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…. http://en.wikipedia.org/wiki/Image:Acrosome_reaction_diagram.svg#file 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 youtube!

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.

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

Now for a break from Power Point to old skool white board scribbles….

Types of Cleavage Holoblastic- Meroblastic- “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

Is all Cytoplasm the Same?? (of course not!!) Heterogeneous organization of the cytoplasm already exists even in the unfertilized egg. Different: Pre-made mRNA Cytoskeleton constructs Endomembrane products BLASTOMERES! …Ultimately producing different kinds of …. ________________ 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 Formation Controlled by a hierarchy of gene activations: 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. …or “HOX” (Indicates a very conservative evolution) D:\bc_campbell_biology_7\0,7052,3117255-,00.html

Human Embryology

And now for some…….. Spectacular Animations: https://www.youtube.com/watch?v=UgT5rUQ9EmQ http://www.yteach.com/ http://www.ted.com/talks/alexander_tsiaras_conception_to_birth_visualized.html http://www.ntd.tv/2016/12/03/9-months-pregnancy-shortened-one-amazing-4-minute-clip/ NOVA: Life's Greatest Miracle Crash Course!