Fertilization, Cleavage and Early Development

Fertilization

Cleavage

Gastrulation

Organogenesis

Function= to activate the egg and combine genetic information. Contact with eggs membrane initiates metabolic reactions which trigger development.

Acrosomal Reaction: Releases hydrolytic enzymes that digest material surrounding the egg (fast block). Hole made in vitilline layer. Contact and fusion of sperm and egg membranes.

Cortical Reaction: Increases Calcium in cytosol and metabolic rate. Hardens vitelline layer (slow black)

Mammals: Fluids in reproductive tract increase sperm mobility. Follicle cells surround cell. Whole sperm taken into egg. Nuclei of sperm and egg don't fuse immediately. First cell division after 12-36 hours

Sea urchin: Lock and key to ensure correct species sperm enters. Nuclei of sperm and egg fuse immediately first cell division at 90 min

Case studies

Rapid cell division without growth. Divides cytoplasm into a single layer of many smaller cells called a bastula, (a ball of cells). Inside is a fluid filled cavity (blastoceol) Yolk determines where blastocoel is located.

Frogs: Polarity of egg defined by yolk before fertilization. Influences body axis development. (fate mapping). Once sperm enters dorsal side is established. First cleavage across gray crescent, forms blastula. Holoblastic cleavage as blastocoel is locates in animal hemisphere where smaller cells are due to yolk placement.

Chickens: Incomplete cell division in yolk rich egg. Meroblastic cleavage. Yolk=egg. White= protein for embryo growth. Small disc of cytoplasm at animal pole. Blastoderm encloses the blastocoel.

Sea urchin: Simplest cleavage, forming a blastula with a central blastocoel.

Case studies

Rearrangement of blastula into 3 layered embryo called a gastrula. With a primitive gut. 3 layers produces are the ectoderm: outer layer. Mesoderm: between others. Endoderm: lines digestive tract.

Mechanisms: Cells change in motility, cells change in shape, cells change in adhesion to other cells and moelcules.

Sea urchin: 1. Migration of mesenchyme cells. 2. Vegetal plate buckles inwards, mesenchym cells migrate through blastocoel. 3. Endoderm forms and cells are sent out to produce ectoderm. 4. Archenteron dragged across blactocoel as filopedia contracts. 5.Fusion of archenteron with blastocoel wall completes mouth anus and primitive gut.

Frogs: 1. Small indented crease. 2. Cell changes shape, pushes in, cells of animal poel spread around outer surface. 3. Balstocoel becomes smalles, archenteron forms, ectoderm spreads. 4. Endoderm replaces blastocoel. 5. Circular balstopore surrounds yolk plug.

Chicken: 1. Some epiblast cells move toward blastoderm midline, detach and move inward toward yolk through primitive streak. 2. Primitive streak thickens, along anterior-posterior axis, equivalent to lip of frog. 3. Hypoblast which doesn't contribute to embryo bust aids development forms a sac around embryo.

Frog: Notochord forms from mesoderm that condenses just above archenteron. Signals from notochord to ectoderm create neural tube region. Neural plate curves in to form neural tube (involves folding and pinching). Neural crest is created. Somites are blocks on the notochord.

Changes in cells shape, folds, splits and clusters,. First organs (notochord forms from mesoderm and neural plate forms from ectoderm)