BRAIN RESEARCH 167

PRENATAL AND EARLY POSTNATAL ONTOGENESIS OF THE HUMAN MOTOR CORTEX: A GOLGI STUDY. I. THE SEQUENTIAL DEVELOPMENT OF THE CORTICAL LAYERS

MIGUEL MARIN-PADILLA

Department of Pathology, Dartmouth Medical School, Hanover, N. H. 03755 (U.S.A.)

(Accepted April 15th, 1970)

INTRODUCTION

PoliakovT, s has pointed out that a definite sequence occurs in the development of the human cerebral cortex and that a distinct relationship appears to exist among the arrival of afferent inputs, the organization of the various cortical layers and the maturation of the cortical neurons. Except for the systematic studies of Poliakov and other Russian investigators TM little is known concerning the prenatal ontogenesis of the human cerebral cortex. The remaining available information on this subject is restricted to isolated reports1-5, 9 of some of the stages of cortical development, which fail to demonstrate the concatenation and the sequential nature of the entire process. Although information is more readily available concerning the postnatal development of the human cerebral cortex1,2, 5 this pertains mostly to the increasing complexity of cortical structures already present at the time of birth and it fails to demonstrate the sequential development of the different cortical structures during the course of cortical ontogenesis. Appreciation of the development of the cortical structures (the different neuronal types and systems of fibers and the cortical lami- nations) during the course of prenatal and early postnatal ontogenesis is a s ine qua non

for the understanding of the structural-functional relationship of the cortex. Further- more, this knowledge is of fundamental importance for interpretation of the unique morphological peculiarities of the structural organization of the cerebral cortex.

This communication reports a series of observations made in a systematic investigation of the prenatal and early postnatal ontogenesis of the human motor cortex. Although this study was confined to the motor cortex and the cases investigated do not represent a complete developmental series, the findings are considered to be representative of the general sequential development of the human cerebral cortex. Many of the observations made in this investigation confirm those reported by Poliakov7, 8. To these some additional findings are now added.

To facilitate their description and analysis, the findings of this investigation are

Brain Research, 23 (1970) 167-183

168 M. MAR1N-PADILLA

separated into 3 categories. These are: (A) observations pertaining to the sequential organization of the cortical layers, including the ordered arrival of the afferent inputs to the cortex and subsequent maturation of some of the most prominent cortical neurons; (B) observations on the development and structure of the basket-pyramidal chains, which are considered to represent intracortical vertical neuronal systems of possible functional significance; and (C) observations on the development and struc- ture of the Cajal-Retzius cells, including evidence contrary to the generally held concept that these cells undergo regressive changes and later disappear hi postnatal ontogenesis. Although these three sets of observations are closely interrelated, individ- ual consideration of each in separate reports is advisable for the sake of clarity. This first report is concerned with the sequential development of cortical layers. Together, these three reports will cover principal features of the prenatal and early postnatal ontogenesis of the human motor cortex.

MATERIALS AND METHOD

The precentral gyrus (area 4) of 3 human fetuses (gestational ages: 5, 7 and 7.5 months) and of 4 infants (postnatal ages: newborn, 2, 2.5 and 8 months) were studied. The 3 fetuses were born prematurely and died soon after delivery because of im- maturity and respiratory failure. The 4 infants studied died of cardio-respiratory failure. The autopsies of these infants were carried out between 1 and 3 h after death. Several tissue blocks transverse to the long axis of the precentral gyrus and perpendic- ular to its surface were obtained from the cerebral cortex of each of the cases. These blocks represent a cross section of the entire gyrus and measure 2.5 mm in thickness. The blocks were stained by the rapid Gotgi method according to the following pro- cedure: immersion during 5 days in a fresh 0.25 ~ osmic-potassium dichromate solution; immersion for 2 days in a fresh 0.75 ~ silver nitrate solution: immersion for 2 days in a fresh 0 .25~ osmlc-potassium dichromate solution: and immersion for 2 days in a fresh 0.75 ~ silver nitrate solution.

Between immersions the blocks were washed in distilled water (1 rain). The blocks were then cut freehand with a razor blade. The sections obtainedwere immersed briefly in absolute ethanol, cleared with oil of cloves, and mounted consecutively with a Damar-xylene solution. Each block yielded 20-24 sections, 150-200/~m thick. Additional methods such as hematoxylin and eosin (H-EL Nissl's, and Weigert's stains were also used for some sections.

When possible, photomicrography was used. Camera lucida drawings were, however, made of neurons and fibers and whenever a lack of clarity of the Golgi preparation did not permit photography of acceptable quality. All drawings were done at approximately x 200 magnification. The cortical location (cortical depth from the surface pia) and the general orientation of all neurons within the sections were also determined and recorded. All the drawings made from each individual case were then assembled together into a final composite picture. The final drawing of each case, therefore, represents a vertical plane of the motor cortex transverse to the long axis of the precentral gyrus. All final drawings are reproduced to the same scale.

Brain Research. 23 (1970) 167-183

ONTOGENESIS OF HUMAN MOTOR CORTEX 169

OBSERVATIONS

Fetus 5 months old. Grossly the cerebral cortex of this infant was smooth and its main fissures were recognizable, but the sulci were indistinct. The frontal, parietal, occipital and temporal lobes were definable. Tissue blocks for Golgi and H -E studies were obtained from the mid-dorsal region of the cortex. They included the entire thickness of the cortex and were 2.5 mm in thickness.

In hematoxylin and eosin preparations of the cerebral cortex of this infant (Fig. 1) the following zones, listed from the pial to the ependymal surface, were recognized : the marginal zone (layer I), the so-called primordial cortical gray zone, the intermediate zone, and the ependymal zone. The marginal zone corresponds to layer I and appears clearly demarcated from the rest of the cortical gray matter. It measures about 50 #m in thickness and is clearly subdivided into an outer and an inner region (Fig. I). The outer region contains more cells and is notable for the presence of very large neurons. These neurons are characterized by their large size (the largest of the cortex at this age) and by their prominent protoplasmic pro- longations. The inner region of layer I is predominantly fibrillar in composition and has few cellular elements. The so-called primordial cortical gray zone corresponds to the future layers II-VI of the cerebral cortex and measures 700-800 #m in thickness. It consists of a thick layer of packed cells arranged in parallel vertical columns sep- arated by thin bands of fibrillar tissue. Although there are no appreciable horizontal laminations of the cortical gray matter, those neurons located at cortical depths between 400-500 #m appear to be slightly more developed than the others. The intermediate zone measures between 1.5-1.7 mm in thickness and appears to be subdivided into an outer and an inner region. The outer region is mainly fibrillar with a predominantly vertical orientation of its fibers. It has few cellular elements. The inner region is characterized by two separate sets of horizontal fibers which seem to orig- inate from the region of the internal capsule. Some of these fibers cross to the other hemisphere through the corpus callosum. The ependymal zone is very cellular and measures between 250-350 #m in thickness.

Golgi preparations of the cerebral cortex of this infant (Fig. I) demonstrate that layer I is more developed than the rest of the cortex. It is subdivided into an outer and an inner region. The outer region is characterized by many well-developed Cajal-Retzius cells. The inner region is composed of many horizontal (tangential) thick fibers which have many thin ascending and descending collaterals. Some of the ascending collaterals become thin horizontal fibers within the outer region. The neurons of the cortical gray matter are still immature at the bipolar stage of development except for those located at cortical depths of 400-500/zm. At this depth the neurons are more developed. These neurons are typical pyramidal cells and they are considered to represent the first recognizable sign of the formation of layer V. Below these partially developed pyramidal cells, and to a cortical depth of about 900/zm, many irregular immature neurons were stained representing the neuronal elements of the developing layer VI.

Several types of fiber (Fig. 1) were found in the Golgi preparation of the cortex

Brain Research, 23 (1970) 167-183

170 M. MARINopAD1LLA

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ONTOGENESIS OF HUMAN MOTOR CORTEX 171

of this infant. Many glial fibers cross the cortex from the ependymal to the pial surface without collaterals. These terminate in a characteristic manner under the pial surface. This peculiar termination of the glial fibers facilitates their identification. Another lYequently encountered type of ascending fiber comes from the white matter, ascends vertically without collateral branches and terminates in the inner region of layer 1 where it becomes horizontal (Fig. 1). These fibers could be followed for long distances in layer 1. The horizontal portion of these fibers has many ascending and descending short collaterals. These fibers come from the deep horizontal systems of fibers in the intermediate layer and are the source of the tangential fibers of layer |. Thin oblique and horizontal fibers (Fig. l) were also stained at cortical depths between 500-700/zm and, therefore, around the developing pyramidal cells of layer V. The cortical location of these fibers suggests that they could represent the beginning of the development of the internal band of Baillarger.

In addition, the two systems of horizontal fibers of the intermediate zone of the cortex were identified. They are prominent structures of the cortex of this infant. They measure between 200-300/~m in thickness and are composed of numerous bundles of fibers. The outer set appears to be richer in fibers than the inner one. In a cross section of the entire brain (Fig. 1) these systems of fibers come from the region of the internal capsule and some of their fibers pass through the corpus callosum to the other cerebral hemisphere. In view of the incomplete development (immaturity) of the efferent neurons of the cortex (pyramidal cells of layer V) it was possible to assume with some degree of certainty that these two systems of horizontal fibers are composed predominantly of afferent fibers to the cortex. Since the fibers come from the region of the internal capsule they probably originate in the subcortical centers of the midbrain (the basal ganglia and the thalamus).

Fetus 7 months old. The cerebral cortex of this infant had already developed the gross external configuration and the sulcal pattern that characterizes the human brain. The precentral gyrus was identified and tissue blocks were obtained from it. Golgi preparations of the motor cortex of this infant (Fig. 2) disclosed the following

Fig. 1. Composite figure of camera lucida drawings made from Golgi preparations of the cerebral cortex of the 5-month human fetus. Included in this figure are: a diagram of a cross section of the brain of this infant, reconstructed from Golgi and H E sections, to illustrate the general disposition of the horizontal systems of fibers of the intermediate zone of the cortex and an enlargement of the cortical region between the vertical bars to illustrate the general structure and the different zones of the cerebral cortex at this age. The general structure of the cerebral cortex of this infant and the stage of development of its neurons, systems of fibers and cortical layers as seen in Golgi preparations are illustrated. Only layers I, V and VI, and possibly the beginning of the formation of layer III, are recognizable in this infant. Layer I appears more advance in development than the rest of the cortical gray matter. The two systems of horizontal fibers of the intermediate zone are prominent structures of the cortex at this age. The fibers of these systems come from the region of the internal capsule and cross the corpus callosum to the other cerebral hemisphere. From these two systems of fibers two ascending types penetrate the developing cortical gray matter. Some of these ascending fibers terminate in the inner region of the marginal zone (layer I) in which region they become horizontal (tangential fibers of layer I). Other fibers terminate around and below the developing pyramidal cells of layer V (internal band of Baillarger). Glial fibers are abundant at this age and they are easily recognizable by their peculiar types of termination under the pial membrane. A. F., afferent fiber; G. C., glial cell; B. V., blood vessel; G. F., glial fiber. Rapid Golgi method. Scale, 100/~m.

Brain Research, 23 (1970) 167-183

172 M. MARtN-PAI)ILLA

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Fig. 2. Composite figure of camera lucida drawings made from Golgi preparations of the motor cortex of the 7-month fetus, representing a transverse section of the precentral gyrus. The general structure of the motor cortex and the stage of development of its neurons, systems of fibersand cortical layers are illustrated. Layer I has already acquired mature characteristics. Layer II is beginning its development. Layer III is developing. Layer IV is beginning its development. Layers V and VI are well established. The system of tangential fibers of layer I, and the external (layer IV) and internal bands (layers V-VI) of Baillarger are recognizable and prominent. These three systems are composed of ascending afferent fibers which come from the white matter and become horizontal at those cor- tical levels. There are other ascending fibers which are seen branching at cortical levels between layers III-V. Thin oblique and horizontal fibers are also seen in layer III. T. F., tangential fibers; Ext. B. B., external band of Baillarger; Int. B. B., internal band of Baillarger; A. F., afferent fibers. Rapid Golgi method. Scale, 100 urn.

r e c o g n i z a b l e c o r t i c a l l ayers : l ayer 1, an i m m a t u r e l ayer I I , a d e v e l o p i n g layer I I I ,

t he ea r ly s tages o f t he d e v e l o p m e n t o f layer IV, a n d layers V a n d VI .

A t this age, l aye r I (Fig . 2) has a l r e ady a c q u i r e d m a t u r e charac te r i s t i c s . I t

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ONFOGENESIS OF HUMAN MOTOR CORTEX 173

measures between 150-200 #m in thickness and appears to be clearly subdivided into

two regions. The outer region is characterized by large Cajal-Retzius cells which are quite abundant. The inner region is composed of many horizontal fibers with ascend- ing and descending collaterals. Layer I I is beginning to develop. The majority of its

neurons are immature at the bipolar stage of development but a few of them have assumed the form of small pyramidal cells. Layer IH is also developing. Many pyram-

idal cells are found that tend to concentrate in its upper and lower regions. Some immature stellate and double-tufted neurons were also stained in layer lII. Layer 1V is difficult to recognize as a cortical lamination at this age. In some of the Golgi sections it is more apparent than in others. A few very immature stellate neurons are encoun- tered at this level. This level of the cortex is represented by a distinct 'space' between the pyramids of lower layer I | I and those of layer V. These observations led to the assump-

tion that the early stages of the formation of layer IV are represented in the motor cortex of this infant. Layer V is composed of apical and basal dendrites, many of which are already covered by a moderate number of dendritic spines. Layer VI has many developing, irregular neurons scattered throughout its territory.

Several systems of fibers (Fig. 2) were stained in the cortex of this infant. The horizontal (tangential) system of the inner region of layer I is composed of numerous thick fibers with ascending and descending collaterals. Another system of thick horizontal fibers was found at cortical depths from 600 to 700 #m. This roughly coincides with the location of the developing layer IV. This system appears to be formed by ascending afferent fibers from the white matter which become horizontal at this level of the cortex. In a few instances (Fig. 2) the fibers of this system were seen to branch at the level of lower layer lIl and layer IV. This system of horizontal fibers, by its location in the cortex and by its apparent relationship to the developing layer IV could represent the early development of the external band of Baillarger. Another system of horizontal fibers located below the pyramidal cells of layer V and in the upper region of layer VI was also stained in the cortex of this infant. This system also appears to be formed by ascending afferent fibers from the white matter and repre- sents the internal band of Baillarger. In addition to these systems of horizontal fibers, ascending afferent axons were found which terminate in the inner region of layer l where they become horizontal fibers and seem to come directly from the deep regions of the white matter. Thin horizontal and oblique fibers were also stained in layer | l l of the cortex of this infant, but none were stained in layer II.

Fetus 7.5 months old. The gross external configuration of the cerebral cortex of this fetus was that of a prematurely born infant. Tissue blocks were obtained from the pre- central gyrus. Golgi preparations of the motor cortex of this infant (Fig. 3) demon- strated that all cortical layers were already established. The motor cortex of this infant was predominantly characterized by the marked development of layer IV.

Layer I seems to be fully developed at this age. The Cajal-Retzius cells are prominent and their horizontal dendrites are markedly developed. The two regions of layer l are well established. The terminal dendrites of the pyramidal cells have penetrated the inner, and have begun to invade the outer, region of layer I. Layer II is developing and marly small pyramids and immature stellate and double-tufted

Brain Research, 23 (1970) 167-183

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Fig. 3. Composite figure of camera lucida drawings made from Golgl preparations of the motor cortex of the 7.5-month human fetus representing a transverse section of the precental gyrus. The general structure of the motor cortex of this infant and the stage of development of its neurons. systems of fibers and cortical layers are illustrated. The motor cortex of this infant is characterized by the marked development of layer IV and by the subdivision of layer III into an upper and a lower region. The tangential fibers of layer I, and t he external and internal bands of Baillarger are prominent structures of the motor cortex of this infant. In addition, afferent fibers are seen branching in all cortical levels (layers I1-VI). Occasional Martinotti cells are seen in the deep layers of the cortex with their ascending axon terminating in layer I. The axon of the Martinotti cells terminates in layer 1 by a peculiar type of branching and gives off many short collaterals to the different layers it crosses. T. F., tangential fibers; Ext. B. B.. external band of Baillarger; Int. B. B., internal band of Baillarger; M. F., Martinotti fiber; M. C.. Martinotti cell: A. F.. afferent fiber. Rapid Golgi method. Scale. 100 #m.

n e u r o n s were s ta ined . Laye r 11I has a l r e ady d e v e l o p e d def in i t ive i n n e r and o u t e r

r eg ions p o p u l a t e d by t w o d i s t inc t t ypes o f p y r a m i d a l cells, t he less d e v e l o p e d m e d i u m -

sized cel ls o f t he o u t e r r e g i o n a n d the m o r e m a t u r e l a r g e r ones o f t h e inne r r eg ion .

L a y e r I I I is a l so c h a r a c t e r i z e d by the p re sence o f m a n y d o u b l e - t u f t e d n e u r o n s with

the i r typ ica l a x o n i c d i s t r ibu t ion . L a y e r IV is well e s t ab l i shed a n d c o n t a i n s n u m e r o u s

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ONTOGENESIS OF HUMAN MOTOR CORTEX 175

typical stellate interneurons (cortical basket cells). Layer IV measures about 200/~m in thickness and appears clearly demarcated between the pyramids of lower layer I II and the pyramids of layer V. Layers V and VI do not show significant changes from what has been described for the 7-month fetus except for the larger size of its neurons.

The horizontal fibers of the inner region of layer I and those of the external and the internal bands of Baillarger are prominent and well established (Fig. 3) at this age. The terminal branchings of afferent fibers were stained in layers I1, l lI (more prom- inently in its lower portion) and in layer V. The axons of Martinotti cells were occasionally stained in the cortex of this infant. These are very thin, ascending, giving off many short collaterals, and terminating in layer I by means of a peculiar branching. This type of branching closely resembles the terminal branches of the apical dendrites of the pyramidal cells. Immature Martinotti cells are recognizable at this time in layers V and VI (Fig. 3). These cells are fully developed at the time of birth. Ascend- ing afferent fibers which come from the deep regions of the white matter and terminate in the inner region of layer I (tangential fibers) were also stained in the cortex of this infant.

Newborn infant. The motor cortex of this infant (Fig. 4) is characterized by the marked development of the pyramidal and the basket cells of the lower portion of layer 11I and of layers IV and V.

Layer I is fully developed (Fig. 4). Layer II is composed of many small pyramidal cells and by developing typical cortical basket cells. The upper region of layer l lI is developing and immature cortical basket cells without pericellular baskets are found at this level. The lower region of layer l l I appears to be more developed than the upper region and contains well-developed double-tufted and cortical basket cells. Many pericellular baskets around the bodies of the large pyramidal cells of the lower portion of layer Il l were stained. Layer IV measures 300 #m in thickness and is fully developed. The cortical basket cells are the predominant neurons of this layer. Layer V is fully developed and, in addition to the giant pyramidal cells, many cortical basket cells and periceIlular baskets were stained. Layer VI is composed of many irregular and polymorphous neurons about which little is known. Of special interest in the motor cortex of this infant is the presence of well-developed Martinotti cells in layers VI and V (Fig. 4). These cells are fusiform neurons having ascending and descending dendrites with a characteristic distribution and an ascending axon which terminates in layer I of the cortex. The ascending axon of the Martinotti ceils gives off many collaterals to layers V, IV, III and II as it crosses them and terminates in layer I by branching in a most peculiar manner. This terminal branching mimics the terminal dendritic branches of the apical dendrite of the pyramidal cells (Fig. 4).

It is virtually impossible to describe adequately the complex pattern of fibers in the motor cortex of this infant, it should be emphasized that in addition to the thick horizontal fibers of the external and internal bands of Baillarger, two new systems of thin horizontal fibers were stained in the motor cortex at cortical depths which coin- cided with lower layer lII and with layer V, respectively. Among the fibers of these two horizontal systems many pericellular baskets were stained. This suggests that the

Brain Research, 23 (t970) 167-183

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Fig. 4. Composite figure of camera lucida drawings made from Golgi preparations of the motor cortex of the newborn infant representing a transverse section of the precentrat gyrus. The general structure of the motor cortex of this infant and the stage of development of its neurons, systems of fibers and cortical layers are illustrated. The motor cortex of this infant is characterized by the marked development of the pyramidal and the cortical basket cells. The cortical basket cells of lower layer HI and layers IV and V are considered to be mature and many pericellular baskets are found at these cortical levels. Immature cortical basket cells are found in upper layer I t i and layer II and hence no pericellular baskets are found at these cortical levels. Martinotti cells are found in layers IV and V and the morphology of their axon and their typical termination in layer I are illustrated. Other neuronal types are also illustrated. M. F.. Martinotti fiber: A. F.. afferent fiber. Rapid Golgl method. Scale, 100/~m.

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Fig. 5. Composite figure of camera lucida drawings made from Golgi preparations of the motor cortex of the 2.5-month-old infant representing a transverse section of the precentral gyrus. The general structure of the motor cortex of this infant and the stage of development of its neurons, systems of fibers and cortical layers are illustrated. The motor cortex of this infant is characterized by the reduction of the thickness of layer VI, by the maturation of the cortical basket cell of upper layer III, and by the marked horizontal growth of the apical dendrites of the pyramidal cells within the outer region of layer I. Pericellular baskets, in addition to those of layers V and IV and the lower portion of layer III, are found in the upper portion of layer III. The cortical basket cells of layer II are immature at this age. Rapid Golgi method. Scale, 100/~m.

178 M. MARt N - P A I ) I LL ,'~

fibers of these two systems are predominantly composed of the horizontal axonic collaterals of the cortical basket cells.

Infant 2.5 months old. There were not sufficient differences between the motor cortices of 2- and 2.5-month-old infants to require a separate descript,~n of each case. The majority of the data described here belongs to the older infant~ The motor cortex of this infant is characterized predominantly by the advanced development of the upper portion of layer III (Fig. 5/. Many well-developed basket cells as well as pericellular baskets were stained at this level of the motor cortex.

Two other findings characterized the motor cortex of this infant. The first

consists of reduction in thickness of layer IV. This phenomenon, rather than being the result of an actual reduction of the layer, appears to be the result of the ingrowth of the pyramidal cells of lower layer III and layer V. Layer IV is obscured by the invasion of pyramidal cells resulting in an apparent reduction of its thickness. In some of the sections layer IV is no longer recognizable and the typical appearance of an agranular cortex becomes evident. The classical concept that the motor cortex is agranular without an appreciable layer IV should therefore be re-evaluated since this feature appears to be the result of a developmental structural modification peculiar to this

region of the cerebral cortex. The second characteristic of the motor cortex of this infant is that layer II seems to be the least developed of the motor cortex at this age.

The fiber composition of the cortex at this age is very complex. Horizontal thin fibers and pericellular baskets were stained in the upper region of layer Ill in addition to those of lower layer I I I and layer V. Also. the cortical basket cells of tipper layer I I I are prominent neurons at this age. Although cortical basket cells were recognized in layer 1I no pericellular baskets were stained at this level of the cortex

Infant 8 months old. The motor cortex of this infant is characterized by its extra- ordinary complexity. The number of fibers has increased considerably in all cortical

layers and the Golgi preparations have become obscure and difficult to analyze. The processes of all the neurons have increased and multiplied considerably. The thickness of the cortex, however, has not changed significantly in comparison with that of the 2.5-month-old infant. This suggests that by this time the motor cortex has already acquired its mature thickness and that it will continue to develop mainly in intracorti-

cal complexity.

D I S C U S S I O N

Although the cases studied do not represent a complete developmental series of

the ontogenesis of the human motor cortex, some general ideas can be drawn from their analysis. Perhaps the most significant observations that can be derived from this study are : (a) the structural organization of the motor cortex follows a distinct se, quence in ontogenesis; (b) the development and subsequent maturation of the efferent cortical neurons and the formation of the cortical layers occur after the arrival o f the

different afferent fibers to the cortex; and (c) the development and subsequent matura- tion of the interneurons parallel the growth and differentiation of the efferent neurons

with which they establish intraeortical neuronal chains.

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ONTOGENESIS OF HUMAN MOTOR CORTEX 179

In the cerebral cortex of the 5-month fetus the advanced development of the horizontal systems of fibers of the intermediate zone contrast sharply with the relative immaturity of the primordial cortical gray matter. The efferent pyramidal ceils of layer V at this age are immature and their axons are too thin to account for the numerous thick fibers which compose these horizontal systems of fibers. This incon- sistency suggests that these systems of fibers are older cortical structures that have developed earlier than the cortical gray matter in ontogenesis. The fibers of these systems come from the region of the internal capsule and probably originate in the

subcortical centers of the midbrain. Therefore, these systems of horizontal fibers must be composed predominantly of afferent fibers to the cortex, it is reasonable to assume that the arrival of these afferent fibers precedes the development and organiza- tion of the primordial cortical gray matter. According to Poliakov s some afferent fibers to the cortex can be recognized as early as the second or third month of intra- uterine life. The subsequent organization of the cortical gray matter into several layers seems to follow the sequential arrival of afferent fibers to the different cortical

levels. The Cajal-Retzius cells are the first neurons to develop and to achieve maturity

in the human motor cortex. Their early development is closely related to, and seems to follow, the early arrival of afferent fibers to the inner region of layer I of the cortex.

The Cajal-Retzius cells, together with these afferent fibers, form the first neuronal chain to be found in the motor cortex. A detailed description of the structure of these cells and of the neuronal chain formed by them will be presented in a separate com- munication. The pyramidal cells of layer V are the next neurons to develop. At 5 months of prenatal life these cells, although immature, can already be recognized. At this time, too, afferent fibers are seen between the developing layers V and VI of the cortex. These fibers, which come from the white matter, become horizontal at this cortical level and constitute the internal band of Baillarger. The arrival of these fibers

to the cortex and the development of the pyramidal cells of layer V are considered to be interrelated. The interneurons of layer IV (the cortical basket cells) are the next group of neurons to develop in the motor cortex. They begin to appear around the 7th month of prenatal life and by 7.5 months are readily recognizable. Their development could be related to the arrival of afferent fibers at that level of the cortex. These fibers also come from the white matter and become horizontal at the level of layer IV. They constitute the external band of Baillarger. The pyramidal cells of the lower portion of layer 11I develop earlier than those of its upper portion in the motor cortex. Their development could be related to the arrival of associative afferent fibers at this level of the motor cortex. The pyramidal cells of the upper portion of layer III seem to achieve Full development postnatally which could be related to the arrival of afferent fibers at this level of the motor cortex. The pyramidal cells of layer II are the last of such cells to develop in the human motor cortex. The development of the interneuron of the different cortical levels of the motor cortex parallels that of the pyramidal cells of the same level. During late prenatal ontogenesis of the motor cortex many and various types of small neurons of Golgi type II appear and develop in all cortical layers.

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180 M. MARIN-PADILLA

The development of the cortical laminations seems to depend upon the arrival of afferent fibers to the cortex and follows a similar ontogenetic sequence. The cortical layers are formed when the neurons of the different cortical levels become organized following the establishment of neuronal chains with the afferent fibers. The sequential appearance of the cortical layers during prenatal and early postnatal ontogenesis of the human motor cortex as found in this study is shown in Table I.

The development of layer IV of the human motor cortex is of special interest. This layer begins to develop around the 7th month of intrauterine life, and by 7.5 months it is well established. Its development seems to follow the arrival of afferent fibers at that cortical level. At the time of birth it is well developed. At 2.5 months of postnatal life layer IV is very thin and in some sections of the motor cortex it is not even recognizable. By the 8th month the motor cortex has achieved its characteristic agranular structure and layer IV is unrecognizable. This feature is caused by the marked development of the pyramidal cells of lower layer III and layer V. These neurons grow considerably during the course of postnatal ontogenesis of the motor cortex and invade the territory of layer IV to such extent that this layer is largely obscured. Its typical neurons, however, can be found around the bodies of the pyrami- dal cells of lower layer III and layer V. These neurons of layer IV, recognized as the cortical basket cells, achieve an extraordinary development in the human motor cortex. These interneurons, with the pyramidal cells, form intracortical vertical neuronal chains which will be described in more detail in a separate communication 6.

Layer VI, of which so little is known concerning its structure and function appears to develop early in prenatal ontogenesis. Its neurons are of various kinds with ascending or descending axons, but are difficult to classify. The early development of the deep cortical layer of the motor cortex and its possible relation to the early arrival of afferent fibers needs further investigation.

The individual developments of the different neurons, systems of fibers and layers of the motor cortex during the course of ontogenesis are better represented by composite drawings than described verbally. The figures which accompany thiscommu- nication were purposely reproduced on the same scale to permit and facilitate the analysis of the development of the different cortical structures during the course of ontogenesis. The growth of the motor cortex as a whole, and the cortical location of its neurons and fibers are also pictorially reproduced in the figures.

The analysis of the sequential development of the human motor cortex presented here is but a superficial outline of a complex event based upon a study of only a few of its basic neurons and afferent fibers. Many more neurons and fibers of various kinds exist in the motor cortex about which practically nothing is known with certain- ty. Further investigations are obviously needed. It should be emphasized that the simplicity of the cerebral cortex during prenatal ontogenesis greatly facilitates the analysis of its structure and permits a better comprehension of its increasing com- plexity as the brain matures. The developmental approach to the study of the cerebral

cortex needs to be encouraged.

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O N T O G E N E S I S OF H U M A N M O T O R C O R T E X 181

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Brain Research, 23 (1970) 167-183

[ 82 M. MAR1N-PAD1LLA

SUMMARY

A Golgi study of the prenatal and early postnatal development o f the human

motor cortex is presented. Observations made in this study justify the following con-

clusions. (A) The arrival of afferent fibers to the cortex precedes the organization of

the primordial cortical gray matter into a recognizable laminated structure. (B) Layer

I is the first level of the cortex to develop and to achieve maturity, its development and

maturation are related to and seem to follow the early arrival of afferent tibers (tangen-

tial fibers) to this level of the cortex. These afferent fibers, with the Cajal-Retzius cells.

form the first distinct neuronal chain to be found in cortical ontogenes~s, q-here is

evidence which supports the concept that this neuronal chain persists throughout life

in the motor cortex. (C) The formation and organization of the cortical layers and the

development and maturation of the efferent neurons are related to the arrival of

afferent fibers to the cortex. The arrival of the different afferent fibers follows a dis-

tinct sequence in cortical ontogenesis. (D) The development and maturation of the

interneurons of the motor cortex parallel that of the efferent neurons with which they

establish intracortical neuronal chains. (E) The human motor cortex is characterized by

the postnatal reduction and eventual disappearance of layer IV. This is a developmental

peculiarity of this area (area 4) of the cortex. It is caused by the marked postnatal

growth of the pyramidal cells of lower layer [11 and layer V which invade the territory

of layer IV to such an extent that this level of the cortex can no longer be recognized

(agranular cortex). (F) The human motor cortex is characterized also by ~he marked

development of pyramidal and basket cells in layers I1. III and V. These two types of

cells form intracortical chains organized into vertical neuronal systems.

ACKNOWLEDGEMENT

This work was supported by Grant HD-03298-08 from the National Institutes of

Health.

REFERENCES

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2 CONEL, J. L., Postnatal Development of the Human Cerebral Cortex, Vols, l-V, Harvard Univ. Press, Cambridge, Mass., 1939, 55 pp.

3 KOELLIKER, A. YON, Handbuch der Gewebelehre des Menschen, Engelman, Leipzig, 1896, pp. 644-650.

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6 MAR1N-PADLLA, M., Prenatal and early postnatal ontogenesis of the human motor cortex. II. The basket-pyramidal system, Brain Research, 23 (1970)185-191.

Brain Research, 23 (1970) 167-183

ONTOGENESIS OF HUMAN MOTOR CORTEX 183

7 POLIAKOV, G. 1., Embryonal and postembryonal development of neurons of the human cerebral cortex. In R. HASSLER AND H. STEeH~N (Eds.), Evolution of the Forebrain, Phylogenesis and Ontogenesis of the Forebrain, Plenum, New York, 1967, pp. 249-258.

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Brain Research, 23 (1970) 167-183