Animal Development

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Animal Development

• Nancy G. Morris
• Volunteer State Community College

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Stages of Embryonic Development

• Two early views of how animals developed from an
  egg competed for supporters until modern
  techniques were developed.
• Preformation suggests that the embryo contained
  all of its descendents as a series of
  successively smaller embryos within embryos.
  This was popular until about the 18th century.
  (Figure 47.1)

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Stages of Embryonic Development

• The second of two early views of how animals
  developed from an egg competed for supporters
  until modern techniques were developed
• Epigenesis from egg to organism, an animals
  form develops gradually.
• Proposed by Aristotle
• Gained support in the 19th century as improved
  microscopy permitted scientists to observe
  embryos as they developed.

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Fertilization

• activates the egg and brings nuclei of sperm and
  egg together
• restores diploidy (from haploid sets of
  chromosomes from two individuals)
• triggers onset of embryonic development

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The Acrosomal Reaction

• Sperm contact eggs jelly coat
• Acrosome releases hydrolytic enzymes
• Penetration (Fig. 47.2)
• Proteins attach to specific receptors on eggs
  vitelline layer species specificity
• The plasma membranes of the egg and sperm fuse

• The acrosomal reaction is the discharge of
  hydrolytic enzymes from a vesicle in the acrosome
  of a sperm cell.

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The Cortical Reaction

• The fusion of egg sperm membranes stimulates a
  series of changes in the eggs cortex known as a
  cortical reaction.

• Chemical reactions change the eggs cortical
  granules.
• Granules fuse with the plasma membrane releasing
  enzymes separating the vitelline layer from the
  plasma membrane.
• Swelling lifts the vitelline membrane forming
  the fertilization membrane.
• Prevents penetration by other sperm

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Activation of the Egg

• Chemical change (increase in Ca2) results in
  metabolic changes that activate the egg cell.
  Figure 47.3

• Cellular respiration protein synthesis rates
  increase.
• Syngamy - sperm nucleus within the egg swells
  merges with the egg nucleus to form the zygote.
• DNA replication begins first division occurs
  about 90 minutes after syngamy.
• Figure 47.5

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Fertilization in Animals

• Capacitation (enhanced sperm function) results
  from secretion in the females reproductive tract.

• Certain molecules on sperms surface are altered
  increasing motility.
• Capacitated sperm must reach the zona pellucida,
  the extra cellular matrix of the egg, containing
  a 3-D network of glycoprotein filaments.
• Microvilli from the egg pull the whole sperm cell
  into the egg cell.

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Basic Developmental Vocabulary

• Fertilization activates egg brings together
  the nuclei of the egg and sperm
• Cleavage partitions the zygote into many smaller
  cells.
• Gastrulation rearranges the blastula to form a
  three-layered embryo with a primitive gut, the
  archenteron.
• Organogenesis is the process by which the organs
  in the animal body form from the three embryonic
  germ layers.

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Basic Developmental Vocabulary

• Blastula a hollow sphere of cells (128 cells)
  formed by cleavage of the morula. The blastula
  contains the blastocoel that is fluid-filled. The
  blastopore is the place where gastrulation
  begins.
• Gastrulation the process leading to the
  creation of the primitive gut or archenteron.
  Invagination at the blastopore results in the
  gut. (Figure 47.9)
• Gastrula transformation of the blastula into an
  embryo possessing 3 germ layers, ectoderm,
  mesoderm, endoderm. (Table 47.1)
• Morula a solid mass of 16-64 cells formed by
  cleavage (Latin for mulberry).

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Figure 47.10Gastrulation in the Frog
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Development of Extraembryonic Membranes

• YOLK SAC develop blood vessels to carry
  nutrients into the embryo
• AMINON encloses the embryo in a fluid-filled
  sac, protecting from desiccation absorbing
  shock
• CHORION cushions the embryo against mechanical
  shock
• ALLANTOIS disposal sac for uric acid

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Figure 47.14Development of extra- embryonic
membranes in a chick
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Mammalian Development

• Fertilization occurs in the oviducts of most
  mammals early development occurs while the
  embryo travels down the oviduct to the uterus.
• Cleavage is relatively slow with the first,
  second, third divisions occurring at 36, 60,
  72 hours.
• At 7 days after fertilization, the embryo
  consists of more than 100 cells arranged around a
  central cavity forming the blastocyst.
• The blastocyst stage reaches the uterus begins
  to implant in the endometrium.

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Early Human Embryo DevelopmentFigure 47.15
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Development of in Human Embryo

• Trophoblast forms the chorion fetal portion of
  the placenta (along with mesodermal tissue)
• Epiblast cell layer forms the three germ layers
  the amnion
• Hypoblast forms the yolk sac

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Early Human Embryo DevelopmentFigure 47.15
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Early Human Embryo DevelopmentFigure 47.15
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Figure 47.14 Convergent extension of a sheet of
cells
A signal causes cells to elongate and crawl
between each other. This results in the
extension of the cell sheet in a direction
perpendicular to the convergence.
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Figure 47.16Change in shape during
morpho-genesis