Topical dose delivery in the reptilian egg treatment model

Abstract

Developing assays to detect endocrine-mediated toxicity from in ovo or in utero exposure is a current challenge in regulatory toxicology. Some species of reptiles exhibiting a unique mode of sex determination, in which the incubation temperature during a critical period determines gonadal sex, have been explored as an in ovo model to screen environmental contaminants for endocrine effects. We critically review published egg-exposure studies and conclude that data regarding the pharmacokinetics of topically applied substances are insufficient to validate dose-response relationships for the effects of chemicals on in ovo endocrine function or gender determination in reptiles. The insufficiencies in these data largely result from methodological failures, including lack of measurement verification, failure to investigate and control extraneous factors affecting the measurements, and lack of independent replication of results. Considerable additional research will be necessary to alleviate these methodological inadequacies. Given the current status of the data, topical treatment of reptilian eggs cannot be considered to be a valid means of establishing causal relationships between chemical treatment and biological outcome.

Molecular Mechanisms of Sex Determination in Reptiles

“…Extra care should be taken in the design of experimental controls when compounds developed for mammals are used in reptiles. Finally, one should validate the assumption that compounds reach the embryo at concentrations that reflect the amount applied to the eggshell [Muller et al, 2007]. Rhen/Schroeder Sex Dev 2010;4:16-28 24…”

Nutrient and Organochlorine Pesticide Concentrations in American Alligator Eggs and Their Associations with Clutch Viability

“…In regard to the transfer of thiamine to the egg yolk, we were successful in increasing the level in the albumin, but our data indicate that thiamine did not move into the yolk compartment and might not have been available to the embryo. Indeed, other studies have also noted problems in achieving target egg yolk concentrations in reptilian eggs via topical treatments (Muller et al 2007a(Muller et al , 2007b. Although our thiamine and oxythiamine treatment experiments did not indicate any effects of treatment, similar studies involving treatment of fish eggs have demonstrated the effects of induced thiamine deficiency and thiamine amelioration on embryo and fry survival (Fitzsimons et al 2001).…”

Experimental exposure of eggs to polybrominated diphenyl ethers BDE‐47 and BDE‐99 in red‐eared sliders (Trachemys scripta elegans) and snapping turtles (Chelydra serpentina) and possible species‐specific differences in debromination

“…Studies using topical egg-dosing techniques for exposing reptile embryos to chemicals have often either not quantified transfer across the eggshell into the egg contents or done so in too few samples to achieve adequate characterization of variability [29]. However, topical dosing techniques can possibly serve as a viable option for controlled embryonic exposure studies of dose-response relationships as long as exposure is verified.…”

“…Schnars and colleagues [27] reported the highest hatching success (61%) after injection in snapping turtle eggs harvested directly from the oviducts of adult females prior to laying, which is a much lower success rate than in studies employing topical dosing techniques. Compared with reptile egg injection techniques, topical dosing methods do not have such a negative impact on hatching success, as shown in several studies reporting hatching success greater than 85% in the control groups [26,28,29].…”

“…Although topical dosing has been shown to be less detrimental to reptile embryos than egg injections, only a few studies have verified the percentage of the topically applied dose (transfer efficiency) that ultimately is incorporated into the egg contents, an issue that must be addressed to determine exposure and effect relationships [29]. The present study sought to quantify transfer of two PBDEs of different molecular weights and log K OW s (BDE-47 and BDE-99, molecular weights 485.8 and 564.7 g/mol, log K OW s 6.81 and 7.32, respectively) from the eggshell into the egg contents of redeared sliders and snapping turtles to validate the method for embryonic exposure and effect studies of PBDEs.…”