Cellular Mechanisms of Development

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Cellular Mechanismsof Development

• Chapter 19

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Overview of Development

• Development is the successive process of
  systematic gene-directed changes throughout an
  organisms life cycle
• -Can be divided into four subprocesses
• -Growth (cell division)
• -Differentiation
• -Pattern formation
• -Morphogenesis

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Cell Division

• After fertilization, the diploid zygote undergoes
  a period of rapid mitotic divisions
• -In animals, this period is called cleavage
• -Controlled by cyclins and cyclin- dependent
  kinases (Cdks)
• During cleavage, the zygote is divided into
  smaller smaller cells called blastomeres
• -Moreover, the G1 and G2 phases are shortened or
  eliminated

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Cell Division
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Cell Division
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Cell Division
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Cell Division
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Cell Division

• Caenorhabditis elegans
• -One of the best developmental models
• -Adult worm consists of 959 somatic cells
• -Transparent, so cell division can be followed
• -Researchers have mapped out the lineage of all
  cells derived from the fertilized egg

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Cell Division

• Blastomeres are nondifferentiated and can give
  rise to any tissue
• Stem cells are set aside and will continue to
  divide while remaining undifferentiated
• -Tissue-specific can give rise to only one
  tissue
• -Pluripotent can give rise to multiple
  different cell types
• -Totipotent can give rise to any cell type

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Cell Division

• Cleave in mammals continues for 5-6 days
• producing a ball of cells, the blastocyst
• -Consists of
• -Outer layer Forms the placenta
• -Inner cell mass Forms the embryo
• -Source of embryonic stem cells (ES cells)

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Cell Division

• A plant develops by building its body outward
• -Creates new parts from stem cells contained in
  structures called meristems
• -Meristematic stem cells continually divide
• -Produce cells that can differentiate into the
  various plant tissues
• -Leaves, roots, branches, and flowers
• The plant cell cycle is also regulated by cyclins
  and cyclin-dependent kinases

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Cell Differentiation

• A human body contains more than 210 major types
  of differentiated cells
• Cell determination commits a cell to a particular
  developmental pathway
• -Can only be seen by experiment
• -Cells are moved to a different location in
  the embryo
• -If they develop according to their new
  position, they are not determined

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Cell Differentiation

• Cells initiate developmental changes by using
  transcriptional factors to change patterns of
  gene expression
• Cells become committed to follow a particular
  developmental pathway in one of two ways
• 1) via differential inheritance of cytoplasmic
  determinants
• 2) via cell-cell interactions

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Cell Differentiation

• Cytoplasmic determinants
• -Tunicates are marine invertebrates
• -Tadpoles have tails, which are lost during
  metamorphosis into the adult
• -Egg contains yellow pigment granules
• -Become asymmetrically localized following
  fertilization
• -Cells that inherit them form muscles

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Cell Differentiation

• Cytoplasmic determinants
• -Female parent provides egg with macho-1 mRNA
• -Encodes a transcription factor that can
  activate expression of muscle- specific genes

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Cell Differentiation

• Induction is the change in the fate of a cell due
  to interaction with an adjacent cell
• If cells of a frog embryo are separated
• -One pole (animal pole) forms ectoderm
• -Other pole (vegetal pole) forms endoderm
• -No mesoderm is formed
• If the two pole cells are placed side-by-side,
  some animal-pole cells form the mesoderm

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Cell Differentiation

• Another example of induction is the formation of
  notochord and mesenchyme in tunicates
• -Arise from mesodermal cells that form at the
  vegetal margin of 32-cell stage embryo
• -Cells receive a chemical signal from underlying
  endodermal cells
• -Anterior cells differentiate into notochord
• -Posterior cells differentiate into mesenchyme

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Cell Differentiation

• The chemical signal is a fibroblast growth factor
  (FGF) molecule
• -The FGF receptor is a tyrosine kinase that
  activates a MAP kinase cascade
• -Produces a transcription factor that triggers
  differentiation
• Thus, the combination of macho-1 and FGF
  signaling leads to four different cell types

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Cell Differentiation
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Cell Differentiation
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Cell Differentiation
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Cloning

• Until very recently, biologists thought that
  determination and cell differentiation were
  irreversible in animals
• Nuclear transplant experiments in mammals were
  attempted without success
• -Finally, in 1996 a breakthrough
• Geneticists at the Roslin Institute in Scotland
  performed the following procedure

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Cloning

• 1. Differentiated mammary cells were removed from
  the udder of a six-year old sheep
• 2. Eggs obtained from a ewe were enucleated
• 3. Cells were synchronized to a resting state
• 4. The mammary and egg cells were combined by
  somatic cell nuclear transfer (SCNT)
• 5. Successful embryos (29/277) were placed in
  surrogate mother sheep
• 6. On July 5, 1996, Dolly was born

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Cloning

• Dolly proved that determination in animals is
  reversible
• -Nucleus of a differentiated cell can be
  reprogrammed to be totipotent
• Reproductive cloning refers to the use of SCNT to
  create an animal that is genetically identical to
  another
• -Scientists have cloned cats, rabbits, rats,
  mice, goats and pigs

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Cloning

• Reproductive cloning has inherent problems
• 1. Low success rate
• 2. Age-associated diseases
• Normal mammalian development requires precise
  genomic imprinting
• -The differential expression of genes based on
  parental origin
• Cloning fails because there is not enough time to
  reprogram the genome properly

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Cloning

• In therapeutic cloning, stem cells are cloned
  from a persons own tissues and so the body
  readily accepts them
• Initial stages are the same as those of
  reproductive cloning
• -Embryo is broken apart and its embryonic stem
  cells extracted
• -Grown in culture and then used to replace
  diseased or injured tissue

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Cloning
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Cloning
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Cloning

• Human embryonic stem cells have enormous promise
  for treating a wide range of diseases
• -However, stem cell research has raised profound
  ethical issues
• Very few countries have permissive policy towards
  human reproductive cloning
• -However, many permit embryonic stem cell
  research

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Cloning

• Early reports on a variety of adult stem cells
  indicated that they may be pluripotent

-Since then these results have been challenged
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Pattern Formation

• In the early stages of pattern formation, two
  perpendicular axes are established
• -Anterior/posterior (A/P, head-to-tail) axis
• -Dorsal/ventral (D/V, back-to-front) axis
• Polarity refers to the acquisition of axial
  differences in developing structures
• Position information leads to changes in gene
  activity, and thus cells adopt a fate appropriate
  for their location

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Drosophila Embryogenesis

• Drosophila produces two body forms
• -Larva Tubular eating machine
• -Adult Flying sex machine axes are
  established
• Metamorphosis is the passage from one body form
  to another
• Embryogenesis is the formation of a larva from a
  fertilized egg

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Drosophila Embryogenesis

• Before fertilization, specialized nurse cells
  move maternal mRNAs into maturing oocyte
• -These mRNA will initiate a cascade of gene
  activations following fertilization
• Embryonic nuclei do not begin to function until
  approximately 10 nuclear divisions later

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Drosophila Embryogenesis

• After fertilization, 12 rounds of nuclear
  division without cytokinesis produces a syncytial
  blastoderm
• -4000 nuclei in a single cytoplasm
• Membranes grow between the nuclei forming the
  cellular blastoderm
• Within a day of fertilization, a segmented,
  tubular body is formed

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Drosophila Embryogenesis

• Nüsslein-Volhard and Wieschaus elucidated how the
  segmentation pattern is formed
• -Earned the 1995 Nobel Prize
• Two different genetic pathways control the
  establishment of the A/P and D/V polarity
• -Both involve gradients of morphogens
• -Soluble signal molecules that can specify
  different cell fates along an axis

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Establishment of the A/P axis

• Nurse cells secrete maternally produced bicoid
  and nanos mRNAs into the oocyte
• -Differentially transported by microtubules to
  opposite poles of the oocyte
• -bicoid mRNA to the future anterior pole
• -nanos mRNA to the future posterior pole
• -After fertilization, translation will create
  opposing gradients of Bicoid and Nanos proteins

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Establishment of the A/P axis

• Bicoid and Nanos control translation of two other
  maternal mRNAs, hunchback and caudal, that encode
  transcription factors
• -Hunchback activates anterior structures
• -Caudal activates posterior structures
• The two mRNAs are not evenly distributed
• -Bicoid inhibits caudal mRNA translation
• -Nanos inhibits hunchback mRNA translation

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Establishment of the D/V axis

• Maternally produced dorsal mRNA is placed into
  the oocyte
• -Not asymmetrically localized
• Oocyte nucleus synthesizes gurken mRNA
• -Accumulates in a crescent on the future dorsal
  side of embryo
• After fertilization, a series of steps results in
  selected transport of Dorsal into ventral nuclei,
  thus forming a D/V gradient

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Production of Body Plan

• The body plan is produced by sequential
  activation of three classes of segmentation genes
• 1. Gap genes
• -Map out the coarsest subdivision along the
  A/P axis
• -All 9 genes encode transcription factors that
  activate the next gene class

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Production of Body Plan

• 2. Pair-rule genes
• -Divide the embryo into seven zones
• -The 8 or more genes encode transcription
  factors that regulate each other, and activate
  the next gene class
• 3. Segment polarity genes
• -Finish defining the embryonic segments

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Production of Body Plan

• Segment identity arises from the action of
  homeotic genes
• -Mutations in them lead to the appearance of
  normal body parts in unusual places

-Ultrabithorax mutants producean extra pair of
wings
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Production of Body Plan

• Homeotic gene complexes
• -The HOM complex genes of Drosophila are grouped
  into two clusters
• -Antennapedia complex, which governs the
  anterior end of the fly
• -Bithorax complex, which governs the posterior
  end of the fly
• -Interestingly, the order of genes mirrors the
  order of the body parts they control

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Production of Body Plan

• Homeotic gene complexes
• -All of these genes contain a conserved 180-base
  sequence, the homeobox
• -Encodes a 60-amino acid DNA-binding domain,
  the homeodomain
• -Homeobox-containing genes are termed Hox genes
• -Vertebrates have 4 Hox gene clusters

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Production of Body Plan
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Production of Body Plan
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Pattern Formation in Plants

• The predominant homeotic gene family in plants is
  the MADS-box genes
• -Found in most eukaryotic organisms, although in
  much higher numbers in plants
• MADS-box genes encode transcriptional regulators,
  which control various processes
• -Transition from vegetative to reproductive
  growth, root development and floral organ identity

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Morphogenesis

• Morphogenesis is the formation of ordered form
  and structure
• -Animals achieve it through changes in
• -Cell division
• -Cell shape and size
• -Cell death
• -Cell migration
• -Plants use these except for cell migration

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Morphogenesis

• Cell division
• -The orientation of the mitotic spindle
  determines the plane of cell division in
  eukaryotic cells
• -If spindle is centrally located, two
  equal-sized daughter cells will result
• -If spindle is off to one side, two unequal
  daughter cells will result

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Morphogenesis

• Cell shape and size
• -In animals, cell differentiation is
  accomplished by profound changes in cell size and
  shape
• -Nerve cells develop long processes called
  axons
• -Skeletal muscles cells are large and
  multinucleated

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Morphogenesis

• Cell death
• -Necrosis is accidental cell death
• -Apoptosis is programmed cell death
• -Is required for normal development in all
  animals
• -Death program pathway consists of
• -Activator, inhibitor and apoptotic
  protease

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Morphogenesis

• Cell migration
• -Cell movement involves both adhesion and loss
  of adhesion between cells and substrate
• -Cell-to-cell interactions are often mediated
  through cadherins
• -Cell-to-substrate interactions often involve
  complexes between integrins and the extracellular
  matrix (ECM)

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Development of Seed Plants

• Plant development occurs in five main stages
• 1. Early embryonic cell division
• -First division is off-center
• -Smaller cell divides to form the embryo
• -Larger cell divides to form suspensor
• -Cells near it ultimately form the root
• -Cells on the other end, form the shoot

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Development of Seed Plants

• 2. Embryonic tissue formation
• -Three basic tissues differentiate
• -Epidermal, ground and vascular
• 3. Seed formation
• -1-2 cotyledons form
• -Development is arrested
• 4. Seed germination
• -Development resumes
• -Roots extend down, and shoots up

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Development of Seed Plants

• 5. Meristematic development and morphogenesis
• -Apical meristems at the root and shoot tips
  generate a large numbers of cells
• -Form leaves, flowers and all other components
  of the mature plant

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Environmental Effects

• Both plant and animal development are affected by
  environmental factors
• -Germination of a dormant seed proceeds only
  under favorable soil and day conditions
• -Reptiles have a temperature-dependent sex
  determination (TSD) mechanism
• -The water flea Daphnia changes its shape after
  encountering a predatory fly larva

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Environmental Effects
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Environmental Effects

• In mammals, embryonic and fetal development have
  a longer time course
• -Thus they are more subject to the effects of
  environmental contaminants, and blood-borne
  agents in the mother
• -Thalidomide, a sedative drug
• -Many pregnant women who took it
  had children with limb defects

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Environmental Effects

• Endocrine disrupting chemicals (EDCs)
• -Interfere with synthesis, transport or
  receptor-binding of endogenous hormones
• -Derived from three main sources
• -Industrial wastes (polychlorinated biphenyls
  or PCBs)
• -Agricultural practices (DDT)
• -Effluent of sewage-treatment plants