5: Cleavage: Becoming Multicellular and Development Consequences

Question 1

first few cellular divisions of a zygote

1. zygote splits along a longitudinal plane
2. also longitudinal, but at _ to the plane of the first
3. _ to the first two and is equatorial in position

Answer 1

90 degrees
perpendicular

these early divisions produce separate cells called blastomeres

Question 2

the rapid, multiple rounds of cell division are termed

Answer 2

cleavage

Question 3

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

Answer 3

blastula

Question 4

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

Answer 4

blastoderm
blastocoel

Question 5

mammals at this stage form a structure called the _ ,
characterized by an inner cell mass that is distinct from the
surrounding blastula

Answer 5

blastocyst

Question 6

each cell within the blastula is called a

Answer 6

blastomere

Question 7

development proceeds at the rate of roughly one cleavage division per
day for the first _

Answer 7

2 days

Question 8

when the embryo consists of approximately 16 cells, it is called a

Answer 8

morula

Question 9

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

Answer 9

compaction

Question 10

during which the individual outer blastomeres tightly adhere through
gap and tight junctions and lose their individual identity when viewed
from the surface

Answer 10

compaction

Question 11

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

Answer 11

E-cadherin

Question 12

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

Answer 12

cavitation

Question 13

cavitation occurs about

Answer 13

4 days after fertilization

Question 14

at the blastocyst stage, the embryo, which is still surrounded by the zona
pellucida, consists of two types of cells:

Answer 14

o Trophoblast – outer epithelial layer
o Inner cell mass – small inner group of cells

Question 15

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

Answer 15

embryonic pole
abembryonic pole

Question 16

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 _

Answer 16

embryo
extraembryonic structures
outer layers of the placenta

Question 17

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

Answer 17

fibroblast growth factor-4

Question 18

The first cleavage of frog’s egg was observed by

Answer 18

Jan Swammerdam in 1738.

Question 19

The entire process of cleavage in frog’s egg was studied by

Answer 19

Pierre Prevost
and Jean Baptiste André Dumas in 1824

Question 20

planes of cleavage are named depending on the position of the

Answer 20

cleavage
furrow

Question 21

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 21

Meridional plane

Question 22

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 22

Vertical plane

Question 23

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 23

Equatorial plane

Question 24

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

Answer 24

Latitudinal plane

Question 25

“yolk rich pole”

Answer 25

Vegetal Pole

Question 26

yolk poor pole

Answer 26

Animal Pole

Question 27

zygotic nucleus is generally displaced towards the _

Answer 27

animal pole

Question 28

# Types of Eggs sparse evenly distributed yolk

Answer 28

Isolecithal

Question 29

# Types of Eggs moderate amount of yolk and often unevenly distributed

Answer 29

Mesolecithal

Question 30

# Types of Eggs dense yolk concentrated at one end

Answer 30

Telolecithal

Question 31

# Types of Eggs yolk concentrated at the middle of the egg

Answer 31

Centrolecithal

Question 32

Isolecithal example

Answer 32

sea urchin and mouse

Question 33

Mesolecithal example

Answer 33

frog

Question 34

Telolecithal example

Answer 34

bird and reptile

Question 35

Centrolecithal example

Answer 35

fly

Question 36

# Cleavage Patterns

Answer 36

1. Total/ Holoblastic Cleavage a. equal holoblastic cleavage b. unequal holoblastic cleavage 2. Meroblastic Cleavage a. discoidal clevage b. superficial cleavage

Question 37

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

Answer 37

Total or holoblastic cleavage

Question 38

in microlecithal and isolecithal eggs, cleavage leads to the formation of **blastomeres of equal size**;

Answer 38

Equal holoblastic cleavage

Question 39

Equal holoblastic cleavage examples

Answer 39

amphioxus and placental mammals

Question 40

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

Answer 40

Unequal holoblastic cleavage

Question 41

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

Answer 41

Meroblastic cleavage

Question 42

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;

Answer 42

Discoidal cleavage

Question 43

Discoidal cleavage examples

Answer 43

e.g. birds and reptiles

Question 44

in centrolecithal eggs, the cleavage is restricted to the peripheral cytoplasm of the egg;

Answer 44

Superficial cleavage

Question 45

Superficial cleavage example

Answer 45

insects

Question 46

Sachs’ Laws proposed by

Answer 46

Julius von Sachs in 1877

Question 47

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 47

Sachs’ Laws

Question 48

Balfour’s Law proposed by

Answer 48

Francis Maitland Balfour in 1885

Question 49

states that “the **speed** or rate of **cleavage** in any region of egg is **inversely proportional** to the amount of **yolk** it contains”

Answer 49

Balfour’s Law

Question 50

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 50

radial cleavage

Question 51

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

Answer 51

spiral cleavage

Question 52

characteristic of deuterostomes

Answer 52

Radial Cleavage

Question 53

deuterostomes

Answer 53

(“mouth second”); e.g. human, starfish

Question 54

Radial Cleavage results in

Answer 54

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) ## Footnote 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

Question 55

characteristic of protostomes

Answer 55

Spiral Cleavage

Question 56

Spiral Cleavage results in

Answer 56

determinant cells (cell that have a determined embryological fate early on during the development of the embryo) ## Footnote determinant cells are programmed to become a specific type of cell, early on during the proces

Question 57

Classification of Cleavages based on the Potentiality of the Blastomeres for the Future Development

Answer 57

a. Determinate b. Indeterminate

Question 58

the developmental fate of each embryonic cell is established very early

Answer 58

Determinate

Question 59

Determinate: if a cell is isolated from the _ stage the embryo will not fully develop

Answer 59

4-cell ## Footnote this is because the fate of each blastomere is predetermined in the early embryonic stage itself

Question 60

examples of determinate

Answer 60

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

Question 61

early embryonic cells retain capacity to develop into a complete embryo if isolated from other cells

Answer 61

Indeterminate

Question 62

Indeterminate: cleavage produces blastomeres which are qualitatively _

Answer 62

equipotential or totipotent

Question 63

when they are isolated, they develop into complete embryos

Answer 63

Indeterminate ## Footnote this is because the fates of blastomeres are not predetermined in the early embryonic period

Question 64

examples of indeterminate

Answer 64

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

Question 65

types of eggs of holoblastic

Answer 65

isolecithal mesolecithal

Question 66

types of eggs of meroblastic

Answer 66

telolecithal centrolecithal

Question 67

patterns of cleavage of isolecithal

Answer 67

1. radial - echinoderms, emphioxus 2. spiral - annelids, molluscs, flatworms 3. bilaterial - tunicates 4. rotational - mammals, nematodes

Question 68

patterns of cleavage of mesolecithal

Answer 68

radial - amphibians

Question 69

patterns of cleavage of telolecithal

Answer 69

1. bilateral - cephalopod molluscs 2. discoidal - fish, reptiles, birds

Question 70

patterns of cleavage of centrolecithal

Answer 70

superficial - most insects

Question 71

occurs in centrolecithal eggs

Answer 71

Superficial Cleavage

Question 72

Superficial Cleavage: the early divisions occur in the surface layer of the egg and cleavage furrows _ extend into the central yolk

Answer 72

do not

Question 73

Superficial Cleavage, the zygote nucleus lies in the _ of the egg

Answer 73

center

Question 74

# Superficial Cleavage it divides repeatedly without the division of the egg cytoplasm o as a result, a large number of _ are formed

Answer 74

nuclei

Question 75

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 75

cytoplasmic localization

Question 76

the organization of the cytoplasm of the egg before and after fertilization is established by the redistribution of ooplasmic determinants

Answer 76

cytoplasmic localization

Question 77

helps seal the developmental fate of each cell's descendants

Answer 77

cytoplasmic localization

Question 78

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

Answer 78

fertilization

Question 79

although fertilization results in union of maternal and paternal genomes, zygotic gene activity is _ until the blastula stage

Answer 79

not required ## Footnote in fact, after fertilization and through cleavage, the maternal, paternal and zygotic genomes are completely dispensible

Question 80

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

Answer 80

zygotic gene transcription

Question 81

# Evidence for maternal mRNA control of early development: 1. maternal trait dominance in _ hybrids 2. cleavage in _ embryos 3. _ inhibition does not prevent cleavage 4. _ inhibition prevents cleavage

Answer 81

interspecific enucleate transcriptional translational

Question 82

# roles of maternal factors 1. processing of the _ , which is necessary for its participation in embryogenesis 2. removal of _ (RNA and protein), 3. robust activation of the _ , which is essential for the development of the embryo beyond two cells

Answer 82

male genome maternal detritus embryonic genome