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

M2

M2

Syn.

Tongue

Ao.

A B

C D

F

Uveocapillaris

E

1

3

Inverted layer of optic cup

Lens

disc

Figure 13–4. The development of the eye from 41/2 to 6 weeks. (A) Stage 13. Early indentation of the retinal and lens discs.

(B) Stage 14. The optic cup and lens pit are evident. (C) Stage 15. The lens pit has become closed off from the surface, forming the lens vesicle. (D) Stage 17. Retinal pigment can be seen in the external layer of the optic cup. The inverted layer of the cup will develop in a manner comparable to that of the wall of the brain (Fig. 14–8). The cavity of the lens vesicle is beginning to become obliterated. Hyaloid blood vessels are visible behind the lens. Early development of the retina. (E) At stage 14, showing layer 1 (the future pigmented layer), layer 3 (the terminal bars of the external limiting membrane), and the thick inverted layer. (F) At stage 15, showing the onset of pigmentation (minute black dots) in layer 1 (as well as two mitotic figures). Only the most external portion of the inverted layer is included in the field selected. Further development can be seen in Figure 23–13. The inverted layer develops in a manner comparable to that of the wall of the brain, as shown in Figure 14–8.

Figs 13 – 6, 7

13 – 5

S.3

Figure 13–7. A hypoglossal root from the neural tube (at left) to somite 3 (at right). The nerve fibers are accompanied by neural crest cells. The hypoglossal roots are not rigidly segmental, some somites receiving two roots. The cells of the primordial hypoglossal nucleus are relatively lateral in position, but that changes during development, as described with Figure 14–9.

The hypoglossal artery participates in the carotico-basilar anastomosis, which is prominent until stage 15 (Fig. 21–22).

NEUROTERATOLOGY: CEREBRAL DYSRAPHIA (ANENCEPHALY)

The most frequent neural tube defects (NTD) are anencephaly and myelomeningocele, which are multifactorial, and the genetic and environmental influences have been reviewed (Detrait et al., 2005).

Cerebral Dysraphia. Anencephaly develops in three phases. The first, cerebral dysraphia (encephaloschisis), is believed to arise as a failure of closure of the neural groove rostrally, an idea supported by many authors. Rupture of the already closed neural tube cannot be entirely excluded in some instances. Cerebral dysraphia arises early, probably as a mesenchymal defect, even as early as stages 8 or 9, i.e., before fusion of the neural folds begins, because elevation of the folds depends on the production of sufficient mesenchyme.

Fusion of the neural folds commences during stage 10, when production of neural crest is only starting. The ganglia of the cranial nerves, derived largely from neural crest, are only minimally affected in anencephaly. A reduction in the size of the ganglia, however, as well as an incomplete chondrocranium, would point to defective distribution of the cephalic mesenchyme, caused probably by a derangement of the mesencephalic neural crest in stages 10 and 11. Moreover, stage 10 is characterized by the first appearance of the optic primordium, and, rarely, anencephaly may be combined with anophthalmia. Usually, however, the eyes are very well developed in anencephaly, indicating that development of the diencephalon at first proceeds normally, although later it degenerates.

It would be extremely difficult to detect a pure cerebral dysraphia at stage 11, because it is only during that stage that the rostral neuropore normally closes. Nevertheless, an example of complete cerebrospinal dysraphia at stage 11, presumably the forerunner of craniorhachischisis totalis, has been described in detail (Dekaban, 1963; Dekaban and Bartelmez, 1964).

Cerebral dysraphia has been described in detail in a twin embryo of stage 13, and reconstructions are available (Muller¨ and O’Rahilly, 1984). The neural tube was open over part of the midbrain and forebrain, although the situs neuroporicus (at the locus of the future commissural plate) was closed. Hence, in future anencephaly, fusion at the “terminal lip” of the rostral neuropore to form the embryonic lamina terminalis (Figs. 11–7A and 13–9) may not be affected. It is possible that a causal relationship exists between neural tube defects and monozygotic twinning.

Communication between the amniotic fluid and the ependymal (future cerebrospinal) fluid is normally eliminated by stage 13 because of neuroporal closure, so that α-fetoprotein no longer diffuses into the amniotic cavity,

except in such conditions as anencephaly and spina bifida cystica.

Phases 2 and 3 of anencephaly are discussed in Chapters 22 and 24 respectively.

MISCELLANEOUS

ANOMALIES

Cebocephaly. Indications of this condition have been found in an embryo attributed to stage 13 (Fig. 13–10). A graphic reconstruction was prepared, from which it could be seen that the notochord did not reach the adenohypophysis, as normally it would. A convergence of the nasal discs indicated a narrow median region of the head. The nasal septum would not develop, and the single nasal cavity would possess a single nostril. Cebocephaly arises earlier than arhinencephaly, in which the brain is almost normal except for absence of the olfactory bulbs. It has been found experimentally in the mouse that in cebocephaly little or no tissue from the medial nasal processes is involved in the development of the nose, lips, and palate. The single nostril in cebocephaly ends blindly, and the eyes are closely set. The brain is holoprosencephalic in type and the forebrain septum is absent or reduced. The corpora striata are not separated from each other.

Arhinencephaly. This term sensu stricto refers to absence of the olfactory bulbs and tracts, irrespective of associations with other developmental malformations, such as holoprosencephaly, which is commonly combined with arhinencephaly. Isolated arhinencephaly would be expected to arise at the time when the nasal discs should begin to form: thickenings normally appear at stage 11, and a nasal field is outlined at stage 12 (Bossy, 1980). This timing, however, depends on the unproven assumption that the olfactory bulb is induced by the olfactory nerve. Holoprosencephaly and arhinencephaly may develop independently of synophthalmia, probably at about stage 11, or perhaps even 10. Although they are commonly linked, holoprosencephaly and arhinencephaly can also arise independently of each other. Siebert et al. (1990) studied 103 cases of this condition and found that arhinencephaly is extremely heterogeneous both clinically and pathologically.

Cerebellar agenesis, although rarely total, could arise as early as stage 13, or even in stage 12. The first part of the cerebellum to develop is material for the hemispheres, whereas the primordium of the vermis becomes defined only at stages 18 and 19.

Abducent-facial palsy (Mobius¨ sequence) presents as a nonspecific, commonly bilateral, mask-like facies. The nuclei of cranial nerves 6 and 7 are usually degenerated, but they may be absent. Most of the motor nuclei of the cranial nerves begin to develop at about stage 13. Other cranial nerves may be involved in the sequence,

T

Figure 13–9. Comparison of normal and deficient closure. (A) The normal sequence of closure of the rostral neuropore involves (1) a caudorostral fusion of the neural folds at the dorsal lip and (2) a fusion at the terminal lip proceeding towards the neuropore. (B) In an embryo of stage 13 believed to be incipiently anencephalic, the fusion of the neural folds is at a standstill. The open region indicates where fusion should have taken place during stage 11. Fusion at the terminal lip, however, has occurred.

and limb defects are frequent. Some of the variable features (e.g., micrognathia and microglossia) are probably secondary. Moreover, some instances of the sequence are considered either to be caused by involvement of peripheral nerves or as a myopathy. Abnormal torsional

and vertical eye movements could be produced by disturbance of the interstitial nucleus (Crawford et al., 1991). That nucleus appears in stage 13 at the ventral transitional area between diencephalon and mesencephalon (Fig. 14–5).

A

11

12

The future cerebral hemispheres (arrow) become identifiable during stage 14 and are delimited from the telencephalon medium by the torus hemispher-

icus internally and by the di-telencephalic sulcus externally. Particularly advanced in the prosencephalon are the hypothalamic, amygdaloid, hippocampal, and olfactory regions. The synencephalon is identifiable. The mesencephalic flexure is less marked and is occupied by the

future tentorium cerebelli. The pontine flexure appears. The cerebellum is formed by the alar plate of the isthmus as well as by that of rhombomere 1. Blood vessels now penetrate the wall of the brain. Various bundles (e.g., the medial and lateral longitudinal fasciculi, the common afferent tract) and a number of nuclei of cranial nerves are present. All 16 neuromeres are now identifiable.