ROSTRAL TRUNCATION OF A CYCLOSTOME, LAMPETRA-JAPONICA, INDUCED BY ALL-TRANS-RETINOIC ACID DEFINES THE HEAD TRUNK INTERFACE OF THE VERTEBRATE BODY/

The effect of all-trans retinoic acid on embryogenesis was studied in a cyclostome, Lampetra japonica, Treatment with 0.05-0.5 mu M retinoic acid on early gastrula and early neurula resulted in loss of the phar ynx and in the rostral truncation of the neural tube, The mouth, phary nx, esophagus, heart, endostyle, and rostral brain were missing with g raded severity. In the severest case, the embryo consisted only of tru nk segments, especially myotomes that extended to the rostral end of t he axis. The effect appeared to be dose-and stage-dependent: Rostral p haryngeal arches were more vulnerable to a lower amount of retinoic ac id, and earlier treatment resulted in severer defects, The initial pro trusion of the anterior axis started equally in control and retinoic a cid-treated embryos, implying that the head morphogenesis is omitted i n treated embryos, By identifying the number of myotomes based on the differentiation of hypobranchial muscles, there seemed to be no myotom es lost by retinoic acid-induced truncation, The rostral truncation, t herefore, was not simply a Limitation of the anterior axis but was res tricted to the ventral portion; only the branchial arches disappeared with normally developing myotomes dorsally. The absent region can be d efined as the vertebrate head in a morphological sense, including the branchiomeric and preotic paraxial regions as well as the heart, The r esults suggest the presence of distinct programs between somitomeric a nd branchiomeric portions of the body, providing a developmental basis for the dual-metamerical body plan of vertebrates. (C) 1998 Wiley-Lis s, Inc.