The temporal and spatial sequence of events involved in joint cavity formation during limb development is dependent upon embryonic movement. Many studies into chick embryo diarthrodial joint development have concluded that a lack of muscular movement results in the fusion of opposing joint elements and absence of articular cavities. This study has addressed the hypothesis that an enhancement of muscle contraction, with drugs such as 4-aminopyridine (4-AP), will accelerate embryonic chick joint cavitation.

Fertilised White Leghorn eggs were treated by methods conforming to UK legislation with sterile-filtered 4-AP at 2 mg ml^-1 in Tyrode solution (TS) applied directly to the chorioallantoic membrane on each day of treatment. Three groups of chicks received 4-AP, group 1 at stage 36-37, group 2 at stages 36-39 and group 3 at stages 36-43. Another three groups of chicks received TS, as a control, at the same times. One day after the final treatment, chicks were killed humanely.

Skeletal movement in control embryos consisted of variable periods of spontaneous muscular activity (approximately 15 seconds per minute studied, range: 10-40 s min^-1). Treatment with 4-AP at stage 36 induced an immediate (within 1 min) increase in the frequency of skeletal movement (greater than 35 s min^-1, range: 35-45 s min^-1) which was sustained for at least 24 h after this initial dose, and maintained by subsequent 4-AP treatments. This increased twitch frequency had no apparent effect on body weight, longitudinal growth or cartilage/bone formation rates in limb skeletal elements when compared to the stage-matched controls. Intriguingly, however, direct measurement of fibre number in hindlimb adductor muscles, showed that prolonged 4-AP treatment (days 10-18) produced significant increases in muscle fibre density compared to the TS control.

In contrast to the well-established cavitation-inhibitory influence of embryonic immobilisation, this study indicates that 4-AP-induced increases in frequency of skeletal movement fails to significantly modify the spatiotemporal pattern of embryonic joint cavity formation in the developing limb, and that such treatment also has little discernable effect on chick weight or skeletal element development and growth. In marked contrast, however, the imposition of defined 4-AP-induced embryonic hyper-mobility for prolonged periods during development stimulates increases in muscle fibre density, suggesting that such treatment may enhance the capacity of post-hatch growth in the chick.