Numbers of AChR clusters contacted by nerves (in percentage of all clusters; A) and sizes of AChR clusters between E13.5 and E15.5 (B). Data from image stacks as illustrated in Figure 2. Note that the number of AChR clusters contacted by axons and their size increase strongly during this period. In contrast, no contacts are made when nerves do not express agrin, nor do clusters grow in the absence of nerves (Hb9^−/−) or agrin (agrn^−/−). Data on agrin mutants are from littermates. Each column gives mean ± SE from 11–16 stacks analyzed, taken from three to four animals, except Hb9^−/− (3–6 stacks, 2 animals), each stack containing 51–556 AChR clusters.