The information presented in the monograph concerns almost exclusively the prenatal development of the human motor cortex. Marín-Padilla M (1983) Structural organization of the human cerebral cortex prior to the emergence of the cortical plate.
Seven-Week-Old Stage
A rich pial anastomotic capillary plexus, with nucleated red blood cells, covers the entire still avascular cerebral cortex (Fig. 3.3c, e). Comparing both embryos, the brain of the other has grown significantly in just a few days (compare Fig. 3.3a, b).
Eleven-Week-Old Stage
Large pyramidal neurons of the SP zone are the largest and most developed neurons of the cerebral cortex at this age. The cortex of this fetus also has immature pyramidal neurons with spineless apical dendrites anchored to the first lamina, smooth somata, and short descending axons (Figure 3.5c).
Fifteen-Week-Old Stage
The arrival of these thalamic corticipetal fibers initiates the increasing functional maturation of gray matter pyramidal neurons. At different levels of the motor cortex there are intrinsic anastomotic capillary plexuses (Fig. 3.9, arrowheads).
Twenty-Week-Old Stage
Thirty-Week-Old Stage
Thirty-Five-Week-Old Stage
A partial view of the intrinsic anastomotic capillary plexus of the gray matter can be seen in Fig. Cataloging all the information of a single fast Golgi preparation of the motor cortex, at this age, would take an additional book.
Forty-Week-Old (Newborn) Stage
Marín-Padilla M (1969a) Origin of the pericellular baskets of the pyramidal cells of the human motor cortex. Marín-Padilla M (1972c) Double origin of the pericellular baskets of pyramidal cells of the human motor cortex.
Developmental Features
Neurons of the human motor cortex are used as models to describe the developmental, morphological and functional characteristics of the mammalian pyramidal neuron. Both types of developmental outgrowths (elongations) invalidate the generally accepted idea, which proposes that the apical dendrites of pyramidal neurons grow from the cell body to the first lamina.
Morphological Features
Given the extraordinary and increasing structural complexity of the neocortex, it is difficult to visualize the long apical dendrites of these neurons could grow (elongate) perpendicularly and thus clean up to the pial surface. On the other hand, since all pyramidal neurons retain their original first lamina dendritic attachment, as cortex thickness increases, all apical dendrites must extend upward (Fig. 4.3). This new view is applicable to all pyramidal neurons throughout the mammalian neocortex regardless of size or location, including those within deep sulci and/or superficial gyri.
Functional Features
For a long time, the anatomical and functional nature of the spine has been the source of controversy, as many consider them merely silver artifacts (Cajal 1923). The ascending axon of these inhibitory neurons reaches the first lamina and branches into a bouquet that mimics the terminal dendrites of the pyramidal neurons. The axons of these interneurons branch into several ascending and descending terminals, which run parallel to the apical dendrites of the pyramidal neuron.
The combined interactions of excitatory and inhibitory synaptic contacts will determine the final function of the pyramidal neuron.
Ascending Functional Maturation
At this age, these pyramidal neurons develop short basal dendrites and a few proximal apical dendritic spines, establishing the first functional P1 pyramidal layer in the motor cortex (Fig. 4.8b). At the same time, the pyramidal neurons above this stratum begin to mature functionally in parallel with the arrival of corticipetal fiber terminals at this level and. Some pyramidal neurons show evidence of cystic degeneration involving the soma and dendrites and severe loss of dendritic spines (c, d).
Degenerating pyramidal neurons with advanced cystic degeneration may appear contiguous with smaller unaffected ones (c).
Descending Function
Marín-Padilla M, Stibitz GR (1968) Distribution of the apical dendritic spines of layer V pyramidal neurons in hamster neocortex. Marín-Padilla M (1969) The origin of the pericellular curves of the pyramidal cells of the human motor cortex. Marín-Padilla M, Stibitz GR, Almy CP, Brown HN (1969) Spinal distribution of layer V pyramidal neurons in man.
Marín-Padilla M (1990) The pyramidal cell and its local circuit interneurons: a hypothetical unit of the mammalian cerebral cortex.
First Lamina Principal Components
Eventually, these first lamina-specific astrocytes will replace radial glia as the main source of endfeet for the EGLM of the expanding neocortex (Chapter 8). During late prenatal development, various small local circuit neurons are also incorporated into the first lamina. Other first components of the lamina include the long horizontal axon terminals of primordial corticipetal fibers, terminal dendritic tufts of zone SP pyramidal neurons, and those of newly recruited pyramidal neurons (Figure 5.1A).
By 24 weeks of gestation, the horizontalization of the dendritic and axonal processes of the C-R cell is at an intermediate stage and is recognized throughout the first layer of the neocortex (Fig. 5.1D).
Cajal–Retzius Cell Unique Morphology
In addition, the long horizontal axon terminals of CR cells are a prominent feature of the first lamina recognized throughout the neocortex (Figures 5.1–5.6). Therefore, neuronal dendritic and axonal morphology, stratification, and distribution are essentially identical and impossible to distinguish in rectangular and/or parallel Golgi preparations of the precentral gyrus. As expected, the profiles of terminal dendrites of pyramidal neurons also remain unchanged and indistinguishable in rectangular and/or parallel Golgi preparations.
A montage of camera lucida drawings of tangentially sectioned Golgi preparations of the motor cortex of 30-week fetuses; illustrate, more clearly, these characteristic neurons dendritic current (Fig. 5.10A) and tangential axon (Fig. 5.10B).
First Lamina Secondary Components
Probably the functional roles of these late-arriving corticipetal fibers represent postnatal events in the maturation of the cortex. Marín-Padilla M (1990) Three-dimensional structural organization of layer I of the human cerebral cortex: A Golgi study. While all pyramidal neurons in the cortex are generated in the ependymal epithelium, the origin of inhibitory interneurons appears to be extracortical.
At birth, different types of inhibitory local circuit interneurons are recognized throughout the pyramidal cell layers of the motor cortex (Marín-Padilla.
The Pyramidal-Basket System
The axon division of a large basket cell could cover a rectangular functional territory that is also flat and perpendicular to the long axis of the gyrus (Figures 6.2 and 6.3). The number and complexity of the baskets are greater than those of the upper pyramidal cell layers (compare with those of Figure 6.2). With good, rapid Golgi preparations it is possible to demonstrate the three-dimensional complex organization of the basket and its apparently empty center occupied by the unstained pyramidal cell body (Figures 6.6a, b, 6.7a and 6.8a-c). ).
The arrows indicate some of the horizontal ends of the fibers involved in the formation of the basket.
The Pyramidal-Martinotti System
Martinotti cells are bipolar interneurons characterized by long ascending and descending dendrites with spine-like projections and by a long ascending axon that crosses the maturing PCP and reaches the first lamina (see Fig. 3.14). Similarly, their distinct terminal axonic florets are also easily recognizable in the first lamina (Fig. 6.9a-c). The most characteristic feature of these local circuit neurons is their ascending axon that reaches and fans into the first lamina, forming a terminal bouquet of spine-like projections (Fig. 6.9a-c and Fig. 3.14a).
The size and distribution of terminal axon tufts of Martinotti neurons mimic the terminal dendritic tufts of pyramidal neurons of their cortical layer.
The Pyramidal-Double-Bouquet System
Furthermore, in tangential fast Golgi preparations of the human motor cortex, the presence of small (6–8) and large (10–12) apical dendrite clusters is a fairly common finding. The size of these tangentially cut cylindrical clusters of apical dendrites also decreases from lower to upper pyramidal cell layers. The diameter of the apical dendrites in these clusters also decreases from lower to upper pyramidal cell layers.
The size and functional territory of the double tufted interneuron increases from upper to lower pyramidal cell layers in parallel with pyramidal neurons in their corresponding layers.
The Pyramidal-Chandelier System
Marín-Padilla M (1974) Three-dimensional reconstruction of the pericellular baskets of the motor (area 4) and visual (area 17) areas of the human cerebral cortex. The perforating vessels within the V-RC represent the extrinsic microvascular system of the cerebral cortex. This distance also determines the size and extent of the intrinsic microvascular system of the cerebral cortex.
This area is quite active during the early embryonic development of the cerebral cortex.
First Lamina Special Astrocytes
Grey Matter Protoplasmic Astrocytes
A complete developmental study of the mammalian cerebral cortex as well as the use of appropriate. Marín-Padilla M (1990b) The pyramidal cell and its local circuit interneurons: a hypothetical unit of the mammalian cerebral cortex. The presence of an additional stratum of P6 pyramidal cells in the motor cortex of the newborn represents a fundamental observation (Chapters 3–9).
It is known that the cerebral cortex of the newborn has one layer less than that.
Cat 25-Day-Old Stage
Original studies of rapid Golgi development of the cat cerebral cortex brought a new idea about the dual origin of the mammalian neocortex (Marín-Padilla 1971). The developing cytoarchitecture of the cat's neocortex during the embryonic period is analyzed in the second chapter of this monograph. The quality and revealing capabilities of selected fast Golgi illustrations of the developing cat motor cortex speak for themselves.
It is important to emphasize that the Golgi observations of the prenatal development of the feline motor cortex presented in this chapter confirm the observations of the prenatal development of the human motor cortex (see Chapter 3).
Cat 30-Day-Old Stage
Prenatal development of the cat's cerebral cortex, as well as that of other mammals, is characterized by an early embryonic and late fetal period. Fetal cytoarchitectonic development of the cat's motor cortex as it develops through the 30.-, 35.-, 40.-, 45.-,. Most of the illustrated neurons represent subplate (SP) zone pyramidal-like neurons with ascending apical dendrites reaching and branching within the first lamina and multiple basal dendrites distributed throughout the SP zone (a–e).
These early SP neurons together with Cajal-Retzius (C-R) cells of first lamina (c) form the essential components of the mammal.
Cat 35-Day-Old Stage
Many white matter fibers have terminal growth cones that progress in opposite directions that represent incoming or outgoing corticofugal fibers (Fig. 11.3). At this age, most corticofugal fibers leaving the neocortex are axon terminals of SP pyramidal-like neurons (Fig. 11.3).
Cat 40-Day-Old Stage
Cat 45-Day-Old Stage
A camera lucida drawing, from rapid Golgi preparations of 45-d-o cat fetuses, reviews the cat motor cortex cytoarchitectonic development at this age, the establishment of P1 and P2 pyramidal cell strata, and the initial regression of the SP neurons' functional impairment . - branches with first lamina (Fig. 11.5b). The cat motor cortex thickness increased due to the PCP expansion, which is already 300 mm thick. The PCP expansion has displaced the SP zone downward, which is now located 600 mm below the pial surface (Fig. 11.5b).
The SP zone neurons, still quite prominent in the motor cortex, began to lose their first lamina functional contacts.
Cat 50-Day-Old Stage
Cat 55-Day-Old Stage
The developmental maturation achieved by the pyramidal neurons of stratum P1, located 500 mm from the pial surface, is remarkable. The pyramidal neurons of the P2 layer have already developed several basal dendrites and those of the P3 layer have begun their ascending functional maturation by developing short basal dendrites (Fig. 11.7a).
Newborn Cat Motor Cortex
Marín-Padilla M (1978) The dual origin of the mammalian neocortex and the evolution of the cortical plate. Marín-Padilla M (1969) Origin of the pericellular baskets of the pyramidal cells of the human motor cortex. Marín-Padilla M (1972a) Prenatal ontogenetic history of the principal neurons of the cat neocortex.
Marín-Padilla M, Stibitz G (1974) Three-dimensional reconstruction of the baskets of the human motor cortex.
