Chapter 47- Animal Development

Fertilization Fertilization- Fusion of gametes (generalized order of events) - Acrosomal Reaction - When the sperm meets the egg, the acrosome releases hydrolytic enzymes which digest the jelly coating of the egg. Acrosomal processes bind to receptors in the egg which initiates “fast block” which prohibits other sperm from fertilizing the egg (polyspermy) by changing the polarity of the egg membrane. - Ends with the entrance of the sperm into the egg

Animation

Fertilization Cortical Reaction - Release of calcium ions will cause cortical granules to release contents into the vitelline layer which will now harden. This hardening is considered the “slow block” to polyspermy. - Egg is now considered “activated”

Specific differences in mammals - Zona pellucida- Extracellular matrix of the egg (digested in the acrosomal reaction) - No known fast block mechanism

Cleavage Cleavage- Rapid cell division - Embryo is considered a morula when it has divided 5-7 times - Fluid filled center is called the blastocoel - Embryo is considered a blastula when it is a hollow ball of cells

Cleavage -Polarity (uneven distribution of goods) of the embryo can be observed in some animals - Vegital pole- Where the yolk can be found, or where there is a high concentration of goods - Animal pole- No yolk, low concentration of goods.

Gastrulation Gastrulation- Formation of a 3 layered embryo with a primitive gut The three layers are called germ layers: - Ectoderm- Outer layer of the gastrula and will form much of the outside of the animal’s body - Mesoderm- Middle layer and will form much of what will be inside of the animal’s body - Endoderm- Inner layer and will line the embryonic digestive tract Animation

- Archenteron- Primitive gut - Blastopore- Anus

Organogenesis Organogenesis- Development of rudimentary organs - The neural plate will fold in on itself to form the neural tube (will develop into the nervous sytem) - Notochord- Skeletal rod that will support the body - Somites- Wandering cells that will develop into individual vertebra and skeletal muscle

Amniotes- Animals that have a fluid cavity around the embryo (most of the higher animals)

Mammalian Development Mammalian Development (specifically humans) - No noticeable polarity - No yolk - The term blastula is replaced with the term blastocyst

Mammalian Development - Extraembryonic membranes - Chorion- Location of gas exchange - Amnion- The fluid around the embryo - Yolk sac- Where early blood cells develop, does not contain yolk - Allontois- Umbilical cord

Other Other Information - Nonamniotic animals (lower animals)- the vegetative and animal poles are decided before fertilization - Amniotic animals- Axes not decided until after fertilization (gravity and pH can play a role in some animals)