The adenylate cyclase activity was analyzed in fetal, early postnatal and adult inner ears of the CBA/CBA mouse and also in approximately one month old inner ears from Shaker -1 and Shaker -2 mice. A comparison was made with the maturation of potassium levels in endolymph as investigated with the X-ray energy dispersive technique. Adenylate cyclase activity in the developing normal inner ear shows two significant periods of increases: from the 16th to the 19th gestational day in both the cochlear and vestibular parts of the labyrinth, and from birth to day 6 after birth in the lateral wall tissues of the scala media. During the first period the anatomical boundaries of the secretory epithelia are developing. The postnatal rise in adenylate cyclase activity correlates with the morphological maturation of stria vascularis at the cellular and subcellular levels and the rise in potassium content of endolymph. The rise of enzyme activity in the cochlear during the maturation of endolymph supports a link between adenylate cyclase and the control of inner ear fluids. Adenylate cyclase activity in stria vascularis/spiral ligament of Shaker -1 and Shaker -2 mice were at normal levels and correlated better with the rather normal morphology of the tissues than the abnormal composition of endolymph in these mutants.
Adenylate cyclase activity and phospholipid labeling were compared during embryonic development of the mouse inner ear in vivo and in vitro. Inner ears were explanted on the 16th gestational day and cultured in vitro for 3-12 days. The gestation time in vivo is 21 days. During the 1st week in vitro there is very little growth of the inner ear with regard to total protein content. In contrast, the labyrinth increases its protein content threefold during the corresponding period of time in vivo. The activity of adenylate cyclase develops parallel in vivo and in vitro until the 19th gestational day whereafter the specific activity of the enzyme in vitro surpasses that of the enzyme in vivo three- to fivefold suggesting a lack of control mechanisms in organ culture. Phospholipids are labeled by 32P in an essentially similar quantitative relationship in vivo and vitro, while some quantitative differences exist. According to the present study the usefulness of the organ culture for the investigation of inner ear development appears limited to a culture period corresponding to an age prior to birth.
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