From www.bloodjournal.org by guest on July 31, 2017. For personal use only.
The
Demarcation
Membrane
System
of the
Megakaryocyte:
A Misnomer?
By J. M. Radley
The
concept
eates
has
that
platelets
been
megakaryocytes
thicker.
cytes
were
and
revealed
extended
exposed
known
from
form
to
ment.3’6 The demarcation
arise
from
the plasma
the
in the form of tubules3
sheets.8
More
recent
fracture
have
supported
tubular
close to the
fenestrated
sheet-like
Shaklai
mation
and
into
idea
arose
and
become
part
involvement
of the surface
of tubules
penetrate
marrow
environ-
the
to
Electron
tion
that
There
is
platelets
cytoplasm
need
such
apparent
transforaligned
then
that
The
therefore
are in fact delineated
by the demarcation
megakaryocytes
organelles
assobut the marginal
not unreasonable
“platelet
if the demarcation
randomly
through
for
areas”
in the
membrane
evidence
that
megakaryocyte
system.
Any
hypothesis
concerning
platelet
formation
must be compatible
with their method
of release
from megakaryo-
Blood.
Vol. 60.
No.
1 (July).
1982
envelops
in
mem-
appropriate
than
“de-
cyte
processes
that
these
extravasculanly
compartment
fragmentation
subsequently
further
studies
are
seen
they
to
thought
is
Attention
has
between
larity
extend
these
and
bridges
and
into
constric-
segments.’4’6
these segments
into
to the
scanning
in which
processes
attenuation
platelets
drawn
are sections
to the parent
with
marrow
into platelet-sized
bridges
linking
release
been
to
undergo
tion at their distal end
Rupture
ofcytoplasmic
fragments
attached
is consistent
of bone
This
microscopic
where
fragmentation
as a result
of the demarcation
a fracture
line.”
Serial
section-
ing, however,
suggests that such
through
processes
that are still
megakaryocytes
sugges-
off from megakaryocompartment
and
undergo
megakaryocyte.’2”3
that
of the bone
into plate-
studies’#{176} led to the
fragments
break
in the intravascular
into platelets,
possibly
membrane
becoming
located
processes
the
circulation.
ultrastructunal
those
simi-
linking
cells
in
the terminal
stages
of cytokinesis,
suggesting
there is a
common
mechanism
for separation
and that extension
of demarcation
membranes
is not involved
in platelet
release.
The
6
demonstration
of attenuated
cytoplasmic
proin sinusoids
with the scanning
electron
microhas led Becker
and Dc Bruyn’4
to propose
that
scope
megakaryocyte
cytoplasm
is
demarcated
than individual
intertwined
and
into
cylindni-
platelets,
compacted
and
that
in the
could
membrane
system
the
cytoplasm.
further
a
the
platelets
“invaginated
cytoplasmic
microscopic
large
sinusoids
is
system
of adjacent
platelets.
oven
a three-dimensional
lets.
that
intravascular
they undergo
cal processes
rather
such processes
are
that
more
produce
certain
sites in micrographs
of mature
as “platelet
areas.”
These
contain
ciated
with the circulating
platelet,
bundle
of microtubules
is absent.
It is
by chance
distributed
thereby
term
the
provide
during
demarcate
The
showed
the
where
electron
fenesfreeze
structure
sheets
Wright’
cesses
arise
was
that
to
system.”
matrix
is envisaged
to result
in platelets
being
enveloped by their plasma
membrane.
Many
authors
(e.g.,
Fig. 6, ref. 5) have designated
to suggest
and
to
is considered
membrane
megakaryocytes
flattened
to explain
the
that parallel
tubules
fuse
together,
the resulting
undergoing
fission
into
two
separate
than
megakaryocyte.
results
evagination
processes
rather
system”
the
functions
undergoes
attenuated
maturing
cytes.
membrane
and adopts
a
deeper
within
the cell.6’9
Tavassoli6
sought
sheets
by proposing
of
from
system
that
platelets.
marcation
that
Ultramem-
system
appears
by invagination,
that
suggested
reserve
putative
brane
is
membrane
on of concentric
studies
involving
the
plasma
form
membrane
deporetracted.
extnacellulan
membrane
membrane
membrane
the
that often
undergo
branching.
Tracer
techhave demonstrated
that
the space
within
the
is patent
lt
When
for many
years’
megakaryocytes,
of tubules
reappeared.
demarcation
formation
to
within
the parent
cell remains
micnognaphs
of megakaryoexistence
of a membrane
system
to be in the
membranes
trated
microtubule
processes
demarcation
it has been
are derived
attenu-
membrane.
to
attenuated
an extensive
the
system
either
long.
bone
microscopy
the cytoplasm,
and the dogma
demarcates
putative
platelets.2
inspection
shows
the demarcation
structural
niques
of mouse
by electron
invaginated
delin-
of formation
Early
electron
permeating
this
system
cysternae
of
system
megakaryocytes
culture
found
the
LTHOUGI-1
that platelets
their mode
uncertain.
were
amount
agents.
became
of
megakaryocytes
such
lymerizing
membrane
cytoplasm
In short-term
that
a limited
brane
the
mature
processes
have
A
demarcation
examined.
marrow.
ated
the
within
and C. J. HaIler
From
the Biological
Research
Unit.
(‘ancer
Institute,
Melbourne,
Australia.
Submitted
Address
Research
bourne
(C)
November
reprint
Unit,
/3,
1981;
requests
to
Cancer
Institute,
3000, Australia.
1982 by Grune & Stratton,
accepted
Dr.
J.
481
Little
March
M.
2, /982.
Radley,
Lonsdale
Biological
Street,
Mel-
Inc.
0006-4971/82/6001-0031$O1.OO/O
213
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214
RADLEY
parent
megakanyocyte.
proposed
earlier
process
A
similar
model
by Bessis,’7
following
by megakaryocytes
formation
had
observations
in vitro.’8
of
How-
ever, ultrastructural
evidence
for cylindrical
arrays
demarcation
membrane
within
megakanyocytes
me
the
role
platelet
plasm
of the
the demarcation
platelets
in the
has led us to reexammembrane
system
in
demarcation
formation
and to consider
how
might
be subdivided
into platelet
enveloped
that the
cyte
This
megakaryocyte.
by membranes.
surface
membrane
suffices
to coven
Simple
of the
less
than
membrane,
demarcate
brane
the
Instead,
that
undergoes
reserve
cytoand
the
surface
may
not
until
the
stage
We have
sought
utilizing
megakanyocyte
it could
provide
evagination
term culture.’8
processes,
the
been
examined,
demarcation
of process
to elucidate
process
After
inducing
ultrastructune
paying
membrane
system.
salt
2% fetal
to
a memduring
the
formation
is
this
matter
formation
attention
in short-
to
calf
depression
slides,
coverslips
using
sealed
were
Alternatively,
of 2 glass
formation
phase-contrast
the
effect
of inhibition
on slides
Co.,
Mo.,
Louis,
at the start
a 4#{176}C
room
cold.
prewarmed
trations
drug
ranging
Australia)
into
microscopy
with
0.1
M
osmium
following
resin.
in
the cells
cut
uranium
In separate
at
the
for
same
electron
acetate
and
Scientific
Corporation,
37#{176}C.These
dishes
gently
transferred
to the
to
to
perfusion
(60 mI/hr.
concen-
Lilly,
fixed
of
Sydney,
for
electron
Postfixation
was
carried
and
out
in
with
for
embedded
were
1%
I hr.
in Spurr
stained
with
2%
citrate.
plastic
Thousand
were
were
were
fragments
diameter
added
I .5% paraformaldehyde
microscopy
experiments,’marrow
in 35-mm
Chemical
been
Oncovin,
dehydrated
I % lead
frag-
formation
by
I hr.
buffer
from
of culture.
of megakaryocytes
Cells
for
cells
of isolated
(Sigma
medium
and
7.4)
in those
on process
determined
zg/mI
by
time-lapse
an hour
had
of exposure
to 0.02
were
using
them
about
gasket.
observed
number
Preparations
was
chamber
rubber
perceived
The
or 50 g/ml)
(pH
culture
was
synthesis
chamber.
buffer
The
cycloheximide
2% glutaraldehyde
which
plus 2% FCS
(Lux
a culture
the depression.
a 2-mI
easily
in the culture
0.2
over
by dislodging
the effect
from
out.
inverted
filmed
after
of the culture.
tetroxide
Sections
aqueous
more
of protein
vincristine
dissected
by a silicone
a fragment.
to which
dissolved
phosphate
in
and
and put in a 37#{176}C
room.
sometimes
were
10 g/ml
to
to fill
in
(BSS)
then
megakaryocytes
of the slide,
to determine
Response
placed
be increased
flick
buffer
were
wax
separated
from
could
a quick
with
by
x C57BL/
submerged
of marrow
that
2% FCS
and
out
was examined
medium
were
processes
migrated
with
St.
plus
processes
The
had
The
BSS
HEPES
fragments
to the slides
(BALB/c
open while
mM
coverslips
microscopy,
photography.
split
15
coverslips
of
of male
were
and
fragments
consisting
The
(FCS)
on glass
HALLER
METHODS
femurs
bones
containing
serum
placed
AND
from
1 1 wk. The
solution
were
ments
retraction
of developed
of megakanyocytes
has
particular
balanced
of all
function
was taken
aged
megakaryocytes
formation
of attenuated
processes
and thereby
envelops putative
platelets.
This implies
that a developing
platelet
would
not become
associated
with its plasma
membrane
reached.
marow
mice,
that
it is likely
that
the
from the invaginated
latter
platelets.
else the
volumes
calculation3
shows
mature
megakaryo-
half
the platelets
it produces.
While
additional
membrane
is derived
plasma
of
has
Bone
6)F,
These
not yet been forthcoming.
Thus,
it is by no means
certain
that
membrane
system
delineates
putative
developing
MATERIALS
been
AND
were
dishes
with
Oaks,
Calif.)
agitated
by
Fig.
1
.
placed
in BSS
loose fitting
and
hand
tops
incubated
at
Megakaryocyte
hourly
in
culture,
photographed
using
phase-contrast.
(a) Four hours
after
setting
up culture
and
immediately
before
exposure
to 4’C. Note extensive
development of attenuated
processes.
which have a “beaded”
appearance. Long arrow points to the
nucleus
of the cell. A detached
process
is visible
above
the
megakaryocyte
(short
arrow)
( x 200).
(b) Same
cell after
2-hr
exposure
to cold.
Processes
are
retracted
around
the nucleus
( x 300).
(c) Same
cell after
having
been
reexposed
at 37’C
for 1 hr. Processes again extended
but not
yet “beaded”
(x 200).
From www.bloodjournal.org by guest on July 31, 2017. For personal use only.
DEMARCATION
This
intervals.
free
MEMBRANE
noticeably
in solution.
attach
Those
themselves
be removed
better
removed
with
suction
showed
of the plastic
dish
through
to
the
by transferring
selectively
were
transferred
After
exposure
with
tubes
supernatent
for electron
microscopy.
and
pipette.
The
easily
cells
inevitably
of the
dish
further
were
the cells
exposed
pelleted
at 4#{176}C
were
and
kept
at
red
to the
Ross.’9
Microscopy
Cytoplasmic
processes
began
to
appear
on
some
megakaryocytes
within
2 hr of initiating
the cultures.
The
megakaryocytes
changed
from
a round
to an
elongated
shape
and then
formed
several
thick
prothat
became
Eventually,
surrounded
attenuated
cesses
further.
nucleus
several
occasionally
uated
ments,
ly,
(Fig.
nucleus
Ia).
underwent
generally
twisting
cell
attenuated
as they
lengthened
the megakaryocyte
consisted
of a
by a thin rim of cytoplasm
and
processes
of varying
lengths
and
branched
processes
in a lateral
was
evident.
were
also
Both
thick
continual
sometimes
atten-
irregular
movebut occasional-
direction,
Slight
and
translocations
seen.
considerably
they
had
retracted
sg/ml
required
of Megakaryocytes
was
difficulty
experienced
retracted
them
as a result
in prepan-
for electron
microsand particularly
if
of exposure
to cold,
because
of the lability
of these cells. Poor adhesion
of
megakaryocytes
under
culture
conditions
has been
descni bed
20 Megaka ryocytes
were
usually
retained
during
preparation
for electron
microscopy
if
they
were
located
within
a marrow
fragment.
Although
the protrusion
of processes
from a fragment
was easily seen under
the light microscope,
it was often
RESULTS
Light
were
ing megakanyocytes
on covenslips
copy ifthey
had developed
processes
fixed
and fixation.
Ruthenium
was carried
out according
processes
Considerable
and
experimentation.
and
zg/ml,
after 30 mm, while a dose level of0.05
70 mm to achieve
the same effect.
Ultrastructure
latter
megakaryocytes
preparations
Cells
to
Other
were
to a new culture
removed,
this temperature
during
centrifugation
was included
in the fixative,
which
and could
preparation
for
of 0.2
no tendency
some
a pipette.
or to vincristine,
the
of Wight
but
a purer
to centrifuge
to cold,
by centrifugation,
method
a Pasteur
plastic
the aspirate
harvesting
of megakaryocytes
processes
the megakaryocytes;
was obtained
processed
the number
formed
to the surface
adhesion
again
215
increased
that
by gentle
showed
SYSTEM
difficult
cells.
tural
to locate
the
To circumvent
observations
enriched
into
preparations
pellets
prior
nucleus
within
the
mass
of other
these difficulties,
most ultrastrucwere
made
on megakanyocytes
that
had
been
centrifuged
in
down
to fixation.
Sections
through
pellets
prepared
from
cells
that
had
been
incubated
at 37#{176}Ccontained
numerous
megakaryocyte
processes
of about
I-I .5 tm diameter
(Fig. 2). The processes
contained
microtubules,
which
of the
The
attenuated
processes
showed
constrictions
at intervals
along
their
length;
from the first signs ofelongation
to the development of these “beaded”-.looking
processes
took I -2 hr.
Process
formation
also occurred
in BSS alone,
but the
“beads”
appeared
to be more
uniform
in size when
BSS
plus
2% FCS
was
used.
Occasionally,
short
lengths
of “beaded”
cultures.
processes
However,
platelets
was not
filmed
by time-lapse
were
fragmentation
found
of
detached
(Fig.
1b).
observed,
although
some cells were
photography
for up to 24 hr.
After
tune of 37#{176}C,
movement
within
2-3 mm.
They
even
more,
rounded
reextend,
sometimes
cell,
and
Ic) although
to develop.
cesses
following
longest
minority
reexposing
cells
of processes
could
initially
appeared
merging
the
Cells
exposure
2 hr
pro-
.‘;,
to a tempera-
to form
they
was
:::
I
be detected
to shorten
completely
but within
about
30 mm,
by 1 hr, process
formation
(Fig.
longer
..
into
Transfer
of slides
to a cold room
(4#{176}C)for
resulted
in marked
retraction
of megakaryocyte
cesses
in
processes
a
began
to
extensive
.
“beaded”
appearance
took
were seen to redevelop
proat 4#{176}C
for up to I 8 hr, the
interval
tested,
but after
were capable
of doing so.
Exposure
of megakaryocytes
resulted
in reversal
of process
this
time
to vincnistine
formation.
With
only
also
a dose
a
‘si#{149}
Fig.
kept
2.
at 37’C
Attenuated
(7 hr after
processes
in pellet
harvested
start of culture)
( x 3360).
from
cells
From www.bloodjournal.org by guest on July 31, 2017. For personal use only.
216
RADLEY
AND
HAILER
3
Fig. 3.
Attenuated
process
of megakaryocyte
were
orientated
longitudinally,
elles
associated
with
the processes
generally
there was relatively
little
In contrast,
exposure
to
processes
cold
for
as well
platelets
(Fig.
appeared
invaginated
fixed
as other
3). The
to
on coverslip.
organ-
surface
be smooth,
membrane.
of megakaryocytes
2 hr were
several
Microtubules
(arrows)
membrane
of
and
fixed after
microns
in
diameter
(Fig.
4). They
had a very irregular
surface
due to the formation
of blebs and villi, the interior
of
which
were amorphous
in appearance.
An extensive
‘I
are orientated
system,
generally
cistennae
that were shown
lular space
by ruthenium
processes.
Formation
be pant
form
to be patent
red staining,
of the mechanism
of retraction.
of dilated
to the extracelpermeated
the
Microtubules
were not present
of the irregular
surface
does
the
surface
lost
In all preparations,
of varying
stages of
cells,
.)
in the
( x 23.000).
in these cells.
not appear
to
The
surfaces
of
megakanyocyte
processes
that
had undergone
retraction as a result
of exposure
to vincnistine
were smooth
(Fig.
5).
Such
megakaryocytes
also
displayed
an
extensive
demarcation
membrane
system.
After
coldtreated
megakaryocytes
were reincubated
at 37#{176}C
for
30 mm,
:O
longitudinally
the
demarcation
its roughness
(Fig.
6).
large
rounded
megakanyocytes
maturation
were
seen.
In these
membrane
system
was
not
0
S
!.
:4e
0
0
‘9’,.
4
‘.
4
‘fl
Fig. 4.
Section
from cells pelleted
after 2 hr at 4’C (7 hr after
start of culture).
Processes
are thicker
than in Fig. 2, and the
surface
is highly irregular.
The demarcation
membrane
system
is
extensive
( x 3360).
Fig. 5.
Cross-section
of retracted
process
of a megakaryocyte
from a pellet of cells prepared
after 2-hr exposure
to 0.2 sg/ml
vincristine.
Cells were kept at 37’C throughout
the culture
period
of 7 hr. Ruthenium
red staining
demonstrates
the extensive
demarcation
membrane
system.
Cysternae
of the latter are generally
dilated.
Narrow
channels
to the surface
are also evident
(arrows).
Microtubules
were absent
in these cells ( x 4800).
From www.bloodjournal.org by guest on July 31, 2017. For personal use only.
DEMARCATION
MEMBRANE
SYSTEM
217
the
microtubules,
although
additional
factors
may
be
21
The
of
reversibility
utilized
to gain
become
enveloped
process
insight
into
by
formation
the
their
way
plasma
clear
from
the results
that
demarcation
membrane
system
with the retraction
of attenuated
cult
to explain
this
demarcation
system
implies
that
between
the
ing
platelet.
posed
Fig. 6.
Megakaryocyte
from
cells
pelleted
and fixed
after
exposure
for 2 hr at 4’C followed
by 0.5 hr at 37’C (fixation
7.5 hr
after
start
of culture).
Serial
sectioning
showed
the retracted
process
on the right to be part of the nucleated
cell. The cell
surface
shows
few protrusions
(compare
with Fig. 4). Ruthenium
red staining
( x 3000).
dilated.
It is thought
that
participated
in process
relative
immaturity.
Exposure
these
were
formation
cells
that
because
had
of
not
their
a
permanent
Shaklai
that
and
demarcation
It is
the basis
platelets,
cytoplasm
resulted
that
for
is
the
this
formed
of the develop-
Tavassoli,6
for example,
from
the
pro-
fission
of
opposite
membranes
of the demarcation
gress
towards
platelet
release
being
system,
promarked
by the
extent
With
extensive
model
to which
fission
had occurred.
it would
be necessary
to invoke
to account
membrane
processes.
for the
modified
for the reappearance
system during
the
such
a
fusion
of the demarcation
retraction
of attenuated
Becker
and De Bruyn,4
in seeking
to account
formation
of processes
(proplatelets),
have
the idea that the demarcation
system
delin-
eates
individual
platelets,
suggesting
instead
that
it
progressively
divides
the cytoplasm
of the developing
cell into cylindrical
processes
that are intertwined
and
compacted
about the nucleus.
study,
when megakanyocytes
to (‘ycloheximide
membrane.
association
and
been
platelets
redevelopment
of the
occurs
concomitantly
processes.
It is diffi-
finding
on
delineates
membrane
has
in which
However,
in the present
were allowed
to develop
When fragments
of bone marrow
were incubated
on
depression
slides in the presence
of cycloheximide
(10
sg/ml
or 50 zg/ml),
processes
were formed
by mega-
long processes
that were then induced
to retract,
the
resulting
ultrastructural
appearance
did not suggest
that the cytoplasm
was composed
of ravelled
up pro-
karyocytes
cess.
We
within
2-3
hr. They
processes
without
attenuated
and
beaded
in the
megakaryocytes
same
way
incubated
After
such cells
hn, the processes
returned
to a
had been exposed
to cold (4#{176}C)for 2
were retracted,
but if the cells were
temperature
of 37#{176}C,the processes
neextended
as
became
extended
cycloheximide.
by
again.
extent
membrane
The
development
vitro, described
many
until
now
given
to
of megakaryocyte
processes
in
years ago by Thi#{233}ryand Bessis’t
little
attention,
appears
to offer
considerable
potential
for studying
the mechanisms
involved
in the megakanyocyte-platelet
transition.
A
useful attribute
of the system
is that with appropriate
treatment,
process
formation
can be reversed.
The
ultnastructune
of
evagination.
DISCUSSION
and
believe
of attenuated
processes
that
develop
in
vitro is similar
to that previously
found in vivo,’6 and it
would
appear
that microtubules
have an integral
role
in their formation
and maintenance.
Process
retraction
when
exposed
to cold, or vincnistine,
is presumably
related
to the depolymerizing
effects
of these agents
on
the
present
results
are
explicable
if the
demarcation
membrane
system
is considered
not to
delineate
platelets
in the developing
megakanyocyte,
but instead,
to represent
stoned plasma
membrane,
the
which
can
Thus,
system
the reappearance
of a demarcation
on retraction
is interpreted
to be due
invagination
already
studies
cation
of
be
the
altered
plasma
by
invagination
or
membrane.
been concluded
by several
of immature
megakaryocytes,
membrane
system
originates
It
investigators,
that the
from the
has
from
demarsurface
membrane
in this way.5’8 Much
other
evidence
attests
to the similarity
of the plasma
and demarcation
membranes,
including
surface
reactivity
to various
stains,2224
freeze-fracture,6’9
and virus
budding
studies.25 While
invagination
is believed
to account
for the
of the demarcation
membrane
in young
megakaryocytes,
there is less certainty
about
its subsequent
origin
development
as cells mature.
Contact
coated
vesicles,
thought
to be derived
complex,
and
the
demarcation
between
from the
membrane
has
dense
Golgi
been
From www.bloodjournal.org by guest on July 31, 2017. For personal use only.
218
RADLEY
seen,
though
their
function
defined.3’9
“Dense
compartments,”
has
not
short
been
fully
stacks
of
paired membranes
in continuity
with the demarcation
membrane
system,
have been observed
in some megakaryocytes
and may
represent
sites
of membrane
assembly.3’5’8’9
However,
although
abundant
at the time
extension
membrane
system
is most intense,
during
early
dense
stages
cesses
may
in the
account
found
present
study,
for the entire
demarcation
cycloheximide
de novo
reformation
was
No evidence
in the
of
retracted
pro-
system.
Certainly,
experiments
in which
was present
in the cultures
indicate
that
If the membrane
cesses undergoes
on the
invagination
it is reasonable
necessary
system.
surface
of attenuated
as the processes
to postulate
they extend.
to be almost
was not
membrane
that
the
reverse
for
proretract,
occurs
The
system
after
ever,
equating
valid,
since
or
demarcation
membrane.
It has been
the open canaliculan
system
of platelets
residue
of the demarcation
membrane
platelet
liberation
the
two
there
has
membrane
is evidence
taken
place.7
systems
from
studies
been
found
demarcation
the plasma
The
that
idea
be associated
membranes.
How-
platelets
do not
become
the
proby
enveloped
systems
in platelets
on a
has
in invagination
relevant
that
been
are
has discussed
in movement
have
of
types
and
filaments
during
of cell,
protrubenances
are
a reserve
of membrane.
microvilli
formed
as a
This is seen,
for example,
during
mitosis
reseeding
of cells that are
and after
harvesting
normally
flattened
culture.3#{176}The
history
evolutionary
cyte is unknown,3’
coincidence
that
geny
have
an
nation
invaginated
an alternative
membrane.
means
at least
occur.32’33
of the
Other
than mere
and its pro-
system.
for the storage
there
is evidence
open
functions
this
This
of surplus
that
canalicular
for
and
during
megakaryo-
more
membrane
In the platelet
of part
of the
but it is probably
both the megakaryocyte
the
as
plate-
as well
how
actin
of membranes
In many
not
microfilaments
in association
with
membranes
and open canalicular
systems
membranes
of megakanyocytes
contraction.
evagisystem
system
in the
such as facilitating
the uptake
of exogenous
and providing
a conduit
for the discharge
endogenous
may not be
on human
with
be
and other
surface
means of creating
platelet,
substances
platelets
that the membranes
of the open canalicular
system
differ
in enzyme
composition
to the plasma
membrane.26
Whether
this difference
originates
in the
platelet
or the megakaryocyte
is unknown.
Possibly
the
dense compartments
may
duction
of these specialized
two
involved
It may
can
outflow
cytoplasm
of the megakaryocyte.
might
be based,
for example,
mechanisms
known.
release,
either
of
of the
provides
involve
development
association
ment
of increasingly
long and narrow
cytoplasmic
bridges
between
putative
platelets.’6
This,
and
the
acquisition
of the discoid
shape
of the platelet
upon
conceivably
the
place does not preclude
the
be an underlying
platelet
region
of the demarcation
membrane
systo become
the open canalicular
system
of
on on the dense
tubule
system.
Close
platelet
as
brane in the maturing
megakaryocyte,
we suggest
it
acquires
its membrane
only as processes
extend
and
become
attenuated.
Adjustment
of the plasma
membrane
will also be necessary
as progression
towards
platelet
release
occurs,
since
it involves
the develop-
could
until
takes
may
specialized
tem destined
the platelet,
lets.4’28 Cohen29
could
participate
Thus, rather
than envisaging
the platelet
completely
enveloped
by its plasma
mem-
inflow
of the
suggested
that
represents
the
membrane
processes
that there
HALLER
recognized.27
suggesting
invagination
redevelopment
of the
synthesis
of proteins
of a demarcation
plasma
substructure
in the
Such a substructure
they are particularly
of the demarcation
they are not found
of its development.
compartments
their
attenuated
possibility
AND
products
gress in unravelling
trolling
the flow
membranes
should
have
also
been
postulated.34
the mechanisms
between
plasma
facilitate
a better
of
Pro-
involved
in conand
invaginated
understanding
of
the role of the demarcation
membrane
in various
diseases.35
Finally,
it will be noted
that the conventional
term
“demarcation
membrane
system”
has been used in this
text. However,
it is suggested
that this is a misnomer;
“invaginated
membrane
system”
would
appear
to be
more
appropriate.
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1982 60: 213-219
The demarcation membrane system of the megakaryocyte: a misnomer?
JM Radley and CJ Haller
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