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

A

L.t. Olf.

D

Di.

A.-H.

Ot.

10

11

Rh.

B

Spinal

cord

E

Thal.d.

Thal.v.

5

C

Hem.

Med. em.

Di.

V4

Rh.

Hypothal

cord

Int. carotid a.

Figure 16–8. Cerebral hemisphere showing the loose distribution of the superficial cells at the periphery. Up to and including stage 16 the neocortical part of the cerebral hemispheres is avascular (Larroche and Jardin, 1985), although it is already surrounded by a prominent “perineural” vascular process (Streeter, 1919; Padget, 1948; Mar´ın-Padilla, 1978, 1988a,b). Bar: 0.2 mm.

Hyp.-th.

sulcus

Figure 16–9. Some two sections more deeply, the surface of the cerebral hemisphere presents widely dispersed cells, indicating progressive differentiation. Bar: 0.2 mm.

The cell columns of the ventricular layer of the diencephalon in Figures 16–9 and 16–10 are high and are penetrated by blood vessels. The cells of the intermediate layer are not as tightly packed as in the telencephalic area shown in Figure 16–11, and this is a clear difference. Peripheral to the intermediate layer, nerve fibers and scattered cells give an impression of a primordial plexiform layer.

. Hipp

Figure 16–11. The hippocampal area, characterized by a marginal layer and a discrete ventricular thickening. Bar: 0.1 mm.

Figure 16–10. The subthalamus, ventral to (below) the hypothalamic sulcus, showing fibers in the marginal layer and capillaries that have penetrated the cerebral wall. Such diencephalic blood vessels are present in most embryos of this stage. Bar: 0.1 mm.

EVAGINATION OF THE NEUROHYPOPHYSIS

Is.

M2

M1

B

Cel. M2

Is.

C

B

C

Rh. 6 5 4 3 2

111

Figure 16–12. Three-dimensional representation of the rhombencephalon and part of the mesencephalon. The orientation of views B and C is shown in the key drawing. (A) The left half seen from the median plane. Intramural fibers of the trochlear nerve are indicated by two, closely set interrupted lines which run in the rostralmost part of the isthmic segment inside a ventricular crest. (B) Block with the roof of the mesencephalon removed, so that part of the floor of the isthmic segment becomes visible. At the left, the cerebellum (mostly Rh. 1) is seen to be much wider than the mesencephalon. (C) Block showing the ventral part of the rhombencephalon, which is composed of rhombomeres 2 to 8.

112

A

1

TA

B C

Longitudinal

NEUROTERATOLOGY: HOLOPROSENCEPHALY

Holoprosencephaly is a term used for a graded series of cerebral and facial anomalies that range from cyclopia to the almost normal. The “entire prosencephalon remains holospheric rather than becoming hemispheric” (DeMyer, 1987); i.e., the prosencephalon remains as a whole, a single, more or less undifferentiated sphere, incompletely cleft into complex halves. In the telencephalon the future septal area and the commissural plate (future anterior commissure and corpus callosum) are missing, and the telencephalic portion of the corpus striatum is not formed (Muller¨ and O’Rahilly, 1989c). At the diencephalic level, disturbances are less evident, except at the level of the optic primordia. The adenohypophysis may be absent or may merely become detached. Its absence may perhaps be related to a disturbance of the prechordal plate.

It is possible that holoprosencephaly involves a fundamental event, such as induction, and has an early origin (probably in stage 8). Absence of the rostral part of the notochord (with consequent cranial defects) could result from faulty interaction with the prechordal plate. At stage 8 a mesenchymal disturbance could already be active. Faulty development of the paraxial mesenchyme from the primitive streak or node could interfere with the elevation of the neural folds. Defective prechordal mesenchyme may fail to produce the normal facial structures and to induce adequate morphogenesis of optic primordia and of the brain. In stage 10 production of the neural crest is beginning. The ganglia of the cranial nerves derived

from rhombencephalic neural crest are smaller in holoprosencephaly. Many basal areas of the forebrain that become distinct in stage 14 are absent in holoprosencephaly, e.g., the olfactory region (paleocortex), the optic stalks, and the neurohypophysial area. Hence, holoprosencephaly is more than a mere failure to “diverticulate” into cerebral hemispheres. In those rare instances where holoprosencephaly is found independently of synophthalmia and arhinencephaly, the origin is possibly at a later time, but nervertheless not later than stage 13 or 14. Such conditions, although variable in their manifestation, would arise during the first 4 or 5 weeks (Fig. 16–14E, F).

Holoprosencephaly is more than 60 times more frequent prenatally than in live births. Examples from stages 13–23 have been described (Yamada et al., 2004). The condition may be autosomal dominant, autosomal recessive, or X-linked recessive, and about half of affected newborns have chromosomal disorders (chiefly chromosome 13, 18, or 21; deletions in chromosome 7 may be implicated). Association with trisomy 13 and triploidy may also occur.

Holoprosencephaly is commonly graded into (1) alobar (a holotelencephalon that lacks division into hemispheres; it can be detected ultrasonically by the demonstration of a single ventricle), (2) semilobar (incomplete cerebral hemispheres), and (3) lobar (well-formed cerebral hemispheres and longitudinal fissure). Alobar holoprosencephaly has been visualized in vivo by 3D ultrasound at 7 12 postfertilizational weeks (Blaas et al., 2000b). The clinical manifestations of holoprosencephaly, together with a survey of holoprosencephalic syndromes and an atlas of 35 cases, can be found in a useful book by Siebert et al. (1990).

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A

Telencephalon

Diencephalon

Medial ventricular eminence

D E

C

G

C h a p t e r 1 6 : EVAGINATION OF THE NEUROHYPOPHYSIS

H

Rostral and caudodorsal growth of the cerebral hemispheres results in deepening of the longitudinal fissure, in which the vessels of the future choroid

plexus develop. The olfactory bulb and tubercle become outlined. The amygdaloid area, which is related to the medial ventricular eminence, contains three to four nuclei.

The first dopamine-producing areas are present. The first indication of a septal nucleus is recognizable. The hemispheric stalk unites the cerebral hemispheres with the ventral thalamus and with the diencephalic part of the medial ventricular eminence. The di-mesencephalic boundary passes between the posterior commissure and the commissure of the superior colliculi (Table 15–2). Gustatory fibers are beginning to separate from the common afferent tract.