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  Tympanic Membrane and Embryonic Disc — Comparable Aspects
  

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By: Arne Enge
Original title: Trommelfell and Embryonalscheibe - vergleichende Aspekte. Der Merkurstab 1996; 49: 375-82. English by A. R. Meuss, FIL, MTA.

Introduction
Discussing main lessons in embryology and anatomy with student teachers and eurythmists in Oslo I became aware of the relationship between the embryonic disc at 2 weeks and the tympanic membrane. As far as I know, no one has so far referred to this. I believe it can open up important and fascinating prospects for our whole understanding of the human being. It may be that the connection between the powers of growth and development that come into play in embryonic development and the sounds and tones we hear is closer than we think, and it is also possible that this will broaden our understanding of pathogenesis.

The embryonic disc
Implantation in the uterine wall is complete at about two weeks after fertilization. The structure that will later be the whole human form is a thin, flat disc at this stage, c. 1.5 mm in diameter and only a few cell layers in thickness (Figs 1, 2a).


Above is the "amniotic cavity" which will later surround the whole form, now cylindrical. On the underside lies the yolk sac. These spaces are enveloped by the developing placenta, in terms of volume the dominant part of the original fertilized ovum. The top of the disc consists of a layer of columnar epithelium, the ectoderm. On the yolk-sac side a layer of cubic cells forms the endoderm. Between these layers lies the mesoderm, a thin layer of loosely organized cells. (The ectoderm will later develop into skin, sense organs and nervous system, the mesoderm into muscles, connective tissue, skeleton, heart, blood vessels and blood cells, the endoderm into the whole digestive system with the different internal organs. In other words, the threefold human being is emerging, as also noted by Karl Koenig and others.)

At this point is it of special interest to note that the external meatus, with its connection to the skin, derives from the ectoderm, whilst the Eustachian tube and tympanic cavity evolve from the direction of the pharynx and are endodermal in origin.

During progressive growth and differentiation in weeks 3 and 4 the emphasis in terms of size is on the ectoderm as the central nervous system is established. A number of authors, Blechschmidt 1 among them, have shown that many essential form processes are stimulated by the ectoderm. Examples are the development of the CNS, growth of the ectoderm as the disc curves, early development of the extremities, and much more. It seems as if generative forces "pour" from the amniotic cavity through the ectoderm into the disc. In a discussion with physicians R. Steiner spoke of a special connection between amniotic cavity and ether body.

Seen from the amnion, the disc is almost circular 15-18 days after fertilization (Fig. 2a), gradually becoming more oval. The primitive streak (Fig. 2b) marks the future longitudinal axis. It extends from the narrow posterior pole to about the center of the disc, to the primitive knot, and then continues in the mesodermal layer as dorsal chord, terminating in the prechordal plate at the broader anterior pole of the disc (Figs 2b, 3a).

This tenuous structure determines all further growth and differentiation of the disc. We might say it shows the way for later formative processes.

Development of the primitive streak and dorsal chord give the originally circular disc a definite shape. Now there is not only above and below, but also anterior and posterior, left and right. Whilst still an open canal connecting with primitive streak and amniotic cavity, the chord penetrates to the yolk sac on its way through the mesoderm. This creates an interesting short- term open connection between amnion and yolk sac, the neurenteric canal (Fig. 3b).


Not only the chord develops, going from primitive streak to mesoderm. The primitive streak also induces the retrograde and lateral migration of mesenchymal cells (Figs 4 a,b). The somites and the lateral mesenchyme evolve from those cells, among other things. They in turn give rise to the spine and the greater part of the musculature of back, rib-cage and abdominal wall. Many aspects of mesenchymal development are still unknown. Here it is of particular interest that the primitive streak and later the chordal process induce the development of central parts of the locomotor system and the human form. The CNS, which also plays a major part in shaping our outer form, appears to take its orientation from these structures, with the neural tube developing parallel to the primitive streak and chordal process.




The tympanic membrane
The tympanic membrane or eardrum is many times the size and thickness of the embryonic disc, but the similarity of form and structure is remarkable. The tympanic membrane has an ectodermal outer and an endodermal inner aspect, and is the area of contact between external auditory meatus and tympanic cavity or middle ear.

Four branchial (pharyngeal) arches develop by the end of the 5th week. Meckel's cartilage, malleus and incus develop from the first arch, the hyoid and among other things part of the pharynx from the arches posterior to it. Between these arches lie the branchial grooves, which gradually disappear almost completely. The primordium of the external meatus derives from the laterodorsal part of the first branchial groove. As the head grows it comes to lie more and more deeply inside it. For a time, the meatus is almost completely filled with loose ectoderm, the meatal plate. This disappears between the 5th and 7th months, and the meatus is then open to the amniotic fluid. The base of the meatus is the ectodermal side of the tympanic membrane which thus is in touch with the amnion, as is the ectodermal aspect of the embryonic disc.

The other parts of the ear develop inside. Let us consider the embryo-genesis of the middle ear. The first pharyngeal pouch narrows laterally, with two narrow canals developing to the left and the right. These go in the direction of the ears and become the Eustachian or auditory tubes on either side. The beginnings of malleus, incus and stapes are already present in a more lateral position. The auditory tube widens into the tympanum as it approaches them.

The endodermal mucosa of the oral cavity lines the canal and gradually also the ossicles and the inner aspect of the tympanic membrane (Figs 5 a & b). As a result the membrane has an endodermal inner layer and an ectodermal outer layer. (The original columnar epithelium of the outer layer is, like the skin, later transformed into squamous epithelium.) Between the two layers lies a mesodermal layer, 3 as in the embryonic disc, but in this case consisting of circular and radial connective tissue and fine blood vessels. The shaft of the malleus is anchored in this layer.

Further similarities
Looking at the tympanic membrane through an otoscope we see further similarities with the embryonic disc seen from above (Figs 6 & 7). The membrane is oval but not entirely symmetrical. The shaft of the malleus can be seen to stand out above the surface longitudinally if we angle the otoscope slightly. It terminates in the umbo. The similarity to embryonic disc, primitive streak and primitive knot is quite striking (Fig. 7).


Sounds reach the tympanic membrane through the external meatus, setting it in motion. The movement is concentrated by the shaft of the malleus and conducted via malleus, incus and stapes, as if through a funnel. Here we have a fascinating parallel to form-giving principles that are poured into the disc from the ectodermal aspect, with the primitive streak and the chord acting like the shaft of the malleus. The oscillations are conducted from the tympanic membrane to the ossicles, whereas in the disc they actually create forms and organs, with the "shaft of the malleus" in the disk pointing the way.

Here the Gospel of John comes to mind: "In the beginning was the word, ... and the Word became flesh and came to dwell among men."

The ENT specialist A. Tomatis has given many interesting details concerning the significance of hearing in pre- and postnatal development. 4 He says, for instance, that the developing human form is like "a shell for a listening creature", and that the most important dimension in the whole of human development is to "listen for the sound of life (in its widest sense)." Doesn't the similarity in the structure of tympanic membrane and embryonic disc reflect the same thing?

Effects on the senses
The relationship can also be seen the other way round, i.e. not the embryonic disc as a tympanic membrane but the tympanic membrane as a persisting embryonic disc. Tomatis has done experiments that show the important role received sounds have for the development and function of the brain. R. Steiner states emphatically that anything which influences us via the ears and other sense organs has an important nourishing and anabolic function even for the internal organs. 5 Some of this has already been established for the eyes, particularly by the work of the ophthalmologist F. Hollwich. 6 Light and dark (and probably colors as well) influence the endocrine glands, for instance, and their profound effects.

We have less information on this where the ears are concerned, but more will no doubt become known as time goes on. It may be that one day similar relationships will be discovered between eyes and endocrine glands. One might also think that the relationship between hearing, breathing and the movements of cerebrospinal fluid have significance. Then we may also ask ourselves what function hormones have in the CSF. Such relationships can give us new impetus in recognizing the importance of working with speech and music. We begin to get an inkling of the efficacy of a number of therapies where speech and music are used in some way. A less pleasant aspect is the question as to the effect of "noise pollution" from all kinds of sources in today's environment, for we are increasingly exposed to this from conception to our last breath.

Clinical aspects
In conclusion, we may consider R. Steiner's frequent references to cancer as an ear developed in the wrong place. 7 We have seen the tympanic membrane to be a structure that "arises anew" in a "roundabout way" (external meatus, meatal plate, Eustachian tube, tympanic cavity) as an organ that is embryonic in character. The outer ear with its cartilage and the inner ear with cochlea and labyrinth may also be seen as structures tending in an embryonic direction. 8 Embryonic form principles enter into the sphere of the ear in a number of ways. Does this show a connection with R. Steiner's statements concerning cancer and ear development? Neoplastic tumors characteristically show cells of an embryonic cast in tissues that originally were highly differentiated.

The aim of this paper was to draw attention to the tympanic membrane as a kind of embryonic disc, though the latter does not come into existence but lets form principles enter into the configuration. On the other hand the embryonic disc may be seen as a tympanic membrane for the creative World Word.

1 Blechschmidt E. Sein and Werden. Stuttgart: Urachhaus 1982.
2 Steiner R. Medical Discussions (in GA 314). Dornach, 21-23 April 1924. Tr. not known. MS translation R92 in Rudolf Steiner House Library, London.
3 Schmidt & Hellstrom. Tympanic membrane structure. ORL 1991; 53: 32-6.
4 Tomatis A. Der Kiang des Lebens. Rowolt 1987.
5 Steiner R. Three Lectures to Doctors (in GA 314). Dornach, 31 Dec. 1923. 1924. Tr. R. Mansell. Long Beach CA: Rudolf Steiner Research Foundation 1990.
6 Hollwich F. The influence of light on metabolism in man and animal. Thieme.
7 Steiner R. Three Lectures to Doctors (in GA 314). Dornach 1 Jan. 1924. Tr. R. Mansell. Long Beach CA: Rudolf Steiner Research Foundation 1990. Steiner R. What can the Art of Healing Gain through Spiritual Science? (in GA 319). Arnhem, 21 & 24 July 1924. Tr. G. Karnow. Spring Valley: Mercury 1986.
8 Schad W. Die Ohrorganisation. Goetheanistische Naturwissenschaft. Stuttgart: Freies Geistesleben 1990.

Other works consulted
Hinrichsen. Human-Embryologic. Springer 1990.
Starck. Embryologic. Thieme 1975.





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