Fibrotic focus is a pivotal morphofunctional unit in developing fibrosis in various tissues. For most fibrotic diseases, including progressive forms, the foci are considered unable to remodel and contribute to the worsening of prognosis. Unfortunately, the dynamics of the fibrotic focus formation and resolution remains understudied. A number of data suggest that the key cell type for focus formation are activated stromal cells marked by fibroblast activated protein alpha (FAPα) due to their high capacity for extracellular matrix (ECM) remodeling. We evaluated the dynamics of fibrotic focus formation and the contribution of the main cell types, including FAPα+ cells, in this process using a murine model of bleomycin-induced lung fibrosis. We revealed the very early appearance of FAPα+ cells in lungs after injury and assumed their important involvement to the myofibroblast pool formation. During the first month after bleomycin administration, FAPα+ cells colocalize with CD206+ M2 macrophages. Interestingly, during the reversion stage, we unexpectedly observed the specific structured foci formed by CD90+FAPα+ cells, which we suggested calling “remodeling foci”. Our findings highlight the crucial role of activated stromal cells in fibrosis initiation, progression, and reversion and provide emerging issues regarding the novel targets for antifibrotic therapy.

Background and Purpose Metabolic chambers are routinely used for urine collection in rodents. In mice, due to small urination volume, evaporation in the metabolic chambers distorts diuresis and urinalysis parameters. We have developed a new technique of chronic bladder catheterization enabling long-term accurate and contamination-free urine collection in awake male and female mice. The study was aimed to illustrate its validity for long-term monitoring of diuresis, pharmacological and pharmacokinetic applications. Experimental Approach Urine was collected in male and female mice implanted with bladder catheters or using the metabolic chambers for 30 days, at 1 h resolution to estimate diurnal variation or at 10 min resolution after water load, desmopressin, furosemide or phenol red administration. Key Results Urine recovery in the metabolic cages drops to 50 % at the typical murine urination volume. Daily diuresis in catheterized mice is more than twice higher as compared to the metabolic cages. The twofold differences in urine flow rate are preserved when the circadian variation of diuresis, the effects of furosemide, desmopressin and water load are estimated using the two techniques. Reliable pharmacokinetic curves for phenol red (90 % recovery) were obtained in catheterized mice. Urine osmolarity, urinalysis and microbiological parameters evidence higher quality of the catheter-collected urine. 30 days after the surgery catheters were patent and had minimal impact on the animals’ heath. Conclusion and Implications Bladder catheterization is a useful tool for pharmacological and toxicological studies that require urine collection in mice.