C5: Cleavage Flashcards
in embryology, it is the first few cellular divisions of a zygote (fertilized egg)
Cleavage
describe the first three dvison of a zygote.
initially, the zygote splits along a longitudinal plane;
the second division is also longitudinal, but at 90 degrees to the plane of the first; third division is perpendicular to the first two and is equatorial in position
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 __ is usually a spherical layer of cells (___) surrounding a fluid-filled or yolk-filled cavity (the ___)
blastula;
(blastoderm);
blastocoel
mammals at this stage (cleavage has produced over 100 cells) form a structure called the ___, characterized by an ___ that is distinct from the surrounding blastula
blastocyst;
inner cell mass
each cell within the blastula is called a __
blastomere
when the embryo consists of approximately 16 cells, it is called a ___ (from the Latin word meaning “___”)
morula;
mulberry
starting after the eight-cell stage, the embryos of placental mammals enter into a phase called ___
compaction
compaction is mediated by concentration of calcium (Ca++)-activated cell adhesion molecules (such as ___), in a ring around the __ surface of the ___
E-cadherin;
apical;
blastomeres
through the activity of a ____, Na+ and water (H2O) move across the epitheliumlike outer blastomeres and accumulate in spaces among the inner blastomeres
sodium (Na+), potassium (K+)–adenosine triphosphatase (ATPase)–based Na+ transport system
the process wherein Na+ and water (H2O) move across the epitheliumlike outer blastomeres and accumulate in spaces among the inner blastomeres is called ___
cavitation
the cavitation process takes about __ days after fertilization
4
the fluid-filled space is known as ___
blastocoele (blastocyst cavity)
once cavitation occurs, and a blastocoeleis formed, the embryo as a whole is known as a ___
blastocyst
the ___ stage, the embryo, which is still surrounded by the zona pellucida, consists of two types of cells (__ and __)
blastocyst;
Trophoblast and Inner cell mass
outer epithelial layer
Trophoblast
small inner group of cells
Inner cell mass
each blastomere at the ___ and the ___ stage contributes cells to both the inner cell mass and the trophoblast
two-cell; four-cell
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 __ give rise to the body of the embryo itself in addition to several extraembryonic structures, whereas cells of the ___ form only extraembryonic structures, including the outer layers of the ___
inner cell mass;
trophoblast;
placenta
there is increasing evidence that ____ acts to maintain mitotic activity in the overlying trophoblast
fibroblast growth factor-4
(a growth factor secreted by cells of the inner cell mass),
fibroblast growth factor-4
The first cleavage of frog’s egg was observed by ____ in 1738.
Jan Swammerdam
The entire process of cleavage in frog’s egg was studied by ___ and _____ in 1824
Pierre Prevost; Jean Baptiste André Dumas
planes of cleavage are named depending on the position of the ___
cleavage furrow
The Planes of Cleavage (4)
1.) Meridional plane
2.) Vertical plane
3.) Equatorial plane
4.) Latitudinal plane
– 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
___is generally displaced towards the animal pole
zygotic nucleus
the ___ of the yolk and its ___ affect the process of cleavage
amount; distribution
types of eggs based on yolk characteristics (4)
- Isolecithal
- Mesolecithal
- Telolecithal
- Centrolecithal
type of eggs that sparse evenly distributed yolk; e.g. ___ and ___
Isolecithal;
sea urchin and mouse
type of egg that has moderate amount of yolk and often unevenly distributed; e.g. __
Mesolecithal;
frog
type of egg that has dense yolk concentrated at one end; e.g. ___ and __
Telolecithal;
bird and reptile
type of egg that has yolk concentrated at the middle of the egg; e.g. ___
Centrolecithal;
fly
Main Cleavage Patterns (2)
- Total or holoblastic cleavage
- Meroblastic cleavage
the cleavage furrow bisects the entire egg; such a cleavage may be either equal or unequal
Total or holoblastic cleavage
2 types of holoblastic cleavage
a. Equal holoblastic cleavage
b. Unequal holoblastic cleavage
cleavage leads to the formation of blastomeres of equal size such that in ____ and ___ eggs,; e.g. __ and ___ mammals
Equal holoblastic cleavage;
microlecithal and isolecithal
amphioxus and placental
cleavage leads to the formation of blastomeres of unequal size such as in ___ and ___ eggs; among the blastomeres there are many small sized micromeres and a few large sized macromeres
Unequal holoblastic cleavage;
mesolecithal and telolecithal
the cleavage furrows are restricted to the active cytoplasm found either in the animal pole (__ egg) or superficially surrounding the egg (___ egg)
meroblastic cleavage
macrolecithal;
centrolecithal
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 ___; e.g. __ and ___
discoidal meroblastic cleavage;
birds and reptiles
___ - in ___ eggs, the cleavage is restricted to the peripheral cytoplasm of the egg; e.g. __
Superficial cleavage;
centrolecithal;
insects
Laws of Cleavage (2)
. Sachs’ Laws;
Balfour’s Law
this law was proposed by ___ in 1877 which 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 (Julius von Sachs)
this lawwas proposed by ___ in 1885 ▪ states that “the speed or rate of cleavage in any region of egg is __ proportional to the amount of yolk it contains”
Balfour’s law (Francis Maitland Balfour);
inversely
cleavage patterns based largely on the orientation of the division planes (2)
radial cleavage;
spiral cleavage
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
occurs such that the resulting daughter cells are located exactly on top of one another;
characteristic of ___
results in ___ cells
Radial Cleavage;
deuterostomes;
indeterminant
the cells divide such that each cell in the top four cell plane is directly over one other cell in the bottom plane
Radial Cleavage
occurs such that the resulting daughter cells are not located exactly on top of one another; instead, they are located at a slight angle;
characteristic of __
results in ___ cells
Spiral Cleavage
protostomes;
determinant
the cells divide at slight angles to one another, so that the none of the four cells in one plane of the eight-cell stage is directly over a cell in the other plane
Spiral Cleavage
Classification of Cleavages based on the Potentiality of the Blastomeres for the Future Development (2)
a. Determinate
b. Indeterminate
the developmental fate of each embryonic cell is established very early; fate of each blastomere is predetermined in the early embryonic stage itself
a. Determinate
in determinate cells, if a cell is isolated from the __ stage the embryo will not fully develop
4-cell
examples of determinate (3)
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; cleavage produces blastomeres which are qualitatively ___ or ___
b. Indeterminate;
equipotential or totipotent
when they are isolated, they develop into complete embryos ▪ this is because the fates of blastomeres are not predetermined in the early embryonic period
b. Indeterminate cells
examples of indeterminate organisms (2)
vertebrates and certain invertebrates such as echinoderms (which produce regulative type of eggs)
this type of cleavage occurs in centrolecithal eggs
Superficial Cleavage
the early divisions occur in the surface layer of the egg and cleavage furrows do not extend into the central yolk
Superficial Cleavage
in this cleavage, the zygote nucleus lies in the center of the egg ▪ it divides repeatedly without the division of the egg cytoplasm
Superficial Cleavage
from the early cell lineage studies and the recognition of mosaic development of mainly invertebrate embryos, the concept of ____ was developed
cytoplasmic localization
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
In cytoplasmic localization, the organization of the cytoplasm of the egg before and after fertilization is established by the redistribution of ___
ooplasmic determinants
examples of such ‘mosaic’ development have been well studied in __ (5)
ascidians, sea urchins, spiralians, Caenorhabditis elegans and also in the vertebrate embryo of Xenopus
after __ cleavage, the rest of the cleavages are ___ and completely delimited cells are formed all over the germinal disc which is termed as __;
this outcome of cleavage called ____ helps seal the developmental fate of each cell’s descendants.
third;
irregular;
blastoderm;
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, ___ activity is not required until the __ stage
zygotic gene;
blastula
after fertilization and through cleavage, the maternal, paternal and zygotic genomes are completely dispensible
▪ this is because in the egg, there exists a stockpile of _____ which govern __ through cleavage to the blastula stage
maternally derived mRNAs;
embryogenesis
following the formation of the blastula, ___ is activated, which carries the embryo through the rest of ___
zygotic gene transcription;
embryogenesis
Evidence for maternal mRNA control of early development (4)
- maternal trait dominance in interspecific hybrids
- cleavage in enucleate embryos
- transcriptional inhibition does not prevent cleavage
- translational inhibition prevents cleavage
____ encode gene products (RNA or protein) that are required in early development prior to zygotic transcription
maternal effect genes
using the rudimentary gene of Drosophila as an example, by definition, whenever, mothers are ____, their progeny are ___ despite being genotypically viable
homozygous mutant;
embryonic lethal
After fertilization, one of the first roles of maternal factors is the processing of the ___, which is necessary for its participation in embryogenesis;
second role is the removal of ____ (RNA and protein), ;
and a third is the robust activation of the ____, which is essential for the development of the embryo beyond two cells
male genome;
maternal detritus;
embryonic genome
targeted ___ experiments in mice have provided researchers with a wealth of insight into candidate maternal factors that participate in the regulation of these essential early steps of development
mutagenesis
“mother’s genotype determines offspring’s phenotype”
maternal effect rule
