Mode of action of teratogenic factors: the effect depends predominantly on the stage of intervention of the agent (chronologic factor) and the genetic constitution (constitutional factor). Several types of sensitivity are listed
Time or stage of sensitivity
Before implantation: external agents, according to their intensity, provoke either completely rreversible lesions or definitive mortal lesions
After implantation and during the entire period of active morphogenesis: this is the principle teratogenic period because a primordium is most sensitive to teratogenic actions at the time of its appearance
a) The same substance can produce different malformations if given at different stages of morphogenesis
b) When more than 1 primordium develop simultaneously, the same agent can result in multiple malformations
Species sensitivity: an agent teratogenic for 1 species may not be so for another
Strain sensitivity: even in the same species, the percentage of malformations seen with any substance can vary according to strain and even the line
Individual sensitivity: even in the same animal litter subjected to a teratogenic influence, certain individuals react differently and may be free of any malformations. Even those malformed are not so to the same degree necessarily. Different metabolic peculiarities may explain these individual variations and also those seen between strains