DETAILED STAGING OF INBRED C57BL 6 MICE BETWEEN THEILERS [1972] STAGE-18 AND STAGE-21 (11-13 DAYS OF GESTATION) BASED ON CRANIOFACIAL DEVELOPMENT/

A detailed staging table of inbred C57BL/6 embryonic mice was develope d to facilitate a study of the stage-by-stage cellular and molecular m echanisms underlying cranial skeletal development and elucidation of t he developmental mechanisms potentially involved in evolutionary chang es in cranial skeletal morphology exhibited by different inbred strain s of mice. Mice were mated for only 2 hr and embryos were recovered ev ery 2 or 4 hr between 11 and 13 days of gestation. Theiler's [1972] st ages 18 through 21 were divided into substages and divisions based on the development of five external structures-the frontonasal area, eyes , auditory meatus, mandibular and hyoid auricular hillocks/pinna, and vibrissae-and three internal histological structures-eyes, tongue, and vibrissae. Each substage and division was designated with a decimal p oint: e.g., substage 20.1 and division 20.11. Embryos were staged usin g the staging table and the relationship of the substages and division s with days of gestation was examined. The results showed considerable intra- and inter-litter variation in stages of embryos, suggesting th at days of gestation are not a good indicator for staging embryos. Our staging table offers a more reliable and precise method to standardiz e embryonic development. Regression analyses of substages on days of g estation showed that the duration of stages increased from stages 18 t o 21. Estimated durations were 3.5 hr for stages 18 and 19, 8.8 hr for stage 20, and 38.8 hr for stage 21. Our staging table also provides b aseline information on development of the frontonasal area (muzzle) an d vibrissae and development and the transformation of the auricular hi llocks into the pinna. The developmental sequence of mystacial and lab ial vibrissae indicated highly regulated differentiation and morphogen esis of vibrissal development at stages 20 and 21. Three hyoid auricul ar hillocks transiently became four hillocks at stage 19.1 before tran sforming to the pinna during stage 21. The second and third hyoid auri cular hillocks were the major contributor to the pinna before stage 21 .2, whereas mandibular auricular hillocks contributed to the pinna fro m stage 21.32 onward. The staging table has already served to demonstr ate stage-specific skeletogenesis of the first arch cartilages in inbr ed C57BL/6 mice and to reveal differences in the onset of timing of sk eletogenesis among inbred C57BL/6, C3H/He and CBA/J mice.