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Development of Invertebrates
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Developmental Sequences, Fertilisation, Development Patterns, Gastrulation, Embryology, Larval Development, Insect Development, Regeneration
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Invertebrate ZoologyWeek 8
Chris Foster
Development
Developmental sequence:
1. Gamete formation, storage development info [oogenesis]
2. Fertilisation
3. Activations of zygote metabolism, translocation of maternal mRNA
4. Cleavage
5. Activation of zygote nucleus, transcription of new zygote specific mRNA
6. Organogenesis
7. Differentiation; cells changing from one type to another
- Frequently involved metamorphosis [differentiated larvae adapted to one set of
functions and change to another]
- Individuals arise from single cell [fertilised egg]
- Continuous process; not all species undergo
- Animal development starts with formation of diploid zygote from 2 haploid
gamete cells.
- Fully developed eggs = large eggs with all materials needed for subsequent
development of new individual [and materials associated with reaction of egg
after sperm contact helps keep out more sperm]
- Stored materials; lipid droplets / mitochondria / abundant ribosome’s rRNA /
cortical structure – granules / alveoli / stored heterogeneous gene transcripts
mRNA
- Invertebrates; clear axial polarity established in oogenesis yolk free “animal” pole
and yolkier “vegetal” pole.
- Drosophila; stems from polarised pattern of egg in its follicle [position of mRNA
stored in oocyte cytoplasm, profound influence in later development]
o Sea urchin experiment shows early morphology controlled by maternal
genes]
Fertilisation;
1. Physical juxtaposition of gametes
2. Surface membrane interaction leading to union of sperm and egg
3. Block of polyspermy
4. Activation of oocyte metabolism
5. Fusion of pronuclei to form diploid genomes of zygote
6. Initiation of early cleavage
- Vitellogenesis = yolk formation, pro/metaphase stages of meiosis
- Marine invertebrates; fertilise in sea water
o Sperm activated by pH change with mix with sea water
o Random movements increase frequency of egg contact
, Invertebrate Zoology
Week 8
Chris Foster
o Close range chemical interactions guide sperm to egg
o Chemotaxis; movement of cells, organism part of one in direction
corresponding to gradient of sub.
o EG; hydroid campanularia; eggs held in flask-shaped “gonangium” tracks
to sperm in water not random but directed forwards mouth. Substance in
mouth at gonangium attract sperm
Close range attraction of sperm towards oocyte. Evidence
annelids / molluscs / tunicates / echinoderms
Chemotaxis not effective at >1/2 egg diameter [other substances
increase sperm motility] EG. Peptides in sea urchin eggs.
- Most eggs have species specific sperm coagulant at surface [of diffuses into
surrounding medium]
- Species specific receptor molecules on sperm head
- Ect-aquasperm; type found marine invert [directly fossiled] has acrosome vesicle
at towards tip [acrosome = cap-like structure over anterior ½ of head in sperm,
vesicle = cell structure]
- Approaches egg surfaces;
1. Vesicle and sperm plasma membrane fuses
2. Releasing enzymes rupturing cell wall
3. Acrosome tubules elongate and penetrate egg coats [single filamentous tubule
– echinoderm / hemichordate / annelid]
o Hole left being in yolk membrane / sperm and egg fuse
o After contact; cortical reaction [visible] spreads radially away from point
of contact
o Fusion membrane of cortical granules raising fertilisation membrane
[cortical granules; regulatory / secretary vesicles in oocyte cortex]
o At universal reaction; lead to emission of jelly coat – egg cannot fuse with
other sperm cells
o 1st cleavage follows predetermined time after fertilisation [changes in
cytomplasmic constituents before]
o Bilaterally symmetrical = movements establish primary access of future
embryos [anterior / posterior / dorsal / ventral quadrants of undivided egg]
o Most organisms axis are fixed / tunicate egg develops asymmetry defining
ventral-dorsal axis shortly after fertilisation.
o Migration of different types of cytoplasm; sperm entry point creates
another point of asymmetry [2nd axis at right angles to animal/vegetal axis
– defines future anteroposterior]
Development Patterns;
- Cells start to divide
- Embryo follow precisely determined pattern of cleavage, cytoplasm of relatively
large egg subdivided into relatively low units
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