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					Ch. 47
From egg to organism, an animal’s form develops
      gradually: the concept of epigenesis

• An embryo is not preformed in an egg; it
  develops by epigenesis, the gradual,
  gene-directed acquisition of form.
  Fertilization activates the egg and brings
   together the nuclei of sperm and egg
• Fertilization both reinstates diploidy and
  activates the egg to begin a chain of metabolic
  reactions that triggers the onset of embryonic
• The acrosomal reaction, which occurs when the
  sperm meets the egg, releases hydrolytic
  enzymes that digest through material
  surrounding the egg.
• Gamete fusion depolarizes the egg cell
  membrane and sets up a fast block to
• Sperm-egg fusion also initiates the cortical
  reaction, involving a signal-transduction pathway
  in which calcium ions stimulate cortical granules
  to erect a fertilization envelope that functions as
  a slow block to polyspermy.
• In mammalian fertilization, the cortical reaction
  hardens the zona pellucida as a slow block to
• SEE Figure 47.5 on page 1002
Cleavage partitions the zygote into
       many smaller cells
• Fertilization is followed by cleavage, a period of rapid cell
  division without growth, which results in the production of
  a large number of cells called blastomeres.
• Holoblastic cleavage, or division of the entire egg, occurs
  in species whose eggs have little or moderate amounts
  of yolk.
• Meroblastic cleavage, incomplete division of the egg,
  occurs in species with yolk-rich eggs.
• Cleavage planes usually follow a specific pattern relative
  to the animal and vegetal poles of the zygote.
• In many species, cleavage creates a multicellular ball
  called the blastula, which contains a fluid-filled cavity, the
Sea Urchin Development Video
Gastrulation rearranges the blastula to form
a three-layered embryo with a primitive gut
• Gastrulation transforms the blastula into a
  gastrula, which has a rudimentary
  digestive cavity (the archenteron) and
  three embryonic germ layers: the
  ectoderm, endoderm, and mesoderm.
• SEE FIGURE47.10 p.1006
  In organogenesis, the organs of the
    animal body form from the three
            embryonic germ
• Early events in organogenesis in
  vertebrates include formation of the
  notochord by condensation of dorsal
  mesoderm, development of the neural
  tube from folding of the ectodermal neural
  plate, and formation of the coelom from
  splitting of lateral mesoderm.
• SEE FIGURE 47.11, p. 1008
Frog Development Video
  Amniote embryos develop in a fluid-
   filled sac within a shell or uterus
• Meroblastic cleavage in the yolk-rich, shelled
  eggs of birds and reptiles is restricted to a small
  disc of cytoplasm at the animal pole.
• A cap of cells called the blastodisc forms and
  begins gastrulation with the formation of the
  primitive streak.
• In addition to the embryo, the three germ layers
  give rise to the four extraembryonic membranes:
  the yolk sac, amnion, chorion, and allantois.
• The eggs of placental mammals are small and
  store little food, exhibiting holoblastic cleavage
  with no obvious polarity.
• Gastrulation and organogenesis, however,
  resemble the processes in birds and reptiles.
• After fertilization and early cleavage in the
  oviduct, the blastocyst implants in the uterus.
• The trophoblast initiates formation of the fetal
  portion of the placenta, and the embryo proper
  develops from a single layer of cells, the
  epiblast, within the blastocyst.
• Extraembryonic membranes homologous to
  those of birds and reptiles function in intrauterine
Morphogenesis in animals involves
 specific changes in cell shape,
     position, and adhesion
• SEE Figure 47.16 and 47.17, p. 1012-1014
• Cytoskeletal rearrangements are responsible for
  changes in both shape and position of cells.
• Both kinds of changes are involved in tissue
  invaginations, as occurs in gastrulation, for example.
• The extracellular matrix provides anchorage for cells and
  also helps guide migrating cells toward their
• Cell adhesion molecules on cell surfaces are also
  important for cell migration and for holding cells together
  in tissues.
The developmental fate of cells depends on
 cytoplasmic determinants and cell-cell
   Fate mapping can reveal cell
 genealogies in chordate embryos
• SEE Figure 47.20, pp. 1014-1015
• Experimentally derived fate maps of
  embryos have shown that specific regions
  of the zygote or blastula develop into
  specific parts of older embryos.
     The eggs of most vertebrates have
cytoplasmic determinants that help establish
 the body axes and differences among cells
            of the early embryo
• SEE FIGURE 47.21, p. 1015
• When cytoplasmic determinants are
  heterogeneously distributed in an egg,
  they serve as the basis for setting up
  differences among parts of the egg and,
  later, among the blastomeres resulting
  from cleavage of the zygote.
• Cells that receive different cytoplasmic
  determinants acquire different fates.
       Inductive signals drive
differentiation and pattern formation
             in vertebrates
• SEE FIGURES 47.22-47.24, pp. 1016-1019
• Cells in a developing embryo receive and interpret
  positional information that varies with location.
• This information is often in the form of signal molecules
  secreted by cells in special "organizer" regions of the
  embryo, such as the dorsal lip of the blastopore in the
  amphibian gastrula and the apical ectodermal ridge of
  the vertebrate limb bud.
• The signal molecules influence gene expression in the
  cells that receive them, leading to differentiation and the
  development of particular structures.

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