Embryonic Development Involves 3 Components: 1. Cell Division- The mitotic increase in the number of cells. 2. Differentiation- The development of specialized cells that are then organized 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!

Fertilization (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 prevents Any other sperm from entering. This is often called the “_________ block to polyspermy.” FAST

Fertilization 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 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 Joining of sperm & egg Joining of sperm & egg sperm head (nucleus) enters egg sperm head (nucleus) enters egg

Stages of Body Building 1. Cleavage: rapid mitotic divisions that partitions the cytoplasm into smaller cells (blastomeres) 2. Gastrulation: drastic rearrangement of cells in blastula to form a three-layered embryo with a primitive gut (archenteron). 3. Organogenesis: the three germ layers develop into rudiments of organs.

Cleavage Repeated mitotic divisions of zygote Repeated mitotic divisions of zygote 1st step to becoming multicellular 1st step to becoming multicellular unequal divisions establishes body plan unequal divisions establishes body plan different cells receive different portions of egg cytoplasm & therefore different regulatory signals different cells receive different portions of egg cytoplasm & therefore different regulatory signals

zygote  morula  blastula zygote  morula  blastula establishes future development establishes future development zygote blastula morula gastrulation Cleavage

Polarity of the Dividing Egg Vegetal Pole- Has the highest concentration of yolk Becomes the posterior portion of the embryo Cleavage occurs more rapidly Animal Pole- Has a lower yolk content Becomes the anterior portion Cleavage occurs more slowly Future Endoderm & Mesoderm Future Ectoderm

Types 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 layers Establish 3 cell layers ectoderm ectoderm outer body tissues outer body tissues skin, nails, teeth skin, nails, teeth nerves, eyes, lining of mouth nerves, eyes, lining of mouth mesoderm mesoderm middle tissues middle tissues blood & lymph, bone & notochord, muscle blood & lymph, bone & notochord, muscle excretory & reproductive systems excretory & reproductive systems endoderm endoderm inner lining inner lining digestive system (intestines, liver, pancreas) digestive system (intestines, liver, pancreas) lining of respiratory, excretory & reproductive systems lining of respiratory, excretory & reproductive systemsGastrulation ectoderm mesoderm endoderm

Is all Cytoplasm the Same?? Heterogeneous organization of the cytoplasm already exists even in the unfertilized egg. Different:Pre-made mRNA Cytoskeleton 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

The 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 Genes Products 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, ,00.html

And now for some…….. Crash Course Animal Development: Ahv0 Ahv0 Ahv0 Spectacular Animations: Spectacular Animations: