The Telencephalon Basic Parts And Morphogenesis

The telencephalic hemispheres evaginate bilaterally from the dorsalmost alar plate of the secondary prosencephalon (Fig. 1). This process includes within each vesicle a portion of the roof plate choroidal tissue, which later builds the choroidal plexi of the lateral ventricles (Figs. 1 and 2F). Rostrally, a thick median wall portion is the site where the telencephalic commissures develop (fibers interconnecting both vesicles; there are three major commissural pathways in mammals—anterior commissure, hippocampal commissure, and corpus callosum; Fig. 5B).

There are two principal subdivisions of the telence-phalic vesicle: the roof, or pallium, and the basis, or subpallium (Fig. 6A). The subpallium consists

Telencephalon Human

Figure 6 Morphological development of the telencephalon. (A-D) These drawings show lateral views of the left telencephalon at four developmental stages (see text). The inset in A shows a cross section and internal division into pallium (Pal) and subpallium (Sp). The olfactory tract is shaded in gray and helps to visualize the enormous growth of the pallium relative to the subpallium. (E) This drawing represents a section through the frontal part of the telencephalon, seen from the front; it shows in perspective the more caudal parts and the position of the temporal lobe. Within the section, the thick black lines mark the palliosubpallial boundary. Many subdivisions are indicated, as well as the lateral olfactory tract (LOT). (F) Similar to the drawing in E, representing a later stage in which further cortical expansions have introduced gyrification, internment of the insular cortex in the Sylvian sulcus (arrows) under the opercular overgrowths (compare with D and inset), and entrance of the temporal lobe (including its ventricular cavity) into the section plane (compare full shape in D). See Table I for abbreviations.

Figure 6 Morphological development of the telencephalon. (A-D) These drawings show lateral views of the left telencephalon at four developmental stages (see text). The inset in A shows a cross section and internal division into pallium (Pal) and subpallium (Sp). The olfactory tract is shaded in gray and helps to visualize the enormous growth of the pallium relative to the subpallium. (E) This drawing represents a section through the frontal part of the telencephalon, seen from the front; it shows in perspective the more caudal parts and the position of the temporal lobe. Within the section, the thick black lines mark the palliosubpallial boundary. Many subdivisions are indicated, as well as the lateral olfactory tract (LOT). (F) Similar to the drawing in E, representing a later stage in which further cortical expansions have introduced gyrification, internment of the insular cortex in the Sylvian sulcus (arrows) under the opercular overgrowths (compare with D and inset), and entrance of the temporal lobe (including its ventricular cavity) into the section plane (compare full shape in D). See Table I for abbreviations.

primarily of nuclei, the so-called basal ganglia (Figs. 7 and 8; note that use of the term "basal" here, or with regard to the amygdala, does not mean an embryolo-gical origin in the neural tube basal plate but, rather, a rough topographic indication of intratelencephalic position; all telencephalic parts are alar). The pallium contains primarily cortical structures but also some pallial nuclei (Fig. 8).

The area occupied by the basal ganglia in the hemisphere differentiates early and then its growth slows, whereas the overlying pallium is capable of prolonged surface growth. The increasing disproportion between these two parts leads to a characteristic morphogenetic deformation that in highly evolved mammals consists of a progressive incurvation and relative diminution in size of the subpallium, forced by anteroposterior and mediolateral expansion of the pallium (Fig. 6). The early forming posterior pole of the vesicle is converted into the temporal pole, which protrudes laterally and rostralwards (TP in Figs. 6B-6E). New anterior and posterior poles appear in parallel, forming the definitive frontal and occipital poles of the hemisphere (FP and OP in Figs. 6A-6E). An outgrowth at the early forming anterior pole forms the olfactory bulb, which gradually becomes displaced under the new frontal pole or orbitofrontal cortex (OB in Figs. 6A-6D). The lateral olfactory tract projecting from the olfactory bulb to the primitive posterior pole (gray in Fig. 6A) is transiently visible at the surface along the whole telencephalon, always just lateral to the palliosubpallial boundary (gray in Figs. 6C and 6D).

Internally, the subpallium forms an intraventricular bulge that affects the shape of the lateral ventricle

Subpallium

Figure 7 Development and subdivisions of the basal ganglia. (A). The common mass of subpallial formations is shown under the pallium. Its caudal part contains prospective subpallial amygdala (AM) and its rostral part the striatum (and pallidum, internally). (B). Early passage of fibers from the internal capsule (arrows) starts to divide the subpallial nuclei. (C, D). At the final stage, external (C) and internal (D) views show the different main portions that are distinguished. The stippled formations in D represent the pallidal components. See Table I for abbreviations.

Figure 7 Development and subdivisions of the basal ganglia. (A). The common mass of subpallial formations is shown under the pallium. Its caudal part contains prospective subpallial amygdala (AM) and its rostral part the striatum (and pallidum, internally). (B). Early passage of fibers from the internal capsule (arrows) starts to divide the subpallial nuclei. (C, D). At the final stage, external (C) and internal (D) views show the different main portions that are distinguished. The stippled formations in D represent the pallidal components. See Table I for abbreviations.

Figure 8 Pallial and subpallial subdivisions shown in schematic cross sections through the middle sector of an undeformed telencephalon (A) and through the amygdaloid complex at the temporal pole (B). The thick black line represents the palliosubpallial boundary. Note the pallium is subdivided into ventral, lateral, dorsal, and medial portions. See Table I for abbreviations.

Figure 8 Pallial and subpallial subdivisions shown in schematic cross sections through the middle sector of an undeformed telencephalon (A) and through the amygdaloid complex at the temporal pole (B). The thick black line represents the palliosubpallial boundary. Note the pallium is subdivided into ventral, lateral, dorsal, and medial portions. See Table I for abbreviations.

(Sp in Figs. 6A, 6E and 6F, 7, and 8). The basal ganglia are traversed by the radiating fibers of the internal capsule (bidirectional thalamocortical axons) and become secondarily subdivided into several portions (Figs. 6E and 6F, 7B-D, 8A, and 8B). Pallial overgrowth also occurs mediolaterally, both in the main frontooccipital body of the hemisphere and in the temporal horn. This leads to the formation of the Sylvian (lateral) fissure and the progressive internment under the frontoparietal and temporal operculae of the piriform (olfactory) and insular cortexes (these are the earliest formed cortical parts, covering laterally the basal ganglia) (Figs. 6D-6F).

Additional adjustments of the pallial surface to more localized bouts of final surface growth lead to the partial or total burial of the oldest formed cortex (anchored by its more advanced connections) in the depth of other fissures (hippocampal, collateral, internal parietooccipital, and calcarine) and constant or variable sulci (central, frontal, parietal, temporal, cingulate, and rhinal). The intervening gyri protrude superficially in interlocked shapes, adapting to available space in the cranium (Fig. 6F). The main sulcal formations serve to separate the frontal, parietal, occipital, temporal, insular, and cingulate/entorhinal lobes; the latter includes the hippocampus. The relative amount of gyrification increases in evolutionarily more advanced mammals. The increase in cortical surface that is found in complex mammals is thought to occur without fundamental changes in the radial organization of the cortex, which is divided into columnar modules of constant dimensions and cell density across mammals. Thus, one evolutionary trend tends to increase the number of cortical columnar modules (presumably improving the analytical capacity of the animal). At the interhemispheric surface, gyrencephalic animals also show a correlatively larger corpus callosum, the main interhemispheric commissure (Fig. 5B).

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