Книги по МРТ КТ на английском языке / The Embryonic Human Brain An Atlas of Developmental Stages. Third Edition. 2006. By Ronan O'Rahilly

D

A

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Lat. em.

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Med.

em.

Figure 18–5. The future amygdaloid complex indicates the developing temporal pole. Three blocks through the ventricular eminences are presented, as shown in the key (D). The di-telencephalic sulcus is marked by a long black arrow. The small arrow points to the hypothalamic sulcus. (A) The lateral ventricular eminence, which is visible adjacent to the roof of the interventricular foramen. (B) Four amygdaloid nuclei, which are arising almost entirely from the medial ventricular eminence. The nuclei present at this stage are (1) cortical, (2) medial, (3) anterior, and (4) basolateral. The medial ventricular eminences are clearly visible. Humphrey’s (1968) claim that the human amygdaloid complex is derived exclusively from the telencephalon is incorrect (Muller¨ and O’Rahilly, 1989b, 2006). The lateral and particularly the medial eminence at stages 18–20 contain large cavities that were observed by earlier investigators. (C) The basal part of the cerebral hemispheres, showing the ventral thalami. The optic stalk enters at the rostral part of the chiasmatic plate. (D) The right cerebral hemisphere and the ventricular eminences are projected onto a median section. The medial eminence indicates the site of the future temporal pole.

Figure 18–12. The caudal part of the nasal sacs. The oronasal membrane (arrowhead) has almost regressed on the other side.

Figure 18–13. The vomeronasal ganglion (v-n), the nerve fibers of which begin in the wall of the vomeronasal organ, which appears bilaterally as an epithelial pocket in the nasal septum. Further details: Muller¨ and O’Rahilly (2004c). The organ continues to enlarge during prenatal life, and measurements have been published (Smith et al., 1997). Its involution postnatally has been queried.

18 – 9

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Figure 18–14. The epiphysis cerebri.

A transverse section to show that the epiphysis now appears as a dorsal evagination of the synencephalon. One so-called follicle is visible. The adjacent part of the diencephalic roof is a portion of the habenular region. The epiphysis is now at Stadium III of Turkewitsch (1933). See Figure 19–16(III). ×214.

and the primordium of the flocculus from the remainder of the alar lamina. The fibers to and from the cerebellum are arranged in two strata, internal and external; the latter is the marginal layer. The fibers to the primordium of the flocculus are mostly vestibulocerebellar (shown by small black arrows at the level of the external cerebellar swelling). (G) To clarify the two sources of the cells that build up the cerebellum:

(1) radially migrating cells produced by the ventricular layer, and (2) tangentially migrating cells from the rhombic lip. The two streams intermigle. The open (white) arrow marks the direction of the afferent fibers to the cerebellum.

A

B

18 – 17

Figure 18–17. Transverse sections through the cerebellum. (A) A laterally situated band of cells that represents the region of the future flocculus. The internal cerebellar swelling (int.) faces the fourth ventricle. (B) The transitional region between the internal (above) and external (below) cerebellar swellings (ext.). Two streams of cellular migration can be recognized and are indicated by arrows in the upper inset drawing:

(1) radially oriented cells produced in the ventricular layer form the main cellular mass, and (2) tangentially oriented cells produced by the rhombic lip form a peripheral band. A layer of nerve fibers is present internal to that band, and another external to it.

Tracts present at stage 18 include the stria medullaris thalami, mamillothalamic tract, medial and lateral tectobulbar, dentatorubral, and the tractus solitarius. The inferior cerebellar peduncles have been reconstructed (O’Rahilly et al., 1988). They contain vestibulocerebellar and trigeminocerebellar fibers, and they reach the region of the dentate nuclei (Fig. 18–3). Collateral vestibular fibers run to the flocculus. The tracts mentioned will be illustrated with the next stage.

Figure 18–18. The sites of production and the migration of neurons in (A) the rhombencephalon, (B) the cerebellum, and (C) the mesencephalon. (A) Block from the hindbrain showing migration from the floor plate to the raphe (1), from the ventricular layer for the motor and sensory (e.g., vestibular) nuclei (2), and from the rhombic lip, (e.g., for the cochlear nuclei (3)). The sensory areas are lateral to the sulcus limitans. (B) Cerebellar migration from both the ventricular layer and the rhombic lip for the primordium of the dentate nucleus (stage 21). Cells from the ventricular layer give rise also to the piriform (Purkinje) cells, and those from the rhombic lip form the external germinal (or external granular) layer (stage 23). (C) Migration in the mesencephalic tegmentum from the ventricular layer, producing first the interpeduncular nucleus and later the ventral tegmental area. (D) The plane of the sections.