The Rostral Diencephalon

As outlined previously, we use the term rostral diencephalon to refer to the extratelencephalic part of the secondary prosencephalon. We hypothesize that it is divided in basal and alar plates across three prosomeres (p4-p6), with its alar plate forming the prethalamus and its basal/floor plates representing the hypothalamus, as originally defined for human embryos by His in 1890 (Figs. 2 and 4). Although the diencephalic prosomeres p1-p3 represent the part of the extratelencephalic forebrain machinery that regulates information processing (and subsequent motor output) of signals coming in from the external world, the rostral diencephalic prosomeres p4-p6 attend mainly to the inner world of viscera, neurohumoral functions, and internal homeostasis. They also provide ascending (motivating) input into the limbic parts of

Figure 5 Schema of connectivity in the rostral and caudal diencephalon. (A) In general, tracts can be mapped within the prosomeric model as aligned with the transverse or longitudinal dimensions of the forebrain components. There are thus basically transverse tracts or tract portions and longitudinal tracts or tract portions. Individual tracts may change direction at specific decision points. This schema does not describe all the tracts present in the forebrain. It only exemplifies the previous general principle by illustrating 20 assorted tracts: 1, Fornix (hippocampomammillary) tract; 2, septohypothalamic tract; 3, stria medullaris (into habenular commissure); 4, optic tract; 5, amygdalohypothalamic tract; 6, supraoptohypophyseal tract; 7, mammillotegmental tract; 8, incertotegmental tract; 9, commissural ventral thalamic tract; 10, mammillothalamic tract; 11, thalamotelencephalic tract; 12, longitudinal alar interconnections; 13, retroflex tract; 14, commissural anterior pretectal tract; 15, posterior commissure; 16, nigrostriatal tract; 17, medial lemniscus; 18, superior cerebellar peduncle; 19, medial forebrain bundle; 20, intergeniculate-suprachiasmatic tract. (B) The various commissures present in the forebrain are illustrated in a median sagittal section. Emphasis is placed in the area where the telencephalic commissures develop, showing in the two schemas on the right the progressive relative increase in size of the corpus callosum over developmental time and from adult rodent to adult man. See Table I for abbreviations.

Early embryonic Adult shape in Man

Figure 5 Schema of connectivity in the rostral and caudal diencephalon. (A) In general, tracts can be mapped within the prosomeric model as aligned with the transverse or longitudinal dimensions of the forebrain components. There are thus basically transverse tracts or tract portions and longitudinal tracts or tract portions. Individual tracts may change direction at specific decision points. This schema does not describe all the tracts present in the forebrain. It only exemplifies the previous general principle by illustrating 20 assorted tracts: 1, Fornix (hippocampomammillary) tract; 2, septohypothalamic tract; 3, stria medullaris (into habenular commissure); 4, optic tract; 5, amygdalohypothalamic tract; 6, supraoptohypophyseal tract; 7, mammillotegmental tract; 8, incertotegmental tract; 9, commissural ventral thalamic tract; 10, mammillothalamic tract; 11, thalamotelencephalic tract; 12, longitudinal alar interconnections; 13, retroflex tract; 14, commissural anterior pretectal tract; 15, posterior commissure; 16, nigrostriatal tract; 17, medial lemniscus; 18, superior cerebellar peduncle; 19, medial forebrain bundle; 20, intergeniculate-suprachiasmatic tract. (B) The various commissures present in the forebrain are illustrated in a median sagittal section. Emphasis is placed in the area where the telencephalic commissures develop, showing in the two schemas on the right the progressive relative increase in size of the corpus callosum over developmental time and from adult rodent to adult man. See Table I for abbreviations.

the telencephalon through the medial forebrain bundle, whereas the caudal diencephalon is connected to the telencephalon via the lateral forebrain bundle (internal capsule). The prethalamus and hypothalamus accordingly are a collection of neural centers that are involved in regulation of homeostasis, reproduction, the autonomic nervous system, and emotional states.

The caudal part of the hypothalamus (p4) includes the mammillary region, the subthalamic nucleus, the lateral hypothalamic area, and part of the posterior hypothalamus (MAM in Fig. 4). The prethalamic (alar) region of p4 reaches dorsally the forebrain roof (choroidal plexus) via the eminentia thalami at the caudal end of the telencephalic stalk (EMT in Fig. 4); this area possibly extends into the telencephalon through the medial amygdala. The p4 alar plate probably includes among its derivatives the subincer-tal area, the paraventricular and supraoptic nuclei (SIA and PV-SO in Fig. 4), and the perireticular nucleus (not shown), with the latter lying just in front of the ventral thalamic reticular nucleus. The eminen-tia thalami received its name due to its protrusion at the back of the interventricular foramen (tight passage interconnecting the prethalamic ventricular space with that of the telencephalic vesicle, or lateral ventricle) and the idea that it belongs to the thalamus. Its neuronal derivatives participate in the bed nuclei ofthe stria terminalis (posterior, or medial, parts) and the stria medullaris. This domain is characteristically traversed superficially by the telencephalic peduncle as it exits the telencephalon, incorporating as well the thalamotelencephalic projections. The fornix tract —hippocampal fibers passing from the caudal aspect of the commissural septum to the mammillary region —follows the dorsoventral dimension of p4 at an intermediate depth (arrow 1 in Fig. 5A).

The intermediate hypothalamus (p5) is represented by the tuberomammillary region, which also contains the premammillary nuclei and the dorsomedial hypothalamic nucleus (TM in Fig. 4). The corresponding alar or prethalamic area extends through the posterior entopeduncular area, the conventional 'anterior hypothalamus,' and anterior entopeduncular area into the telencephalic stalk (PEP, AH, and AEP in Fig. 4).

The rostral hypothalamus (p6) starts ventrally with the neurohypophysis, median eminence, and arcuate nucleus (floor plate derivatives) and includes the basal plate tuberal area, with the ventromedial hypothala-mic nucleus and the anterobasal nucleus (retrochias-matic area) (NH and TU in Fig. 4). The rostral prethalamic alar plate continues through the supra-

chiasmatic and posterior/anterior preoptic regions into the telencephalic stalk (SCH, POP and POA in Fig. 4).

The mammillary area is a complex of nuclei with a variety of inputs, including the fornix fibers from the hippocampus, as a part of the limbic circuit of Papez. These nuclei send their major outputs through the basal plate of p3, p2, and pi to the brain stem tegmental nuclei (mammillotegmental tract), with a collateral projection to the anterior dorsal thalamus, via the mammillothalamic tract (arrows 7 and 10 in Fig. 5A); these are also part of the Papez limbic circuit, which returns from there to cingulate and hippocam-pal cortex.

The tuberal/infundibular region and pituitary stalk are traversed by neuroendocrine fibers carrying vaso-pressin and oxytocin from magnocellular secretory cells in the overlying alar plate (supraoptic and paraventricular nuclei) to the neurohypophysis (arrow 6 in Fig. 5A); the neurohypophysis liberates these substances into the bloodstream. The median eminence contains a profusion of nerve terminals that secrete the proteins that regulate the release of various hormones from the anterior pituitary (corticotropin, thyrotropin, gonadotrophin, and growth hormone) into the hypophyseal portal capillaries. These regulator proteins arise from discrete groups of parvocellular neurosecretory neurons in the anterior hypothalamus and preoptic area. Other modulation of the pituitary occurs directly via axon terminals of dopamine neurons in the arcuate nucleus. The anterobasal nucleus bridges the retrochiasmatic midline area as a bed nucleus of the postoptic (supraoptic) commissures (poc in Fig. 5B) and is the rostralmost component of the basal plate.

The prethalamus can be subdivided into two superposed longitudinal tiers across prosomeres p4-p6. The lower tier coincides with the subpial course of the optic tract and the optic chiasm (Figs. 2 and 5A). It contains the suprachiasmatic nucleus (p6; involved in the control of circadian rhythms in homeostatic functions), the posterior entopeduncular area (including the migrated posterior entopeduncular nucleus; p5) and the subincertal area (p4). Little is known about the functions of the more caudal area. The upper tier is the so-called optoeminential domain, which lies just under the telencephalic stalk and telencephalon impar (un-evaginated parts continuous with telencephalic structures, such as the telencephalic preoptic and median septal areas; Ti in Fig. 2). The optoeminential domain starts rostrally (p6), with a prethalamic preoptic area found around the optic stalk recess (POP in Fig. 4). It includes the anterior hypothalamic area (p5), dorsal to the PEP, and the supraoptic and paraventricular nuclei, together with the derivatives of the eminentia thalami (p4; posterior bed nucleus striae terminalis and bed nucleus striae medullaris). At the boundary with the telencephalic stalk, this domain is traversed longitudinally by the stria medullaris, which continues caudalwards into the diencephalic roof (epithalamus) (arrow 3 in Fig. 5A). The overlying telencephalic stalk domain (telencephalon impar) consists of cell groups found at the peduncular transition into the evaginated telencephalic vesicles. From caudal to rostral, these contribute to the amygdala, the extrapyramidal system [entopeduncular nucleus (or internal segment of globus pallidus in highly evolved mammals), substan-tia innominata, and basal nucleus of Meynert], the anterior preoptic area/diagonal band formation, and the telencephalic preoptic area.

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