Short communication: Initial evidence supporting existence of potential rumen epidermal stem and progenitor cells.

The bovine rumen epidermis is a keratinized multilayered tissue that experiences persistent cell turnover. Because of this constant cell turnover, epidermal stem cells and their slightly more differentiated daughter cells, epidermal progenitor cells, must exist in the stratum basale of rumen epidermis. To date, these 2 epidermal cell populations and any unique cellular markers they may possess remain completely uncharacterized in the bovine rumen. An important first step in this new research area is the demonstration of the relative abundance and existence of markers for these cells in rumen tissue. A related second step is to document rumen epidermal proliferative responses to an extrinsic signal such as nutrient concentration within the rumen. The objectives of this experiment were to evaluate the extrinsic effect of diet on (1) gene expression of 6 potential rumen epidermal stem or progenitor cell markers and (2) rumen epidermal cell proliferation within the stratum basale. Twelve preweaned Holstein heifers were fed either a restricted diet (R) or an enhanced diet (EH). Animals on R received a milk replacer (MR) diet fed at 0.44kg of powder dry matter (DM)/d (20.9% crude protein, 29.8% fat, DM basis) and EH received MR at 1.08kg of powder dry matter/d (28.9% crude protein, 26.2% fat, DM basis). All calves had access to a 20% crude protein starter and were weaned during wk 7 of the experiment. Lifetime DM intake was 0.73kg of DM/calf per day for R (5.88 Mcal of net energy/calf per day) and 1.26kg of DM/calf per day for EH (10.68 Mcal of net energy/calf per day). Twenty-four hours before slaughter heifers received an intravenous dose of 5-bromo-2'-deoxyuridine to label proliferating cells. Heifers were slaughtered at 8 wk of age, and rumen samples from the ventral sac region were obtained and stored in RNA preservative and processed for routine histology. Quantitative real-time reverse transcriptase PCR was used to analyze relative abundance of genes. Candidate genes for markers of epidermal stem and progenitor cells were β1-integrin (ITGB1), tumor protein p63 (TP63), keratin-14 (KRT14), Notch-1 (NOTCH1), Leu-rich repeat-containing G protein-coupled receptor 5-expressing (LGR5), and musashi-1 (MSI1). All genes were detected in the rumen tissue; ITGB1 was increased in EH compared with R. 5-Bromo-2'-deoxyuridine immunohistochemistry revealed that both R and EH rumen tissue had proliferating cells within the stratum basale of the rumen epidermis at the time of analysis. The EH diet resulted in an additive effect on cell proliferation. The percentage of cells in the stratum basale synthesizing DNA in preparation for mitosis nearly doubled (23.8±2.4% for EH vs. 14.7±2.0% for R) compared with calves fed R. This work represents the first attempt at characterizing rumen epidermal stem and progenitor cells. We demonstrated the relative abundance and existence of potential markers in rumen tissue and showed a rumen epidermal proliferative response to the extrinsic stimulus of nutrient concentration in the form of diet.