Chapter 19: Animal development
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Cell behaviour
During development, cells
• proliferate
– divide to produce new cells
•
undergo apoptosis
– programmed cell death to remove cells
•
differentiate
– form different types of cells
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Embryonic development
Following fertilisation, the zygote passes through
• cleavage
– rapid cell division
•
gastrulation
– development of basic features
•
organogenesis
– formation of organs from tissues
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Cleavage
•
Zygote divides rapidly
– zygote does not change size
– cells are reduced in size with each division
•
Cleavage follows predictable pattern
– becomes less predictable as division proceeds
•
Ends with formation of blastula
– hollow ball of cells (blastomeres)
– fluid-filled cavity (blastocoel)
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Fig. 19.1: Pattern of cleavage in sea
cucumber
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Patterns of cleavage
•
Pattern influenced by amount of yolk
– physical barrier to cleavage
– displaces mitotic spindle
•
Small amount of yolk (e.g. sea urchin)
– symmetrical pattern of division
•
Intermediate amount (e.g. frog)
– uneven distribution

vegetal pole (most yolk)
 animal pole (least yolk)
– division slower in vegetal pole, resulting in larger
blastomeres at that end
(cont.)
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Fig. 19.2: Cleavage in the frog
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Patterns of cleavage (cont.)
•
Large amount of yolk (e.g. birds)
– other cell contents displaced
– blastodisc or blastoderm
•
Mammalian eggs
– eutherians almost yolk-free (nourishment from placenta)
– cell division slow
– distinct pattern of cleavage

inner cell mass (blastocyst) that gives rise to embryo
 outer layer produces placenta
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Fig. 19.4: Cleavage in mouse
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Maternal control of cleavage
•
Zygote genome does not control cleavage
– enucleated zygotes divide normally
•
•
Materials required for cleavage provided to egg
during oogenesis
Cleavage distributes materials unevenly
– different blastomeres receive different materials in
different amounts
– influences development
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Gastrulation
•
Blastula with undifferentiated blastomeres
– no specialised tissues
– no organs
•
During gastrulation
– development of basic features of adult body plan

germ layers: ectoderm, mesoderm, endoderm
 body cavities: archenteron, coelom
 bilateral symmetry
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Primary germ layers
•
Ectoderm
– outer layer of gastrula

•
becomes outer body covering and nervous system
Mesoderm
– intermediate layer of gastrula

•
becomes tissues and organs
Endoderm
– inner layer of gastrula

becomes lining of gut and organs associated with gut
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Sea urchin gastrulation
•
At vegetal pole, epithelial cells flatten to form
vegetal plate
– primary mesenchyme cells migrate towards animal pole
•
Invagination to create cylindrical cavity
– archenteron (cavity)
– blastopore (opening)
•
Secondary mesenchyme cells migrate into
blastocoel and contact inner surface of blastoderm
– eventually fill remaining blastocoel
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Fig. 19.5a–e: Gastrulation in sea urchin
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Fig. 19.5f–j: Gastrulation in sea urchin
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Clawed frog gastrulation
•
At animal pole, presumptive mesoderm folds into
cavity
– involution
– earliest cells to do this give rise to notochord
•
Ectodermal cells grow over presumptive endoderm
– epiboly
•
Animal hemisphere cells (ectoderm) enclose
vegetal cells (endoderm)
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Fig. 19.6: Gastrulation in clawed frog
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Morphogenesis
•
•
Generation of pattern and form during
development
Changes in cell shape
– action of cytoskeleton
– actin and myosin microfilaments
•
Changes in cell adhesion
– protein adhesion molecules on cell surface
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Organogenesis
•
Development of organs from tissues
• Mechanisms
– thickening and folding of tissue

example: formation of neural tube
– disaggregation and migration

example: nerve cells, connective tissues
– localised cell proliferation

example: digits in amphibians
– localised apoptosis (cell death)

example: digits in mammals
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Neurulation
•
Development of nervous system in vertebrates
– earliest organ system in embryo
•
Ectoderm thickens along dorsal midline
– neural plate
•
•
•
Folds to form neural groove
Neural folds meet and fuse to form neural tube
Neural tube separates from ectoderm
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Fig. 19.11: Neurulation
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Neural crest development
•
Neural crest cells (from neural folds)
– change from epithelial cells to mesenchymal cells
– disaggregate and migrate through other tissues
•
Give rise to
– sensory nerve cells
– autonomic nerve cells
•
Contribute to
– adrenal glands
– connective tissues of head
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Mechanism of cell migration
•
Migration path of neural crest cells determined by
extracellular matrix (ECM) molecules
– fibronectin
– laminin
– collagens
•
Cells follow ECM molecule pathways
– adhere via receptors on cell surface
•
Change in nature of receptors on cell surface
– ends migration
– promotes aggregation
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Limb formation
•
Limbs develop from buds of ectoderm and
mesoderm
– ectoderm thickest at tip of bud
– causes underlying mesoderm to proliferate
– bud elongates
•
Tissues
– cells aggregate to form cartilage
– those that form muscle migrate in from around neural
tube and aggregate around cartilage
•
Digits arise from local proliferation or apoptosis
depending on organism
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Cell lineages
•
Cells differentiate
– develop specific form and function
•
Stem cells can give rise to one or more types of
cells
– unipotent (one cell type)
– pluripotent (two or more cell types)
•
Terminally differentiated cells cannot give rise to
others type of cells
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Tissue maintenance
•
Tissues are repaired or replaced by
– division of differentiated cells

examples: liver cells, lining of blood vessels
– proliferation from stem cells

examples: red blood cells, lining of intestine
– some tissues are not replaced

examples: nerve cells, oocytes, cardiac muscle
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Regulating development
During development, individual blastomeres respond to
• internal signals
– within blastomere
– cytoplasmic factors in different blastomeres influence fate of
those blastomeres during development
– example: in sea squirts (phylum Chordata), blastomeres
with myoplasm become muscle cells
•
external signals
– from other blastomeres or extracellular matrix
– other cells regulate cell fate (induction)
– example: in clawed frogs (phylum Chordata) animal and
vegetal cells interact to induce mesodermal tissues
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Genetic regulation
•
•
Activity of genes in a developing embryo controlled
by internal or external signals
Genetic activity causes cells to
–
–
–
–
–
divide
change shape
change connections with other cells
undergo apoptosis
differentiate
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Pattern formation
•
In many animals, spatial arrangement of tissues is
along
– anterior–posterior axis
– dorsal–ventral axis
•
Repeated structures or segmentation along
anterior–posterior axis
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Pattern formation in Drosophila
•
Drosophila (fruit fly)
– segmented body plan along A–P axis

head
 thorax (three segments)
 abdomen (eight segments)
•
Genes of pattern formation
– segmentation genes
– homeotic (Hox) genes
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Early development in Drosophila
•
No cytokinesis during first thirteen mitotic divisions
– syncytium with multiple nuclei
•
Nuclei migrate to periphery of egg
– cell membranes enclose each nucleus
•
Pole cells at one end of embryo become germ line
– remainder of cells become cellular blastoderm
(cont.)
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Early development in Drosophila
(cont.)
•
•
Maternal genes establish polarity of embryo along
A–P and D–V axes
Bicoid gene determines A–P axis
– bicoid mRNA remains at anterior pole
– diffusion gradient of bicoid protein
– different concentrations of bicoid protein cause nuclei to
express different sets of genes
•
Morphogens
– regulatory proteins with a concentration-dependent effect
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Segmentation in Drosophila
•
•
Once axes are established by maternal-effect
genes, segmentation genes are induced
Gap genes
– establish spatial organisation that leads to segmentation
•
Pair-rule genes
– pattern embryo into discrete segments
•
Segment-polarity genes
– give rise to repeated structures
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Hox genes in Drosophila
•
Hox genes determine identities of segments
• Hox genes in Drosophila in two clusters on a
single chromosome
– Antennapedia complex

five genes
– Bithorax complex

•
three genes
Combination of gene activity determines identity of
individual segments
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Evolution of body plans
•
Homologues of Drosophila Hox genes found in all
major animal phyla
– specify regional identity along A–P axis
•
Conservation of structure, arrangement and
pattern of expression of Hox genes between
insects and vertebrates
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