1987 Developmental Stages In Human Embryos - Stage 16: Difference between revisions
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* Approximately 11–14 mm | |||
* Approximately 37 postovulatory days | |||
==Summary== | |||
External: the nasal pits are coming to face ventrally and to disappear from profile view; retinal pigment is becoming visible externally; pharyngeal arch 2 is more massive and more conspicuous, whereas arch 3 is receding from the surface; auricular hillocks are beginning to appear; the thigh, leg, and foot are becoming distinguishable. | External: the nasal pits are coming to face ventrally and to disappear from profile view; retinal pigment is becoming visible externally; pharyngeal arch 2 is more massive and more conspicuous, whereas arch 3 is receding from the surface; auricular hillocks are beginning to appear; the thigh, leg, and foot are becoming distinguishable. | ||
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O'Rahilly R. and Müller F. Developmental Stages in Human Embryos. Contrib. Embryol., Carnegie Inst. Wash. 637 (1987).
| Online Editor Note |
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| O'Rahilly R. and Müller F. Developmental Stages in Human Embryos. Contrib. Embryol., Carnegie Inst. Wash. 637 (1987).
The original 1987 publication text, figures and tables have been altered in formatting, addition of internal online links, and links to PubMed. Original Document - Copyright © 1987 Carnegie Institution of Washington.
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- 1987 Stages: Introduction | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | References | Appendix 1 | Appendix 2 | Historic Papers | Embryonic Development
| Historic Disclaimer - information about historic embryology pages |
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Stage 16
- Approximately 11–14 mm
- Approximately 37 postovulatory days
Summary
External: the nasal pits are coming to face ventrally and to disappear from profile view; retinal pigment is becoming visible externally; pharyngeal arch 2 is more massive and more conspicuous, whereas arch 3 is receding from the surface; auricular hillocks are beginning to appear; the thigh, leg, and foot are becoming distinguishable.
Internal: foramen secundum is present in the heart; a definitive mesentery has appeared and intestinal rotation is commencing; the pelvis of die ureter is becoming bipartite; no longitudinal fissure is found yet between the cerebral hemispheres; the neurohypophysial evagination is developing in the more advanced embryos; the lens pit is D-shaped; thickenings for the semicircular ducts are appearing.
Size and Age
As more-advanced and larger embryos are considered, there appears to be an increasing range in variability of size. If, however, size be regarded from the standpoint of the percentage variation in the greatest length of the embryos, the size range in these groups is not greatly different from those at previous stages. This should be taken into account in considering both actual size differences and differences resulting from the condition and treatment of the embryos.
The greatest length is most frequently 8–11 mm, but about one-fifth of the embryos are around 7 mm and slightly fewer are 11–12 mm. The size of the chorion is very variable, and there is much overlapping with the preceding stage. The range is from 15 to 48 mm.
The age is believed to be approximately 37 postovulatory days. Specimens that may well belong to this stage have been recorded as the 38th day (Tandler, 1907) and the 40th day (v. Hayek, 1934).
External Form
The general proportions and shape of the head of the embryos at this stage are illustrated in figures 16-1 to 16-5. It will be seen that these are fairly uniform, largely because of the brain, the main parts of which can be recognized in the intact embryo (O'Rahilly, Müller, and Bossy, 1982). The mesencephalic and thalamic regions are relatively longer and more prominent than they were in the preceding stage. It is in the details of special regions, however, that developmental progress can be most sharply identified, and in this group such distinctive characteristics are to be found in the nose, eye, hyoid arch, and especially in the limb buds.
The nasal areas made their appearance earlier as two bilaterally symmetrical, discrete, and separate plates. In the present stage they are still separate and discrete. When they acquired their elevated borders, in earlier specimens, the discs faced laterally and their concave floors could be seen in profile views of the embryo. In the less advanced members of stage 16 a part of the floor of the nasal pit can still be seen in profile views, but in more advanced members, because of the increased relative thickness of the surrounding tissues, the pits face ventrally and their floor is no longer visible. One sees only the prominent lip which now forms their lateral boundary. In all the specimens of the group the marginal fold (nasal wing) now overhangs the floor of the depression. It is further to be noted that, in the embryos belonging to the more advanced half of the group, a nasofrontal groove is making its appearance and marks the frontal border of the nose, being exaggerated in shrunken specimens.
The pigment in the external layer of the retina is now visible macroscopically in the intact specimen in the more advanced members of the group. It is best seen in the fresh state or in specimens fixed in formalin.
As a consequence of the widening of the head, the two nasal pits turn ventrally and are brought relatively nearer to each other. Similarly the dorsal segments of the mandibular and hyoid arches are pressed laterally while their ventral ends are drawn medially and thereby partially disappear from the profile view. On that account these arches and the formation of the face must be studied in decapitated specimens and reconstructions. In the present stage the hyoid arch is not bent under toward the median plane as markedly as the mandibular arch, and consequently it is more conspicuous than the latter in profile views. It also appears more massive because of the superficial cell proliferations that constitute the precursors of the auricular hillocks. In stage 16 they can be seen on the hyoid arch in every specimen. These superficial masses are not to be confused with the primary grooves that subdivide the mandibular and hyoid masses as a whole into dorsal and ventral segments. Except in the less advanced members of the group, the third pharyngeal arch has receded from the surface and is covered over by its more bulky neighbor.
The cervical sinus, an indication of which may be detected in some embryos at stage 13, is clearly visible by stage 14. Its floor is formed by arches 3 and 4 (O’Rahilly and Müller, 1985). The cervical sinus is smaller by stage 16. Contrary to Frazer (1926), it would seem that pharyngeal arch 3 is no longer visible on the surface after stage 16. The operculum frequently shown in drawings is incorrect.
In the preceding stage, one could distinguish a rounded hand region as a discrete part of the upper limb bud, distinct from the arm and shoulder. In stage 16 the hand region has become differentiated into a central carpal part, around the edge of which there projects a thick crescentic flange, which is to form the digital plate. In transparent specimens one can usually see the marginal vein coursing along the rim of the latter. The lower limb bud, although bearing some resemblance to the upper bud, has its own characteristics from the outset. The two are never identical. They appear so at first, but that is only because in their simpler form there are fewer features to be compared. Closer study, however, reveals their respective individualities. In the present stage, three growth centers are visible in the lower limb bud, subdividing it into three principal regions. These are (1) a rostrolateral part, in which the femoral and obturator nerves of the lumbar plexus terminate, (2) a caudomedial part, in which the peroneal and tibial nerves of the sacral plexus terminate, and (3) the foot region, to the base of which a part of the tibial nerve already extends. Roughly these represent the thigh, leg, and foot. The pelvis is not yet clearly set off from the thigh. Outline forms of the lower limb bud, as found in stages 15–17, are shown in figure 16-4.
Spinal ganglia are evident on the surface throughout the whole length of the trunk. Somites can be seen distinctly in the caudal part of the trunk.
Nervous System
(fig, 16-10) The ascending fibers in the dorsal funiculus of the spinal cord reach the level of the medulla oblongata.
Rhombomeres are still present. The alar lamina is developing, and its upper part, the rhombic lip, is very dense. Cells of the general and special visceral efferent nuclei of the cranial nerves (5, 7, 9, 10, 11) are still migrating from medial to lateral. The reticular formation is better outlined than in stage 15. The growth of the cerebellar primordium in width and height deepens the groove between it and the mesencephalon. A well-pronounced isthmus rhombencephali is evident.
The mesencephalon still comprises two parts, and the button-like formation visible in stage 14 is still present in the roof between the two portions. The peripheral part of the trochlear nerve is present in all embryos.
The posterior commissure is beginning to develop. The dorsal and ventral thalami are separated by a groove adjacent to the marginal ridge (fig. 16-10). A neurohypophysial evagination is apparent in half of the embryos but is not very distinct. The medial (diencephalic) ridge of the corpus striatum is directly continuous with the lateral (telencephalic) ridge. The cerebral hemispheres are bulging laterally, but no longitudinal fissure is present yet. The interventricular foramen is wide and is limited dorsally by the ventral thalamus. In the more advanced embryos the area epithelialis and sulcus limitans hippocampi become visible. The most developed telencephalic area is the amygdaloid part. Cajal-Retzius cells, with fibers tangential to the surface of the cerebral vesicles, are present. Olfactory fibers running from the nasal pit to the brain are developed in half of the embryos. The olfactory tubercle becomes apparent. Fibers of the medial forebrain bundle form.
The cells of the sympathetic nervous system form a solid trunk.
Eye
(fig. 16-11) The growth of the lens body results in an approximately D-shaped lens cavity. Perilental blood vessels (tunica vasculosa lentis) are visible. Retinal pigment is shown in figure 18-12A.
Ear
(fig. 16-11) The endolymphatic appendage is long. Thickenings in the wall of the main, or vestibular, portion of the otic vesicle presage the appearance of the semicircular ducts. The ventrocaudal portion of the otic vesicle, although not yet marked off, is the primordium of the cochlear pouch.
Embryos of Stage 16 Already Described
- No. BR, 9.75 mm G. L., 8.8 mm. Summarized by Tandler (1907), who, on the basis of the coital history, gave the age as the “38th day.” About stage 16.
- 9-mm embryo. The peripheral nervous system was described by Masy (1955) in this embryo of stage 16.
- Huber No. 3, 10 mm. The nuclei of origin of the cranial nerves and the peripheral nervous system were described by Streeter (1908a,b). An advanced example of stage 16.
- Wetzel (We) embryo, 10 mm. Discussed by v. Hayek (1934) in regard to age, which, based on the coital history, was given as the “40th day.” Probably about stage 16.
- Carnegie No. 6516, 10.5 mm (corrected). Double ureters described by Wharton (1949).
- No. H60, University of Missouri, 11 mm. Described briefly by Bonnot and Severs (1906). Probably belongs to stage 16.
References
| Online Editor Note |
|---|
| O'Rahilly R. and Müller F. Developmental Stages in Human Embryos. Contrib. Embryol., Carnegie Inst. Wash. 637 (1987).
The original 1987 publication text, figures and tables have been altered in formatting, addition of internal online links, and links to PubMed. Original Document - Copyright © 1987 Carnegie Institution of Washington.
|
Search Pubmed
- 1987 Stages: Introduction | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | References | Appendix 1 | Appendix 2 | Historic Papers | Embryonic Development
| Historic Disclaimer - information about historic embryology pages |
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Cite this page: Hill, M.A. (2024, May 23) Embryology 1987 Developmental Stages In Human Embryos - Stage 16. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/1987_Developmental_Stages_In_Human_Embryos_-_Stage_16
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
