Development of a morphologically-based scoring system for postimplantationNew Zealand white rabbit embryos

Rodent whole-embryo culture (WEC) systems are welt-established, as are seve ral corresponding morphological scoring systems. Recently, WEC techniques f or rabbits have been developed, creating the need for a morphological evalu ation system in this species. Consequently, we developed a gestational-age- based quantitative morphology evaluation system for rabbit embryos. Detaile d descriptions of 21 embryonic structures, as collected from gestational da y (gd) 9-13 rabbit embryos, formed the basis for this evaluation system. Th ese descriptions were then developed into specific criteria for assigning n umerical scores to quantify the degree of development of each embryonic str ucture. The overall morphologic score was calculated as the average of the individual structure scores. To make the system as informative as possible, the numerical scale of the scoring system was gestationally age-based (i.e ., range of potential scores was 9.0-13.0). The scoring system was then app lied in the evaluation of New Zealand White (NZW) rabbit embryos explanted on gd 9 and cultured for 48 hr. Embryos grown in vitro developed normally, but at a slightly slower rate in vitro than in vivo, as evidenced by the lo wer morphology score (10.4 in vitro, 11.0 in vivo) and measures of growth ( somite number, total protein, and head length). This work firmly establishe s the normal archetype of embryonic development in the gd 9-13 NZW rabbit a nd provides an important tool for the advancement of mechanistic studies of rabbit embryos developing both in vivo and in vitro. Teratology 59:88-101, 1999. (C) 1999 Wiley-Liss, Inc.