Development

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Development
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Fertilization
Egg (female gamete) (n)
Sperm (male gamete) (n) fertilization Zygot
e (2n)
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Embryo Development
The single cell zygote must go through a vast
number of changes to become a multicellular
organism with differentiated cells.
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Cleavage
A series of rapid mitotic divisions occur without
growth or gene expression
The cells form a ball with a fluid filled center
called a blastula
Day 1
Day 2
This forms a solid ball of cells called a morula
16 32 cells
Day 3
Day 4
Day 5
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Blastula

• Does not have an opening
• Blastocoel fluid filled center
• Blastomeres outer cells to become chorion
• Inner cell mass will become the embryo

Blastula (has no ICM)
Morula
blastocoel
Inner Cell Mass in Blastocyst
Blastomeres (prechorion)
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Human Development
Fertilization and morula formation occur in the
oviduct. The blastula forms as the
developing embryo enters the uterus
Implantation
Blastocyst secretes enzymes allowing it to
imbed in the uterine lining (day 7 -
13)
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Implantation
Human Chorionic Gonadotropin (HGC) (secreted by
the embryo) maintains the corpus luteum and
production of progesterone
The
placenta will secrete its own progesterone later.
(takes over in weeks 10-12)
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Implantation
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Gastrulation

• starts when cells on one side pulled into the
  embryo (invagination)
• ends with the formation of a gastrula

Day 16
Blastopore (opening)
Gastrula
three tissue layers (ectoderm mesoderm and
endoderm) and a primitive digestive tract called
the archenteron
invagination
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Gastrulation
All tissues and organs will develop from these
three layers (called germ layers). Cell fate has
been determined. Gene expression begins
as differentiation starts.
ectoderm
endoderm
mesoderm
archenteron
blastopore
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Gastrulation in Humans
Inner Cell Mass produces an embryonic disc
Primitive streak - Site of invagination
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Neurulation

• Initiating the Nervous System
• Ectoderm becomes nerve chord and brain
• Induction by notochord
• Cell migration forms peripheral nerve tissue
• Begins development of body segmentation
• Somites from mesoderm form precursors to
  vertebrae ( 21 days)

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Embryonic Germ Layers
Cell differentiation begins to occur at the
blastula stage. Different types of cells use
(express) different parts of the DNA code
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Extra Embryonic Tissues
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Placenta
Site of exchange of nutrients and waste
Embryo part forms from chorion Maternal part
forms from uterine tissues Blood of mother and
embryo or fetus never mix
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Umbilical Cord
Stretches between placenta and fetus
Carries oxygen and nutrients to fetus Carries
carbon dioxide and wastes to placenta
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Embryo Development
Fifth Week
Limb buds form, head (brain) enlarges, sensory
organs noticeable
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Embryo Development
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Embryo Development
Human
Pig
Chick
Fish
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Fetal Development
Embryonic stages occur in the first eight
weeks. All organs are initiated in the embryonic
stage Fetal development occurs from week nine
through week 40 Marked by a great increase in
size and weight
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12 16 weeks
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Figure 29.24
24 weeks
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Parturition
Three stages 1. Expansion of the cervix 2.
Delivery of the fetus 3. Expulsion of the
placenta (afterbirth)