Paper - Pharyngeal end of Rathke's pouch
|Embryology - 27 May 2019 Expand to Translate|
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Pharyngeal end of Rathke's Pouch
By J. Ernest Frazer, F.R.C.S.,
Lecturer and Senior Demonstrator of Anatomy at King’s College, London.
Rathke’s pouch, the upgrowth that meets the infundibular process from the fore-brain, can certainly be considered as derived from the area of the epiblastic stomodaeum in the embryo, though most frequently the buccopharyngeal membrane has given way before the growth can be recognised.
When considering its subsequent fate it can be divided into a cranial and pharyngeal part; an intermediate portion connects these two parts, passes through the growing base of the skull, and usually is the ﬁrst part to disappear.
The fate of the pharyngeal part of the pouch is not deﬁnitely dealt with in the text-books, and the ﬁnal position of its end implanted in the mucous membrane is not, as far as I am aware, accurately known : confusion has arisen between it and other naso-pharyngeal structures, such as Seessel’s pocket, though this -is a purely pharyngeal derivative, associated in some way probably with the notochord (where this comes into relation with the roof of the pharynx in the occipital region). The ultimate situation of the site of Rathke’s outgrowth is of interest when endeavouring to follow the method of development of the several parts of the nose, naso—pharynx, and palate ; and this paper is a contribution to the information available on the matter, founded on observations made while working out the development of these regions.
In addition to the diagram shown, there are models illustrating the position of the structures in the 5th, 6th, 7th, 8th, 9th, and 10th weeks and linear reconstructions of later stages.
At the end of the ﬁrst or beginning of second month the pouch of Rathke forms a canal, with a relatively large lumen, bordered by a thick cell-lining. It opens into, or just in front of, the angle where the roof of the mouth-cavity joins that of the pharynx, and is surrounded by condensed mesenchyme. The condition is shown in fig. 1, in which the notochord can also be seen lying on the roof of the pharynx, above the situation of the second arch and tuberculum impar: the relations of the pouch and notochord to each other and to the pharyngeal cavity are also seen in a model of the part, in which the notochord can be observed to lie centrally between the two Eustachian recesses.
- Eustachian recesses - This term was employed to designate the early state of the haryngeal exclusion that forms the tympanum and tube, in a paper on their early deve opment read before the Anatomical Section B.M.A., Annual Meeting 1910, B.M.J., 15th October 1910.
Fig. 1. Sagittal section, 12 mm. (camera lucida). H., hind-brain; F., fore-brain ; P., pericardium ; M., mouth (placed in front of mandibular arch) ; L., upper part of opening larynx.
The diagrams in ﬁg. 2 are drawn to scale from models, and are intended to illustrate the position of the pouch and notochord in a median sagittal section of each embryo, the anterior end being to the right. The situation of the internal nasal openings is also indicated, and it can be seen that these tend rapidly to assume a higher position, until they come to open on a level with the roof of the pharynx.
In the 6th week the pharyngeal part of the pouch shows ‘a slight lengthening forwards, so that it lies for a very short distance on the roof before becoming continuous with the epithelial lining. This is the first indication of the carrying forward of the “ opening ” of the pouch that goes on continuously in association with the growth of the septum nasi.
In the 7th and 8th weeks the elongation of the pharyngeal portion becomes slightly more marked, and its lower end is only a little distance above the level of the top of the nasal openings, as these have extended upwards. The palatine processes are beginning to meet behind the premaxillae.
By the 9th week the tops of the nasal openings have nearly come up to the level of the roof of the pharynx, and the lower end of the “pouch ” is between them, i.e. it is on the back of the thick nasal septum. The size of the nasal opening has been much diminished by the meeting of the palate folds and their fusion with the septum for a considerable distance, and thus it comes about that the lower end of the diverticulum is implanted in the back of the septum about half way between the levels of roof and ﬂoor of each opening.
In the 10th week, not only have the palate-folds fused far back, and probably also with a little more of the postero-inferior part of the septum, but the upper limit of the nasal opening is now at the level of the roof of the pharynx : as a result, the pharyngeal part of the pouch, considerably elongated, is ﬁxed to the septal edge a comparatively short distance above the palate.
The notochord, where it lies on the roof of the pharynx, maintains its general relation to R_athke’s diverticulum, and at ﬁrst also to the Eustachian recess. As the opening of the recess becomes smaller relatively to the size of the pharynx, this notochord area is found to extend further back than the opening, and, in the 3rd month model, in which atrophy of the posterior part of the recess (tube) has occurred, the line extends a considerable distance back on the roof, though its front end is nearly opposite the upper border of the tube.
These changes, in conjunction with the gradual alteration in the direction of the cavity, lead to the notochord area of the pharynx being placed on the upper part of its back wall, separated by an interval from the region that holds the remains of Rathke’s pouch, running down the back of the septum.
The later arrangement of the parts concerned is well seen in serial sections through the head of a foetus of the latter half of the 4th month ;the intermediate portion of the diverticulum has mostly disappeared in the cartilaginous basis, but can still be found close to the upper and lower surfaces.
Below the base the pharyngeal part is clearly deﬁned as a small strand of cells that runs through the loose tissue above the roof of the pharynx, downwards and forwards, on to the back of the nasal septum, Where it lies near the free edge, and becomes continuous with the epithelial lining just above the level of the palate (ﬁg. 3). This may, I think, be taken as showing the ﬁnal situation of the pharyngeal remnant of the pouch.
The notochord can only be followed with diﬁiculty at this stage, but the position of the point where its front end runs into the base is shown in the ﬁgure: it lies on the roof below this, and re-enters the base much lower down than the drawing extends.
At still later stages, my sections have not been satisfactory so far as concerns the pharyngeal remnant: these sections were made from old dissecting-room subjects, hardened by formalin, and it may be that fresh specimens would give more deﬁnite results.
In the 7th month the upper part of the pharyngeal remnant of the pouch was found, but it could not be traced down, and in a foetus at term it was not possible to locate the cell-strand definitely.
Fig. 3. The mucous, membrane of the right nasal opening is shown cut, marking the upper limit of the opening. Traced from a linear reconstruction. x 18. 4 months.
In an adult septum the posterior soft part was examined, and a cystic dilatation of a strand of degenerate cells was found at the upper end, hardly on the free septum : the cyst was lined by somewhat cubical cells, and was continued down for a little distance as a cell-tract, and was then lost. This may represent a dilatation of the remnant: certainly the deﬁnite cylinder of degenerated cells that ran up from it was, to my mind, very suggestive of such an explanation.
There are other structures in ‘this free edge of mucous membrane deserving a little attention. In the lower half of the edge there is a deep linear groove or sulcus running down the free edge nearly to the palate, lined by columnar cells, and having the openings of various gland‘-ducts in it at intervals,—one very large medial mass opened into its lower end.
The signiﬁcance of this groove—if it has any value—is very doubtful; it is not visible, or even suggested, in the 4th month sections, but it is present in part in those of the later months, but in these no connection could be traced between it and the upper remnant.
The structures I have mentioned may only be individual variation, and the same explanation may perhaps account for a couple of minute cartilaginous nodules also present in this septum.
One of these is situated on the left side, close to the free margin, near the top of the groove, and is a long nodule, 1.5 mm. by about 0.25 mm. in size. The other is a shorter and thicker particle, lying lower down and much further forward on the right-hand side of the back border of the vomer; it is about 1 x 5 mm. in size. Both nodules are embedded in ﬁbrous tissue, and their appearance suggests that they are formed by chondriﬁcation in that tissue.
I have found no indication of them in any earlier specimens.
- They were not found in another adult septum : this specimen had no dilatation of the large strand of cells in its upper part, and the groove in the lower half was not at all well marked.
A few words may be said about the appearance of the strand of cells at different stages. The intermediate portion of the pouch lies in the sixth week in a large space in the cellular basis cranii, is distinctly compressed by the forming cartilage in the seventh week, and the nuclei of its cells in the eighth week stain very feebly. It seems to increase in size a little after this, but has partly disappeared by the fourth month, but I presume it may remain occasionally, and occupy the canal sometimes found in the bone. The pharyngeal part is enlarged slightly immediately below the base, then gets smaller, and enlarges again in the earlier stages before reaching its end. In the later stage the portion that lies above the pharynx is drawn out and smaller, and the lower enlargement is at the top of the septum, and in the fourth month another narrow strand is continued on from this down the septum.
The upper part of the strand has its cells radially arranged round a possible very minute lumen, but this arrangement cannot be made out in the lower part, at any rate in the older stage. It is surrounded by a vascular network, that increases in size as growth proceeds.
The object of this communication can be shortly summed up by saying that an endeavour has been made to show the ﬁnal position of the pharyngeal part of Rathke’s point to be along the back of the nasal septum, with the site of its original opening just above the palate. It has attained this situation largely by the upward change of level of the nasal openings, but also, without doubt (as shown by its elongation forward), as a. result of some traction exercised on it during and after the second month.
The traction is probably due to wedges of ‘cell-masses that grow inwards and upwards on each side of the pouch from the maxillary mesenchyme: they carry vessels and nerves with them, and seem to form the prominence of the back part at least of the median elevation between the nasal capsules. The two processes meet round the pharyngeal stalk of the pouch, and seem to draw it forward in their downgrowth. I have not as yet followed these processes in detail, and do not propose to enter further into their history; they can be seen in the earlier part of the second month, and seem to be continuous with the laterally-placed maxillary mass, but their value may not be What I suppose, and I mention them here because they appear to me to be responsible partly for the carrying forward of the pharyngeal end of the diverticulum.
Cite this page: Hill, M.A. (2019, May 27) Embryology Paper - Pharyngeal end of Rathke's pouch. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Pharyngeal_end_of_Rathke%27s_pouch
- © Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G