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Vertebrate
Development
Biology II:
Form and Function
The six stages of vertebrate development
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Fertilization
Cleavage
Gastrulation
Neurulation
Neural crest formation
Organogenesis
Stage of vertebrate
development (I)
Stage of vertebrate
development (II)
Fertilization (I)
• Entry of sperm cell
induces activation
– prevents other
sperm from
entering
– Intitiates second
meiotic division of
egg nucleus
– Induces polarity
Fertilization (II)
Fertilization in sea urchins
Sperm penetration
Polarity in early embryos
Cleavage
• Division of first cell to many within ball of same volume
(morula) is followed by hollowing of that ball to a
blastula. Form of cleavage and blastulation depends
on orientation of yolk and nucleus
– In primitive chordates, division is even, towards a
symmetrical blastula composed of cells of equal
size
– In amphibians, holoblastic cleavage leads to
assymetrical blastula
– In reptiles and birds, meroblastic cleavage occurs,
resulting in a cap of cells on top of the yolk
– In mammals, holoblastic cleavage occurs, creating
a trophoblast containing a blastocoel, with inner
disc of cells equivalent to a blastodisc
Yolk distribution in amniotic eggs
affects blastula development
Holoblastic cleavage
• Cells with little yolk,
and central
nucleus, develop
evenly
Uneven cleavage
• In frog cells, there is
more yolk, and nucleus
of fertilized egg is to
one side:
– Yolk slows division,
so areas of low yolk
content divide
quicker, and create
smaller cells (see
here, front)
– Areas of high yolk
content divide more
slowly, and give rise
to larger cells
Meroblastic cleavage
• Occurring in reptiles, birds and mammals, an uneven
division of cells causes a cap of cells on top of the yolk
Blastula of mammals and birds
• Cap of cells develops into a blastodisc
• Blastocoel develops in mammals, surrounded by
trophoblast
Gastrulation
• Invagination of outer layer of cells to inside of
the blastula is known as gastrulation,
resulting in the formation of the gastrula
• Type of gastrulation is a function of type of
blastula…
• End result is three types of germ layer tissue
- endoderm, mesoderm and ectoderm
Gastrulation in the lancelet
Gastrulation in the frog
Gastrulation in birds
Gastrulation in mammals
Neurulation and neural crest formation
• Formation of neural fold (primitive streak) above
notocord, begins a channel that eventually seals on
the dorsal surface, forming neural groove
– Mesoderm derived tissue close to notocord develop
into somites, giving rise to muscles, connective
tissue and vertebrae
• Layer of cells on dorsal surface of groove form neural
crest, responsible for formation of several important
organs
– Associated patches of ectoderm tissue derive into
placodes, which evetually result in important
neurally related organs
Neural tube formation (I)
Neural tube formation (II)
Induction
• Proximity of a cell to certain other cells controls its
development - Spemann and Mangold
• Cells whose fate can be predicted are termed
‘determined’
• Cells whose fate cannot be reversed are termed
‘committed’
• Since all cells initially can become any tissue...
...Development is a process of progressive restriction
of gene expression
Spemann and Mangold’s dorsal
lip transplant experiment
Induction of the vertebrate eye
Organogenesis
• Ontogeny recapitulates phylogeny
• (and a quick word about extraembryonic
membranes)
Derivation of major tissue types
Embryonic development of vertebrates (I)
Embryonic development of vertebrates (II)
Extraembryonic membranes - Chick embryo
Extraembryonic membranes mammalian embryo (I)
Extraembryonic membranes mammalian embryo (II)
The placenta
Human
development
Developing human at 4 weeks
Developing human at 7 weeks
Developing human at 3
months
Developing human at 4
months
Ultrasound at 5 months
Delivery position of foetus