Development

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Development Gamete formation ? Fertilization ?
Cleavage ? Gastrulation ? Organogenesis
? Growth
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Determination- cell becomes committed to
that pathway of differentiation Cytoplasmic
localization Induction
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Fertilization Restoration of diploidy Activation
of egg Parthenogenesis sperm not required Some
species of salamanders and fishes sperm
required for egg activation but not genetic
material
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Egg enlarges due to yolk reserves Also lots of
protein synthesis machinery Morphogenesis
determinants (transcription factors) Maturing
egg becomes bloated with RNA germinal
vesicle Most of this takes place during meiosis I
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Sea urchin fertilization
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Sperm and egg nuclei fuse immediately in sea
urchins In mammals, fusion occurs after first
cell division Sperm receptors are present on
vitelline envelope species- specific Important
in marine environments! But seem to be universal
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Prevention of polyspermy Upon sperm
binding membrane potential changes in
egg (fast block) cortical reaction- cortical
granules are released into space between
plasma membrane and vitelline envelope removes
sperm and forms fertilization envelope
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How does mammalian fertilization
differ? Internal Capacitation- female secretions
enhance sperm motility Egg surface is protected
by secreted follicle cells Extracellular matrix
is called zona pellucida sperm receptors are
located there Timing of sperm and egg nuclei is
different
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After fertilization Lots of DNA and protein
synthesis (mRNA was made earlier and
stored) Cytoplasm is reorganized determinants
are positioned to direct trasncription during
development
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Cleavage patterns are consistent within
species But there are several different
patterns Affected by quantity and distribution
of yolk Genetic control of symmetry of cleavage
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Isolecithal yolk is distributed evenly
throughout egg cleavage furrow goes straight
through (holoblastic) Echinoderms, tunicates,
many mammals (including humans as well as
marsupials) actual cleavage patterns may
vary note mouse and sea star Mesolecithal
moderate amount of yolk in middle cleavage is
holoblastic but faster at animal pole than
vegetal pole Many more cells seen at animal
pole amphibians
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Telolecithal eggs- yolk concentrated at
vegetal pole diving cells lie on top of yolk
mass birds, reptiles, some amphibians,
cephalo- pods, monotreme mammals meroblastic
(partial) cleavage Meroblastic cleavage also
seen in insect eggs, but yolk is in the middle
(centrolecithal) Therefore cleavage pattern is
affected
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Effect of yolk on development If mother does not
supply nourishment, yolk does Limited yolk
(e.g., aquatic invertebrates) zygotes quickly
develop into free-swimming larvae Indirect
development larva is distinct part of sequence
between embryo and adult will undergo some form
of metamorphosis Direct development enough
yolk is present to nourish throughout formation
of juvenile
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Direct development in mammals placental
development nourishment obtained form mother
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Patterns of cleavage radial bilateral (unique
to tunicates) spiral rotational (most
mammals) forms trophoblast placenta formed on
outside, inner cell amss (embryo) formed on
inside
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Meroblastic cleavage Telolecithal-
discoidal blastoderm is originally formed as a
single layer of cells Centrolecithal-
superficial cleavage around rim of
egg cytoplasmic cleavage occurs after several
rounds of mitosis Some nuclei migrate to form
pole cells (later germ cells)
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(Drosophila)
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Protostomes vs deuterostomes
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Modifications of these groups mammals have
radial cleavage reptiles, birds and some fishes
discoidal vertebrates form coelom in a
basically schizocoelous (splitting)
pattern Cleavage eventually results in
formation of blastula (blastocyst)
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Gastrulation General mechanisms changes in
cell motility changes in cell shape cellular
organization formation of cell layers Patterns
of gastrulation are affected by presence of yolk
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Cnidarians and ctenophorans are
diploblastic (endoderm and ectoderm
only) Coelom formation schizocoel (splitting)
seen in protostomes vertebrates form this way
too, probably because of yolk otherwise,
deuterotomes are enterocoelous
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(chick)
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Aqueous environment required for
development How, in terrestrial
animals? fluid-filled sac amnion (birds,
reptiles, mammals) fertilization is
internal gastrulation is similar in all
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Mechanisms of development Delayed-nucleation
experiment (Spemann, early 1900s) All cells
contained same nuclear information Cytoplasmic
determinants are essential for normal development
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How do cells differentiate? Cytoplasmic
specification (not so much in mammals) Cell-cell
induction adhesion molecules (extracellular
matrix) paracrine signaling Importance of each
varies among animals
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Deuterostomes protostomes
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Cell induction 1920s- Spemann and Mangold
newts grafted dorsal lip from one embryo
onto another Dorsal lip is critical formation
of other parts of the embryo
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Differentiated cells can induce
adjacent, undifferentiated cells Thus
sequential patterns of development
occur induction cell movement cells
adhesions cell proliferation
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Gene expression during development Initially
from stored mRNAs transcribed from maternal
DNA Usually takes process to blastula stage or a
little beyond Then switches to zygote DNA
homeotic genes Model animal Drosophila genes
are conserved throughout animal kingdom
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What is a Hox gene? Contains a homeobox that
enables it to bind to DNA as a transcription
factor (3 ?-helices) Remainder of the protein
helps determine which gene it regulates Some of
these gene products help regulate body part
development (homeotic genes) So far, homeotic
genes have been found only in animals
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Model animals Drosophila, nematodes frogs
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Genes are arranged linearly Found on third
chromosome in fruit fly Mice and humans have
four clusters, each on different
chromosome Clusters are expressed in different
parts of the body Gene expression could be
blocked by antibodies to homeobox proteins
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Organogenesis Pattern formation Positional
information Techniques for study tissue
culture hormones growth factors genetic
engineering
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Heart is first functioning organ formed from
mesoderm, guided by endoderm Beating heart seen
by day 2 in the 21-day incubation of a chick
embryo- before there is blood or blood vessels!
(ventricle, then atria) How else will food be
delivered throughout the growing embryo?