5: Cleavage: Becoming Multicellular and Development Consequences Flashcards
first few cellular divisions of a zygote
- zygote splits along a longitudinal plane
- also longitudinal, but at _ to the plane of the first
- _ to the first two and is equatorial in position
90 degrees
perpendicular
these early divisions produce separate cells called blastomeres
the rapid, multiple rounds of cell division are termed
cleavage
after the cleavage has produced over 100 cells, the embryo is called a
blastula
the blastula is usually a spherical layer of cells (_ )
surrounding a fluid-filled or yolk-filled cavity (the _ )
blastoderm
blastocoel
mammals at this stage form a structure called the _ ,
characterized by an inner cell mass that is distinct from the
surrounding blastula
blastocyst
each cell within the blastula is called a
blastomere
development proceeds at the rate of roughly one cleavage division per
day for the first _
2 days
when the embryo consists of approximately 16 cells, it is called a
morula
starting after the eight-cell stage, the embryos of placental mammals enter into
a phase called
compaction
during which the individual outer blastomeres tightly adhere through
gap and tight junctions and lose their individual identity when viewed
from the surface
compaction
compaction mediated by concentration of calcium (Ca++)-activated cell adhesion
molecules (such as _ ), in a ring around the apical surface of
the blastomeres
E-cadherin
through the activity of a sodium (Na+
), potassium (K+
)–adenosine
triphosphatase (ATPase)–based Na+
transport system, Na+
and water (H2O)
move across the epitheliumlike outer blastomeres and accumulate in spaces
among the inner blastomeres
cavitation
cavitation occurs about
4 days after fertilization
at the blastocyst stage, the embryo, which is still surrounded by the zona
pellucida, consists of two types of cells:
o Trophoblast – outer epithelial layer
o Inner cell mass – small inner group of cells
the end of the blastocyst that contains the inner cell mass is known as
the _ , and the opposite end is called the _
embryonic pole
abembryonic pole
cells of the inner cell mass give rise to the body of the _ itself in
addition to several _ , whereas cells of the trophoblast form only extraembryonic structures, including the _
embryo
extraembryonic structures
outer layers of the placenta
there is increasing evidence that _ (a growth factor secreted by cells of the inner cell mass), acts to maintain mitotic activity in the overlying trophoblast
fibroblast growth factor-4
The first cleavage of frog’s egg was observed by
Jan Swammerdam in 1738.
The entire process of cleavage in frog’s egg was studied by
Pierre Prevost
and Jean Baptiste André Dumas in 1824
planes of cleavage are named depending on the position of the
cleavage
furrow
the plane of the cleavage lies on the animal vegetal axis;
it bisects both the poles of the egg; the egg is divided into two equal halves
Meridional plane
the cleavage furrows may lie on either side of the meridional
plane; the furrows pass from animal to vegetal pole; the cleaved cells may be
unequal in size
Vertical plane
this cleavage plane bisects the egg at right angles to the
main axis; lies on the equatorial plane; divides the egg into two halves
Equatorial plane
similar to the equatorial plane, but it lies on either side of
the equator; also called as transverse or horizontal cleavage
Latitudinal plane
“yolk rich pole”
Vegetal Pole
yolk poor pole
Animal Pole
zygotic nucleus is generally displaced towards the _
animal pole
Types of Eggs
sparse evenly distributed yolk
Isolecithal
Types of Eggs
moderate amount of yolk and often unevenly distributed
Mesolecithal
Types of Eggs
dense yolk concentrated at one end
Telolecithal
Types of Eggs
yolk concentrated at the middle of the egg
Centrolecithal
Isolecithal example
sea urchin and mouse
Mesolecithal example
frog
Telolecithal example
bird and reptile
Centrolecithal example
fly
Cleavage Patterns
- Total/ Holoblastic Cleavage
a. equal holoblastic cleavage
b. unequal holoblastic cleavage - Meroblastic Cleavage
a. discoidal clevage
b. superficial cleavage
the cleavage furrow bisects the entire egg;
such a cleavage may be either equal or unequal
Total or holoblastic cleavage
in microlecithal and isolecithal eggs,
cleavage leads to the formation of blastomeres of equal size;
Equal holoblastic cleavage
Equal holoblastic cleavage examples
amphioxus and placental mammals
in mesolecithal and telolocithal eggs, cleavage leads to the formation of blastomeres of unequal size; among the
blastomeres there are many small sized micromeres and a few large sized macromeres
Unequal holoblastic cleavage
the cleavage furrows are restricted to the active cytoplasm found either in the animal pole (macrolecithal egg) or superficially
surrounding the egg (centrolecithal egg)
Meroblastic cleavage
since the macrolecithal eggs contain plenty of yolk,
the cytoplasm is restricted to the narrow region in the animal pole; hence
cleavage furrows can be formed only in 3 the disc-like animal pole region;
such a cleavage is called discoidal meroblastic cleavage;
Discoidal cleavage
Discoidal cleavage examples
e.g. birds and
reptiles
in centrolecithal eggs, the cleavage is restricted to
the peripheral cytoplasm of the egg;
Superficial cleavage
Superficial cleavage example
insects
Sachs’ Laws proposed by
Julius von Sachs in 1877
states that “cells tend to divide into equal daughter cells” and “each new division plane tends to intersect the preceding plane at right angles” (acts to maintain the spheroidal shape of blastomeres)
Sachs’ Laws
Balfour’s Law proposed by
Francis Maitland Balfour in 1885
states that “the speed or rate of cleavage in any region of egg is inversely proportional to the amount of yolk it contains”
Balfour’s Law
simplest pattern; in which successful division planes
are at 90 degree angles relative to each other; this results in the
blastomeres aligned directly over or to the side of one another
radial cleavage
- the division planes are not at 90-degree angles,
resulting in blastomeres that are not aligned directly over or beside one another
spiral cleavage
characteristic of deuterostomes
Radial Cleavage
deuterostomes
(“mouth second”); e.g. human, starfish
Radial Cleavage results in
indeterminant cells (cells that can individually give rise to a
complete embryo, and they don’t have a determined embryological fate early
on during the development of the embryo)
you can take a single cell from a developing embryo, and given the right condition, that single cell can give rise to a whole embryo; e.g. experiments done with frog embryo
characteristic of protostomes
Spiral Cleavage
Spiral Cleavage results in
determinant cells (cell that have a
determined embryological fate early on during the development of the
embryo)
determinant cells are programmed to become a specific type of cell, early on during the proces
Classification of Cleavages based on the Potentiality of the Blastomeres for the
Future Development
a. Determinate
b. Indeterminate
the developmental fate of each embryonic cell is established very early
Determinate
Determinate: if a cell is isolated from the _ stage the embryo will not fully develop
4-cell
this is because the fate of each blastomere is predetermined in the early
embryonic stage itself
examples of determinate
annelids, mollusks and ascidians (which produce mosaic type of eggs)
early embryonic cells retain capacity to develop into a complete embryo if
isolated from other cells
Indeterminate
Indeterminate: cleavage produces blastomeres which are qualitatively _
equipotential or
totipotent
when they are isolated, they develop into complete embryos
Indeterminate
this is because the fates of blastomeres are not predetermined in the early embryonic period
examples of indeterminate
vertebrates and certain invertebrates such as echinoderms (which produce
regulative type of eggs)
types of eggs of holoblastic
isolecithal
mesolecithal
types of eggs of meroblastic
telolecithal
centrolecithal
patterns of cleavage of isolecithal
- radial - echinoderms, emphioxus
- spiral - annelids, molluscs, flatworms
- bilaterial - tunicates
- rotational - mammals, nematodes
patterns of cleavage of mesolecithal
radial - amphibians
patterns of cleavage of telolecithal
- bilateral - cephalopod molluscs
- discoidal - fish, reptiles, birds
patterns of cleavage of centrolecithal
superficial - most insects
occurs in centrolecithal eggs
Superficial Cleavage
Superficial Cleavage: the early divisions occur in the surface layer of the egg and cleavage furrows
_ extend into the central yolk
do not
Superficial Cleavage, the zygote nucleus lies in the _ of the egg
center
Superficial Cleavage
it divides repeatedly without the division of the egg cytoplasm
o as a result, a large number of _ are formed
nuclei
an invariant cleavage pattern divides the embryo into different cell
lines that differentiate according to the information they obtain from
differentially distributed cytoplasmic factors
cytoplasmic localization
the organization of the cytoplasm of the egg before and after fertilization is
established by the redistribution of ooplasmic determinants
cytoplasmic localization
helps seal the
developmental fate of each cell’s descendants
cytoplasmic localization
acts as a trigger to initiate a program of events starting with
cleavage, and continuing with gastrulation and neurulation, etc
fertilization
although fertilization results in union of maternal and paternal genomes, zygotic gene activity is _ until the blastula stage
not required
in fact, after fertilization and through cleavage, the maternal, paternal and
zygotic genomes are completely dispensible
following the formation of the blastula, _ is activated, which carries the embryo through the rest of embryogenesis
zygotic gene transcription
Evidence for maternal mRNA control of early development:
- maternal trait dominance in _ hybrids
- cleavage in _ embryos
- _ inhibition does not prevent cleavage
- _ inhibition prevents cleavage
interspecific
enucleate
transcriptional
translational
roles of maternal factors
- processing of the _ , which is necessary for its participation in embryogenesis
- removal of _ (RNA and protein),
- robust activation of the _ , which is essential for the development of the embryo beyond two cells
male genome
maternal detritus
embryonic genome
