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

A

B

Amnion

Neural plate

Artifact

Primitive pit

Primitive Notochordal streak

process

Neurenteric

canal Endoderm

C

Mesoderm

Endoderm

Figure 8–5. The formation of the primitive streak at stage 8a. (A) Median section of an embryonic disc of nearly 1 12 mm in length. (B) A higher powered view of the section shown above. The neurenteric canal connects the amniotic cavity with the umbilical vesicle temporarily. It is an important landmark in stages 8 to 10 (Table 8–1). (C) Transverse section through the primitive groove and primitive streak, which attains its greatest length at this stage.

A

B

A

B

C

C

D

E

D

E

Figure 8–6. Serial sections through an excellent example of stage 8. The level of the sections is indicated on the reconstructed median section.

(A) The prechordal plate in transverse section shows as many as eight rows of cells that differ from the endodermal cells in being larger and more nearly spherical, and in containing numerous granules. The neural ectoderm at this level shows a very faint indentation, the beginning of the neural groove. The prechordal and neural plates are separated by a basement membrane. (B) The neural groove in an area rostral to the notochordal process. (C) The primitive pit and its continuation with the notochordal canal of the notochordal process. (D) The primitive node. (E) The primitive groove and streak. At this caudal level the mesenchyme between the surface ectoderm and the endoderm is far greater in quantity than at rostral levels. Blood vessels are not yet present. Bar = 0.03 mm.

Figure 9–1. Left lateral and dorsal views of an embryo of stage 9. Striking changes have occurred since the preceding stage.

The embryonic disc is now clearly an elongated embryonic body, set off from the surface of the umbilical vesicle (the so-called yolk sac). Various features are indicated in Figure 9–2 A,B.

In the lateral view, the “waist” of the embryo can be seen to be the site of considerable lordosis. The cephalic and caudal portions of the body are elevated from the surface.

In the dorsal view, the region of the head, which is broadest at the level of the mesencephalon, is clearly joined to the caudal eminence by a “waist,” in which the first (occipital) somites are appearing. Caudally the greatest width is at the level of the primitive node, the site of which is shown in Figure 9–2. (In the present figure the artist has placed it too far caudally.) Cephalic to the node, the embryo has elongated greatly. Beyond the embryo, the umbilical vessels and the allantoic diverticulum have been sectioned. The neural folds bound a long and deep neural groove. Preotic and postotic sulci are very faint (compared with those in rodents).

This is another example of an elegant drawing by James F. Didusch, artist to the Carnegie Collection.

A

Rhombencephalon

B

Figure 9–2. (A) dorsal, (B) right lateral, and (C) median views of an embryo of stage 9. The cephalic end is to the right. The first (occipital) somites are stippled. The otic discs are indicated by interrupted lines; C is a graphic reconstruction prepared from transverse sections.

Although no part of the neural tube has yet formed and no so-called brain vesicles are present, the three major divisions of the brain recognized by von Baer are distinguishable. The prosencephalon is almost exclusively diencephalic. The mesencephalon is at the site of a marked bend, the mesencephalic flexure. In C, the rhombencephalon, which extends from the otic to the somitic region, can be seen to comprise four subdivisions: rhombomeres A, B, C, and D, the last (first recognized by Muller¨ and O’Rahilly, 1983) representing the hypoglossal region. These rhombomeres are real entities (as shown by immunological methods in the chick). The asterisk indicates the future cerebrospinal junction. A faint groove is visible between the mesencephalon and Rh.A. It is clearest in a dorsal view (Fig. 9–2A), where it separates the wider midbrain from the narrower hindbrain. It corresponds to the preotic sulcus in rodents.

The first sign of the production of neural crest can be found in a few embryos of stage 9. It occurs at the level of the mesencephalon and the rhombencephalon. In addition, the production of general mesenchyme continues. The prechordal plate participates in the formation of the head and probably provides cardiac mesenchyme. The primitive streak, however, seems to have ceased as a site of proliferation, and its functional role has been replaced by that of the caudal eminence. The primitive node is shown in its correct position (more rostrally than in Fig. 9–1).

A

Umbilical vesicle

C, D

B

Allantoic diverticulum

A B

Neural ectoderm

Umbilical

vesicle

C

D

Figure 9–4. Regression of the primitive streak in stage 9. (A) A coronal section. The epiblast has mostly been replaced by cuboidal surface ectoderm. The dark tissue rostral to the umbilical vesicle (and mostly at the right side of the photograph) is neural ectoderm. (B) A transverse section through the rostral part of the primitive streak. The median, invaginating flask-shaped and spherical cells are still numerous, and the epiblast is still thick laterally. The open arrow indicates the primitive groove. (C) A transverse section through the neurenteric canal (black arrow) in an other embryo in which the cuboidal surface ectoderm has almost replaced the former epiblast. (D) The neurenteric canal (black arrow), epiblast, and mesenchyme of the primitive streak in the embryo shown in B. Modified from Muller¨ and O’Rahilly (2004a, Cells Tissues Organs) by permission of S. Karger AG, Basel.