321release their contents into the underlying yolk cell, forming an extraembryonic YSL
(Fig. 7.3a) (Betchaku and Trinkaus 1978 ; Ballard 1982 ; Kimmel and Law 1985a).
In contrast to other vertebrates, the cells in the teleost blastoderm and early gastrula
are loosely packed and “constitute a population of individually motile cells,” accord-
ing to Trinkaus (p. 357) (Trinkaus 1996 ). In zebrafish, for instance, cells move in
random directions for distances up to 70 μm during the late blastula stages (Kane
and Kimmel 1993 ; Concha and Adams 1998 ; Bensch et al. 2013 ). Movements are
even more dramatic in the killifish blastoderm. Killifish blastoderm cells start bleb-
bing at the end of the cleavage stages, and utilize an actin-based amoeboid-like
mechanism to translocate in random directions up to 150 μm before gastrulation
begins (Trinkaus 1973 ; Fink and Trinkaus 1988 ).
In teleosts, gastrulation begins when epiboly movements start and cells migrate
toward the vegetal pole to cover the extraembryonic yolk (Fig. 7.4d, black arrows)
(Trinkaus 1996 ). During gastrulation, cells converge toward the future dorsal side of
the embryo, resulting in a thickened region called the embryonic shield (Oppenheimer
1959 ). As in amphibians, cells at the dorsal midline extend to change the shape of
the embryo from a sphere to a rod (Solnica-Krezel 2005 ). The mechanism by which
cells internalize during gastrulation in teleosts has been controversial. Teleosts lack
bottle cells and do not forma classic, amphibian-like blastopore. Instead, the func-
tion of the bottle cells may be performed by the YSL and a group of Non-
internalizing, highly Endocytic Marginal cells (NEM), also called the dorsal
forerunner cells (D’Amico and Cooper 2001 ; Feldman et al. 2002 ). The NEM cells
may physically constrain the possible movements of internalizing cells. Early stud-
ies were confounded by the fact that the easiest cells to label and visualize, the EVL,
never internalize. Some of the first investigations of teleost gastrulation reported
that deep cells undergo involution in sea bass, trout, and salmon (Goette 1873 ; His
1878 ; Wilson 1891 ). At the same time, other studies in trout and cod concluded that
deep cells internalized by delaminating from the blastodisc (Stricker 1865 ; Reineck
1869 ; Ryder 1884 ). The first cell labeling experiments in teleosts found no evidence
of involution, invagination or ingression in trout (Ballard 1966a). Instead, the evi-
dence supported a novel model, in which cells deep in the center of the blastoderm
migrate outward during gastrulation and form a hypoblast due to convergence
movements during epiboly (Ballard 1966b).
The ability to record the behavior of individual cells in live embryos provided the
most detailed description of how teleost cells are internalized during gastrulation.
The large nuclei of cells in the cyprinid fish, Barbus conchonius (Rosy Barb) make
it very easy to follow the behavior of single cells under a microscope using DIC
optics (Wood and Timmermans 1988 ). In this species, the presumptive mesoderm
and endoderm clearly involute. Similar results were obtained in the Atlantic herring
(Hill and Johnston 1997 ). By contrast, live imaging of the early stages of gastrula-
tion in the killifish revealed that cells internalize by ingression, not involution
(Trinkaus 1996 ). The behavior of individual cells during gastrulation has been ana-
lyzed in the greatest detail in zebrafish (Kimmel and Law 1985b; Warga and Kimmel
1990 ; Shih and Fraser 1995 ). When epiboly movements bring cells to the equator,
precursors of mesoderm and endoderm involute and migrate towards the animal
7 Establishment of the Vertebrate Germ Layers