Talk:1987 Developmental Stages In Human Embryos - Stage 13

From Embryology


Fig. 13-1. (A–C) Embryo No. 6473 is among the less advanced embryos of the group and is on the borderline of the preceding stage. Because of the translucency of its tissues, much of the internal structure can be seen, particularly the details of the cardiac and hepatic regions. Aortic arches 2 and 3 course obliquely through their respective pharyngeal arches, and it can be seen that the form of the latter is not simply caused by the presence of the arches. The trigeminal and vestibulofacial ganglia, as well as the otic vesicle, can be seen in A. B is a “top” view showing the high degree of specialization of the different parts of the brain wall, and also the attachment of the trigeminal and vestibulofacial ganglia, along with the otic vesicle. Cavity of fourth ventricle is deeper and narrower than appears in the photograph.

(D) Embryo No 6469, showing especially well the optic evagination, the otic vesicle, and the condensed unsegmented mesenchymal strip, extending caudalward from the upper limb bud and probably supplying the body wall musculature.

(E,F) Embryo No. 8066, showing a straighter posture than usual. The form of the upper limb buds is seen particularly well in E.

(G) Embryo No. 7433. The roof of the fourth ventricle is thinner in two areas, which are separated by a strip of more-opaque tissue which crosses transversely from the region of one vestibulofacial ganglion to the other.

(H) Embryo No. 7433 photographed in situ, showing the character of the umbilical vesicle and its relations to the connecting stalk.

(I) Embryo No. 8119. An opaque Bouin specimen in an excellent state of preservation. Belongs to the second half of this stage. With the exception of B and H, all photographs are enlarged to the same scale.

Fig. 13-2. (A–D) Two well-preserved embryos fixed in Bouin's fluid. Because of the opacity of the tissues, the surface details are well shown. Most of the photographs used in this study were made by Chester F. Reather.

(A,B) Embryo No. 7889, belonging to the first half of stage 13. It is coiled more closely than usual, and this explains its measurement of 4.2 mm. In B the outlines of the nasal plate are distinguishable.

(C,D) Embryo No. 7618, belonging to the more advanced members of the group and showing development of the upper limb buds. The pharyngeal arches in both embryos show individual and detailed characteristics which appear to be unrelated to the aortic arches that pass obliquely through them. Transmitting vascular channels would seem to be no more than a passing and minor function of the conspicuous structures. All photographs are enlarged to the same scale.

Fig. 13-3. The main channels by which the blood is returned to the heart. The right hepatocardiac vein is shown particularly well. The coelomic surface over it is modified in a manner that increases the permeability between the coelom and this channel as it empties into the sinus venosus. This greater permeability is most marked at this stage and disappears later. Based on Born reconstructions of No. 836, made by Osborne O. Heard under the supervision of H. M. Evans. The number of somites is now believed to be 32 pairs rather than 30.

Fig. 13-4. The endocardium and the endothelium of the large arteries. The angiogenesis of the capillary networks throughout the embryo would seem to be a response to their immediate environment. Although the heart and the aortae begin as capillary networks, they soon show individualities that indicate a higher order of specialization. Their form is controlled by a number of factors, including their response to the rapid accumulation of blood plasma, and the hydrostatic effects consequent on the specialization of the cardiac wall, which seems to be limited normally to a defined area of coelomic mesenchyme. Based on Born reconstructions of No. 836, made by Osborn O. Heard.

Fig. 13-5. Ventral view of the blood vessels of the hepatic region and their communications at the venous end of the heart. The umbilical veins still empty bilaterally into the sinus venosus. Based on Born reconstructions and serial tracings of No. 836.

Fig. 13-6. A sagittal section through the heart, in the plane of the atrioventricular canal. The drawing shows in simplified form the character of the cardiac wall and the difference between the atria and the ventricles. The cardiac mesenchyme (“jelly”) in the myoendocardial interval persists as endocardial cushions that serve as an atrioventricular valve. Similarly it persists in the conus cordis and truncus arteriosus, aiding in preventing backflow from the aortic arches. Based on serial sections and a Born reconstruction of No. 836.

Fig. 13-7. (A-D) The histogenesis of the ventricular wall of the heart during stages 12–14. The myocardium begins as a specialized area of the visceral coelomic wall and is two or three cells in thickness. The surface cells are the less specialized germinal cells.

(A) No. 6079, stage 12. The myocardium is now four or five cells in thickness and constitutes a muscular plate that is separated by cardiac jelly from the endocardium.

(B) No. 8066, stage 13. The internally situated myocardial cells have differentiated rapidly and form loops and strands that indent the endocardium.

(C) No. 7618, advanced stage 13. The myocardial plate is well defined.

(D) No. 6830, stage 14. Residual germinal cells are at the surface, covering the myocardial plate, deep to which is the trabecular layer, which will be the first to acquire fibrillae and striations.

Fig. 13-8. Section through the liver, showing its blood vessels and the spreading of the hepatic trabeculae. The specialization of the hepatocardiac veins and the overlying coelomic wall is now, at its height. In this less advanced embryo (No 836) the umbilical veins still pass around the liver to reach the sinus venosus. Section 8-3-3.

Fig. 13-9. The digestive epithelium of a less advanced embryo of stage 13. As yet the surrounding mesenchyme shows little change beyond a moderate cellular proliferation. Caudal to the stalk of the umbilical vesicle the intestine is less advanced, thereby participating in the rostrocaudal gradient of differentiation that characterizes the embryo as a whole. Based on Born reconstructions of No. 836, made by Osborne O. Heard under the supervision of H. M. Evans.

The small drawings show the progressive development of the right and left lung buds during stage 13. In each case a cross indicates the separation point between the respiratory and digestive tubes (i.e., between the beginning trachea, clearly visible in No. 800, and the esophagus).

Fig. 13-10. Form and size of the brain at stages 11–13, drawn to the same scale. Various neural areas are indicated according to current interpretation. A section (at higher magnification) through the midbrain at stage 13 (the level marked A–B) shows the appearance of the neural wall at this time. Based on reconstructions by Osborne O. Heard.

Fig. 13-11. Sections through the optic vesicle in four embryos, selected to show the range of development in stage 13. The major parts of the evaginated wall are already determined and, of these, the retina is the most advanced. The proliferation of its cells results in its bulging into the lumen of the evagination, and its outer surface is marked by its widening nucleus-free marginal zone. Where the retinal disc is in contact with the skin ectoderm, the latter has been stimulated to form the lens disc, which will become indented in the next stage. The angioblastic activity preceding the formation of the vascular coat can be seen in No. 7889. All drawings are at the same scale.

Fig. 13-12. Otic vesicles of four embryos, illustrating the range of development during stage 13. The vesicle is now closed, but the less advanced members still have some remnant of the ectodermal stalk. In the more advanced specimens the stalk has disappeared and the endolymphatic appendage is marked off as a recess from the main cavity of the vesicle. The endolymphatic appendage is apparently determined even before the vesicle is closed. In the more advanced specimens the surrounding mesenchyme is reacting by exhibiting the first steps in the formation of the otic capsule. All drawings are at the same scale.


The development of the vena cava inferior in man

Charles F. W. McClure andElmer G. Butler