Paper - An abnormal human embryo 11 mm long
|Embryology - 17 Oct 2019 Expand to Translate|
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Dodds GS. An abnormal human embryo 11 mm long. (1929) The Anat. Rec. 199-208.
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An Abnormal Human Embryo 11 mm Long
G. S. Dodds
School of Medicine, West Virginia University
Three Figures (1929)
The embryo herein described was presented by Dr. G. R. Maxwell, of Morgantown, West Virginia, on October 6, 1927. There was a history of spontaneous abortion after a pregnancy of four months, as measured by the menstrual history. This was the first pregnancy.
The chorionic vesicle was intact and showed abundant vascularity in the maternal part of the placental attachment. The amniotic ﬂuid was about normal in amount and of a yellowish color. The embryo and umbilical cord were very pale and showed no indication of vascular-ity. Evidently, the embryo had been dead for some time before abortion, but no external evidences of maceration or injury were apparent. Before preservation, both lateral and ventral views of the embryo were drawn, the outlines being made with the aid of a camera lucida (figs. 1 and 2). After this, it was preserved in 5 per cent formalin. At a later time it was cut into serial sections 15 p thick by the paraifin method. The detailed studies were made from these sections by means of graphic reconstruction.
The crown—rump length of the embryo, as measured before preservation, was 11 mm., which would indicate an age of six Weeks. Moreover, those external and internal structures which are normal are also like those of an embryo of this age.
Though some breaking down of tissue had taken place, there had not been sufficient decomposition to change the external form of the embryo, nor was there evidence that it had suffered an eonsiderable mechanical injury either before or after birth. The enveloping layer of stratified squamous epithelium is about four cells in thickness and in a fair state of preservation and everywhere intact. Most of the internal structures are well enough preserved to permit recognition of the form of tlie organs and something of their histological structure. The brain and spinal cord had suffered the most. Though the form of these organs could be clearly traced by the persistent meninges, the nervous tissues within had been reduced mostly t.o a mass of fragments. The ganglia and peripheral nerves were in a better state of preservation, so that it was possible to trace most of the larger nerves with certainty.
As niay be inferred from figures 1 and 2, the external anomalies are largely confined to the region of the head and neck. The same is true of the internal abnormalities. Both external and internal organs of the trunk are esseiitially like those of the average embryo six weeks old.
The head, instead of having the enlarged, nodding shape which is typical for this age, is a cilindrical continuation of the trunk, showing nothing’ of the form of the normal head. The. eyes appear at about the usual position, but all the other usual landmarks of the face are wanting. There is no external evidence of the branchial arches. The most conspicuous feature in the region where the face might to be is a wide, shallow depression, which, on account of its internal, eonneetions with the esophagus and trachea, must be considered the mouth. The large transverse ridge below the mouth doubtless represents the lower jaw. Well forward on the ventral surface of the head is another opening, a small pore, which probably represents the united nasal pits. On the ventral surface of the head betiween the eyes the fresh specimen showed clearly a pair of eloiigzzted, opaque, white spots, the structure and relations of which will be described later.
The chief interest. of the specimen lies in the face and the brain and cranial nerves.
Fig. 1 Lateral View of abnormal human embryo 11 mm. long.
Fig. 2 Ventral View of embryo shown in figure 1.
The entire absence of ext.ernal structures typically associated with the face in embryos of this age is apparent from figures 1 and 2. A correct interpretation of the structures present is by no means easy, partly because of their very peculiar nature and partly because disintegration of tissues in some places makes it impossible to distinguish between a.ct.ual structures and artefacts.
Fig. 3 Drawin r showin certain internal strut-tures of the enibrvo )i1'tur(*<l in is 5 . I figures ] and 2. Based on graphic reconstruction made frmn serial sections.
The wide oral cavitty continues inward as a narrowing space which extends caudad a short distance, where it divides into the trachea. and esophagus (fig. 3). The innermost part. of the undivided cavity must represent the pharynx, but just how much of it cannot be determined.
There are two lateral openings from the anterior part of the oral ope11i1'1g', which, as seen from the front, suggest the external nares (fig. 2). Internal relations, however, indicate that they may not be thus explained. They lead into an extensive cavity, quite unlike any cavity normally found in the human embryo, which seemingly represents a much distorted modification of the oral and nasal cavities (fig. 3). It is lined, for the most part, with a single layer of squamous cells.
The shape of this cavity may be described as follows: From each‘ of the lateral openings a narrow canal extends cephalad. After a short course, these two canals unite into a single cavity, which in cross-section is shaped somewhat like an inverted T, having two wings extending laterally just beneath the ventral surface of the head and a keel extending dorsadtoward the brain (fig. 3). From the caudal end of the keel there extends caudad a slender canal which lies just dorsad to the oral opening and the pharynx. It ends blindly. From the dorsal margin of the keel of the main cavity there is given off a small diverticulum which lies close to the infundibular diverticulum of the brain, in such a position that it evidently is Rathke’s pouch. This relation lends support to the view that the main cavity must represent the oral pit. At a point a short distance in front of the two epithelial spots the cavity contracts into a narrow canal, which. soon expands again into a broad, shallow space opposite the anterior end of the brain. In this broader region it opens to the exterior through a single pore, which probably is the nostril. Immediately opposite this opening the epithelium of the roof of the cavity becomes very tall, in contrast to the ﬂat cells lining the greater part of the cavity. From this epithelium there extends to the anterior end of the brain what looks like a short, thick nerve, although the preservation is rather bad in this region and its nature cannot be determined with certainty. The structure strongly suggests an olfactory epithelium and the olfactory nerve. This epithelium has a median longitudinal ridge which partially divides the nasal pore, suggesting that it may possibly be formed by the fusion of two nasal pits. Covering the anterior end of the nasal pore is a thin ﬂap of epithelium which stands up as a hollow keel extending cephalad for a short distance, where it ends blindly. The position of the pore, cephalad of the eyes, suggests the condition in cyclopia, in which the nasal opening is usually single and above the eyes. There is not, however, any suggestion of fusion of either the eyeballs or the optic nerves.
Certain premuscle masses must be described in connection with the face. In the region opposite the eyes there are two good-sized masses, one lying to each side of the deep keel of the oral cavity. These might be supposed to represent a development of the maxillary process of the first arch. The maxillary branch of the trigeminal nerve ends in this mass, but the mandibular branch also terminates here — a relation which might be taken to indicate that this mass represents both maxillary and mandibular processes, even though it lies wholly cephalad of the mouth.
The large, horsesh0e—shaped ridge below the mouth, the lower jaw, is also occupied by a large premuscle mass, which by its position would be supposed to be derived from the mandibular process of the first arch. In this mass are two pairs of slender precartilage rods, which are probably Meckel’s cartilage and the hyoid cartilage (fig. 3). The presence of these two cartilages indicates that this mass represents not only the mandibular process, but also includes the second arch and possibly the succeeding ones. This premuscle mass shows a median elevation in the ﬂoor of the pharynx just in front of the opening of the trachea, which apparently represents the tongue.
In this mass are some good—sized nerves, but it has not been possible with certainty to trace any of them out of the mass so as to learn where they come from. The mandibular nerve definitely does not enter it. The facial nerve seems to end in the region of the pharynx near the lateral ends of the mass and could not be definitely seen to enter it, though there are defects in the sections at crucia.l places, which prevent a definite decision. Neither the glossopharyngeal nor the vagus could be traced into it. At one place it seems as if the brachial plexus sends a nerve in.to the mass which connects with the la.rge branching nerves in it, but imperfections of histological detail prevent a11 unequivocal a.nswer. So it is that the nerve supply of this very interesting region cannot definitely be determined further than to say that the mandibular nerve, which should enter it, does not do so, and that the others which might be expected to supply it cannot with certainty be traced to it, nor can it be definitely determined just what is the termination of these nerves on account of imperfections of histological detail a11d defects in the sections.
The Brain and Cranial Nerves
The spinal cord is probably about normal in size for an embryo of this size, but the brain is somewhat smaller than normal, the various part.s being of about the usual length, but usually slender (fig. 3). The amount of mesenchyme surrounding the brain is excessive, for, whereas in the normal embryo of this age the brain is almost in contact with the ectodermal covering of the head, so that the form of the brain is very evident externally, in this specimen it does not at all affect the external contour of the head.
The brain gives the impression of unusual elongation, partly on account of its attenuation and partly beca.use the three ﬂexures are less marked than usual. Striking features are the unusual length of the isthmus and of the ventral extension of the brain from which the optic nerves arise. This latter has the form of a long slender process with a deep keel.
The cranial nerves are for the most part about normal, but there are certain interesting peculiarities.
The nervus terminalis
A large pair of nerves arise from the anterior end of the brain in close proximity with what has already been described as the olfactory nerves. These nerves run caudad for a short distance, parallel to the ventral surface of the head, and after a short course, end iii a pair of large ganglia. Beyond the ganglia no nerve fibers could be identified. The suggestion is made that this is the nervus terminalis.
Associated with the ganglion of this nerve are two peculiar structures, the ‘epithelial spots’ and the ‘epithelial bodies’ (figs. 2 and 3). The epithelial spots in the fresh embryo appeared as a pair of white spots, broadest at the anterior end and tapering to a point posteriorl_v. Sections show them to be areas of thickened epithelium, Where several layers of squamous cells are superimposed. Beneath the larger end of each spot the epithelium is depressed in the form of a short. conical stump extending into the underlying mesenchyme. Extending from this depression to the caudal end of the corresponding ganglion is a strand of fibrous mesenchyme.
Close to the caudal end of each ganglion, embedded in the premuscle mass, is a rounded mass of epithelial cells. At its caudal end the mass is drawn out into a slender process which extends to the surface as a slender strand of epithelial cells. This reaches the surface just under the caudal end of the corresponding epithelial spot.
It is hardly safe to suggest what may be the significance of these epithelial structures, except to say that if the ganglia with which they are associated are actually those of the terminal nerves, the epithelial structnres may possibly represent Jacobson’s organ.
The olfactory nerve
In connection with the discussion of the nasal cavity, there has already been given a description of what has been taken for the olfactory nerve and which arises from the brain in close association with the terminal nerve.
This nerve arises from the ventral recess of the dienceph— alon near its distal end and runs directly laterad to the eyeball.
T/2.6 0(:nl(m/ ofor 4‘z<*.rvc
This nerve arises from the customary place on the brain and ends in the region of the eyeball.
The trochlear nerve ru11s tl1e usual course from the dorsal surface of the isthmus to the region of the eyeball. In this embryo the right nerve is much stouter than the left one, being much larger t.l1a.11 is usual for this nerve.
The abrlmrcns ncrve
This nerve arises at the usual place from the ventral side of the medulla and runs to the region of the eyeball.
The trigenmzal nerve
This nerve arises from the usual place on the side of the hindbrain and runs a short course to the large semilunar ganglion. From the ganglion arise the usual three nerves, each of which runs about the course that might be expected, except that the mandibular nerve, in following this course, does not arrive at the mandibular process, which lies far caudad. It, as well as the other two branches, ends in structures which lie above the mouth.
The facial and auditory names
The facial and auditory nerves arise from the usual position on the lateroventral surface of the medulla oblongata as a large trunk in which no division between the two nerves can be detected. After a short course, the nerve enters the ganglion, which, like the nerve, does not show a definite division into two parts. From the cephalic side of the ganglion the facial nerve emerges, almost immediately to divide into the small chorda tympani nerve which runs ventrad along with the mandibular branch of the trigeminal, and a larger branch which runs caudad to end in the same region as the glossopharyngeal and the vagus. Its exact termination could not be determined, as stated i11 a11 earlier paragraph. The chorda tympani fails to reach the rudiment of the tongue, which lies farther caudad.
The caudal portion of the ganglion gives off a small branch to the vestibular part of the otic vesicle (fioa 3), and from it some fibers also run to the curved. cochlear part of the otic vesicle. The otic vesicle has about the form to be expected at this age.
An unknoiwn nemze
There is a good—sized nerve without a ganglion which arises from the ventrolateral surface of the medulla about midway between the auditory and glossopharyngeal nerves. It runs in company with the glossopl1ar_vn.geal and vagus nerves to the region of the pharynx.
The glossopharyngeal /nerve
This nerve arises as a small trunk from the side of the medulla. oblongata. It runs ventrolaterad a short. distance and enters an elongated ganglion. Beyond the ganglion it is a good—sized nerve which ends in the region of the pharynx.
The vagus nerve
The vagus nerve originates from the side of the medulla oblongata. It soon enters a large ganglion, beyond which it runs ventrolaterad to the region of the pharynx. The large portion of this nerve which runs to the thoracic and abdominal viscera is easily traced.
The accessory avzerive
There is a series of rootlets and small ganglia along the side of the medulla caudad to the vagus nerve. They are connected by a longitudinal nerve which unites with the vagus near its origin. This is clearly the accessory nerve, but there could not be identified any nerve leaving the vagus which could be interpreted as a continuation of this nerve.
The hypoglossal nerve
The several roots of the hypoglossal nerve are clearly seen arising from the ventrolateral surface of the medulla. They converge in the usual manner and can be traced through the foramen in the occipital cartilage.