Pallium (neuroanatomy)
Encyclopedia
In a neuroanatomy context, the word pallium refers to the layers of gray and white matter that cover the upper surface of the telencephalon
in vertebrates. The non-pallial part of the telencephalon builds the subpallium. In basal vertebrates the pallium is a relatively simple three-layered structure, encompassing 3-4 histogenetically distinct domains, plus the olfactory bulb. It used to be thought that pallium equals cortex and subpallium equals telencephalic nuclei, but it has turned out, according to comparative evidence provided by molecular markers, that the pallium develops both cortical structures (allocortex and isocortex) and pallial nuclei (claustroamygdaloid complex), whereas the subpallium develops striatal, pallidal, diagonal-innominate and preoptic nuclei, plus the corticoid structure of the olfactory tuberculum. In mammals, the cortical part of the pallium registers a definite evolutionary step-up in complexity, forming the cerebral cortex
, most of which consists of a progressively expanded six-layered portion isocortex, with simpler three-layered cortical regions allocortex
at the margins. The allocortex subdivides into hippocampal allocortex, medially, and olfactory allocortex, laterally (including rostrally the olfactory bulb and anterior olfactory areas).
The general layout or Bauplan of the pallium is already clearly defined in animals with relatively simple brains, including lampreys, sharks and amphibians. In teleost
fish, reptiles, birds, and mammals, the pallial architecture is greatly modified (sharply divergently in fish), with differential growth and specialization of diverse sectors of the conserved pallial Bauplan. In all vertebrate brains, the telencephalic forebrain consists of two hemispheres, joined at the midline by a region called the septum. The septum is continuous with the preoptic area across the plane defined by the anterior commissure; it is largely subpallial, but also contains a small pallial portion, where the hippocampal commissure forms, which is contiguous to the medial pallium. The telencephalic part of the rostral choroidal tela (roof plate continuous caudally with a diencephalic part) is inserted at the back of this commissure at a locus where mammals show the subfornical circumventricular organ, and extends laterally over the interventricular foramen into a wing-shaped medial telencephalic territory, the so-called choroidal fissure. Here the choroidal tissue is attached to the fimbria of the hippocampus (also known as the cortical hem area), bordering lengthwise the medial pallium. At its rostral and caudal ends, the medial pallium contacts the ventral pallium, which builds the pallial portion that contacts the subpallium across the pallio subpallial boundary, observed at the lateral telencephalic wall. Inside the ring formed thus by the medial and ventral pallium there is a sort of island that contains the dorsal and lateral pallial portions. In older literature the pallium used to be subdivided only into three zones, called the medial pallium, dorsal (or dorsolateral) pallium, and lateral pallium. The old lateral pallium encompassed the modern lateral and ventral parts of the pallium. The medial pallium is the progenitor of the mammalian hippocampus
, and is thought to be involved in spatial cognitive mapping and memory formation across a broad range of species. The lateral and ventral pallium is the progenitor of the mammalian piriform cortex
, and has an olfactory function in every species in which it has been studied. The evolutionary diversifications and specialization in functions of the dorsal pallium have been more difficult to decipher. It is widely believed to be the progenitor of the bulk of the mammalian cerebral cortex, although the evidence for this is considered by some anatomists not yet to be conclusive.
Importantly, the lateral and ventral parts of the pallium produce also deep to their respective sectors of subpial olfactory cortex sets of pallial nuclei, the neurons entering the claustrum, rostrally, and the pallial amygdala, caudally. The concept of hypopallium refers to this histogenetically unitary complex of olfactory (piriform) cortex and deep pallial nuclei. In reptiles and birds the hypopallium becomes differentially enlarged (largest in crocodiles and birds, whose olfactory cortex gets nevertheless reduced), whereas in mammals it becomes reduced to the claustroamygdaloid complex and relatively enlarged olfactory (prepiriform and piriform) cortex.
The evolution of the dorsal pallium is not fully understood yet. Some authors hold that it largely contributes to the mammalian hippocampal allocortical and parahippocampal mesocortical (transitional) areas. Others postulate it directly transforms into the six-layered isocortex (neocortex) characteristic of mammals, and still others suppose that medial and lateral parts of the dorsal pallium contribute (perhaps with some contributions from the lateral pallium) to the alternative allocortical and isocortical fates.
The pallial amygdala contains mainly the so-called basolateral amygdala, encompassing the lateral, basolateral (basal) and basomedial (accessory basal) nuclei, plus the anterior, amygdalopiriform and posterolateral corticoid areas at its surface. The medial pallium also may contribute to the pallial amygdala, forming the amygdalohippocampal nucleus and the posteromedial corticoid area. It has been postulated that the neurons forming the nucleus of the lateral olfactory tract derive from the dorsal pallium and migrate tangentially into its final position caudal to the olfactory tuberculum. Situated ventral to the pallium in the basic vertebrate forebrain plan (though representing a topologically rostral field in neural plate fate maps) is another region of telencephalic gray matter known as the subpallium, which is the progenitor area for the basal ganglia
, a set of structures that play a crucial role in the executive control of behavior. The subpallium region has distinct striatal, pallidal, diagonal and preoptic subregions, which are stretched obliquely between the septal midline and the amygdala at the posterior pole of the telencephalon. At least the striatum, pallidum and diagonal domains extend into the amygdala, representing there the subpallial amygdala, forming its central and medial nucleis, as well as the amygdaloid end of the bed nucleus stria terminalis complex.
The amygdala
thus encompasses an heterogeneous group of subpallial nuclei and hypopallial olfactory and amygdalohippocampal corticonuclear cell masses which are on the whole heavily involved in emotion and motivation. The pallial portions build the analytic or perceptual end of this complex, whereas the subpallial portions represent the corresponding output or efferent functional pole.
The olfactory bulb is a peculiar pallial outgrowth (maybe induced by the primary olfactory fibers afferent to it, coming from the sensory neurons developed in the olfactory placode) whose projection neurons (the mitral and tufted neurons) are pallial in origin and accordingly excitatory. In contrast, the superfial periglomerulary neurons, various intermediate interneurons and the deep granule cells are all of subpallial origin and migrate tangentially out of the striatal part of the subpallium (apparently froma dorsal subsector of this domain) through the so-called rostral migratory stream into the olfactory bulb. These extremely numerous subpallial cells are all inhibitory. The olfactory bulb is thus singularly formed by a minority of autochtonous pallial neurons and a majority of inmigrated inhibitory subpallial cells (it is nevertheless classified as a part of the ventral pallium). There is also a modified accessory olfactory bulb at the base of the principal one, which is associated specifically to incoming afferents from Jacobson's organ found at the nasal septum. The accessory olfactory pathway is maximally developed in some reptiles (e.g., snakes) and is lost in birds.
) envelops most of the telencephalon, due to extensive surface expansion of the isocortex. The telencephalic pallium has been described classically as having three parts: the archipallium
, the paleopallium
and the neopallium, but these concepts are now considered obsolete, having been substituted by the concept of medial pallium, dorsal pallium, lateral pallium and ventral pallium mentioned above under pallial Bauplan. It used to be said in anatomy textbooks that pallium equals cortex and subpallium equals telencephalic nuclei, but it has turned out, according to molecular markers, that the pallium develops both cortical structures (allocortex and isocortex) and pallial nuclei (claustroamygdaloid complex), whereas the subpallium develops striatal, pallidal, diagonal-innominate and preoptic nuclei, plus the corticoid structure of the olfactory tuberculum.
. However, the pallial portions do not show a visible lamination. They already have a mixture of glutamatergic (excitatory) and GABAergic (inhibitory) neurons, whereas the subpallium is largely populated by inhibitory neurons. This structure is very similar to that found generally in anamniotes, though cartilaginous fishes do show a layered arrangement of their pallial neurons.
Telencephalon
The cerebrum or telencephalon, together with the diencephalon, constitutes the forebrain. The cerebrum is the most anterior region of the vertebrate central nervous system. Telencephalon refers to the embryonic structure, from which the mature cerebrum develops...
in vertebrates. The non-pallial part of the telencephalon builds the subpallium. In basal vertebrates the pallium is a relatively simple three-layered structure, encompassing 3-4 histogenetically distinct domains, plus the olfactory bulb. It used to be thought that pallium equals cortex and subpallium equals telencephalic nuclei, but it has turned out, according to comparative evidence provided by molecular markers, that the pallium develops both cortical structures (allocortex and isocortex) and pallial nuclei (claustroamygdaloid complex), whereas the subpallium develops striatal, pallidal, diagonal-innominate and preoptic nuclei, plus the corticoid structure of the olfactory tuberculum. In mammals, the cortical part of the pallium registers a definite evolutionary step-up in complexity, forming the cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
, most of which consists of a progressively expanded six-layered portion isocortex, with simpler three-layered cortical regions allocortex
Allocortex
The allocortex is a part of the cerebral cortex characterized by fewer cell layers than the neocortex The allocortex (also known as heterogenetic cortex) is a part of the cerebral cortex characterized by fewer cell layers than the neocortex The allocortex (also known as heterogenetic cortex) is a...
at the margins. The allocortex subdivides into hippocampal allocortex, medially, and olfactory allocortex, laterally (including rostrally the olfactory bulb and anterior olfactory areas).
The general layout or Bauplan of the pallium is already clearly defined in animals with relatively simple brains, including lampreys, sharks and amphibians. In teleost
Teleostei
Teleostei is one of three infraclasses in class Actinopterygii, the ray-finned fishes. This diverse group, which arose in the Triassic period, includes 20,000 extant species in about 40 orders; most living fishes are members of this group...
fish, reptiles, birds, and mammals, the pallial architecture is greatly modified (sharply divergently in fish), with differential growth and specialization of diverse sectors of the conserved pallial Bauplan. In all vertebrate brains, the telencephalic forebrain consists of two hemispheres, joined at the midline by a region called the septum. The septum is continuous with the preoptic area across the plane defined by the anterior commissure; it is largely subpallial, but also contains a small pallial portion, where the hippocampal commissure forms, which is contiguous to the medial pallium. The telencephalic part of the rostral choroidal tela (roof plate continuous caudally with a diencephalic part) is inserted at the back of this commissure at a locus where mammals show the subfornical circumventricular organ, and extends laterally over the interventricular foramen into a wing-shaped medial telencephalic territory, the so-called choroidal fissure. Here the choroidal tissue is attached to the fimbria of the hippocampus (also known as the cortical hem area), bordering lengthwise the medial pallium. At its rostral and caudal ends, the medial pallium contacts the ventral pallium, which builds the pallial portion that contacts the subpallium across the pallio subpallial boundary, observed at the lateral telencephalic wall. Inside the ring formed thus by the medial and ventral pallium there is a sort of island that contains the dorsal and lateral pallial portions. In older literature the pallium used to be subdivided only into three zones, called the medial pallium, dorsal (or dorsolateral) pallium, and lateral pallium. The old lateral pallium encompassed the modern lateral and ventral parts of the pallium. The medial pallium is the progenitor of the mammalian hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
, and is thought to be involved in spatial cognitive mapping and memory formation across a broad range of species. The lateral and ventral pallium is the progenitor of the mammalian piriform cortex
Piriform cortex
In anatomy of animals, the piriform cortex, or pyriform cortex is a region in the brain.-Anatomy and function:The piriform cortex is part of the rhinencephalon situated in the telencephalon....
, and has an olfactory function in every species in which it has been studied. The evolutionary diversifications and specialization in functions of the dorsal pallium have been more difficult to decipher. It is widely believed to be the progenitor of the bulk of the mammalian cerebral cortex, although the evidence for this is considered by some anatomists not yet to be conclusive.
Importantly, the lateral and ventral parts of the pallium produce also deep to their respective sectors of subpial olfactory cortex sets of pallial nuclei, the neurons entering the claustrum, rostrally, and the pallial amygdala, caudally. The concept of hypopallium refers to this histogenetically unitary complex of olfactory (piriform) cortex and deep pallial nuclei. In reptiles and birds the hypopallium becomes differentially enlarged (largest in crocodiles and birds, whose olfactory cortex gets nevertheless reduced), whereas in mammals it becomes reduced to the claustroamygdaloid complex and relatively enlarged olfactory (prepiriform and piriform) cortex.
The evolution of the dorsal pallium is not fully understood yet. Some authors hold that it largely contributes to the mammalian hippocampal allocortical and parahippocampal mesocortical (transitional) areas. Others postulate it directly transforms into the six-layered isocortex (neocortex) characteristic of mammals, and still others suppose that medial and lateral parts of the dorsal pallium contribute (perhaps with some contributions from the lateral pallium) to the alternative allocortical and isocortical fates.
The pallial amygdala contains mainly the so-called basolateral amygdala, encompassing the lateral, basolateral (basal) and basomedial (accessory basal) nuclei, plus the anterior, amygdalopiriform and posterolateral corticoid areas at its surface. The medial pallium also may contribute to the pallial amygdala, forming the amygdalohippocampal nucleus and the posteromedial corticoid area. It has been postulated that the neurons forming the nucleus of the lateral olfactory tract derive from the dorsal pallium and migrate tangentially into its final position caudal to the olfactory tuberculum. Situated ventral to the pallium in the basic vertebrate forebrain plan (though representing a topologically rostral field in neural plate fate maps) is another region of telencephalic gray matter known as the subpallium, which is the progenitor area for the basal ganglia
Basal ganglia
The basal ganglia are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus and other brain areas...
, a set of structures that play a crucial role in the executive control of behavior. The subpallium region has distinct striatal, pallidal, diagonal and preoptic subregions, which are stretched obliquely between the septal midline and the amygdala at the posterior pole of the telencephalon. At least the striatum, pallidum and diagonal domains extend into the amygdala, representing there the subpallial amygdala, forming its central and medial nucleis, as well as the amygdaloid end of the bed nucleus stria terminalis complex.
The amygdala
Amygdala
The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...
thus encompasses an heterogeneous group of subpallial nuclei and hypopallial olfactory and amygdalohippocampal corticonuclear cell masses which are on the whole heavily involved in emotion and motivation. The pallial portions build the analytic or perceptual end of this complex, whereas the subpallial portions represent the corresponding output or efferent functional pole.
The olfactory bulb is a peculiar pallial outgrowth (maybe induced by the primary olfactory fibers afferent to it, coming from the sensory neurons developed in the olfactory placode) whose projection neurons (the mitral and tufted neurons) are pallial in origin and accordingly excitatory. In contrast, the superfial periglomerulary neurons, various intermediate interneurons and the deep granule cells are all of subpallial origin and migrate tangentially out of the striatal part of the subpallium (apparently froma dorsal subsector of this domain) through the so-called rostral migratory stream into the olfactory bulb. These extremely numerous subpallial cells are all inhibitory. The olfactory bulb is thus singularly formed by a minority of autochtonous pallial neurons and a majority of inmigrated inhibitory subpallial cells (it is nevertheless classified as a part of the ventral pallium). There is also a modified accessory olfactory bulb at the base of the principal one, which is associated specifically to incoming afferents from Jacobson's organ found at the nasal septum. The accessory olfactory pathway is maximally developed in some reptiles (e.g., snakes) and is lost in birds.
In humans
The human pallium (cloak in LatinLatin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
) envelops most of the telencephalon, due to extensive surface expansion of the isocortex. The telencephalic pallium has been described classically as having three parts: the archipallium
Archipallium
In anatomy of animals, the archipallium is the oldest region of the brain's pallium.The archipallium is often considered contiguous with the olfactory cortex, but the extent of the archipallium varies among species. In older species, such as fish, the archipallium makes up most of the cerebrum...
, the paleopallium
Paleopallium
In anatomy of animals, the paleopallium is a region within the telencephalon in the brain.It is a primitive and evolutionarily ancient region of the brain, first appearing in amphibians....
and the neopallium, but these concepts are now considered obsolete, having been substituted by the concept of medial pallium, dorsal pallium, lateral pallium and ventral pallium mentioned above under pallial Bauplan. It used to be said in anatomy textbooks that pallium equals cortex and subpallium equals telencephalic nuclei, but it has turned out, according to molecular markers, that the pallium develops both cortical structures (allocortex and isocortex) and pallial nuclei (claustroamygdaloid complex), whereas the subpallium develops striatal, pallidal, diagonal-innominate and preoptic nuclei, plus the corticoid structure of the olfactory tuberculum.
In amphibians and other anamniotes
In amphibians, the telencephalon distinctly shows medial, dorsal, lateral and ventral parts of the pallium, plus striatal, pallidal, diagonal and preoptic parts of the basal nucleiBasal ganglia
The basal ganglia are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus and other brain areas...
. However, the pallial portions do not show a visible lamination. They already have a mixture of glutamatergic (excitatory) and GABAergic (inhibitory) neurons, whereas the subpallium is largely populated by inhibitory neurons. This structure is very similar to that found generally in anamniotes, though cartilaginous fishes do show a layered arrangement of their pallial neurons.