Experimental cryosurgery investigations in vivo

Abstract

Cryosurgery is the use of freezing temperatures to elicit an ablative response in a targeted tissue. This review provides a global overview of experimentation in vivo which has been the basis of advancement of this widely applied therapeutic option. The cellular and tissue-related events that underlie the mechanisms of destruction, including direct cell injury (cryolysis), vascular stasis, apoptosis and necrosis, are described and are related to the optimal methods of technique of freezing to achieve efficacious therapy. In vivo experiments with major organs, including wound healing, the putative immunological response following thawing, and the use of cryoadjunctive strategies to enhance cancer cell sensitivity to freezing, are described.

Introduction

Cryosurgery is the use of freezing temperature to elicit a specific reactive response in tissue. The nature of the response depends upon the severity of the freezing injury. Minor freezing injury causes an inflammatory response, but severe freezing kills cells and results in destruction of tissue. An understanding of the mechanisms of cell and tissue injury due to freezing is only about sixty years in development. Though experience with cold injury and therapy is described in ancient manuscripts and in experiments with frostbite in the 19th century, most current knowledge is based on investigations that began in the 1940’s, initially with frostbite and then with the use of freezing techniques to preserve cells or produce local lesions for physiological investigations. By early in the 1960’s, experimental studies had shown that cell and tissue death from freezing was due to direct cell injury and to the vascular stasis that developed shortly after the thawing period. The relative importance of these two mechanisms of injury was a matter for debate. Investigators of frostbite were impressed by vascular stasis produced by freezing, but investigators of cryopreservation focused on ice crystal formulation and its deleterious effects. Nevertheless clearly both mechanisms are operative in freezing injury of any type or cause.

Experiments pertinent to cryosurgery on local tissue injury by freezing, which also began about sixty years ago, produce lesions which resemble frostbite. Investigations in vitro and in vivo have shown that the major mechanisms of injury are due to direct cell damage, which begins as tissues falls into hypothermia, and the failure of the microcirculation which develops after thawing [60]. The considerable research, which has been done in vitro has provided detailed information on the effects of freezing on cells. But substantial differences may exist between the effects of freezing cells in vitro versus freezing in vivo. The in vitro work, performed in the absence of the vascular factor of injury, permits close examination of the effect of the diverse facets of the freezing and thawing cycle. Whether directed at cell preservation or destruction, the in vitro work is of substantial importance and recently has clarified the molecular-based mechanisms in the complex cell death cascade [11]. Nevertheless, in cryosurgery, the effect of freezing on the vasculature is clearly a critical and perhaps dominant cause of cell death. With thawing, the microcirculatory failure in the previously frozen tissue is progressive, resulting in vascular stasis in about 1 h, which insures cell death from ischemia. Therefore research in vivo by cryosurgical techniques is critical to knowledge of freezing for therapy.

The range of investigations in vivo is substantial and covers every facet of cryosurgery. Many experiments focus on the technical features of cryosurgery, that is, the manipulation of the freeze–thaw cycle to enhance destruction or to achieve a cell-selective response. Other work investigates the monitoring of the freezing process in the tissue, principally by imaging, but also by other techniques. Many investigations detail the effects of freezing tissue in terms of wound healing which varies with the structure of the tissue. Recent research has investigated adjunctive therapy with cytotoxic drugs, chemical agents or irradiation as a means of enhancing the efficacy of cryosurgical treatment. Though experiments in vitro and clinical results are mentioned occasionally to support or verify the work in animals, this review focuses on the diverse aspects of cryosurgical research in vivo.