Section 10 — Developmental Genetics 



covered. The degeneration produces an ex 

 vacuo hydrocephalus which is clearly visible in 

 the living animal. The time of onset of the 

 anomalies varies, but classification is usually 

 possible at birth. The most susceptible parts of 

 the brain are the cerebral hemispheres and the 

 olfactory lobes. In later stages the epithelium of 

 the nose and the trachea also degenerates. The 

 involvement of the skull is believed to be second- 

 ary. The homozygotes generally die before 

 reaching maturity, but if they live on they are 

 always sterile. 



tically reduced. Those seen are small and pyk- 

 notic. In adjacent areas where myelin is intact 

 the oligodendrocytes are normal in size and 

 shape and are lined up in rows along the nerve 

 fibers. An astrocytic reaction is not apparent 

 although some cells with two or more nuclei were 

 seen. Microgliacytes are plentiful. Perivascular 

 cuffing by macrophages is not a prominent 

 feature, but is occasionally present. 



Supported by U.S. Public Health Service 

 Grant GM 05921-05. 



10.45. (D.) Hereditary Cerebral Degeneration in the 

 Mouse. Gillian M. Truslove and M. S. Deol 

 (London, Great Britain). 



A demonstration showing the extent of the 

 fibre degeneration in the cerebral hemispheres 

 caused by a new recessive gene {cerebral degener- 

 ation, symbol cb) in the mouse. A series of 

 drawings based on sections through normal and 

 cb/cb brains with degeneration and hydrocepha- 

 lus. Photographs to show the distribution of the 

 cells in the nasal epithelium in different regions 

 of the nose. High power photographs showing 

 the normal and abnormal mucous membrane 

 and also differences in the bone structure of the 

 cranium. 



10.46. Cellular Changes in the Central Nervous 

 System of Dilute-Lethal Mice. Diane E. Kel- 

 ton (Amherst, U.S.A.). 



Previous study of the effect of the dilute-lethal 

 gene (d 1 ) on the central nervous system showed 

 that the sequence and degree of myelinization 

 did not differ among DD, Dd l and d l d l mice from 

 4 to 25 days of age. Death of d x d l mice occurs at 

 18 to 20 days of age. The Marchi method showed 

 increasing amounts of degenerating myelin in 

 the vestibulospinal, spinocerebellar and tecto- 

 spinal systems of d l d l mice with increasing age. 

 The degenerating myelin appeared within a day 

 or two of the onset of myelinization. Current 

 studies indicate that although myelin disinte- 

 grates partially or completely in scattered foci 

 there is little or no destruction of axis cylinders, 

 nor is an axonal reaction of nerve cells observed. 

 Little cellular change is seen at 10 days of age. 

 By 20 days of age in the areas where the myelin 

 sheaths have disappeared the number of identi- 

 fiable interfascicular oligodendrocytes is dras- 



10.47. A Developmental Analysis of Strong's Luxoid 

 Mutant. Paulinus F. Forsthoefel (Detroit, 

 U.S.A.). 



Deficient growth of the facial processes and 

 anterior components of the chondrocranium 

 first detected at 12 days of gestation restricts the 

 forward growth of the brain resulting in excessive 

 upward expansion of the telencephalon and 

 mesencephalon. The pressure exerted by the 

 upward pushing brain inhibits hair follicle 

 development and ossification of the frontal and 

 parietal bones over the dorsal part of the brain. 

 Tension exerted on the tissues about the eyes by 

 the outward-pushing mesencephalon results in 

 defective formation of the eyelids. Excessive 

 growth of the preaxial margins of the footplates 

 can be detected at 11 \ days in the forelimb and 

 at 12 days in the hind limb. The pattern of 

 blastematal condensations in the limbs and 

 pelvis combines excesses with deficiencies, 

 resulting in Polydactyly, radial reduction and 

 duplication, reductions of the pubis and tibia. 

 Deficient skeletal elements lag in chondrification 

 and ossification and show weak alkaline phos- 

 phatase and ascorbic acid activity. The posterior 

 belly wall and the anterior half of the genital 

 phallus show deficient growth. Epidermis never 

 covers the belly between the phallus and the root 

 of the umbilical cord. Hair follicles (guard, awl, 

 auchene, zigzag) form at the normal times on 

 the dorsum, but in early stages appear to be 

 retarded. The failure of hair to persist on the 

 dorsum for several weeks after birth is probably 

 due to a weakness in their structure. The primary 

 effect of the Strong luxoid mutation is on 

 growth in localized body regions. 



Research supported by U.S. Public Health 

 Service Grant C-3613. 



184 



