Файл: The Embryonic Human Brain An Atlas of Developmental Stages. Third Edition. 2006. By Ronan O\'Rahilly-1.pdf
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C h a p t e r 1 1 : CLOSURE OF THE ROSTRAL NEUROPORE |
D1 |
D2 |
M |
Rh.
1
2
3
4
5
6
Figure 11–4. Sagittal section through an embryo at stage 11 (17 somites) showing the rostral neuropore, through which the lumen of the neural tube still communicates with the amniotic cavity. (The thin line of the amnion is visible above.) Further caudally, the wall of the neural tube shows portions of several rhombomeres. Also recognizable are the oropharyngeal membrane and foregut, and the heart, which presents myocardial mantle, cardiac jelly (appearing here as an almost clear space), and endocardium.
CLOSURE OF THE ROSTRAL NEUROPORE |
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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.
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C h a p t e r 1 1 : CLOSURE OF THE ROSTRAL NEUROPORE |
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.
CLOSURE OF THE ROSTRAL NEUROPORE |
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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
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Neuropore |
C |
Optic sulcus |
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C h a p t e r 1 1 : CLOSURE OF THE ROSTRAL NEUROPORE |
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D |
E |
A |
B |
C |
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Figure 11–8. Development of the notochord and the floor plate.
(A)After the notochord has developed from the dorsal part of the notochordal process of stages 7 and 8, its cross-sectional shape changes.
(B)It appears as the notochordal plate in stage 9 (Fig. 9–5B). (C, D) It has the form of an inverted U in stages 10 and 11 (Figs. 10–11 and 11–9). (E) Next it appears as a rod in stage 12 (Fig. 10B).
Notochord
Figure 11–9. The notochord in this embryo of stage 11 with 16 pairs of somites still has the form of an inverted U at the level of rhombomere 5. Bar: 0.05 mm.
Figure 11–10. Secondary development of the body. The caudal part of the notochord, caudal to the site of the former neurenteric canal, which is formed from the caudal eminence, in an embryo with 17 pairs of somites. The neural groove above is still open. Bar: 0.04 mm.
CLOSURE OF THE ROSTRAL NEUROPORE |
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A
Aorta
Heart
BCapital v.
RV
LV
Internal carotid a.
C
2
1
Figure 11–11. The early blood vessels. The movement of the blood would be merely ebb and
Otic disc flow. The still open brain would not yet be dependent on a blood supply. (A) At stage 9 (2, 3
somites). (B) At stage 10 (10 somites). (C) At stage
11 (14 somites).
Aorta
Omphalo-
mesenteric v.
Aorta
Umbilical v.
Umbilical
plexus
Cardinal vv.
Cono-
truncus L. & R. ventricles
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C h a p t e r 1 1 : CLOSURE OF THE ROSTRAL NEUROPORE |
NEUROTERATOLOGY:
ENCEPHALO(MENINGO)CELE
Encephalocele and encephalomeningocele may arise, at least in some instances, from a failure in separation of the brain from the surface ectoderm in later stages. This failure in separation would be caused by a disturbance of the mesencephalic neural crest, which is at the height of its formation at stage 10. This mesenchyme is believed to be the main source for the development of the face, and insufficient production of it may lead to a narrow facies, as seen in the fetal alcohol syndrome.
Encephaloceles are, in the Western Hemisphere, most frequently found in the occipital region. They may perhaps be caused by a lack of the basal mesenchyme originally between the neural tube and the notochord; its quantity is accumulating rapidly.
Fronto-ethmoidal encephaloceles may be at or near the situs neuroporicus (as proposed by Hoving from studies
of the rat). It has been speculated (by Hoving, 1993) that insufficient cell death (apoptosis) at the terminal lip of the rostral neuropore (e.g., from retinoic acid deficiency) might result (immediately after fusion of the neural folds) in persistent attachment of the surface ectoderm to the neural ectoderm, producing a subsequent cranial defect and a fronto-ethmoidal encephalocele.
An interesting example of a telencephalic diverticulum in a 35 mm fetus has been recorded (Bossy, 1966). It was situated in the region of the former situs neuroporicus and was considered to represent cerebral dysraphia with secondary encephalocele.
Nasal Glioma. Insufficient cell death in the region of the rostral neuropore causes a persistent “surplus of unintended surviving cells” that “might be represented by heterotopias like nasal gliomas” (Hoving, 1993).
C H A P T E R 12
STAGE 12: CLOSURE OF THE CAUDAL NEUROPORE AND THE BEGINNING OF SECONDARY NEURULATION
Approximately 3–5 mm in Greatest Length;
Approximately 31 Postfertilizational Days
The rostral neuropore is closed, although the situs neuroporicus can frequently be detected and is probably at the future commissural plate in the mid-
dle of the embryonic lamina terminalis. The closure (during stage 11) allows the formation of the telencephalon medium to occur. The mesencephalon consists of two neuromeres (M1 and M2), and the mesencephalic flexure is a right angle. The rhombomeres have important
relationships to the cranial ganglia: Rh.5 is associated with the otic vesicle, and Rh.D is level with the four occipital somites. The first nerve fibers are differentiating, particularly those of the future lateral and ventral longitudinal fasciculi. The hypoglossal nucleus has appeared and four to five intramural hypoglossal roots are present. The caudal neuropore closes during stage 12, and its final prenatal site is at the level of somitic pair 31 (future vertebral level S2).
The Embryonic Human Brain: An Atlas of Developmental Stages, Third Edition. By O’Rahilly and Muller¨ Copyright C 2006 John Wiley & Sons, Inc.
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62 C h a p t e r 1 2 : CLOSURE OF THE CAUDAL NEUROPORE AND THE BEGINNING OF SECONDARY NEURULATION
Figure 12–1. Right lateral view of an embryo of stage 12 with the brain superimposed. The number of somitic pairs was 28; the range in number at this stage is 21–29.
M
Di.
T
Figure 12–2. Right lateral view of the brain. The features shown are the optic vesicle, the trigeminal and facio-vestibulocochlear ganglia, the otic vesicle, and the glossopharyngeal and vagal-accessory ganglia. The brain now occupies about 40% of the length of the neural tube. Expansion of the brain stretches the surface ectoderm (O’Rahilly and Muller,¨ 1985) so that entrance of mesenchymal cells may be hindered. The prechordal mesenchyme is caudal to the optic vesicles, where the orbital muscles will develop. The optic neural crest (Fig. 12–5) is now at the height of its development. The neural crest contributes to the trigeminal and facio-vestibulocochlear ganglia. Crest cells from the region of ganglia 9–11 migrate, by way of pharyngeal arch 3, towards the aortic sac. Moreover, the surface (epipharyngeal) ectoderm gives rise to cells that may contribute to the facial, inferior glossopharyngeal, and vagal ganglia. The hypoglossal cell cord is forming from the occipital somites and, in the more advanced embryos of this stage, it begins to enter pharyngeal arch 4.
CLOSURE OF THE CAUDAL NEUROPORE AND THE BEGINNING OF SECONDARY NEURULATION |
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Figure 12–3. Reconstruction of the brain. The asterisk indicates the junction with the spinal cord. The rostral neuropore has closed. The site of final closure (the situs neuroporicus) is probably at the future commissural plate in the middle of the embryonic lamina terminalis.
The initial formation of the telencephalon medium normally depends on the closure of the rostral neuropore during stage 11. The prosencephalon now consists of three parts: the telencephalon, D1, and D2. The telencephalon medium, which is rostral to the well-defined neuromere D1, is recognizable externally by its situation between the nasal discs (Fig. 12–7). D1 is still characterized mainly by the optic vesicles. The semilunar opening of the optic ventricle into the third ventricle is indicated by shading. D2 is related to the adenohypophysial primordium and now shows the first sign of the mamillary recess.
The mesencephalon consists of two neuromeres: M1 and M2. The mesencephalic flexure is a right angle between the forebrain and the hindbrain. The sulcus limitans is seen to traverse the midbrain.
The various rhombomeres have distinctive features. For example, the floor is expanded at the level of Rh.2, 4, and 6. Rh.5 is related to the otic vesicle, and the ganglia of the future cranial nerves (which are projected onto the median view) are important relationships of Rh.2, 4, 6, and 7. Rh.D is at the level of somites 1–4. The first nerve fibers are now differentiating. Apart from short fibers in ganglia 5 and 7/8, the fibers are mostly related to cells that will form the nucleus of the lateral longitudinal fasciculus. Moreover, fibers of the future ventral longitudinal fasciculus are discernible in Rh.7. Four intramural hypoglossal roots are present, i.e., the hypoglossal nucleus has developed and some hypoglossal neural crest is still present dorsally. The relative length of the rhombencephalon has been decreasing since stage 9.
Figure 12–4. An almost median section of the brain showing rhombomeres 2–7 and two occipital somites.
64 C h a p t e r 1 2 : CLOSURE OF THE CAUDAL NEUROPORE AND THE BEGINNING OF SECONDARY NEURULATION
Optic
crest
Opt. D1
Chiasmatic
plate
Oropharyngeal
epithelium
Figure 12–5. Optic vesicles and D1 in a horizontal section. A mesenchymal sheath separates the optic vesicles from the surface ectoderm. Optic neural crest is evident and gives the optic vesicle the appearance of a “frightened hedgehog” (Bartelmez) externally (Fig. 12–2). Little or no mesenchyme is present between the dorsal part of D1 and the surface ectoderm.
Fig. 12 – 5
M
5
7,8v
Fig. 12 – 6
Ot.
Figure 12–6. Rhombencephalon. Several cranial ganglia and the otic pits are visible. The surface ectoderm covering the ventral part of pharyngeal arch 1 (arches 2 and 3 are also visible here) is thicker and gives off cells that join the trigeminal ganglion. The ectoderm is a part of the ectodermal ring (O’Rahilly and Muller,¨ 1985).
CLOSURE OF THE CAUDAL NEUROPORE AND THE BEGINNING OF SECONDARY NEURULATION |
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Situs neuroporicus Tel. medium Nasal disc
Optic
crest
D1
Figure 12–7. Optic vesicles and D1 in a horizontal section of another embryo. Optic neural crest is evident. At this time the diencephalon consists of two neuromeres: D1, related to the optic vesicle, and D2, related to the adenohypophysis. The site of the closed rostral neuropore is conspicuous. The nasal disc becomes bilaminar and becomes shifted laterally. Bar: 0.1 mm.
Sternberg (1927), who placed the rostral neuropore in the region of, and probably dorsal to, the commissural plate, ventured to identify the situs neuroporicus even up to the end of the embryonic period, at which time he showed it between the eyes and at the root of the nose.
D2
Fig. 12 – 8
Fig. 12 – 7
Rh.
2
Rh.
5
5
7,8v
10,11 |
Figure 12–8. Rhombencephalon of still another |
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embryo. Several cranial ganglia and the otic |
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vesicles are visible. The uppermost part of the |
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Occipital |
photomicrograph depicts the mesencephalon and |
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is followed by rhombomeres, which are clearly |
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somite |
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delimited by evaginations. The trigeminal |
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ganglion is related to Rh.2, the facial to Rh.4, the |
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glossopharyngeal to Rh.6, and the vagal and |
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accessory to Rh.7. The part of the brain that is |
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related to the four occipital somites is Rh.D. Bars: |
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0.1 mm. |