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

150 C h a p t e r 1 9 : CHOROID PLEXUS OF THE FOURTH VENTRICLE AND THE MEDIAL ACCESSORY OLIVARY NUCLEUS

Figure 19–1. An embryo of stage 19 surrounded by the intact amnion (limiting the amniotic cavity) and the bisected leafy mass of the chorion (limiting the extra-embryonic coelom). The fourth ventricle is clearly visible “behind” the cerebellar plate. The umbilical vesicle appears as a small sphere at the lower right-hand corner. Villous stems are visible in the future chorion frondosum. Photography by Mr. Chester F. Reather.

152 C h a p t e r 1 9 : CHOROID PLEXUS OF THE FOURTH VENTRICLE AND THE MEDIAL ACCESSORY OLIVARY NUCLEUS

a

(a) Sulcus

limitans

(b) Hypothalamic

sulcus

b

(c) Marginal ridge

c

Figure 19–3. Graphic reconstruction prepared from transverse sections to show a median view of the brain. The asterisk indicates the junction with the spinal cord. Clearly the cerebral hemispheres have grown rostrally, and hence the prosencephalic septum (inset) has increased in size. The nuclei in the septum will be discussed with the next stage. The interventricular foramen is narrow and is bounded mainly by the ventral thalamus (dorsally), the medial ventricular eminence (ventrally), and the hippocampal region (rostrally). Some embryos possess a paraphysis, which opens into the telencephalic part of the third ventricle. The choroid plexus becomes visible by ultrasonography in vivo at 7 postovulatory (9 postmenstrual) weeks.

Figure 19–4. Median section of the brain and the related cartilaginous skeleton. The chiasmatic and commissural plates are important landmarks. The choroid fold of the fourth ventricle is evident, and the subdural mesenchyme is a fluid-filled meshwork. Among the injected arteries, the basilar is especially well visible. Fine vessels can be seen in the area of the future falx cerebri. The dorsal funiculus appears as a thick bundle in the spinal cord. The median raphe (His, 1890), known as the septum medullae rhombencephali, consists of sagittally arranged fibers derived from the embryonic floor plate and representing the adult floor plate (Sidman and Rakic, 1982). In the photomicrograph, it is clearly visible above the basilar artery.

The basicranium (reconstructed by O’Rahilly and Muller,¨ 1984b) now consists of an occipital part (developed from sclerotomes 1–4) and a sphenoidal part, which has grown rostrally to reach the adenohypophysis. Although not visible here, the orbital wing of the sphenoid (mainly the postoptic root) is forming.

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Figure 19–3. (Continued ) The di-telencephalic borderline is indicated by an interrupted line between the velum transversum and the preoptic sulcus. The first optic fibers arrive in the chiasmatic plate but do not yet cross to the opposite side. The commissure shown is formed by supraoptic fibers. The epiphysis cerebri is adjacent to the posterior commissure and contains several follicles. At stages 19–21 it corresponds in general to Stadium IV of Turkewitsch (1933), as pointed out by O’Rahilly (1968). The globus pallidus externus has developed in the subthalamus. The dorsal and ventral thalami are well-separated from each other by the marginal ridge associated with the sulcus medius at the ventricular side and with the zona limitans intrathalamica (Fig. 18–6). The dorsal thalamus possesses an intermediate layer. The epithalamus is well-delimited, and its habenular nucleus (Fig. 19–13) has sent fibers (the habenulo-interpeduncular tract) to the midbrain since stage 17. The fibers arriving in the habenular area constitute the stria medullaris thalami, and arise mostly in the nucleus ovalis of the diencephalon (Figs. 19–9 and 19–23). Medial and lateral ventricular eminences, seen partly by transparency, are shown in the inset.

The isthmic groove between the mesencephalon and the cerebellum is indicated by a short arrow. The lateral recess and choroid fold of the fourth ventricle are shown. The floor of the rhombencephalon is thickened, and this will become more pronounced with the addition of further fibers in subsequent stages. The medial accessory olivary nucleus (Fig. 19–4) is beginning to form in between the roots of the hypoglossal nerve, and its site is indicated schematically by a projection onto the median plane. The lemniscal decussation is visible.

The characteristic hairpin bend of the mesencephalic flexure is formed by the metencephalon and the diencephalon, and indicates the site of the future pontine and interpeduncular cisterns of the subarachnoid space. The apex of the flexure marks the exit of the oculomotor nerves. The pontine flexure is eliminated later, so that the medulla and pons are in a straight line postnatally.

154 C h a p t e r 1 9 : CHOROID PLEXUS OF THE FOURTH VENTRICLE AND THE MEDIAL ACCESSORY OLIVARY NUCLEUS

Figure 19–5. Section through the neural tube, clarified in the key below the photomicrograph. The three zones of the neural tube are identified. A spinal

ganglion is visible at the left (medial to the neural arch). The key at the left shows the formation of a spinal nerve.

An atlas of the developing spinal cord is available (Bayer and Altman, 2003).

Spinal n.

Figure 19–6. Scheme of the basal nuclei showing the development of their diencephalic and telencephalic components. Stages of appearance are also included. Late in the embryonic period, the amygdaloid complex as well as the globus pallidus externus will occupy telencephalic territory. Whether the amygdaloid complex and the claustrum should be included with the other basal nuclei is doubtful. The basal nuclei are best defined as the basal structures affected pathologically in so-called extrapyramidal motor diseases.

Figure 19–7. Reconstruction of the developing basal nuclei (transitional between stages 18 and 19). (A) Medial view of the right half of the forebrain, in which the medial wall of the right cerebral hemisphere is shown as if transparent. The region of the amygdaloid nuclei is projected as an oval. (B) Dorsal view of the cerebral hemispheres, with the right hemisphere opened to illustrate its basal aspect from the interior. The medial ventricular eminence has almost completely invaded telencephalic territory (cf. Fig. 19–3). The lateral eminence represents the main primordium of the future corpus striatum (Fig. 19–6). From O’Rahilly, Muller,¨ Hutchins, and Moore (1988).

The medial ventricular eminence has been found in the rat to resemble the globus pallidus histochemically far more than it does the lateral eminence, and it is connected to the dopamine pathway that leads from the ventral tegmental region of the midbrain to the forebrain.

The nucleus accumbens develops as a separate entity far earlier than in rodents, and projects as a thickening into the ventricular cavity. It was shown by Hewitt (1958, Fig. 3) in an early human fetus, where it appeared as the elevation known as the “paraterminal body.” It is regarded by various authors as a ventromedial extension of the caudato-putamen complex. Its superficial component includes the nucleus of the diagonal band (Kuhlenbeck, 1973). The nucleus accumbens has been shown in the human to contain dopamine, and it resembles the caudato-putamen complex in its histochemical development (Brana et al., 1995).

TABLE 19–1. Comparison of Medial and Lateral Ventricular Eminences