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pole along the inside of the embryo (Fig. 7.4d, white arrows) (Warga and Kimmel
1990 ). Imaging of zebrafish embryos by light sheet microscopy confirmed that cells
around the margin internalize by the “wheeling-in” type of movement, as if they
were moving around a blastopore lip (Keller et al. 2008 ). The movements in zebraf-
ish are often referred to as “involution” in the literature because they are reminiscent
of the movements during amphibian gastrulation. A significant difference, however,
is that cells internalize individually in teleosts rather than as an epithelial sheet, as
in amphibians. Therefore, a more accurate term to describe this type of internaliza-
tion movement is “synchronized ingression” (Kane and Adams 2002 ; Adams and
Kimmel 2004 ; Keller et al. 2008 ). In synchronized ingression, cells in the blasto-
derm undergo a coordinated epithelial-to-mesenchymal transition as they internal-
ize at the margin, similar to the process of subduction described for axial and
paraxial mesoderm in some species of salamander (Purcell and Keller 1993 ; Shook
et al. 2002 ). Imaging studies revealed that cells within the shield also internalize by
a more traditional form of ingression (Keller et al. 2008 ). Thus, zebrafish utilize two
distinct methods of internalization in different regions of the embryo: Cells around
the margin internalize as a group by synchronized ingression, while some cells in
the shield internalize individually by ingression. The seemingly contradictory
observations from different teleost species could reflect true species-specific differ-
ences in the mechanisms of internalizing mesoderm and endoderm. Alternatively,
the apparent contradictions could simply be an artifact of investigators focusing on
different regions of the embryo that use different methods of internalization.
Detailed imaging studies of other teleosts are necessary to determine if the results
in zebrafish are representative of all teleosts.
7.3.4 Non-teleost Fish
Lampreys (Lampetra) are jawless fish that sit at the base of the vertebrate tree, well
before the origin of the hinged jaws observed in all cartilaginous and bony fish
(Kuratani et al. 2001 ). Sturgeon (Acipenser) and Bichers (Polypterus) are two fami-
lies of ray-finned fish that evolved soon after they branched off from the Sarcopterygian
lineage, which produced the tetrapods (Hurley et al. 2007 ). Embryos from these
three groups undergo holoblastic cleavage, and form a blastocoel (Bolker 1993a;
Bartsch et al. 1997 ; Takeuchi et al. 2009b). The vegetal blastomeres in lampreys and
bicher are extraembryonic cells specialized for the purpose of providing nutrients
during embryogenesis and do not contribute to the embryo (Takeuchi et al. 2009b).
Gastrulation in the sturgeon has been very well described (Ballard and Ginsburg
1980 ; Bolker 1993a). The mesoderm and endoderm internalizes during gastrulation by
involution, which begins with the formation of a blastopore in the dorsal margin. The
lip of the blastopore is formed by constriction of bottle cells, which drive the involution
of mesoderm and draw the endoderm directly inside the embryo. This process strongly
resembles that described for Xenopus and other amphibians, but there are some slight
differences. First, in sturgeon the notochord is formed after gastrulation by the ingres-
W. Tseng et al.