Lecture 16. Organising a Body Plan in Multicellular Organisms Flashcards
What can give rise to complex multicellular organism ?
A fertilised egg cell
How do most multicellular organisms arise ?
Sexual reproduction
What does sexual reproduction ensure ?
Genetic variation and adaptability in the population
How is a diploid individual formed ?
Two haploid gametes produced by parents fuse
What process takes places in the production of gametes ?
Meoisis
What does meiosis achieve ?
Halving of the genetic material
What is gametogenesis ?
Produces a haploid sperm and egg
What are the two functions of fertilisation ?
- Brings haploid gametes together to form the new individual
- Activates the developmental processes in the egg
How are different cell types produced ?
Progressively as the embryo develops
What is special about a fertilised egg cell ?
It is the only cell that naturally has the capacity to form a new unique individual
What are model organisms use for ?
Researchers study development in model organisms to identify general principles
What are some examples of invertebrate model organisms ?
- Drosphilia melanogaster
- Caenorhabditis elegans
- Echinodermata
What are some examples of vertebrate model organisms ?
- Xenopus laevis - frog
- Gallus - chick
- Mus musculus - mouse
- Danio rerio - zebrafish
What is an example of a plant model organism ?
Arabidopsis thaliana
What are the steps involved in building a new organism ?
- Fertilisation
- Cleavage
- Gastrulation
- Neurulation
- Organogenesis
What does cleavage form ?
A hollow ball or disk of cell eg. blastula, blastoderm
What is gastrulation ?
Turns the blastula into a three germed layer
What is neurulation ?
A special type of organogenesis - it sets cells as and forms the entire nervous system
What does a three layered embryo have ?
A primitive gut called a gastrula
What are the three germ layers called ?
Ectoderm, mesoderm, endoderm
Where is the ectoderm ?
Outer layer
Where is the mesoderm ?
Middle layer
Where is the endoderm ?
Inside layer
What does gastrulation lead to ?
The generation of germ layers placed in the correct relative positions in the embryo
What does gastrulation result in ?
- Elaborate cell movement
- Differentiation of cells into three types
- Positioning of cell layers in the gastrula which allows them to interact in new ways
- Communication between cells that are now different to one another
What is the first operational organ system in the developing embryo ?
The circulatory system
What are the three major inter-related processes involved in building a complex organism from a single fertilised egg cell ?
- Cell division
- Cell differentiation
- Morphogenesis
What is the basis of cellular differentiation ?
Differential gene expression
What is the turn on and turn off genes in specific cells controlled by ?
Cell specific transcription factors that bind to gene specific enhancer regulatory elements
What is morphogenesis ?
The emergence of shape and structure in the body plan of the developing embryo
How do we define position in a 3D object ?
By co-ordinates along 3 axes
How do cells receive information about their relative position ?
- Localisation of cytoplasmic determinants
2. Induction
What is induction ?
Where one group of cells influences the development of a neighboring group of cells
How is induction achieved at a molecular level ?
Through cell signaling
How can genes be turned on and off ?
In response to signals that a cell receives
What is meant by competence ?
The ability to respond to a signal
What do maternal effect genes do ?
Establish poles and axes
What do segmentation genes do ?
Divide the body into segmental units
What happens in mutants of homeotic genes ?
Structure characteristic of a particular part of the animal arise in the wrong place
What does Antennapedia mutant do ?
Leads to the formation of legs at the site of antenna
What are transcription factors ?
Master genes that guide developmental events
What are hox genes ?
A special set of such genes that encode transcription factors, they are the genes mutated in homeotic mutants. They control the identity of body parts
What is the vertebrate limb a good model for ?
How positional information is organised leading to morphogenesis
What are some points about the vertebrate limb ?
- Formed relatively late in development
- External position
- Obvious 3D pattern
What are secondary fields ?
Regions of the embryo that emerge or are defined after the primary body plan is laid down
What has to be established to pattern the secondary field ?
A new set of positional cues
What marks out the future skeleton of the limb ?
Cartilage condenses with the mesenchyme
How does cartilage form bone ?
It ossifies
What is an organiser ?
A group of cells that influences the development of surrounding tissue
How is the organiser function achieved ?
Induction
What are the two important organisers in the limb ?
- The apical ectodermal ridge
2. The zone of polarising activity
What happens if the apical ectodermal ridge is removed ?
The limb ceases to grow
What does the removal of the apical ectodermal ridge at different stages lead to ?
The trunification of the limb at the different points along the p/d axis
Where is the zone of polarising activity located ?
In the posterior mesoderm of the developing limb bud
What happens when the zone of polarising activity is transplanted to the anterior ?
It induces a duplication of the A/P pattern of digits, a mirror image of the normal digits
What was the zone of polarising activity shown to be ?
A dose dependent phenomenon since transplantation of only some cells lead to partial duplication
What lies downstream of p/d and a/p signaling in limb patterning ?
Hox genes
What is produced by the zone polarising activity to pattern along the a/p axis ?
A classic morphogen
