MOUSE EMBRYO 451 



Under natural conditions the mammalian embryo is not readily accessible and cannot be 

 observed during development, and allow it to survive more than a few hours as can be 

 done with the frog and chick. Experiments with the mammalian embryo must therefore 

 be conducted somewhat differently. A few suggestions follow: 



1. Cell Dissociation and Tissue Culture : The mouse embryo is excellent material 

 for such studies (see Ex. 28, 29). The 9-day embryo has a diameter of about 1 

 millimeter, can be easily dissected out, and is as sterile as possible until con- 

 taminated by the operator, the air or instruments. Later stages would have fur- 

 ther development of certain organ systems. 



2. Dietary Effects: It is a simple matter to regulate the food intake of the pregnant 

 mouse, either with respect to quantity or quality. Vitamin imbalance will cause 

 drastic effects at certain stages of development, and deprivation of essential 

 minerals can affect both mother and litter members. 



3. Chemical Effects : Certain drugs are highly toxic, such as trypan blue, insulin, 

 etc. , and will alter development in the embryo and/or fetus without killing (in 

 low doses). Any endocrine imbalance has dramatic effects. Try COo and O2 

 shifting. 



4. Traumatic Effects : While the mammalian embryo is cushioned, and efficiently 

 protected, there is evidence that moderate handling of the pregnant mouse at cer- 

 tain stages of gestation will result in drastic effects on the embryos. Electric 

 shock, ultra-sonic vibration, etc. have not been fully investigated, and may well 

 cause congenital anomalies. 



5. X-lr radiation : Most Colleges and Universities have available adequate x-ray 

 equipment for such studies. X-rays have the advantage of being highly penetrat- 

 ing, and highly damaging to the early embryo without concomitant damage to the 

 mother. It is suggested that mice at 8. 5 to 9. days gestation be exposed to 150 r 

 whole body x-rays (dose at level of gravid uterus) and examined by Caeserian 

 section at 18. 5 days for congenital anomalies which are invariably present. 



6. Genetic Effects : The literature on mouse genetics is accumulating rapidly, and 

 fertility between divergent strains is generally so good that the study of lethal 

 genes that kill before birth comes within the scope of this book. 



EXPERIMENTAL DATA: 



The variety of effects of experimental procedures on the mammalian embryo ranges from 

 unrecognized but nevertheless demonstrable anomalies (heart or kidneys), through gross 

 anomalies (brain hernias, anencephalies, malformations of either end) to fetal deaths, 

 early embryonic deaths and resorptions, to failure even to divide or implant. Thus, it is 

 generally necessary to examine the fetuses prior to the time of expected delivery. This 

 is because in the mouse, contrary to other (and possibly less wise) mammals, the female 

 usually kills and devours its abnormal offspring at birth. The pregnant mouse at 18. 5 

 days may have its cervical vertebrae dislocated (see above) and the uteri opened quickly. 

 If the fetuses are placed in warm saline they will be active for some time. 



In every instance it is necessary to sinnultaneously dissect control mice and compare 

 litter members with those experimentally treated. This is because size variations 

 (stunting) will not otherwise be recognized. Further, some strains of mice are genetically 

 predisposed to develop certain anomalies such as eye defects, overlapping teeth, etc. more 

 than do other strains. In quantitative experiments, no less than 100 offspring for each set 

 of categories should be examined, in addition to controls. 



RESULTS: 



The results of any mammalian embryo experiment may be statistical (weight, sex, size 

 differences) and can well be illustrated by compilation of data and by photographs. It may 



