19-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Chapter 19: Animal development

19-2Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-3Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Embryonic development

Following fertilisation, the zygote passes through• cleavage

– rapid cell division

• gastrulation– development of basic features

• organogenesis– formation of organs from tissues

19-4Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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)

19-5Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-9Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 19.4: Cleavage in mouse

19-10Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-11Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-12Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-13Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-14Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 19.5a–e: Gastrulation in sea urchin

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Fig. 19.5f–j: Gastrulation in sea urchin

19-16Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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)

19-17Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-19Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-20Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-23Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-24Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-25Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-26Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-27Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-28Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-29Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-30Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-31Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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.)

19-32Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-33Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-34Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

19-35Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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