Scientific Background

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The complete reproductive cycle of Xenopus spans up to 18 mo from the earliest stages of oogenesis to the sexually mature adult. Owing to its popularity as an embryonic model, the long arduous path to adulthood in Xenopus has been thoroughly documented (see Deuchar, 1966, Deuchar, 1975; Nieuwkoop and Faber, 1967, Nieuwkoop and Faber, 1994; Dumont, 1972). For an excellent overview of early events in amphibian development—especially Xenopus —the reader is referred to Scott Gilbert’s textbook Developmental Biology (1994). A beautiful color atlas depicting early events in Xenopus development is also available (Hausen and Riebesell, 1991). Moreover, a detailed fate map of the 32-cell Xenopus embryo has been published (Dale and Slack, 1987). Much of our knowledge about the molecular processes mediating early Xenopus development has been obtained using microinjection protocols; some of these achievements are presented in Table 1 . A brief description of the principal developmental events pertaining to the use of microinjected Xenopus oocytes and embryos are presented in the next section.

Table 1 Developmental Processes Studied in Microinjected Xenopus Embryos

Process	Molecules examined (ref.)
Gastrulation and morphogenetic cell movements	Goosecoid (1) ; nodal-related (2)
Mesoderm induction and dorso-ventral axial patterning	TGF-β receptor (3) ; Xwnt-8 (4) ; Raf-1 (5) ; FGF receptor (6) ; FGF receptor/ activin (7) ; BMP (8) ; N-cadherin (9) ; GSK-3 (10) ; brachyury/pintallavis (11)
Neural induction and antero-posterior axial patterning	Noggin (12) ; XASH (13) ; follistatin (14) ; activin/activinR (15) ; XIPOU 2 (16) ; Xotz2 (17) ; hedgehog (18) ; Xlim-1 (19)
Neuronal differentiation	Pintallavis (20) ; vhh-1 (21) ; neuroD (22)
Myogenesis	MyoD (23) ; MyoD (24) /Myf5
Somitogenesis	Vimentin/desmin (25)
NMJ development	AChE (26) ; AChR (27) ; synapsin I (28) ; synapsin Ha (29) ; synaptophysin (30)
Cell adhesion	EP-cadherin (31) ; B-catenin (32)
Gap junctional communication	Wnt-1 (33) ; connexins (34)
Tissue-specific gene regulation transcriptional	E12 (35)/MyoD ; distal-less-like-2 (Xdll-2) (36) ; TFIIIA (37) ; TFIID (3&)/MEF2/XMyoD
Tissue-specific gene regulation posttranscriptional	AChE (39)

^a References 1 , Niehrs et al. (1993); 2 , Smith et al. (1995); 3, Amaya et al. (1991); 4, Christian and Moon (1993); 5, MacNicol et al. (1993); 6 , Bhushan et al. (1994); 7, Cornell and Kimelman (1994); 8, Graff et al. (1994); 9 , Holt et al. (1994); 10 , He et al. (1995); 11 , O’Reilly et al. (1995); 12 , Lamb et al. (1993); 13 , Ferreiro et al. (1994); 14 , Hemmati-Brivanlou et al. (1994); 15 , Hemmati-Brivanlou and Melton (1994); 16 , Witta et al. (1995); 17 , Blitz and Cho (1995); 18 , Lai et al. (1995); 19 , Taira et al. (1995); 20 , Ruiz i Altaba et al. (1993); 21 , Roelink et al. (1994); 22 , Lee et al. (1995); 23 , Hopwood and Gurdon (1990); 24 , Ludolph et al. (1994); 25 , Cary and Klymkowsky (1995); 26 , Ben Aziz-Aloya et al. (1993a); Seidman et al. (1994); 27 , Shapira et al. (1994); 28 , Lu et al. (1992), Valtorta et al. (1995); 29 , Schaeffer et al. (1994); 30 , Alder et al. (1995); 31 , Heasman et al. (1994); 32 , McCrea et al. (1993); 33 , Olson et al. (1991); 34 , Paul et al. (1995); 35 , Rashbass et al. (1992); 36 , Morasso et al. (1993); 37 , Rollins et al. (1993); 38 , Leibham et al. (1994); 39 , Seidman et al. (1995).