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Induced C Left Lip and Face Shape in Mice

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Induced C Left Lip and Face Shape in Mice INDUCED C LEFT LIP AND FACE SHAPE IN MICE Harry Rajchgot Human Genetic 5 Sector M. Sc. Department of Biology Harry Rajchgot Human Genetic s Sector, M. Sc. Department of Biolog y ABSTRACT 6- AMINO:-.l'ICOTINAMIDE- INDUC ED C LEFT LIP AND EMBR YONIC FACE SHAPE IN MIC E Studies of 6-aminonicotinamide-induced median cleft lip in the C 5ïBL/ 6 strain of mice indicated that the rate of differentiation of the nasal placode is not a factor in the pathogene sis of this defect. After treatment, neithe r greater divergence of the incipient nasal placodes nor a wider embryonic face shape was seen, and all embryonic head measurements were reduced. The latte r observation sugge sts a drop in mitotic rate. Reduction in early nasal placode thickness and an increase in variance of its width, together with an apparent retardation in neural cre st ce 11 mig ration by t reatme nt, suggest a threshold model in which the interaction of· these factors pro­ duce critically small nasal placodes, resulting in abnormal mergence and consequent median cIe ft lip. Selection for two lines of mice, one responding to ~-aminonicotinamide t reatment specifically with median cleft lip, the othe r with late raI c le ft lip, now in their seventh generation, is presently successful for the forn,er, but not for the latter. Comparisons of morphological stage at the time of treatn,ent and embryonic face shape sho·..... ed no diHerences between the t",.. o lines at the present time. 6-AMINONICOTINAMIDE-INDUCED C LEFT LIP AND EMBRYONIC FACE SHAPE IN MICE by Harry Rajchgot A THESIS Subrnitted to The Faculty of Graduate Studies and Research In Partial Fulfillment of the Requirements for the Deg ree of Maste r of Sc ience Human Genetic s Sector Department of Biology McGill University Montreal July, 1971 @) Harr1 Raj cbgot 1971 TABLE OF CONTENTS Page 1. lnt roduction l Il. Lite rature review 2 A. C left lip 2 B. Neural cre st mig ration 6 C. 6-Aminonicotinamide 30 Ill. Expe rime nta l h ypothe si s 33 IV. Materials and Methods 37 A. Maintenance 37 B. Expe riments 37 1. Early embryology of the lip 37 a. AnimaIs 37 b. Experimental design 38 2. Se lection expe riment 47 a. AnimaIs 47 b. Backg round of the expe rimental procedure 47 c. Present status of the experiment 50 V. Results 52 A. Pathogenesis of induced median cleft lip 52 1. lnduced median cleft lip 52 2. Early development of the nasal placode 53 3. Rate of development of the nasal placode in treated and untreated embryos 54 a. :--:asal placode stage and somite numbe r 55 b. Ch ronological age and nasal placode stage 55 c. Ch ronological age and somite numbe r (.3 4. Effects of (,-aminonicotinamide on the early nasal placode (,(. 5. Effects of ~,-animonicotinamide on the face at the oblon1! and crescent sta1!es of the nasal 'P lac ode ïe t.. Effects of t.-an1Înonicotinan-1ide on neural l-rest cclI migration il -11- Page B. Selection expe riment 81 1. Method of selection 81 2. lnduced c left lip frequency 81 3. Spontaneous cleft Hp frequency 96 4. Morphological stage at the time of treatment 98 S. Embryos examined on Day 10, 20 hours 99 a. Morphological stage 99 b. Face shape 104 VI. Discussion 105 A. Effect of 6-aminonicotinamide on the C S7B L face 105 B. Success of selection n8 VII. Summary 125 VIII. Ack now ledge me nt s 127 IX. Appendices A-E 128 X. Bib 1iog raph y 139 1. INTRODUCTION The specific embryological mec.hanisms which are involved 10 the production of cleft lip are unknown. Embrvonic face shape ha s bee n implicated as one pos sible factor influencing predisposition to this malformation, as it has been found that certain strains which differ in face shape also differ in both their spontaneous c1eft lip frequencie s and in their induced cleft lip re sponse to various te ratogens. The se te ratogens lead to different types of c1eft lip depending on the time of administration during gestation. One way a teratogen may produce a cleft lip is by influencing face shape. The present thesis studies normal embryonic face shape and its relation to c1eft lip predisposition as well as the effects of a te ratogen on face shape. -2- II. LITERATURE REVIF:W A. C left lip Cleft lip, in mice, results from the abnormal development of the fronto- nasal p roce sse s. ln the ea rly embryonic lHe of the mouse (Patten, 1911; Trasler, 1968), the nasal placodes are originally h'.'o flaltened oval l..ulges lying in antcro-inferior positions on either side of the midline of the head. As growth proceeds, the mesodermal tissue of the placodes proliferates to a greater degree near the cir­ cumference than at the center of the oval. This results in a deprC's­ sion which will later form the external nare or nostrils. This de­ pression, the nasal pit, divides the nasal placodc into lateral and medial clements. At ils posterior limit, the nasal pit b<.:comcE extended as a caudally-directe:d invagination. This invagination liés superior to the area ...... here the surface epithelia al the posterior ends of thcse lateral and medial processcs are continuolls ,vith one another. This epithelium infC'rior ta the invagination of the: naEéd pit is known as the isthmus. The epithelia of the: lateral and mediéd procC'sses which line the invaginatio:1 of the: nasal pit bccome opposéd inCeriorly to form a double layer of epithelium lying in the antero­ postt:rior pl<~n('. This double layer is the ndsdl fin, and lies im- -3- The invagination of the nasal pit later forms the nasal passage, leading from the external to the internaI nare. The internaI nare will later open into the nasopharynx superior to the secondary or ha rd pa late. At this stage the inte rnal pit or primary choana invaginate s until it is separated from the invagination of the nasal pit by a double layer of ectode rm, the oronasal membrane, one laye r being continuous with the ectode rmal lining of the nasal pit, the othe r continuous with the ectode rmal lining of the inte rnal pit. The nasal fin now disappears as the two epithelia which make it up separate. As development proceeds, the poste rior portions of the late ral and medial proce s se s come to oppose one another in a postero-anterior wave. Their epithelia come into contact with one another to form a double epithelium separating their mesodermal cores. Shortly after these surface epithelia lining the sides of the nasal pit come in contact, they break again in an postero-anterior direction and fusion of their mesodermal cores occurs. The mesodermal cores of the medial and late raI proce s se s thus become continuous. Event­ ually, the opening of the nasal pit is quite small. and its posterior in .... agination connects it to the internaI pit. By this time, the oronasal rnen1brane separating the tv.:o pits has broken do".. -n, 50 that the two nov.: form one continuous nasal passage. -4- In some cases, the normal fusion of the lateral and medial nasal processes does not occur. This may be because they are too w idely divergent. As the embryo continuesto grow, the processes failto fuse and the isthmus is torn. The consolidated isthmus normally forms the upper lip and primary palate. Thus lack of fusion of various degrees results in a lateral cleft of the lip and primary palate of varying degrees of severity. Reed (1933) suggested that lateral cleft lip in the mouse results from insufficient pre ssure of the maxilla ry proce s s on the late raI na sa 1 proce ss, re sulting in abnormal fusion of the late raI and medial nasal processes with a resultant lateral cleft lip. Stark (1955) proposed that a deficiency of mesodermal tissues in eithe r of the se proce s se s may p revent normal pe net ration of meso­ de rm afte r apposition and fusion of thei r adjacent epithe lia. This may lead to a lateral cleft lip. Trasler (1968) has postulated that it is the degree of separation of the late raI and medial proce s se s which dete rmine s '\... hethe r a lateral cleft lip will result. There is quantitative cvidence (Trasler, unpublished) that the frequency of spontaneous lateral cleft lip in certain strains of mice can be correlated with the degrce of di .... er- ).!ence of their lateral and medial nasal processes (i. e. their embryonic -5- face shape). The middle of the upper lip in mice becomes continuous by merging of the two medial nasal processes (Patten, 1971), and sub­ sequent overgrowth by the anterior portions of the maxillary processes. Smith and Monie (969) administered methyl salicylate, trypan blue, and 9- methyl PGA to rat embryos. Examination of these embryos at various times after treatment revealed lateral and medial clefts of the lip and primary palate as well as maxillary clefts. The authors proposed that a me sench ymal defieiency as well as failure of merging may result in indueed median cleft lip. The mesoderm of the nasal processes, as of most of the head, develops from neural crest eeUs, which have immigrated into this region (McAlpine, 1955). One mutant gene, dancer (Oc), in mice usually cause s a white blaze on the forehead, always produee s abnormal development of the inner ear, and results in lateral eleft lip in homozygotes.
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