Download ORIGIN OF THE PERICELLULAR BASKETS OF THE PYRAMIDAL

BRAIN RESEARCH
633
O R I G I N OF T H E P E R I C E L L U L A R BASKETS OF T H E P Y R A M I D A L CELLS
OF T H E H U M A N M O T O R CORTEX: A GOLG1 STUDY
MIGUEL MARIN-PADILLA
Department of Pathology, Dartmouth Medical School, Hanover, N.H. 03755 (U.S.A.)
(Accepted February 24th, 1969)
INTRODUCTION
The identification of a distinct histological structure with a specific function is
the most significant objective in the study of the nervous system. This objective, unfortunately, has been met on only a few occasions. Perhaps the best known and the
one which best illustrates this objective has been the discovery of the inhibitory
function of the basket cells of the cerebellum and the hippocampusl, 2. Cajal established
for the first time the histological structure of these neurons and of their connections.
In 1888 (refs. 3, 7) he described, in Golgi preparations, the pericellular baskets of the
Purkinje ceils and their origin from stellate neurons with short axon, the basket cell,
located in the molecular layer. In 1893 (Refs. 4, 7) he also described, in Golgi preparations, the pericellular baskets of the pyramidal cells of the hippocampus and their
origin from stellate neurons with short axon, the basket cell, located in the stratum
oriens. The morphological characteristics of the basket cells of the hippocampus were
later described in greater detail by Lorente de N611. These two systems have a similar
structure consisting of an intrinsic stellate neuron characterized by the termination of
its axon in an intricate axonic plexus around, and in contact with, the soma of several
large neurons of projection, thus forming a distinct pericellular basket. This peculiar
structure is today associated with inhibition in the cerebellum and in the hippocampus.
Cajal also described a third type of basket around the cortical pyramidal cells of
the visual and the motor cortices of infants 5-7. He compared them 7, structurally, with
the baskets of the cerebellum and the hippocampus. He suggested 5 that the most likely
origin of these baskets were the stellate neurons with horizontal axons encountered in
layers III-V of the cortex. With an ingenious method (chronically isolated slabs of
neocortex in which only their vascular supply is maintained) Szent~tgothaPZ, 13 has
been able to prove that the axo-somatic contacts are mainly contributed by cortical
neurons with short axons. He suggested that the pericellular baskets formed around
the pyramidal cells are derived from stellate neurons with axons not longer than 1-2
mm in length. Cajal further suggested 7 that perhaps afferent axons may also contribute
to the formation of the baskets but concluded that more investigations were needed
Brain Research, 14 (1969) 633-646
634
xR MARtN-PAt)Jt.~..\
before any definite conclusion could be drawn. The possibility that the baskets of the
pyramidal cells may represent a component of a cortical inhibitory system has been
already suggested 9,l°,1~,1a. This possibility is further supported by the electron
microscopic studies of the type ofaxo-somatic contacts in pyramidal cells o f the cortex s.
Exceot for two original drawings of Cajal 7 of the baskets of the pyramidal cells
of the motor cortex of man, the available information about them remains incomplete
in such areas as: the structure of the baskets, their location and distribution, and the
type of terminal contact which they make with the soma of the pyramidal cells. More
importantly, little is known about the distribution and structural characlcristics of the
cortical stellate neurons which participate in their formation. The present communication reports the observations made in Golgi preparations of the human cerebral
cortex concerning the structure and the distribution of the pericellular baskets of the
pyramidal cells. It also describes the morphological characteristics of the cortical
stellate neurons which form the baskets.
MATERIAL AND METHOD
The motor areas (area 4) of the cerebral cortex of a 7-month fetus, a premature
infant, a newborn, a 2-month-old infant and an 8-month-old infant have been studied.
Several tissue blocks from the motor cortex of these infants were obtained between 1
and 3 h after death. All infants died in cardio-respiratory failure. The blocks obtained
measured approximately 2.5 mm in thickness, 10 mm in length and 5 mm in width.
All the blocks were stained by the rapid Golgi method according to the following
procedure: immersion during 5 days in a fresh 0.25 ~o osmic-dichromate solution;
immersion for 2 days in a fresh 0.75 ~o silver nitrate solution; immersion for 2days in
a fresh 0.25 ~ osmic-dichromate solution ; and immersion for 2 days in a fresh 0.75 ~,,
silver nitrate solution. The tissue blocks were washed (1 min)in distilled water between
solutions. The blocks were cut by free hand with a razor blade, cleared with oil ofcl0ve
and mounted with a Damar-xylene solution. 20-24 sections, 150-200 # thick, were
obtained from each block. A total of 380 sections were prepared and studied.
OBSERVATIONS
7-month/btus. The motor cortex of this infant was characterized by its embryonic
stage of development and did not disclose pericellular baskets. Only pyramidal cells
were found in this cortex, some localized deep in the cortex and thus establishing the
first indication of the layer V. The other pyramidal cells were distributed throughout
the superficial cortical regions without a clear lamination. No stellate cells were f o u n d
in the cortex of this infant.
Premature infant. The motor cortex of this infant was well developed and all its
cortical layers and cell types were recognized. The pyramidal cells were distributed
throughout the cortex with two distinct concentrations in layers III a n d V . Typical
stellate cells were found among the bodies of the pyramidal cells. The terminal portions of the axons of these stellate cells became so fine that they failed t o b e properly
Brain Research, 14 (1969) 633-646
THE CORTICAL BASKET CELL
635
impregnated by silver and could not be followed. Probably because of this no pericellular baskets were found in this infant.
8-month-old infant. The motor cortex of this infant was characterized by its
extraordinary complexity of axonic fibers and dendrites. Pericellular baskets were seen
on rare occasions. Any a~.tempt to follow the fibers which form them to the cell of
origin, through the complexity of this cortex, failed to give clear and positive results.
Typical stellate cells were also found among the bodies of the pyramidal cells but,
again, to follow their axons to their termination was a difficult if not an impossible
task in this complex cortex.
Newborn and 60-day-old infants. The motor cortices of these 2 infants proved to
be optimal for the detailed analysis required for this study. Pericellular baskets were
abundant and the complexity of the cortex, at this age, permitted an accurate analysis
of their location, structure, terminal type of contact with the soma of the pyramidal cells
and, more importantly, their origin from typical stellate cells with short axon.
The human motor cortex at this age (first 2 months of extrauterine life) is well
developed. It measures 2000 # thick and is organized in the following manner: layer 1:
150 #; layer II: 250 #; layer III: 500 #; layer IV: 250/z; layer V: 300 #; layer VI:
500 #. Although the motor cortex is classically identified and characterized by the
lack of a clearly defined layer IV (granular layer), in this study a layer IV is considered
at a cortical depth ranging between 950 and 1200 # from the surface pia. This is based
on the observation that typical stellate neurons with short axon appeared to concentrate more frequently at that cortical depth than in any other cortical region. They
concentrate above the bodies of the giant pyramidal cells of layer V and below the
bodies of the large pyramidal cells of lower layer l Il. The stellate cells appeared at that
cortical depth in small groups of several neurons (Figs. 6, 7), but failed to constitute a
clearly defined layer. The bodies of the large and giant pyramidal cells of the motor
cortex appeared at cortical depths ranging from 800 to 1400/~. This particular distribution of the pyramidal cells in the motor cortex and their abundance also explains the
absence of a clearly defined layer IV such as the one which characterizes the primary,
visual, auditory and sensory areas of the cortex.
Location and distribution of the cortical baskets
In Golgi preparations of the human motor cortex at this age (first 2 months of
extrauterine life) pericellular baskets are found frequently. They are located at a
cortical depth ranging between 800 and 1400/z from the surface pia. They coincide in
location and cortical distribution with the bodies of the large and giant pyramidal cells
of the lower layer III and layer V respectively. In spite of this general cortical distribution, the baskets appear to concentrate in two main locations or cortical depths at 900
/z and at 1400 # from the surface pia. These two locations of the baskets indicate two
clearly different populations of them. Those located around the 900/~ cortical depth
are smaller, roughly triangular in shape and depict a very simple structural organization. The baskets located around the 1400 # cortical depth are larger, triangular or
fusiform, with apical and basal prolongations and they have a rather complex
Brain Research, 14 (1969) 633-646
636
~1. \l\t< N- ' , \ J ~ l l
~
Figs. 1 and 2. Pericellnlar baskets of layer V o f the m o t o r cortices of a n e w b o r n a n d ot ;~ ~0-dav-old
infant respectively. Cortical basket cells (arrows) appeared invariably associated with the basket~.
The structure, fiber c o m p o s i t i o n a n d shape of the baskets are clearly seen. Notice the a b u n d a n c e of
horizontal axonic fibers a m o n g the baskets. T h e r e is an appreciable increase in the total n u m b e r oi
baskets, in their complexity a n d in horizontal fibers between birth (Fig. I ) a n d 2 m o n t h s of age I Fig. 2).
R a p i d Golgi m e t h o d , scale 100/~.
Brain Research, 14 (1969) 633-646
THE CORTICAL BASKET CELL
6.,7
Fig. 3. Detail o f the fibrillar constitution of a basket of layer V of the m o t o r cortex of a 60-day-old
infant. T h e p h o t o m i c r o g r a p h s represent 6 different levels of the s a m e basket to s h o w its intricate
structure, the f u s i f o r m dilatations of the fibrillae and the terminal r o u n d heads of some. T h e fusiform
dilatations a n d the terminal heads are considered to represent synaptic contacts with the s o m a of the
pyramidal cell. R a p i d Golgi m e t h o d .
Brain Research, 14 (1969) 633-646
structure. The distribution of the cortical baskets in two main populations is best seeJ~,
in the motor cortex of the newborn infant. At 2 months of age this double corticai
distribution is less apparent due to an increase in size and complexity oflhc b~skets and
to an increase in their total number. At this age the cortical baskets are found throughout the general cortical distribution of lower layer III and layers IV and V. Fhe limits
of layer IV become less apparent at this age also. The baskets are more abundant
around the giant pyramidal cells of layer V than in any other cortical region.
Structure of the cortical baskets
The delicate and fine structure of the cortical baskets can be easily obscured in
Golgi preparations by overstaining, the presence of silver precipitates, and the overlapping with other structures. It is only in certain good Golgi preparations in which the
baskets are stained and can be thoroughly analysed. For reasons which remain unknown the cortical baskets are stained only in those Golgi preparations where the
pyramidal cells which they surround are not stained. On the other hand. when the
pyramidal cells are well stained the baskets are not distinguishable.
At low microscopic magnification the cortical baskets appeared simply as
localized roughly triangular concentrations of axonic terminals clearly distinguishable
from the surrounding structures. At medium microscopic magnification (Figs. l, 27
the baskets are seen to be formed by a concentration of many incoming axonic
terminals reproducing the morphology of the bodies of the pyramidal cells around
which theyare formed. At high microscopic magnification the incoming fibers approach
and enter into the baskets from all directions. They branch in several short and fine
fibrillae which constitute the basket itself. The incoming axons are seen after giving off
the fine fibrillae to a basket to continue their path into other baskets or other cortical
regions. On a few occasions an incoming axon terminates solely in a given basket. The
baskets of the lower layer I11 are relatively simple in their structure. The baskets of the
layer V are q aite complex structures characterized by apical and basal prolongaticns
which appear to involve the first portion of the apical and the basal dendrites of the
giant pyramidal cells. Some large baskets have also another prolongation which, by
its location, appears to involve the first portion of the axon of the giant pyramidal cell
(Fig. 3). At this very high microscopic magnification the fine fibrillae which constitute
the basket are best seen. The fibrillae terminate by small round heads ('boutonterminaux') of approximately 1-1.5 ,u in diameter (Fig. 3). The fibrillae have also
many fusiform dilations which resemble contact 'en passant' of approximately 1 # in
diameter (Fig. 3). A schematic representation of a large basket (layer V) around a
giant pyramidal cell is shown in Fig. 4.
Origin of the baskets: the cortical basket cells
In Golgi preparations of the motor cortex the baskets appeared to be associated
invariably with a distinct stellate neuron with short axon, the cortical basket cell (Figs.
1, 2). They are medium-sized stellate neurons distributed at a cortical depth ranging
Brain Research, 14 (1969) 633-646
THE CORTICAL BASKET CELL
639
++ ++ ~
++
\
~
+
~
,
~
+
Fig. 4. Schematic representation of a large pericellular basket around the body of a giant pyramidal
cell of the human motor cortex, demonstrating the incoming axons, the fibrillar constitution of the
basket and the terminal heads and fusiform dilatations of the fibrillae. The apical, basal and axonal
prolongations of the basket are also demonstrated.
Brain Research, 14 (1969) 633-646
640
~,1. ~ I / \ I ~ t N - P A D I t [,~
III
M.C.N.B. 2 II 3
Depth 1200p
,i,
i-
m
T
IV
T
m
V
m
Vl
Fig. 5. Camera lucida drawing of a basket cell from the motor cortex of a newborn infant. It demonstrates beautifully the two types of dendrites, the distribution of the axon, and the axonic collaterals
forming baskets around the pyramidal cells of the layers III and V, which characterize the histological
structure of the basket cells of the human motor cortex. Several incoming axons are seen establishing
axo-spinodendritic synapses (S). T, horizontal axonic fibers.
l~rain Research, 14 (1969) 633-646
641
THE CORTICAL BASKET CELL
J
MC 60o21il2
I
lOOp
!
'0
.tO0
II
iO0
IlZS~
I[I
IV?
¢.
000
V
1500
Yl
zooo~
Fig. 6. Camera lucida drawing of a group of cortical basket cells to demonstrate their structural
characteristic and cortical location. The horizontal axonic (T) fibers of these neurons were cut and,
therefore, their termination is missing. Several axonic collaterals (c) arising from the horizontal fibers
terminate in the pericellular baskets. (Motor cortex of a 60-day-old infant.)
Brain Research, 14 (1969) 633-646
642
x.]. M A R I N - P A I ) I I _ I
,
i
i
X
E
'190
,,
~
,,, 000
'
SO0
Vl
ZOOOp
Fig. 7. Camera lucida drawing of the two cortical basket cells to demonstrate the length of two
complete horizontal axonic (T) fibers of one of them. These fibers measure approximately 1000/~ in
length and give many axonic collaterals (c) for the baskets around the pyramidal cells of lower layer
III and layer V. The cortical location of the neurons is represented in microns from the surface pia.
The structural characteristics of the basket cells are clearly demonstrated. The slight oblique position
of the basket cells and of the pericellular baskets are reflections of the curvature of the cortical
convolution (insert). (Motor cortex of a 60-day-old infant.)
from 850 to 1400/~ from the surface pia, among the bodies o f the large and the giant
pyramidal cells of the motor cortex, In spite of this general cortical distribution they
appeared to be more abundant at a cortical depth between 950 and 1200 u from the
surface pia establishing a poorly defined layer IV (Figs. 5-7).
The cortical basket cells o f the human motor cortex (Figs. 5-7) are distinct neurons characterized by both the distribution of their dendrites and the behavior o f their
Brain Research, 14 (1969) 633-646
THE CORTICAL BASKET CELL
643
axon. Several dendrites, mainly vertical and horizontal, radiate from the body of the
neuron. The vertical dendrites (Figs. 5-7) are a prominent feature of the cortical basket
cells. The superior vertical dendrite crosses layer III and terminates in layer II. The
inferior vertical dendrite crosses the layer V and terminates in layer VI. Both of these
dendrites may measure up to several hundred microns in length. The horizontal dendrites cross the cortex at the same cortical depth as that of the neuronal body and are
shorter than the vertical ones. The dendrites of the cortical basket cells are thin, long
and moderately covered by dendritic spines. Perhaps the most significant and distinctive structural characteristic of these cells is the behavior of their axon (Figs. 5-7). The
axon may be ascending or descending, depending on the location (cortical depth) of
the neuron. It branches almost immediately after arising from the neuronal body into
several horizontal fibers of great length at different cortical levels. These horizontal
fibers may measure up to 1000/z in length or more. From these horizontal fibers many
ascending and descending axonic collaterals arise which terminate in the pericellular
baskets. Either they terminate in the basket by branching into fine terminal fibrillae or,
after giving off terminals to a basket they continue to other baskets. The same basket
cell can enter into the formation of baskets around the pyramidal cells of lower layer
Ill as well as around the giant pyramidal cells of layer V (Fig. 5). A single basket cell
participates in the formation of many pericellular baskets. The soma of the cortical
basket cells has clearly distinct spines.
The horizontal axons of the basket cells are found crossing the cortex at depths
which correspond to the cortical location of the bodies of the large as well as the
giant pyramidal cells of the motor cortex. The horizontal axons of the basket cells in
Golgi preparations are clearly distinguishable from horizontal afferent axons which
are also quite abundant at the same cortical depth. The afferent axons are thicker and
have few or no collaterals until they finally branch profusely at a given region in a very
characteristic manner. The axons of the basket cells are thinner and characterized by
fine ascending and descending axonic collaterals.
DISCUSSION
The present communication confirms original observations made by Cajal in
1899. To those it adds new information concerning the structure, the distribution and
the origin of the baskets of the pyramidal cells of the human motor cortex.
The basket cell found in the human motor cortex is structurally similar to the
basket cells of the cerebellum and the hippocampus. It is a stellate neuron with short
a×on characterized by its long vertical and shorter horizontal dendrites, by its cortical
location and by the behavior of its axon. The dendrites of the cortical basket cell are
thin, long and moderately covered by spines. The cortical basket cells are distributed
from lower layer I!! to layer V. However, they are more abundant in layer IV where
they are found in small groups representing perhaps the neuronal elements of this
poorly defined layer of the motor cortex. The axon of the basket cell is its most
distinctive characteristic. It branches in several long horizontal fibers at different
cortical levels. Many ascending and descending collaterals arise from the horizontal
Brain Research, 14 (1969) 633-646
o~
I
Stellate
short axon
Motor cortex
Projective neurons.
Stellate
short axon
Hippocampus
*
Stellate short
axon
Type
Cerebellum
Location
Horizontal fibers
with collaterals
terminating in
many baskets
Horizontal fibers
with collaterals
terminating in
many baskets
Horizontal fibers
with collaterals
terminating in
many baskets
,4xon
M O R P H O L O G I C A L C H A R A C T E R I S T I C S OF T H E B A S K E T C E L L S
TABLE
With spines
With spines
With spines
Dendrites
Same regional
distribution as
dendrites o f
Purkinje cells 3.7
Same regional
distribution as
dendrites of
pyramidal cells
of hippocampus 7, ~
Same regional
distribution as
apical & basal
dendrites of large
& giant pyramids
Distribution
o f dendrites
Neocortical
pyramidal cell*
Hippocampal
pyramidal cell*
Purkinje cell*
Receptive neuron
Unknown
perhaps inhibitory
Inhibitory (1)
Inhibitory (2)
Function
7-
i
,>
4~
4~
THE CORTICAL BASKET CELL
645
fibers and terminate in fine fibrillae in the pericellular baskets. The fibrillae of a basket
contact the soma of the pyramidal cell probably by its terminal heads and by its
fusiform dilatations thus establishing axo-somatic synapses. The fibrillae also make
contacts with the first portions of the axon, the apical and the basal dendrites of the
pyramidal cells.
The same cortical basket cell can enter in the formation of baskets around the
bodies of the large and the giant pyramidal cells of the lower layer III and layer V
respectively. This indicates perhaps that the baskets around these two types of pyramiJal cells belong to the same cortical system. A basket is formed by the axonic
collaterals of many bskets cells, and a single basket cell enters in the formation of
many baskets.
Typical basket cells are already found shortly before birth in the motor cortex,
and possibly the baskets are being formed at that age also. At the time of birth the
motor cortex has well developed basket cells and the baskets are abundant, more so in
layer V than in lower layer 1II. At 2 months of age the baskets have increased considerably in structural complexity and in total number. The basket cells at this age
show the rich arborization of their axon. This age represents the best time for a detail
analysis of the structure of the basket cells and of the pericellular baskets. The structural complexity of the motor cortex of older infants makes any detailed analysis of
the baskets themselves and of their origin from typical stellate neurons nearly impossible.
The structural similarities of the basket cells of the motor cortex, the cerebellum
and the hippocampus (Table I) give further support to the hypothesis that the baskets
of the cortical pyramidal cells are perhaps part of an inhibitory system.
While the possibility that afferent axons may also contribute to the formation of
the baskets of the cortical pyramidal cells has not been confirmed in the present study,
it has also not been ruled out. Undoubtedly, more investigations are needed.
A few words about the rapid Golgi method appear to be in order. The clarity,
completeness and elegance of this method in demonstrating the finer structure of the
central nervous system have not been surpassed by any of the neurohistological
methods available today. It is because of this that the rapid Golgi method,in spite of its
occasional failure and capricious nature, should be encouraged in the study of the
structure of the nervous system. The knowledge derived from such Golgi studies
facilitates a better understanding of the structure and gives a frame of reference for
ultrastructural and neurophysiological studies.
SUMMARY
A Golgi study of the structure, distribution and origin of the pericellular baskets
of the pyramidal cells of the human motor cortex is presented. This study has disclosed
the presence in the motor cortex of a distinct type of stellate neuron which forms
pericellular baskets around the bodies of the pyramidal cells of the lower layer III and
layer V. These cortical basket cells are characterized also by their long horizontal and
vertical dendrites. They are distributed from layers Ill-V, but they concentrate more
Brain Research, 14 (1969~ 633-646
646
~ :~t'\ ff,i N-|'ADII I~
commonly
i n l a y e r 1V o f t h e m o t o r c o r t e x , p e r h a p s r e p r e s e n t i n g t h e ~ t ~ r o n a J ele-
m e n t s o f t h i s p o o r l y d e f i n e d c o r t i c a l layer. T h e s t r u c t u r e o f t h e pericell u l a r b a s k e t s i~
d e s c r i b e d i n d e t a i l . It is s u g g e s t e d t h a t t h e c o r t i c a l b a s k e t cell h e r e d e s c r i b e d m a y
represent an inhibitory neuron of the human motor cortex.
ACKNOWLEDGEMENT
This work was supported by Grant GM
10210 f r o m t h e N a t i o n a l I n s t i t u t e s o f
Health.
REFERENCES
1 ANDERSEN, P., ECCLES. J. C.. AND LOYNING, Y., Recurrent inhibition in the hippocampus with
identification of the inhibitory cell and its synapses, Nature (Lond.), 198 (1963I 540-542.
2 ANDERSEN, P., ECCLES, J, C., AND VOORHOEVE,P. E.. Inhibitory synapses on somas of Purkinje
cells in the cerebellum, Nature (Lond.?, 199 (1963) 655-656.
3 CAJAL,S. R. Y, Estructura de los centros nerviosos de las ayes, Rev. trim. Hist. norm. pat., 1 (1888).
4 CAJAL, S. R. Y, Estructura del asta de Ammon y fascia dentata. An. Soc. esp. Historia nat.. 22
(1893).
5 CAJAL, S. R. Y, Estudios sobre la corteza cerebral humana. Estructura de la corteza motriz del
hombre y mamiferos. Rev. trim. Microgr., 4 (1899) 117-200.
6 CAJAL, S. R. Y, Estudios sobre la corteza cerebral humana. Corteza visual. Rev. trim. Microgr.. 4
(1899) 1-63.
7 CAJAL, S. R Y. Histologie du SystOme Nerveux de rHomme et des Vert~brds, Vol. 2. Maloine,
Paris, 19ll. p p 1-71: 555-559; 733-761. (Contains a rGsume of the former papers.~
8 COLONNIER,M., Synaptic patterns on different cell types in the different laminae of the cat visual
cortex. An electron microscope study, Brain Research, 9 (1968) 268-287.
9 COLONNIER. M., The structural design of the neocortex. In J. C. ECCLES (Ed.), Brain and Conscious
Experience. Springer, New York. 1966, pp. 1-18.
10 ECCLES, J. C.. The Physiology o f the Synapses, Academic Press, New York. 1964. pp. 213-214.
11 LORENTEDE NG, R.. Studies on the structure of the cerebral cortex. II. Continuation of the study of
the ammonic system, J. Psychol. Neurol. (Leipz.), 46 (1934) l 13-177.
12 SZENTAGOTHAI.J., The synapses of short local neurons in the cerebral cortex. Syrup. Biol. hung., 5
(1965) 251-276.
13 SZENTAGOTHAI,J., The use of degeneration methods in the investigation of short neuronal connections. In M. SINGER AND J P. SCHADt~ (Eds.), Degeneration Patterns in the Nervous" System.
Progress in Brain Research, Vol. 14, Elsevier. AmsterdarrL 1965, pp. 1-32.
Brain Research, 14 (1969) 633-646