Файл: The Embryonic Human Brain An Atlas of Developmental Stages. Third Edition. 2006. By Ronan O\'Rahilly-1.pdf
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A
Notochordal plate and notochord
UMBILICAL
VESICLE
Endoderm
C h a p t e r 9 : NEUROTERATOLOGY
S1 |
D |
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Notochordal |
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plate |
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C |
GUT |
Neural |
GUT |
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plate |
E |
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AMNIOTIC
CAVITY
F
UMBILICAL
BVESICLE
NORMAL
DIASTEMATOMYELIA
DORSAL ENTERIC CYST
GUT
Figure 9–8. The neurenteric canal and anomalies that are probably related to it. (A), (B) Median and transverse sections through a four-somite embryo of stage 10 showing the neurenteric canal. (C) After normal closure of the canal. (D) A few weeks later (at stage 23). (E) The formation of a split notochord. (F) Persistence of the neurenteric canal.
C H A P T E R 10
STAGE 10: THE NEURAL
TUBE AND THE OPTIC
PRIMORDIUM
Approximately 2–3.5 mm in Greatest Length;
Approximately 29 Postfertilizational Days
When approximately five somitic pairs are present, the neural folds begin to fuse in the adjacent rhombencephalic and spinal regions, frequently
in several places simultaneously. A portion of the neural tube is thereby formed, and mitotic figures are found near its cavity. The diencephalon consists of the future thalamic region (D2) and an optic portion (D1) in which the optic sulcus appears. The right and left optic primordia, which are visible for the first time at this stage, are connected by the chiasmatic plate (primordium chiasmatis; torus opticus). The lateral parts of the forebrain beyond the chiasmatic plate belong to the telencephalon, which is visible for the first time at this stage. Several regional components of the neural crest can now be distinguished, and the formation of the crest in the head is probably at its greatest.
Stage 10 is usually listed as 22 days, but may be as much as a week older.
The neural crest, widely dispersed groups of cells, arises at the neurosomatic junction. In the head they include mesencephalic, trigeminal, facial-vestibular, glossopharyngeal, vagal, and hypoglossal crest, and at least half of these also receive contributions from epipharyngeal
placodes, which have been considered as special “islands” in the somatic ectoderm.
Summary
Two de novo sites of fusion of the neural folds appear in succession: α in the rhombencephalic region (stage 10) and β in the prosencephalic region, adjacent to the chiasmatic plate (stage 11). Fusion from site α proceeds bidirectionally (rostrad and caudad), whereas that from β is unidirectional (caudad only). The fused regions terminate in neuropores, of which there are two: rostral and caudal. Fusion β begins later than fusion α, but the rostral neuropore closes before the caudal.
Accessory loci of fusion are (1) inconstant, (2) variable in position, and (3) exhibit no specific pattern, so that they are not “multiple sites” of fusion.
Important
The telencephalon medium can be distinguished from the diencephalon at this stage, i.e., much earlier than commonly stated.
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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40
S4
C h a p t e r 1 0 : THE NEURAL TUBE AND THE OPTIC PRIMORDIUM
Figure 10–1. A 10-somite embryo of stage 10. (A) Left dorsolateral view. Various features are identified in the inset, according to the authors’ interpretation, including the eight rhombomeres (numbered) and the first four somites (in gray). M, midbrain. Ph., first pharyngeal arch. The beginnings of the telencephalon (Fig. 10–3) are not visible in these views. (B) Left lateral view. These two superb illustrations were made by James F. Didusch for Dr. George Corner.
S4
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M |
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Ph. |
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44
5 5
6 6
7 7
8 8
A B
Figure 10–2. Dorsal and right lateral views of an embryo of stage 10. The number of somitic pairs at this stage ranges from 4 to 12; the embryo shown here has 10 pairs. An extensive portion of neural tube has already formed, leaving parts of the neural groove exposed.
(A) The dorsal view shows the neural folds fused in the adjacent rhombencephalic and spinal regions. Fusion begins when approximately 5 somitic pairs are present. Accessory loci of fusion are sometimes seen. The neurenteric canal is still present in the least advanced embryos. The caudal eminence (a continuation of the primitive streak) constitutes about one-fifth of the embryonic length. It continues the production of mesenchyme and its surface ectoderm forms the caudal part of the future neural plate during stage 10. (B) The right lateral view shows the developing pharyngeal arches and the cardiac region, represented here by the pericardial cavity. The surface ectoderm is contributing to the otic plate (dotted outline) and the epipharnygeal area of the future vagal ganglia.
THE NEURAL TUBE AND THE OPTIC PRIMORDIUM |
41 |
M
D2
D1
Tel.
Figure 10–3. Graphic reconstruction prepared from transverse sections to show a median view. An end-on view is shown on the right. The major divisions of the brain are distinguishable in the largely still-open neural groove. The mesencephalic flexure has decreased (from approximately 150◦ to 104◦ ) since the previous stage, contributing to a ventral bending of the prosencephalon and the shaping of the future head. The diencephalon comprises two portions: D2, the future thalamic region, and D1, in which the (left) optic sulcus can be seen for the first time. D1 possesses a thick floor that forms the median bridge between the two, laterally placed, optic primordia. This is the chiasmatic plate (primordium chiasmatis) or torus opticus. The optic sulci may extend to the median plane and end in a pit, the “postoptic recess” (Johnston, 1909), which is the caudal boundary of the chiasmatic plate. The lateral parts of the forebrain extend further rostrally than the chiasmatic plate. This is the telencephalon, evident for the first time at this stage in embryos of 7–12 somitic pairs. At this stage and in stages 11 to 14 (Bartecko and Jacob, 1999) the notochord extends rostrally to the oropharyngeal membrane (Muller¨ and O’Rahilly, 1985), so that induction of the floor plate is possible as far rostrally as neuromere D2. This is the future epinotochordal part of the brain. Neuromere D1, which is induced by the prechordal plate (Fig. 10–7), is prenotochordal in position. Pax 3 is expressed in the neural groove and in the closed part of the neural tube at stage 10 (and at stage 15 in the brain stem and spinal cord) in the human embryo (Gerard´ et al., 1995). Pax 6 is important for the development of the eye.
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C h a p t e r 1 0 : THE NEURAL TUBE AND THE |
D
A B C D
M C
P B
Rh. A
10
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
11 |
M2 |
M1
T
D2
D1
12
D2
13
OPTIC PRIMORDIUM
16 neuromeres:
Telencephalon
Diencephalon 1
Rostral parencephalon
Caudal parencephalon
Synencephalon
Mesencephalon 1
Mesencephalon 2
Isthmus
Rhombomeres 1-8
Isth.
Cbl
Par.
Syn.
Par.c.
Par.r.
14
Figure 10–4. The neuromeres of the human embryo at stages 9 to 14. Neuromeres are morphologically identifiable transverse subdivisions perpendicular to the longitudinal axis of the embryonic brain and extending onto both sides of the body.
Primary neuromeres are six larger divisions that appear early (stage 9) in the open neural folds: prosencephalon (P), mesencephalon (M), and rhombomeres A, B, C, and D.
Secondary neuromeres are smaller subdivisions that are found both before (stages 10 and 11) and after (stages 12–14) closure of the neural tube.
In all, 16 secondary neuromeres develop: telencephalon medium (T), diencephalon 1 (D1), rostral parencephalon, caudal parencephalon, synencephalon, mesencephalon 1 (M1), mesencephalon 2 (M2), isthmus rhombencephali, and rhombomeres 1 to 8. These are detectable at stages 14 to 17. A longitudinal organization begins to be superimposed on the neuromeres at stage 15, and five longitudinal zones can be discerned in the diencephalon (Table 18–1). Although in some instances territories of gene expression follow the morphological neuromeres, in others they may cross interneuromeric boundaries.
N e u r a l t u b e
THE NEURAL TUBE AND THE OPTIC PRIMORDIUM |
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5
7 |
A |
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A
8
Rhomben10,11 cephalic
crest
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B |
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Spinal |
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0.1 |
C |
D
D
ESpinal crest
E
0.1
Figure 10–5. The neural crest at stage 10. The levels of the neural tube are shown in the left-hand column. At rostral levels (A), crest material appears before closure of the neural groove, whereas at levels B to E, the contrary holds. The crest cells, which leave the neural plate where the basement membrane is interrupted, contribute to ganglia of cranial nerves and migrate to the general mesenchyme to provide ectomesenchyme for the skull and face.
Also arising at the neurosomatic junction are otic crest and nasal crest, and the latter gives origin to vomeronasal crest and terminalis crest. Finally, arising from the retina, which is in continuity with the neural ectoderm of the diencephalon, is the optic crest (Table 10–3).
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C h a p t e r 1 0 : THE NEURAL TUBE AND THE OPTIC PRIMORDIUM |
Neural
crest
10 – 8 |
7 |
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6
M
Di.
T
Figure 10–6. In the upper part of the photomicrograph, the mesencephalic neural crest can be seen migrating on each side from the neural ectoderm. The sites of neural crest emigration show discontinuity of the basement membrane. The optic sulcus is indicated by an arrow.
Figure 10–7. Neural crest is visible above as it emerges from rhombomere 1. The median plane is occupied by the
Chiasmatic floor of the neural groove. The prechordal plate shows as a plate small dark projection on each side of the lower portion of the
photomicrograph above the chiasmatic plate. The very large bilateral blood vessels on each side of the neural plate are the first aortic arches, which arise from the paired dorsal aortae, visible in Figures 10–11 and 10–12.
THE NEURAL TUBE AND THE OPTIC PRIMORDIUM |
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Crest
Ao.
Pharynx
Notochord
Figure 10–8. The facial crest, rather than migrating from the neurosomatic junction, appears to arise directly from the neural ectoderm of the future neural tube (arrows in the key). Such an origin resembles that of the mesencephalic (Fig. 10–6) and trigeminal (Fig. 10–7) crest, although it is less pronounced here.
TABLE 10–1. The Terminology, Development, and Derivatives of the Neuromeres in the Human Embryo
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Secondary |
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Neuromeres |
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Neuromeres |
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Stage 9 |
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Stages |
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10 |
11 |
12 |
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13 |
14 |
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P |
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T medium |
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D1 |
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D2 |
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Parenc. |
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Rostral |
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Caudal |
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Synenc. |
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M |
M1 |
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M2 |
a |
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Isthmus |
a |
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Rh. A |
Rh. 1 |
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Rh. 2 |
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Rh. 3 |
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Rh. B |
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Rh. 4 |
Rh. C |
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Rh. 5 |
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Rh. 6 |
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Rh. 7 |
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Rh. 8 |
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Rh. D |
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Derivatives
Telenceph.
Dienceph.
Mesenceph.
Isthmus
Cerebellum
Pons
Med-
ulla
a The distinction between M1 and M2 can be followed up to and including stage 17.