Embryonic Development

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Chapter 27

• Embryonic Development
• TEST TUESDAY

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Embryonic Development

• The First Two Major Phases
• 1. Cleavage rapid succession of cell division
• 2. Gastrulation adds more cells to embryo

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Cleavage

• Rapid Divisions
• Produces a multicellular embryo from zygote
• Blastocoel fluid filled cavity forms in center
• Blastula end of cleavage, hollow ball

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Cleavage
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• 27.11 Gastrulation produces a three-layered
  embryo
• In gastrulation
• Cells migrate inward and form a rudimentary
  digestive cavity
• The resulting gastrula
• Has three layers of cells

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• Development of the frog gastrula

Figure 27.11
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• What happens during cleavage?
• What marks the end of cleavage?
• What new cavity is created during gastrulation?
• What marks the end of gastrulation?

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• 27.12 Organs start to form after gastrulation
• After gastrulation
• The three embryonic tissue layers give rise to
  specific organ systems

What two new structures are present that were not
present in gastrulation?
Figure 27.12A
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• The three embryonic tissue layers
• Develop into a number of different organs and
  tissues in an adult

Table 27.12
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• A structure known as the neural plate
• Forms the neural tube, which will become the
  spinal cord

Figure 27.12B
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• After neural tube formation
• The somites and the coelom form
• Somites
• mesoderm give rise to segments
• Coelom
• body cavity

Figure 27.12C
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• 27.13 Changes in cell shape, cell migration, and
  programmed cell death give form to the developing
  animal
• Cells of the ectoderm
• Fold inward during neural tube formation

Figure 27.13A
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• Programmed cell death, or apoptosis
• Is a key developmental process in which cells die

Why is this beneficial?
Spaces between fingers and toes Development of
nervous and immune system
Figure 27.13B
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• 27.14 Embryonic induction initiates organ
  formation
• In a process called induction
• Adjacent cells and cell layers influence each
  others differentiation via chemical signals

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• Induction during eye development

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• 27.15 Pattern formation organizes the animal body
• Pattern formation
• Is the emergence of the parts of a structure in
  their correct relative positions

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• Involves the response of genes to spatial
  variations of chemicals in the embryo
• Three demensional

Figure 27.15A
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• Experimental evidence has revealed
• That vertebrate limbs have zones of cells that
  provide positional information to other cells

Figure 27.15B
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HUMAN DEVELOPMENT
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• 27.16 The embryo and placenta take shape during
  the first month of pregnancy
• Pregnancy, or gestation
• Is the carrying of developing young within the
  female reproductive tract

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• An Overview of Developmental Events
• Human development
• Begins with fertilization in the oviduct

Cleavage starts about 24 hours fertilization 6-7
days embryo reaches uterus (blastocyst) Implanted
in uterus in about 1 wk
Figure 27.16A
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• Cleavage produces a blastocyst
• Whose inner cell mass becomes the embryo

Figure 27.16B
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• The blastocysts outer layer, the trophoblast
• Implants in the uterine wall
• One week

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• Meanwhile, the four extraembryonic membranes
  develop
• The amnion, the chorion, the yolk sac, and the
  allantois
• Gastrulation underway 9 days after

Figure 27.16D, E
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• Roles of the Extraembryonic Membranes
• About a month after conception
• The extraembryonic membranes are fully formed

Figure 27.16F
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• The embryo floats in a fluid-filled amniotic
  cavity
• Surrounded by the amnion
• The chorion and mesoderm cells from the yolk sac
• Form the embryos part of the placenta
• Cells from chorion secrete HCG levels so high
  some released in urine
• The allantois
• Forms part of the umbilical cord

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• The Placenta
• The placentas chorionic villi
• Absorb food and oxygen from the mothers blood to
  nourish the embryo

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• 27.17 Human development from conception to birth
  is divided into three trimesters
• Human development is divided into three
  trimesters
• Each about three months in length

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• The First Trimester
• The most rapid changes
• Occur during the first trimester

Figure 27.17A
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• By 9 weeks
• The embryo is called a fetus

Figure 27.17B
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• Second Trimester
• The second trimester
• Involves an increase in size and general
  refinement of the human features
• Corpus Luteum no longer needed

Figure 27.17C
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• At 20 weeks
• The fetus is about 19 centimeters long, and
  weighs half a kilogram
• Very active
• Eyes open, toes , fingers, hair

Figure 27.17D
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• Third Trimester
• The third trimester
• Is a time of rapid growth, which ends in birth
• Changes to adapt to environment outside
• Circulatory and Respiration for breathing
• Muscles and Bones

Figure 27.17E