1. Embryonic Development
AP Biology
Unit 6

2. Fertilization The joining of sperm and egg  creates a zygote
Occurs in the fallopian tube (oviduct)
Cleavage starts after fertilization
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3. 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.
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4. 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
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5. General Stages of Development
Cleavage
Blastula (Blastocyst) Formation
Gastrulation
Organogenesis (including neurulation)
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6. 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.
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7. 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
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8. Gastrulation
Stage in which cells move to form tissue layers and body axes.
Animation
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9. 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
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10. Gastrulation Ectoderm (outer most layer)
Becomes nervous system, skin, hair, nails, sweat and oil glands
Archenteron = earliest stage of digestive tract
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11. Organogenesis
Neurulation (initiation of the nervous system) occurs at the beginning of organogenesis
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12. 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
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13. Neurulation
Notochord = mesoderm that supports the developing embryo  becomes vertebrae
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14. 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)
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15. 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
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