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22 Cards in this Set

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What are the body axes?
The vertebrate body has 3 principal axes

Anteroposterior - head to tail
Dorsoventral axis - back to belly
Left-right axis - certain internal organs are not symmetrically distributed
Describe pattern formation of the body.
How do cells make a body with pattern?

- to organise themselves into a body, cells must know where they are in relation to other cells of the embryo
- This is achieved by giving each cell a positional value in relation to the principal embryonic axes, often in response to a morphogen gradient/s
What are the methods of cell to cell molecular communication.
Endocrine factors - blood carried
Paracrine factors - influence cells in immediate vicinity
Autocrine factors - influence the cells that produce it
Direct membrane to membrane receptor/ligand
What are key molecules driving development (signalling molecules?? Describe.
Signaling molecules
- normally proteins
- often members of large families (grouped by structural homology)
- receptor to nucleus (signal transduction)
- concentration - dependent effects
- concentration gradients (gradients of morphogens) critically important in development
- A morphogen is an inductive signal that elicits different responses at different concentration thresholds
- In response the target cell may produce a different protein, produce proteins in different concentrations and change its behaviour (divide, die, move, differentiate etc)
Describe the steps of signalling.
- signal molecule (ligand) produced
- signalling molecule binds to transmembrane receptor
- membrane to nucleus signal transduction pathway
- changed pattern of gene expression
What are some families of signalling molecule ligands?
TGF-beta superfamily (more than 20)
- TGF-betas themselves
- bone morphogenetic proteins (BMPs - more than 9)
- Activins
- Inhibins
- Mullerian Inhibitory substance (MIH)

Fibroblast growth factors (FGFs - more than 20)

Hedgehogs
- Sonic (Shh)
- Indian (Ihh)
- Desert (Dhh)
- Tiggywinkle (Thh)
What are key molecules driving development (transcription factors)? describe.
Transcription factors
- Molecules that initiate patterns of gene expression
- Bind promoters
- Bind RNA Polymerase 11
- Regulate RNA production
- Two types
> generic - common to many cell types
> organ and development specific - guide differentiation of particular organs (e.g. key to stem cell differentiation)
Describe axis specification.
Axis Specification
- The eggs of all vertebrates are radially symmetrical
- Axis specification is therefore a symmetry-breaking process
- In Xenopus symmetry breaking is provided by the site of sperm attachment to the egg.
- In mammals, the blastocoel forms asymmetrically with the inner cell mass displaced to one end. This defines the future dorsoventral axis. Origins of anteroposterior polarity are unknown - signals from the uterus?
What is xenopus?
Xenopus (L., strange foot) are a genus of carnivorous frog native to Africa. There are 15 species in the Xenopus genus, all aquatic with unwebbed fingers and fully webbed feet, with claws on three toes.[1] The best-known species belonging to this genus is Xenopus laevis, which is commonly studied as a model organism.
What is segmentation?
- The establishment of an axis is often followed by segmentation
- Segmentation involves the axis being divided into a repetitive series of similar but independent developmental units e.g. segmentation of insect bodies, somites of vertebrates

In humans
- differential expression of sets of genes along the long axis apparent in the 3rd week of development
- segmentation physically apparent in the 4th week
- segmentation becomes less obvous with development
- segmentation persists in the adult as vertebrae, ribs spinal nerves, etc
Define somites.
Longitudinal segmentation (somites)

In the developing vertebrate embryo, somites (or primitive segments in older texts) are masses of mesoderm distributed along the two sides of the neural tube and that will eventually become dermis (dermatome), skeletal muscle (myotome), and vertebrae (sclerotome). They originate from paraxial mesoderm which, towards the end of the third gestational week, becomes organized into loose masses of cells called somitomeres. Driven by changes in the expression of adhesion molecules, somitomeres compact and bud off to form the somites. Approximately 44 somites form and give rise to the bones of the face, vertebral column, associated muscles, and overlying dermis.
What do homeotic genes do?
- Determine regional characteristics
- Impart regional or specific characteristics to a part of the body e.g. head, wings, antennae, legs
- Contain homeodomains but not all homeodomain-containing genes are homeotic
What are homeodomain proteins?
- Transcription factors
- Contain a highly conserved homeodomain of 61 amino acids (a helix-loop-helix domain)
- The 183 nucleotides that code for the homeodomain are collectively called a hoemobox.
- homeobox regions first described in the homeotic genes found in the Antennapedia and bithorax complexes of Drosophila.
What is Drosophila?
Drosophila is a genus of small flies whose members are often called small fruit flies, or more appropriately vinegar flies, wine flies, pomace flies, grape flies, and picked fruit-flies. A second insect family, the Tephritidae are also called fruit flies; they feed on unripe or ripe fruit. One species in particular, Drosophila melanogaster, has been heavily used in research in genetics and is a common model organism in developmental biology. Indeed, the terms "fruit fly" and "Drosophila" are often used synonymously with D. melanogaster in modern biological literature. The entire genus, however, contains about 1,500 species and is very diverse in appearance, behavior, and breeding habitat.
Describe homeobox genes.
Drosophila
- 8 homeobox-containing genes in 2 clusters on 1 chromosome

Human and mouse
- homeobox genes are called Hox genes in vertebrates
- At least 38 genes are arranged in 4 clusters on 4 chromosomes (2, 7, 12, 17)
- 13 paralogous groups
- Hox genes are members of the large family of homeobox genes
- Hox genes demarcate regions of the embryo, rather than particular tissues or cell types.
How is the expression of homeobox genes regulated?
- genes are activated and expressed sequentially along chromosome
- genomic organisation of the Hox genes is co-linear with their expression in the various body regions (i.e. pattern on gene = pattern on body)
- genes at 3' end of complex are expressed anteriorly and those at teh 5' end are expressed more posteriorly
- In general, loss-of-function mutations result in posterio-ro-anterior transformations
- Gain-of-function mutations result in anterior-to-posterior transformations
- Genes can be switched by various factors
> FGF selectively switches caudal genes
> retinoic acid activates more posterior genes
> etc
Describe how function can be interfered with in Xenopus.
xenopus (interference with function)
- antibody to XIH Box 1 administered
- expansion of hindbrain in loss of function manipulation
Describe Left/righ asymmetry.
- Vertebrates appear bilaterally symmetrical
- But, some of their internal organs are arranged asymmetrically (e.g. heart, vasculature, gut, lungs)
- the body plan has distinct left and right sides
- the constancy of this organisation indicates that left and right must be specified during development. It is not an accident.
What are organisers?
- An organiser is any region of an embryo that directs the development of surrounding tissues, usually by acting as a source of inductive signals
- An organiser is responsible for generating the axes of a developing structure and thus initiating pattern formation
- The term organiser (unqualified)usually refers to teh primary embryonic organiser
- The organiser has multiple roles in axis specification, pattern formation and morphogenesis
> zebrafish - embryonic shield
> amphibians - spemann organiser
> birds - hensen's node
> mammals - the node
What are the steps to left/right asymmetry?
- Initial symmetry breaking (establishment of L-R coordinator)
- Asymmetric signalling initiated in the organiser
- Induction of asymmetric gene expression in paraxial mesoderm
- Induction of asymmetric gene expression in lateral plate mesoderm
- Establishment of midline block to prevent signals leaking to the contralateral side
- Asymmetric expression of transcription factors in developing organs
What is the link between genes and L/R asymmetry?
- About 1/10000 people have situs inversus - L/R asymmetry is abnormal
- Situs inversus totalis is when there is perfect reversed asymmetry - usually healthy
- however, axis reversal is only partial (heterotaxis) and often involves cardiopulmonary defects
- Link betwen laterality defects and microtubular proteins in patients with Kartagener's syndrome who have situs inversus and respiratory disease
- Due to defect in iv gene which transcribes a microtubule motor protein called left-right dynein (LRD)
- In early mouse development, LRD is expressed bilaterally in the node, and many cells in the node contain monocilia.
- recently shown that the beating of the cilia in the node generates a net leftward flow of perinodal fluid
- in iv mutant mice the nodal monocilia are immotile and thre is no net flow of nodal fluid
- iv mutant mice show heterotaxis
- Current thinking: Nodal flow breaks the initial bilateral symmetry, probably by causing the accumulation of a specific determiniant on the left side of the node
- In the mouse nodal and lefty2 genes are expressed specifically in the left lateral plate mesoderm
What is heterotaxis?
partial axis reversal