The primary function of the placenta is to act as an interface between the dam and fetus. The anatomic structure of the chorioallantoic placenta in eutherian mammals varies between different animal species. The placental types in eutherian mammals are classified from various standpoints based on the gross shape, the histological structure of the materno-fetal interface, the type of materno-fetal interdigitation, etc. Particularly, the histological structure is generally considered one of the most useful and instructive classifications for functionally describing placental type. In this system, three main types are recognized according to the cell layers comprising the interhemal area: (1) epitheliochorial type (horses, pigs and ruminants), (2) endotheliochorial type (carnivores) and (3) hemochorial type (primates, rodents and rabbits). The number of cell layers in the interhemal area is considered to modify the transfer of nutrients between maternal and fetal blood and is one of the important factors with respect to the difference in placental permeability between animal species. Therefore, in reproductive and developmental toxicity studies, careful attention should be paid to the histological structure of the interhemal area when extrapolating information concerning placental transfer characteristics to different animal species.

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The placenta plays a pivotal role in fetal growth, and placental dysfunction and injury are associated with embryo/fetal toxicity. Histological examination of the rat placenta for safety evaluation provides valuable clues to the mechanisms of this toxicity. However, the placenta has specific and complex biological features unlike those of other organs, and placental structure dramatically changes depending on the time during the gestation period. Thus, time-dependent histopathological examination of the rat placenta should be performed based on the understanding of normal developmental changes in morphology and function. The placentas of rats and humans are both anatomically classified as discoid and hemochorial types. However, there are differences between rats and humans in terms of placental histological structure, the fetal-maternal interface, and the function of the yolk sac. Therefore, extrapolation of placental toxicity from rats to humans should be done cautiously in the evaluation of risk factors. This review describes the development, morphology, physiology, and toxicological features of the rat placenta and the differences between the rat and human placenta to enable accurate evaluation of reproductive and developmental toxicity in studies.

ABSTRACT. Paramylon is a -1,3-D-glucan isolated from Euglena gracilis Z. This study was designed to evaluate the suppressive effects of the oral administration of paramylon on the development of atopic dermatitis (AD)-like skin lesions induced by repeated application of 2,4,6-trinitrochlorobenzene (TNCB) in sensitized NC/Nga mice. The effects of paramylon were assessed by measuring macroscopical and histopathological findings of skin, ear swelling, serum levels of total IgE, interleukin-4 (IL-4) and interferon- (IFN-) and IL-18 and IL-12 contents in the skin lesions. Oral administration of paramylon inhibited the development of AD-like skin lesions as exemplified by a significant decrease in dermatitis scores for the back, ear swelling and hypertrophy of the skin, infiltration of inflammatory cells in the skin, and serum IgE levels. Oral administration of paramylon reduced serum levels of both IL-4 and IFN- and IL-18 and IL-12 contents in the skin lesions. Oral administration of paramylon did not cause weight loss, as was observed with prednisolone. These results suggest that paramylon inhibits the development of AD-like skin lesions in NC/Nga mice by suppressing both the T-helper (Th) 1 and Th 2 cell responses. Our results indicate that paramylon treatment could provide an effective alternative therapy for the management of AD.KEY WORDS: atopic dermatitis, -1,3-D-glucan, interferon-, interleukin-4, paramylon.