Embryonic Embryonic DevelopmentDevelopmentInvolves 3 Components:

1. Cell Division-The mitotic increase in the number of cells.

2. Differentiation- The development of specialized cells that are thenorganized into tissues & organs.

3. Morphogenesis-Physical processes that give shape to the organism.…involves movement & arrangement of cells

and apoptosis (programmed cell death)

All triggered by…..

FERTILIZATION!

FertilizationFertilization (as studied in Sea Urchins)(as studied in Sea Urchins)

1. Acrosomal Reaction-The discharge of hydrolytic enzymes from the Acrosome of the sperm cell (via exocytosis)

Proteins at the tip of the sperm attaches to specific receptors on egg’s “vitelline layer”, digests it, then contacts the plasma membrane.

When the membranes fuse, it depolarizes the membrane and preventsAny other sperm from entering.

This is often called the “_________ block to polyspermy.”FAST

Fertilization Fertilization Continued….Continued….

2. Cortical Reaction-Fusion of sperm & egg membranes causes activation of a signal transduction pathway:

-Releases Ca++ ions from the egg’s E.R. which results in the release of…

“Cortical Granules”

Cortical Granules fuse with plasma membrane and release their contents into the “perivitelline space”

Osmosis causes it to separate

Eventually hardens to become a “fertilization membrane”

This is often called the “_________ block to polyspermy”.SLOW

Fertilization FinaleFertilization Finale

3. Activation of the egg

The release of Ca++ also signals an increase in cellular:

Respiration

and

Protein synthesis

“pre-made” mRNA begins translation immediately.

Fertilization Fertilization Joining of sperm & egg Joining of sperm & egg

sperm head (nucleus) enters eggsperm head (nucleus) enters egg

Stages of Body BuildingStages of Body Building

1.1. CleavageCleavage: rapid mitotic divisions : rapid mitotic divisions that partitions the cytoplasm into that partitions the cytoplasm into smaller cells (blastomeres) smaller cells (blastomeres)

2.2. GastrulationGastrulation: drastic rearrangement : drastic rearrangement of cells in blastula to form a three-of cells in blastula to form a three-layered embryo with a primitive gut layered embryo with a primitive gut (archenteron). (archenteron).

3.3. OrganogenesisOrganogenesis: the three germ : the three germ layers develop into rudiments of layers develop into rudiments of organs.organs.

CleavageCleavage Repeated mitotic divisions of zygoteRepeated mitotic divisions of zygote

1st step to becoming multicellular1st step to becoming multicellular unequal divisionsunequal divisions establishes body plan establishes body plan

different cells receive different portions of egg cytoplasm & different cells receive different portions of egg cytoplasm & therefore different regulatory signalstherefore different regulatory signals

zygote zygote morula morula blastula blastula establishes future developmentestablishes future development

zygote

blastulamorula

gastrulation

CleavageCleavage

Polarity of the Dividing Polarity of the Dividing EggEgg

Vegetal Pole-Has the highest concentration of yolkBecomes the posterior portion of the embryo

Cleavage occurs more rapidly

Animal Pole-Has a lower yolk contentBecomes the anterior portion

Cleavage occurs more slowly

Future Endoderm & Mesoderm

Future Ectoderm

Types of CleavageTypes of Cleavage

Holoblastic-“Complete” cell division (Membranes are fully formed before next division)

Usually in cells with little to moderate levels of yolk

Meroblastic-Cleavage is “incomplete”(Membranes are not fully formed before next division)

Usually in yolk-rich eggs

Establish 3 cell layersEstablish 3 cell layers ectodermectoderm

outer body tissuesouter body tissues skin, nails, teeth skin, nails, teeth nerves, eyes, lining of mouthnerves, eyes, lining of mouth

mesodermmesoderm middle tissuesmiddle tissues

blood & lymph, bone & notochord, blood & lymph, bone & notochord, musclemuscle

excretory & reproductive systemsexcretory & reproductive systems endodermendoderm

inner lininginner lining digestive system digestive system (intestines, liver, pancreas)(intestines, liver, pancreas)

lininglining of respiratory, excretory & of respiratory, excretory & reproductive systemsreproductive systems

Gastrulation Gastrulation

ectoderm

mesoderm

endoderm

Is all Cytoplasm the Is all Cytoplasm the Same??Same??

Heterogeneous organization of the cytoplasm already exists even in the unfertilized egg.

Different: Pre-made mRNACytoskeleton constructs

Endomembrane products

…Ultimately producing different kinds of …. ________________BLASTOMERES!

With those blastomeres then having cell to cell interactions initiated by cytoplasmic differences to begin the morphogenesis.

It’s all part of a phenomenon known as…. Pattern Formation

Pattern FormationPattern FormationThe development of an organism’s spatial organization.

In other words…..Putting the right organs in the right places!

Controlled by a hierarchy of gene activations:

1. Egg Polarity Genes-Maternal genes that set the polar axis of future embryo.

2. Segmentation Genes-Regulate the origin of segmentation once the anterior to posterior axis is defined.

3. Homeotic Genes-Specify for the type of appendages specific for that segment.

Homeotic GenesHomeotic GenesProducts of these genes are regulatory proteins that bind to DNA.

Affect selective gene expression.

“HOMEOBOX” genes-Specific sequences in homeotic genes which are very similar in ALL eukaryotes.

(Indicates a very conservative evolution)…or “HOX”

D:\bc_campbell_biology_7\0,7052,3117255-,00.html

And now for some……..And now for some……..

Crash Course Animal Development: Crash Course Animal Development: http://www.youtube.com/watch?http://www.youtube.com/watch?v=k_9MTZgAhv0 v=k_9MTZgAhv0

Spectacular Animations:Spectacular Animations:

http://simbryo.stanford.edu/previewindex.html#

http://health.howstuffworks.com/adam-200129.htm

http://www.hhmi.org/biointeractive/stemcells/animations.html

http://www.yteach.com/