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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 17
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 17
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 17
General Stages of Development
• Cleavage
• Blastula (Blastocyst)
Formation
• Gastrulation
• Organogenesis
(including neurulation)
Slide 5 of 17
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 17
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 17
Gastrulation
• Stage in which cells move to form tissue
layers and body axes.
• Animation
Slide 8 of 17
Gastrulation
• Endoderm (inner most 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 17
Gastrulation
• Ectoderm (outer most layer)
– Becomes nervous system, skin,
hair, nails, sweat and oil glands
• Archenteron = earliest stage of
digestive tract
Slide 10 of 17
Organogenesis
• Neurulation (initiation of the nervous
system) occurs at the beginning of
organogenesis
Slide 11 of 17
Steps of Neurulation
1. Neural plate forms as ectoderm
thickens and flattens on the
posterior side.
2. Edges of the neural plate move
towards each other to form the
neural tube
3. The edges of the plate fuse together
to form a tube
4. The neural tube detaches from the
rest of the ectoderm
Slide 12 of 17
Neurulation
• Notochord = mesoderm
that supports the
developing embryo 
becomes vertebrae
Slide 13 of 17
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 17
Image taken without permission from http://medicalimages.allrefer.com/large/spina-bifida-degrees-of-severity.jpg
Fetal Surgery for Spina Bifida
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 16 of 17
Development at a Cellular &
Molecular Level
• Normal embryonic development regulated by
– Expression of genes for tissue-specific proteins
– Induction: nearby cells release signal
molecules that trigger a specific path of
development in other cells
– Hox genes: control the expression of many
other genes  involved in pattern/sequence
development.
– Apoptosis: certain cells undergo cell death to
form structures like fingers and toes.