A lung defect in cd44^−/−; met^+/− mice. (A) Histology of lungs obtained shortly after birth from mice with the indicated genotypes. (B) Floating test for lungs on water.

In situ hybridization of brain tissue. In situ hybridization of sagittal brain sections of mouse pups at birth with CD44, HGF, and c-Met probes. VRG indicates the ventral respiratory group. An overview of the whole brain section is shown in the left panels, the area of the VRG is shown in higher magnification in the right panels. Hybridization with sense CD44 mRNA is shown in the lower panels (ctrl). Hybridization with sense HGF and c-Met mRNA gave similar results.

Severe impairment of inhibitory synaptic transmission in preBötC neurons in cd44^−/−; met^+/− mice. (A) Representative recordings of miniature glycinergic and GABAergic IPSCs (mIPSCs) in brain stem preBötC neurons. (B and C) Average frequency (B) and amplitude (C) of mIPSCs. Data are expressed as means ± SEM. Statistical significance of differences between means was assessed with Student t tests. The level of significance was set at P of <0.05; n.s., not significant. The numbers indicate the number of neurons/mouse.

Motoneurons in facial nucleus and nucleus ambiguus and nerve fibers in phrenic nerves. (A) Nissl-stained paraffin sections of motoneurons in the facial nucleus and nucleus ambiguus of the brain stem of E19 and 3-week-old mice with the indicated genotypes and semithincross sections of the phrenic nerves of 3-week-old mice. Scale bars for facial motoneurons equal 500 μm, for motoneurons of the nucleus ambiguous, 200 μm, and for phrenic nerves, 50 μm. (B) Digitally generated overview (a, b) and high-magnification images (c, d) of phrenic nerve cross sections of E19 mice. In phrenic nerves of cd44^+/−; met^+/− mice (a, c), a 1:1 ratio of Schwann cell to axon is very frequent, as indicated in the boxed area in panel a and by arrows in panel c. Occasionally, early myelination is found (white arrowhead in panel c). Bundles of axons that are in direct apposition and surrounded by a common Schwann cell sheath (arrow in panel a) are comparatively rare. In phrenic nerves of cd44^−/−; met^+/− mice, a 1:1 Schwann cell-to-axon ratio is found only occasionally (arrow in panel b), while bundles are more often seen (arrowheads in panels b and d). Scale bars for panels a and b, 5 μm; for c and d, 1 μm.

Severe impairment of overall network activity in preBötC neurons in cd44^−/−; met^+/− mice. (A) Representative recordings of sPSCs in brain stem preBötC neurons. (B) Average frequency of sPSCs. Data are expressed as means ± SEM. Statistical significance of differences between means was assessed with Student t tests (InStat; GraphPad Software Inc.). The level of significance was set at P of <0.05. The numbers indicate the number of neurons/mouse. (C) The sPSC average frequency of cd44^+/− mice in panel B was set to 100% and was compared to the sPSC frequency of individual cd44^−/−; met^+/− mice.

Severe impairment of excitatory synaptic transmission in preBötC neurons in cd44^−/− mice. (A) Representative recordings of miniature glutamatergic EPSCs (mEPSCs) in brain stem preBötC neurons. (B and C) Average frequency (B) and amplitude (C) of miniature glutamatergic EPSCs. Data are expressed as means ± SEM. Statistical significance of differences between means was assessed with Student t tests. The level of significance was set at P of <0.05. The numbers indicate the number of neurons/mouse.