Paper - Extroversion of the cerebral hemispheres in a human embryo
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Hunter RH. Extroversion of the Cerebral Hemispheres in a Human Embryo. (1934) J Anat. 69(1): 82-5. PMID: 17104519
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Extroversion of the Cerebral Hemispheres in a Human Embryo
By Richard H. Hunter, M.D., M.Cu., Pu.D., M.R.LA.
From the Department of Anatomy, Queen’s University, Belfast
The specimen I am about to describe was obtained from a tubal pregnancy; it measured 35 mm. c.R. length. It presented on superficial examination what appeared to be enormously dilated cerebral hemispheres in which the cerebral tissue was directly exposed to the surface without any covering of either ectodermal or mesodermal derivatives (fig. 1). Dark brown-coloured “fringes” were present at the posterior part of each hemisphere, and these fringes had the appearance of the choroid plexus. Coronal sections were cut serially of the whole head region, and these presented certain unusual characters.
Fig. 1. Lateral view of the embryo showing the exposed cerebral hemispheres. Note the dark fringes of the choroid plexus.
Sections through the mid-brain region of the head showed a deep V-shaped depression in the middle line, and on either side of the depression was a tumour-like mass of tissue, containing in its centre a mass of nerve fibres, and on its outer portions a thick layer of nerve cells. This tissue apparently represented the thalamus, the sections presenting an appearance exactly similar to that seen in the developing thalamus of the same growth period. The cerebral hemispheres were much larger than normal for an embryo of this size, and overhung the eyes. The frontal, parietal and squamo-temporal regions of the skull were lacking, but the chondro-cranium was normal in its form and state of development. The margins of the brain tissues were continuous with the skin margins of the head. The mesoderm from the head proper extended into, and partially filled, the “cavity” of the hemispheres. This tissue, at first sight, appeared to be developing choroid plexus, but it did not contain the rich development of blood vessels associated with the latter, nor was its covering layer composed of the cubical ependymal cells which are seen on the normal choroid plexus.
Further back in the series of sections frill-like evaginations of cerebral tissue were seen at the line of junction of the cerebral hemispheres with the thick mass of thalamus on each side. Under the high powers of the microscope this tissue was found to consist of the typical cubical-shaped layer of covering cells, with the usual core of vascular mesoderm of the normal choroid plexus. These frill-like evaginations were the dark brown-coloured “fringes”? seen on nakedeye examination.
In all the sections of this series the lamination of the hemispheres was in the reverse order from that seen in the developing hemisphere of the normal human embryo of 35 mm. c.R. length, 1.e. an outer fibrous layer and an outer cellular layer, with an inner fibrous layer and an inner cellular layer. In this specimen the order of the lamination was reversed, and the normal inner cellular layer was exposed on the surface of the hemisphere, while the fibrous layer, normally seen on the outer surface, was placed next the cavity of the ventricle on the inner surface of the hemisphere. The optic cups were normally developed as well as the optic nerves, chiasma and tracts. The otic vesicles were differentiated in the usual manner into cochlear and vestibular portions, and these were surrounded by cartilage of the petrous-temporal bone. The middle and external ears also appeared normal in development. The Gasserian ganglia were developed normally, and their nerve roots arose in the usual position from the pons, on each side at the junction of this part of the brain stem and its middle cerebellar peduncle. The cerebellum had not yet taken its adult form, but the embryonic lateral masses, rhombic lips and fourth ventricle choroid plexus were present in that stage of development usual for an embryo of 35 mm. c.R. length. The lumen of the spinal cord was continued upwards into the fourth ventricle, and thence through the Sylvian aqueduct to open to the exterior between the tumour-like masses of the thalami. The brain was thus abnormal in its development only in the fore-brain region.
The appearance of this specimen suggests that the margins of the fore-brain portion of the medullary plate had rolled outwards, instead of inwards, at an early stage of development, so that the normal internal surface of this end of the medullary tube tissue became everted (fig. 2). The thalami were thus brought to the surface where they lay exposed in the median plane of the hemisphere, with the choroid plexus extending outwards in the direction of what would have been the interior of the lateral ventricles if the normal inversion of the medullary plate had occurred. The margins of the nerve tissue retained their connection with the skin ectoderm, and in section the two tissues, medullary (nerve) tissue and the ectoderm (skin) tissue, were found to be continuous with one another. The line of junction was not sharply demarcated and the embryonic skin cells blended imperceptibly into the nerve cells of the cerebral tissue. A similar relation between the neural and dermal tissue occurs normally in early development. Bremer (1) describes it in the 4 mm. stage of a human embryo. In it he states that the closure of the medullary tube is “incomplete in the anterior part of the head, and also for a considerable distance from the end of the tail, giving both an anterior and a posterior neuropore.”’ The fore-brain he describes as being divided into two parts, one of which includes the anterior neuropore marked by the “rolled”? ectoderm where it joins the skin, as it did in the specimen which I have just described.
Fig. 2. Diagrams to illustrate the method of eversion of the medullary lips, with the resultant exposure of the thalami, and choroid plexus.
This specimen appears to be an anomaly of an exceedingly rare nature. Schwalbe (2) makes no reference to the condition, and in the literature I can find only one case which resembles it. It is described by Sternberg (3) in an embryo of 5 mm. c.R. length. But in Sternberg’s specimen two openings occurred in the cerebral area, one in the mid-brain and one in the fore-brain region. The skin ectoderm he described as being in direct continuity with the tissue of the brain wall in the region of these two openings, just as brain and skin junction were in my specimen. Sternberg’s specimen would appear to be either an early stage in the development of extroversion of the brain, or, as it is such a young embryo, it is more likely to be a case of delayed, or of abnormal method of closure of the neuropore. From notes in a personal letter from Dr George L. Streeter, I learn that in the collection at the Carnegie Laboratory of Embryology, Baltimore, Md., there is an embryo of the 16-somite stage in which there is a defect in the development of the fore-brain similar to that which I have here described.
The condition would appear to be a closely similar one to that of spina bifida aperta, i.e. the flat spine form in which the spinal cord is uncovered, but affecting the cephalic end of the medullary plate region only, instead of the usual caudal end. My specimen also differs from true spina bifida in that the neural tissue has undergone the differentiation of normal development. But in spite of these differences the aetiology of the two conditions, spina bifida and ‘extroversion of the fore-brain, would appear to be the same. Wheeler (4), in discussing the aetiology of spina bifida, suggests that it is due to failure of the neural tube to separate from the skin, i.e. that the skin connection inhibits the differentiation of the nerve elements from the medullary plate by a process unexplained by him. My specimen does not support such a view, as the brain tissue is differentiated into the layers normal for an embryo of this size, and as already stated there is a persistence of the neuro-dermal junction.
- An unusual case of extroversion of the fore-brain is described in a human embryo of 35 mm. c.R. length, obtained from a tubal pregnancy.
- Unusual features noted are a reversed order of the normally developed cellular-fibrous layers of the cerebrum, and a persistence of the neuro-dermal junction, and exposure of the thalami to the surface.
(1) Bremer, J. L. (1906). Amer. J. Anat. vol. v, p. 459.
(2) ScHWALBE, E. (1909). Die Morph. der Missbild. des Mensc. u. der Tier. T. m1, L. a. Jena. (3) SrernBerG, H. (1926). Z. f. Anat. u. Hntwick. Bd. txxxu, 8. 763.
(4). WHEELER, T. (1918). Contr. Embryol. Carneg. Instn. vol. vu, p. 91.
Cite this page: Hill, M.A. (2019, October 18) Embryology Paper - Extroversion of the cerebral hemispheres in a human embryo. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Extroversion_of_the_cerebral_hemispheres_in_a_human_embryo
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