An experimental approach to the development of the ear capsule in the turtle, Chelydra serpentina

Transcription

1 /. Embryol. exp. Morph., Vol. 13, Part 2, pp , April 1965 Printed in Great Britain An experimental approach to the development of the ear capsule in the turtle, Chelydra serpentina by M. J. TOERIEN 1 From the Department of Anatomy, University of Stellenbosch INTRODUCTION THE effects of the elimination of the otic placode or otocyst on the development of the cartilaginous ear capsule and columella auris have been observed in several fishes, amphibians and birds and are summarized by de Beer (1937), Yntema (1955) and Benoit (1957). More recent work includes that of Benoit (1960) and Toerien (1963). Except in the fish Acipenser (Filatow, 1930) the development of a cartilaginous ear capsule is completely suppressed following the removal of the otocyst. The effect of the operation on the development of the columella is not so clear and the resultant deficiency in the medial aspect has been interpreted in various ways. Yntema's (1965) method for using turtle eggs has made the early stages of reptilian embryos available for experimental studies. The purpose of the present investigation, therefore, is to study the effect of the removal of the placode on the development of the auditory region in yet another vertebrate class. MATERIAL AND TECHNIQUES The eggs of the snapper turtle {Chelydra serpentina) were obtained from two sources: (1) Some females were collected under Prof. Yntema's supervision from the shores of small lakes in the vicinity of Syracuse, N.Y., and (2) some were obtained from the Lemberger Co. of Oshkosh, Wisconsin. The eggs and embryos were handled according to the methods devised by Yntema (1965). The relevant series of experiments consisted of the complete removal of the right auditory placode by means of glass needles. Embryos containing twelve to twenty pairs of somites were used. The operations on the 1 Author's address: Department of Anatomy, University of Stellenbosch, Stellenbosch, South Africa. 10

2 142 M. J. TOERIEN younger embryos are relatively easy but in the older ones the head is covered by the amniotic fold and this presents some difficulties. Successful operations on the otocysts of the 20-somite embryos were carried out by means of two techniques. (1) The otocyst was approached by pulling the fold forwards over the head, and (2) by incising the membrane over the placode. After removal of the otocyst the embryos were incubated at 30 C. for 30 days. This is about half the total incubation time at this temperature. Serial sections of the heads, 16/x in thickness were prepared and were studied by means of graphic reconstructions. RESULTS AND DISCUSSION Of the fourteen specimens investigated, half contained no vestige of a true ear capsule on the operated side. In the other half the ear on the operated side was small and incomplete. In these latter specimens the pedicle of the otocyst appears to have been incompletely removed. In addition to the snapper turtles, one painted turtle embryo (Chrysemys pycta) underwent the same operation and also showed the complete absence of an auditory capsule. The stage of the embryos used in this experiment does not appear to affect the outcome of the operation. In all age groups similar results were obtained. The elimination of the ear capsule affects the configuration of the chondrocranium in the following ways: On the operated side the (1) pro-otic window (incisura antotica) is considerably larger, (2) part of the floor of the ear capsule is retained, (3) cranial nerves VII, XI and X do not pass through foramina after leaving the brain, (4) the eighth cranial nerve and membranous labyrinth are absent, (5) an isolated metotic cartilage is always present, and (6) the footplate of the columella is deficient. A few of these points need some elaboration. The floor of the auditory capsule The part of the floor of the capsule immediately posterior to the anterior acoustic foramen appears to be of basal plate cartilage (Kunkel, 1911, 1912b). This region develops in spite of the absence of the rest of the capsular walls (Text-fig. 1). This would also mean that the ventral rim of the fenestra ovalis is formed by the basal plate (Text-fig. 2). The more lateral part of this floor plate is usually partly separated from the rest of the skull base on the operated side. This deficiency in the plate is found dorsal to the internal carotid artery, posterior to the facial nerve ganglion (geniculatum) and ventral to the medial tip of the columella. A deficiency also occurs in some specimens on the unoperated sides and the cartilage lateral to the deficiency seems to represent Kunkel's (1912a) 'stapes inferior'. This author, and also de Beer (1937), raised the question of the possible

3 Development of the ear capsule in Chelydra 143 homology of this structure with the operculum of amphibians. This is unlikely since the operculum in amphibians is almost certainly of capsular origin and, with the capsule, fails to develop after extirpation of the otic placode (Luther, 1927). The posterior boundary of the basal part of the capsule floor is more difficult to ascertain but it probably ends at the anterior edge of the metotic fissure, i.e. at the Aud. cap. VIII Ant. for. Endol. D. VI For. VIII Post, for, Column, f.pl. IX Fl. rec. sc. tymp. X Hypoper. Occ. ar. Hyp. TEXT-FIG. 1. Dorsal view of graphic reconstruction of posterior part of skull of Chelydra serpentina. Roof of ear capsule, occipital arches and lateral parts of columellas removed. Aud. cap., auditory capsule; Colum. f. pi., columella footplate; Endol. D.,endolymphatic duct; Fl. rec. sc. tymp.,floorof recessus scalae tympani and roof of hypoperiotic space; Hyp., hypglossal nerve; Hypoper., hypoperiotic space; Occ. ar., occipital arch; VI For., foramen for abducent nerve; VII, facial nerve; VIII Ant. for., anterior foramen of auditory nerve; VIII Post, for., posterior foramen of auditory nerve; IX, glossopharyngeal nerve; X, vagus nerve. level where the glossopharyngeal nerve leaves the cranium. In addition to the recessus scalae tympani, the metotic fissure in the turtle is occupied by several structures which obscure the junction between basal plate and capsule: (1) A bar of cartilage between the basal plate and ear capsule separates the glossopharyngeal nerve from the medial opening of the recessus scalae tympani, and (2) a 'hypo-perilymphatic' duct (Nick, 1912) is developed as a posteroventral

4 144 M. J. TOERIEN diverticulum of the periotic duct. The cartilaginous walls of this diverticulum not only separate the glossopharyngeal nerve from the lateral opening of the Aud. cap. Quad VII Gang Column. Bas. pi. A. car. int. Quad VIII Post br. - VII Hyom. br. Colum. Bas. pi. A. car. int. TEXT-FIG. 2. Cross-section through skull of Chelydra serpentina. A. car. int., internal carotid artery; Bas. pi., basal plate; Colum., columella auris; Quad., quadrate; VII Gang., geniculate ganglion; VII Hyom. br., hyomandibular branch of facial nerve; VIII Post. Br., posterior branch of auditory nerve. Other abbreviations as in Text-fig. 1. recessus scalae tympani but also the periotic canal partly from the hypoperiotic duct (Text-fig. 1). According to de Beer (1937) the cartilaginous pillar between the internal glossopharyngeal opening and the medial opening of the recessus scalae tympani TEXT-FIG. 3. Lateral view of graphic reconstruction of posterior part of operated side of Chelydra skull showing the absence of cartilaginous bars between the 9th and 10th cranial nerves. Hyp. for., hypoglossal nerve foramina; Occ. ar., occipital arch. Other abbreviations as in previous text-figures. is an upgrowth of the basal plate (possibly representing a preoccipital neural arch) which joins the medial wall of the capsule. In the absence of the auditory capsule, i.e. on the operated side, this bar of cartilage is entirely absent (Text-figs. 1 & 3). It is therefore part of the capsule, or its development is induced by the capsule.

5 Development of the ear capsule in Chelydra 145 No trace of the periotic duct or its cartilaginous walls are present on the operated side. This would be remarkable if these walls are modified occipital arches or cranial ribs (de Beer, 1937). As in the case of the posterior wall of the internal glossopharyngeal opening, these structures are therefore either of capsular origin or their development is induced by the labyrinth. These experiments cannot solve the problem of the possible intracapsular course of the facial nerve (Shaner, 1926) but it is clear that it does not leave the skull through the basal plate. The metotic process This process has not been identified as such in the Chelonia. Kunkel (1911, 19126), and Shaner (1926) figure a structure (crista parotica) in the chondrocrania of Emys lutaria and Chrysemys marginata respectively which appears to be a metotic process. In Dohrer's (1919) illustration of the chondrocranium of Aud. cap. Colum. Quad. Metot. cart. TEXT-FIG. 4. Ventral view of graphic reconstruction of posterior part of skull of Chelydra. Part of quadrates removed. Aud. cap., basal plate part of auditory capsule; Metot. cart., metotic cartilage; Metot. proc, metotic process. Other abbreviations as in previous text-figures. Chelydra serpentina it is quite obvious but the structure is not discussed or annotated. According to de Beer & Barrington (1934) a metotic process is only found in birds and crocodiles (the subcapsular process of Shiino, 1914). The postero-lateral process on the ventral aspect of the auditory capsule in both the snapper and painted turtles can, on its relationship to neighbouring structures, only be identified with the metotic process of birds. It is, however, much less developed and not, at least not at this stage of development, in contact

6 146 M. J. TOERIEN Quad. Colum. Metot. cart. TEXT-FIG. 5. Ventral view of graphic reconstruction of quadrate and columella auris of Chelydra. Abbreviations as in previous text-figures. Quad. B.P. Aud. cap Colum. Metot. proc. Aud. cap. Metot. cart. lmm TEXT-FIG. 6. Ventral view of graphic reconstruction of posterior part of skull of Chrysemys pycta. Abbreviations as in previous text-figures.

7 Development of the ear capsule in Chelydra 147 with the basal plate and occipital arch. (See Engelbrecht (1958) for a more recent discussion on the configuration of this structure in birds.) The fact that it is always present in the absence of an ear capsule is proof enough that it is not merely a process of the auditory capsule as Crompton (1953) claims for the penguin. On the operated side the metotic cartilage was found to be either free Colum. lat. pi. Metot. cart. Colum. st. Att. quad Hyp. for. Cond. TEXT-FIG. 7. Lateral view of graphic reconstruction of posterior part of operated side of Chrysemys. Att. quad., plane of attachment of metotic cartilage to quadrate; Colum. lat. pi., lateral plate of columella auris; Colum. St., stylus columellae; Cond., occipital condyle. Other abbreviations as in previous text-figures. (Text-fig. 4), attached to the columella (Text-fig. 5) or to the tectum synoticum (cartilaginous roof of skull). In the painted turtle it is posteriorly fused with the quadrate for a short distance (Text-figs. 6 & 7). The footplate of the columella auris In all specimens in which the otic placode was successfully removed the footplate of the columella is incomplete. This is consistent with the results of similar experiments on the tetrapods so far investigated (Reagan, 1915, 1917; Luther, 1927; Violette, 1931; Toerien, 1963). The important problem of the possible contribution of chondrocytes from the capsule to the footplate of the columella will be dealt with elsewhere. However, it is interesting to note that, with very few exceptions, the footplate is not absent on the operated side. Although very irregular, at least a few footplate cells usually surround the medial end of the columellar rod. It therefore seems more likely that the capsule determines the extent of the footplate development but not its presence or absence.

8 148 M. J. TOERIEN SUMMARY 1. With the exception of part of the floor which is of basal plate origin, the cartilaginous ear capsule is absent following complete extirpation of the otocyst at the somite stages. 2. Cartilaginous structures occupying the metotic fissure are similarly absent. 3. A metotic cartilage is present in Chelonia and develops independently of the ear capsule. 4. The full development of the stapes footplate is inhibited by the absence of the ear capsule. RESUME Un examen experimental du de'veloppement de la capsule de Voreille chez la tortue Chelydra serpentina 1. A l'exception de la partie basale qui trouve son origine dans la plaque basale, la capsule cartilagineuse de l'oreille fait defaut apres ablation complete de l'otocyste au stade de 12 a 20 somites. 2. Les structures cartilagineuses dans la fissure mediane sont egalement absentes. 3. Du cartilage median se presente chez Chelonia, et se developpe independamment de la capsule de l'oreille. 4. Le developpement complet du plancher de l'etrier est inhibe par l'absence de la capsule de l'oreille. ACKNOWLEDGEMENTS The experiments on which these results are based were carried out under the direction of Professor Chester L. Yntema of the Department of Anatomy, State University of New York Upstate Medical Center at Syracuse. I am most grateful to him for his guidance and assistance while a guest in his laboratory as International Post-doctoral Research Fellow of the National Institutes of Health. I am much indebted to Mr D. de Beer of this department who assisted in the preparation of the slides. REFERENCES BENOIT, J. (1957). Sur un example de cascade d'inductions dans l'organogenese de vertebres: la genese de l'oreille interne. Ann. Biol. 33, BENOIT, J. A. A. (1960). Etude experimentale des facteurs de l'inductions du cartilage otique chez les embryons de Poulet et de Truite. Ann. Sci. nat. Zool. 12, CROMPTON, A. W. (1953). The development of the chondrocranium of Spheniscus demersus with special reference to the columella auris of birds. Acta zool., Stockh. 34, DE BEER, G. R. (1937). The Development of the Vertebrate Skull. Oxford: Clarendon Press. DE BEER, G. R. & BARRINGTON, E. J. W. (1934). The segmentation and chondrification of the duck. Phil. Trans. 223, DOHRER, J. (1919). Die Entwicklung der Paukentasche bei Chelydra serpentina. Morph. Jb. 97, ENGELBRECHT, D. v. Z. (1958). The development of the chondrocranium of Pyromelana orix orix. Acta zool., Stockh. 39,

9 Development of the ear capsule in Chelydra 149 FILATOW, D. (1930). Entwicklungsmechanische Untersuchungen an Embryonen von Acipenser giildenstddii und Acipenser stellatus. Arch. EntwMech. Org. 122, KUNKEL, B. W. (191J). Zur Entwickelungsgeschichte und vergleichenden Morphologie des Schildkroten-schadels. Anat. Am. 39, KUNKEL, B. W. (1912a). On a double fenestral structure in Emys. Anat. Rec. 6, KUNKEL, B. W. (19126). The development of the skull of Emys lutaria. J. Morph. 23, LUTHER, A. (1927). Entwicklungsmechanische Untersuchungen am Labyrinth einiger Anuren. Comment, biol., Helsingf. 2, NICK, L. (1912). Das Kopfskelet von Dermochelys coriacea L. Zoo I. Jb. 33, REAGAN, F. P. (1915). A genetic interpretation of the stapes, based on a study of avian embryos in which the development of the cartilaginous otic capsules has been experimentally inhibited. Anat. Rec. 9, REAGAN, F. P. (1917). The role of the auditory sensory epithelium in the formation of the stapedial plate. /. exp. Zool. 23, SHANER, R. F. (1926). The development of the skull of the turtle, with remarks on fossil reptile skulls. Anat. Rec. 32, SHUNO, K. (1914). Das Chondrocranium von Crocodilius mit Beriicksichtigung der Gehirnnerven und der Kopfgefasse. Anat. Hefte, 50, TOERIEN, M. J. (1963). Experimental studies on the origin of the cartilage of the auditory capsule and columella in Ambystoma. J. Embryol. exp. Morph. 11, VIOLETTE, H. N. (1931). Experiments upon the middle ear of Rana. Thesis, Yale University. YNTEMA, C. L. (1955). Ear and nose. In Analysis of Development (eds. Willier, Weiss and Hamburger). Philadelphia and London: Saunders. YNTEMA, C. L. (1965). Procurement and use of the turtle embryo for experimental procedures. Anat. Rec. (in press). (Manuscript received 18th September 1964)