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in fixed tissue by a chemical reaction that produces a dark colored precipitate
(Weisblat et al. 1978 ). This method also has some weaknesses. It is labor intensive,
since the embryos have to be sectioned to visualize the internal cells carrying the
label (Jacobson and Hirose 1978 , 1981 ; Hirose and Jacobson 1979 ). Since HRP is a
protein, it is degraded over time in live cells and therefore cannot permanently mark
a cell and its descendants. Modern fate mapping techniques in amphibians utilize
nontoxic, nondegradable fluorescent dyes conjugated to sephadex beads, injected
into cleavage stage embryos (Cooke and Webber 1985 ; Dale and Slack 1987a;
Moody 1987a, b). These are stable, nondiffusible compounds that permit visualiza-
tion of labeled cells in live embryos. The fate maps produced by these methods
revealed that the conserved morphology of amphibian blastulae reflects a deeper
homology in the organization of cell fates, which is exemplified by the Xenopus fate
map (Fig. 7.5a). Endodermal tissues, such as the gut, pancreas and liver, are derived
from the large, yolky cells at the vegetal pole of the blastula, while the ectodermal
tissues like the neural tube and epidermis are derived from cells at the animal pole,
above the blastocoel (Fig. 7.5a). Mesodermal tissues, like the skeletal muscle, heart,
and bone are derived from the cells around the equator, or the marginal zone.

Xenopus Zebrafish

Chicken Mouse

ab

cd

Fig. 7.5 Blastoderm stage fate maps. (a) Diagram of a Xenopus blastoderm fate map, after those
constructed by injecting cells with a fluorescent lineage tracing dye at the 32-cell stage (Cooke and
Webber 1985 ; Dale and Slack 1987a; Moody 1987b). (b) Diagram of a zebrafish blastoderm fate
map, after those constructed by injecting single blastomeres with lineage tracing dye at the blasto-
derm stage (Kimmel et al. 1990 ; Shih and Fraser 1995 ; Melby et al. 1996 ; Warga and Nusslein-
Volhard 1999 ). (c) Diagram of a chicken blastoderm fate map, after those constructed by labeling
cells with DiI or by homotypic transplantation of quail tissue into chick embryos (Hatada and Stern
1994 ; Callebaut et al. 1996 ). (d) Diagram of a fate map of a mouse pre-gastrula stage epiblast, after
experiments injecting single blastomeres with an HRP-lineage tracer followed by ex vivo culturing
of the embryo (Lawson et al. 1991 ). Blue = ectoderm, red = mesoderm, green = endoderm, grey =
extraembryonic structures

7 Establishment of the Vertebrate Germ Layers