fine structure of the octopus retina

15 Pages·2003·1.65 MB·English

FINE STRUCTURE OF THE OCTOPUS RETINA
TOSHIYUKI YAMAMOTO, M.D., KYOJI TASAKI, M.D., YOSHIO
SUGAWARA, and AKIRA TONOSAKI, M.D.
From the Department fo Anatomy and the Department fo Physiology, Tohoku University loohcS fo
Medicine, Sendai, Japan
ABSTRACT
The fine structure of the visual and the supporting cells and of the blood capillaries in the
octopus retina is described. Lamellatcd structures contained in the proximal segment of
the visual cell consist of compact arrays of dense membranes each of which is quintuple-
layered and divides at its margins into two thinner sheets or membranes which are connected
directly with the agranular or granular cndoplasmic reticulum. Proximal to the deeper
extremitics of the rhabdomcres, the lateral plasma membranes of two adjoining visual cells
contact each other forming a quintuple-layered compound membrane, which results in
occlusion of the intercellular space. The central layer of the compound membrane is of high
density, os that thc membrane, as a whole, appears to be a single thick layer at low magnifi-
cations. The supporting cells are connccted with the neighboring visual cells by two typcs
of junctions. Long slender processes extend from the supporting cells to the surface of the
retina through narrow spaccs among the distal segments of the visual cells. The capillary
cndothclial cclls are charactcrized by luminal surfaces irregularly contoured and by lateral
surfaccs clahorately interdigitatcd. The functional significancc of the close contact between
adjoining visual cclls is discussed.
INTRODUCTION
The cephalopod retina, which develops from an Other neuronal elements, such as bipolar or
ectodermal invagination in the head region, is in ganglion cells, are not present in the retina of the
some respects remarkably different in construction cephalopod. Because of such structural simplicity
from the vertebrate retina. As revealed by light and the superficial arrangement of the rhabdomes,
microscope observations (6, 22, 28), the pigmented the cephalopod retina has been one of the most
visual cell is comprised of both a proximal and a favorable materials used not only for studying the
distal segment. The nucleus of the cell is situated comparative physiology of the retina but also for
in the former, and rhabdomeres are carried by the the investigation of the photoreceptive mechanism
latter on its two opposite lateral surfaces. Four in primary sensory cells (4, ,7 ,71 23-25).
rhabdomeres from four different cells combine to The recent studies on the retinal ultrastructure
form a prismatic rhabdome, the long axis of which in the cephalopoda by Wolken (26), Moody and
points towards the incident light. From the Robertson (14), and Moody and Parriss (13) have
infranuclear portions of the proximal segments demonstrated that the rhabdomeres consist of
long axonal fibers arise and dendritic collaterals closely packed hexagonal arrays of tubules, 300
spread into the plexiform or deepest layer of the to 1000 A in diameter, arranged approximately
retina. The optic nerve is composed of these axonal perpendicular to the light path. In these investiga-
fibers originating directly from the visual cells. tions, however, attention has been focused ex-
543
clusively on the rhabdomes, and only little one third of the distance from the proximal surface
description has been presented of the fine structure to the distal surface of the retina. The visual cells
of the other retinal elements. The presence of the are long and slender and carry rhabdomeres on
lamellated systems of membranes in the proximal the two opposite lateral surfaces of their distal
segments of visual cells was reported by Zonana segments. Proximally the rhabdomeres do not
(29) and Nishioka, Hagadorn, and Bern (15). reach the basement membrane but end a short
Zonana also made observations on the limiting distance from it, The innermost portion of the
membrane of pigment granules, the fibers in the distal segment devoid of the rhabdomere was called
plexiform layer, and the structure of the rhabdo- by Young (28) the spindle region or basal spindle,
mere. and the pigment granules are densely distributed
The present paper will treat the submicroscopic in it.
structure of the visual and the supporting cells as On the distal surface of the basement membrane,
well as of the blood capillaries in the octopus retina. heavily pigmented supporting cells are placed in a
The fine structure of the lamellated systems of single layer and are separated from one another
membranes and especially the specialized inter- by the basal spindles. Each spindle region of the
connection between adjacent visual cells will be distal segment is connected, through a small
discussed in particular detail in connection with opening in the basement membrane, with a
the lateral spread of the retinal potential which proximal segment containing the nucleus at its
has been found by Tasaki, Norton, and Fukada center. A network of blood capillaries lies imme-
(23). diately proximal to the basement membrane.
THE VISUAL CELLS: The nuclei of the
MATERIALS AND METHODS
visual cells are nearly oval in shape but of slightly
Adult octopuses, Octopus vulgaris, were obtained uneven contour (Figs. 2 and 7). They have a few
locally during the period from October 1963 to distinct nucleoli, and chromatin substance is
January 1964. Eyes were removed in a lighted room, accumulated over the inner surface of the nuclear
immediately after decapitation, and their posterior envelope and around the nucleoli. In the cytoplasm
walls with the retinal layer were dissected out and
of the proximal segment there are numerous
cut into small pieces. The specimens were fixed for 2
mitochondria, a small amount of granular endo-
hours in ice cold 1 per cent osmium tetroxide solution
plasmic reticulum, free ribosomes, and multi-
(adjusted to pH 7.3 with sodium phosphate buffer)
vesicular bodies. The Golgi complex of this cell
with glucose added to it (12). After fixation, the
specimens were dehydrated in a series of increasing has a supranuclear position. The plasma mem-
concentrations of ethanol and embedded in Epon 812 branes on the lateral surfaces of adjoining cells
(10). face each other across a thin intervening space,
Sections were cut on a Porter-Blum microtome with which varies in width (usually in the range of 100
a glass knife and stained either with Millonig's lead to 150 A) from place to place because of the
solution 1( l) for 51 to 20 minutes or with 3 per cent meandering coupes of the apposed plasma mem-
aqueous solution of uranyl acetate for 3 hours. A
branes.
Hitachi HS-6 electron microscope was used for the
A few profiles of glial fibers appear among the
observations.
For a survey of the tissue with a light microscope, proximal segments (Fig. 2). They have a cytoplasm
thick sections 1( to 2 ~) were cut from Epon blocks of low density with only rudimentary vesicular
and stained with 1 per cent toluidine blue in 1 per and filamentous structures. As illustrated in Fig.
cent aqueous solution of borax. Moreover, paraffin
14, these glial fibers increase considerably in both
sections, 5 # in thickness, of specimens fixed in I0 per
number and thickness as they approach the
cent neutral formol were also used after they had been
stained with hematoxylin-eosin. basement membrane. No specialized intercellular
junction has been observed between two adjoining
OBSERVATIONS
proximal segments, nor between the glial fiber and
The diagram of Fig. l, showing the general features the proximal segment.
of the octopus retina, was drawn on the basis of One of the most striking features of the proximal
our findings in both light and electron microscopy. segments is the presence of myelin-like lamellated
A distinct basement membrane, which runs systems of membranes. These structures extend
parallel to the retinal surface is located at about throughout the whole length of the segments and
346 THE JOURNAL OF CELL BIOLOGY • VOLUME ~5, 1965
FmtraE 1 A schematic three-dimensional reconstruction of the octopus retina. The glial fibers and the
limiting membrane which covers the vitreal (upper in this diagram) surface of the retina are not depicted.
B, basement membrane; BS, basal spindle portion of the visual cell loaded with numerous pigment gran-
ules; ,C blood capillary; D, distal segment of the visual cell carrying rhabdomeres (R) on its lateral sur-
faces; J, joined plasma membranes between two adjoining basal spindles; N, nucleus of the visual cell; PL,
plexiform layer of the retina; Pr, proximal segment of the visual cell; PS, tip portions of processes from the
supporting cells (S).
,Or'OMAMAY TASAKI, ,ARAW~..CUS DNA TONOSA~ Fine Structure of Retina 347
FIGImE ~ A low-power electron micrograph of an oblique section through the level of visual cell nuclei
(N). The cytoplasm of the visual cell contains many mitochondria (M) and one or two lamellated bodies
(L). ,C blood capillary; ,G glial fibers. <) 8000.
frequently can be traced a little way into the widely in regions where the cytoplasmic matrix is
basal spindles and the plexiform layer. interposed between them.
At relatively low magnifications, the lamellated Unlike the myelin in vertebrate Schwann cells,
structures appear to consist of compactly layered these lamellated structures are not connected
dense membranes which number in some places with the plasma membrane. Each dense membrane
only few but more commonly up to fifty (Figs. 3 divides at its margin into two thinner sheets
to 5). At the level of the nucleus, which in most or membranes which limit a small vesicle or a
cells is covered with only a small quantity of the somewhat irregular cistern (Figs. 3 and 4). These
cytoplasm, the membranes in these lamellated cistern-limiting membranes are often observed to
structures tend to decrease in number and often carry ribosomes on their outer surfaces and to
disappear. Their periodicity varies from 160 to represent a part of the granular endoplasmic
250 A in regions of relatively close packing, but reticulum. It is rare for the membranes of the
two adjoining membranes may be separated more lamellated structures to be connected with the
348 THE JOURNAL OF CELL BIOLOOY • VOLUME 0~5, 1965
ERUGIF 3 A portion of the proximal segment of the visual cell provided with myelin-like lamellated struc-
tures (L) and mitochondria (M). Each dense membrane divides, at its margin, into two thinner mem-
branes which enclose a small vesicle (a) or a somewhat irregular cistern carrying ribosomes (b). Sometimes
thinner membranes derived from two or three dense membranes enclose together a common cistern (e).
PM, apposed plasma membranes of two visual cells. <) .000,2~3
ERUGIF 4 An electron micrograph illustrating the marginal dividing of the dense membranes
in lamellated structures (L). Thinner membranes originating from the dense membranes enclose small
vesicles (a) or ribosome-carrying cisterns (b). The dense membraneisn dicated by c and d divide into two
thinner sheets or membranes which unite again into thicker membranes. PM, apposed plasma mem-
branes separating two adjoining visual cells. <) 70,000.
,OTOMAMAY ,IKASAT ,ARAWAGUS DNA IKASONOT Fine Structure of Retina 349
nuclear envelope (Fig. 5). As shown in Fig. 4, appear as wavy dark strands in Epon-embedded
sometimes the separated sheets of a dense mem- and toluidine blue-stained sections of OsO4-fixed
brane can be reapposed to form one dense mem- materials (Fig. 7), whereas in paraffin-embedded
brane again. Although the Golgi lamellae are sections of formol-fixed materials they are hardly
often seen located in parallel and close to the recognizable.
lamellated structure, no direct relation between • As shown in Fig. 8, the basal spindles of the
them has yet been found. visual cells, like the proximal segments, are pro-
vided with abundant cell organelles, though a
large number of pigment granules appear here and
the myelin-like structures disappear. These
pigment granules are spherical or ellipsoid in
shape, and encapsulated with a limiting mem-
brane. Distribution of the pigment granules is not
strictly confined to the distal segments, but a few
granules are also contained in the most distal
portions of the proximal segments.
As the apical portions of the supporting cells
taper away, the basal spindles are brought into
close contact with one another. At relatively low
magnifications, as illustrated in Figs. 8 and 12, the
plasma membranes of adjoining visual cells are
regularly observed to join together forming a
thicker membrane, about 130 A across, with
obliteration of the intercellular space. Conse-
quently, the adjoining cells are connected with
each other; the boundary between them is repre-
sented by only a single membrane. In normal
sections the joined membranes are invariably seen
extending as far as the deepest ends of the rhabdo-
meres, though temporary separations into two
apposed membranes bordering an extremely
narrow gap can occur in some places.
The left inset in Fig. 21 is a high-power view of
the contacting plasma membranes. At such high
ERUGtF 5 Lamellated structure (L) in the juxtanu- magnification, the substructure of the compound
clear cytoplasm of a visual cell. At a point indicated by membrane thus formed is observed to be like that
the arrow one of the dense membranes divides into two of the dense membranes of the lamellated struc-
sheets continuous with tlm nuclear envelope (NE). ,M tures in the proximal segments. This compound
mitochondria; N, nucleus of tile visual cell. <) 80,000. membrane is quintuple-layered: the central dense
layer represents the fused outer leaflets of the two
adjoining cell membranes, and is seen, in both
At high magnifications, the substructure of the
lead- and uranyl-stained preparations, to be more
dense membranes of the lamellated structure is
dense than the lateral dense layers. Accordingly,
disclosed. Each membrane consists of three dense
at low magnifications, the compound membrane
layers separated by two interposed light zones, and
as a whole appears to be a single thick layer.
hence is quintuple-layered (Fig. 6). The central
The rhabdomere-carrying portions of the
layer is more dense than the lateral layers and is
distal segments contain only a small amount of
formed by the fusion of the two inner leaflets of the
vesicular and filamentous structures as well as a
cytoplasmic (or unit) membranes enclosing a
vesicle or a cistern. The total thickness of the few pigment granules (Fig. 10). The rhabdomere
quintuple-layered compound membrane measures is composed of closely-packed tubules, 300 to 800
about 130 A. A in diameter. As noted by Moody and Robertson
In the light microscope, the lamellated structures (14) and Moody and Parriss (13), the membranes
350 THE JOURNAL OF CELL BIOLOGY - VOLUME ~5, 1965
of two neighboring tubules in the rhabdomere THE SUPPORTING CELLS: They are located
adhere to form a quintuple-layered compound in a single layer on the distal surface of the base-
membrane, in the same way that the plasma ment membrane. Their nuclei are somewhat
membranes of two adjoining basal spindles come irregular in shape and homogeneously granular
into contact (Fig. 9). The over-all thickness of with a small nucleolus in the middle (Fig. 14).
this membrane measures about 130 A in our The juxtanuclear cytoplasmic matrix appears
specimens. finely granular and is of moderate density. It
6 FIGtrEE High-power view of a lamellated body illustrating the quintuple-layered structure of the mem-
brane (arrows). Lead-stained. X ~5,000.
ERUGIF 7 Light micrograph of an Epon-embedded and toluidine blue-stained section through the
proximal segments of the visual cells. The lamellated bodies (L) are represented as dark wavy strands in
this specimen. ,G glial fibers; N, nuclei of the visual cells. X 4~50.
As seen in Fig. 11, at the apex of each distal contains a large number of pigment granules and
segment are distributed numerous small-sized a few organelles, such as mitochondria, granular
mitochondria and a considerable amount of endoplasmic reticulum, and Golgi complex. The
pigment granules. Here, the agranular endo- pigment granules in the supporting cells are smaller
plasmic reticulum of vesicular form is also well in size, on an average, than those in the visual cells.
developed. The rhabdomeres terminate a little Distally towards the lower extremities of the
below the upper ends of the segment~. Distal to rhabdomeres, the supporting cell bodies taper
these points, the lateral surfaces of adjacent conspicuously and a number of long slender cyto-
segments come to face each other or the "limitans plasmic processes project out from the distal
fibers" (PS, Fig. 11) may be interposed between surface of each cell (Fig. 13). These processes have
them. Here again, the plasma membranes of two a filamentous core and extend to the vitreal
adjoining distal segments show adhesion. surface of the retina through the narrow inter-
,OTOMAMAY ,~KASAT ,AaAWAGUS DNA IKASONOT Fine Structure of Retina 153
cellular spaces among the rhabdomere-carrying none has been seen among the distal segments.
distal segments (Fig. 10). The filamentous cores They form a network just proximal to the base-
are especially distinct in both root and tip portions ment membrane.
of the processes. The tip portions, called the The cytoplasm of the endothelial cells contains
limitans fibers (6, 29), are markedly swollen and a few organelles, such as mitoehondria and vesicu-
fall up the shallow hollows among the domed lar and filamentous elements, and the nucleus is
apices of the distal segments (Figs. 1 and 11). homogeneously granular. As shown in Fig. 14,
Their free surfaces are provided with closely these ceils are characterized by irregularly con-
arranged microvilli. In accordance with Zonana's toured luminal surfaces and by elaborate inter-
observation (29), the limiting or hyaloid mem- digitations of the apposed lateral surfaces of
brane covering the vitreal surface of the retina adjoining cells The intercellular space between
appears as a thick layer (3 to 5/~) of an amorphous interdigitated plasma membranes is 100 to 250 A
material (Fig. 11). across and is filled with a dense amorphous
Immediately proximal to the proximal extremity material. The whole outer surface of the cell is
of the rhabdomere (Fig. 13) or of the joined lined with a thin basement membrane; moreover,
plasma membranes of two adjoining basal spindles the luminal surface is covered with the inner
(Fig. 12), the apical portion of the supporting cell basement membrane of Hama (8).
body is connected with the neighboring visual
DISCUSSION
cell by a desmosome-like junction. The two
apposed plasma membranes are arranged nearly The presence of the myelin-like tamellated systems
parallel to each other with an intercellular space of membranes in the proximal segments of octopus
of 100 to 150 A between them. They are curved visual cells has also been pointed out by Nishioka,
slightly in most cases and measure 0.2 to 0.4 # in Hagadorn, and Bern (15). A similar type of
length. At this level, a high concentration of organized system of membranes has been observed
dense amorphous material in the subjacent cyto- by these authors in the process-bearing cells of the
plasmic matrices is characteristic. epistellar body of the same animal which may be a
The second type of intercellular junction is vestigial photoreceptive organ, and also by
immediately proximal to the above mentioned one Zonana (29) in the squid retinula cells. Zonana
(Figs. 21 and 13). Throughout the second junction, has noted that each dense membrane displays the
the apposed plasma membranes run conspicuously triple-layered unit structure about 60 A in over-
wavy courses strictly parallel to each other. The all thickness, though electron micrographic
intercellular space between the two outer leaflets evidence of it was not published in his paper.
of the plasma membranes measures about 100 A Our findings indicate that each dense mem-
and is occupied by a moderately dense amorphous brane of the lamellated structure is formed by the
material which shows faint cross-striations with close contact of two cytoplasmic (or unit) mem-
the periodicity of 150 to 200 A. A weak increase branes, such as the membranes of the granular
in density occurs in the subjacent cytoplasmic and the agranular endoplasmic reticulum and of
matrices. Since the second type junctions con- the nuclear envelope; and that it is quintuple-
sistently appear in normal sections, they are likely layered, about 130 A in over-all thickness, the
to form continuous belts surrounding the apical central dense layer representing the fused inner
portions of the supporting cells. leaflets of the membranes enclosing a vesicle or a
THE BLOOD CAPILLARIES: As described by cistern.
Young (28), blood capillaries extend distally as Some types of lamellated bodies in retinas of
far as the basement membrane of the retina and various animals have been described. For example,
FmURE 8 A perpendicular section through the basal spindle portions of visual cells.
Lateral plasma membranes of adjoining basal spindles (BS) are joined into one thicker
membrane (J) with resultant obliteration of the intercellular space, while the plasma mem-
brane of the supporting cell (S) is separated by a narrow space from that of the neighboring
basal spindles. M, mitochondria; P, pigment granules; R, rhabdomeres. X 18,000.
352 THE LANRUOJ FO CELL YGOLOIB • EMULOV ,5~ 1965
,OTOM~2M~,Y ,IKASAT ,Aa(cid:127)WAGUS DNA !KASONOT Fine Structure of Retina 353
Porter and Yamada (18) reported that the pigment by a faint intermediate line. On the other hand,
epithelial cells of the frog's retina have myeloid the central layer of the compound membrane
bodies comprised stacks of paired membranes between adjoining basal spindles is of high density,
(spacing between the pairs of membranes being of so that the membrane as a whole looks like a single
the order of 70 A) and shaped like biconvex lenses; thick layer at low magnifications. In this respect,
and that, at their margins, the membranes are con- the contrast between the two types of membrane
nected with elements of the endoplasmie reticulum. contact is distinct. Recently, the high density of
Because the paired membranes resemble, in their the central layer of the compound membrane has
thickness and spacings, those which make up the been seen by Robertson (20) in the permanganate-
rod outer segments, these authors regarded them as fixed Mauthner cell synapses of the goldfish.
intracellular photoreceptors relating to the pig- However, when OsO4 fixation had been applied,
ment migration. The lamellated bodies in the the central layer also appeared as a faint line.
octopus retina, however, are covered with the The functional significance of the quintuple-
dense pigment layer, so it seems unlikely that they layered cell int~rconnections has been discussed
are related to a photoreceptive mechanism. hitherto by a number of investigators. In the
Lamellated bodies in the pigment epithelial optic nerves of rats, mice and toads, Peters (16)
cells of both cat (1) and albino rat (2) appear to found plasma membrane contacts to occur between
be fundamentally different in structure from those either adjacent oligodendrocytes or adjacent
in the cephalopod visual cells. The former are capillary endothelial cells; between adjacent
represented as a kind of inclusion body encapsu- myelin sheaths; as well as between the myelin
lated with a limiting membrane. sheath and gila cells. He suggested that one
In the octopus retina the lateral plasma mem- function of the contacts may be to prevent sub-
branes of two adjoining basal spindles come into stances from passing between cells, so that they
contact to form a quintuple-layered compound must pass through cytoplasm.
membrane, which results in occlusion of the Gray (5) described closed contacts between
intercellular space; the central dense layer repre- glial processes of the clear type around blood
sents the merged outer leaflets of the two plasma capillaries in brain tissue. He noted that these
membranes. This type of intercellular junction is contacts are possibly situated at strategic positions
basically similar in structure to the tight junction to seal off the extracellular spaces, so that meta-
or zonula occludens in various epithelia (3). The bolites are forced to traverse glial cytoplasm.
occurrence of such areas of membrane contact has Farquhar and Palade (3) observed the tight
been observed also in some other tissues, as referred junctions in some epithelia to function as a diffusion
to in the paper of Farquhar and Palade (3). barrier or "seal" between the intercellular space
The tight junction in OsO4-fixed sections looks, and the lumen of the organ.
at relatively low magnifications, just like an area Karrer (9) speculated that the contacts between
of what appears to be an extreme narrowing of adjacent striated muscle cells in the thoracic and
the intercellular gap which sometimes is bisected the pulmonary veins of mice might be of signifi-
ERUGIF 9 A high-power electron micrograph of a cross-section through tubules of the
rhabdomere. The limiting membranes of adjoining tubules contact each other to form a
quintuple-layered compound membrane (arrows). Uranyl acetate-stained. X 186,000.
ERUGIF 01 Some processes (PS) from supporting cells run through an intercellular space
between the distal segments of visual cells. Filamentous cores are seen in the processes. P,
pigment granule in the distal segment; R, rhabdomeres. <) 15,000.
ERUGIF 11 A perpendicular section through the surface area of the retina. The tip portion
of a supporting cell process (PS) is provided with microvilli and filamentous core .)CF(
Apices of the distal segments are heavily loaded with pigment granules (P) and mito-
ehondria (M). LM, limiting membrane covering the vitreal surface of the retina; R, rhab-
domere. X 17,000.
354 THE LANnVOJ FO CE~L YOOLOIB • EMULOV ,5~ 1965