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The fine structure of a lateral recess of the subarachnoid space in the rat.

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Subarachnoid Space in the R a t
Department of Anatomy, University of North Dakota,
Grand Forks, North Dakota
This study reports an electron microscopic investigation of the
spinal meninges of the rat as they exist between the dorsal and ventral nerve
roots in upper thoracic and cervical levels of the vertebral column. Between these
roots, at a point where they penetrate the spinal dura, the leptomeninges are
characterized by a histological pattern comparable to that elsewhere comprising
the subarachnoid angle. However, this region is modified and possesses a number of heretofore unknown histological features. The most noteworthy is a lateral
recess of the subarachnoid space.
The meninges located between the nerve roots do not form a definite lateral
boundary for the subarachnoid space. Here the subarachnoid space opens into a
lateral recess which extends peripherally between the dorsal and ventral nerve
roots. Conspicuous amounts of cellular debris are collected within the lateral recess. Numerous free macrophages congregate here. The lateral recess may be a
communicating pathway between the central and peripheral nervous systems by
way of the endoneurium of the nerve trunk.
The subarachnoid angle, where nerve the dorsal root and neighboring areas reroots leave the subarachnoid space, is an main unclear in this contribution. Shanarea of transition in the coverings of pe- thaveerappa and Bourne ('62, '63, '66), in
ripheral nerve. It is suspected to be a path- a series of studies dealing with the ''periway of communication between the neural epithelium," interpreted the piasubarachnoid space and the periphery. arachnoid of the root sheath as being idenNevertheless, the histological fine structure tical with the cellular component of the
of the portion of this area between the dor- perineurium. Gamble ('64) described the
sal and ventral roots seems not to have pial sheath (root sheath) as essentially the
been reported. A preliminary survey made same as perineurium, of which he conin this laboratory (Haller and Himango, sidered it a direct central continuation.
'70) indicates that this site possesses a mod- However, the histology of the subarachnoid
ified histological pattern comparable to that angle was later shown to be more complex
found elsewhere in the subarachnoid angle than could be accounted for by simple con(McCabe and Low, '69). Accordingly its tinuity with more peripherally located
peculiar histology is described in this paper sheaths (Andres, '67a; McCabe and Low,
as a variation of the known fine structure '69; Haller and Low, '71). While the deeper
layers of the perineurium could sometimes
that characterizes this particular area.
The publications dealing with the fine be traced directly into the root sheath the
structure of the spinal meninges and re- more superficial ones separated from the
lated tissues were reviewed briefly by
Received Aug. 5, '70. Accepted Feb. 5, '71.
McCabe and Low ('69). Notably very few
1 The project was supported by Public Health Service
Training grant 5TI-GM-1014, from the National Instiof them dealt with histological patterns at tutes of General Medical Sciences. The terminal phases
the work were aided by grant NS 09363, United
the subarachnoid angle. Benke and Roh- of
States Public Health Service.
lich, in 1963, examined the dorsal and
ZNDEA Title IV National Defense Fellowship 66and trainee on Public Health Service training
ventral surfaces of the dorsal root where it 07809.1
grant 5TI-GM-1014, from the National Institute of
Medical Sciences. This work was done in parleft the subarachnoid space. However, the
fulfillment of the requirements for the degree of
identity of the cellular layers surrounding tial
Master of Science.
ANAT. REC., 171: 1-20,
nerve at the subarachnoid angle and came
to lie between the arachnoid and dura
mater. Here the cytoplasm of these cells
became electron lucent. They formed a
readily recognizable layer separated from
the arachnoid by a dense extracellular line.
In this location they were first observed by
Pease and Schultz ( ’ 5 8 ) who called them
“dural mesothelium” and later by Andres
(’67) who coined the term “neurothelium.”
Another structural feature sometimes present at the subarachnoid angle consisted of
the turn-back of the superficial layer of the
arachnoid to form the superficial layer of
the root sheath (McCabe and Low ’69).
According to Haller and Low (’71) the root
sheath, rather than being a simple continuation of the perineurium, was divisible into
deep and superficial layers. The deep layer
resembled structurally modified perineurium and the superficial layer resembled the
arachnoid or pia.
Between the exits of the dorsal and ventral roots the linings of the subarachnoid
space represent variations of histological
patterns found in the dura-arachnoid and
the root sheath. The relationships of these
modified linings to a lateral recess of the
subarachnoid space are described in this
Fixation. Seven white rats of the
Sprague-Dawley strain were used for this
study. The perfusion by buffered aldehydes
consisted of modifications of the methods
of Palay et al. (’62) and Rosen, Basom and
Gunderson (’67). First, a washout solution
containing procaine hydrochloride (0.1 % )
was administered to prevent vasoconstriction (Forssman et al., ‘67). This procedure,
lasting approximately one minute and 20
seconds, was immediately followed by a 20
minute perfusion with buffered aldehydes
prepared according to Karnovsky’s formula
Dissection. The vertebral column was
excised and the surrounding muscle was
removed to facilitate dissection. A laminectomy was performed which made it possible to insert a razor blade in a sagittal
plane through the spinal cord, thereby splitting it and the surrounding dura-arachnoid
complex into left and right halves. The vertebral bodies were cut longitudinally, thus
completing a division of the vertebral col-
umn and its contents. The nerve roots were
severed at a point adjacent to the cord
which was then removed to expose the dorsal and ventral roots as they penetrated the
dura. The dura was cut transversely into
sections so that each piece contained one
dorsal and one ventral root. The cutting
surface of a surgical needle was used to
free the dura from the surrounding connective tissue within the intervertebral foramen. Adjacent vertebrae were separated by
scissor cuts to allow the nerve trunks to be
severed. This made possible the removal of
the desired tissue which lay partly within
the intervertebral foramen. Each piece of
tissue thus obtained included a segment of
the nerve trunk, dorsal and ventral roots,
dorsal root ganglion, associated meninges
and connective tissue (fig. 1 ) .
Microscopy. The tissue was post-fixed
in cacodylate-buffered osmium tetroxide
( 2 . 0 % ) for one hour (Karnovsky, ‘65).
This was followed by alcohol and propylene
oxide dehydration, then embedding in Epon
812 in a 6 : 4 ratio (Luft, ’61). Each piece
of tissue was oriented for longitudinal sectioning of the nerve roots as they left the
subarachnoid space. Thick sections (1 p )
were stained with toluidine blue and examined in a light microscope for more precise orientation. Thin sections for electron
microscopy were stained conventionally
with lead (Venable and Coggeshall, ’65)
and uranium salts (Greenlee et al., ’66).
These were examined in a Philips EM-200
electron microscope.
Terminology. A description of the fine
structure of the linings of the subarachnoid
space involves problems of nomenclature.
Simple descriptive terms are well suited to
a primarily morphological study of this
area and are less controversial than names
implicating a particular germ layer origin
or suggesting an epithelial organization.
Hence, the electron lucent layer of cells
found between the arachnoid and the dura
is herein called the “intermediate cellular
layer” although specific names such as
“dural mesothelium” (Pease and Schultz,
’58), “neurothelium” (Andres, ’67a) and
“perineural epithelium” (in the root sheath;
Shanthaveerappa and Bourne, ’62) have
been used. In the literature the linings of
the subarachnoid space are collectively
designated “mesothelium,” (Weed, ’20)
and “mesenchymal epithelium” (Bloom and
Fawcett, ’68). Fine structure tends to confirm the mesodermal nature of these linings
but contraindicates the epithelial connotation (Frederickson and Haller, ’71). The
histological organization of epithelium in
fine structure (Farquhar and Palade, ’63,
’64, ’66) is clearly different from that of
the tissues lining the subarachnoid space.
Therefore the descriptive terms, arachnoid,
root sheath and pia are used in this paper.
to the loci in figures 9 through 12. Figure
5 shows the histological organization of the
roof and floor of the lateral recess and corresponds to figures 13 through 15.
The leptomeninges display their characteristic histology in all areas except where
the roots make their exit from the subarachnoid space. Figure 6 illustrates the morphologic features of the dura-arachnoid complex that are typical of areas removed from
the nerve exits. The arachnoid is attenuated and fairly dense with many lacunae.
Underlying it is a dense extracellular line
Although the general relationship be- of variable thickness (fig. 6). An intermeditween the dorsal and ventral roots is similar ate cellular layer that is considerably less
throughout the length of the spinal cord, dense lies between it and the dura mater.
their relationship with the meninges is The dura is made up chiefly of unit colvariable. In the sacral, lumbar and lower lagen fibrils and constitutes the tough,
thoracic portions of the vertebral column outermost meningeal covering. This histothe dorsal and ventral roots pass out of the logical pattern prevails around most of the
subarachnoid space through separate open- central nervous system.
ings in the dura mater. However, in upper
Some details of the histology of the
thoracic and cervical levels this point of arachnoid membrane are illustrated in figexit is not clearly defined. The thoracic and ures 7 and 8. The arachnoid membrane is
cervical ‘areas receive major attention in usually two or three cells thick and posthis study.
sesses small lacunae containing extracelluFigure one is a sketch of the tissues lar connective tissues (figs. 6-8). Intermitstudied as they appear when the gross dis- tent fragments of basal lamina line these
section is complete. The dorsal and ventral lacunae. The cells of the arachnoid typinerve roots penetrate the dura mater and cally contain a large number of mitochoncontinue peripherally as a spinal nerve. dria, due to the attenuation of the cells,
Ventral to the dorsal root where it pene- are closely associated with the plasmalemtrates the dura is the meningeal lip and mae (fig. 6). An irregular cleft of dense
ventral to this lip is the mouth of the lateral extracellular material intervenes between
recess of the subarachnoid space. Figure 2 the arachnoid cells and the much more luis a sketch of a vertical section through a cent cells of the intermediate cellular layer
tissue block comparable to that represented (figs. 3 , 6-8).
in figure 1. The subarachnoid space, the
The root sheath that covers the nerve
tissue space, the meningeal lip and the roots as they traverse the subarachnoid
lateral recess are represented in the same space bears superficial resemblance to both
relationship as in figure 1. The location of arachnoid and perineurim but has been
the electron micrographs constituting fig- recognized on close examination to possess
ures 6 through 15 (plates 1-6) are indi- a histological identity of its own (Haller
cated by numbers in figure 2. The differ- and Low, ’71). Typical root sheath histolences in histological fine structure found ogy is illustrated in figure 9. The number of
among these areas are described in detail cellular layers varies from 2 to 12, but the
in the following paragraphs.
sheath is usually three to four layers thick.
Figures 3 through 5 are simplified The outer layers are loosely arranged but
sketches of the fine structure of critical the inner ones are compact. A basal lamina
areas examined in this study, as depicted separates the root sheath from the endoat lower magnification in figure 2. Figure 3 neurium. Connective tissue is found in the
represents a typical dura-arachnoid com- lacunae between cell layers but the lacunae
plex in the area ilhstrated by figures 6 are not as discretely organized as in the
through 8. Figure 4 represents the area arachnoid membrane. As the root sheath is
around the meningeal lip and corresponds traced along the ventral surface of the
Fig. 1 Gross representation of dorsal root, ventral Toot and dura-arachnoid complex. This sketch
represents the tissuc block as it appears after the gross dissection. The subarachnoid space lies to
the left of the dura-arachnoid complex and the tissue space to the right. Between the dorsal root
along the top of the diagram and ventral root along the bottom are found the meningeal lip and
the mouth of the lateral recess. Figure 2 represents a longitudinal section through a comparable
dorsal root and approaches the meningeal
lip, it displays histological changes that
transform it into an organization that is essentially indistinguishable from the duraarachnoid complex.
Figures 10 and 11 illustrate the modification of the root sheath on the ventral surface of the dorsal root as the meningeal
lip is approached. The cellular layers assume an irregular, “piled up” appearance
with numerous lacunae containing connective tissue. The surface of the meningeal lip is made up of cells of the arachnoid membrane that enclose lacunae filled
with unit collagen fibrils (fig. 12). An
intermediate cellular layer separates these
surf ace cells from the underlying collagenous connective tissues. The dense extr acel-
lular cleft is poorly developed or absent.
The organization of the underlying connective tissues closely resembles that of
the dura mater. As the meningeal lip is
traced laterally along the roof of the lateral
recess, its histological organization becomes indistinguishable from the duraarachnoid complex that largely surrounds
the subarachnoid space.
The meningeal lip is separated from the
dorsal surface of the ventral root by a portion of the subarachnoid space that extends
peripherally underneath the dorsal root
ganglion as the lateral recess (figs. 2,4, 12).
This extension of the subarachnoid space
is clearly defined in its medial portion (fig.
1 3 ) but does not seem to have a definite
lateral termination. It does not continue
Fig. 2 Longitudinal section of dorsal and ventral roots at their point of exit from the subarachnoicl space. This drawing represents a section through a block comparable to that depicted in figure
1 and is represented in the same plane. The numbers indicate selected fields which are presented
by electron micrographs i n figures 6 to 15. Numbers 6 to 8 represent the dura-arachnoid complex.
Numbers 9 to 11 represent the root sheath of the dorsal root near the meningeal lip (number 12).
Numbers 13 to 15 identify with the lateral recess and its surface linings.
laterally as an uninterrupted channel in
the plane of any section. Instead it forms
a maze of lacunae bounded by irregular
and intermittent areas of cellular contact.
The latter seem to be incidental rather than
accompanied by junctional complexes.
This portion of the lateral recess is filled
with a conglomeration of cellular debris
(figs. 5, 14). Free macrophages characterize the extent of the lateral recess (fig. 15).
These cells possess large, dense, usually
oval inclusions suggesting phagocytosed
material. Although the lateralmost extent
of the lateral recess cannot be determined
by simple morphological examination, the
recess clearly underlies the full length of
the dorsal root ganglion (fig. 2).
This investigation describes the special
histological construction of the lining of the
subarachnoid space between the dorsal and
ventral roots in the cervical region of white
rats. Starting from a point on the ventral
surface of the dorsal root close to its point
of emergence from the subarachnoid space
and tracing laterally the root sheath terminates in a loose tangle of cells that projects
into the subarachnoid space (figs. 9-11).
These cells form Iacunae. This structural
arrangement extends into a compact mass
of tissue herein called the meningeal lip
(figs. 2, 4, 12). The individual layers of
this lip suggest the construction of a duraarachnoid complex such as that which lines
the greater part of the central nervous system (fig. 12). Traced farther laterally the
lip becomes continuous with a typical duraarachnoid complex (figs. 5, 13). Ventral to
the meningeal lip is the mouth of the lateral recess of the subarachnoid space which
underlies the base of the dorsal root ganglion (fig. 2). The lateral recess does not
have a definable lateral extent, but instead
becomes compartmentalized by contacts,
apparently incidental rather than junctional, between cells of the dura-arachnoid
above and the ventral root sheath of the
ventral root below.
It is now evident that the histological
organization of the dura-arachnoid complex (arachnoid, intermediate cellular
layer, dura; figs. 3, 6-8) differs from that
of the root sheath, which consists chiefly of
multiple layers of attenuated cells (fig. 9).
However, there are points of continuity between the two where nerve roots leave the
subarachnoid space. The situations at the
dorsal surface of the dorsal root and the
ventral surface of the ventral root are described and diagrammed in a previous publication (McCabe and Low, '69; their figs.
18, 19). A similar transition occurs from
root sheath to dura-arachnoid complex at
the meningeal lip where the suabarachnoid
space extends laterally into the lateral recess (figs. 2, 4). Here the superficial layers
of the root sheath extend laterally as the
arachnoid membrane which forms the roof
of the lateral recess. At the point of transition a heavy concentration of unit collagen
fibrils forms the bulk of the meningeal lip.
This lip is separated from its arachnoid
covering by thinly attenuated cells that are
indistinguishable from the intermediate
cellular layer of a typical dura-arachnoid
complex (figs. 4, 12). The floor of the
lateral recess is an uninterrupted continuFig. 3 The dura-archnoid complex. This sketch
represents a more detailed view of the area illustrated in figuies 6 to 8 and ( a t lower power) in
figure 2 . The subarachnoid space (SS) is lined
by arachnoid membrane (AM) and more deeply
by a n intermediate cellular layer (ICL). A dense
intercellular line ( D ) of a very variable thickness
separates the two. Above is the dura mater (DM)
which consists chiefly of unit collagen fibrils,
here represented in cross section. Compare with
figures 6 to 8.
Fig. 4 The root sheath and the meningeal
This sketch has been simplified to emphasize relationships among basic layers. Endoneurium (EN) is above. To the left the root
sheath (RS) is multilayered and approaches the
meningeal lip (ML). The loose arrangement of
the arachnoid that is illustrated i n figures 10 and
11 has been largely omitted. The bulk of the
meningeal lip is made up of unit collagen fibrils.
The surface layers of the meningeal lip are
formed by arachnoid membrane ( A M ) and intermediate cellular layer (ICL) depicted as i n figure
3 (except for the dense lin- which is very poorly
developed i n this area). The main portion of the
subarachnoid space ( S S ) is below and to the left
while the lateral recess (LR) is below and to the
right. Compare with figures 9 to 12.
Fig. 5 The lateral recess. The lateral recess
is bounded above by a dura-arachnoid complex,
here represented by arachnoid membrane (AM)
and intermediate cellular layer (ICL). These are
essentially the same as elsewhere i n this complex.
Below is the multi-layered root sheath (RS) of
the ventral root. Particulate material ( P M ) is
found i n the lateral recess ( L R ) . The macrophage
( M ) in the lateral recess a t the left is a common characteristic of this area. Compare with
figures 13 to 15.
Figures 3-5
ation of the root sheath of the ventral root
(figs. 4, 12, 1 3 ) and presents no special
morphological features.
A number of observations made in the
course of this investigation may merit some
speculation as to their significance. The
thin but distinct layer of electron lucent
cells, herein called the intermediate cellular layer, is a nearly constant feature of the
dura-arachnoid complex. McCabe and Low
('69) describe it at the subarachnoid angle,
where i t is continuous with the perineuriurn
of peripheral nerve. However, it does not
account for all of the perineurial layers
(McCabe and Low, '69; their fig. 19). If
this layer is connective tissue i t is an unusual sort because of its cellular coherence
and lack of open interstitial space. The
dense intercellular cleft existing between
the intermediate cellular layer and the
deepest layer of arachnoid appears in the
preparations of Pease and Schultz ( ' 5 8 ) ,
Andres ('67a) and McCabe and Low ('69).
It's thickness is highly variable (figs. 3 , 5,
7, 8). It is filled with an electron dense
material that clearly differs from the usual
basal lamina. It's significance is unknown
but its discrete localization indicates that
both arachnoid and the intermediate cellular layer keep i t enclosed in a restricted
area. It may be interpreted as an unusual
expression of extracellular connective tissue.
The general morphology of the lateral
recess and the presence of macrophages in
this area are more provocative of speculation since communication between the subarachnoid space and the tissue space (endoneurial) of peripheral nerves has been
suspected to exist for nearly a century (Key
and Retzius, 1876; Elman, '23; Weed, '38).
Yet physical evidence for such a pathway
seems to have been lacking. The limitations inherent in the process of tissue fixation prior to the use of balanced buffered
solutions, together with the limited resolving power of the light microscope, make it
unlikely that the lateral recess could have
been distinguished from an artifact of preparation even if it had been visualized prior
to electron microscopy. But the possibility
of artifactitious origin is eliminated by the
ability to preserve intact the roof and floor
of the recess itself, the former being dura-
arachnoid and the latter root sheath. Although cellular debris is plentiful in the
lateral recess there is no sign of forcible
separation of roof and floor. Debris, although common here, is not generally encountered in the subarachnoid space. This
is consistent with the suspicion that there
is slow lateral flow of the cerebrospinal
fluid toward the "catch basin" that would
exist where the roof and floor of the lateral
recess come into contact (fig. 14). The
presence of macrophages in this area, in
spite of their virtual absence elsewhere in
tht subarachnoid space, is also consonant
with the idea of communication between
the peripheral endoneurium and the subarachnoid space. As toxins pass along the
peripheral nerve from the periphery, the
lateral recess would represent the first entry
of these substances into the subarachnoid
space (Frederickson and Haller, '71). It is
here that macrophages guard against septic
conditions reaching the central nervous
system. Of interest in this connection is
Essick's ('20) observation that the lining
cells of the subtrachnoid space turn into
macrophages in the presence of foreign
particulate material. Although admittedly
speculative, physiological communication
between endoneurim and subarachnoid
space is suggested by both debris and macrophages in the area of the lateral recess.
The connective tissue coverings associated with nervous tissue in the unique area
of the body covered by this report may be
interpreted under three distinct morphological headings ;( 1) dura-arachnoid complex, ( 2 ) root sheath and ( 3 ) peripheral
nerve sheaths. The dura-arachnoid complex may be understood to cover the central
nervous system coextensively with the dura
mater. The root sheath covers peripheral
nerve roots in their traverse of the subarachnoid space. The peripheral nerve
sheaths; epineurium, perineurium and endoneurium envelop peripheral nerves without essential variation until the subarachnoid space is encountered centrally. The
critical area of change and relationships
among these three entities occurs at the
subarachnoid angle, as described by
McCabe and Low ('69). The lateral recess
of the subarachnoid space constitutes an
interesting variation of the relationships of
these histological entities to each other.
The authors wish to thank Dr. Frederick
R. Haller, Miss Carol Soutor, Miss Elvina
Rolette and Miss Kristine McDonald for
their valuable assistance during the course
of this work.
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6 Dura-arachnoid complex. The subarachnoid space (SS) is bordered
by the arachnoid membrane (AM). Some lacunae (L) are present
between cells of the arachnoid. A dense line of variable thickness ( D )
lies between the arachnoid membrane and the intermediate cellular
layer (ICL). The dura mater (DM) is made up chiefly of densely
packed unit collagen fibrils. x 23,000.
7 Arachnoid membrane, dense line and intermediate cellular layer. An
extracellular dense line of irregular thickness ( D ) separates the
arachnoid membrane (AM) from the underlying intermediate cellulzir
layer (ICL). x 22,000.
Typical arachnoid membrane with lacunae and mitochondria. The
arachnoid membrane (AM) typically contains large numbers of
lacunae ( L ) filled with connective tissue components. This micrograph shows the large number of mitochondria ( M ) which also typify
the arachnoid membrane. A n irregular dense line, somewhat less well
developed than in figure 7, is indicated by the tip of the marker on
the right. It separates the arachnoid membrane from the intermediate
cellular layer. x 18,000.
William A. Himango and Frank N. Low
Typical root sheath. The nerve root sheath (RS) i s generally three
to four layers thick with the outermost layers loosely arranged. The
root sheath contains a less orderly arrangement of lacunae (L) t h a n
does the arachnoid membrane. It is separated from the endoneurium
(E) by a basal lamina (basement membrane; BM). A myelineated
nerve ( M N ) occupies much of the field. X 35,000.
Modified dorsal root sheath. The region of the ventral surface of
the dorsal root close to the meningeal lip (fig. 2) consists of loosely
arranged cells which reach into the subarachnoid space (SS). Lacunae
( L ) and mitochondria ( M ) are numerous. The nuclei ( N ) are
rounded or irregular in shape and are not characteristic of typical
root sheath or arachnoid membrane. x 12,000.
Lateral limits of dorsal root sheath. Modified tissue as its exists o n
the ventral surface of the dorsal root contains loosely arranged cells
which extend into the subarachnoid space (SS). At its most lateral
limits it is underlaid by a n intermediate cellular layer (ICL) and
dura mater (DM). Nuclei ( N ) in this region are irregularly lobulated.
x 11,000.
William A. Himango and Frank N. Low
12 Meningeal lip and edge of ventral root. The surface tissue of the
meningeal lip consists of a n arachnoid membrane (AM) with
numerous lacunae (L). Separating the arachnoid membrane from the
dura (DM) is an intermediate cellular layer (ICL). The meningeal
lip projects toward the root sheath (RS) of the ventral root ( V R ) . The
portion of the subarachnoid space between the meningeal lip and the
ventral root sheath (fig. 2 ) forms the mouth ( M R ) of the lateral
recess (LR). The greater part of the subarachnoid space is below and
to the left of the figure. X 17,000.
William A. Himango and Frank N. Low
13 Lateral recess. This micrograph of the medial portion of the lateral
recess (LR) represents a field immediately peripheral to figure 12.
The lateral recess here shows a peripheral extension (PE). Both are
roofed over by a n arachnoid membrane ( A M ) and a n intermediate
cellular layer (ICL). The floor of the lateral recess is formed by the
root sheath (RS) of the dorsal surface of the ventral root. The lining
layers of both roof and noor possess characteristic lacunae. Foreign
particulate matter ( P M ) is characteristic of the lateral recess.
x 12,000.
Peripheral extent of lateral recess. This more lateral portion of the
lateral recess (LR) is cluttered with particulate matter ( P M ) that is
not found elsewhere i n the areas examined. The arachnoid membrane
( A M ) forms the upper border of the lateral recess while the root
sheath of the ventral root (RS) is below. X 32,000.
William A. Himango and Frank N. Low
Free macrophage. This free macrophage (FM) is located within the
lateral recess (LR). It contains a typical lobulated nucleus ( N ) and
numerous dense inclusions. Cells of this sort are frequent and conspicuous here but are rare in other areas examined. x 21,000.
William A. Himanga and Frank N. Law
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structure, space, lateral, subarachnoid, rat, fine, recess
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