Cleavage

Embryology

1. Cleavage and Fate
Maps
Embryology
Bio 415 LECTURE
4

2. Cleavage stage and and Fate Maps
Cleavageisthefirstphaseofembryonicdevelopmentbymitoticcell
division.
Introduction
Levels of Cleavage types:
 Full splinting or cleavage.
 Partial splinting or cleavage.
 Surperfacialcleavage or cleavage.
Cleavage stages:
2 cell stage 4 cell stage 8 cell stage 16 cell stage 32 cell
stage Morula Blastula.
Fate maps:
In amphibians
In birds
In Mammals

3. Diagram explaining the development of embryos within the
Fallopian Tubes (cleavage stages)

4. Cleavagestage

5. •Afterfertilizationandformationofthezygote(thefertilizedova)beginsthefirst
stagesofearlyembryonicformation,whichconsistsof:
Aseriesofindirectdivisions(mitotic)knownasthephaseofcleavageor
(segmentation)wherethefertilizedeggisdividedintoseveralsuccessivedivisions
producedBlastomeres.
•Atthebeggingofcleavagetheblastsareinitiallylargeandthenbecome
smallerwiththeprogressintheprocessofsplintingorcleavagetoformblastula
attheendofthecleavagephases.
Cleavage Introduction

6. beforethefertilizedeggbeginstheprocessofsplinting,itpreparesinternallyby
increasingthefollowing:
1.Increasingthecompositionofprotein.
2.MultipliesGeneticmaterialorDNAchromosomes.
3.Formationofthefertilizationmembrane.
4.Formationofthehyalinelayer(protectionorpreservationoftheblastomeres
together).
Cleavage Introduction

7. What is cleavage?
Cleavageisarapidseriesofmitoticdivisionsthatoccurjustafterfertilization.
Therearetwocriticalreasonswhycleavageissoimportant:
1.Generationofalargenumberofcells:
Thatcanundergodifferentiationandgastrulationtoformorgans.
2.Increaseinthenucleus/cytoplasmicratio:
Eggsneedalotofcytoplasmtosupportembryogenesis.Itisdifficultor
impossibleforonenucleustosupportahugecytoplasm,andoocytesareone
ofthelargestcellsthatexist.Onesmallnucleusjustcannottranscribe
enoughRNAtomeettheneedsofthehugecytoplasm.
Alargernucleustocytoplasmicratioisoptimalforcellfunction.Cell
divisionoccursrapidlyafterfertilizationtocorrectthisproblem.

8. 1.During the process of cleavage ,the cell cycle consists of only 2 phases:
The (S-phase DNA Synthesis) the manufacture of genetic material.
The cellular division phase (M-phase) only.
2.The cell in cleavage does not go through a first or second interval (G1,G2 phases)
as in the normal cell cycle.
3.The cell division is regular at begging and then followed by multiple divisions that
cannot be pursued.
4.The cleavage leads to a multicellular early embryo
Cleavage character
Fig.Comparingbetweencellcyclein
earlycleavageandsomaticcell.

9. 5.Cleavage does not include an increase in the size of the fetus (about the same size
as the ovum ).
6.The embryonic form remains as it is the general form is the same (except the
appearance of the blastula cavity) At the end of the cleavage.
7.The time of the cleavage does not depend on the amount of the yolk, it varies by
the difference of living organisms:
•In the mammals' ova the first cleavage stage takes 24 hours which is the lowest
eggs in the yolk.
•In the amphibians 1st cleavage done within 3 hours.
•In the embryo of birds completes the stages of cleavage before the female lays the
egg.
Cleavage character

10. 1.Thefirstlevelofsplintingpassesthroughthemainaxisofthefertilizedovastarting
fromtheanimalpole(thelocationofthenucleustothevegetativepole(producestwo
cellsortwoblastomers).
2.Thesecondlevelofcleavage:Radialcleavagemaybeperpendiculartothefirst
divisionsuchasseaurchinandamphibiansinbothblastomers,andmayberotating
(Rotationalcleavage)dividedontheaxislengthandtheotheronthewidthoftheaxis
asinmammals.
3.Thethirdlevelofsplinting:horizontalonthefirstandsecondsplitsandproduces
eightcellorblastomersuppergroupoftheanimalhalfpoleoftheembryo(inwhich
thenucleuswaslocated)alowergroupofthehalfofthevegetativepole(asin
amphibians)
4.Thefourthcleavage:composefrom4divisionsintheuppercellgroupandanotherin
thelowercellgroupproduces16cells.
5.Thefifthcleavageisasetof16celldivisionsineachblastomeartoproduce32cells
6.Thesixthcleavage:asetof32celldivisiontogivethestageofMorula.
7.Theseventhcleavageisasetofdivisionsresultingintheblastulastage.theendof
thecleavagestage
Cleavage Levels

11. Radial cleavage Rotational cleavage
Type of cleavage, Radial cleavage in sea urchin embryo And Rotational cleavage as in mammals.

12. Cleavage differs from normal mitoses in 2 respects
1.Blastomeres do not grow in sizebetween successive cell divisions as they do
in most cells. This leads to a rapid increase in the nucleus / cytoplasmic
ratio.
2.Cells undergoing cleavage have mainly S and M phases of the cell cycle
(little or no G1 or G2).
3.Cleavage occurs very rapidly, and mitosis and cytokinesis in each round of
cell division are complete within an hour. Typical somatic cells divide much
more slowly (several hours to days) and even the fastest cancer cells divide
much slower than occurs in a zygote during cleavage.
4.Cleavage differs in different types of eggs. The presence of large amounts of
yolk alters the cleavage pattern, leading to incomplete cleavage that
characterizes birds and reptiles.

13. Eggs are classified by how much yolk is present
1.Isolecithaleggs(iso=equal)haveasmallamountofyolkthatisequally
distributedinthecytoplasm(mostmammalshaveisolecithaleggs).
2.Mesolecithaleggs(meso=middle)haveamoderateamountofyolk,andthe
yolkispresentmainlyinthevegetalhemisphere(amphibianshave
mesolecithaleggs).
3.Telolecithaleggs(telo=end)havealargeamountofyolkthatfillsthe
cytoplasm,exceptforasmallareaneartheanimalpole(fish,reptiles,and
birds).
4.Centrolecithaleggshavealotofyolkthatisconcentratedwithinthecenterof
thecell(insectsandarthropods).
Two areas of interest:
How does the process of cleavage differ in different organisms?
What mechanisms regulate cleavage?

14. Thepatternofcleavageofthezygotedependsuponthepatternof
yolkdistribution.
1.Holoblasticcleavageoccursinisolecithaleggs(mammals,seaurchins).The
entireeggiscleavedduringeachdivision.
2.Meroblasticcleavageoccurswheneggshavealotofyolk.Theeggdoesnot
dividecompletelyateachdivision.Twotypes:
a.Discoidalcleavageislimitedtoasmalldiscofcytoplasmattheanimal
pole.Alloftheyolkfilledcytoplasmfailstocleave(characteristicof
telolecithaleggssuchasbirds).
b.Superficialcleavageislimitedtoathinsurfaceareaofcytoplasmthat
coverstheentireegg.Theinsideoftheeggthatisfilledwithyolkfailsto
cleave(centrolecithaleggssuchasinsects).

15. Typical cleavage patterns of isolecithal, mesolecithal,
telolecithal and centrolecithal eggs

16. •In eggs where the yolk is distributed regularly and equally,
such as most mammals, sea urchins.
•The amount of the yolk is small
•The cells is totally divided
•The whole cells are equal in size (cells in the animal pole are
smaller).
sea urchins. cleavage
1-Holoblasticequal cleavage

17. Amphibians have mesolecithalor telolecithal eggs and
undergo holoblastic cleavage with unequal blastomeres
Amphibianeggshavealotofyolk,however,theyarestillabletoundergoholoblastic
cleavage,butproduceunequalblastomeres
The1stcleavageismeridional,asisthe2nd.
The3
rd
cleavageisequatorial.Thecleavageisdisplacedtowardtheanimalpoledueto
theyolk.Thisresultsin4smallanimalblastomeresand4largevegetalblastomeres.
Morula(morum=mulberry)atthe16to32cellstagetheembryoiscalledamorula
becauseitlookslikeamulberry.Witchwilltransfertoblastulastage.

18. The blastula formation = the last stage of cleavage
Blastula= will form at the end of cleavage stage, it from the 128 cell stage
onward the amphibian embryo is a blastula.
Its has a cavity called the blastocoelbetween the animal and vegetal pole.
The outer surface of the amphibian blastula has cells connected by specialized
cell junctions.
1.Tight junctionscreate a seal that isolates the outside of an embryo from the
inner layer. Tight junctions polarize the apical and basal surfaces. The basal
portions of cells start secreting into the blastocoel.
2.Desmosomesattach the blastomeres together on the outside.
3.Gap junctionsconnect all surface blastomeres.

19. Fig: The cleavage stages in amphibian embryo

20. In eggs with a large amount of yolk, the levels of cell division or cleavage
do not pass through cells and there are two types:
a-Discoidal
Is incomplete as in reptiles and birds where the cell division is confined to
the highest area of the egg. Where the fertilization occurs in the active
cytoplasm (germinal disk). (or blastoderm-cytoplasm free of the yolk and
floats on it -in the form of a small disk)
b -Partial cleavage ( or Meroblastic)
It is incomplete cell division or cell splitingas in fishwhere the cleavage
occurs at the top of the egg or some of the surface dividing cell are
complete, while the underneath dividing cells are incomplete, and
connected with the yolk.
2-Discoidal or Meroblastic Cleavage

21. Discoidal cleavage in birds

22. Meroblastic cleavage in fish

23. 3-Superficial Cleavage
SuperficialCleavageoccurin
insect
TheYolkinthecentralofthe
ova
Theseveralcleavagehapend
inthenuclouseAndthe
neclousmoved tothe
cytoplasmarea.
Inthesurfaceoftheovum
andeachneouloussuronded
bycytoplasm.
Thecellsforminthesurface
oftheegg.

24. •Theresearcherswereabletodrawavirtualmaponthephaseoftheblastula
stage.
•Theblastulaaredistributedinto3areas(animalpole,vegetalpoleandthe
middleoftheblastula),theyfollowthefateofeachparticulargroupofareas
oftheblastulawhentheygrowuplater.
•Theresearcher(Vogt)wereabletodrownthefatemapbyspecialvitaldie
tostainthe3areaofblastulastage(itsnottoxictotheembryocell),(the
vitaldieNeutralred,Nilebluesulphate,andJanusgreen),thentheyfollow
thefateofeachstainedgroupofcell.
1.Theanimalpolewilldeveloptotheectodermwitchwillgivetheneural
tubeandtheskin(epidermis).
2.Thevegetalpole(thelowerpartoftheblastula)willformtheendoderm
witchwilldevelopandgivetheelementarycanal,lungs.
3.Themiddleoftheblastulawillgivethemesodermthatwilldevelopand
givethedermis,heart,muscles,bones,kidney.Seefig.
Fate Maps

25. Fig: Frog Embryo Fate Maps at the blastula stage

26. Fig: Different Embryos Fate Maps at the blastula stage

27. Fig: Embryos Fate Maps at the blastula stage