ENDOCYTOSIS AND EMBRYONIC INDUCTION
Tseng Mi-pai and Zhou Mei-yun (Shanghai Institute of Cell Biology,Academia Sinica)
In the study of primary induction inamphibian embryos,the transport of theinducing factors to the reactive cells has at-tracted much attention (Grunz and Staubach,1979;Toivonen,1979).but still remains tobe a problem unsolved. Attempt has been made with in vitro ex-periments to elucidate this problem.Ectodermexplants of early gastrula of Cynops orien-talis were treated with the crude extract ofguinea pig bone marrow (BME) which hasbeen proved to be a potent mesodermal inductor(Toivonen,1953).After the treatment withthe concentration of 3,000 μg/ml for 12 hours,the ectoderm explants were fixed immediatelyin order to see the uptake of the inductivefactors.Two series of control were carriedout:(1) isolated ectoderm treated with bovineserum albumen of the same concentration andfor the same length of time,and (2) ecto-derm cultured for the same period withoutany treatment. All the ectoderm explants were fixed with2.5% glutaraldehyde,and both freeze-etchingreplicas and ultrathin sections were preparedand examined with Zeiss EM 109. Observations of the freeze-etching repli-cas showed that endocytotic vesicles were offrequent occurrence on the lateral membraneof the ectoderm cells treated with the BME(Fig.1).They were quite numerous in somecells while in others only a few could befound.On the lateral membrane of ectodermcells of the two control series endocytotic ve-sicles were not observed. In the ultrathin sections successive stepsof the formation of the vesicles have beenrecorded (Figs.2—4).By the characteristic feature of the presence of the fine protrusionson the cytoplasmic surface of the vesicles,they were recognized as the coated vesicles(Anderson and Kaplan,1983).In the cellswith numerous coated vesicles,multi-vesicu-lar bodies could usually be encountered (Fig.2).In the control series treated with bovineserum albumin and the series without anytreatment,vesicles can be found only in afew cells. It can be concluded that in case of in-duction of heterogeneous inductors,endocy-tosis by means of the coated vesicles may playan important role.Although the inductivesubstance used was a crude extract yet incomparison with the control series and inconsideration of its inductive effect,it can beaccepted that the inductive factors have beentaken into the cell through the process of en-docytosis.Whether this phenomenon also oc-curs in vivo conditions certainly deserves fur-ther verification.