THE EVOLUTIONARY ORIGIN OF THE MAMMALIAN ISOCORTEX: INSIGHTS FROM MOLECULAR DEVELOPMENTAL BIOLOGY

Francisco Aboitiz, Daniver Morales and Juan Montiel

 

Affiliation:

Programa de Morfología, Instituto de Ciencias Biomédicas, Facultad de Medicina, Universidad de Chile.

 

Address:

1027 Independencia Ave., P.O. Box 70079, Santiago 7, Chile.

Tel: (56-2) 735 7068; Fax (56-2) 678 6264

E-mail: faboitiz@machi.med.uchile.cl

 

 

 

 

SHORT ABSTRACT

A long-standing debate in comparative neuroscience concerns the identification of homologues to the mammalian isocortex in other vertebrates. One perspective, based on similarities in the telencephalic sensory projections between birds/reptiles and mammals, asserts that the isocortex partly arose from a structure denominated dorsal ventricular ridge, that is present in the avian/reptilian brain. Alternatively, this paper presents some proposals that are based on recent results on the expression of regulatory genes in the developing telencephalon. The evidence suggests that the isocortex arose mostly from the reptilian dorsal pallium. A scenario for the origin of the isocortex is also provided which is consistent with the reviewed evidence.

LONG ABSTRACT

The isocortex is a distinctive feature of the mammalian brain, which has no clear counterpart in the cerebral hemispheres of other amniotes. Historically, there have been long-standing controversies regarding possible homologues of this structure in reptiles and birds. In these vertebrate classes, a structure denominated dorsal ventricular ridge develops in the lateral aspect of the hemisphere and, like the mammalian isocortex, receives ascending auditory and visual tectofugal projections. On these grounds, it has been postulated that part of the dorsal ventricular ridge is homologue to part of the mammalian isocortex (i.e. the auditory and the extrastriate visual cortices). Dissenting views have claimed that the dorsal ventricular ridge originates from a topographically different part of the hemisphere than the isocortex, and therefore there is no embryonic similarity between these two structures. Furthermore, recent evidence on the expression patterns of regulatory genes strongly suggests that a large part of the dorsal ventricular ridge arises from a region denominated the intermediate territory or ventral pallium, which in mammals gives rise to parts of the amygdalar complex among other structures. Considering that embryological criteria are in some cases more reliable to determine homology than comparisons of adult states, we are inclined for the developmental approach, which prescribes non-homology between the isocortex and the dorsal ventricular ridge. Additionally, we suggest a scenario for the origin of the isocortex as an expansion of the reptilian dorsal cortex, which is consistent with current evidence.

Keywords: Development, evolution, dorsal cortex, dorsal ventricular ridge, forebrain, homology, isocortex, regulatory genes, vertebrates, Wulst.

1. Introduction

The use of molecular techniques in the study of the development of the nervous system has proved to be particularly fruitful to determine genes involved in the specification of distinct brain components. These data may also offer interesting insights into brain evolution, especially when morphological evidence yields inconsistent results regarding cross-species homologies. In this paper, we discuss evidence for the specification of presumptive territories in the embryonic telencephalon, on the basis of expression patterns of regulatory genes. We propose that recent results in this area provide a new scope to understand a major evolutionary transition such as the origin of the mammalian isocortex, and a new criterion for developmental homology of brain components. Additionally, we suggest a scenario, consistent with developmental evidence, for the sequences of neurobiological events that gave rise to the isocortex.

2. The problem of isocortical origins

The mammalian isocortex is a character unique to mammals in several respects. First, it has undergone an enormous expansion, especially in the tangential domain (Rakic 1988). Second, it has a six-layered architecture which differs from the three-layered array of simpler telencephalic laminar structures such as the hippocampal formation, the olfactory cortex and the reptilian cortices (Supèr 1998). Although in other vertebrates there are some expanding telencephalic structures that receive a similar sensory input, in no case such a conspicuous laminar arrangement has been observed (Striedter 1997).

There have been important disagreements as to which components of the non-mammalian telencephalon can be compared to the isocortex. This problem is complicated by the intrincate topography of the hemispheres in some vertebrate classes, and by the absence of a single criterion to establish homology of neural structures. Commonly used criteria for similarity are connectivity (Karten 1969; 1997; Medina & Reiner 2000), neurochemistry (Reiner 1991; 1993), and embryonic origins (Källén 1951; Striedter 1997; Smith Fernández et al. 1998; Puelles et al. 1999). Unfortunately, when dealing with the homologues of the isocortex, these different approaches have been inconsistent.

This paper focuses on the comparison between reptilian and mammalian brains. The telencephalon of reptiles (and birds) is characterized by a periventricular structure denominated dorsal ventricular ridge (DVR), which receives many thalamic sensory projections. We will address the issue of a possible homology between the DVR and the isocortex (Karten 1969; 1997), and will review recent molecular evidence suggesting a different type of interpretation. Then, the general organization of the mammalian and reptilian brains will be discussed in light of these new findings. Our main argument along this paper is that the evidence on regulatory gene expression provides a notable example of how molecular developmental biology may help to clarify important issues in the evolution of the nervous system.

 

 

3. A brief history of the pallium

The cerebral hemispheres can be subdivided in a dorsal part or pallium, and a ventral part or subpallium (see Fig. 1). In agnathans (jawless vertebrates), the olfactory bulbs project heavily upon the whole pallial surface (Northcutt 1996a; Northcutt & Puzdrowski 1988; Wicht & Northcutt 1992; 1993; Wicht 1996), but in gnathostomes, which are characterized by the possession of jaws, the olfactory projection occupies a much more restricted portion of the pallium, being usually confined to the lateral aspect of this structure (Ebbesson & Heimer 1970; Smeets 1983; Northcutt & Kaas 1995). The acquisition of predatory lifestyles by the early gnathostome vertebrates, involving the further development of other sensory modalities, implied the progressive development of ascending visual, somatosensory and lateral line afferents (Northcutt and Puzdrowski 1988; Wicht & Northcutt 1992; 1993; Wicht 1996). The expansion of these sensory projections, which are relayed to the hemispheres via the diencephalon, was concomitant with the enlargement of the telencephalic components receiving the respective inputs (Northcutt & Puzdrowski 1988; Wicht & Northcutt 1992; 1993; Wicht 1996; Northcutt 1981; Striedter 1997). With the exception of amphibians, which are considered to have a secondarily simplified brain (Northcutt 1981; Neary 1990), this phenomenon is also evident among terrestrial vertebrates.

In reptiles and birds, many non-olfactory sensory projections terminate in a prominent periventricular structure denominated dorsal ventricular ridge (DVR; Fig. 1) (Ulinski 1983; Ten Donkelaar 1998). The DVR is the most expansive telencephalic component of reptiles and birds and is a main integratory center in their brains. It consists of an anterior part (ADVR) and a posterior or basal part (PDVR). The ADVR receives much of the sensory input, and its output is directed mainly to the corpus striatum and to the PDVR. The latter (corresponding to the archistriatum in birds) has been compared to parts of the mammalian amygdala and projects mainly to the hypothalamus (Ten Donkelaar 1998; Lanuza et al. 1998; Lanuza et al. 1999). Beside the DVR, the reptilian pallium has a three-layered cortex, consisting of a medial and a dorsomedial moiety (both comparable to the mammalian hippocampal formation), plus a lateral (olfactory) cortex (Ulinski 1990), and finally a dorsal cortex (the Wulst in birds) located between these two, part of which receives visual projections from the dorsal lateral geniculate nucleus, as well as some somatosensory input (Medina & Reiner 2000).

FIGURE 1

Mammals are characterized by the possession of the isocortex, which during development originates at least in large part from the dorsal pallium (Rakic 1988; 1995; Northcutt & Kaas 1995). Recent studies in mammals indicate that additionally, cells originating in the embryonic corpus striatum migrate tangentially in a dorsal direction and become incorporated into the isocortex, mostly as GABAergic interneurons (Anderson et al. 1997a; 1999). The isocortex receives ascending sensory input from the thalamus and projects to the hippocampus and to the amygdala, as well as sending output to several lower brain centers including the thalamus, corpus striatum, several brainstem nuclei and the spinal cord.

4. Diverging concepts of homology in pallial organization

4.1. Connectional and neurochemical criteria

Karten and collaborators ( Nauta & Karten 1970; Shimizu & Karten 1993; Karten 1997) and more recently Medina and Reiner (2000) have proposed that the Wulst of birds and the dorsal cortex of reptiles (see Fig. 1) are homologues to the primary visual cortex of mammals since both receive visual projections from the dorsal lateral geniculate nucleus or its avian equivalent, the OPT nucleus (the so-called thalamofugal visual pathway; Fig. 2). Medina and Reiner (2000) also argue that the mammalian somatosensory and motor cortices have an homologue in the avian Wulst and in the reptilian dorsal cortex. In addition, the DVR of birds/reptiles has been considered comparable to the mammalian isocortical auditory and extrastriate visual areas (Nauta & Karten 1970; Shimizu & Karten 1993; Karten 1997; 1968). The latter proposal was raised on the grounds that both, the auditory radiation and the visual projection from the optic tectum (the tectofugal visual pathway, which is parallel to the thalamofugal pathway; see Fig. 2) end in the avian/reptilian DVR, and in the mammalian auditory and extrastriate visual cortices, respectively. Furthermore, relying on similarities in intrinsic connectivity, these authors proposed that the distinct isocortical laminae corresponded to different components of the avian DVR: the ecto, neo and archistriatum of birds would be homologues of the mammalian isocortical layers IV, II-III, and V-VI, respectively (Nauta & Karten 1970; Shimizu & Karten 1993; Karten 1997; Veenman et al. 1995). This hypothesis implies that the mammalian isocortex has a dual origin, one from the dorsal cortex of reptiles and corresponding to the striate visual cortex (associated to the thalamofugal visual pathway), and the other from a structure homologous to the DVR of reptiles and corresponding to the auditory cortex (receiving the auditory projection) and to the extrastriate visual cortex (associated to the tectofugal visual pathway).

FIGURE 2

 

 

 

A similar interpretation has been provided by Reiner (1993), who has argued that in terms of neurotransmitter contents, the DVR resembles the reptilian dorsal cortex and the mammalian infragranular isocortical layers. The granular and supragranular isocortical layers differ from these structures in that some of their cells possess neurotransmitters (CCK8, VIP and acetylcholine) that are absent in reptiles and in the infragranular isocortical layers. To Reiner (1993), this suggests that the DVR has a similar origin as the reptilian dorsal cortex, and these are comparable to the infragranular isocortical layers of mammals. This perspective differs from the hypothesis of Karten and collaborators in that the isocortical supragranular layers would have no counterpart in the avian/reptilian brain. Nevertheless, based on hodological evidence and on the distribution of GABAergic terminals and receptors, Reiner and collaborators described similarities between the avian archistriatum and the infragranular isocortical layers of mammals, and between central components of the avian neostriatum (a major component of the avian ADVR) and the granular and supragranular layers of the isocortex (Veenman et al. 1994a; 1994b; 1995), which somehow agrees with Kartenís proposal.

Other connectional evidence points to important differences between the reptilian DVR and the mammalian isocortex. First, the mammalian extrastriate visual cortex receives an important input from the primary or striate visual cortex (Montero 1993; Rosa 1999). Although in reptiles, projections from the dorsal cortex to the DVR have been described (Ten Donkelaar 1998; Ulinski 1990), these do not exert a significant influence over the latter. Second, the mammalian isocortex projects reciprocally to the entorhinal cortex and from there to the hippocampus (Van Hoesen 1982; Rosene & Van Hoesen 1987; Insausti 1993), while in reptiles few if any connections have been reported from the DVR to the hippocampus (Ulinski 1983; Ten Donkelaar 1998; Ulinski 1990). Note that in this regard, the mammalian isocortex resembles more the reptilian dorsal cortex, which has important connections with the medial/dorsomedial (hippocampal) cortices (Ulinski 1990; Fig. 2). Finally, sensory projections do not always end in comparable structures. In amphibians, the auditory and tectal visual pathways terminate in the corpus striatum, which is clearly not homologue of either the isocortex or the DVR (Butler 1994). Thus, connectivity and neurotransmitter information yield inconsistent clues as to the possibility of homology between the DVR and the isocortex.

 

 

4.2. Developmental criteria

A homology criterion that has in our view produced less ambiguous results is developmental origin. Northcutt (1996b) has already underlined the importance of the ontogenetic context in evolutionary considerations, as originally proposed by Garstang (1922). More precisely, in a lucid discussion of the different approaches to the problem of homology, Striedter (1997) quotes Russelís (1916) considerations favouring similarity of development as a strong criterion for homology. In Russelís view, the generalized morphology and topographic relations are shown most clearly in early developmental stages, which facilitates cross-species comparisons. This assumption is valid in cases in which there is cross-species conservatism of embryonic processes while adult morphology tends to diverge. Alternatively, in cases of embryological diversity with adult conservatism, perhaps adult structures and relations may be a better criterion for homology (Aboitiz, 1995; 1999). In a similar line, Striedter (1997) has called attention to complement embryological information with phylogenetic data in order to discern true homologies from instances of independent evolution or homoplasy. In the case of the amniote telencephalon, phylogenetic evidence points to a notable conservation of early embryonic structure with adult diversification (Aboitiz, 1995; 1999; Striedter, 1997), which puts weight on embryological comparisons as a more reliable criterion for homology.

During embryogenesis, the DVR (especially its anterior part) develops from the lateral pallium, in a position deep to the olfactory cortex (Källén 1951; Striedter et al. 1998), while most of the isocortex originates from the dorsal pallium. This is supported by studies of expression patterns of regulatory homeobox-like genes in the embryonic forebrain. These have revealed a conserved mosaic organization in which the different compartments develop into specific brain components in the adult (Puelles & Rubenstein 1993; Gellon & McGinnis 1998; Moens et al. 1998; Seo et al. 1998). In the embryonic mammalian telencephalon, distinct markers for pallial and for subpallial regions have been detected. The corpus striatum (a subpallial structure that borders the lateral aspect of the pallium; Fig. 1) arises from the embryonic lateral and medial ganglionic eminences, which are located in the lateral subpallium and express the marker genes Dlx-1 and Dlx-2 (Anderson et al. 1997b). The cerebral cortex arises mostly from the embryonic pallium and is characterized by the expression of genes of the Emx and the Otx families (Fig. 1) (Puelles & Rubenstein 1993; Simeone et al. 1992; Pannese et al. 1998; Mallamaci et al. 1998; Acámpora & Simeone 1999). Smith Fernández et al. (1998) identified for the first time an intermediate territory (IT) in the equatorial region of the hemisphere, between the pallium and the subpallium of amphibians, reptiles, birds and mammals, which does not express either the Emx-1 or Dlx-1 markers but is largely positive for the gene Pax-6 (Smith-Fernández et al. 1998). A more recent report (Puelles et al. 1999) has confirmed the existence of the IT (which has been denominated ventral pallium, VP, by these authors), extending its definition by showing that, like other pallial regions, the IT/VP also expresses the regulatory marker gene Tbr-1. In reptiles and birds, an important part of the ADVR develops from the IT/VP (Fig. 1), while in mammals the basolateral amygdalar complex, and perhaps parts of the olfactory cortex Ėamong other structures- derive from this region (Smith-Fernández et al. 1998). Previous concepts proposing homology between the reptilian ADVR and the mammalian basolateral amygdalar complex (Bruce & Neary 1995) or the adjacent endopiriform nucleus (Striedter 1997) may find support in these findings.

Smith-Fernández et al. (1998) have argued that, in reptiles and birds, the IT/VP remains as a distinct neuroepithelial zone until late development, period in which it gives rise to most of the ADVR. On the contrary, in mammals this territory has been described as producing only the above mentioned early-generated components. In later embryonic stages, the mammalian intermediate territory is obliterated between the Emx 1-positive and the Dlx 1/2-positive zones, disappearing from the neuroepithelial surface (Smith-Fernández et al. 1998). This suggests that in mammals there are no structures comparable to those late-generated components in the avian/reptilian intermediate territory (i.e. the DVR), which agrees with the concept that in mammals there are no strict homologues of the reptilian anterior DVR, or at least of a large part of it (Aboitiz 1992).

Puelles et al. (1999) seem to disagree with the concept of the IT/VP disappearing from the neuroepithelial surface in mammals, although they admit that this territory is considerably compressed between the lateral pallium and the developing striatum. Undoubtedly, further studies are strongly needed to clarify the developmental fate of the IT/VP in mammals. In any case, for this component to contribute to isocortical development as predicted by Kartenís (1968; 1997) hypothesis, a massive tangential migration of neurons should take place from the IT/VP to the dorsal pallium, producing the visual extrastriate and the auditory cortices. There is evidence that many isocortical GABAergic cells originate in the subpallial corpus striatum and migrate dorsally into the isocortex (Anderson et al. 1997a), which raises the possibility that some cells from the IT/VP also migrate to the dorsal pallium. Our own view is that in mammals, the relative reduction of this proliferative territory when compared to the expansion of the embryonic dorsal pallium contrasts with the relatively well-developed IT/VP in reptiles and birds, making the latter unlikely to make such a significant contribution to the isocortex as predicted by Kartenís hypothesis. Furthermore, if the auditory and extrastriate cortices derive from the IT/VP, these areas should be largely negative for Emx-1, which is characteristic of the developing cortex and not of the IT/VP.

In this context, one additional issue that requires further study concerns the homologies between parts of the mammalian amygdalar complex and the reptilian PDVR/avian archistriatum. Swanson and Petrovich (1998) divide the mammalian amygdala into pallial (cortical and basolateral amygdala) and subpallial (central and medial amygdala) moieties. According to Smith-Fernández et al. (1998), the reptilian PDVR/avian archistriatum express pallial markers and are comparable to the corticomedial and central amygdala of mammals, while Puelles and colleagues (Puelles et al. 1999) argue that only the posterior archistriatum is pallial. An additional interpretation, based on hodological studies, is that the reptilian PDVR is homologue to the mammalian laterobasal amygdala (Lanuza et al. 1998). According to Dubbeldam (1998) the avian archistriatum consists of a sensorimotor moiety that receives projections from the ADVR, and an amygdalar moiety. Perhaps embryological studies will help clarify the compartmentalization and homologies of the mammalian amygdala and the reptilian PDVR/avian archistriatum.

Another intriguing result is that the hyperstriatum ventrale (a structure located ventral to the Wulst in birds, and which has been considered to belong to the avian DVR), expresses Emx-1 during development (Fig. 1), suggesting that it may derive from the dorsal or lateral pallium and not from the DVR. Puelles et al. (1999) argue that in the avian lineage, components of the lateral and dorsal pallium have been incorporated into the DVR. An alternative description would be that in birds, the DVR comprises a smaller proportion of the pallium than has been considered before.

5. A scenario for isocortical origins: olfaction, the hippocampus and the thalamofugal visual system

Developmental evidence therefore suggests that the reptilan DVR and the mammalian isocortex (including auditory and visual striate and extrastriate isocortices) derive from different telencephalic components. Although an explanation for the evolutionary development of the reptilian DVR is still needed, we suggest that the mammalian isocortex originated in the dorsal pallium by virtue of the relations of the dorsal cortex with the medial/dorsomedial cortex (hippocampus) and the olfactory cortex (Fig. 2). This proposal is partly based on Lynchís (1986) original hypothesis of the origin of the isocortex. Briefly, associative networks between the dorsal cortex and the olfactory system, via the hippocampus, became increasingly important to develop multisensorial maps of space. This may have triggered the expansion of the dorsal cortex as a recipient of not only the visual thalamofugal but also the auditory and visual tectofugal sensory projections.

It has been repeatedly proposed that in ancestral mammals, olfaction was an important sensory modality (Jerison 1973; 1990; Kemp 1982). Endocasts of mesozoic mammals indicate relatively large olfactory bulbs and perhaps an elevated rhinal fissure (Jerison 1990), suggesting a large olfactory cortex in relation to the rest of the pallium. Likewise, in small-brained insectivores and in some marsupials, olfactory-related structures occupy a much larger proportion of the volume of the brain than is the case in larger-brained species with a well-developed isocortex (Voogd et al. 1998; Finlay & Darlington 1995; Finlay et al. 1998; Stephan 1983).

The mammalian olfactory cortex is reciprocally connected with the hippocampus, and has been postulated to engage in associative interactions with other sensory modalities which are mapped in the isocortex and project to the hippocampus (Lynch 1986). Although it has long been considered that the hippocampus encodes mainly visual spatial memory (Best & White 1998; Rawlins 1999), recent evidence indicates that it also represents olfactory information (Myers & Gluck 1996; Dusek & Eichenbaum 1998; Wood et al. 1999), and that there is an interleaved segregation of spatial and nonspatial information along the length of this structure (Hampson et al. 1999). This suggests that nonspatial cues (like odors) may be used in the elaboration of spatial maps in the hippocampus.

A circuit connecting the medial (hippocampal), the dorsal (receiving the visual thalamofugal pathway) and the olfactory cortices already exists in reptiles, and may have been put to use by the first mammals to make relatively elaborate, largely olfactory-based, representations of space, in which specific odors labeled particular places and routes (Lynch 1986). Nevertheless, the contribution of the visual system became undoubtedly neccessary in the elaboration of more precise maps of space. The dorsal cortex, receiving visual information from the thalamofugal visual pathway, may have become an important sensory processing system in the early mammalian brain (Aboitiz, 1992). The reviewed developmental evidence suggests to us that other parts of the isocortex (visual extrastriate and auditory) may have originated from an expansion of the dorsal cortex and perhaps parts of the lateral cortex, to accommodate the incoming visual thalamofugal and auditory inputs that became increasingly involved in associative interactions with the primary visual cortex, and with the olfactory system through the hippocampus. In reptiles on the other hand, the thalamofugal visual pathway does not play a dominant role for processing visual information (Ulinski 1990). In this vertebrate class, the more important tectofugal pathway and the auditory system project into the DVR, which apparently does not participate in associative networks with the hippocampus.

6. Fossil brains

Endocast information indicates that early mammal-like reptiles (therapsids) had narrow, tubular hemispheres with no signs of telencephalic expansion (Kemp 1982; Hopson 1979; Quiroga 1980). Increase in brain size, resulting from a generalized growth of the isocortex, occurs in the more recent fossil mammal Triconodon (Rowe 1996a; 1996b; Fig. 3). The full enlargement of the brain coincides with the detachment of the auditory bones from the mandible to form the mammalian middle ear (Rowe 1996a; 1996b), and perhaps with the development of auditory (and visual tectofugal) projections into the isocortex. Interestingly, Morganucodon, a primitive mammal from the early Jurassic, and taxonomically intermediate between Triconodon and smaller-brained, more primitive therapsids, shows only partial expansion of the brain. In this species, widening of the occipital parts of the hemispheres can be observed (Rowe 1996a; 1996b; Fig. 3). Although admittedly endocast information can be difficult to interpret, if we consider that the primary visual cortex tends to be located in the occipital lobes, this feature may perhaps be attributed to the early development of the primary visual or striate cortex in Morganucodon. If this is correct, it would be consistent with our model of an early expansion of parts of the dorsal cortex, receiving the thalamofugal visual pathway, followed by the expansion of adjacent cortical areas, which received the tectal visual and the auditory inputs.

FIGURE 3

 

 

7. Different patterns of brain organization in reptiles and mammals?

The confluence of the visual thalamofugal and the visual tectofugal pathways in the mammalian isocortex suggest different patterns of brain organization in mammals and in reptiles/birds (see Fig. 2). In mammals, the primary visual cortex projects to the extrastriate cortical areas (which receive the tectofugal visual projection), and these, together with the auditory and somatosensory cortical areas, project (through a series of successive corticocortical projections) to the hippocampus and the amygdala, to process different types of mnemonic information (spatial and emotional, respectively; Lynch 1986; Maren 1999). In reptiles and birds, although a circuit exists associating the olfactory, the dorsal (visual thalamofugal) and the medial/dorsomedial (hippocampal) cortices, the more important projections from somatosensory, auditory and visual tectal systems end in the ADVR, which is in turn connected primarily to the PDVR (comparable to parts of the mammalian amygdalar complex). In other words, in reptiles and birds two relatively separate systems exist for processing sensory information: one receiving thalamofugal visual information and somatosensory projections, which is connected primarily with the hippocampus, and the other, receiving auditory and tectofugal visual information, which is more related to the amygdalar system (see Fig. 2). Thus, in mammals, the hippocampus may receive a much heavier sensory projection than is the case in reptiles and birds, who may rely more on amygdalar components (PDVR/archistriatum) than on the former to process certain types of mnemonic information. [In this context, it has been found that in homing pigeons, hippocampal lesions disrupt certain types of spatial learning such as using the sun compass directional information, while the capacity to learn on the basis of landmark beacons remains spared (Gagliardo et al. 1996). This suggests that not all forms of spatial memory depend on the hippocampus in birds.] We are proposing that the difference in hippocampal sensory input between reptiles/birds and mammals was a key factor in the development of the isocortex from the dorsal pallium of mammals.

8. Final comment

We have reviewed developmental evidence for a separate origin of the mammalian isocortex and the reptilian DVR. The latter appears to be related to ventral pallial structures of mammals such as the basolateral amygdala and/or the endopiriform nucleus (ventral claustrum), while the isocortex originates largely from the dorsal pallium. Connectional evidence indicating similarity of sensory input in the reptilian ADVR and the mammalian auditory and extrastriate isocortex is weakened by other hodological evidence that suggests different interpretations.

The above proposals have been complemented with a scenario describing the origin of the isocortex from the dorsal pallium based on the reciprocal relations of this structure and the hippocampus. One important assumption held by several workers is that the modern reptilian brain represents a stage similar to that of the ancestral mammals. On the other hand, the developmental evidence suggests to us that the reptilian and mammalian brains may have diverged very early in their organization. The scenario proposed here is an attempt to describe the sequences of events that may have led to this early separation of the reptilian and mammalian brain architectures. In our view, the main significance of these scenarios is that they provide an evolutionary framework which may guide future studies of the embryology and structure of the cerebral cortex.

Acknowledgements

Supported by FONDECYT Grant 1970294 and by a gift from EMEC S.A. and ENAEX S.A. to Francisco Aboitiz. We wish to acknowledge Alexander Vargas for interesting discussions. Claudia Andrade provided secretarial help.

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Figure Legends

Fig. 1. The cerebral hemispheres of reptiles, birds and mammals (only one hemisphere is shown; lateral is to the right), indicating the pallium (dark grey), which during development expresses the marker genes Emx 1/2, Otx 1/2, Pax-6 and Tbr-1, and gives rise to the Wulst (W) and hyperstriatum ventrale (HV) of birds, and to cortical structures in reptiles and mammals. The subpallium (light gray), expresses Dlx-type genes during embryogenesis and gives rise to the corpus striatum (STR) among other structures. Standard grey indicates the intermediate territory or ventral pallium, which is largely positive for the genes Emx-2, Pax-6 and Tbr-1 and gives rise to the anterior dorsal ventricular ridge (ADVR) of reptiles, to the neostriatum (N) of birds (which corresponds to a large part of the ADVR), and to the laterobasal amygdala (AM) of mammals. In each vertebrate class, the projection sites of the auditory pathway (a), and the visual pathways via the lateral geniculate (vg), and via the optic tectum (vt), are indicated. D, dorsal cortex; ISO, isocortex. Based on Smith Fernández et al. 1998 and Puelles et al. 1999.

Fig. 2. Diagrams summarizing some visual and olfactory projections in mammals and in reptiles. Visual projections originate in the retina (RET) and project in two separate pathways: the thalamofugal path (TaF), which is directed to the dorsal lateral geniculate nucleus (LGN) and from there to the dorsal cortex (DCx) in reptiles and to the primary visual cortex (V1) in mammals. The second visual route is the tectofugal pathway (TeF), that projects to the optic tectum (or superior colliculus, OT) and from there to the reptilian anterior dorsal ventricular ridge (ADVR), and to the the extrastriate visual cortex (ESt) of mammals, via the nucleus rotundus (R ) in reptiles, and the pulvinar (Pu) in mammals. Projections from the olfactory bulb (OB) end in the olfactory cortex (OCx), which projects to the medial/dorsomedial cortex (M/DMCx) in reptiles and to the hippocampus (HP) in mammals. The reptilian dorsal cortex (DCx) has connections with the M/DMCx. In mammals, the primary visual cortex (V1) exerts control over the extrastriate visual areas (ESt); all sensory isocortical areas indirectly project to the hippocampus (HP). The reptilian posterior dorsal ventricular ridge (PDVR) is related to parts of the mammalian amygdalar system and receives projections from the ADVR.

Fig. 3. Endocasts of mammal-like reptiles and primitive mammals, indicating the progressive increase in brain size. Note that in Morganucodon, expansion of the posterior part of the hemisphere can be observed. More advanced mammals like Triconodon show a more complete expansion of the hemispheres. Based on Rowe (1996a; 1996b).