Axial-skeletal defects caused by Carbaryl in Xenopus laevis embryos

Embryotoxic effects of Carbaryl (CB), a widely used carbamate insecticide, was evaluated by modified Frog Embryo Teratogenesis Assay-Xenopus (FETAX), coupled with a histopathological screening of the survived larvae. X. laevis embryos were exposed to 1, 2, 4, 8, 16 and 24 mg/L CB from stage 8 to stage 47. From an estimated LC50 of 20.28 mg/L and TC50 of 8.43 mg/L a TI of 2.41 was derived, indicating that CB is to be considered teratogenic for X. laevis embryos. The most characteristic terata, classified as abnormal tail flexure, involved a significant percentage of larvae from 1 mg/L CB onward, reaching 100% at 24 mg/L CB. Histopathological screening revealed tail musculature and notochord as the main targets for CB. Skeletal muscle lesions consisted of myotomes reduced in size, showing myocytes with disorganized contractile systems and irregular myosepta, coupled with disarranged myocyte apexes. Notochords from CB exposed larvae appeared wavy or bent, with irregular connective sheaths and histologically characterized by protrusions of fibrous matrix and inclusions of ectopic cell masses. This axial-skeletal damage was hypothesized to be related both to the inhibition of acetylcholinesterase, with consequent muscular tetanic spasms, and to disorders in the organization of the connective tissue matrix surrounding the notochord.