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The Continuity of Life II:

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Title: The Continuity of Life II:


1
Chapter 45
  • The Continuity of Life II
  • Development

http//cmgm.stanford.edu/biochem118/images/Stem20
Cell20Slides/0120Tree20of20Development.jpg
2
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.

http//www.stanford.edu/group/Urchin/path.html
3
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.

http//www.saintbrendan.com/cdnjuly/graphjul/seaur
ch7.jpeg
4
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.

http//students.biology.lsa.umich.edu/bio208_17/Co
mmon20Development20Stages.htm
5
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
6
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
7
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.

http//vacquierlab.ucsd.edu/Images/2urchinspawn.jp
g
8
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.

http//www.nap.edu/readingroom/books/test_site/moo
re/chap10.html
9
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

10
The Development of the Amphibian (cont)
http//www.nap.edu/readingroom/books/test_site/moo
re/chap10.html
11
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.

http//www.backyardnature.net/pix/frogsits.jpg
http//www.herpnet.net/Minnesota-Herpetology/salam
anders/images/SalamanderSpotted_2.jpg
12
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!
http//www.maf.govt.nz/photos/gifs/93.jpg
13
Development of the Chick (cont)
http//www.park.edu/bhoffman/courses/bi342/more/ch
ickdev.htm
14
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

15
http//www.mic-d.com/gallery/darkfield/images/chic
k96hr.jpg
16
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.

17
Organogenesis (cont)
http//138.238.136.106/comp/embryo/
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