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

g c

Raphe magnus nuclei

Somatic afferent

Visceral afferent

Visceral efferent

Somatic efferent

Inferior cerebellar peduncle

Figure 23–29. Dorsal view of a reconstruction of the rhombencephalon (silver-impregnated), showing nuclei and tracts. The caudal direction is towards the top of the page. The right embryonic half is on the left of the drawing. The cerebellum has not been included. The white line on the right-hand side of the drawing represents the cut edge of the roof of the fourth ventricle. Black triangles indicate the levels of the reconstructed cross sections in Figure 23–30. The nucleus funiculi teretis (Kappenkern of the genu of the facial nerve) is included. The two (ascending and descending) intramural parts of the facial nerve are indicated by the numeral 7.

The inset shows the four vestibular nuclei.

The rhombic lip, an important proliferative area in the alar plate, is the source of two superficially situated migratory areas: the so-called olivo-arcuate caudally, and the pontine rostrally. Two of the three migratory areas found at stage 23 have been reconstructed (Muller¨ and O’Rahilly, 1990c, Fig. 1). The term “olivo-arcuate migration” (Essick, 1912), however, is unsatisfactory, because it has been shown (in the monkey) that the olivary nuclei arise mainly from the ventricular layer.

The rhombencephalon at this stage is already very complicated and comparable in many respects to that of the newborn (Fig. 26–5). Its rapid development suggests an early onset of functional activity.

7

Vestibulo-

spinal tr.

and cuneo-

Figure 23–30. Reconstructed coronal sections of the rhombencephalon. The levels of the sections (A)–(F) in rostrocaudal succession are indicated in Figure 23–29 by black triangles. (A) Section showing the nuclei of the trigeminal nerve and the transition to the cerebellum via the inferior cerebellar peduncle. The position of the trigeminal nuclei here is conditioned by the slight obliquity of this slice. The inset in the lower right-hand corner of the page shows fiber bundles of the motor part, of the intermediate, and of the sensory portion of the trigeminal nerve. (B) Area of the abducent and facial nerves, showing the superior olivary nucleus, and the superior vestibular nucleus and its fibers to the medial longitudinal fasciculus. (C) Entry of the cochlear nerve. Ventral and dorsal cochlear nuclei are lateral to the inferior cerebellar peduncle. The fibers from the dorsal cochlear nucleus that run medially constitute the intermediate acoustic striae. A dagger indicates the tectobulbar and tectospinal tracts. (D) Entry of the sensory glossopharyngeal fibers that run directly to the tractus solitarius. Olivocerebellar fibers can be seen passing towards the inferior cerebellar peduncle. (E) and (F) Inferior olivary, vagal, accessory, and hypoglossal nuclei. A small bundle of corticospinal (pyramidal) fibers is present in all the sections.

Lat. corticospinal

Motor

neurons

Ant. corticospinal

Figure 23–32. The pyramidal decussation in the transitional region of the medulla oblongata and spinal cord.

Figure 23–33. The arteries in a dorsal view, with a key drawing on the left. This is the first published reconstruction of the arteries at stage 23. The right and left sides were reconstructed separately, and the two sides are slightly different. Most cranial nerves, the optic chiasma, and part of the optic tracts are shown. The main components of the circulus arteriosus have been present since stage 16 (Fig. 16–5) and the circle has been complete since stage 19 (Fig. 21–23). The arteries to the choroid plexus of the lateral ventricles come from one of the deep branches of the anterior cerebral artery (anterior choroid a.) and from the posterior cerebral artery (posterior choroid a.). Two anterior communicating arteries are present, a frequent finding later in life. Apart from differences in the proportions, the arterial pattern resembles closely that of the adult.

Figure 23–35. Examples of in vivo ultrasonic images at 6–7 12 postfertilizational weeks, i.e., during the embryonic period.

(A) 12.9 mm. Obliquely coronal section showing cerebral hemispheres. (B) 15 mm. Sagittal view showing mesencephalic flexure. (C) 15 mm. Another obliquely coronal section showing hemispheres.

M

Figure 23–35. (Continued ) (D) 18 mm. Sagittal view showing the spinal cord. (E) 25 mm. Coronal section, the “hole in the head” view. Courtesy of Dr. Harm-Gerd Blaas, Trondheim, Norway.

TABLE 23–1. Some Examples of Genes Already Known to Be Implicated in the Development of the Human Nervous System