Transcript Hox - McGraw Hill Higher Education

Chapter 20: Animal development
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20-1
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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20-2
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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20-4
Fig. 20.3: 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
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20-6
Fig. 20.4: 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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20-8
Fig. 20.6: Cleavage and tissue
formation in the 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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20-10
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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The three 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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20-12
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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20-13
Fig. 20.7a–e: Gastrulation in sea
urchin
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20-14
Fig. 20.7f–h: Gastrulation in sea
urchin (cont.)
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20-15
Xenopus (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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20-16
Fig. 20.8: Gastrulation in the frog
Xenopus
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20-17
Question 1:
All of the following occur during early cleavage of
an animal zygote EXCEPT:
a) The developing cells undergo mitosis
b) The nuclear-to-cytoplasmic ratio of the cells
increases
c) The ratio of surface area to volume of cells
increases
d) The embryo grows significantly in mass
e) The developing cell undergoes cytokinesis
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20-18
Mechanisms of 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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20-19
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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20-20
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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20-21
Fig. 20.13: Neurulation
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20-22
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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20-23
The mechanism of neural crest
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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20-24
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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20-25
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
other types of cells
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20-26
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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20-27
Genetic regulation of development
• 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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20-28
Pattern formation
• In many animals, spatial arrangement of tissues is
along the
– anterior–posterior (A–P) axis
– dorsal–ventral (D–V) axis
• Repeated structures or segmentation along
anterior–posterior axis
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20-29
Molecular genetics of segmentation
in 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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20-30
Early embryonic 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
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20-31
Early embryonic 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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20-32
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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20-33
Homeotic (Hox) genes in Drosophila
• Hox genes determine identities of segments
• Hox genes in Drosophila are 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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20-34
Hox genes and the evolution of body
patterns
• Homologues of Drosophila Hox genes are 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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20-35
Question 2:
How would you expect Hox genes to be expressed in
millipedes, where all segments bear similar legs, as
compared to insects?
a) The leg-specifying Hox genes would be expressed in all leg-bearing
segments.
b) Hox gene expression would be the same as in insects, but the
interpretation of segmental identity would have evolved.
c) The Hox code is unlikely to be at all similar between millipedes and
insects, so the expression of Hox genes would be impossible to
predict.
d) Other genes would have to have evolved to specify legs in
millipedes, since millipedes would form wings on thoracic segments
if they expressed Hox genes.
e) Millipedes probably do not have or use Hox genes to specify identity
along the A–P axis.
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20-36
Summary
• Three main cellular processes in animal
development: cell proliferation, differentiation and
apoptosis
• Generation of the patterns and forms of the mature
body structure is termed morphogenesis
• The main processes of embryogenesis are
cleavage, gastrulation and cell shape change
• Development is regulated by differential gene
expression, cytoplasmic determinants and cell
signalling
• Stem cells can divide and generate one or more
different types of cells
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
20-37