Download Apoptosis - Sir Joseph Williamson

Apoptosis
Apoptosis (Programmed Cell Death)
 Cells are not immortal.
 Cells can only undergo a limited number of
cell division:
 usually about 50
 they 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.
3
 During apoptosis:
 the cytoskeleton is broken down by enzymes
 the nuclear envelope and DNA fragment
 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 phagocytosed
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!
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.
Cell shrinks as it disintegrates. The blebs
containing cytoplasm and organelles are
phagocytosed (engulfed) by phagocytes
and digested. Process is quick and
prevents damage to surrounding tissues (if
necrosis, enzymes and toxic products can
cause extensive damage).
5
Apoptosis or programmed cell death
Phagocytosis
7
8
 Apoptosis is controlled by a range of
signalling molecules, e.g. cytokines,
hormones and even chemicals like nitric
oxide.
 It is part of normal development.
 Consider the changes that take place in the
metamorphosis of amphibian:
 the larva (tadpoles) are aquatic, have external
gills and a tail
 the adults have lungs, legs and no tail
 the gills and tail are reabsorbed due to
apoptosis
The life cycle of an
amphibian involves
metamorphosis, with
reabsorption of the gills and
tail. Apoptosis is responsible
for this.
Life cycle of amphibian.
 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 opposite).
Failure of apoptosis leads to
syndactyly or fused digits, as seen
here on the foot.
Hand of newborn infant showing complete syndactyly of two fingers.
12
 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