Developmental Biology
Life Cycles and Developmental
Stages
Developmental Stages
Dividing embryogenesis into stages allows
for easier description of embryonic events
Stages should follow logical, observable
changes in the embryo
Refinement of classical embryological
stages with the advent of molecular
technology has occurred
Developmental Stages
 Early Development
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Fertilization
Cleavage
Gastrulation
Neurulation
 Later Development
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Organogenesis
Larval molts
Metamorphosis
Aging
Developmental Stages
Early Development
• Fertilization
• Cleavage
• Gastrulation
• Neurulation
Later Development
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Organogenesis
Larval molts
Metamorphosis
Aging
 Attraction of sperm to egg
• species specificity
• attraction at a distance
 Fusion of sperm and egg
• activation of egg to
complete meiosis
• prevention of polyspermy
 Fusion of pronuclei
 Rearrangement of cortical
cytoplasm
Developmental Stages
 Early Development  Rapid cell divisions
• Fertilization
• Cleavage
• Gastrulation
• Neurulation
 Later Development
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Organogenesis
Larval molts
Metamorphosis
Aging
• Drosophila 8’, Xenopus 20’
– no transcription
– S and M phases only
• mammals 12-24 hr, chick 612 hr
– transcription occurs
 Produces a hollow embryo
• blastocoel
 mesoderm induction
• signals from endoderm
induce ectodermal cells
Developmental Stages
 Early Development  Formation of gut
• Fertilization
 Complex cell movements
• Cleavage
to arrange trilaminar
embryo and set up body
• Gastrulation
plan
• Neurulation
 Later Development
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Organogenesis
Larval molts
Metamorphosis
Aging
• Major movements are of
the mesodermal cells
• Positioning of mesodermal
cells to interior of embryo
Developmental Stages
 Early Development  Cell movements occur in
distinct ways in different
• Fertilization
genera
• Cleavage
 Invertebrates
• Gastrulation
• Neurulation
 Later Development
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Organogenesis
Larval molts
Metamorphosis
Aging
• invagination of cells on the
future ventral side
 Vertebrates
• fish & amphibians
– spherical embryos
– involution of mesoderm on
future dorsal side at
blastopore lip
Developmental Stages
 Early Development  Vertebrates
• Fertilization
• Cleavage
• Gastrulation
• Neurulation
 Later Development
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•
•
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Organogenesis
Larval molts
Metamorphosis
Aging
• Reptiles, birds and some
mammals (humans)
– disc embryos
– delamination of ectoderm to
form endoderm & mesoderm
– delaminating cells involute at
Henson’s node, primitive streak
and groove form & progresses
along dorsal midline
• mammals (mouse)
– cup embryos
– ingression at the dorsal midline
(primitive streak) as in chick
and human
Developmental Stages
 Early Development  Invertebrates
• Fertilization
• Cleavage
• Gastrulation
• Neurulation
 Later Development
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Organogenesis
Larval molts
Metamorphosis
Aging
• Echinoderms
– unusual CNS
• Arthropods
– form CNS from region from
neural cells that are
displaced from the
neuroectoderm
Developmental Stages
 Early Development  Vertebrates
• Fertilization
• Cleavage
• Gastrulation
• Neurulation
 Later Development
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Organogenesis
Larval molts
Metamorphosis
Aging
• Dorsal ectoderm is induced
by the notochord to
become neural tissue
• Ectoderm turned into
neural tissue is called the
neural plate
• Neural plate cells change
shape & fold into neural
tube
Developmental Stages
 Early Development  neural crest
• Fertilization
• Cleavage
• Gastrulation
• Neurulation
 Later Development
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Organogenesis
Larval molts
Metamorphosis
Aging
• transition zone between
neural plate & ectoderm
becomes neural crest
• crest cells leave dorsal
neural tube & migrate
throughout embryo
differentiating into multiple
cell types
– melanocytes – pigment cells
– peripheral nervous system –
sensory and motor nerves
– adrenal gland
Developmental Stages
 Early Development
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•
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•
Fertilization
Cleavage
Gastrulation
Neurulation
 Later Development
• Organogenesis
• Larval molts
• Metamorphosis
• Aging
 Invertebrates
• Highly variable
depending on phyla
• Arthropods (insects)
– Imaginal discs
– Specialization of
segments
Developmental Stages
 Early Development  Vertebrates
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Fertilization
Cleavage
Gastrulation
Neurulation
 Later Development
• Organogenesis
• Larval molts
• Metamorphosis
• Aging
• Complete body plan is
apparent at end of
neurogenesis
• Pharyngula
– All vertebrate embryos
similar appearance
• Major landmarks
– branchial arches/visceral
arches/pharyngeal pouches
– segmented neural tube
– segmented axial mesoderm –
somites
Developmental Stages
 Early Development  Forming organs
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Fertilization
Cleavage
Gastrulation
Neurulation
 Later Development
• Organogenesis
• Larval molts
• Metamorphosis
• Aging
• gut tube, proctodeum
(anus), stomodeum
(mouth)
• pronephros (kidney)
• dorsal aortas
• heart
• liver diverticulum
• eye anlagen
• otic vesicles
(ears/tympanic
membranes)
• cartilage deposition
Developmental Stages
 Early Development  Invertebrates
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•
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Fertilization
Cleavage
Gastrulation
Neurulation
 Later Development
• Organogenesis
• Larval molts
• Metamorphosis
• Aging
• Arthropods and
Nematodes
– Series of larval stages
– Several orders of magnitude
increases in organismal size
– Ecdysysis – shedding of
exoskeleton/epiderm
• Holometabolous arthropods
– Imaginal discs
– Precursor to adult organ
– Hormonally induces to develop
during larval stages
Developmental Stages
 Early Development  Arthropods
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Fertilization
Cleavage
Gastrulation
Neurulation
 Later Development
• Evagination of imaginal
tissues
 Echinoderms
• Massive revision of body
plan
 Chordates
• Organogenesis
• Larval molts
• Metamorphosis
• Aging
• Tunicates
– Loss of chordate structures
• Cephalochordates
• Amphibians
– Major changes to respiratory
system & digestive tract
Developmental Stages
 Early Development  Cell senescence
 Regeneration
• Fertilization
• Cleavage
• Gastrulation
• Neurulation
 Later Development
• Organogenesis
• Larval molts
• Metamorphosis
• Aging