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

Thalamus dorsalis

Hypothalamic sulcus

Ext. germinal layer

Flocculus

Internal cerebellar swelling

Rhombic

lip

Dentate

nucleus

Sulcus

limitans

Fibers

Ventricular

& intermediate layers

Figure 23–19. A section through the interpenduncular fossa, which appears in the middle of the photomicrograph. The dorsal thalamus, which has enlarged considerably, is evident at this level and is characterized by a dark (subventricular) layer and a clearer superficial (intermediate) layer. The matrix is still in its migrational period (Richter, 1965). Its ventricular zone is still thicker than that of the ventral thalamus. A mesenchymal sheet, the primary meninx, is noteworthy between the diencephalon and the cerebral hemispheres.

The cerebellum has developed an external germinal layer, the extent of which has been reconstructed (Muller¨ and O’Rahilly, 1990b, Fig. 2B; Muller¨ and O’Rahilly, 1990c, Fig. 1A). The fibers in the cerebellum are mostly from the inferior cerebellar peduncle, rostral to which the dentate nucleus can be distinguished. (See also the scheme in Fig. 21–20.) A small rectangular section through the interpeduncular fossa, containing numerous small blood vessels, is visible below the third ventricle.

Figures 23–20 to 23–22 show sections through the frontal part of the neopallium of a silver-impregnated embryo. The levels are indicated in Figure 23–5. The bars represent 0.05 mm.

Subpial layer

Cortical plate

Pia mater

Subplate

Intermediate layer

Figure 23–21. An overall view of an area that shows the cortical plate, which is situated within the former cell-sparse primordial plexiform layer (Fig. 21–7). The plate now consists of five or more cellular rows that have migrated radially from the ventricular layer. The elements of the cortical unit and those of the proliferative unit form an ontogenetic column (Rakic). The following are present from the ventricular towards the pial surface: ventricular layer, intermediate layer, subplate, cortical plate, and subpial layer. Tangentially arranged nerve fibers are seen in the subpial layer, which is future cortical layer 1, and also between the cortical plate and the intermediate layer (shown also in Fig. 23–22). The latter fibers are specific thalamic afferents, which are transiently in the subplate before they invade the cortical plate. The early monoamine projection to the subplate that has been shown at about this time is believed to be permanent, although direct data in the human are lacking. Fibers from catecholamine neurons enter the intermediate layer (Zecevic and Verney, 1995). Nigrostriatal fibers arrive in the hemispheres at 8 weeks (Freeman et al., 1991). Other afferents are from the brain stem and the basal part of the prosencephalon.

The labels for the different zones shown here are based on the fact that the layer internal to the cortical plate is rich in tangential fibers, but poor in cells, which is characteristic of the subplate. The space between the pia mater and the subpial layer is artifactual.

Subpial layer

Cortical plate

Subplate

Figure 23–22. The peripheral part of Figure 23–21 [enlarged area, indicated by rectangle]. Numerous silver-impregnated, tangentially arranged neurites are evident within the subplate. On its first appearance, a part of the cortical plate contains early-generated subplate cells. Later, those cells leave the cortical layer and come to reside in the subcortical layer. They may give off the descending axons that run in the intermediate zone and reach the internal capsule. Although formerly it was thought that only layers 2 to 5 develop from the cortical plate, it is now generally believed that layer 6 is also derived from the plate (Mrzijak et al., 1988).

The first synapses in the brain appear in the two parts of the former primordial plexiform layer in human embryos and fetuses. Already at stage 19, that layer contains horizontal cells corresponding to Cajal–Retzius cells (Larroche and Houcine, 1982), which form the earliest synapses in the neopallium of the human brain (Choi, 1988), probably at stage 17.

Figure 23–24. The hippocampus and adjoining areas. Adjacent to 1, the multilayered epithelial lamina, is 2, the area dentata. Here the cell columns of the ventricular layer have become looser, and an intermediate layer is not present. The hippocampus sensu stricto, 3, which reaches the temporal pole at this stage, is characterized in general, as compared with the neocortex, by wide marginal and intermediate layers and a narrow ventricular layer. The cell columns of the ventricular layer are loose, and adjacent to them are cells, the axes of which resemble those of the ventricular layer; they represent probably future pyramidal cells (7). The tangenitally arranged cells in the marginal layer are subpial or marginal cells (6), which were also observed in this early phase of hippocampal development by Humphrey (1965) and Kahle (1969). The distances between individual cells in the intermediate layer of the hippocampus sensu stricto are greater than in any other part of the telencephalon. In the absence of special techniques (e.g., radioactive marking) it is scarcely possible to decide whether marginal cells of the present scheme correspond to a first wave of early-generated pyramidal cells or simply represent migrating cells from the area dentata. The mesocortex, 4, is between the hippocampus sensu stricto and the neopallium, and possesses the narrowest intermediate layer. The hippocampus sensu lato comprises the areas marked 2, 3, and 4.

Mesenchyme is noteworthy between the telencephalic surfaces at the right and the diencephalon at the left. Bar: 0.1 mm.

6

3

2

1

A

Mesocoele

Area membranacea

rostralis

caudalis

V4

Inf. horn

V3

Olfactory ventricle

B

Lateral recess of V4

"Blindsack"

Figure 23–27. The ventricular system.

(A) A left lateral view showing the now small interventricular foramen, and the overlap between the lateral and third ventricles. Anterior and inferior horns (part of the C formation) are well defined, but the posterior horn has not yet appeared. The future aqueduct is becoming delineated by two narrow regions, one of which is at the isthmus rhombencephali. Compared with the corresponding areas in stages 17 and 18, the epithelium of the areae membranaceae caudalis et rostralis is cuboidal rather than flat. No apertures are yet present in the roof of the fourth ventricle. An asterisk marks the obex. (B) A dorsal view of the ventricular system. Two pairs of semicircles can be seen: the left and right lateral ventricles rostrally, and the lateral recesses and the main part of the fourth ventricle caudally. The third ventricle is foreshortened in this perspective, whereas the future aqueduct and its bilateral Blindsack are seen in their full extent. The area choroidea of the roof of the telencephalon medium consists of two cellular rows and is the thinnest portion of the lamina terminalis.

An area membranacea rostralis and an area membranacea caudalis appear at stage 18. The caudal area has also been termed the saccular ventricular diverticulum (Wilson, 1937) and the central bulge (Brocklehurst, 1969). It is possible that the area permits the passage of cerebrospinal fluid (as suggested in the mouse), in which case a functional rather than a structural median aperture could be said to be present at this time. It has been claimed that the median and lateral apertures (of Magendie and Luschka) are present at 20 and 220 mm, respectively (Brocklehurst, 1969), but further study of this region is required.

It is likely, as has been suggested, that the cerebrospinal fluid is nutritive while vascularization is progressing, while the wall of the brain is increasing in thickness, and while the lateral ventricles are decreasing proportionately. This is supported by the relatively late appearance of capillaries that penetrate the wall of the brain, namely at stages 12 (hindbrain), 13 (midbrain), and 16 (cerebral hemispheres).

O’Rahilly and Muller¨ (1990, Table 1) have provided measurements of the ventricular system at this stage. A three-dimensional reconstruction, prepared ultrasonically, of the ventricles in vivo in an embryo of 29 mm (Blaas et al., 1998) confirms the accuracy of the more detailed graphic reconstruction shown here.

A

Vascular plexus

B A

X4

Mesencephalon

Mesocoel

V4

Alar lamina

B

Internal

cerebellar Sulcus swelling limitans

Ventricular layer

P–L fissure

Ext. germinal layer

Figure 23–28. The cerebellum at the end of the embryonic period.

(A) Photomicrograph of a section through the isthmus (above) and the mesencephalon (below). The decussation of the trochlear nerves (X4) develops in situ in the isthmus and does not migrate caudorostrally, as has been claimed. (B) A more ventral section, in which caudal is shown uppermost. The external cerebellar swelling (at the right) presents the rhombic lip and the flocculus, the latter now being covered by a thin sheath of external germnal layer, which is given off from the rhombic lip. An adjacent new and smaller swelling (unnamed by both Hochstetter and Larsell) contains the primordium of the dentate nucleus and (in more ventral sections) the inferior cerebellar peduncle. Numerous blood vessels penetrate as far as the ventricular layer.

246 C h a p t e r 2 3 : THE BRAIN AT THE END OF THE EMBRYONIC PERIOD

Taenia

Sulcus limitans

D

X4

Taenia

of V4

Ext. germinal Sulcus layer of Rh. 1 limitans

X4

Sulcus limitans

Roof of isthmus

Plane of section of photomicrographs

Vermis

Flocculus

(nodule)

Figure 23–28. (Continued). (C) A reconstruction of the roof of the isthmus (the future superior medullary velum) and rhombomere 1. The external germinal layer (gray) presents a slight median elevation, which is the beginning of the vermis.

Below, a reconstruction of the cerebellar hemispheres already showing areas of fusion.

(D) A reconstruction (based partly on Hochstetter, 1929) showing the cerebellar hemispheres from behind.

On the right a median outline and a superior view of the isthmus and cerebellum indicating (in mauve) the plane of section of the photomicrographs on the facing page. The uppermost three photographs show the isthmus and vermis between the mesencephalic Blindsacks of the two sides. The lowermost view demonstrates fusion between the two cerebellar hemispheres.