Paper - Developmental horizons in human embryos group X

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Heuser CH. and Corner GW. Developmental horizons in human embryos. Description of age group X, 4 to 12 somites. (1957) Carnegie Instn Wash Publ 611, Contrib. Embryol., 36: 29-39.

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Developmental Horizons in Human Embryos - Description of Age Group X (4 to 12 somites)

by Chester H. Heuser and George W. Corner

The studies of C. H. Heuser were supported in part by a research grant, RC-4212, from the National institutes of Health, U. S. Public Health Service.


For convenience in classifying human embryos as to state of structural organization and age, the late George L. Streeter (i945, 1948, 1949, 1951) set up 23 age groups to indicate levels of development from the fertilised egg to embryos measuring 30 mm. in length, or approximately 47 days old. Beginning with stage xi, ovulation age 24 ± 1 days, Dr. Streeter carried the work forward through the remaining older groups. In preparing a survey of embryos of the first 3 weeks of development, it is convenient to begin with the oldest specimens of the period and thus consider the age group immediately preceding the one first completed by Dr. Streeter.

Age Group X, 4 to 12 Somites

External Form, Size, and Age

The most conspicuous event during age group x is the fusion of the neural folds. In the younger specimens the neural groove is open through its whole length. Fusion of the folds begins at about the stage of 7 somites, and by the end of the period the neural tube is closed from the otic region of the rhombencephalon to about the level of the latest-formed somite. Other important changes of form occur during stage x. The embryo progressively elongates, and the yolk sac continues to expand. At the same time the cranial portion of the neural folds grows forward and becomes distinctly elevated; the tail fold appears at the beginning of the period and overrides the yolk sac, and finally the whole embryo rises above the level of the yolk sac. The visceral components of the head begin to appear. Toward the end of the period, the increasing size of the heart makes the pericardial region a prominent feature of the external form.

The task of compiling for the “Horizons” series a survey of this stage has been lightened by free use of the admirable study of human embryos, covering a slightly longer period of development (2 to 16 somites), that was published by Bartelmez and Evans in 1926. Their study, which was concerned chiefly with the early development of the nervous system, included all relevant embryos then in the Carnegie and the University of Chicago Collections, and incorporated references to specimens in other laboratories. Since it appeared, 3 additional embryos of stage x have been acquired by the Carnegie Collection; the University of Chicago specimens have been deposited there; and 5 embryos have been described from other laboratories. In all, counting both fully described specimens and those that have merely been cited incidentally, about 36 embryos having 4 to 12 pairs of somites are known to have been examined by the embryologists of the world. Of these, I3 are now in the Carnegie Collection (including 2 which are not good enough for use in the “Horizons”), and 6 more are represented there by tracings, photographs, or models. Four of the remainder have been described in monographic form (Orts Llorca, 1934; Politzer, 1930; Schenck, 1954; Sternberg, 1927), so that they can be compared in almost all details with those available at the Department of Embryology in Baltimore. Arey’s (1938) comprehensive study of the first somite in human embryos has also been very useful for the present undertaking.

Unfortunately, only one specimen of this period has been received, in the unfixed condition or recently preserved, at the Carnegie laboratory since the development there of expert photographic methods such as produced the superb photographs which Streeter was able to present with his account of horizon xi.

Representative specimens of this age group are illustrated in figures I and 2. These outlines, showing the dorsal aspect and the median longitudinal section of each specimen, are enlarged to the same scale. As pointed out by Bartelmez and Evans (1926), the cranial flexure is present in all embryos of this period. A dorsal fiexure may or may not be present. The curvature of the dorsal profile varies from a gentle convexity through all degrees of concavity from the least possible curve to a deep, sharp kink. Bartelmez and Evans (1926) and Streeter (1942) have discussed the significance of this variation as seen in human and rhesus embryos of the early somite stages. On the whole, the evidence indicates that, whereas extreme dorsal Hexion must be regarded as an artifact, anything from a gentle convexity to a moderate dorsal concavity must be considered normal.

The modeling of the head shows certain significant features. At about the stage of 7 somites the optic sulcus appears as a shallow groove on the external surface of the forebrain (fig. 1). It becomes much more prominent as it extends rostrally in later specimens, but at the end of this stage (no. 3707, 12 somites) the optic primordium, as seen in sections, is still only a thickened ectodermal plate indented by the sulcus.

In discussing the visceral regions of the head we are forced to depend, for lack of good photographs of the youngest stages, upon the evidence afforded by the ....

(insert p32 here)

In the second step (stage of 7 to 8 somites, illustrated by Davis’ fig. 17 [no. 4216], fig. 20 [no. 4439], and fig. 22 [no. 391]), the two endothelial tubes have fused in the regions of the aortic bulb and bulbus cordis, and also progressively in the ventricular region; the fusion becomes complete at the 8-somite stage, the atria still being left paired. The single ventricular tube thus formed is at first practically straight, then assumes the gentle beginnings of an 5 curve. Embryos that have become available since Davis’ work (no. 6330, 7 or 8 somites; Politzer, 8 somites; West, no. 4983, 8 somites) all agree closely with the foregoing account.

The enveloping myocardial wall is also a single tube as far as the ventricular region, but distinct bulges, to the left cranially and to the right caudally, give evidence of the imminent curving while the endoeardium is as yet scarcely curved.

The third step is the definite appearance of the 5 curve of the bulbus cordis and ventricular endocardial tube. The curvature, which is not marked in some of the 9-somite embryos (Eternod “Du Ga,” no. 4439; Wilson H. 98, no. 7251), is apparent in the 9-somite Veit specimen, no. 4251, and well developed in no. 5074 (10 somites). Davis illustrates it in the 12-somite embryo no. 3707.

In the myocardial wall, deep sulci separate the bulbar region from the twisted ventricular segment, and the latter from the right atrium.

The heart is ready to begin the propulsion of blood through the pathways that are becoming available by the formation of the first and second aortic arches at about the end of horizon x.

Nervous System

The nervous system, because of its simplicity at this period, offers few salient features that are useful in the classificatory description at which these Horizons aim. The elevation of the head fold has been mentioned in the introductory paragraph.

The otic placode (otic plate or disk) could probably be seen by direct inspection of intact embryos of the horizon in the unfixed condition or after recent formol fixation. As Bartelmez (1922) showed, the otic placode appears very early in development and affords an excellent landmark for analyzing the early embryonic brain. It has been identified in sections in all phases of this horizon, early and late. In the oldest specimen, no. 3707, the plate is perhaps just beginning to show the invagination, preparatory to formation of the otic cyst, which is characteristic of horizon xi.

As to the closure of the neural groove, some of the newer specimens help to throw light upon the time of this event and the level at which it begins. The earliest embryo, as defined by a precise count of the somites, in which closure of the folds is under way is the 6-somite Huber embryo, Michigan no. 71. Professor Arey states (personal communication) that in this specimen the neural folds are in relatively close relation throughout most of the somite region. Actual fusion extends from the middle of somite 2 to the middle of somite 3. If, as Bartelmez supposes (personal communication), the West embryo, no. 4923, has only 5 or 6 somites instead of 8 as tentatively counted by West, it would be the earliest known embryo showing closure. The 6-somite Farris embryo, no. 8244, has a completely open neural groove. The embryo “Ludwig” (Streiter, 1951), ostensibly of 7 somites, is out of line with all others of comparable somite number; the extensive closure of the neural tube, taken into account with other advanced features, throws doubt on the somite count.

The level at which fusion begins is evidently variable. Bartelmez and Evans (1926) located it at about the 4th somite, basing their conjecture on an apparently impending closure at that level in the Pfannenstiel embryo of 5 or 6 somites, and also on retrospective calculation from no. 4216 (7 somites), in which the tube is closed from the middle of the 3d to the middle of the 6th pair of myotomes. In Politzer's 8-somite embryo, the fusion, which must have begun at about the same level, has reached two somites farther caudally, extending from the middle of the 3d to the middle of the 8th myotome. (This statement takes account of Arey’s correction of the somite count; compare his article of 1938 and the list of embryos, page 37 of the present paper.) West's embryo has only a short closure opposite the 6th somite. Since the first contact occurs wherever the curving edges of the infolding ridges meet, it is to be expected that, when additional specimens having 6 or 7 somites are obtained, some scattering of the level of first contact from the 2d to the 7th somite will be found.

The rate of spread of the closure in both directions was tabulated by Bartelmez and Evans (1926). As they point out, the caudal extension of the closure proceeds at about the same rate as the formation of new somites, so that the margin of the posterior neuropore is usually opposite the latest pair of somites. Rostrally, progress is slower and apparently more variable. At the end of the period of horizon X, no. 3707, closure has reached well into the hindbrain region (rhombomere 3). The tosomite embryo no. 5074, which may now be added to the tabulation, is no exception, the closed part of the neural tube reaching from the caudal end of the 10th somite to the level of the otic plate.

Notochord and Primitive Streak

By the time embryos are 3 weeks old, the cephalic region has greatly increased in size, and in all members of horizon x the primitive node and streak are limited to the relatively short caudal part of the hotly. At the head

Heuser1957 fig03.jpg Heuser1957 fig04.jpg Heuser1957 fig05.jpg Heuser1957 fig06.jpg Heuser1957 fig07.jpg Heuser1957 fig08.jpg

Cite this page: Hill, M.A. (2024, May 28) Embryology Paper - Developmental horizons in human embryos group X. Retrieved from

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