesis for its effect.