Download Development and Apoptosis

Body Plans of Eukaryotes
In any multicellular organism, development is controlled and coordinated and, more often than not, cells
end up where they are meant to be. The development follows a body plan and is under genetic control.
The genes which control the body plan are called homeobox genes. Homeobox genes are 180 base pairs
long and code for small protein transcription factors (60 amino acids long – remember the triplet
coding?). These transcription factors bind to DNA allowing it to be transcribed and ‘switch on’ many
genes in a cascade effect. Homeobox (or Hox) genes have been discovered in fungi, plants and animals
from fruit fly to humans. The following outline shows how they are important in controlling the body
plan of fruit fly.
The fruit fly has the body plan of a typical insect, i.e. the adult has a segmented body with a head
section, a thorax (three segments T1, T2 and T3) and an abdomen. Each of the thorax sections has a
pair of legs; there is a pair of wings on T2 and a pair of balance organs on T3. Hox genes control the
polarity of the body, polarity of the segments and the development of individual segments.
Thorax
Abdomen
Head
Pair of wings on T2
Compound eye
Antennae
Pair of balance organs on T3
Paired legs on T1, T2 and T3
Hox genes of the fruit fly, Drosophila melanogaster.
Public domain image (PhiLiP).
The Antp (antennapedia) homeobox gene codes for a transcription factor that switches on all of the
genes that allow leg development. It is switched on in thorax sections 1, 2 and 3 and usually switched
off in the head segment. If switched on in the head segment, a pair of legs develop instead of antennae
(producing the antennapedia condition). Similarly, the Ubx (ultrabithorax) homeobox gene usually stops
development of wing production on thorax segment 3. If mutated, an extra pair of wings is grown
instead of the usual balance organs. You can view a series of animations showing the role of homeobox
genes in Drosophila melanogaster (the fruit fly) by connecting to:
http://www.dnalc.org/view/16760-Animation-37-Master-genes-control-basic-bodyplans.html
The situation is similar in humans where Hox A11 and Hox D11 switch on the genes for development of
the forelimb. It has been suggested that thalidomide may have switched off the homeobox genes for
limb development and so caused the birth defects typical of thalidomide use. Thalidomide can certainly
insert itself into DNA and inhibits production of new blood vessels in limb buds.
SJWMS Biology
Apoptosis or Programmed Cell Death
Cells are not immortal. They can only undergo a limited number of cell division (usually about 50) and
then die. It is not the type of death that is caused by damage or trauma, but an orderly series of events
leading to cell death. This ordered or programmed cell death is called apoptosis.
During apoptosis the
cytoskeleton is broken
down by enzymes, the
nuclear envelope and DNA
fragments and the cell
membrane begins to bleb
– instead of invaginating
as it does during
phagocytosis, it forms
little balloons that are
pinched off from the cell
surface membrane. The
blebs (vesicles) are then
phagocytosed. The key
events are summarised in
the diagram.
1. Normal cell, capable of dividing for a
number of cycles (the Hayflick
constant, about 50 divisions). After
this, a series of ordered steps results
in the death of the cell – not the same
as necrosis!
2. Cell begins to break down. Enzymes
digest the cytoskeleton and the
cytoplasm becomes dense. The
chromatin condenses and DNA
fragments. Blebs (extrusions of the
cytoplasm) appear containing
cytoplasmic material.
The process is controlled
by a range of signalling
molecules (cytokines,
hormones and even
chemicals like nitric oxide)
3. Cell shrinks as it disintegrates. The
and is part of normal
development. Consider
blebs containing cytoplasm and
the changes that take
organelles are phagocytosed
place in the
(engulfed) by phagocytes and
metamorphosis of
digested. Process is quick and prevents
amphibians. The larvae
damage to surrounding tissues (if
(tadpoles) are aquatic,
necrosis, enzymes and toxic products
have external gills and a
can cause extensive damage).
tail. The adults have
lungs, legs and do not
have a tail. The tissues of
The events of apoptosis or programmed cell death
the gills and tail are
reabsorbed due to
apoptosis. Similarly, during limb development in humans, the tissues have to be rearranged. Apoptosis
allows for the separation of the digits (fingers and toes). Failure of this leads to syndactyly, i.e. fused
digits (toes in the case of the illustration).
The balance between too much cell death and too little is important.
Too little and tissue accumulates (tumour); too much and there is cell
loss and tissue degeneration.
Failure of apoptosis leads to syndactyly or
fused digits, as seen here on the foot.
Image licensed under the Creative
Commons Attribution ShareAlike 3 Licence
SJWMS Biology