Animal Development

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Animal Development

• Chapter 47

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Preformation

• How does an egg become an animal?
• Until the end of the 18th century, the prevailing
  theory was that the embryo was a miniature
  infant.
• This idea of preformation also included the
  thought that each embryo contained all of the
  descendents as smaller embryos.

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Much like a Russian Nesting Doll
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• Another version of preformation included the idea
  of a homunculus- the sperm contains a preformed
  infant which grows.

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Epigenesis

• Another theory proposed by Aristotle 2,000 years
  earlier was that of epigenesis.
• The form of an animal emerges gradually from a
  relatively formless egg.
• Microscopy allowed scientists to witness the
  progressive development of embryos- thereby
  validating Aristotles theory.

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Embryonic Development Stages
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Fertilization

• Fertilization in vertebrates is, of course, the
  union of two haploid gametes to reconstitute a
  diploid cell - a cell with the potential to
  become a new individual.
• Fertilization is a not a single event. Rather, it
  is a series of steps that might be said to begin
  when egg and sperm first come into contact and
  end with the intermingling of haploid genomes.

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Events of Fertilization

• Contact recognition between sperm and egg
• Regulation of sperm entry into the egg. Only one
  can enter - others inhibited from entering
• Fusion of genetic material
• Activation of egg metabolism to start development

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Sperm Capacitation

• Freshly ejaculated sperm are unable or poorly
  able to fertilize.
• Rather, they must first undergo a series of
  changes known collectively as capacitation.
• Capacitation is associated with removal of
  adherent seminal plasma proteins, reorganization
  of plasma membrane lipids and proteins.

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• Capacitation occurs while sperm reside in the
  female reproductive tract for a period of time,
  as they normally do during gamete transport.
• Capacitation appears to destabilize the sperm's
  membrane to prepare it for the acrosome reaction

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Acrosomal Reaction

• Binding of sperm to the zona pellucida (egg
  membrane) is the easy part of fertilization.
• The sperm then faces the daunting task of
  penetrating the zona pellucida to get to the
  oocyte.
• Evolution's response to this challenge is the
  acrosome - a huge modified lysosome that is
  packed with zona-digesting enzymes and located
  around the anterior part of the sperm's head -
  just where it is needed.

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• The acrosome reaction provides the sperm with an
  enzymatic drill to get throught the zona
  pellucida.
• The same zona pellucida protein that serves as a
  sperm receptor also stimulates a series of events
  that lead to many areas of fusion between the
  plasma membrane and outer acrosomal membrane.
• Membrane fusion (actually an exocytosis) and
  vesiculation expose the acrosomal contents,
  leading to leakage of acrosomal enzymes from the
  sperm's head.

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• As the acrosome reaction progresses and the sperm
  passes through the zona pellucida, more and more
  of the plasma membrane and acrosomal contents are
  lost.
• By the time the sperm traverses the zona
  pellucida, the entire anterior surface of its
  head, down to the inner acrosomal membrane, is
  denuded.

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• The constant propulsive force from the sperm's
  flagellating tail, in combination with acrosomal
  enzymes, allow the sperm to create a tract
  through the zona pellucida.
• Once a sperm penetrates the zona pellucida, it
  binds to and fuses with the plasma membrane of
  the oocyte.

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Egg Activation

• Prior to fertilization, the egg is in a quiescent
  state, arrested in metaphase of the second
  meiotic division.
• Upon binding of a sperm, the egg rapidly
  undergoes a number of metabolic and physical
  changes that collectively are called egg
  activation.
• Prominent effects include a rise in the
  intracellular concentration of calcium,
  completion of the second meiotic division and the
  so-called cortical reaction.

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The Zona Reaction

• The cortical reaction refers to a massive
  exocytosis of cortical granules seen shortly
  after sperm-oocyte fusion.
• Cortical granules contain a mixture of enzymes,
  including several proteases, which diffuse into
  the zona pellucida following exocytosis from the
  egg. These proteases alter the structure of the
  zona pellucida, inducing what is known as the
  zona reaction. Components of cortical granules
  may also interact with the oocyte plasma
  membrane.

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• The critical importance of the zona reaction is
  that it represents the major block to polyspermy
  in most mammals.
• This effect is the result of two measurable
  changes induced in the zona pellucida
• The zona pellucida hardens.
• Sperm receptors in the zona pellucida are
  destroyed

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The Zona Reaction Animation
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Stages of Development

• In animals, one can usually distinguish 4 stages
  of embryonic development.
• Cleavage
• Patterning
• Differentiation
• Growth

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Cleavage

• Mitosis and cytokinesis of the zygote, an
  unusually large cell, produces an increasing
  number of smaller cells, each with an exact copy
  of the genome present in the zygote.
• However, the genes of the zygote are not
  expressed at first. The activities of cleavage
  are controlled by the mother's genome that is,
  by mRNAs and proteins she deposited in the
  unfertilized egg.
• Cleavage ends with the formation of a blastula.

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Patterning

• During this phase, the cells produced by cleavage
  organize themselves in layers and masses, a
  process called gastrulation. The pattern of the
  future animal appears
• front and rear (the anterior-posterior axis)
• back side and belly side (its dorsal-ventral
  axis)
• left and right sides.

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• There is little visible differentiation of the
  cells in the various layers, but probes for
  cell-specific proteins reveal that different
  groups of cells have already started on specific
  paths of future development.
• Gastrulation forms three major "germ layers"
  ectoderm, mesoderm, and endoderm.
• By gastrulation, the genes of the zygote genome
  are being expressed.

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Late Gastrulation in the Frog
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Differentiation

• In time, the cells of the embryo differentiate to
  form the specialized structures and functions
  that they will have in the adult.
• They form neurons, blood cells, skin cells,
  muscle cells, etc., etc.
• These are organized into tissues, the tissues
  into organs, the organs into systems.

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Growth

• After all the systems are formed, most animals go
  through a period of growth. Growth occurs by the
  formation of new cells and more extracellular
  matrix.

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Germ Layers II

• Each of these will have special roles to play in
  building the complete animal.
• Some are listed in the table on the next slide.

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• Germ-layer origin of various body tissues.

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Eggs and Zygotes have Animal and Vegetal Poles

• Egg cells are very large
• Sea urchin 70 to 150 microns
• Human 100 microns
• Frogs fishes, some insect eggs 1000 to 2000
  microns(1-2 mm)
• Birds reptiles millions of microns (many cm)
• Eggs store materials needed for development of
  the embryo
• Yolk lipids, carbohydrates and proteins
  organized into granules

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• Yolk settles to bottom of egg, producing a
  gradient of stored material
• Top of egg, with little yolk, is called the
  animal pole
• Bottom of egg, rich in yolk, is called the
  vegetal pole
• Polar axis goes from animal to vegetal pole

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• Eggs have different amounts of yolk
• Large animals developing outside mothers body
  (birds, reptiles) have large eggs with lots of
  yolk
• Large animals developing within mother's body
  (mammals) have small eggs with very little yolk
  they get their food from the mother through the
  placenta
• Animals which develop into small feeding larvae
  (sea urchins, sea stars) also have small, simple
  eggs
• Frogs and fish are intermediate in egg size and
  yolk content
• Almost all of the zygote volume comes from the
  egg, giving the zygote an animal vegetal pole

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Embryological Development

• To go from a single-cell to an organism the
  embryo must repeatedly divide by mitosis
• The early set of rapid cell divisions is called
  cleavage

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In a Series of Mitotic Divisions the Zygote
Becomes a Hollow Ball (Blastula)

• The first set of cleavage divisions are
  synchronized and there is no cell growth between
  divisions
• The size of the embryo does not change, but the
  egg material is partitioned into more and more
  cells
• DNA synthesis does occur between divisions since
  each new cell needs a nucleus.
• The cells arrange themselves into a ball
  (blastula called blastocyst in mammals) with the
  cell layer surrounding the fluid-filled interior
  (blastocoel)

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The Archenteron

• After the blastula is finished the wall folds
  inward at one point
• Forms a tube, the archenteron or primitive gut
• The opening to the archenteron is called the
  blastopore
• Cells at the animal pole grow and spread over
  outer surface, forcing other cells inward through
  the blastopore
• In bird mammal embryos there is a long furrow,
  the primitive streak instead of a blastopore

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