Massive loss of mid- and hindbrain neurons during embryonic development of homozygous lurcher mice

Abstract

The mouse neurological mutant lurcher (Lc) results from a semidominant mutation. Heterozygous Lc/+ mice are viable but ataxic because Lc/+ Purkinje cells die by apoptosis within the first 3 weeks of life. Lc/Lc mice die shortly after birth. To aid in understanding the function of the lurcher gene product, we have examined the embryonic development of homozygous lurcher animals. The ratio of +/+:Lc/+:Lc/Lc animals did not deviate significantly from the expected 1:2:1. Homozygous lurcher mice at P0 were found to be normal under gross morphological examination. However, these mice weighed less, lacked milk in their stomach, and died within the first day of life. No resorbed embryos were found at embryonic day (E) 17.5, indicating that all homozygous lurchers survived until birth. Histological examination of P0 animals revealed that in homozygous lurcher mice the patterning of the brain is normal but that there has been a massive loss of hindbrain neurons during embryonic development. A particularly conspicuous consequence of the Lc/Lc genotype at birth is the complete absence of large neurons comprising the trigeminal motor nucleus. These neurons arise normally and are maintained until E15.5. However, beginning at E15.5 large numbers of pyknotic cells are evident in the trigeminal motor nucleus, suggesting that these cells die coincident with their terminal differentiation in the developing hindbrain. Because the trigeminal motor nucleus controls muscles required for suckling, these results suggest an explanation for the neonatal death of homozygous Lc animals. These data demonstrate that the severe and dose-dependent developmental consequences of lurcher gene action result from degeneration of distinct neuronal populations on maturation in the developing CNS.

Fig. 1.

P0, Newborn animals from Lc/+ intercross. Left, +/+; center, Lc/+;right, Lc/Lc.

Fig. 2.

P0, Paraffin sections of neonatal brains fromLc/+ intercross. Top panels, A low magnification view showing patterning of mid- and hindbrains at birth. At higher magnification (middle panels) the decreased cellular density within the hindbrain of Lc/Lc embryos is evident.Bottom panels, At high magnification, the motor nuclei of V cranial nerve from Lc/+ mice and its absence in equivalent area of the brain in Lc/Lc animals.Me, Medulla; Cb, cerebellum. Arrowspoint to trigeminal motor neurons. Magnification: A,B, 6×; C, D, 20×; E,F, 400×.

Fig. 3.

Paraffin sections of E12.5 embryos from theLc/+ intercross. Low magnification views of sagittal sections of whole E12.5 Lc/+ and Lc/Lcembryos revealed no differences in gross anatomy. In particular, patterning of the Lc/Lc brain appears normal.

Fig. 4.

Paraffin sections of E12.5 brains fromLc/+ intercross. Left panels present low magnification views of the hindbrain region containing trigeminal motor nucleus (outlined) in Lc/Lc andLc/+ animals. Higher magnification views showing trigeminal motor neurons (arrows) are shown in the right panels. Magnification: A, B, 20×;C, D, 400×.

Fig. 5.

Paraffin sections of E15.5 brains fromLc/+ intercross. Low magnification views of the hindbrain region containing trigeminal motor nucleus (outlined) are presented in the left panels. Higher magnification pictures showing trigeminal motor neurons (arrows) are presented in the right panels. Magnification: A,B, 20×; C, D, 400×.

Fig. 6.

Paraffin sections of E16.5 brains fromLc/+ intercross. Low magnification views of the hindbrain region containing trigeminal motor nucleus (outlined) are shown at the left. Higher magnification views showing trigeminal motor neurons (arrows) in Lc/+ animals and a large number of pyknotic profiles (arrows) inLc/Lc animals are shown on the right. Magnification: A, B, 20×; C,D, 400×.