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Nerve fibers in the area postrema of cat rabbit guinea pig and rat.

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NERVE FIBERS IN THE AREA POSTREMA O F
CAT, RABBIT, GUINEA P I G AXD RAT'
C. D. C L E M E N T E 2 AND V. L. VAN BREEMEN
Ilepartments o f Anatomy, Univrrsity College London, England, and University of
Colorado School of Medacine, Denver, Colorado
FIFTEEN FIGURES
INTRODUCTION
The area postrerna is a highly vascular but small region of
the brain stem situated at the level of the obex in the floor
of the fourth ventricle. This area can easily be identified from
the surrounding rricdulla and is composed principally of blood
vessels and parerichynial cells whose cytoplasmic processes
attach to vascular walls. It has been said that in the human
neurons exist in the area postrema (Wilson, '06 ; Cammerrrieyer, '47), and Cammermeyer felt that tliese neurons contained a "rnclanin-like pigment." King ( '371, on the other
hand, concluded that neurons did not exist in the area postrerria of adult cats, stating further that nerve fibers were
only rarely seen arid that the area postrema did not haw
neural connections with other regions in blie brain.
It mas first sl10~.\7n
by Wislocki and Putnam ( '20, '24) that
tlie area postrenia bccairie vitally stained after the systemic
administration of trypan blue of Prussian blue dyes. I n this
respect this region differed froin most other areas of the
brain but was similar to the supraoptic crest, the neurohypophysis and the hypophysial stalk, the intercolumnar
tubci.cle (Putnani, '22) o r subforriical organ (Pines, '26) and
A preliminary report of this work was presented before the Physiological
Society of Great Rritan, March 26, 1954. J. Ph)siol., 19'4, 23 P.
2 Bank of America Giannini Foundation Fellow, Present Address : Department
of Anatomy, University of California at Lo8 Aiigeles School of Medirine, Los
Arigeles 24, California.
(i3
66
C. D. CLEMENTE AND V. L. VAN BBEEMEN
the pineal body. This property of selective permeability in
the central nervous system has led to a general acceptance of
the existence of a blood-brain barrier.
Although the function of the area postrema has not been
established, it is of interest to note that Ranson and Killingsly
( ’16) recorded marked and immediate depressions of blood
pressure upon direct faradic stirnulation of this area in cats.
On the other hand, stimulation only 3 mm away from the area
postrema ( a t the apex of the ala cinerea or the fovea inferior)
resulted in a marked elevation of blood pressure. King ( ’37)’
claiming that nerve fibers were only rarely seen in the region,
was “certain that the effect on the blood pressure obtained in
cats by Ranson and Billingsly through stimulation of the
area postrema could not have resulted from ncxi-ve irnpnlses
arising in this area.” Cammermeyer ( ’47) favored the view
that the cellular elements of the area postrema were neurosecretory in nature while Wislocki and Leduc ( ’52) suggested
that the area postrema represented a “thickened specially
differentiated portion of the subeperidymal plate, ” and agreed
with King that the predominant cells were related to ependymal spongioblasts.
Recently it has been repostulated that a central coordinating region for emesis exists in the dorsal medulla (Borison
and Wang, ’49; Wang and Borison, ’50). From elect&al
stimulation and ablation studies, it has been concluded that
the “vomiting center” is located at the dorsolateral border
of the lateral reticular formation, including the tractus solitarius and its nucleus. It was also felt that the area postrema
may be associated with a “chemotrigger mechanism” initiating the vomiting response (Borison and Brizzee, ’51; Brizzee
and Borison, ’53).
Most interpretations on the functional significance of the
area postrema have been suggestive of some neural niechanism, and yet the existelice of nerve fibers in the region has
not been thoroughly established. For this reason the areae
postremae of several species of animals have been studicd
NERVE FIBERS I N THE AREA POSTREMA
67
by silver staining methods, and the present comniunication
reports the anatomical findings.
MATERIALS AND METHODS
Histological observations were made on adult cats, rabbits,
guinea pigs and rats. Animals were sacrificed by perfusion
with physiological saline followed by 10% formalin according
to tlie method of Koenig, Groat and Windle ('45). After
perfusion, the brain stem containing the intact area postrema
was dissected, and further fixation was carried out in different
ways. Some tissues were allowed t o remain in formol-saline
f o r 48 hours, while others were placed in absolute ethanol o r
Carnoy's fixative for 6 to 12 hours. Tissues were embedded
in paraffin and cut serially a t 8 p. General observations such
as location, size and relationships of the area postrema were
made o n sections stained with hematoxylin and eosin, while
a modification of the Holmes' ('42) slide silver technique was
used for nerve fiber staining. The Holmes' technique gave
the finest results in preparations perfused with formol-saline
and hardened in Carnoy 's fixative. The modification (suggested by I<.TI;. Guillery ) involved heating the hydroquinonesodium sulfite reducing solution to about 50°C. The reducing
process required two t o three minutes. Other cytological
stains used in this study included the buffered thionin technique ( Windle, Rhines and Rankin, '43) and the iron heniatoxylin nuclear stain.
OBSERVATIOPU'S
Distinguishable from the surrounding brain stein by its
rich vascularity and loose structure, the area postrema in the
cat is located in a slightly different position from that in the
rabbit, guinea pig and rat. I n the cat the area postrema is
bilateral and projects as two small symmptrical masses of
tissue from the ventrolateral walls into the fourth ventricle,
just rostra1 to the obex in the medulla (fig. 2). It varies in
length from approximately 1.3 to 1.6 mm and, continuing
68
C. D. CLEMENTE AND V. L. VAN B R E E M E S
caudally to the obex, the two “horns” of tissne converge at
the midline forming a V-shaped organ (fig. 1).
I n the rabbit, guinea pig and rat the area postrema is not
bilateral, but is situated in the midline and forms the dorsal
wall over the central canal just caudal to the extent of the
fourth ventricle (figs. 1, 3, 7). In this position it overlies the
alae cinereae and is bounded by the hypoglossal nuclei, the
dorsal nuclei of the vagal nerves and the nuclei of the solitary
tracts. I n the rabbit the area postrema is about 0.6 to 0.8 mm
in length, whereas in the guinea pig and the adult rat it
extends for approximately 0.5 mm.
As the ependymal lining of the fourth veiitricle coritiiiues
over the area postrema, a morphological ti-ansition hecomes
apparent in the ependymal cells. The characteristie ciliated
border is lost arid the ependyrnal cells arc no longer coIiirni~a~’,
but assume a more cuboidal shape (fig. 15). ?’he cell I)odic>sof
one characteristic cell type in the area postrema of the cat,
the glialoid cell, vary in shape and size. Tlicse cclls, however,
have oval-shaped nuclei of consistent size (5 to 7 p in diameter)
which are markedly similar to the nuclei of tlie epeiitlyrnal
cells (figs. 14, 15). Elongated cytoplasmic extensions of tlie
NERVE FIBEES I N T H E AREA POSTREMA
69
glialoid cells attach to the walls of blood vessels. These modified ependymal cells exist in greatest numbers throughout the
area postrema of the cat.
A second cell type, less numerous than the glialoid cell,
can usually be identified in the area postrema of the cat.
Although the cell bodies resemble small neiirons seen in the
medullary regions immediately surrounding the area postrenia, and the cytoplasm contains thionin chromophilic material, argyrophilic fibers cannot be seen originating from
these cells. Nevertheless, these cells have been called neurons
(Brizzee and Neal, '54).
The area postrema of the cat, rabbit, guinea pig and rat
was found to be richly innervated. Tissues impregnated by
the Holrnes ' sliver technique revealed nerve fibers varying
in diameter from one to 8 p. The fibers iormed a pathway
hetween the surrounding medulla and the area postrema in
eveiy animal studied, regardless of the species. A t the lateral aspects of the organ fascicles of nerve fibers assembled
to form discrete tracts (figs.4, 5, 6, 8, 9). On serial sections
the fibers could be traced to the nearby nuclei of the solitary
tracts and the dorsal nuclei of the vagal nerves. However,
it could not be ascertained that the fibers arose from these
areas. I t seemed certain that the fibers did not arise from
within the area postrema.
Upon entering tlic area postrema the fibers became dispersed and coursed freely among the parenchymal cells (figs.
10, 13, 14), often dividing into scvcral branches. Some terminated on the walls of blood vessels (figs. 4, 5, 6 ) , while
others ended in intimate contact with the parenchyinal cells
(figs. 12, 13, 14). No specialized terminals could be found.
Many of the nerve fibers contained argyrophilic nodular
swellings. These were not of uniform size or spacing and often
three o r 4 were seen along a short course of nerve (figs. 11,
1 2 ) . The nodules occurred more frequently on fibers coursing
within the area postrema, although a few were seen also on
the axons in the lateral tracts.
70
C. D. CLEMENTE AXI) V. L. VAN BREEMEN
COMMENT
I n every mammal in which information is available on the
subject, the area postrema exists as a highly vascular region
in the dorsal medulla. Its position and size vaq- slightly in
different species. Generally it can be said to lie in the floor
of the fourth ventricle near the obex. The functional significance of this area has given rise to much speculation, and indeed anatomists have not even been able to agree on the
types of cells that exist in this region. It is generally believed, however, that the main cell type in the area postrema
is a neuroglial element, possibly derived from epend-yma,
and containing the staining characteristics of the protoplasmic astrocyte. Wilson ( '06) described neurons in the
area postrema of hrimans, whereas King ('37) not only stated
that neurons were lacking in the area postrema of cats, but
also that the region was devoid of nerve fibers and neural
connections. Since King's work, however, Cammermeyer ( '46)
described nerve fibers in the area postrema of the human arid
Borison and Rrizzee ('51) described them i n the cat. Both
felt that probably there were neural connections with other
medullary areas.
The observations reported here clearly confirm those of
Borison and Rrizzee ('51) and more recently Brizzee and
Neal ('54) on the existence of nerve fibers in thc arca postrema of the cat. Furthermore, it has been shown that the
area postrema of the rabbit, guinea pig and rat contain an
abundant innervation and that the nerve fibers enter the
area postrema at a comparable point in each of the 4 species
studied. Within the organ the fibers course among the parenchymal cells and terminate on cell surfaces and on blood
vessel walls in a manner suggesting (1) a true secrctomotor
glandular innervation or ( 2 ) an afferent or receptor phenomenon associated with either the vascular system or the
parenchymal cells of the area postrema on the one hand and
some other region in the medulla on the other. No conclusivc
evidence exists to indicate which mechanism the described
nerve fibers serve, nor has it been established whether the
NERVE FIBERS IN T H E AREA POSTREMA
71
nerves are motor o r sensory. Experimental methods must be
employed to determine the site of origin of these fibers. It
seems quite certain, however, that the nerve fibers do not arise
from within the area postrema, but enter the lateral aspect
of the organ from elsewhere in the central nervous system.
SUMM$RP
The morphology of the area postrema has been studied in
cats, rabbits, guinea pigs and rats by various staining methods. Silver impregnated material revealed many nerve fibers
within the area postrema. Fascicles of fine unmyelinated
fibers and thicker fibers entered the lateral aspects of the area
postrema from adjacent bulbar regions. Within the area postrema the fibers were seen to course freely among the parenchymal cells and to terminate on the walls of blood vessels
and on parenchymal cell surfaces. The functional significance
of these neural elements in the area postrema is yet to be
c stablished.
ACKNOWLEDGMENT
The authors acknowledge with thanks the technical assistance of Rlessrs. R. Ansell and J. Armstrong from the Department of Anatomy, University College, London, England.
LITERATURE CITED
BORISON,
H. L., AND K. R. BRIZZEE 1951 Morphology of emetic chemoreceptor
trigger zone in r a t medulla. Proc. Soc. Exp. Bjol. and Med., 77:
38-42.
BORISON,I€. L., AND S. C. WANG 1949 Functional localization of central coordinating mechanism f o r emesis in cat. J. Neurophysiol., 12: 305-313.
RRIZZEE,K. R., AND H. L. BORISON1952 Studies on the localization and
morphology of the chemoreceptor trigger (C.T.) zone i n the area
postrema of the cat. Anat. Rec., 212: 13-14 (Abstract).
BRIZZEE,K. R., A N D L. M. NEAL 1954 A reevaluation of the cellular morphology of the area postrema in view of recent evidence f o r a
chemoreceptor function. J. Comp. Neur., 100 : 41-61.
CAMMERMEYER,
J. 1947 Is the human area postrema a neuro-vegetative nucleus? Acta Anat., 8: 294-320.
HOLNES, W. 1952 A new method f o r the impregnation of nerve axons i n
mounted paraffin sections. J. P a t h . and Bact., 5 4 : 132-136.
72
C. D. CLEMENTE A N D V. L. VAN BREEhIEN
KING, L. S. 1937 Cellular iriorphology iii the a r m postrema. J. Conip. Neur.,
66: 1-22.
KOENIG,
H., R. A. GROATA N D W. F. WINDLE 1945 A physiologicnl :Lpproacli
to perfusion fixation of tissues with formalin. Stain Tech., 30: 13-20.
PINES,J. L. 1926 e b e r eiii bisher unbeobaclitetes Gcbilde im Gehirn einiger
Siiugetiere: Das subfornicale Organ des I T 1 Ventrikel. J. f . Psychol.
u. Neur., 34: 186-193.
PUTNAM,T. J. 1922 The intercolumnar tuherclc, a n undescribed area in tlir
anterior wall of the third vrntricle. Johns Hopkins Hosp. Eull., 3 8 :
181-182.
RANSON,S. W., AND P. R. RILLIKGSLY 1916 Vasomotor reactions from stimulation of the floor of the fourth ventricle. Studies in vasomotor reflex amb
111. Am. J. Physiol., d 2 : 85-89.
WANG,S. C., A K D H. L. BORISON 1950 The vomiting centcr: A critical experi
mental analysis. Arch. New. and Psychiat., 63: 928-941.
WILSON,J. R. 1906 On the anatomy of the calamus region in the human bulb;
with the account of the hitherto undescrihed “ nucleus postrema. ”
J. Anat. (London), 4 0 : 210-241, 357-386.
WINDLE,W. F., R. RHINESAND J. RANKIN 1943 A Nissl method using buffered
solutions of thionin. Stain Tech., 18: 77-86.
WISLOCKI,G. B., AND E. LEDUC 1952 Vital staining of the hematoenccphalic
barrier by silver nitrate and trypan blue, aiid cytological comparisons
of the neurohypophysis, pineal body, area postrema, intercolumnar
tubercle aiid supraoptic crest. J. Comp. Neur., 96: 371414.
WISLOCKI,G. E., A N D T. J. PUTNAM
1920 Note on the anatomy of the area(’
postrcmx. Anat. Rec., 1 9 : 281-286.
1924 Further observations on the anatomy of the arcne postremae. Anat. Rec., 27: 151-156.
PLATES
PLATE 1
EXPLANATION
OF FIGURES
2
In this figure is shown the area postrema (QP) in a c a t and its relation
t o the 4th ventricle ( I V ) . Note the many blood vessels (bv) in this highly
vascular structure. T h r outlined region in the upper right is shown i n
higher magnification i n figure 5. Holmes ’ silver method, magnific;ttioii X 60
3
Here is shown the area postrema ( A P ) of a rat. It is s e a to lie in the
floor of the 4th ventricle ( I V ) . The ventral boundary of the area postrema is designated by the arrows, and the ontlined regioii in the upper
right I S shown i n higher magnification in figme 6. Holmes’ silvrr mctliod,
magnification X 60.
4 The arrows indicate a blood vessel in the area postrema of a r a t and shou
iicrve fibcrs tcrmiiiating oii a i d i n the vicinity of the blood vessel. Holmcs’
silver method, magnification
X 300.
5 This figure shows a dctail of the outlined region in figure 1. The area postrema ( A P ) is seen t o contain a large number o f nerve fibers, many o f
which can be traced from the medulla (on the right) and which often
terminate on blood vessel walls (arrows). Holmes’ silver method, mngnifica
tion X 360.
6
This shows a detail of thc outlined area in figure 2. The rat, similar t o the
c a t (fig. 5), shows fibers enteriag the area postrema from the surrounding
medulla and terminating on the walls of blood vessels (arrow). IIolnies ’
silver method, magnification X 360.
74
PLATE 1
75
A detail of tlie outlined region ( a ) i n figure 7. Here can be seen nerve fibers ( n f ) appearing a s a conipact buiidlc coilnecting the area postrema (to t h e right) and the medulla (to t h e l e f t ) . Blood vessels (bv) are also designated. Bodian
silver nietliod, magnification X 360.
A detail of the outlined region (b) in figure 7. This shows ail area comparable to figure 8, b u t on tlie opposite side of the
area postrema aiid again illustrates similar bundles of fibers ( n f ) riiteriiig the area postrema from the surrounding medulla. Godian silver method, magnification X 360.
8
9
10 A detail of outlined region ( c ) i n figure 7 . Kote the iiitiiiiaey with which the iierve fihers ( n f ) course niuonyst the glinloid cells (gc), often termiriatiiig 011 cell surfaces. Bocliari silver method, niagnificatioil X 360.
The three arrows indicate the extent of the area postrema in this sectioii of the niedulla i n a guinea pig. The outlined
areas (a, b aiid c ) a r e sliowii i n higher magnification i n figures 8, 9 and 1 0 respectively. Bodinn silver method, magiiification X 60.
O F FIGURES
7
EXPLANATION
PLATE 2
C . D . C I i E M E N T E A B D V. I). V 9 X H & E E I i I N
NERVE F I A E R S IN THE A R E A POSTREMA
PLATE 2
PLATE 3
EXPLANATION OF FIGURES
11 This figure illustrates under high magnification a nerve fiber i n the area
postrema of a rat showing the argcntophilic nodules ( n ) which were ofteii
seen along the course of nerve fibers in this region. Holnies’ silver method,
magnification approximately X 850.
12
A high magnification photomicrograph of the area postrema of a cat showing
a large nerve fiber with nodules ( n ) and the glialoid cells. Holmes’ silver
method, magnification approximately X 430.
13
Glialoid cells (gc) a r e seen with nerve terininnls (t) ending directly on the
cell walls. The nucleus and the cytoplasm of the cells are not in perfect
focus with the nerve terminals. IIolmes silver method, magnification q proxiniately x 850.
’
14 This photomicrograph shows a cluster of glialoid cells (surrounded by
arrows) i n the area postrema of tlie cat and a n abundant numbcr of nerve
fibers coursing within the cell cluster. This typifies tlie general histologic
structure throughout the area postreinn of tlie cat. Holincs’ silver method,
magnification X 450.
15
Here is illustrated the region of transition between the ependgmal cells (ep)
liiiiiig the 4th ventricle ( I V ) arid the glialoid cells (gc) of the area postrenia ( A P ) . Notc t h a t the ependymal cells lose their ciliated border and
become flattened. Note also tlic similarity i n size and impregnatibn of the
nuclei in the glialoid cells of the area postreina and the eperidyrnal cells
IIolnies silver method, mngiiification x 450.
’
78
NZRVE F I B E R S I N THE AREA POSTREMA
C. D . CLEMENTE A N D V. L. VAN BRERMEN
PLATE 3
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