Embryonic Development AP Biology Unit 6

Fertilization The joining of sperm and egg  creates a zygote Occurs in the fallopian tube (oviduct) Cleavage starts after fertilization Slide 2 of 15

Acrosomal Reaction Occurs in order for sperm to fertilize the egg the sperm releases digestive enzymes from the acrosome in its head to digest into the egg. When the sperm and egg membranes come into contact, they fuse and allow the sperm to enter. receptors on the egg membrane that are species-specific (usually) and ensure correct fertilization. Slide 3 of 15

Sperm Entry into Egg Once the sperm enters the egg, many different things happen Egg blocks any more sperm from entering Egg is metabolically activated to start development (DNA Replication, more Protein Synthesis and Cellular Respiration) Egg and sperm nuclei fuse = diploid nucleus Slide 4 of 15

General Stages of Development Cleavage Blastula (Blastocyst) Formation Gastrulation Organogenesis (including neurulation) Slide 5 of 15

Cleavage Rapid cell division Not much time for cells to grow in between divisions Cells at this stage are totipotent stem cells—can become any kind of cell. Slide 6 of 15

Blastula Formation A blastula (hollow ball of cells) forms when some of the cells in the center pull away from one another to form a fluid filled cavity in the center (blastocoel) Also called a blastocyst It is at this stage when it implants into the uterus Slide 7 of 15

Gastrulation Stage in which cells move to form tissue layers and body axes. Animation Slide 8 of 15

Gastrulation Endoderm (inner most layer) Mesoderm (middle layer) Forms digestive tract, respiratory tract, pancreas liver Mesoderm (middle layer) Forms muscle, bones, excretory system, circulatory system, most of reproductive system Slide 9 of 15

Gastrulation Ectoderm (outer most layer) Becomes nervous system, skin, hair, nails, sweat and oil glands Archenteron = earliest stage of digestive tract Slide 10 of 15

Organogenesis Neurulation (initiation of the nervous system) occurs at the beginning of organogenesis Slide 11 of 15

Steps of Neurulation Neural plate forms as ectoderm thickens and flattens on the posterior side. Edges of the neural plate move towards each other to form the neural tube The edges of the plate fuse together to form a tube The neural tube detaches from the rest of the ectoderm Slide 12 of 15

Neurulation Notochord = mesoderm that supports the developing embryo  becomes vertebrae Slide 13 of 15

Neural tube defects Anencephaly (folds don’t fuse at the top forebrain can’t form properly) Spina Bifida (folds don’t fuse at the bottom  can cause major spinal problems as amniotic fluid flows over the open area) Slide 14 of 15 Image taken without permission from http://medicalimages.allrefer.com/large/spina-bifida-degrees-of-severity.jpg

Development in Mammals As a mammalian fetus develops, it also forms a series of membranes that surrounds the embryo These membranes (like the amnion) function to help nourish and protect the embryo. The formation of these membranes is thought to be a evolutionary adaptation to living on land Slide 15 of 15