Developmental Biology for UG course Lifesciences

1. CLEAVAGE IN ANIMALS
By
DR. SANJUKTA DAS
DR. KHANGEMBAM CHERITA DEVI
DEPARTMENT OF ZOOLOGY
KIRORI MAL COLLEGE, UNIVERSITY OF DELHI

2. What is
Cleavage?
• It is the first embryological stage in life cycle of all metazoans.
• It follows fertilization process and is a dynamic event.
• It lays the ground for the next stage of embryogenesis that is
Blastula.
• It refers to repeated mitotic divisions of the fertilized egg into
large number of cells.
• The cell division occurs in rapid succession.
• The cell products of cleavage is called as blastomeres.

3. Characteristics of
Cleavage division
Like a mitotic division cleavage involves nuclear division (Karyokinesis) followed by
cytoplasmic division (Cytokinesis) producing smaller nucleated cells called blastomeres.
The first cleavage of zygote divides it in to 2 blastomeres. Then two blastomeres
undergo cleavage to form 4, 8,16, 32 and so on.
The rate of cell divisions during cleavage is very fast.
The cell-cycle duration in cleavage is short as compared to somatic cell cycle duration.
[This is due to absence of gap phases (G1 and G2)]
Thus, cleavage is initially biphasic consisting of S (synthetic phase) and M ( Mitotic
phase).
Cleavage starts from animal pole and extends towards vegetal pole.

4. Fig. Increase in cell number in frog due to cleavage (between the blastula stage and
the beginning of gastrulation)—After Sze 1953.
Increase in blastomere numbers in early development of frog (Rana pipiens)

5. Outcomes of
Cleavage
Increase in cell numbers of the embryo.
Generation of many copies of genome.
Segregation of cytoplasmic contents into blastomeres.
Volume of embryo does not change, hence
blastomeres become smaller.
Increase in the nucleocytoplasmic ratio (ratio of
nuclear volume to cytoplasmic volume).
Cytoplasmic volume does not increase , rather large
volume of cytoplasm is divided into smaller
blastomeres formed due to cleavage.

6. Significance of Cleavage
It contributes to multicellularity of metazoans at an early stage of metazoan development.
It sets the stage for the future embryonic processes.
Increasing the nucleocytolasmic ratio– ratio of nuclear vol to cytoplasmic vol. , critical
cellular parameter is necessary for efficient turnover of RNA and Protein.
In certain embryos as that of Xenopus and Drosophila, it has been shown that increase in
nucleocytoplasmic ratio is crucial in timing the activation of certain genes.
Rapid cleavage reduces the risk of predation when the embryo remains immobile

7. Pattern and types of Cleavage : Yolk distribution and amount
Animal forms differ in their cleavage patterns- the relative sizes of blastomeres
and their configuration are different.
Pattern of cleavage is established by amount of yolk and its distribution
Types of egg based on the amount of yolk
• Microlecithal egg: small /little amount of yolk. e.g., Marsupials
• Mega/Macrolecithal egg: Large amount of yolk e.g., some fishes, reptile and bird
• Alecithal egg: absence of yolk. e.g., human egg

8. Types of Egg and Types of Cleavage: Animals' eggs are characterised and classified in terms of amount
of yolk and its distribution. Presence ofyolk also signifies the polarity of egg.
Type of cleavage Pattern of cleavage Animals
Isolecithal
Little yolk , Evenly distributed
Holoblastic
Egg completely cleaved
Radial
Bilateral
Spiral
Rotational
Echinoderms
Ascidians
Molluscs
Mammals
Mesolecithal
Moderate amount
Of yolk, mostly in vegetal
Hemisphere
Telolecithal
Large of amount
Of yolk except for blastodisc at
animal pole
Holoblastic
Meroblastic
Egg incompletely
Cleaved
Radial
Discoidal
Blastomeres
Form disc on
yolk
Amphibians
Most fishes
Birds
Reptiles
Centrolecithal
Yolk concentrated in centre of the
egg
Meroblastic Superficial Insects and other arthropods

9. Types of cleavage
Based on the amount of yolk the cleavage is
complete or partial
A) Holoblastic – Complete cleavage – in this case
the cleavage plane completely passes through the
egg bisecting it into two blastomeres.
• So, the holoblastic cleavage results in the
formation of two equal size blastomeres.
B) Meroblastic – Incomplete cleavage – In this
case the yolk does impede the complete division
of the egg
Fig. Holoblastic cleavage Fig. Meroblastic cleavage

10. Types of Holoblastic Cleavage: Radial , Bilateral and Spiral
cleavage
i. Radial type of cleavage
• It is found in echinoderms
• The plane of first division is meridional passing through the
main axis.
• The second division is also meridional but at right angle to
the first division .
• Thus, the first four blastomeres all lie side by side.
• The third division is at right angles to the first two divisions.
• If each of the blastomeres of the upper tier lie over the
corresponding blastomeres of the lower tier , the pattern
of the blastomeres is radially symmetrical.
• E.g. Sea Cucumber Fig. Radial cleavage with almost equal size
blastomeres in sea cucumber (after
Selenka from Korschelt, 1936)

11. ii. Bilateral Cleavage
• It is found in Ascidians.
• The eggs of ascidians have anteroposterior polarity besides
animal and vegetal polarity due to asymmetrical distribution of
cytoplasmic components.
• The first cleavage is meridional, and the second is also meridional
again but it does not pass through the centre; slightly posterior
being parallel to the animal-vegetal axis resulting in the formation
of two large blastomere and two small at the posterior side.
• At this point, cleavage has only one plane, the median plane, so
the embryo is bilaterally symmetrical. Subsequent cleavage makes
the bilateral arrangement of blastomeres more pronounced.
A
B
C
Fig. Bilateral Cleavage in Ascidians

12. iii. Spiral cleavage
• In this type all the blastomeres of the upper tier are shifted in
the same direction in relation to the blastomeres of the lower
tier , so that they come to lie not over the corresponding
vegetal blastomeres , but over the junction between each two
of the vegetal blastomeres.
• This arrangement comes about not as a result of secondary
shifting of the blastomeres , but because of oblique positions
of the mitotic spindles, so that from the start the two daughter
cells do not lie one above the other.
• The four spindles during the third cleavage are arranged in sort
of a spiral. This type of cleavage is therefore called the spiral
type of cleavage.
E.g. Annelids, Molluscs , Nemerteans and some of the
Planarians ( the Polycladida).
Fig. Spiral cleavage in Mollusc

13. Types of Meroblastic cleavage: Discoidal and Superficial Cleavge
i. Discoidal cleavage
• This type of cleavage is found in many birds, reptiles and fishes that have
telolecithal eggs .
• Because of the presence of large amount of yolk the cytoplasm remains in
the form of a disc ,,called blastodisc ,above the yolk in the animal pole.
• Initially all the cleavage planes are vertical and all the blastomeres lie in
one plane only.
• The cleavage furrow begins to form at the animal pole, but instead of
bisecting the egg completely , it stops at the yolk.
• With more number of cleavage including horizontal ones the blastomeres
become arranged over the uncleaved yolk in the form of a patch of cells.
Fig. discoidal cleavage of hen’s egg (surface view)

14. ii. Superficial cleavage
• This type of cleavage is found in Insects having centrolecithal
egg.
• the cleavage starts when the nucleus is in the centre of the egg
surrounded by small amount of cytoplasm.
• The karyokinesis happens without cytokinesis.
• Thus multiple number of nuclei are formed all remaining in the
undivided central mass of cytoplasm..
• Then the nuclei start moving away from the centre and each
nucleus get surrounded by a small amount of central cytoplasm
. And start moving toward the surface of the egg, as a result the
surface layer of the embryo become syncytium with numerous
nuclei in an undivided layer of cytoplasm, then the cytoplasm get
subdivided forming real cells.
Fig. Superficial cleavage in
insects

15. Mechanism Cleavage –formation of cleavage furrow
• The cleavage pattern of an embryo , the relative size and spatial
arrangement of its blastomere is controlled most directly by the
positioning of cleavage furrow.
• Cleavage begins from animal pole and then extends to the vegetal pole.
• In holoblastic cleavage, cleavage furrow constricts like tightening belts
around the entire cell.
• In eggs that undergo meroblastic, cleavage furrows begin as infoldings of
the egg plasma membrane at the animal pole.

16. Contractile Ring
• The mechanical agent for cleavage is contractile ring.
• Formation of contractile ring takes place during cleavage since after
cleavage it disappears.
• Under confocal microscope, contractile ring appear as distinct band of
0.1 µm wide.
• It is made up of actin and myosin filament that lie below the plasma
membrane.
• The position of the contractile ring at the same plane previously
occupied by the metaphase plate. Hence, it is perpendicular to the
axis of mitotic spindles.

17. Cleavage furrow: the contraction of contractile ring generates cleavage furrow
Under electron microscope, cleavage furrows show a thin, dense layer
beneath the plasma membrane, consisting of a bundle of actin and myosin
filaments.
The filaments of the contractile ring are anchored to proteins embedded in
the plasma membrane, so that constriction of the ring causes the
membrane to furrow.
The force exerted by the contractile ring is most likely generated by the
sliding of the actin and myosin filaments past one another , a mechanism
similar to the action of these proteins in muscle contraction.
Cleavage furrow begins from the animal pole extends towards vegetal
pole.
Furrow

18. MBT: Mid Blastula Transition
• MBT represents the late stage of cleavage.
• When the embryo enters MBT stage, some new properties are added to
the cleavage of embryo.
• During this phase, cleavage transitions from synchronous cleavage to
asynchronous cleavage.
• Initially the cell cycle of cleavage is biphasic, but, with the onset of MBT G1
and G2 phases are added to the cell cycle.
• Duration of cell cycle gets elongated in MBT.
• As the cytoplasmic components are used up during cleavage, the embryo
nucleus begin to synthesize cellular components towards MBT.
• The new mRNAs are transcribed. Some of these, encodes the proteins
necessary for gastrulation in the later stage.

19. References
• Analysis of Biological Development; Second Edition by Klaus Kalthoff
• Developmental biology ; Tenth edition by Scott F. Gilbert
• Foundations of Embryology; Sixth Edition by Bruce M Carlson
• An Introduction to Embryology; fourth Edition by B.I. Balinsky
• Pictures from Common Creative licences