Paper - The Development of the Pars Membranacea Septi in the Human Heart
Embryology - 6 May 2024    Expand to Translate
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Odgers PN. The Development of the Pars Membranacea Septi in the Human Heart. J Anat. 1938 Jan;72(Pt 2):247-59. PMID 17104688
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The Development of the Pars Membranacea Septi in the Human Heart
By P. N. B. Odgers
From the Department of Human Anatomy, University of Oxford
THE accounts which have been published of the development of this region of the heart, the method in which the interventricular foramen is occluded to form the membranous part of the septum, or whether indeed this occurs at all, have been so vague or so contradictory that a re-examination of the question seemed worth while.
For this investigation I was able to study human embryos of 11.2, 11.4-, 12.5, 14, 14.5, 15.5, 17, and 17.5 mm. in crown-rump length. Of these the 14 mm. embryo was the least valuable for my present purpose, as the heart is over-distended with blood in this specimen. I have therefore not included it among the detailed descriptions which follow. Reconstruction models were made of this region in the 11.2, 11.4, 14.5, 15.5, and 17 mm. specimens.
11.2 mm Embryo
At this stage (Text-fig. 1) the two bulbar ridges (Frazer) or the proximal bulbar swellings (Tandler) are seen united above to form the bulbar septum, but soon separating below into right and left moieties. The right one (R.B.R.) can be traced down on the right ventricular wall to become continuous with the anlage of the right or inferior cusp of the tricuspid valve, while the left one (L.B.R.) joins the right side of the interventricular septum and on that soon disappears. Where they are fused and for a few sections after their separation the two ridges are seen to be of equal size, but at a. lower level the right one soon appears to be not only the more extensive but the more substantial of the two.
The interventricular foramen (I. V.F.) is seen in thirty sections, i.e. its cranio-caudal diameter is 300 p.. It is bordered ventrally by the inter ventricular septum (I. V.S.) and dorsally by the fused auriculoventricular cushions (hereafter abbreviated into the A.-V. cushions). Above, the dorsal (cranial) border of the interventricular septum joins the fused A.-V. cushions about their centre, while below it joins their right margin.
Below the caudal border of the foramen the septum is fused to the inferior A.-V. cushion in fourteen sections (i.e. for 140 p.). This must presumably represent the amount of closure of the interventricular foramen which has already occurred since about the 7 mm. stage. It may be noted here, however, that in our 7 mm. embryo, where the interventricular septum is represented by a
low ridge which does not yet make contact with the A.-V. cushions, the cranio-caudal -measurement of the interventricular foramen is only 220 μ in contrast
to 300 μ in the specimen under consideration. This illustrates how diflicult it
is to compare accurately the condition of any single region in different embryos.
There cannot be in the nature of things any constant initial size of foramen to
be obliterated. Secondly, the foramen may not be diminishing steadily with
the age of the embryo. Rather is it reasonable to suppose that it shares in the
general growth and that its gradual closure is a compromise between two
competitive processes.
Text-fig. 1. Drawing of a model of this region of the heart in an 11-2 mm. embryo ( x 100) after removal of the ventral wall of the bulbus and right ventricle and of ahnost the whole of the interventricular septum, viewed from the right side. R.B.R. right bulbar ridge; L.B.R. left bulbar ridge; R.S.T. right superior tubercle of the A.-V. cushions; R.I.T. right inferior tubercle of the A.-V. cushions; I. V.S. interventricular septum; I . V.F. interventricular foramen; R.A.V. right auriculoventricular orifice; A.0. aortic channel; P.A. pulmonary artery.
It is interesting to observe the method of closure here. In the three sections
immediately below the foramen this is seen to be taking place by a narrow
tongue—shaped proliferation of the right border of the A.-v. cushion making
contact with and extending Ventralwards on the right side of the septum
(Pl. I, fig. 1). From the left margin of this proliferation contact with the septum
gradually broadens, but there are ten sections below the foramen before the
dorsal border of the interventricular septum and the A.-V. cushion are joined.
The right and left edges of the fused A.-V. cushions are seen to project as
prominent swellingsto form the medial boundaries of the right and left
auriculoventricular orifices respectively. Further, in both these swellings
superior and inferior tubercles can be distinguished corresponding presumably to the original dorsal and ventral components of the fused cushions. Over the
upper border of the right superior tubercle (R.S.T.), which is well cranial to the
upper limit of the right auricular opening, the aortic channel passes to gain the
left ventricle through the upper and dorsal portion of the interventricular
foramen above the level of the lowest attachment of the left bulbar ridge to the
interventricular septum.
11.4 mm Embryo
I suspect that this embryo is older than its crown-rump length suggests. There is no record of its length before fixation, and in estimating this from an enumeration of the sections I expect an insuflicient allowance was made for shrinkage. The model (Text-fig. 2) begins below the level of the bulbar septum and the two bulbar ridges appear quite separated. The left one (L.B.R.) is seen, as before, to join the interventricular septum well to the right side and ventral to its dorsal border. On this it soon disappears. The right ridge (R.B.R.) is the more prominent of the two and, as it is traced caudalwards, is found to become continuous with the projection of the commencing anterior cusp of the tri- cuspid valve. Between the two bulbar ridges are seen the marked elevations of the right tubercles of the fused A.-V. cushions, and now the separation between the superior (R.S.T.) and inferior (R.I.T.) is much more pronounced than in the earlier embryo.
Text-fig. 2. Drawing of a model of this region of the heart in an 11-4 mm. embryo ( x 100) a. ter
removal of the ventral wall of the right ventricle and the greater part of the interventricular
septum, viewed from the right side. R.B.R. right bulbar ridge; L.B.R. left bulbar ridge;
.R.S.T. right superior tubercle of the A.-V. cushions; R.I.T. right inferior tubercle of the A.-V.
cushions; I . V.S. interventricular septum; I. 17.1‘. interventricular foramen; R.A. V. right
auriculoventricular orifice.
The relation of the right bulbar ridge to these tubercles must be described in some detail. While superiorly the two structures are quite separate, farther down the left margin of the right bulbar ridge gradually joins the right side of the lower portion of the superior and the upper portion of the inferior tubercle. In this way a part of the bulbar cavity is cut off and is found to be continuous below with that of the right auricle. This must mean that the right bulbar ridge has grown across the upper end of the right auriculoventricular orifice (R.A. V.) to meet the right A.-V. tubercles. In conformity with this, while the latter orifice measures 190 p. in its vertical extent in the 11-2 mm. embryo, it is now only patent for 40 μ. The interventricular foramen (I. V.F.) is very much smaller than in the earlier specimen. It is now only seen in eight sections, i.e. its craniocaudal diameter is only 80p.. Caudal to its lower margin the ventricular septum and the inferior A.-V. cushion are fused for 320 μ in contrast to 140 μ in the first embryo. Further, a comparison with the 11.2 mm. specimen shows that the foramen is not only less extensive but is more obviously divided now into two parts. That portion of it cranial to and dorsal to the lower end of the left bulbar ridge is certainly forming the aortic channel, while the caudal and ventral part of it is being gradually obliterated. The closure of the foramen can be observed to be taking place here by the proliferation of the inferior tubercle in a cranial direction and it is this proliferation which makes the sulcus, separating the two tubercles, so much more pronounced (Pl. I, fig. 2). The microphotograph is that of the fourth section cranial to the foraminal closure and shows the rapidly extending cushion tissue bare of endocardium in contrast to the interventricular septum, which is still covered by it.
14.5 mm Embryo
(Text-fig. 3).
The bulbar septum is now very much more extensive and forms the cranial border of the still patent interventricular foramen. The junction between its two component ridges can be distinguished as a more deeply staining seam, and by tracing this down it can be seen that it is the left bulbar ridge (L.B.R.) which joins the interventricular septum, as it does in the earlier embryos, but now it does so practically at its dorsal border and nothing like so much to its right side compared with the 11-4 mm. specimen. It soon fades out on the septum and does not appear to contribute any cushion tissue to it or to take any part in closing the foramen. The right bulbar ridge (R.B.R.), as it is traced caudalwards, is much the more extensive of the two. Just above the level of the interventricular foramen its left border is seen to be forming the greater part of the right wall of the aortic channel, while its right border bulges out prominently to form the anterior cusp of the tricuspid valve. Projecting into the aortic channel (A.0.) from its dorsal wall is the superior portion of the right tubercle of the A.-V. cushions (R.S'.T.) ; it is impossible now to define accurately the division between superior and inferior tubercles. The interventricular foramen (I.V.F.) is patent in four sections (20 μ in thickness). It is bounded now cranially, as has been said, by the bulbar septum, caudally by A.-V. cushion tissue, on its dorsal border by the right tubercle of the A.-V. cushions (R.S. T.) and on its ventral border by the interventricular septum (I. V.S.). The microphotographs show how the foramen is being closed. In P1. I, fig. 6, cushion tissue is seen proliferating from the A.-V. cushion, while a small disk-shaped mass of similar tissue caps the interventricular septum to the right of its dorsal border. In the next sections below this one this proliferation becomes more extensive until it reaches and fuses with the tissue capping the septum (Pl. I, fig. 7), so that the succeeding sections show a single fused mass joining the A.-V. cushion to the septum.
Text-fig. 3. Drawing of a model of this region of the heart in a. 14-5 mm. embryo ( x 100) viewed obliquely from above and from the left side. R.B.R. right bulbar ridge; L.B.R. left bulbar ridge; R.S.T. right superior tubercle of A.-V. cushions; I. V.S. interventricular septum; I . V.F interventricular foramen; A.0. aortic vestibule; RA .0. bulbo-auricular channel.
The only interpretation which can be put on these appearances is that proliferating A.-V. cushion tissue is growing up from below on the ventral (septal) boundary of the interventricular foramen. These facts are shown in Text-fig. 4, which is a diagrammatic enlargement in silhouette of this region.
Further, P1. I, fig. 7, shows that just at the caudal boundary of the foramen
the left border of the right bulbar ridge is joining the tissue proliferating from
the A.-V. cushion rather than the cushion itself as it was seen to do in the
11-4. mm. embryo. In either case, however, this junction is cutting off a bulbo-auricular channel which is far larger in thepresent specimen than in any of my others and which still communicates inferiorly with the right auricle.
12.5 mm Embryo
In this specimen, while the interventricular foramen is more extensive, 180;; in its vertical diameter, in some particulars the development of this region is more advanced. The right bulbar ridge and the superior tubercle of the A.-V. cushions, which seems to be poorly developed here, have blended above. It is not until the level of the first section which shows the closure of the interventricular foramen that a narrow cleft is seen to be separating the two (Pl. I, fig. 3). When this narrow cleft is traced caudalwards through the succeeding four sections (40 p) it is found to communicate with the right auricle and this is all that is seen of the much larger bulbo-auricular channel of the 14-5 mm. embryo.
A microphotograph of the next section below the first one (Pl. I, fig. 4-) shows that the right bulbar ridge is in close coaptation at once with the right tubercle of the A.-V. cushions, with cushion tissue capping the interventricular septum, and with cushion tissue connecting them.
Text-fig. 4. A. 17.0. A.-V. cushions from which tissue is proliferating to fill up from below the inter-ventricular foramen, which is black, and to climb up on the dorsal border of the interventricular septum, I . V.S. R.B.R. right bulbar ridge joined with the left bulbar ridge, L.B.R., to form the bulbar septum and the cranial boundary of the foramen. R.B.R.’ is the lower margin of the right bulbar ridge at its junction with the proliferating A.-V. cushion tissue.
In the next three sections the connexion between the right tubercle and the
cushion tissue capping the septum gradually seems to be more and more pronounced. They are still demarcated, however, from each other by a distinct
notch on either side (Pl. I, fig. 5). This notch becomes indistinct in the course of
the next few sections and then the A.-V. cushions appear as a uniform mass in
contact with the interventricular septum. The origin of the horn-shaped cap
of cushion tissue on the interventricular septum (Pl. I, fig. 3), which can be
recognised in the three sections cranial to this level, might be thought to be
doubtful from the appearances presented by the microphotographs (P1. I,
figs. 3-5). In this series it is seen to be in contact with the right bulbar ridge
10 μ rostral to the level at which it is connected with the right A.-V. cushion,
but two facts make its origin from the latter certain. As it is traced caudal-wards it obviously increases in size, which would surely be impossible if it
were a derivative of the bulbar ridge. Further it is found to actually fuse with
the right A.-V. cushion, while it is simply in coaptation with the right bulbar
ridge, which does not appear to contribute anything to its formation.‘
I think therefore it can be truly regarded as having proliferated from the A.-V. cushions and the appearances presented here thus fall into line with those exhibited in the previous, 14-5 mm., specimen.
15.5 mm Embryo
As in the 14-5 mm. embryo, the bulbar septum is seen here to form the cranial boundary of the interventricular foramen. The seam between its two constituents is distinct here, and the left bulbar ridge fuses with the dorsal border of the interventricular septum (Pl. II, fig. 8) and is contiguous with the cushion tissue which caps it (Pl. II, fig. 9). The foramen is now 30 ft in its craniocaudal diameter.
Above the level of the interventricular foramen the right bulbar ridge and
the right superior tubercle of the A.-V. cushions are completely fused, and it is
only at the caudal border of the foramen that a cleft appears which separates
them (Pl. II, fig. 10). While this cleft separates the right bulbar ridge from the
A.-V. cushion, it still permits the former to meet the cushion tissue which has
proliferated to fill the foramen. As it is traced caudalwards the cleft gradually
becomes wider and ends in the posterior portion of the right auriculoventri-
cular orifice. The condition found here is almost exactly similar to that seen
in the 12-5 mm. specimen and represents the remains of the bulbo-auricular
channel of the earlier embryos, but owing apparently to a gradual extension
cranialwards of the tricuspid orifice, the cleft is more correctly described now
as communicating with that orifice rather than with the auricle.
In the left wall of the cleft immediately posterior to the junction of the
right bulbar ridge and the cushion tissue capping the interventricular septum,
a notch may be seen. This notch can be seen in seven sections (70 p.) caudal to
the interventricular foramen and is in continuity above with its lower margin.
It represents without doubt, I consider, the partial separation of the A.-V.
cushion from the cushion tissue proliferated from it and is in the same situation
as the notches referred to above in my description of the 12-5 mm. specimen.
17 mm Embryo
At this stage it is becoming less easy to distinguish the different masses of cushion tissue, the bulbar septum and the A.-V. cushions.
The interventricular foramen is just completely closed, although in one section this is only effected by two strands of single cells, while in the section next below this the foramen is closed on either side with its central part still unobliterated. In both these sections the closing foramen appears as a cleft between two masses of cushion tissue. Above this level, as in the 15-5 mm. embryo, the right bulbar ridge and the superior tubercle of the A.-V. cushions are completely fused. At the level of the closing foramen, While the)‘ are still connected, they can be distinguished from each other, but in the next section below this a deep cleft divides them, as the right bulbar ridge separates off to enter into the formation of the right cusp of the tricuspid valve. For six
Cite this page: Hill, M.A. (2024, May 6) Embryology Paper - The Development of the Pars Membranacea Septi in the Human Heart. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_Development_of_the_Pars_Membranacea_Septi_in_the_Human_Heart
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