Toxic substances can pass to a developing fetus through the blood and by simple diffusion across the placental membranes. Currently, it is uncertain whether the placenta can actively prevent the transfer of toxicants from the mother to the fetus. However, the placenta has demonstrated biotransformation properties which could reduce exposure of some substances to the fetus (Klaassen and Rozman, 1991). The effect of an absorbed toxicant will depend on the ability of fetal tissue to concentrate the specific substance. For example, in laboratory mammals the livers of newborns and fetuses will not accumulate some foreign substances and will show much reduced toxicant levels in comparison to maternal liver tissue (Klaassen and Rozman, 1991). Because of a poorly developed blood-brain barrier, the fetal brain may be more susceptible to some toxicants.
In mammals, the effect of a toxicant on the fetus will also depend on the stage of development when exposure occurs (Manson and Wise, 1991). The difference between reversible and irreversible damage often is measured in days. Irreversible effects may be lethal (abortion or stillbirth is induced) or nonlethal (retarded or delayed growth of specific organ systems).
As stated in Chapter 1, it is difficult to verify tissue damage from exposure to crude oil in marine mammals. To date, chlorinated hydrocarbons (DDT, PCBs) have been implicated in reproductive disorders in pinnipeds (St. Aubin, 1990). Little information is available on the direct toxicological effects of petroleum hydrocarbons on the reproductive system, pregnancy, or fetal development. Following the EVOS, petroleum hydrocarbon concentrations were measured in the tissues of a newborn pup from a lightly oiled female sea otter in the rehabilitation center. Polycyclic aromatic hydrocarbons (PAHs) were found in the lungs, liver, and kidneys of the pup. Accumulation was especially marked in the kidney, which showed a mean PAH concentration that was three times the levels in other tissues (T. M. Williams, unpublished data). Although mortality of the pup may have been independent of oil contamination, maternal transfer of petroleum hydrocarbons to the developing fetus appears possible.
In mammals, some toxic agents may be transferred to milk by simple diffusion across blood vessels in the mammary glands. This, in turn, creates a pathway of exposure to the nursing animal. The high lipid content of milk promotes the accumulation of lipid-soluble toxicants including many petroleum hydrocarbon compounds. The problem is exacerbated in marine mammals, which have an exceptionally high concentration of lipids in their milk (Worthy, 1990). Colostrum (the secretion of the mammary glands immediately following parturition) contains an even higher lipid content than milk. Species differences in the excretion of lipid-soluble toxicants through milk will depend on the proportion of milk fat derived from the circulation versus milk synthesis in mammary tissue (Klaassen and Rozman, 1991).
Although the results are not conclusive, milk transfer of petroleum hydrocarbons has been implicated for both pinnipeds and sea otters. St. Aubin (1990) suggests that the effects of petroleum hydrocarbon exposure through nursing is heightened in immature animals due to their comparatively low levels of detoxifying enzymes. A colostrum sample taken from a heavily oiled sea otter showed high levels of total paraffinic hydrocarbons (941 ppm; T. M. Williams, unpublished data). Ideally, it would be beneficial to analyze the milk of oiled marine mammals before allowing newborns to nurse in rehabilitation centers.
