“Dendroarchitectonics”: From Santiago Ramón y Cajal to Enrique Ramón-Moliner or vice versa?

The fundamental neuroanatomical work of Santiago Ramón y Cajal (Fig. 1) from around the turn of the nineteenth to the twentieth century, such as his influence on the neuron theory and the idea of nervous system plasticity, is still highly relevant [1‐4]. One of his favorite topics was the scientific question of what makes the human cerebral cortex with its pyramidal cells special and different from other species and how ontogenetic development relates to phylogeny. Thus, he and many others carried out comparative neuroanatomical studies to solve the old question of the structural basis of cognitive and mental skills such as creativity, which is still under debate [5‐12]. Indeed, the multilayered neocortex allows for complex information processing with intricate abilities such as cognitive or motor functions [13]. More than half a century after S. R. y Cajal's work, Enrique Ramón-Moliner (Fig. 2), adopting the same morphological perspective and studying various species' nervous systems, arrived at a different model in terms of an integrative phylogenetic-ontogenetic interpretative framework. Understanding the findings that have led to this change serves as the basis for generating future scientific hypotheses. Thus, we discuss these morphological findings and their implications on ontogenetic development and phylogenetic history. For the presentation of the relevant work of S. R. y Cajal, we use the annotated and edited translation from Spanish—with the additions of the French version—into English by P. and T. Pasik. In keeping with this, S. R. y Cajal 1904 (vol. 2, second part) corresponds to Pasik and Pasik, 2002 (vol. 3).

When describing the olfactory apparatus, S. R. y Cajal distinguished the first-order olfactory center, the olfactory bulb, where the central processes of the olfactory mucosa's bipolar collector cells terminate. The second- and third-order olfactory centers comprise the lateral olfactory tract, anterior olfactory nucleus/frontal cortex subjacent to the lateral olfactory tract, parahippocampal gyrus/pyriform lobe, including the (medial) entorhinal cortex, (pre-)subiculum, and others. With the fourth-order olfactory centers, S. R. y Cajal distinguished a hippocampal gyrus from a dentate gyrus (for their layers, see Table 1) [14]. Accordingly, the hippocampal gyrus is a thin and simplified band of the cerebral cortex. Its free border apparently is covered by the cavity of the dentate gyrus, which is an even more simplified cortical structure. These investigations were built upon earlier studies, such as the work of Sir Grafton Elliot Smith (1871–1937), who was a distinguished Australian anatomist interested in Egyptian mummies and paleoneurological diseases [15, 16]. Early in his professional life, in 1896, he demonstrated that in lower mammals (such as the platypus), there are regions of the hippocampal gyrus where the zone of granules of the dentate gyrus appears in continuation with the hippocampal pyramidal cells [17].

S. R. y Cajal and other authors at his time explored the comparative anatomy of the cerebral cortex (Table 1). He specifically commented in detail on fish, amphibians, reptiles, birds, and small mammals [14]. In fish, the presence of a pallium or cortex was, at least generally, denied. They only have the basal region of the cerebrum, corresponding to the pyriform lobe, parolfactory regions, and corpus striatum of higher vertebrates. The cerebral cortex of amphibians would be a most simple construction, representing a rudiment of the hippocampal gyrus. It consists of three fundamental layers: the zone of epithelial cells, the zone of granules or pyramidal cells, and the molecular or plexiform zone. The latter is the thickest and comprises two formations: first, the dendritic plexus, which is characteristic of the plexiform or molecular layer of humans and higher vertebrates and occasionally contains neurons (layer poor in cells of Meynert); second, the projecting and association axons. This superficial arrangement of fibers recalls that of the funiculi of the spinal cord. Although less accentuated, this array can also be seen in certain regions of the mammalian cerebrum, such as the extrinsic fibers of the hippocampal and dentate gyri or the lateral olfactory tract covering the temporal cortex. In reptiles, the axon of the pyramidal cell turns toward the inner surface of the brain and gives off collaterals below instead of above the soma. The cerebrum of birds shows an enormous size of the corpus striatum or fundamental ganglion, which is adherent to the cortex proper except in the medial aspect of the hemispheres. In this area, a prolongation of the ventricle separates these two central nervous system (CNS) structures. The cerebral cortex of birds is composed of the ependymal layer, the layer of inner stellate cells, the layer of large pyramidal and stellate cells, the layer of small stellate cells, and the molecular or plexiform layer. The cerebrum of small mammals such as rodents would include, in addition to the ependyma and white matter, the following cortical zones: the layer of ovoid or polymorphic cells, layer of large pyramidal cells, layer of medium pyramidal cells, layer of small pyramidal cells, and the plexiform layer. This layer array is a simplification of the cortex in humans and other gyrencephalic mammals (which are mammals with a folded cortex). These also show a granular zone, and their cerebrum includes (when leaving aside regional differences) the following layers in addition to the ependyma and white matter: fusiform cell layer, inner medium pyramidal cell layer, inner large pyramidal cell layer, dwarf pyramidal cell and stellate cell layer (or granule cell layer), outer medium and large pyramidal cell layer, small pyramidal cell layer, and the plexiform layer (layer poor in cells of Meynert, molecular layer). The granular cell layer shows massive growth in the human cortex compared to other gyrencephalic mammals [14].

According to S. R. y Cajal, the morphological simplification, beginning mainly in rodents and peaking in birds, reptiles, and amphibians, applies to the number of layers or the numbers of differentiated regions and, in particular, to the morphological features of nerve cells with dedifferentiation into this direction [14]. However, the neuronal radial orientation (with tufts emerging from the outward pole of the pyramidal cells) and the plexiform layer (with the tufts contacting with afferent fibers) would be a constant feature. With the extrapolated, high activity in the order of function of the pyramidal cell, he introduced the name "psychic cell," also called "psychomotor cell" [2], for these neurons. As to the cerebral cortex, any pyramidal neuron or other long-axon cell with a radial stem sends a dendritic tuft of ramification to the plexiform layer, but most short-axon neurons do not have a dendritic representation in the first layer [14]. S. R. y Cajal contended that the short-axon cells would function as "electric condensers of neural energy," transforming latent into active energy by an arriving current via an afferent fiber. This "dynamic reserve" would be the basis for outstanding human mental abilities and neural processes, which happen later than external excitation, such as memory or ideation [6, 14]. S. R. y Cajal considered the bitufted cells, a form of short-axon cells, as one of the essential features of the human cortex, given their high abundance; he found these cells rarely in dogs and cats, with less delicate features than humans.

Through investigations of pyramidal neurons in the cerebral cortex of animal series, embryonic and juvenile stages in the mammalian histogenesis, S. R. y Cajal postulated correspondence between the phylogeny and ontogeny [4, 14] (Fig. 3A). He generalized this idea to neuroglial cells. Based on structural similarities of "adult" forms of various vertebrates and mammalian ontogenetic stages, he contended that a given ontogenetic phase of the pyramidal neurons represents a phylogenetic phase. In keeping with this, the different forms of developing human neurons would be close to the "adult" forms of amphibians and reptiles. Figure 3A  shows the stages of the ontogenetic development of mammals and the adult forms of the pyramidal cells from various vertebrates (frog, reptile, mouse, and human). For the ontogenetic development of humans (Fig. 3A), he discriminated a neuroblast phase (neurons with an axon processing from the cell body), followed by the (secondary) bipolar phase (cells with a thicker, varicose process from one pole and a finer process emerging from the other pole). Pyramidal cells with basilar dendrites, axon collaterals, and centripetal fibers follow. However, some ontogenetic stages (e.g., bipolar cells) would not correspond to a phylogenetic phase since the individual development is characterized by a continuous development with more transitional structures. In phylogeny, some forms would be present rather transiently [14].

Substantial advancement has been achieved in our scientific understanding of phylogeny and ontogeny over the past centuries, and the ideas about concerted vs. mosaic phylogenetic development (i.e., coordinated vs. independent variation) are still a matter of debate [26, 27]. In the 1960s and 1970s, E. Ramón-Moliner appreciated the impact of nervous function on the morphological specialization of cells leading to segregation and loss of diffuse (receptive) contacts [18, 20, 23]. S. O. E. Ebesson's "parcellation theory," raised in the 1980s, suggested that the nervous system becomes more complex during phylogeny, and likewise ontogeny, not by one system invading another but by parcellation (segregation–isolation) followed by selective loss of connections of the newly formed subsystems [28, 29]. Accordingly, overlapping circuits are more common in primitive (generalized) as compared to specialized brain organizations. He also stated that neocortical equivalents have been present since the start of vertebrate phylogenesis. However, brains may have less dense connections per se due to their increasing size [30]. Moreover, E. Ramón-Moliner stressed that, during phylogeny, a trend reversing the parcellation theory occurs [31]. For example, the (lower layer of the) colliculi superiores of mammals possess many heterogeneous connections with the reticular formation [23]; in contrast, in reptiles or amphibians, the superior colliculi have more demarcated borders, being more segregated. Herewith, he rejected the idea that the ontogenetic loss of connections always corresponds to adult ancestral forms.

Since humans and mice have homologous cells allowing for direct comparisons, studies on these animals have been used to investigate cortical development (e.g., [32]); however, these results cannot simply be transferred to humans [33]. The cortex of rodents, such as mice, has its own phylogenetic history. At the turn of the nineteenth to the twentieth century, S. R. y Cajal postulated a correspondence between phylogeny and ontogeny of the cerebral cortex, arguing that a given ontogenetic phase of the pyramidal neurons represents a phylogenetic phase [14]. He contended that ontogenetic phases of "the" cerebral pyramidal cell (the "psychic cell") would correspond roughly to "adult" forms as they phylogenetically appear in invertebrates/lower vertebrates. In contrast, more than half a century later, in reconstructing the probable course of events in the phylogenetic history that led to the dendroarchitectonic families employing a three-stage scheme, E. Ramón-Moliner did not consider ontogeny as a mere recapitulation of the phylogeny [18, 21]. Indeed, the lepto- or isodendritic configurations, the pools of undifferentiated neurons, remain diffusively distributed in the mammalian nervous system (such as the brainstem and periventricular areas of the cerebrum). Furthermore, the lophodendritic neurons and their development into pyramidal cells, characteristic of the cerebral cortex and indispensable for the most elaborate patterns of behavior and function, are no "new arrivals" but represent one of the earliest neuronal configuration trends. Neurons with subpial tufts are present not only in the telencephalon but also in the brainstem of lower vertebrates [20]. Patterning the neuraxis of vertebrates means a constant process over a long time in ontogenesis [34]. In terms of the terminal neurogenesis (i.e., the "birth" of cells following the final precursor cell division, yielding a neuroblast for further differentiation), various neurons with low morphological specialization, e.g., brainstem areas (raphe complex, superior colliculi, or locus ceruleus), and a high degree of morphological specialization (e.g., Purkinje cells) emerge at around the same in both monkeys and rats [35]; various limbic structures including hippocampus and amygdala show early development in monkey and synchronous neurogenesis with rats later on; and isocortical neurons terminally developed either at around the same time (layer VI), sequentially (layer V) or delayed (layer II-III) in monkeys as compared to rats. The earliest generated cell layers in the cerebral cortical development comprise preplate cells, with the Reelin secreting Cajal–Retzius cells being its most prominent component [36]. The latter cells are, in fact, a general feature of all vertebrates; however, there is an increasing Reelin signaling with developing cortical complexity, which might have contributed to the basic mammalian cortical pattern. The pallium in nonmammalian amniotes has a different architecture than its mammalian homolog, i.e., the six-layered neocortex; therefore, the capability to generate an orderly sequence of distinct neocortical cells was thought to have emerged in mammals [37]. However, it was shown that layer-specific neuron subtypes do exist in the chick pallium. Thus, the emergence of layer-specific neuron subtypes predates the development of the laminar architecture, suggesting that mammals and avians share the neocortical neuron subtypes [38]; also, their "common ancestor" may show a similar neuronal repertoire. Indeed, there is an astonishing degree of similarity, but not identicalness, in the features at the very early phases of development [39‐41], whereas at later stages, brain development diverges in the various vertebrate groups [13, 27, 42‐46]. The hypothesis of ontogeny as a "replay" of the phylogenetic development, i.e., phylogeny generating ontogeny with a highly conserved embryonic stage common to all vertebrates, is historically closely linked with the name Ernst Heinrich Philipp August Haeckel (1834–1919) [27, 30, 47, 48]; this hypothesis has also been referred to as the so-called biogenetic law. However, there are highly variable morphological features of vertebrate embryos forerunning essential differences in the adult structures [45]. More recent evidence also rejected the ontogeny recapitulating phylogeny idea by examining fishtails fossils [49]; instead, the final structure is ruled by differential growing.

In ontogeny, early developmental features of different neuronal cell types show a higher degree of similarity than their adult forms' features. While an increasing morphological specialization is associated with the reconstructed, projected phylogenetic development as an abstract whole, phylogenetically "primitive neurons" such as those found in the reticular formation are present at later phylogenetic stages; conversely, phylogenetically "new neurons," such as the cortical pyramidal cell, may be found early during phylogeny. Thus, in higher mammals' adult nervous system, phylogenetically "old primitive" neurons can coexist with phylogenetically "new complex" neurons. These results corroborate the rejection of the interpretative framework of ontogeny as a simple, speedy repetition of the phylogeny. The re-interpretation of ontogenetic and phylogenetic similarities and differences of neuronal morphologies and the historic underpinnings thereof may provide a useful framework for refining scientific hypotheses.