Paper - The vascular drainage of the endolymphatic sac and its topographical relation to the transverse sinus in the human
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The Vascular Drainage of the Endolymphatic Sac and its Topographical Relation to the Transverse Sinus in the Human
Department of Embryology, Carnegie Institution of Washington, Johns Hopkins Medical School, Baltimore, Maryland
In a previous paper (Streeter ’14) dealing with some experimental studies on amphibian larvae, it was shown that in the tadpole the endolymphatic sac always lies in close apposition to the membranous roof of the hind-brain. This relation exists not only in normal specimens, but it was also found that in specimens Where the ear vesicle had been rotated or transplanted by operative procedure, the endolymphatic sac in the subsequent selfcorrection of posture, succeeds in most cases in attaching itself to the membranous chorioidal roof in the normal manner.
This interesting topographical relation of the endolymphatic sac in the tadpole, induced the writer to examine more closely the endolymphatic sac in later human embryos, and it is the purpose of the present paper to outline the results of such a study in embryos from 20 mm. to 240 mm. crown~rump length.
It has long been known that in elasmobranchs the endolymphatic appendage opens directly on the surface of the body and that the surrounding sea—water can thereby pass directly through the endolymphatic duct to the cavities of the labyrinth. The arrangement that we have referred to as existing in the tadpole, suggests that we have there quite a different source of access for the endolymph. At any rate, it is evident that the contact existing between the endolymphatic sac and the membranous roof of the hind-brain affords favorable structural conditions for an interchange of substances between the cerebro—spina1 ﬂuid and the endolymph, either by diffusion or by a secretory activity of the separating epithelial membranes. The endolymphatic appendage also in the human embryo serves.as an absorption-apparatus or one for regulating the endolymph, that is, if we may judge from its structural and topographical characteristics. The condition, however, in human embryos becomes somewhat more complicated than that in the tadpole in that here the sac is separated very early from the chorioidal membrane by the development of the dura mater and the intervening arachnoid-pial membrane. Instead of attaching itself to the membranous roof of the hind-brain, the sac projects against one of the large veins of the dura mater. Furthermore, it does not apply itself directly against the vein wall, but is separated from it by an intervening capillary plexus, which in turn drains into the vein. As far as the writer knows the character and connections of this endolymphatic capillary plexus is described here for the ﬁrst time. As to its functional signiﬁcance we must for the present limit ourselves to the above suggestion and in the following paper attention will be directed only to its morphology as seen in the typical stages of its development.
Material And Methods
The specimens which were examined microscopically in connection with this study consist of a group of human embryos, measuring from 21 mm. to 240 mm. (crown-rump) long, that is, from about the eighth to the twenty—eighth week of fetal life. They all belong to the Collection of the Department of Embryology of the Carnegie Institution of Washington. The specimens in most cases had been injected with India ink through the umbilical vein and had been prepared in serial sections. In some cases after injection and ﬁxation they were dissected so as to make total preparations which were rendered transparent in Wintergreen oil and were examined under the binocular microscope. For purpose of topographical determinations, proﬁle reconstructions were made of several of the embryos that had been cut serially and in some instances the structures were modelled after the Born Wax-plate method. These will be speciﬁed under their separate descriptions. Although other embryos Were examined the following list includes those that were chosen as best representing the stagesof growth of the endolymphatic sac and its blood-vessels.
EMBRYO NO. °M£:§;::MP D1R:§;f§§%sFss';:;:oN vnscumn INJECTION 460 21 mm. 40 p. trans. India ink. Wax-plate reconstruction 632 24 mm. 100 p. sagit. India ink. Proﬁle reconstruction 449 34 mm. 100 u sagit. India ink. Serial examination 96 50 mm. 100 u sagit. 0 Proﬁle reconstruction 448 52 mm. 100 ;u sagit. India ink. Serial examination 458 54 mm. 0 India ink. Cleared specimen Left side 1018 130 mm‘ 50.1.; trans. India ink. Proﬁle reconstruction Right side 0 India ink. Cleared specimen 1131 240 mm. 100 p: trans. 0 Serial examination
In the opinion of the earlier embryologists the endolymphatic appendage represents the last portion of the ear vesicle that is attached to the skin, and which becomes drawn out into a stalklike elongation as the vesicle recedes from the surface. They further pointed out that it corresponds to the narrow tube found in Selachians that passes dorsally through the cartilagenous skull to reach the surface of the head where it opens and thereby constitutes a canal that leads from the outside directly to the labyrinth. In this instance the ear Vesicle remains attached to the skin throughout the Whole period of its development. In other vertebrates it persists only as an embryological remnant of varying size that terminates as a blind sac under the dura mater and is apparently of no further use (Balfour ’81, Hoffman ’90, Hertwig ’98).
This was the prevailing view regarding the endolymphatic appendage until results that conﬂicted with it were reported by Poli ’97 and Netto ’98. These investigators found that in reptiles and amphibians it is the lateral surface of the ear vesicle that is last to be detached from the skin, at a place clearly remote from the dorsal tip that gives origin to the endolymphatic duct. It was also found that in some cases the endolymphatic appendage does not make its appearance until after the detachment from the ectoderm is completed. Keibel ’99 was strongly inﬂuenced by the condition existing in the embryo of the chick, Where the separation of the otic vesicle from the ectoderm occurs relatively late and in fact the last point of attachment does occur at the dorsal tip of the endolymphatic appendage, and he therefore supported the original view of Balfour ’81. He quite correctly defends the opinion that the tube in Selachians connecting the inner ear With the ectoderm is the same as the endolymphatic duct of the vertebrates. The conditions found in amphibians by Netto ’98, where the endolymphatic duct does not develop until a considerable time after the complete detachment of the ear vesicle, he explains as a shifting in the time of occurrence of the ontogenetic as compared with the phylogenetic processes.
Subsequently the origin of the endolymphatic sac was carefully reviewed by Krause ’01 who had an abundance of material for a comparative anatomical study. He showed that in reptiles the point of separation of the ear vesicle from the ectoderm has nothing to do with the dorsal pointed end of the vesicle from which the endolymphatic duct arises. While in birds, as described by Keibel ’99 and others, it corresponds exactly to the tip of the endolymphatic duct. In mammals it also corresponds approximately to the tip of the endolymphatic duct, but here the duct does not form until after or just at the completion of the detachment of the ear vesicle. In other words the separation point of the ear vesicle is a variable one and is not to be confused With the question of the homology of the endolymphatic duct. As regards the latter, Krause concludes that the endolymphatic duct of higher vertebrates is completely homologous with the canal that connects the labyrinth in Selachians with the surface of the head.
This, in brief, is the present status of our information regarding the endolymphatic duct in its general embryological aspects. As to its histology and blood supply We are primarily indebted to Boettcher ’69. This investigator made razor—serial sections of the endolymphatic appendage of the adult cat and new—born babe. He, first of all, established the fact that it does not degenerate in mammals as was thought by contemporary investigators, but develops further and persists through life as an epithelial canal that connects with the two Vestibular sacs, and forms an important part of the labyrinth. The terminal part spreads out (new-born. babe) into a ﬂattened sac 0.6 mm. wide, and is embedded in the connective tissue of the dura. This sac he describes as made "up of cuboidal pavement epithelium, closely under which, and sometimes resting directly against it, are found capillary loops ﬁlled with red blood cells. The Walls of the sac are somewhat irregular, due to the presence of small epithelial pockets which project outward into the periosteum or bone, and also papilla-like processes or folds which extend into the lumen of the sac. Both Varieties are provided with capillary vessels. The capillaries are described as losing themselves in the periosteum. In another place he describes the small Vessels of the vestibular aqueduct at its bony exit as uniting to form a common stem that empties into the inferior petrosal sinus. These signiﬁcant observations of Boettcher have received scant attention from subsequent writers and do not seem to have resulted in further investigation of these interesting conditions.
Hasse ’73 to whom We owe the generally accepted terms ‘endolymphatic duct’ and ‘endolymphatic sac,’ and who contributed many observations on the anatomy of the labyrinth, speaks of the endolymphatic appendage as a tube extending from the labyrinth to the cranial cavity where it either ends blindly as an ‘epicerebral lymph space’ or opens into the general epicerebral lymph space (p. 768). Elsewhere (p. 792) he describes a small funnel-shape ﬂaring process of the endolymphatic sac that penetrates through a small opening in the dura and there fuses with the arachnoid, thus establishing a communication between the ‘cavum endolymphaticum’ and the ‘cavum epicerebrale.’ The function of the endolymphatic appendage, according to Hasse, is threefold; 1, the sac, during embryonal life, is an epithelial secretory organ that furnishes the endolymph; 2, in the adult, it is either a closed sac that secures new materials for the endolymph by endosmosis from the epicerebral spaces, or it is an open sac through which the epicerebral ﬂuid ﬂows directly into the chambers of the labyrinth; 3, the endolymphatic sac is a reservoir for endolymph which serves as an expansion tank that relieves the pressure when it becomestoo great in the labyrinth. The investigators who have studied the blood supply of the labyrinth do not seem to have directed much attention to the vascularization of the endolymphatic appendage. They have done little more than to conﬁrm the observation of Cotugno, made a century and one half ago, that a vein draining the vestibule and the canals accompanies the endolymphatic duct and empties into one of the dural sinuses. The most careful description is that of Siebenmann ’94 who showed, as others had done for the aquaeductus cochleae, that the veins of the vestibular aqueduct (endolymphatic appendage) though originally accompanying the duct, become separated later in their own bony canal, which he designated as‘ the ‘canalis accessorius aquacductus vestibuli.’ Eichler ’92 who studied the blood-vessels of the human labyrinth conﬁned his attention to the cochlea. Shambaugh ’03 describes the endolymphatic duct as incased by capillaries which are supplied by an arteriole coming usually from the posterior vestibular artery, and are drained by a vein that empties into the transverse vestibular vein. Where the endo lymphatic sac was preserved it was found to be drained by a small dural vein.
Endolymphatic Appendage During First Two Months
The features with which we are chieﬂy concerned in the present paper do not become established until toward the end of the second month (embryos over 30 mm. long). A review, however, will be brieﬂy made of the form and relations of the endolymphatic appendage prior to that time. For a more detailed description with illustrations the reader is referred to a paper previously published on the development of the membranous labyrinth (Streeter ’06) and to a recent paper on the dural sinuses in which special attention is given to the topography of the labyrinth at its different stages (Streeter ’15).
Fig. 1 Proﬁle reconstruction showing the topography of the membranous labyrinth and the endolymphatic appendage in a human embryo 24mm. long (No. 632, Carnegie Collection). The principal head veins are shown in solid black. Enlarged about 4 diameters.
In embryos 4 mm. long the ear vesicle consists of a simple slightly elongated spherical sac that lies in the space between the primary head vein and the lateral wall of the hind-brain. At its dorsal end can be recognized a rounded pouch-like projection which is quite distinctly marked off from the rest of the vesicle. This is the early endolymphatic appendage. It is in relation both with the brain wall andpthe skin, but is separated from them by a scant amount of mesenchyme, in which can be seen minute blood-vessels that communicate with the middle and posterior dural plexuses. The appendage points toward the rhombic lip, but does not quite reach its dorsal margin.
In its subsequent growth the endolymphatic appendage rapidly becomes more clearly differentiated from the remainder of the labyrinth. It takes on a slender tubular form, whereas the vestibular part of the labyrinth expends into a voluminous triangular pouch. The tubular character of the endolymphatic appendage is pronounced in embryos from 9 mm. to 14 mm. long. By its elongation it passes over the rhombic lip and in 14 mm. embryos we ﬁnd the tip of it overlapping the Ventrolateral part of the thin chorioidal roof of the fourth ventricle. It, however, does not lie in direct contact with this membrane as is the case in tadpole larvae, but is always separated by a thin layer of the surrounding mesenchyme.
At about the time of the closing—off of the semicircular canals (embryos 15 mm. long) the simple tubular form of the endolymphatic appendage is gradually modiﬁed by the expansion of its distal half into a ﬂattened fusiform sac, which from then on is recognized as the endolymphatic sac as distinguished from the remaining proximal part, the endolymphatic duct, that connects it with the rest of the labyrinth. The endolymphatic sac lies lateral and caudal to that part of the chorioidal membrane that is to form the lateral recess of the fourth ventricle. It lies close against it, but is always separated from it by the tissue that is to form the arachnoid and dural membranes.
Fig. 2 Proﬁle reconstruction showing the topography of the membranous labyrinth and endolymphatic appendage in a human fetus 50 mm. long (No. 96, Carnegie Collection). The endolymphatic sac is partly covered by the transverse sinus, Which with the other head veins is shown in solid black. Enlarged about 4 diameters.
Simultaneously with the formation of the semicircular canals and the differentiation of the endolymphatic sac there occurs an alteration in the large dural veins in this neighborhood that plays an important part in its topography. This consists in the replacement of the primary head vein by a more dorsally situated longitudinal channel. The middle dural plexus instead of draining, as formerly, into the primary head Vein drains caudalward into the posterior dural plexus. Soon afterward the anterior dural plexus, in a similar manner, changes its direction of drainage and instead of continuing to drain into the cephalic end of the primary head vein, it unites with the middle dural plexus and they both drain into the posterior dural plexus and through it into the internal jugular vein. Due to these alterations in the drainage of the anterior and middle dural plexuses the greater part of the primary head vein disappears and we ﬁnd it replaced by the more dorsally situated channel that is to become the transverse sinus. This channel forms in a groove in the dorsal margin of the otic capsule. Topographically it passes longitudinally in the space between the two vertical canals and the endolymphatic sac. -The general relation of these structures is shown in ﬁgures 1 and 2 which are reproduced from the paper previously referred to (Streeter ’15). The canals are separated from the sinus by their cartilagenous envelope. The endolymphatic sac, however, like the transverse sinus itself does not become encased by cartilage and lies against the median wall of the latter, separated from it only by a small amount of loose embryonic connective tissue in which both are embedded. This close relation which becomes established between the endolymphatic sac and the transverse sinus in 18 mm. embryos, continues as a permanent condition. At ﬁrst (ﬁg. 1) when the endolymphatic sac has a vertical posi— tion, it completely overlaps the median surface of the sinus. Subsequently as the cranium enlarges, this part of its Wall is crowded outward and downward into a more horizontal position and partakes in the formation of the ﬂoor of the posterior cerebral fossa. We then ﬁnd the endolymphatic sac resting on the dorsal surface of the sinus and furthermore the sinus becomes relatively larger than the sac and is then only partly overlapped by the latter.
Though closely related to the chorioidal membrane of the lateral recess, the endolymphatic sac becomes more and more clearly separated from it as the dural and arachnoidal tissues become differentiated. On the other hand, though resting against the transverse sinus, there is a scant amount of loose embryonic connective tissue separating the two. Running through the meshes of this connective tissue can be seen blood capillaries that form a plexus which empties into the transverse sinus. This plexus anastomoses with the vessels of the labyrinth by communications along the endolymphatic appendage. It also an astomoses with the posterior dural plexus. These blood-vessels and their communications can be recognized in embryos 20 mm long, but subsequent to that they rapidly increase in size and importance, and in embryos 50 mm. long obtain a characteristic appearance which We shall now proceed to describe.
Fig. 3 Sagittal section through the ear region of a human fetus 52 mm. long (No. 448, Carnegie Collection). The blood vessels are injected with India ink and are represented in solid black. The endolymphatic sac is outlined as a clear space and surrounding it can be seen its dense capillary plexus and the manner in which this drains into the transverse sinus. Enlarged about 10 diameters.
Endolymphatic Appendage During the Third Month
The topography and vascular drainage of the endolymphatic sac in embryos about 50 mm. (crown-rump) long are shown in ﬁgures 2, 3 and 4. In ﬁgure 2 can be seen the general posture of the labyrinth and the relation of its component parts to the dural sinuses. This ﬁgure is drawn from a proﬁle reconstruction of the labyrinth, dural veins and central nervous system in an embryo 50 mm. long (No. 96, Carnegie Collection). The reconstruction was prepared by projecting the serial sections on transparent papers which were then superimposed and all traced on one sheet. It will be noted that the endolymphatic sac passes upward so that its dorsal one-third rests against the median surface of the transverse sinus, opposite the chorioidal roof of the ventricle of the hind-brain. It does not project above the sinus as in the younger stage shown in ﬁgure 1.
A section through this region is shown in the accompanying ﬁgure 3. This is a portion of a sagittal section through a human enbryo 52 mm. long (No. 448, Carnegie Collection). Before the embryo was prepared in serial sections its vascular system was injected with India ink through the umbilical vein. This injection mass is shown in the drawing in solid black. The section passes antero-posteriorly through the lateral part of the cerebellum, and includes a portion of the ventricle with the chorioidal villi projecting into it. At the base of the villi there is a collection of the injection mass which apparently is an extravasation. This is separated from the endolymphatic sac and its vessels by the dura which is already fairly well outlined, though it is not represented in the drawing.
The feature to which particular attention should be given is the capillary plexus surrounding the endolymphatic sac. Its general character is indicated, and it can also be seen that it drains by several outlets into the transverse sinus. On following it through the sections of the series it is found that it completely envelops the endolymphatic sac and duct. It can be traced centrally within the cartilage as a ﬁnely meshed tubular covering of the duct extendingvto the region where the duct arises from the utricle and saccule. At this point it anastomoses with the vessels of the vestibular part of the labyrinth. The vessels belonging to the vestibule and canals are more sparse; a portion of the cochlea,’ however, seems equally as well provided as the endolymphatic appendage. It is to be remembered that We are dealing with an injected embryo and the meshes of this plexus are doubtless distended, so that the picture we obtain shows them more prominently than would be the case in uninjected material. The topography and communications of the endolymphatic blood plexus are shown more completely in ﬁgure 4. This is an outline drawing of the labyrinth and its blood—vessels in a human embryo 54 mm. long (No. 458, Carnegie Collection). The blood-vessels were injected With India ink and after ﬁxation the head of the embryo was dissected and the desired portions of it were dehydrated and cleared in wintergreen oil. Figure 4 shows the right labyrinth as seen in such a specimen. The injected vessels in the region of the vestibulo—cochlear junction are shown in solid black and also their continuation into the endolymphatic plexus inclosing the endolymphatic duct. The continuation of the plexus toward the lateral sinus is shown in stipple.
Fig. 4 Camera. lucida drawing showing the endolymphatic plexus and its communications in a. human fetus 54 mm. long (N o. 458, Carnegie Collection). The blood vessels were injected with India ink and the whole rendered transparent with Wintergreen oil. A portion of the plexus was removed to show the contained endolymphatic sac, and part of the sac was also removed in order to show the drainage of the plexus. The vessels of the rest of the labyrinth are only ﬁlled in far enough to show their communication with the endolymphatic plexus. Enlarged about 17 diameters.
In the region of the endolymphatic sac a part of the plexus is represented as cut away. The greater part of the sac is also cut away in order to expose more completely the outer leaf of the plexus, that intervenes between the endolymphatic sac and the sinus, and its characteristic communications with the sinus. The sac is quite ﬂat and when it is intact it corresponds in contour to that portion of the plexus that has been left. The reader will be able to form a picture of the whole apparatus by imagining the rest of the sac back in place and covered in by the inner leaf of the plexus.
From an examination of ﬁgures 2, 3 and 4, we see, therefore, that in embryos about 50 mm. long the endolymphatic appendage consists of a narrow duct that widens out into a broad ﬂattened sac that lies between the chorioidal membrane of the lateral recess and the transverse sinus. It is separated from the former by the dura and is separated from the latter by the endolymphatic plexus. This plexus consists of thin walled capillaries which everywhere inclose the duct and sac. In the distended state, as in injected specimens, they virtually constitute a surrounding sheet of blood inclosed in endothelium, since the openings in the mesh are, as a rule, narrower than the blood channels themselves. There is some tendency at this time, and it becomes more marked later on, to the formation of principal channels in this plexus. The plexus anastomoses centrally with the other blood vessels of the labyrinth. Distally it drains by several openings into the transverse sinus. In addition it anastomoses with a coarser plexus of veins that lies between the dura and the cartilaginous skull in the neighborhood of the sinus. In this same region there are some small arteries of the dura mater that seem to communicate by minute branches with the endolymphatic plexus. There were very few of these and their arterial nature could not be determined with certainty.
Endolymphatic Appendage at End of Fourth Month
The endolymphatic plexus gradually changes its character as we advance to older fetuses. Instead of a fairly uniform meshwork that envelops evenly all parts of the appendage, part of it takes the form of larger and simpler channels that become more or less separated from the remainder of the plexus while the latter continues as a ﬁne meshwork closely applied to the surface of the appendage. The ﬁner plexus drains into the larger channels which in turn drain into the transverse sinus.
In order to determine the topography and vascularization of the endolymphatic appendage at this period, a well hardened fetus, 130 mm. crown-rump length, was selected in which the blood-Vessels had been injected through the umbilical Vein with India ink (No. 1018, Carnegie Collection). The part of the skull on each side containing the labyrinth was removed, care being taken to preserve the dura. The specimen from the right side was dehydrated and cleared in Wintergreen oil and studied as a transparent specimen. The left one was decalciﬁed and cut in serial sections and a proﬁle reconstruction was made of the labyrinth and larger vessels. By combining the reconstruction with the study of the transparent specimen it was possible to ascertain very deﬁnitely the relations of the structure with which we are concerned.
A camera lucida drawing of the endolymphatic plexus and its connecting vessels is shown in ﬁgure 5, as they are seen in the cleared specimen mentioned above. In the same drawing is introduced a proﬁle reconstruction of the endolymphatic appendage prepared from serial sections of the other labyrinth. From an examination of this ﬁgure it will be seen that the endolymphatic appendage is divisible into a duct and a sac. The duct is further divisible into a proximal ﬂaring portion and a narrow portion that connects this with the sac. It can be seen in sections that the proximal ﬂaring portion possesses thin walls that show a tendency to be thrown in folds. The endolymphatic sac consists of a ﬂattened blind pouch with a rounded contour. Microscopic examination shows that its Walls consist of a single layer of cuboidal epithelium which is uniform throughout the sac except at its distal extremity where it narrows into a tubular process whose epithelium retains the embryonic character. In its general topography the endolymphatic sac maintains its former relations and its distal part is found overlapping the dorsomedian wall of the transverse sinus.
On examining the endolymphatic plexus in ﬁgure 5 it will be seen that it has undergone certain changes as compared with the younger stage shown in ﬁgure 4. A vascular plexus still enyelops the appendage everywhere. This consists of a thin walled endothelial networli whose meshes vary in size and pattern and lie closely against the epithelial wall of the appendage. In the drawing only the more prominent loops are shown; besides these there are everywhere small anastomosing capillaries that intervene between them. The network as a whole is richer over the sac and over the-proximal ﬂaring portion of the duct and is more scant over the narrow portion of the duct. Running through the plexus there‘ are a few larger channels that have been separated out. These form main drainage channels that become partially detached from the general plexus, though the latter continues to anastomose with them at frequent intervals. One of these is the so-called ‘vena aquaeductus vestibuli.’ This forms along the borders of the endolymphatic duct. It may be regarded as having a group of tributaries from the remainder of the labyrinth. These are numerically indicated in ﬁgure 5 as follows: ‘3’ is a vein draining the dorsal surface of the utricle from where it curves around at the base of the crus commune to join the endolymphatic system; ‘4’ drains the plexus belonging to the medial wall of the utricle; ‘5’ drains the plexus of the posterior ampulla and the adjacent posterior surfaces of the utricle and saccule; ‘6’ indicates a group of anastomosing vessels from the median wall of the saccule through which it also communicates with the cochlear system. Opposite the narrow part of the endolymphatic duct these Various channels are assembled into two vessels of which the one along the posterior margin of the duct is the principal one, and the one that persists as the V. aquaeductus vestibuli. Tracing it upward we ﬁnd it receiving large tributaries from the plexus of the endolymphatic sac and at the same time enlarging into a wide channel along the caudal margin of the sac. In addition to the tributaries from the endolymphatic plexus it receives several tributaries from the plexus underlying the surrounding dura, such as ‘1’ in ﬁgure 5. It empties into the transverse sinus by one or two openings in conjunction with adjacent dural veins.
Fig. 5 Proﬁle reconstruction of the endolymphatic appendage in a human fetus of 130 mm. crown-rump length (No. 1018, Carnegie Collection). Combined with it is a camera lucida drawing of the endolymphatic plexus, with its connections, made from the other labyrinth of the same specimen which had been cleared in oil. The numerals indicate communications of the endolymphatic plexus with other veins: 1 and 2, dural veins; 3, vein draining plexus on dorsal surface of utricle; 4, from plexus on median surface of utricle; 5, from posterior ampulla and adjacent part of utricle and saccule; 6, veins from median surface of saccule and cochlea. Enlarged 17% diameters.
In describing this plexus and the vena aquaeductus vestibuli it is simpler to think of the blood stream as ﬂowing all in one direction, that is, toward the transverse sinus. In reality it is quite possible that, due to mechanical conditions, the plexus of the proximal part of the duct drains backward into the vessels of the rest of the labyrinth and in common with them through the veins of the cochlear aquaeduct. The natural drainage of the sac, however, is toward the transverse sinus. Under these con ditions the narrow part of the duct is a ‘divide’ from which the blood ﬂows in both directions, and through the same v. aquaeductus vestibuli.
Endolymphatic Appendage in Embryos During Seventh Month
To represent late fetal conditions of the endolymphatic sac, a fetus was selected weighing 948 gms., in formalin, and measuring 240 mm. crown-rump length (No. 1131, Carnegie Collection). The head of the fetus was removed and divided in bilateral halves. On one side a dissection was made exposing the endolymphatic sac which was done by carefully reﬂecting the dura. The form of the sac and its relation to the transverse sinus was found to be essentially the same as that shown in ﬁgure 5, so a drawing of it will not be repeated. On the other side of the specimen the dura was raised in one mass together with all the soft tissues between it and the bone; this included the endolymphatic sac, the periosteal vessels and part of the terminal portion of the transverse sinus. This was then embedded and prepared in serial sections, in a plane longitudinal to the duct and transverse to the sac. A simplified drawing of one of these sections is shown in ﬁgure 6.
In the drawing the endolymphatic sac is shown in heavy black stipple. It consists of a ﬂattened sac embedded in the connective tissue that forms the substratum of the dura. Its distal portion overlaps the dorso-median surface of the sinus as in the previous stage. One new feature is found that was not present in the younger stages and that is that the epithelial wall of the sac projects irregularly in small longitudinal folds apparently thereby offering greater surface area. A characteristic fold of this kind is cut through in the section shown in ﬁgure 6. Such a fold gives the appearance of a double sac but tracing it through the sections shows that it is only an out-pocket whose lumen communicates with that of the main sac.
Fig. 6 Section through the endolymphatic sac showing its relation to the dura and blood vessels in a human fetus measuring 240 mm. crown-rump length (No. 1131, Carnegie Collection). Endolymphatic sac is stippled dark. Blood vessels are shown in plain white. The endolymphatic plexus is more dense on the median or upper surface of the sac; on the lateral or lower surface the plexus is partly replaced by the main channel through which it drains into the transverse sinus. ‘V. d.,’ a large dural vein; ‘Art.,’ artery. The arachnoidal surface of the dura is intact, but the bony surface was torn in the removal of the specimen from the bone. Enlarged 15 diameters.
The dura mater merges gradually into a somewhat loose substratum of connective tissue that attaches it to the bony skull. This is schematically represented in the drawing and the raggedness of the bony surface of the dura is due to the diﬂiculty in detaching the dura from the bone and also in part to the irregularity of the bone. In the meshes of the connective tissue of the dura are found numerous blood vessels which are shown in the drawing as white spaces. The largest of these is transverse sinus. A portion of its wall is missing having been injured in the removal of the dura from the bone. Around the endolymphatic sac is a thick plexus of thin walled veins which apparently is the same as the endolymphatic plexus which we have studied in the younger specimens. At the caudo-lateral surface of the sac they open into a large channel which in turn drains into the transverse sinus. This is the channel that follows along the endolymphatic duct and is known as the vena aquaeductus vestibuli. Other dural veins anastomose with it, but its primary communication is with the venous plexus of the endolymphatic sac. As this specimen did not include the intraosseus portion of the endolymphatic appendage the proximal connections of these veins could not be studied.
From the above study of the endolymphatic appendage in human embryos the principal features in its development, topography and vascularization may be summarized as follows:
The endolymphatic appendage makes its appearance at the dorsal tip of the otic vesicle in embryos about 4 mm. long, whereupon it rapidly enlarges, forming an elongated tube that extends upward toward the chorioidal roof of the hind—brain. As it does this it becomes differentiated into two subdivisions :-the distal half spreads out forming a broad ﬂattened blind pouch, the saccus endolymphaticus ; the proximal half, the ductus endolymphaticus, forms an elongated narrow tube connecting the distal part with the remainder of the labyrinth. The main features in this differentiation are completed in embryos 30 mm. long and at the same time the topographical relations of the appendage have assumed practically the adult conditions.
A prominent factor in the topography of the endolymphatic sac is its relation to the transverse sinus. The characteristic ﬂattened form of the sac and the establishment of the sinus are to be seen at about the same time. From then on the sac always lies with its ﬂat surface applied against the median wall of the sinus, or the dorso-median wall as the base of the skull becomes more ﬂattened out. The sac does not become incorporated With the rest of the labyrinth in the cartilaginous capsule, but like the sinus lies exposed in the ﬂoor of the posterior cerebral fossa and is covered in only by the dura mater.
Throughout the greater part of foetal life the endolymphatic appendage is ensheathed by a vascular plexus, the plexus endolymphaticus, which anastomoses on the one hand with the vessels of the rest of the labyrinth and on the other hand with the transverse sinus into which it drains through several openings.
This plexus makes its appearance at about the time of the differentiation of the appendage into its adult subdivisions of duct and sac. It can be plainly recognized in embryos 30 mm. long. In embryos 50 mm. long, it is Well developed and at that time it forms a closely meshed Web completely investing the appendage, whereby the latter is virtually inclosed in a sheet of blood from which it is separated only by the endothelium of the blood spaces.
In the course of its further enlargement and development in embryos 100 mm. long and over, the endolymphatic plexus becomes resolved into a few principal channels connected with which there remain parts of the original plexus. The plexus persists notably in the neighborhood of the endolymphatic sac.
One of the most constant channels that are developed through the endolymphatic plexus is the one forming the so-called vena aquaeductus vestibuli. This forms along the side of the endolymphatic duct and the posterior margin of the endolymphatic sac, and it constitutes a direct communication between the vascular plexus surrounding the labyrinth on the one hand, and the transverse sinus on the other. It may be a single or multiple channel. Through it is drained the plexus of the endolymphatic sac and also some of the dural veins of the immediate neighborhood.
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Cite this page: Hill, M.A. (2021, April 14) Embryology Paper - The vascular drainage of the endolymphatic sac and its topographical relation to the transverse sinus in the human. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_vascular_drainage_of_the_endolymphatic_sac_and_its_topographical_relation_to_the_transverse_sinus_in_the_human
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