Vertebrate Development

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Vertebrate Development

• Chapter 51

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Material to be covered (for Tri-C)

• Describe the events of each of the three stages
  of fertilization in an advanced vertebrate.
• Compare the cleavage patterns, describe the
  appearance of the blastula and indicate how
  gastrulation proceeds in primitive chordates,
  aquatic vertebrates and reptiles/birds/mammals.
• State the tissues produced by the three germ
  layers endo-, meso- and ectoderm.
• Understand the developmental processes that occur
  during neuralation.
• Explain Haeckels biogenic law, ontogeny
  recapitulates phylogeny.

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Material to be covered (for Tri-C)

• Understand the importance of extra-embryonic
  membranes in terrestrial vertebrate development.
• Describe the characteristic events of each
  trimester of human pregnancy and of postnatal
  development.
• Use vertebrate models to understand embryonic
  development.
• Discussion and evaluation of bioethical issues
  related to embryology cloning, stem cells and
  in vitro fertilization.
• Describe the ways that cells can signal each
  other.
• Differentiate between intracellular receptors and
  cell surfaces receptors.

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Outline

• Stages of Development
• Cell Cleavage Patterns
• Gastrulation
• Developmental Process During Neurulation
• How Cells Communicate During Development
• Embryonic Development-Vertebrate Evolution
• Extraembryonic Membranes
• Human Trimesters
• Birth and Postnatal Development

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Stages of Development

• Fertilization
• combination of gametes
• Cleavage
• series of extremely rapid mitotic divisions
• Gastrulation
• series of extensive cell rearrangement
• Neuralation
• the process where tissue forms a neural tube
• Organogenesis
• the process where cells interact with one another
  and rearrange themselves to produce tissues and
  organs
• Gametogenesis
• the development of gametes
• often not complete until the organism matures
• varies greatly within animal kingdom
• Maturity
• Larvae pupae adult
• metamorphosis

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Fertilization

• Penetration
• glycoprotein-digesting enzymes in acrosome of
  sperm head
• Activation
• events initiated by sperm penetration
• chromosomes in egg nucleus complete second
  meiotic division
• triggers movement of egg cytoplasm
• sharp increase in metabolic activity

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Stages of Development

• Nuclei fusion
• The third stage of fertilization is fusion of the
  entering sperm nucleus with the haploid egg
  nucleus to form the diploid nucleus.

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Mammalian Reproductive Cells
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Vertebrate Development Review

• Formation of blastula
• water drawn into cell mass forming a hollow ball
  of cells - blastula or blastocyst
• Gastrulation
• some cells of blastula push inward, forming a
  invaginated gastrula
• invagination creates main axis of vertebrate body
• Has an animal pole and a vegetal pole
• Animal pole end forms external tissues
• Vegetal pole form internal tissues
• embryo now has three germ layers

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Vertebrate Development Review

• Neurulation
• zone of ectoderm thickens on dorsal surface of
  embryo
• neural tissue rolls and forms neural tube
• cell migration
• variety of cells migrate to form distant tissues

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Vertebrate Development Review

• Organogenesis
• basic body plan established
• tissues develop into organs
• embryo will grow to be a hundred times larger

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Vertebrate Development
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Vertebrate Development
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Cell Cleavage Patterns

• Initial cell division, cleavage, is not
  accompanied by an increase in the overall size of
  the embryo.
• morula - mass of 32 cells
• Each cell is a blastomere.
• eventually a blastula is formed
• The pattern of cleavage is influenced by the
  presence of yolk
• Animal Pole small amount of yolk
• Vegetal pole large amount of yolk

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Cell Cleavage Patterns

• Primitive chordates
• holoblastic cleavage - egg contains little or no
  yolk, and cleavage occurs throughout the whole
  egg
• Amphibians and advanced fish
• Eggs contain much more cytoplasmic yolk in one
  hemisphere than the other.
• large cells containing a lot of yolk at one pole,
  and a concentrated mass of small cells with very
  little yolk at the other pole.

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Holoblastic Cleavage
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Cell Cleavage Patterns

• Reptiles and birds
• eggs composed almost entirely of yolk
• cleavage only occurs in polar cytoplasm
• meroblastic cleavage
• Mammals
• contain very little yolk
• holoblastic cleavage
• inner cell mass forms developing embryo
• outer sphere, trophoblast, enters endometrium

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Meroblastic Cleavage
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Cell Cleavage Patterns

• Blastula
• Each cell is in contact with a different set of
  neighboring cells.
• Interactions are a major factor influencing
  developmental fate.

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Gastrulation

• Certain groups of cells invaginate and involute
  from the surface of the blastula during
  gastrulation.
• By the end of gastrulation, embryonic cells have
  rearranged into three primary germ layers
• ectoderm
• mesoderm
• endoderm

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Gastrulation

• Gastrulation in primitive chordates
• surface of blastula invaginates into the
  blastocoel
• eventually inward-moving wall pushes up against
  the opposite side of the blastula
• produces embryo with two cell layers
• outer ectoderm
• inner endoderm
• mesoderm forms later between the ectoderm and
  endoderm

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Gastrulation in a Lancet
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Gastrulation

• Gastrulation in most aquatic vertebrates
• Yolk-laden cells of the vegetal pole are fewer
  and much larger than the yolk-free cells of the
  animal pole.

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Frog Gastrulation
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Gastrulation

• Gastrulation in reptiles, birds, mammals
• no yolk separates two sides of embryo
• lower cell layer differentiates into endoderm and
  upper layer into ectoderm without cell movement
• primitive streak

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Mammalian Gastrulation
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Developmental Processes During Neurulation

• Tissue differentiation begins with the formation
  of the notochord and the hollow dorsal nerve
  cord.
• neurulation
• After the notochord has been laid down,
  ectodermal cells above the notochord invaginate,
  forming the neural groove down the long axis of
  the embryo.
• edges move toward each other and fuse creating
  neural tube

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Mammalian Neural Tube Formation
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Developmental Processes During Neurulation

• On either side of the developing notochord,
  segmented blocks of mesoderm tissue called
  somites form.
• Ultimately, somites give rise to muscles,
  vertebrae, and connective tissues.
• Mesoderm in the head region remains connected as
  somitomeres and form striated muscles of the
  face, jaws, and throat.

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Developmental Processes During Neurulation

• Neural crest
• Edges of neural groove pinch off and form the
  neural crest.
• Nearby clusters of ectodermal cells thicken into
  placodes.
• Gill chamber
• Some of the neural crest cells form cartilaginous
  bars between the embryonic pharyngeal slits.
• forms efficient pump

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Developmental Processes During Neurulation

• Elaboration of the nervous system
• Some neural crest cells migrate ventrally toward
  the notochord and form sensory neurons in the
  dorsal root ganglia.
• others become specialized Schwann cells

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How Cells Communicate During Development

• Inductions between the three primary tissue types
  are referred to as primary inductions.
• Inductions between tissues that have already been
  differentiated are called secondary inductions.

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How Cells Communicate During Development

• Nature of development decisions
• Some cells become determined early in
  development.
• At some stage, every cells fate becomes fixed
  (commitment).
• not irreversible, but rarely reverses under
  normal conditions

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Embryonic Development - Vertebrate Evolution

• Ontogeny recapitulates phylogeny
• Embryological development (ontogeny) involves the
  same progression of changes that have occurred
  during evolution (phylogeny).
• Homework (due by wednesday) find out if this
  makes any sense. (turnitin.com)
• Do you agree or disagree with this statement?
  Why?
• Does science agree with this statement? Why?

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Vertebrate Embryonic Development
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Extraembryonic Membranes

• Fluid-filled amniotic membrane an adaptation to
  terrestrial life
• amniotic membrane an extraembryonic membrane
• Extraembryonic membranes, later to become fetal
  membranes, include the amnion, chorion, yolk sac,
  and allantois.

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Extraembryonic Membranes
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First Trimester

• First trimester
• fourth week - organ development
• organogenesis
• most women not yet aware of pregnancy
• Fetal Alcohol Syndrome

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First Trimester

• Second month - morphogenesis
• limbs assume adult shape
• major organs become evident
• embryo is about one inch in length
• Third month - completion of development
• now referred to as fetus
• nervous system and sense organs develop
• all major organs established

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Second and Third Trimesters

• Second trimester - growth
• bone formation occurs
• covered with fine hair (lanugo)
• by the end of the sixth month, baby is one foot
  in length
• Third trimester - pace of growth accelerates
• weight of fetus more than doubles
• most major nerve tracts formed within brain
• by end, fetus is able to survive on own

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Birth and Postnatal Development

• Uterus releases prostaglandins
• begin uterine contractions, but then sensory
  feedback from the uterus stimulates the release
  of oxytocin from the mothers pituitary gland
• rate of contraction increases to one contraction
  every two or three minutes
• strong contractions, aided by the mothers
  pushing, expels the fetus

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Birth and Postnatal Development

• Nursing
• Milk production, lactation, occurs in the alveoli
  of mammary glands when they are stimulated by
  prolactin.
• milk secreted in alveolar ducts which are
  surrounded by smooth muscle and lead to the
  nipple
• first milk produced after birth called colostrum
  - rich in maternal antibodies
• Milk synthesis begins about three days following
  birth.

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Birth and Postnatal Development

• Postnatal development
• Babies typically double their birth weight within
  a few months.
• Neuron production occurs for six months.
• allometric growth

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