Книги по МРТ КТ на английском языке / The Embryonic Human Brain An Atlas of Developmental Stages. Third Edition. 2006. By Ronan O'Rahilly-1
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C h a p t e r 1 7 : THE FUTURE OLFACTORY BULBS AND THE FIRST AMYGDALOID NUCLEI |
Di.
Th.d.
Marg. ridge
Hemisphere |
Th.v. |
Hyp.-th. sulcus
Opt. stalk
Di.
Marg.
ridge
Hyp.th. sulcus
N.- H.
A.- H.
Tentorium
Ggl 5
Dural limiting layer
Subarachnoid space
Figure 17–16. The diencephalon is visible and the dorsal thalamus can be distinguished from the ventral. The two are separated by the marginal ridge (zona limitans intrathalamica of Kuhlenbeck) and are delineated towards the hypothalamus sensu lato by the hypothalamic sulcus (Table 18–1). The ventral thalamus (which possesses an intermediate layer) is more advanced than the dorsal thalamus. (In the mouse, the zona limitans intrathalamica has been found to be clearly delineated by gene expression.) The optic cup is surrounded by condensed material that represents the primordia of the orbital muscles. The retinal (“choroid”) fissure of the optic cup is closing. Its continuation on the optic stalk, however, is still open and allows the ingrowth of optic fibers, which begin to develop in this stage. A hyaloid vessel can be seen in the optic stalk. The part of the hemisphere shown contains the amygdaloid area. This figure closely resembles illustrations in various articles (Sidman and Rakic, 1982, Fig. 1–41) and textbooks in which the interpretation is incorrect: a section that includes all five parts of the diencephalon as well as the eyes cannot also pass through the mesencephalon.
M2
Sulc. lim. |
Tectum |
3
Rh.2
Rh.3
Rh.4
Ot.
Figure 17–17. The neurohypophysis and the adenohypophysis are in close apposition, as they are from the beginning of their development, although this is frequently not appreciated. The primordium of the tentorium is attached at the surface of the adenohypophysis.
Figure 17–18. The sulcus limitans in the mesencephalon separates the tectum from the tegmentum. The area of the oculomotor nucleus belongs to M2 (future inferior collicular region). Rhombomeres can still be recognized by ventricular grooves. Thinnings appear in the wall of the otic vesicle; the thicker areas represent the future semicircular ducts.
THE FUTURE OLFACTORY BULBS AND THE FIRST AMYGDALOID NUCLEI |
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A
p
i
v
B
PM
GL
p
C. R. Cells
MZ
Figure 17–19. The primordial plexiform layer. (A) The earliest afferent (probably catecholamine) fibers in the human embryo can be detected in the primordial plexiform layer of the future temporal cortex at 6 weeks (stage 17). This is the cortex overlying the primordium of the amygdaloid region. Catecholamine fibers may perhaps participate in early synaptogenesis in the subplate of the human embryo (Larroche, 1981). Axodendritic and axosomatic synapses have been found in the cerebral hemispheres before the appearance of the cortical plate (Choi, 1988). Abbreviations: v, ventricular layer; i, intermediate layer; p, primordial plexiform layer with large neurons. Bar:
20 µm. (B) Epon section (1 µm) of the telencephalic wall of an unstaged human embryo of 6 weeks. The primordial plexiform layer contains scattered Cajal–Retzius cells. The pia mater is visible at the top of the photomicrograph. ×128. (C) Higher-power view of the primordial plexiform layer showing Cajal–Retzius (CR) cells. A glial barrier (the glia limitans, GL) is formed by cell processes at the pial (PM) surface. The glial barrier and the basement membrane are thought to be critical to the migration and final positioning of the neurons and to the differentiation of the laminar cortical pattern
Cwithin the developing neopallium (Choi, 1994). ×697. (D) Another higher-power view showing Cajal–Retzius cells with ovoid nuclei, prominent nucleoli, and abundant cytoplasm. A mitotic figure is evident in the lower left-hand corner. ×697. B, C, and D are reproduced by courtesy of Ben H. Choi, M.D., Ph.D., University of California, Irvine.
i
D
v
mitotic figure
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C h a p t e r 1 7 : THE FUTURE OLFACTORY BULBS AND THE FIRST AMYGDALOID NUCLEI |
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Figure 17–20. Median reconstructions at |
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(A) stage 15, (B) stage 16, and (C) stage 17. |
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The now small opening to the optic ventricle |
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is indicated in solid black. The solid |
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horizontal line in A refers to a section that |
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would include all five longitudinal zones of |
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D2 (Table 18–1). Asterisks in A-C denote the |
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sulcus medius. A′-C′ are key drawings to |
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show the limits of the neuromeres; Dl and Ml |
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Ep. |
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are stippled. Tracts shown by interrupted |
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lines are in A′ the habenulo-interpeducular |
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tract, in B′ the tract of the posterior |
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commissure, in C′ from top to bottom the |
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tract of the posterior commissure, the |
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habenulo-interpeduncular tract, and the |
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tract of the zona limitans intrathalamica. y, |
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commissure of the superior colliculi; z, |
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posterior commissure sensu stricto. |
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Bars: A, 0.2 mm; B, 0.22 mm; C, 0.4 mm. |
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From Muller¨ |
and O’Rahilly (1997b, Cells |
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Thalamus (dorsal & ventral) |
Tissues Organs) by permission of S. Karger |
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Medial eminence |
AG, Basel. |
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Lateral eminence
B
B′
C
C′
THE FUTURE OLFACTORY BULBS AND THE FIRST AMYGDALOID NUCLEI |
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A
M2
M1
Syn.
Zona
limitans Lateral intrathalamica ventricle
B
Hippocampus
C B A
C
V3
Optic stalk
V3
Olfactory bulb
Olf. n.
Figure 17–21. Important neuromeres in lateral to medial sagittal sections (cf. Fig. 17–20). The blood vessels, which appear here in white, penetrate the ventricular layer. (A) The cavities of the synencephalon and of the two mesencephalic neuromeres are visible. (B) and (C) More medially, the third ventricle (V3) and its connections to the optic stalk and to the lateral ventricle become evident.
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C h a p t e r 1 7 : THE FUTURE OLFACTORY BULBS AND THE FIRST AMYGDALOID NUCLEI |
NEUROTERATOLOGY
Dandy–Walker Syndrome. This ill-understood complex includes dysgenesis of the vermis, cystic dilatation of the fourth ventricle, a high tentorium, and hydrocephaly. It is different from the Arnold–Chiari malformation (Gardner et al., 1975). Lack of patency of one or more of the apertures in the roof of the fourth ventricle is frequent. A number of features are probably secondary rather than primary. Maldevelopment of the ventricular roof (the rostral membranous area) is believed to be an important factor in the development of the abnormality. The time of origin of the condition is probably in the embryonic period proper, likely
later than that of the Arnold–Chiari malformation, perhaps 6 to 8 weeks.
Diagnostic studies and clinical experiences can be found in a treatise by Raimondi et al. (1984).
Dysgenesis of the Optic Nerve. The optic nerves are elaborations of the optic stalks and their persistence depends on the ingrowth of optic nerve fibers. In the absence of such an ingrowth (e.g., because of premature closure of the fissure in the stalk), the optic nerve does not develop.
Neuronal Ectopia. Disruption of the pia–glial barrier may lead to the migration of neurons and glial cells into the subarachnoid space (Choi, 1987).
C H A P T E R 18
STAGE 18: THE FUTURE
CORPUS STRIATUM, THE
INFERIOR CEREBELLAR
PEDUNCLES, AND THE
DENTATE NUCLEUS
Approximately 13–17 mm in Greatest Length;
Approximately 42 Postfertilizational Days
The head is now more compact and the cerebral hemispheres are slightly flattened in the future insular region. The olfactory bulb and tubercle are sepa-
rated by a sulcus. The C-shaped hippocampus, accompanied by the area dentata, reaches the olfactory region. The lateral ventricular eminence of the future corpus striatum (Fig. 19–6) is distinct. Four amygdaloid nuclei are present in the region of the medial ventricular eminence, so that the archistriatum is now identifiable. The interventricular foramina are relatively narrower. The prosencephalic septum is recognizable. Optic fibers form, although they
do not yet enter the chiasmatic plate. The red nucleus is present and the substantia nigra is beginning to develop. The external cerebellar swellings represent the flocculi. The internal cerebellar swellings contain the dentate nuclei. The inferior cerebellar peduncles reach the region of these nuclei. The primary sensory nuclei of the rhombencephalon are now present. Choroid plexuses develop in the lateral as well as in the fourth ventricle, so that the production of cerebrospinal fluid sensu strico can now begin. The vomeronasal organ, nerve, and ganglion have appeared. At least two semicircular ducts are isolated.
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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C h a p t e r 1 8 : THE FUTURE CORPUS STRIATUM, THE INFERIOR CEREBELLAR PEDUNCLES |
M
Di.
T
Figure 18–1. Right lateral view of the brain. The cerebral hemispheres are slightly flattened in the area overlying the corpus striatum, i.e., in the future insular region. Frontal and temporal poles are indicated in the key by α and γ , respectively. The optic cups are assuming a more ventral position.
The pontine flexure is deep, in association with shortening of the head. The bilateral cerebellar swellings (ausserer¨ Kleinhirnwulst of Hochstetter, 1929) represent the flocculi, which are delineated by the posterolateral fissure. The rhombic lip is part of the external cerebellar swelling and is characterized by “nonsurface mitotic” figures, which are found also in the otic region, where the rhombic lip seems to participate in the formation of the cochlear nuclei. The ventral outpocketing of the isthmus was named Isthmushocker¨ and Tuberculum interpedunculare (dagger) by Hochstetter (1919). It is identified also in the key to Figure 17–2. It serves as an important landmark for the localization of the interpeduncular nucleus, because dopaminergic cells develop rostral to this ventral projection. The semicircular ducts are becoming isolated one from another, and at least two are distinct, which is typical for this stage. The main events in the development of the ear have been summarized elsewhere (O’Rahilly, 1983b).
The inset includes an end-on view showing the cerebellar plate, mesencephalon, epiphysis cerebri, diencephalon, and cerebral hemispheres. The cerebellar plate is still slightly wider than the combined cerebral hemispheres. The posterolateral or dorsolateral fissure (Larsell, 1947) or sulcus transitorius (Hochstetter, 1929) is marking off the rhombencephalic lip, including the anlage of the flocculus (Larsell, 1947), whereas according to Hochstetter this area represents the external cerebellar swelling and is more than the primordium of the flocculus. Comparison between Hochstetter’s figures and the reconstructions of the present authors would suggest that only the most lateral part of the external cerebellar bulge is the primodium of the flocculus, and that it probably corresponds to the Randstreifen, which gives rise also to the nodule
and to the inferior medullary velum.
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Sulcus |
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Hypothalamic |
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Marginal |
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Figure 18–2. Graphic reconstruction prepared from transverse sections to show a median view of the brain. The asterisk indicates the junction with the spinal cord. The olfactory bulb is separated from the olfactory tubercle by a groove, the sulcus circularis rhinencephali (Fig. 18–9). The bulb shows a recess, and the olfactory ventricle is forming.
In the inset, the medial wall of the left hemisphere is shown as if transparent, so that the C-shaped hippocampus can be seen to reach the olfactory region. It is accompanied by a small rim of area dentata, adjacent to which the area epithelialis is visible. In the future corpus striatum, a groove (the “intereminential” sulcus) separates the medial from the lateral ventricular eminence. The interventricular foramina become relatively narrower, chiefly because of the enormous growth of the medial ventricular eminence. Each foramen is bounded rostroventrally by the commissural plate and the medial ventricular eminence (Fig. 19–6), and dorsocaudally by the ventral thalamus. The medial eminence continues to invade telencephalic territory, as seen rostral to the interrupted line that extends from the preoptic recess to the velum transversum.
The prosencephalic (forebrain) septum is the area between the embryonic lamina terminalis and the olfactory bulb. It can be regarded as a junctional region where hippocampal, amygdaloid, and olfactory germinal matrices meet. What has been termed the septum verum (Andy and Stephan, 1968) is the basal part of the medial walls of the hemispheres. Hence it is formed when the hemispheres expand beyond the lamina terminalis, beginning at stage 17. The fibers of the supraoptic commissure, which are fine and pale, can readily be distinguished from the optic fibers, which appear coarse and black. Although optic fibers are present in about half the embryos of stage 18, they do not yet enter the chiasmatic plate. The adenohypophysial pouch is now closed off from the pharynx, with which it is connected by a solid epithelial cord.
The caudodorsal part of the midbrain is wide, presaging the appearance of a cul-de-sac (hinterer Mittelhirnblindsack of Turkewitsch, 1935). The midventral proliferative area participates in the formation of the red nucleus and the substantia nigra. The latter has been stated to develop in the monkey at a time that would correspond to human stages 18–21.
The internal cerebellar swelling of Rh.1 is more prominent than the tegmentum. It contains the dentate nucleus (reconstructed by O’Rahilly et al. 1988), the site of which is shown here by an interrupted line.
Choroid folds are present bilaterally in the fourth ventricle of nearly half of the embryos, and their villi present cylindrical cells characterized by protrusions into the ventricular cavity.
A photomicrograph of a median section has been published (Muller¨ and O’Rahilly, 1990a, Fig. 1B).
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Stage 12
16
13
17
14
15
18
Figure 18–3. Right lateral views of the brain from stage 12 to stage 18 showing the mesencephalic flexure and the form of the mesencephalon.
B
D
C
A
Figure 18–4. The cerebral hemispheres and relationships between the diencephalon and the telencephalon. The di-telencephalic border (arrowheads) is shown in (A)–(C). The orientation of the four views is summarized in the inset drawing. (A) Ventral view showing olfactory and optic areas. The adenohypophysis, neurohypophysis, nasal sacs, and medial and lateral ventricular eminences (interrupted lines) are projected onto the ventral surface, and the chiasmatic plate is marked. The supraoptic commissure is formed by fibers of the preoptico-hypothalamic tract (dagger). (B) Dorsal view. Newly arisen nuclei in the dorsal thalamus are marked by asterisks. Medial and lateral ventricular eminences are indicated by interrupted lines. (C) Frontoventral view showing the relationship between the olfactory and amygdaloid areas. (This view is foreshortened in comparison with A because of its different angle of projection.) (D) Lateral view of the future temporal pole with the projected amygdaloid nuclei (hatched) of the right cerebral hemisphere.