1194.2 Main Patterns of Embryonic Cleavage in Vertebrates:
Holoblastic Versus Meroblastic
Early embryonic cell division patterns in vertebrates can be broken into two broad
categories, holoblastic cleavage (e.g., most amphibians and mammals) and mero-
blastic cleavage (e.g., birds, reptiles, and teleost fishes) (Fig. 4.1). In holoblastic
cleavage, the entire egg undergoes cellularization, and yolk platelets are either
absent (e.g., in mammals) or present as cytoplasmic inclusions that partition among
cells (e.g., in amphibians). Cleavage furrows encompass the embryo completely,
from the animal pole (corresponding to the region containing the meiotic spindle in
the oocyte) to the vegetal pole. In meroblastic cleavage, in contrast, cell division
does not divide the embryo in its entirety. Instead, embryonic cells divide in the
animal pole independently of the vegetally located yolk, with cells typically remain-
ing syncytial to the yolk cell for a period of time that varies among species. We
would like to point out that pure holoblastic or meroblastic cleavage patterns are
only idealized extremes. We will discuss that many embryos cleave with an inter-
mediate geometry, in which the entire embryo cleaves but the strong asymmetry of
yolk leads to a bias of cleavage planes. In any of these cases, dividing cells are
called blastomeres regardless of whether they contain yolk.
In all vertebrates, determination of the animal and vegetal poles of the egg devel-
ops during oogenesis, with the oocyte typically containing a pre-existing animal
region with distinct properties (see Chap. 5 ). This animal region will contain the
nuclear DNA after fertilization and, in embryos with meroblastic cleavage, accumu-
lates yolk-free ooplasm after fertilization. The vegetal region of the egg, in addition
to acting as the site for yolk storage in meroblastic species, is also an essential
player in early embryogenesis, containing signals involved in early axial patterning
Fig. 4.1 Types of embryonic cleavage. Holoblastic cleavage encompasses the entirety of the
embryo, involving meridional planes that cleave through the animal and vegetal poles of the
embryo. Meroblastic cleavage involves an early separation between cells at the animal pole and
yolk at the vegetal pole of the egg, and cell furrows encompass only the animal-most region of the
embryo, leaving the yolk intact
4 Vertebrate Embryonic Cleavage Pattern Determination