04.PhasesofEmbryonicdevelopment.ppt

phases of Embryonic development

1. Phases of Embryonic Development

2. 7 steps:
1.gametogenesis
2.fertilization
3.cleavage
4.blastulation
5.gastrulation
6.neurulation
7.organogenesis

3. 1.Gametogenesis
isaprocessbywhichthediploidgermcells
undergoanumberofchromosomaland
morphologicalchangestoformmaturehaploid
gametes.
Animalsproducegametesdirectlythroughmeiosis
inorganscalledgonads.Malesandfemalesofa
speciesthatreproducessexuallyhavedifferent
formsofgametogenesis:
spermatogenesis(male)intestesproducesperms.
oogenesis(female)inOvaryproduceova.

5. Structure of sperm

6. Structure of ovum of sea urchin

7. Common terms
AnimalPole:thepole(end)oftheeggwhere
yolkisleastconcentrated.
Animalhemisphere:thehemisphereoftheegg
whereanimalpoleislocated.
Vegetalpole:thepole(end)oftheeggwhere
yolkisthemostconcentrated.
Vegetalhemisphere:thehemisphereoftheegg
wherevegetalpoleislocated.

8. 2.Fertilization:
istheprocesswherebytwosexcells(gametes)fusetogether
tocreateanewindividualwithgeneticpotentialsderived
frombothparents.
Havetwoseparateactivity:
1.Combiningofgenesderivedfromthetwoparents.
2.Creationofneworganisms.
Thus
Thefirstfunctionis:Transmitgenesfromparentsto
offspring.
Thesecondis:initiatereactionsintheeggcytoplasmthat
proceeddevelopment.Also,
Restorationofthediploidnumberofchromosomesreduced
duringmeiosis.

9. It have 4 major steps:
1.Sperm contacts the egg
2.Sperm or its nucleus enters the egg, and
regulation of sperm entry .
3.Sperm and egg nuclei fuse
4.Egg becomes activated and developmental
changes begin

10. 3. Cleavage
Istheprocessofrepeatedrapidmitoticcelldivisionsofthe
zygote(unicellularstructure)toformtheBlastula
(multicellularstructure).
TheproducedcellsnamedBlastomeres.
Duringthisstagethesizeoftheembryodoesnotchange,
theblastomeresbecomesmallerwitheachdivision.
Thetype&patternofcleavagedifferfromspeciesto
species.
continuesdivisionstoformaballof32cellscalledthe
morula.
Themorulacontinuesdivisionstoformthehollowblastula
withuptoseveralhundredcells.
Thecavityoftheblastulaistheblastocoel.

11. Figure 47.8x Cleavage in a frog embryo

12. 4. Blastulation
Theresult(endperiod)ofcleavage.
Theproductionofamulticellularblastula
Blastulacellsarecalledblastomeres.
Acavityformswithintheballofthecellscalled
theblastocoel.

13. Blastula of frog

14. Sea urchin blastula

15. Human blastula

16. Starfish development, unfertilized egg.
2 blastomeres.
Starfish development,
nonmotile blastula.
4 blastomeres.
16 blastomeres. 32 blastomeres.
morula

17. 5. Gastrulation
Themorphogeneticprocesscalledgastrulation
rearrangesthecellsofablastulaintoathree-
layered(triploblastic)embryo,calleda
gastrula,thathasaprimitivegut.
Itmeansrearrangementofblastulacellsthat
transformstheblastulaintoagastrula.
Theblastuladevelopsaholeinoneendand
cellsstarttomigrateintothehole;thisforms
thegastrula
Characterizedbycellmovement.
Blastocoelisgraduallydisappearandanew
cavityisformedGastrocoel.

18. Thegastrulaisathree-layeredembryo
Theformationofthreeprimaryembryonicgerm
layers
Endoderm(inner)
Mesoderm(middle)
Ectoderm(outer)
Thepatternofgastrulationisaffectedbytheamount
ofyolk.
Thecellsatthevegetalpoleinvaginate,initiating
gastrulation.

21. Gastrulation in a Frog Embryo

22. Figure 47.10 Gastrulation in a frog embryo

23. 6. Neurulation
formationofadorsal,hollowneuraltube
by
ectodermalcellsflattenintoneuralplate
thecenteroftheplatesinksformingneuralgroove
edgeofplateiselevatedtoformneuralfolds
neuralfoldsfuseandformneuraltube
anteriorenddevelopsintobrain
posteriorenddevelopsintospinalcord
Chordates Only

25. Neurulation

26. The Neural Crest
The neural crestis a
critical structure that
guides formation of several
organ systems
The neural crest forms on
either side of the point of
fusion
Its cells migrate to form the
dorsal root ganglia, the
postganglionic sympathetic
neurons, many sense organs
and all pigment-forming
cells

27. Organogenesis
Organogenesisistheformationoftheorgans
Thelayersaregermlayers;theyhavespecificfates
inthedevelopingembryo:
Endoderm
Theinnermostlayer
Goesontoformthegut
Mesoderm
Themiddlelayer.
Goesontoformthemuscles,circulatorysystem,blood
andmanydifferentorgans
Ectoderm
Theoutermost
Goesontoformtheskinandnervoussystem

29. Organogenesis Begins With
Development of the Nervous System
The nervous system is the first
organ system to develop.
The notochord grows and
induces overlying ectoderm to
form the neural plate.
Cells of the neural plate fold to
form the neural groove and
the surrounding neural folds
fuse to form the neural tube.
The anterior portion forms the
brain; the rest forms the spinal
cord.

30. 7. Organogenesis
Development of organs from three primary germ layers
Ectoderm forms:
skin and associated glands, nervous
system.
Mesoderm forms:
muscles, skeleton, gonads, excretory
system, circulatory system.
Endoderm forms:
lining of digestive tract, liver,
pancreas, lungs.

32. Figure 47.11 Organogenesis in a frog embryo

33. Somatic &Germ Cells
SomaticCells:
Foundinallbodytissuesexceptgonads.
Containdiploidnumbersofchromosomes(2N).
Replacementofdeadcells
Reproducebymitoticdivision.
Functions:
1.Responsibleforformationofdifferentsystemand
organs.
2.Haveotherspecificfunctions
e.x.:Muscularsystemhasmyoplastforcontractionand
relaxation.
Nervoussystemhasneuronsfortransmissionof
impulses.

34. digestivesystemhavesecretorycellsforsecretionof
enzymesfordigestion.
bonehaveosteoplastsmakehardeningofbone.
Lunghavecellsforrespiration.
GermCells:
Foundonlyingonads(testes&ovary)
Containhaploidnumberofchromosomes(1N)
Reproducebymeioticdivision(meiosis).
Function:Formationofgametes(male&female)

35. Primordial germ cells
Appearsinthewalloftheendodermallayerofthe
yolksacduetotheirlargesizeandhighcontentof
alkalinephosphatase,andmigratebyamoeboid
movementtowardthehindgutepitheliumand
thenthroughdorsalmesenteryreachtothe
primordiaofthegonads(primitivesexglands).
Becomerecognizableat24dayspost-fertilization.
Invadingthegenitalridgesinthe6
th
weekof
development.
Theprimordiaofthegonadsgiveachemostatic
attractantforthegermcells.

36. Alsomaybethegermcellsdirectedtowardthe
gonadsbythematureofcellularandnon-cellular
microenvironmentthatsurroundthem.
Orbytissuerearrangementthatoccurintheearly
embryos.
Iftheyfailtoreachtheridges,thegonadswillnot
develop.
So,ithavetheinductiveinfluenceonthe
developmentofgonadsintoovaryandtestes.
Determinesexoftheembryo.
Theywillformdifferentstagesofspermatogenesis
andoogenesisinthefuture.
Foundinadultingonads(testesandovary)

38. Basic Developmental Vocabulary
Fertilization–activatesegg&bringstogetherthe
nucleioftheeggandsperm.
Cleavagepartitionsthezygoteintomanysmaller
cells.
Gastrulationrearrangestheblastulatoforma
three-layeredembryowithaprimitivegut,the
archenteron.
Organogenesisistheprocessbywhichtheorgans
intheanimalbodyformfromthethreeembryonic
germlayers.

39. Basic Developmental Vocabulary
Blastula–ahollowsphereofcells(128cells)formedby
cleavageofthemorula.Theblastulacontainstheblastocoel
thatisfluid-filled.Theblastoporeistheplacewhere
gastrulationbegins.
Gastrulation–theprocessleadingtothecreationofthe
primitivegutorarchenteron.Invaginationattheblastopore
resultsinthegut.
Gastrula–transformationoftheblastulaintoanembryo
possessing3germlayers,ectoderm,mesoderm,endoderm.
Morulaasolidmassof16-64cellsformedbycleavage.