Paper - An anencephalic human embryo 16.5 mm long
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Dodds GS. and Deangelis E. An anencephalic human embryo 16.5 mm long. (1937) Anat. Rec. 67(4): 499-505.
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An Anencephalic Human Embryo 16.5 mm Long
G. S. Dodds And Eugene Deangelis
School of Medicine, West Virginia University
The embryo herein described was secured from a spontaneous abortion by a woman 17 years old. It was brought to our laboratory soon after birth with the membranes intact. Inasmuch as we have been unable to ﬁnd a recorded description of an early anenccphalus of this type we feel justiﬁed in publishing an account of this embryo.
Its crown—rump length was 16.5 mm., and its external form, except for the anomalies of the head, was that of an embryo of about 7 weeks, being clearly in the transition from embryonic to fetal form. There was no conspicuous external evidence of decomposition. Microscopic study showed that there had be-en marked post mortem changes, but the condition of the tissues was still such that internal structures could be deﬁnitely identiﬁed.
The trunk and limbs were essentially normal, both externally and internally. The external ear was about as would be expected in an embryo of this stage of development. There was double harelip, and internal defects of palate and nasal septum.
The conspicuous deformity, as seen in figure 1, involved the upper portion of the head, which had a form simulating that of a brain consisting of cerebral hemispheres with prominent olfactory lobes and a cerebellum with two lobes. Microscopic examination showed this peculiar structure to be composed of highly Vascular mesenchyme, quite loose withi11, but ending above in a thin, denser, non~cellular layer (see fig. 2, just below the word ‘l\-larg’). At the lateral margin of this mesenehymal mass, the epidermal layer of the skin, consisting of a single layer of cuboidal cells, terminated in a ragged, partly detached edge, the location of which is shown by the dotted line in ﬁgure 1.
Fig.1 Posterior and lateral views of anencephalic monster. Drawings made after preservation in formalin. Shows mesenchymal mass simulating the form of a brain. In it were seen many blood vessels before it was placed in formalin. The dotted line traces the torn edge of the epidermal layer of the skin. The irregular sheet of tissue. attached to the top of the head is primitive nervous tissue. The opening into the spinal canal was ﬁlled with similar tissue.
At the nape of the neck was a good sized opening leading downward into the spinal canal. From it there protruded a ﬁocculentr mass which became lost during the preparation of sections, but it was doubtless a broken down portion of the spinal cord. The persisting portion of the spinal cord was seen in sections to occupy the lower end of the spinal canal and to be in a fair state of preservation. At the margin of the opening into the spinal canal, the epidermal layer of the skin terminated, just as it did at the edge of the mesenchymal mass.
Probably the most signiﬁcant feature of this embryo was a wrinkelcd sheet of tissue loosely attached to the top of the head (ﬁg. 1). Microscopic examination showed this to be composed entirely of primitive nervous tissue. Here and there were seen small fragments of this same tissue adhering to the surface of the mesenchymal mass.
This sheet“ of tissue had the typical structure of the primitive neural tube, consisting of a thin, highly cellular ependymal layer, followed by a thick mantle la.yer with less abundant cells, and a marginal layer with no cells (ﬁgs. 2 and 3). The ependymal layer faced outward, away from the head, while the marginal layer faced inward toward the mesenchymal mass, from which it had become separated at most places. This sheet of nervous tissue had a thickness of about 250 in. Its histological structure was quite like that of the neural tube of the embryo as seen in the lower portion of the spinal cord, except that the spinal cord had become closed in the normal fashion.
We interpret the structures just described as indicating that the anencephalic condition of this embryo was produced by failure of the neural plate in the cephalic region to form a closed neural tube. It seems probable that the fragment of nervous tissue adhering to the top of the head is the remnant of the cephalic portion of the neural plate, which at one time covered the entire mesenchymal mass and was directly continuous at its margin with the epidermal (ectodermal) layer of the skin. It no doubt also extended downward into the spinal canal, where the neural groove had become closed to form the spinal cord.
We have no explanation for the great thickening of the mesenehyme underlying the cephalic portion of the neural plate, nor for its peculiarly suggestive form. It is evident, however that had this mesenehyme been of moderate, uniform thickness throughout its whole extent, the head of this monster would have resembled very closely that of the
Fig. 2 Photomicrograph. X 90, through neural plate into mesenchymal mass. Shows ependymal, mantle and marginal layers of neural plate. In mesenehymal mass it shows blood vessels, and a nerve approaching the surface.
Fig. 3 Photomierograph, X 400, showing structure of epeudymal and mantle layers of neural plate.
common ‘form of full term anencephalie acranial monster, in which the cranial Vault and brain are Wanting and the covering of the ﬂattened top of the head resembles the meninges without an epidermal covering. Certainly the condition observed in this embryo would furnish a reasonable beginning for such a condition should development continue to full term.
Certain observations concerning the nerves and ganglia are of interest. The spinal cord was ﬂanked by the spinal ganglia in the usual manner, and both dorsal and ventral roots of the spinal nerves were present. In the cervical region several ganglia were united into one elongated mass from which arose several nerves. These nerves, together with their corresponding ventral roots, united to form a rather imperfect cervical plexus. From this plexus arose the phrenic nerve
Fig. 4 Graphic reconstruction of head showing nerves and ganglia.
The cranial nerves and ganglia were traced with considerable certainty. In general it may be said that the cranial nerves had their apparent origins in the surface of the mesenchymal mass. Their ﬁbers penetrated its surface by deﬁnite, foramen—like openings, and one of them, the oculomotor, actually connected with the nervous sh.-eet where it was still in contact with the mesenchyme (ﬁg. 4). Figure 2 shows a photomicrograph of one of these nerves as it approaches the surface. The following nerves were found.
The olfactory nerves had their origins in some fragments of nervous tissue lying in the ﬁssure between the mesenchymal hemispheres. They coursed downward to the nasal mucosa.
The optic nerves could be traced backward from the eyeballs until they united in the optic chiasma, just as they pierced the superﬁcial mesenchyme at the bottom of the median ﬁssure. l/Vithin the eyeball the optic cup showed good development of both pigmented and retinal layers.
Just back of the optic chiasma, close to the nasal cavity, was a median mass of epithelial cells which we consider to be the anterior lobe of the hypophysis. We did not observe anything that seemed to represent the posterior lobe.
A pair of nerves without ganglia arose from the surface of the mesenchyme near the vertex, and coursed through the mesenchyme, past the hypophysis to terminate in the region of the eyeball. These are probably the oculomotor nerves. The enlargement on this nerve, shown in ﬁgure 4 is not ganglionic in nature and there is no corresponding enlargement on the other nerve of the pair.
In the region behind and below the eyes were two large pairs of ganglia which belonged to the trigeminal nerves. Each of them had several roots arising from the surface of the mesenchyme in locations shown in ﬁgure 4. The uppermost of these ganglia gave off two nerves which were evidently the ophthalmic and maxillary branches of the trigeminal nerve, While the other ganglion gave rise to the mandibular branch.
A smaller ganglionic mass connected with the second of these ganglia could not be identiﬁed.
In the lower pharyngeal region there were a pair of small ganglia from which arose nerves running into the thorax along with the sympathetic trunks, evidently the vagus nerves.
Neither ganglia nor nerve ﬁbers representing the facial or glossopharyngeal nerves could be found, and there seemed to be complete absence of internal ear and auditory nerve and ganglion.
The presence of the ganglia of the trigeminal and vagus nerves in this embryo shows that there had been in the cranial region some development of the neural crest, or of the placodes which contribute to the ganglia, or of both.
It is of interest to note that the poorly deve-loped neural plate had, in some measure behaved like a normal neural tube: From it had grown out the optic vesicles to produce optic nerves and retina. From it had also grown one pair of efferent nerves, the oculomotor. Deﬁciency of the neural plate is indicated by the absence of other efferent nerve trunks which if present ought readily to be recognized.
We believe that the anencephalic condition in this embryo arose by a failure of the neural plate in the cranial region to develop into a closed tube. Thus we explain the absence of brain and cranium.
The defect must have begun at least as early as the fourth week.
It seems reasonable to suppose that in at least the extreme types of full term anencephalic monsters, the mode of origin might Well be just what is indicated in this early specimen, rather than the result of a ‘catastrophe’ or ‘trauma’ after the brain had become Well formed. In this connection We believe that our specimen presents a crucial case.
Cite this page: Hill, M.A. (2024, February 24) Embryology Paper - An anencephalic human embryo 16.5 mm long. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_An_anencephalic_human_embryo_16.5_mm_long
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