Metamorphosis-related changes in leukocyte profiles of larval bullfrogs (Rana catesbeiana)

Abstract

Metamorphosis in amphibians is a fascinating phenomenon that offers physiologists a number of opportunities to study aspects of tissue growth and lysis in animals. In larval amphibians, tissues are formed early on during larval growth, then are vastly reorganized and broken down during metamorphosis, and the effects of this phenomenon on the relative distribution of white blood cells in circulation (i.e., leukocyte profiles) was first examined in the 1920s. Nearly a century later, there has been little work to continue this research avenue, although our knowledge of the roles of certain leukocyte types has improved. Moreover, recent interest in counts of leukocytes in amphibians for environmental monitoring purposes emphasizes the need to understand how white blood cells naturally vary throughout larval life. In this study, I examined blood smears from wild-caught larval and adult bullfrogs (Rana catesbeiana) of varying developmental stages and quantified the proportions and abundance (estimated from blood smears) of all leukocyte types at each stage. Lymphocytes were the most common cell found and appeared most abundant during the early growth phases of larvae. Neutrophils were the second most common cell and were also associated most with earlier phases. Eosinophils appeared directly associated with metamorphosis, generally increasing in abundance to a peak at metamorphic climax. Monocytes were also found most frequently in individuals experiencing metamorphic climax. Results in this study concerning eosinophils are consistent with those of urodeles and point to their role in the breakdown of tissues, which may resemble their role in modulating inflammation responses.