C5: Cleavage

Question 1

in embryology, it is the first few cellular divisions of a zygote (fertilized egg)

Answer 1

Cleavage

Question 2

describe the first three dvison of a zygote.

Answer 2

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

Question 3

these early divisions produce separate cells called __

Answer 3

blastomeres

Question 4

the rapid, multiple rounds of cell division are termed __

Answer 4

cleavage

Question 5

after the cleavage has produced over 100 cells, the embryo is called a ___

Answer 5

blastula

Question 6

the __ is usually a spherical layer of cells (___) surrounding a fluid-filled or yolk-filled cavity (the ___)

Answer 6

blastula;
(blastoderm);
blastocoel

Question 7

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

Answer 7

blastocyst;
inner cell mass

Question 8

each cell within the blastula is called a __

Answer 8

blastomere

Question 9

when the embryo consists of approximately 16 cells, it is called a ___ (from the Latin word meaning “___”)

Answer 9

morula;
mulberry

Question 10

starting after the eight-cell stage, the embryos of placental mammals enter into a phase called ___

Answer 10

compaction

Question 11

compaction is mediated by concentration of calcium (Ca++)-activated cell adhesion molecules (such as ___), in a ring around the __ surface of the ___

Answer 11

E-cadherin;
apical;
blastomeres

Question 12

through the activity of a ____, Na+ and water (H2O) move across the epitheliumlike outer blastomeres and accumulate in spaces among the inner blastomeres

Answer 12

sodium (Na+), potassium (K+)–adenosine triphosphatase (ATPase)–based Na+ transport system

Question 13

the process wherein Na+ and water (H2O) move across the epitheliumlike outer blastomeres and accumulate in spaces among the inner blastomeres is called ___

Answer 13

cavitation

Question 14

the cavitation process takes about __ days after fertilization

Answer 14

4

Question 15

the fluid-filled space is known as ___

Answer 15

blastocoele (blastocyst cavity)

Question 16

once cavitation occurs, and a blastocoeleis formed, the embryo as a whole is known as a ___

Answer 16

blastocyst

Question 17

the ___ stage, the embryo, which is still surrounded by the zona pellucida, consists of two types of cells (__ and __)

Answer 17

blastocyst;
Trophoblast and Inner cell mass

Question 18

outer epithelial layer

Answer 18

Trophoblast

Question 19

small inner group of cells

Answer 19

Inner cell mass

Question 20

each blastomere at the ___ and the ___ stage contributes cells to both the inner cell mass and the trophoblast

Answer 20

two-cell; four-cell

Question 21

the end of the blastocyst that contains the inner cell mass is known as the ___, and the opposite end is called the ___

Answer 21

embryonic pole;
abembryonic pole

Question 22

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 ___

Answer 22

inner cell mass;
trophoblast;
placenta

Question 23

there is increasing evidence that ____ acts to maintain mitotic activity in the overlying trophoblast

Answer 23

fibroblast growth factor-4

Question 24

(a growth factor secreted by cells of the inner cell mass),

Answer 24

fibroblast growth factor-4

Question 25

The first cleavage of frog’s egg was observed by ____ in 1738.

Answer 25

Jan Swammerdam

Question 26

The entire process of cleavage in frog’s egg was studied by ___ and _____ in 1824

Answer 26

Pierre Prevost; Jean Baptiste André Dumas

Question 27

planes of cleavage are named depending on the position of the ___

Answer 27

cleavage furrow

Question 28

The Planes of Cleavage (4)

Answer 28

1.) Meridional plane 2.) Vertical plane 3.) Equatorial plane 4.) Latitudinal plane

Question 29

– 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

Answer 29

Meridional plane

Question 30

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

Answer 30

Vertical plane

Question 31

this cleavage plane bisects the egg at right angles to the main axis; lies on the equatorial plane; divides the egg into two halves

Answer 31

Equatorial plane

Question 32

similar to the equatorial plane, but it lies on either side of the equator; also called as transverse or horizontal cleavage

Answer 32

Latitudinal plane

Question 33

yolk-rich pole

Answer 33

Vegetal Pole

Question 34

yolk-poor pole

Answer 34

Animal Pole

Question 35

___is generally displaced towards the animal pole

Answer 35

zygotic nucleus

Question 36

the ___ of the yolk and its ___ affect the process of cleavage

Answer 36

amount; distribution

Question 37

types of eggs based on yolk characteristics (4)

Answer 37

1. Isolecithal 2. Mesolecithal 3. Telolecithal 4. Centrolecithal

Question 38

type of eggs that sparse evenly distributed yolk; e.g. ___ and ___

Answer 38

Isolecithal; sea urchin and mouse

Question 39

type of egg that has moderate amount of yolk and often unevenly distributed; e.g. __

Answer 39

Mesolecithal; frog

Question 40

type of egg that has dense yolk concentrated at one end; e.g. ___ and __

Answer 40

Telolecithal; bird and reptile

Question 41

type of egg that has yolk concentrated at the middle of the egg; e.g. ___

Answer 41

Centrolecithal; fly

Question 42

Main Cleavage Patterns (2)

Answer 42

1. Total or holoblastic cleavage 2. Meroblastic cleavage

Question 43

the cleavage furrow bisects the entire egg; such a cleavage may be either equal or unequal

Answer 43

Total or holoblastic cleavage

Question 44

2 types of holoblastic cleavage

Answer 44

a. Equal holoblastic cleavage b. Unequal holoblastic cleavage

Question 45

cleavage leads to the formation of blastomeres of equal size such that in ____ and ___ eggs,; e.g. __ and ___ mammals

Answer 45

Equal holoblastic cleavage; microlecithal and isolecithal amphioxus and placental

Question 46

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

Answer 46

Unequal holoblastic cleavage; mesolecithal and telolecithal

Question 47

the cleavage furrows are restricted to the active cytoplasm found either in the animal pole (__ egg) or superficially surrounding the egg (___ egg)

Answer 47

meroblastic cleavage macrolecithal; centrolecithal

Question 48

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 ___

Answer 48

discoidal meroblastic cleavage; birds and reptiles

Question 49

___ - in ___ eggs, the cleavage is restricted to the peripheral cytoplasm of the egg; e.g. __

Answer 49

Superficial cleavage; centrolecithal; insects

Question 50

Laws of Cleavage (2)

Answer 50

. Sachs’ Laws; Balfour’s Law

Question 51

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)

Answer 51

. Sachs’ Laws (Julius von Sachs)

Question 52

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”

Answer 52

Balfour's law (Francis Maitland Balfour); inversely

Question 53

cleavage patterns based largely on the orientation of the division planes (2)

Answer 53

radial cleavage; spiral cleavage

Question 54

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

Answer 54

radial cleavage

Question 55

the division planes are not at 90-degree angles, resulting in blastomeres that are not aligned directly over or beside one another

Answer 55

spiral cleavage

Question 56

occurs such that the resulting daughter cells are located exactly on top of one another; characteristic of ___ results in ___ cells

Answer 56

Radial Cleavage; deuterostomes; indeterminant

Question 57

the cells divide such that each cell in the top four cell plane is directly over one other cell in the bottom plane

Answer 57

Radial Cleavage

Question 58

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

Answer 58

Spiral Cleavage protostomes; determinant

Question 59

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

Answer 59

Spiral Cleavage

Question 60

Classification of Cleavages based on the Potentiality of the Blastomeres for the Future Development (2)

Answer 60

a. Determinate b. Indeterminate

Question 61

the developmental fate of each embryonic cell is established very early; fate of each blastomere is predetermined in the early embryonic stage itself

Answer 61

a. Determinate

Question 62

in determinate cells, if a cell is isolated from the __ stage the embryo will not fully develop

Answer 62

4-cell

Question 63

examples of determinate (3)

Answer 63

annelids, mollusks and ascidians (which produce mosaic type of eggs)

Question 64

early embryonic cells retain capacity to develop into a complete embryo if isolated from other cells; cleavage produces blastomeres which are qualitatively ___ or ___

Answer 64

b. Indeterminate; equipotential or totipotent

Question 65

when they are isolated, they develop into complete embryos ▪ this is because the fates of blastomeres are not predetermined in the early embryonic period

Answer 65

b. Indeterminate cells

Question 66

examples of indeterminate organisms (2)

Answer 66

vertebrates and certain invertebrates such as echinoderms (which produce regulative type of eggs)

Question 67

this type of cleavage occurs in centrolecithal eggs

Answer 67

Superficial Cleavage

Question 68

the early divisions occur in the surface layer of the egg and cleavage furrows do not extend into the central yolk

Answer 68

Superficial Cleavage

Question 69

in this cleavage, the zygote nucleus lies in the center of the egg ▪ it divides repeatedly without the division of the egg cytoplasm

Answer 69

Superficial Cleavage

Question 70

from the early cell lineage studies and the recognition of mosaic development of mainly invertebrate embryos, the concept of ____ was developed

Answer 70

cytoplasmic localization

Question 71

an invariant cleavage pattern divides the embryo into different cell lines that differentiate according to the information they obtain from differentially distributed cytoplasmic factors

Answer 71

cytoplasmic localization

Question 72

In cytoplasmic localization, the organization of the cytoplasm of the egg before and after fertilization is established by the redistribution of ___

Answer 72

ooplasmic determinants

Question 73

examples of such ‘mosaic’ development have been well studied in __ (5)

Answer 73

ascidians, sea urchins, spiralians, Caenorhabditis elegans and also in the vertebrate embryo of Xenopus

Question 74

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.

Answer 74

third; irregular; blastoderm; cytoplasmic localization

Question 75

___ acts as a trigger to initiate a program of events starting with cleavage, and continuing with gastrulation and neurulation, etc.

Answer 75

fertilization

Question 76

although fertilization results in union of maternal and paternal genomes, ___ activity is not required until the __ stage

Answer 76

zygotic gene; blastula

Question 77

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

Answer 77

maternally derived mRNAs; embryogenesis

Question 78

following the formation of the blastula, ___ is activated, which carries the embryo through the rest of ___

Answer 78

zygotic gene transcription; embryogenesis

Question 79

Evidence for maternal mRNA control of early development (4)

Answer 79

1. maternal trait dominance in interspecific hybrids 2. cleavage in enucleate embryos 3. transcriptional inhibition does not prevent cleavage 4. translational inhibition prevents cleavage

Question 80

____ encode gene products (RNA or protein) that are required in early development prior to zygotic transcription

Answer 80

maternal effect genes

Question 81

using the rudimentary gene of Drosophila as an example, by definition, whenever, mothers are ____, their progeny are ___ despite being genotypically viable

Answer 81

homozygous mutant; embryonic lethal

Question 82

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

Answer 82

male genome; maternal detritus; embryonic genome

Question 83

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

Answer 83

mutagenesis

Question 84

"mother's genotype determines offspring's phenotype"

Answer 84

maternal effect rule