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

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Figure 10–11. The neural groove of rhombomere 2. Ventral to it, the folding notochordal plate is visible and, on each side, a dorsal aorta. Transverse anastomoses between the right and left dorsal aortae have been recorded at stage 11, and fusion has definitely begun at stages 13 and 14.

Ot.

Figure 10–12. The neural tube of rhombomere 5. Mitotic figures are visible near the cavity of the tube. The otic plates appear on each side as ectodermal thickenings. Extramural blood vessels are forming ventrally and laterally, adjacent to the wall of the brain, but they do not yet give off intramural branches. A histological distinction between the perineural arteries and veins becomes apparent by stage 20 and for the intramural vessels during trimester 2.

D2

D1

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Figure 11–3. Graphic reconstruction prepared from transverse sections showing a median view of the brain. The asterisk indicates the junction with the spinal cord. The rostral, or cephalic, neuropore is indicated by a thick line between two arrowheads. Each of the two neuropores, rostral and caudal, measures about 0.5 mm in length (mean of 24 embryos). The rostral neuropore closes within a few hours during stage 11, when about 20 somitic pairs are present. (The embryo illustrated here has 17.) The thin surface ectoderm overlying the dorsal aspect of the neural tube has not been included in the drawing.

D1 still consists mostly of optic primordium. The chiasmatic plate is first at the rostral end of the neural plate (13 pairs of somites), as in stage 10. When 14 or more somitic pairs have appeared, however, fusion at the “terminal” lip (Fig. 11–7A) of the neuropore results in the formation of a floor for the telencephalon medium. This represents the future lamina terminalis and commissural plate.

In D2, the neural floor adjacent to the adenohypophysial primordium is the neurohypophysial region. Both components of the hypophysis cerebri develop in contact with each other, so that no migration or growth of one to meet the other occurs. The level of the adenohypophysial primordium is caudal to that of the chiasmatic plate, and its site is clearly defined by the oropharyngeal membrane, which has not yet ruptured (in 11 out of 16 embryos).

The percentage of length occupied by the mesencephalon in relation to that of the whole brain remains almost constant from stage 9 up to and including stage 11. The rhombencephalon takes up approximately three-quarters of the brain in length. The relative increase in prosencephalic length is slightly less than in stage 10. Cranial ganglia and neural crest are projected onto the median view. Eight rhombomeres are distinguishable (Fig. 10–4B).

The notochord or the notochordal plate is still closely related to the neural tube or the neural plate, so that it is possible that induction of the floor plate could continue along almost the whole length of the neural axis.

Figure 11–5. Drawings of the rostral end of the neural tube in seven embryos of stage 11, to show the progressive closure of the rostral neuropore. Under each is shown a horizontal section taken at the level indicated by the horizontal lines. The embryos shown had 13, 14, 16, 17, 17, 17, and 20 somitic pairs, respectively. Modified from Streeter, as reproduced by O’Rahilly and Muller¨ (1987a). The rostral neuropore, which is still open when 19 pairs of somites are present, is closed when 20 pairs have formed (O’Rahilly and Gardner, 1979, Table 2). See also Figure 12–12.

The rostral neuropore is not considered to be the “front” end (das wahre ursprungliche¨

Ende) of the neural tube, which is frequently taken to be the infundibulum or the infundibular recess (Dart, 1924, in agreement with von Baer). There is much to be said, however, for Johnston’s (1909) view that “in all vertebrates the anterior end of the head is the point at which the brain plate meets the general ectoderm at the same time that it comes into contact with the anterior end of the entoderm. This point is marked in the adult by the optic chiasma.”

Abbreviations: A-V, atrioventricular canal; C-T, conotruncus; LV, left ventricle; RA, right atrium; RV, right ventricle; SV, sinus venosus.

Figure 11–6. Drawings and photomicrographs to illustrate the closure of the rostral neuropore in an embryo of stage 11. Basically, the neuropore closes bidirectionally, i.e., from (site β) its “dorsal lip” (near D2) and simultaneously from (site α) its “terminal lip” (in the telencephalon, adjacent to the chiasmatic plate). The surface ectoderm participates in the closure at the terminal lip, and the area of fusion of the neural folds lies between the two nasal discs. The right and left components of the neural ectoderm (stippled) and those of the surface ectoderm (in blue) seem to fuse simultaneously across the median plane. The bar represents 0.02 mm. A key drawing showing the lips and the plane of sections A and H is provided in Figure 11–7 A. From O’Rahilly and Muller¨ (1989a).

The phenomenon of closure at the “terminal lip” is ignored in most publications on embryology. In the chick embryo, near the time of fusion of the neural folds, occlusion of the lumen of the neural tube occurs and has been considered to be necessary for the growth of the brain. In the human embryo, however, occlusion is infrequent during stage 11 and has been found only when the rostral neuropore is still open (Muller¨ and O’Rahilly, 1986a), thereby precluding any hydrodynamic function. Moreover, growth of the brain is practically at a standstill during stages 11 and 12 (Muller¨ and O’Rahilly, 1987,

Table 3).

The closure at the dorsal lip is much simpler. It concerns the fusion of both the unilaminar surface ectoderm and the unilaminar neuroectoderm, represented schematically in Figure 10-5E.

Figure 11–7. Views of the rostral neuropore and the beginning optic vesicles at stage 11.

(A) End-on and median views as a key to Figure 11–6. Tel. indicates the surface ectoderm overlying the telencephalon. In the median view, the thin surface ectoderm overlying the dorsal lip has not been included in the drawing. The two horizontal lines show the plane of sections A and H. (B) End-on and right lateral views as a key to the plane of section of the photomicrograph, C. (C) The optic vesicles are being formed by further deepening of the optic sulci in D1. Optic crest is developing from the external layer of the optic vesicle and is met by migrating mesencephalic neural crest. The two form the sheath of the optic vesicle, i.e., of the future optic cup. The basement membrane is interrupted where the cells of the optic crest leave the neural ectoderm. Optic crest is formed by mitotic division in the superficial cells of the optic vesicle, an exception to the general rule that mitosis in the neural tube occurs adjacent to the ventricle. The rostral neuropore is visible at the bottom of the photomicrograph. Bar: 0.15 mm.

Mesencephalon