Principles of animal development Flashcards
Development is
self-organising
Describe animal fertilisation
- meiosis forms haploid gametes
- sperm provides the paternal genome
- large variation in the amount of material supplied to the oocyte
What can the oocyte provide?
- ‘yolk’
- organelles
- RNAs and proteins
- extra nutrition
What is the yolk?
constituent molecules and energy supplies for synthesis and growth
Describe the early cell divisions - the basics
- holoblastic cleavage
- first three divisions perpendicular
- radial cleavage
Describe radial cleavage
- first division separates left and right sides of the body
- occurs in many bilaterians
- typical of deuterostomes
Give a radially cleaving organism
Xenopus laevis (African clawed frog)
Describe protostome cleavage
- spiral cleavage
- third cell division may be twisted clockwise or anticlockwise
dextral spiral cleavage
clockwise twisting of third cell division
sinistral spiral cleavage
anti-clockwise twisting of third cell division
Describe the early cell divisions - the specifics
- equal or unequal, depending on the taxon
- blastomeres inherit different portions of the original oocyte cytoplasm
- exaggerated in unequal cleavage
Describe equal early cell divisions
first 4 blastomeres of similar size
Describe unequal early cell divisions
blastomeres of very different sizes
Describe gastropod mollusc cleavage
direction of spiral cleavage determines which way the shell coils
Describe teloblastic cleavage
- cleavage restricted to only part of the fertilised egg
- occurs in embryos with a lot of yolk
- as in teleost fish
Describe the blastula
- hollow ball
- blastocoel
- blastoderm
- varied across species
Describe gastrulation
- making layers
- dorsal lip
- blastopore
- archenteron
- yolk plug
Describe the late blastula
- animal pole and vegetal pole
- marginal zone
- blastocoel
- yolky vegetal cells
Describe organogenesis
- ## making tissues and systems
Describe the three germ layers
- localised by gastrulation
- go on to make the organ systems
What does the ectoderm make?
nervous system and the epidermis
What does the endoderm make?
- gut and associated digestive and respiratory organs
What does the mesoderm make?
muscles, skeleton, excretory system and gonad
Describe development control
- differentiation
- pattern formation
- morphogenesis
Describe differentiation
process by which cells come to display a specific phenotype related to a specific set of functions
Describe pattern formation
process of organizing cells in time and space
Describe morphogenesis
movement of cells or groups of cells to form shapes and structures
Give some examples of morphogenesis
- gastrulation
- neural crest migration
Describe myogenesis
- formation of muscle tissue from myoblast precursors
- dividing myoblasts multiply using growth factors
- cell multiplication ceases and cells align
- cells fuse into myotubes that show muscle-specific proteins
- spontaneous contraction of muscle fibre begins
Describe the genetics of myogenesis
- MyoD
- homologous in most animals
MyoD
- encodes bHLH transcription factor
- expression can be sufficient to convert cultured fibroblasts into myoblasts
Describe patterning
spatiotemporal organisation of differentiated cells
What are the two patterning mechanisms?
- lineage-dependent mechanisms
- organising fields of cells
Describe patterning by asymmetric cell division
- cells differentiate according to the axis of cell division
- explains local patterns of differentiation when few cells are involved
Describe assymetric cell division
- asymmetric distribution of cytoplasmic determinants
- cell division
- different daughter cells
Describe C. elegant development
- adult hermaphrodite has 959 somatic cells
- lineage of every cell is fully prescribed, always the same
- every cell at every stage now has a name
Describe patterning by signalling
cells differentiate according to their position relative to a signal, irrespective of their lineage
Describe Wolpert’s ‘French Flag’ model of positional information relating to a morphogen gradient
- position of each cell defined by morphogen concentration
- positional value interpreted by cells at thresholds
- pattern forms
Describe patterning by morphogen gradient in Drosophila bicoid
- maternal bicoid mRNA anchored at the anterior end of the oocyte
- when it is translated the protein diffuses and forms a gradient
- activates different genes at different concentrations, leading to stripes of gene expression
- the first step to making a segmented body
Describe patterning along embryo axes
- patterning along the AP axis by bicoid
Describe bicoid
- encodes a transcription factor
- this protein diffuses to form the gradient
- only works because early Drosophila development is syncytial
- zygote nucleus divides many times without cytokinesis
Describe intercellular signalling
- most animal embryos are fully cellular
Describe the mediolateral axis
bilateral symmetry
Describe development along the DV axis in zebrafish
Bmp signalling
Describe development along the AP axis in zebrafish
Wnt
Describe the Hox genes - the basics
- intermediate transcriptional state
- cells acquire a ‘positional identity’ relative to a signal without this immediately leading to differentiation
