lecture 2 - 01/10/24 Flashcards
What is fertilisation?
Fusion of sperm and egg
What is cleavage?
mitotic cell division after fertilisation
- no time for cells to increase in size
What are the 2 types of cleavage?
spiral and radial
Describe spiral cleavage
seen in protostomes animals
cell pattern rotates by 45 degrees
e.g. pond snail
Describe radial cleavage
seen in deuterostomes animals
cells stack in an aligned manner
e.g. sea urchin
Describe the blastula stage
- dividing cells are aligned in the outer layer
- inside of the embryos forms a cavity called
‘blastocoel’ - cells in blastula stage are not easily
differentiated
What does the blastocoel contain?
salt water
What are the 3 germ layers?
ectoderm
mesoderm
endoderm
What is a germ layer?
- a group of cells which are specified to
become certain cell fates - formed at the late blastula stage
What does the ectoderm form in humans?
neutral plate
- forebrain
- midbrain
- hindbrain
- spinal cord
neutral crest
placodes
- eyes
- nose
- ear
epidermis (skin)
What does the mesoderm form in humans?
muscle
bone
kidney
heart
blood
What does the endoderm form in humans?
intestine
pharynx
liver
stomach
Describe the trend between complexity and number of germ layers
As number of germ layers increases, complexity increases
What is the term for 2 germ layers?
diploblastic
What is the term for 3 germ layers?
triploblastic
What does evolution of germ layers allows?
it allows the development of different tissues with complex functions
How many germ layers does porifera have?
no germ layers
How many germ layers does Ctenophora and Cnidaria have?
2
diploblastic
How many germ layers do the Bilateria have?
3
triploblastic
Describe Gastrulation
rearrangement of cells
- involution and internalisation of mesoderm
and endoderm cells
specification of cell fates
- ectoderm > epidermis, neuron
- mesoderm > muscle, blood etc.
- endoderm > digestive system
What is radial symmetry?
one main axis
What is bilateral symmetry?
Plane of symmetry runs from anterior to posterior.
Explain the evolution of symmetrical animals
asymmetrical –> radially symmetrical –> bilaterally symmetrical
What is the exception that breaks the bilaterally symmetrical rule in the bilateral?
echinoderms are bilateria but have lost bilateral symmetry
Why is bilateral symmetry important?
It facilitated dorsal-ventral differentiation of more complex tissue development
What groups are in protostomia?
ecdtysozoa
lophotorochozoa
What is the difference between cleavage pattern in protostomes and deuterostomes?
protostomes = spiral cleavage
deuterostomes = radial cleavage
What is the difference between cell fates during cleavage in protostomes and deuterostomes?
protostomes = determinate cleavage - tissue e.g. skin, muscle
deuterostomes = indeterminate cleavage - complete larvae
Protostomes vs deuterostomes mouth or anus first?
protostomes - blastopore forms mouth
deuterostomes - blastopore forms anus
Do acoelomates have a body cavity?
no body cavity
space between gut and muscular wall filled with masses of cells - mesenchyme
Do pseudocoelomates have a body cavity?
yes - pseudocoel
muscle (mesoderm derived) present only on outer side of pseudocoel
no mesoderm surrounding internal organs
Do coelomates have a body cavity?
yes - coelom
internal organs surrounded by muscular tissue - peritoneum
coelom is enclosed both inside and out by mesoderm
Explain the evolution and complexity of body cavity
acoelomate –> pseudocoelomate –> coelomate
gets more complex –>
How do earthworms move?
peristalsis
bristles anchor the worm and muscles manipulate the fluid in the coelom flattening and lengthen sections of the animal allowing it to move
What is a hydrostatic skeleton?
closed body compartment filled with fluid under pressure
Are fluids compressible?
fluids are relatively uncompressible
