THE ANATOMICAL RECORD 206:307-312 119851 Hypotelorism in Trisomy 1-Producing Mice HELMUT v DOMARUS Hospital for Kiefer- und Gesschtschirurgie, Medszsnische Hochschule Lubeck, 0-2400Lubeck, Federal Republic of Germany ABSTRACT Trisomy 1 embryos in mice are smaller in all dimensions, showing a developmental retardation as compared with euploid mice. Very rarely the trisomic embryos develop a typical hypotelorism with holoprosencephalon and missing olfactory nerves. Corneal distances, angles between the optic nerves and further microscopic examination showed no intermediate forms between the trisomy 1 embryos and the rare trisomy 1 embryo with hypotelorism. There seems to be a threshold, beyond which the major developmental derangement occurred. This is an experimental model showing parallels to the occasional varying phenotypes in human trisomies. Laboratory mice normally have 20 pairs of acrocentric chromosomes. Some feral mice, however, show a number of metacentric chromosomes originated from two acrocentric chromosomes which fused at the centromere in the form of a Robertsonian whole arm translocation (Gropp and Winking, 1981). Seven pairs of such metacentric chromosomes were found in M. m. poschiauinus in southeastern Switzerland (Gropp et al., 1970). By appropriate crosses in the Lubeck laboratories they produced males with two defined metacentric chromosomes as for example Rb 1Bnr with the arms of chromosomes 3 and 1 and Rb 10 Bnr with the arms of chromosomes 10 and 1. Due to segregational meiotic disorders (Fig. l), when crossed with NMRI-laboratory mice bearing all acrocentric chromosomes, they produced trisomies and monosomies (Gropp, 1975). MATERIALS, METHODS, AND RESULTS In a personal series of 248 embryos from this trisomy 1-producing model, we observed by chromosome analysis 53 embryos with trisomy 1and 195 euploid embryos. The female NMRI mice were mated overnight with males which had the metacentric chromosomes Rb 1 Bnr and Rb 10 Bnr (Fig. 1).The mice were kept a t a temperature of 20-22"C, humidity 50-6070, a 12-hour daylnight rhythm from 6 AM to 6 PM, and fed water and food ad libitum. Gestational day 1 started on the morning of a vaginal plug. The embryos had to be investigated at day 13 and day 14 as embryos of trisomy 1 die after this period. 1983 ALAN R LISS. INC The investigation was carried out to examine possible malformations in trisomy 1 embryos. Trisomic embryos were smaller in all dimensions, showing only a developmental retardation as compared to the euploid embryo (v. Domarus, 1977). One of the 53 trisomic embryos showed a marked hypotelorism with a median supraorbital proboscis (Fig. 2a,b). This observation is repeatable as another four trisomy 1 embryos with hypotelorism were observed in the same Lubeck laboratories' in the early seventies in a series of approximately 100 trisomic embryos used for other purposes. This event therefore occurs in a roughly estimated 2 4 % of the trisomic embryos. Measurements of corneal distances taken from standardized photographs were shorter than in euploid embryos but were congruent with other dimensions of the trisomic embryos, therefore demonstrating nothing but the developmental retardation (v. Domarus, 1977). Having ruled out a possible position effect (v. Domarus and Louton, 19811, we selected from all viable trisomic embryos (n = 33) six pairs of euploid and trisomic embryos with the aid of random figures, each pair from one litter. On horizontal histological sections in the plane of the optic nerves we measured the angle between the optic nerves anterior to the chiasm (Fig. 3a,b). 'Personal communication: Gropp and Zimmerrnann, Institut fur Pathologie der Medizinischen Hochschule Lubeck, F.R.G. Received November 5, 1982; accepted March 22, 1983. 308 H. v. DOMARUS a) 3 - 3 1 TRISOMY 1 Fig. 1. a ) Diploid male with the two fused chroinosomes 311 (~ Rb 1 Bnr) and l0il I = Rb 10 Bnr). Only six chromosomes out of diploid set 2 11 = 40 shown i n the diagram. b) Two out of foul possible haploid sperm segregants. c) Normal female, haploid set. Only two sets of 10 1 10 MONOSOMY three chromosomes each out of n = 20 shown in the diagram. d) When crossing b with c, trisomics and monosomies can be produced. The simultancously produced euploid embryos are not included in the diagram (see b). Fig. 2 . Trisomy 1 embryo, day 14, with marked hypotelorisni and il median proboscis. Lateral (a) and frontal (b) views. HYPOTELORISM IN TRISOMY 1 MICE Fig. 3. Horizontal microscopic sections in the Plane of the optic nerves of an euploid (a) and a nonhypoteloric trisomy 1 embryo 6)to meamre the angle between the optic nerves anterior to the chiasm. Both embryos, day 14. There are no detectable defects of the main anatomic 309 structures in the trisomic embryo. Eye (l), optic nerve (2),diencephalon ( 3 ) ,pituitary gland (41,internal carotid artery (5), trigeminal ganglion (6),upper cardinal vein (7), “anlage” of the ear (8). 310 H. v DOMARUS Repeated measurements of the same angles differed between zero and four degrees. The figures in Table 1 are mean values. As seen in Table 1, there was not the slightest hint of smaller angles for the trisomic embryos, which would have indicated a possible tendency towards hypotelorism. Therefore we did not pursue these measurements with a larger number of embryos. Furthermore, the olfactory nerves were well differentiated in the trisomic embryos. Serial microscopic sections of the hypotelorism embryo showed a n optic stalk only on one side (Fig. 4).Autolytic signs were minimal, although the embryo was probably already avital when rescued. There was one holencephalon (Fig. 5 ) , and a n olfactory bulb was not detectable. DISCUSSION The development of malformations depends on the genetic background of the animals used and can differ from one strain of laboratory mice to another (fiasler, 1968).It is quite possible, therefore, that producing TABLE 1. Angle between optic nerues anterior to chiasm' Euploid Trisomic Euploid Trisomic Euploid Trisoniic Euploid Trisomic Euploid Trisomic Euploid Trisomic Embryo no. Angle hetween optic nerves 2,17915 2,17914 2,22015 2,22016 2,22418 2,22414 2,23211 2,23212 2,23411 2,234110 2,242/13 2,24213 145O 147" 147" 157" 159.5" 159.0" 162 160" 153" 155" 152.5" 154.5" 'Comparison hetween euploid and Lrisomic embryos each pair from same litter. pairs selected from vital t r i s o m i c embryos with aid of random figures. trisomy 1 embryos in another strain of mice could increase or decrease the number of hypoteloric embryos. Hypotelorism has to our knowledge not been described in laboratory mice. In this experimental model it did occur but as an Fig. 4. Trisomy 1 embryo, day 14, with hypotelorism. Eye (11, optic stalk (2)and diencephalon ( 3 ) HYPOTELORISM IN TRISOMY 1 MICE 311 Fig. 5 Trisomy 1 cmbryo, day 14, with hypotelorism showing holoprosencephalon. Prosen cephalon (l),dicncephalon ( 2 ) . exception and in trisomic embryos only, somies allows the study of endogenous terawithout any intermediate forms. Neither the togenesis. Therefore, as one of many other angle of the optic nerves (Table 1) nor the examples, we see a parallel between the occasional arrhinencephaly and cyclopia in huintercorneal distances or a possible lacking “anlage” of the olfactory nerves (v. Domarus, man trisomy 13 (Buhler et al., 1962; Miller et al., 1962; Singh et al., 1974; Warkany, 1977) gave any clues toward a development of cyclopia or hypotelorism in the trisomy 1 19711, and our results, which throws some light on the variation of phenotypes in huembryos, which seems to indicate a threshold man trisomies. for the development of hypotelorism. Apart from the typical outer aspect our LITERATURE CITED embryo showed a holencephalon and missing olfactory nerves on microscopic sections, fur- Buhlcr, E.. I. Bodis, B. Rossier and G. Stalder (1962) Trisomie 13-15 mit Cebocephalie. Ann. Paediatr., ther underlining the very close resemblance (Basel) 199:198-205. with the human pathology (De Myer, 1975). De Myer, W. (1975) Median facial malformations and We therefore believe that this is experimentheir implications for brain malIiiimations. In: Morphogenesis and Malformation of Face and Brain. D. tal evidence that a trisomy does favor the Bergsma, ed. Alan R. Liss, Inc., New York, pp. 155development of rare spontaneous malformations. No other malformations but hypotelo- v. 181. Domarus, H. (1977) Morphologische Befunde der rism were found in trisomy 1 embryos. AlKopfregion bci der Trisomie-l der Maus. Habilitationsschrift Lubecli, pp. 1-111. though hypotelorism occurred only very occasionally, there seems to be a strong link v. Domarus, H., and T. Louton (1981) The position effect of embryos in trisomy-l producing mice. Teratology, between trisomy 1 and this particular mal24,315-319. formation. In contrast to the experiments Gropp, A . (1975) Chromosomal animal model of human disease. Fetal trisomy and developmental failure. In: with exogenous teratogenic factors (X-rays, Teratolo~y,Trends and Applications. C.L. Berry and corticosteroids, vitamin A), this model of tri- 3 12 H. v DOMARUS D.E. Poswillo, eds. Springer-Verlag, B e r l i d e i d e l bergmew York, pp. 17-33. Gropp, A., and H. Winking (1981) Robertsonian translocations: Cytology, meiosis, segregation patterns and biological consequences of heterozygosity. Symp. Zool. SOC. Lond., 47:141-181. Gropp, A,, U. Tettenborn, and E. v. Lehmann (1970) Chromosomenvariation vom Robertson’schen Typus bei der Tahakmaus, M. poschiavinus und ihren Hyhriden mit der Laboratoriumsmaus. Cytogenetics, 9:923. Miller, J.Q., E.H. Picard, M.K. Alkan, and P.S. Gerald (1962) Neurologic manifestations of 13-15 trisomy: A chromosome abnormality in arrhinencephaly. Trans. Am. Neurol. Assoc., 87:51-56. Singh, D.N., R.A. Wiscovitch, R.A. Osborne, and G.R. Hennigar (1974)Partial trisomy 13 in a case of cyclopia with 13114 translocation. Clin. Genet., 5,218-222. Trader, D.G. (1968)Pathogenesis of cleft lip and its relation to embryonic face shape in AIJ and C 57 BL mice. Teratology, 1:33-50. Warkany, J. (1971) Congenital Malformations. Year Book Medical Publishers, Chicago, pp. 201-206.