SBS 306 Molecular Cell Biology

1
SBS 306Molecular Cell Biology

• Lectures 11
• Development of the fertilised egg 1

2
Ontogeny recapitulates Phylogeny?

• In other words that embryonic development
  re-traces an animals evolution. Not true but
  sometimes does yield insights

3
Development of an internal space, the coelum

• The most primitive animals such as sponges or
  hydra have only two layers of cells (protective
  epidermis and digestive gastrodermis separated
  by a gelatinous mesoglea. Higher animals have
  three layer. The most primitive have no body
  cavity, the more advanced have a body cavity
  which is completely lined by mesoderm and use
  this as an attachment point for organs

4
Two worms
5
The right leg in the wrong place.
The fly below has a mutation in the antennopedia
gene and has legs where antennae should be
6
Animals which have been used to study development

• The fruit fly Drosophila melanogaster
• The nematode Caenorhabditis elegans
• The zebra fish
• The toad Xenopus laevis
• The chicken
• The mouse
• Study of development of the mouse is difficult as
  the embryo is rapidly surrounded by membranes.
  The development of birds closely resembles that
  of mammals so this is what we will concentrate on.

7
Development in history

• Primitive animals such as nematodes are basically
  sausages with tubes through them
• Development of segmentation in animals such as
  worms eases evolution
• Development of a central nervous system (nb
  convergent evolution at work here, both insects
  and vertebrates have a CNS but while the spinal
  cord is dorsal, the insect equivalent is ventral)
  makes for control
• Development of appendages on the segments. These
  can diverge to give limbs, wings, tentacles etc.
  again speeding evolution

8
Speed v Accuracy

• The majority of lower animals produce very large
  numbers of eggs which are released into the
  environment.
• The environment contains many predators to eat
  those eggs.
• Hence development in lower animals is speeded up
  as far as is possible because even a newly
  hatched animal can make some attempt to avoid
  predators an egg can do nothing for itself.
• It also follows that a small number of errors in
  development will not have any impact on the
  population

9
Optimisation

• There are several ways in which development can
  be speeded up.
• Hard wiring. In animals such as nematodes
  which use this strategy the fate of cells is
  almost totally determined from the time
  fertilisation onwards. This means that there is
  little or no opportunity to correct errors

10
A differentiated cell

• 2) Storing the mRNAs needed for development as in
  flies, and amphibians. This markedly lowers the
  energy demands of the developing embryos and also
  means that the fate of cells in the developing
  embryo is largely determined at leisure before
  fertilisation

11
Fate maps
The result is that there can be very rapid
development and, indeed, the fate of the cells
and their descendents can be mapped on the
surface of the blastula.
12
And a special trick from insects

• Development in a syncytium. In the case of
  drosophila nuclear division occurs but there is
  no cytokinesis. The end results are a single
  cell with about 6000 nuclei anchored in place by
  cytoskeletal elements. An unknown signal
  initiates ingrowths in the plasma membrane which
  turn these into separate cells
• The advantage is that normally a cell signalling
  to its neighbours will produce a signal which
  binds to a receptor on the target cell which
  initiates events which end in the binding of a
  regulatory protein to DNA. In a syncytium the
  nuclei can signal to each other directly

13
Embryogenesis in Xenopus

• 1) Tight junctions form between the outer cells
  and fluid is pumped into the centre forming a
  hollow ball of cell the blastula. The fate of
  the different cells is decided at this stage
  although they still look identical
• 2) An indentation forms on the surface of the
  blastula at the junction between animal pole and
  vegetal pole-derived cells and the cells of the
  vegetal pole invaginate as do cells from
  immediately above the pore. The former will form
  endoderm, the latter mesoderm. The structure is
  now called a gastrula and the process that forms
  it gastrulation

14
And yet more

• 3) The gastrula is extended by development of a
  rod, called the notochord and the epidermis on
  the back of the embryo folds in to form the
  neural tube which will give rise to the CNS. At
  the stage the embryo is called a neurula
• 5) Blocks of connective tissue (the somites)
  develop on either side of the notochord and
  marked the segmentation of the animal
• The later stages do not concern us here

15
Problems of living on land

• The water is a friendly environment in which to
  develop. Swimming is a simple way to move, food
  comes in all sizes. Accordingly animals living
  in water generally hatch their young at a very
  immature stage.
• On land, movement I more difficult, food less
  easy to find. Hence land animals must hatch at a
  much later stage and this is especially true of
  the lower animals where maternal care is limited
• This in turn led to the development of large
  yolky eggs

16
Early development in chicken
17
Developing in flatland

• The yolk of a bird, reptile or duck billed
  platypus egg is far to large to divide in the
  same way as a frogs egg. After fertilisation
  furrows develop which dissect the area containing
  the nucleus and the non-yolk cytoplasm away from
  the yolk. Cell division now occurs so that on
  laying there is a flat disk of cells, the
  blastoderm, lying on the yolk.
• There is a gap between the central part of the
  blastoderm and the yolk. Cells grow in to cover
  this forming a layer called the hypoblast. The
  upper layer of cells now thins and forms the
  single layer epiblast

18
Gastrulation in Chickens
19
Continued

• A pore now forms in the epiblast at what will be
  the bottom of the embryo. This moves upwards
  towards the head leaving behind the primitive
  streak. Cells from the epiblast move inwards
  through the streak forming the mesoderm
• In addition to forming the mesoderm, the cells
  immigrating through the primitive streak displace
  the hypoblast from the surface of the embryo and
  form the primitive endoderm
• This forms a flat three-layered embryo. The
  hypoblast will give rise to the extra-embryonic
  structures.

20
Introducing Hensens Node

• The cells which will form the primitive streak
  are committed as shown by transplantation
  experiments
• When the primitive streak reaches near to the top
  of the developing embryo a dense group of cells,
  Hensens node develops at the head end and moves
  backwards.
• The movement of Hensens node initially has no
  obvious effect but after a time differentiation
  of the mesoderm over which the node has passed
  becomes apparent

21
Somites etc.

• The initial structures to appear are first the
  notochord, a cellular rod which stiffens the
  embryo and guides development and the somites.
• Somites are blocks of mesodermal tissue which can
  give rise to the vertebrae, the muscles and the
  dermis.
• Passage of Hensens node commits the cells to
  develop not just as somites but a particular
  somite (eg 3rd thoracic) but the positional
  information is at least in part laid down before
  Hensens node develops

22
Everything at once
Because differentiation follows the passage of
Hensens node the head of the embryo may be quite
well diffentiated when the base has just the
primitive streak