The Continuity of Life II:

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Chapter 45

• The Continuity of Life II
• Development

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The Continuity of Life II Development

• Each life began with the fusion of a tiny motile
  sperm cell and a larger but structurally simple
  egg.
• The fertilized egg is transformed into a complete
  organism which closely resembles its parents
• This process involves growth (increase in size),
  differentiation (specialization of cells,
  tissues, and organs), and morphogenesis (shaping
  of the adult body form).
• Development is most clearly explained through
  concrete examples.

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Development of the Sea Urchin

• Sea urchins have been a favorite of embryologists
  to study because
• their eggs are produced in large numbers
• they are fertilized and develop internally,
  making it possible to study them under relatively
  simple lab conditions
• both egg and developing embryo are nearly
  transparent, so its easier to observe early
  events without with out disrupting them
• the process is rapid
• In about 48 hours, the zygote develops into a
  free swimming larval form, known as the pluteus.

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Fertilization and Activation of the Egg

• development begins with fertilization of the egg
  by the sperm
• sperm cell of sea urchin consists of an acrosome
  at anterior tip, tightly packed nucleus,
  cytoplasm, and long flagellum.
• egg cell is much larger than the sperm, and is
  surrounded by an outer membrane known as the
  vitelline envelope.
• embedded in the vitelline envelope and on its
  surface are species-specific protein receptors
  that participate in the binding of sperm to egg.
• ultimately the cell membranes of the sperm and
  egg make contact, they fuse, and the sperm
  nucleus enters the cytoplasm of the egg.

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mmon20Development20Stages.htm
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Fertilization and Activation of the Egg (cont)

• fertilization has at least four consequences
• changes take place on the surface of the egg to
  prevent the entry of an additional sperm. the
  increase in concentration of free Ca ions
  triggers a series of reactions that cause the
  vitelline envelope to lift off the surface of the
  egg. The protective outer membrane that forms
  from the vitelline envelope is know as the
  fertilization membrane.
• the egg is activated metabolically, evidenced by
  increase in protein synthesis and rise in oxygen
  consumption.
• genetic material of the male is introduced into
  the female gamete
• egg begins to divide by mitosis, with the
  centrioles of the sperm participating in the
  organization of the mitotic spindle.

http//www.stanford.edu/group/Urchin/path.html
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From Zygote to Pluteus

• zygote divides once an hour for 10 hours, in a
  process known as cleavage. the embryo at this
  stage is called a morula.
• Na ions are pumped from the cells into the
  extracellular spaces, followed by this osmotic
  flow of water. this creates a fluid filled cavity
  called a blastocoel in the center of the embryo.
  when the blastocoel is fully formed, the embryo
  is now called the blastula, and its cells are
  called blastomeres.
• the formation of the blastula is followed by a
  process known as gastrulation, which gives rise
  to the primative gut.
• gastrulation in the sea urchin begins with the
  formation of the blastopore, an opening into the
  blastula
• primary mesenchyme-cells that break loose near
  the blastopore and move over the interior surface
  towards the opposite pole.
• the entire new cell layer closest to the
  blastopore turns inward, moving through the
  blastocoel, forming a new cavity called the
  archenteron.
• the archenteron will develop into the digestive
  tract, and the blastopore into the anus.
• once gastrulation is complete, evidence of cell
  differentiation can be seen.

http//www.stanford.edu/group/Urchin/path.html
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The Influence of the Cytoplasm

• the sea urchin egg contains a relatively small
  amount of yolk which is concentrated in the lower
  (vegetal) half of the egg. the upper half is
  called the animal half
• at cleavage the first two cell divisions run
  perpendicular. these 4 cells are separated, and
  each can develop into normal pluteus larva.
• during the third cleavage division, the cytoplasm
  of the animal and vegetal half are separated as
  four cells split across the equator, producing an
  eight cell embryo.

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Development of the Amphibian

• the eggs of frogs and many other amphibians are
  laid in shallow water and are fertilized
  externally, hence they are easy to be readily
  observed.
• the amphibian however contains a large amount of
  yolk. the animal half and vegetal half of the egg
  differ markedly in appearance.
• in experiments by Hans Spermann in early 1900s
  it was shown that the cytoplasm associated with
  the gray crescent is of critical importance in
  the later development of the embryo. these
  experiments provide further evidence that
  differences in the cytoplasm and partially in
  localized factors play a major role in
  determining the course of early development.
• when larger amount of yolk are present, the egg
  divides unevenly, forming larger cells in the
  vegetal hemisphere.
• the amphibian blastocoel is small and usually
  off-center.
• the blastopore appears as a crescent shaped slit,
  and always forms at the boundary between the gray
  crescent and the vegetal hemisphere.
• the formation of the blastopore marks the
  initiation of gastrulation. cells at the dorsal
  lip change shape and sink below the surface as
  they move to the interior.
• the direction in which the migrating cells move
  is the future anterior-posterior axis of the
  animal.

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The Development of the Amphibian (cont)

• in the course of gastrulation, the primary
  embryonic tissues (endoderm, mesoderm, ectoderm)
  become arranged in a three-layered pattern.
• floor of archenteron is composed of yolk laden
  endodermal cells. its roof consists of endodermal
  cells that have been pushed and pulled to the
  interior by a sheet of mesodermal cells that lies
  above it along the axis of the embryo.
• this sheet of mesoderm includes cells destined to
  form the notochord and is called the
  chordamesoderm.
• at the sides of the archenteron, other mesodermal
  cells have slipped between the ectoderm and the
  endoderm, forming the lateral plate mesoderm.
• neural ectoderm-sheet of ectoderm that will give
  rise to the brain and spinal cord
• epidermal ectoderm-ectoderm covering rest of the
  gastrula, will give rise to the epidermis of the
  skin.
• by end of gastrulation, chordamesoderm has formed
  the notochord, and neural ectoderm has begun to
  thicken, forming the neural plate.
• neural plate ridges curve up and in, forming
  neural groove. they meet to form the neural tube,
  which pinches off from rest of ectoderm.
• somites-blocks of tissue formed from
  differentiated notochord split

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The Development of the Amphibian (cont)
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The Development of the Amphibian (cont)

• differentiation is the result of the selective
  activation and inactivation of specific genes in
  the nucleus of a cell.
• although cells have become differentiated, they
  retain their full developmental potential.
• in the amphibian, the first tissue to develop a
  unique character is the dorsal lip of the
  blastopore. it is the key to events that follow.
• dorsal lip of the blastopore called the organizer
  because of its effect on organizing tissues in
  the embryo.
• embryonic induction- occurs when two different
  types of tissue come in contact with one another
  in the course of development and one tissue
  induces the other to differentiate.
• induction takes place if chemical exchanges are
  allowed, and if prevented it does not occur.

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Development of the Chick

• the hens egg is different then any of the
  previous eggs examined.
• it is surrounded by a shell, which permits it to
  develop on land, as well as a system of
  membranes. it also contains a large amount of
  yolk.
• the yolk of a hens egg is so large and dense
  that cleavage doesnt involve most of the egg
  mass.
• the only part of the fertilized egg that cleaves
  a thin layer of cytoplasm that sits on top of the
  yolk and contains the nucleus.
• cleavage of this thin layer produces a
  lozenge-shaped blastula known as a blastodisc. it
  is made up of many cells and the cells are
  divided into two layers the upper layer
  (epiblast) and the lower layer (hypoblast).
• within a short period of time a visible line, the
  primitive streak, appears on the surface of the
  blastodisc. it is essentially an elongated
  blastopore

I am chicken, hear me Roar!
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Development of the Chick (cont)
http//www.park.edu/bhoffman/courses/bi342/more/ch
ickdev.htm
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Extraembryonic Membranes of the Chick

• eggs develop in water. in the amniote egg, this
  water is held in a set of membranes formed from
  the tissues of the embryo, known as the
  extraembryotic membrane.
• the endoderm is derived from the cells of the
  hypoblast, and the mesoderm and ectoderm are from
  cells in the epiblast.
• yolk sac- membrane of mesoderm and endoderm
  surrounding the yolk. nutritive function
• amniotic cavity- the amniotic folds of ectoderm
  and mesoderm fused together surrounding the
  embryo
• fusion of the amniotic folds creates two
  membranes, each made up of a layer of ectoderm
  and mesoderm and separated by the extraembryotic
  coelom. the inner membrane is the amnion and the
  outer membrane is the chorion.
• allantois- posterior pouch formed from the
  primitive hindgut and composed of endoderm and
  mesoderm. first function is excretion.
• chorioallantoic membrane- formed by the fusion of
  allantoic wall with the chorion. acts as an
  efficient respiratory membrane for the embryo
  during its later development

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Organogenesis The Formation of Organ Systems

• organogenesis is the later stages of development
  after cleavage and gastrulation.
• it begins with the inductive interaction between
  ectoderm and underlying chordamesoderm. each of
  the 3 primary tissues formed during gastrulation
  then starts to undergo growth, differentiation,
  and morphogenesis.
• ectoderm neutral tube stretches out along the
  dorsal surface growing and becoming longer and
  thinner as the embryo gets bigger. differentiate
  into cartiledge and give rise to the hollow
  vertebral column
• secondary induction- inducing tissue is the
  result of the primary induction of dorsal
  ectoderm by dorsal mesoderm.
• mesoderm differentiates into sclerotome cells
  (skeletal elements), dermatome cells (skin), and
  myotome cells (musculature).
• endoderm differentiates into tissues of the
  respiratory and digestive tracts and a number of
  related organs.
• morphogenesis the processes include
  increase/decrease in rates of cell growth and
  division, changes in cell adhesion with neighbor
  cells, deposition of extracellular materials,
  changes in cell shape by extension/contraction.
• pattern formation- developmental process that
  gives rise to structural differences of the body.
  

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Organogenesis (cont)
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