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and cell specification that are transmitted to blastomeres (see Chaps. 6 – 8 ). In the
following sections, we explore the factors that govern holoblastic and meroblastic
cleavage patterns, especially the substantial variation observed within these two
broad categories.
4.3 Cellular Mechanisms Underlying Cell Cleavage
Pattern Determination
The pattern of early cell division in an embryo influences parameters of the forming
blastula, such as the number of cells, depth of stacking within a cell layer, and rela-
tionship to extraembryonic spaces and structures. The cleavage pattern depends on
cell division planes in individual blastomeres. Here, we address cellular mecha-
nisms that influence the furrow plane and which lead to the cellular arrangement of
the early embryo.
4.3.1 Induction of Cell Cleavage Plane by Chromatin
and Amphiasters
Typically, the cell divides perpendicular to its longest axis into two equal daughter
cells. At the core of this process is the spindle apparatus (Fig. 4.2) with the two
microtubule asters (amphiasters) emanating from its centrosomes. The spindle
apparatus is an arrangement of microtubules that organize around the microtubule-
organizing centers (MTOCs) and the DNA during M phase of the cell cycle and is
responsible for chromosome segregation during cell division. Kinetochore microtu-
bules connect the spindle pole to chromosomes. The microtubules between the
poles that are not connected to the kinetochore are called interpolar microtubules.
While we currently know little about microtubule length distribution in the mitotic
spindles of early development, in the closely related meiosis II spindles, interpolar
microtubules are very short compared to spindle length (Needleman et al. 2010 ).
Another set of spindle-associated microtubules are astral microtubules, which ema-
nate outward from each of the MTOCs in a radial fashion. Typically, microtubule
themselves are arranged with their minus ends pointing toward the closest MTOC
in the spindle pole. As described below, signals both from chromatin at the spindle
midzone and from astral microtubules are involved in furrow induction and there-
fore cleavage plane determination.
Early observations indicated a correlation between the appearance of the furrow
plane and the location of the spindle apparatus, with the furrow forming perpen-
dicular to the spindle at a location equidistant from the spindle poles (Hertwig
1893 ). This plane normally corresponds to that of aligned chromosomes during the
preceding metaphase, as well as the region where astral microtubules overlap.
A. Hasley et al.