Observations of human fetal otoconial membranes

Microprobe analysis of human otoconia

Utricular and saccular otoconia (N = 510) from 1 fetus (39th gestational week; Trisomy-18), 2 babies (1 1/2 and 2 months) and 3 adults (58, 61 and 77 years) were analysed with an energy dispersive X-ray microprobe. In all otoconia the elemental composition showed an extremely high concentration of Ca but also Na, Mg, P, S, Cl and K were identified. The presence of non-Ca elements was interpreted as being due to a protein nucleus of the otoconia. Non-Ca elements are more prominent during fetal age and early post-natally than later in life. Old age otoconia contained high concentrations of P which indicates that a ' phosphatization ' of aging otoconia occurs, i.e. calcite otoconia are transformed in situ into apatite by reaction with phosphate ions. The elemental composition of otoconia in Trisomy-18 was similar to that in early postnatal inner ears.

SEM observations on development of human otoconia during the first trimester of gestation

The early development of human otoconial membranes was studied by scanning electron microscopy and X-ray microanalysis; 87 specimens, ranging from 7 to 12 weeks gestational age were examined. A few otoconia, which contained calcium, were present in the utricle by the end of the seventh week. By 8 weeks, both the saccule and utricle contained otoconia and differentiation of the macular neuroepithelia was well underway. At all stages, the utricular otoconia appeared more mature and were more varied in size and shape than saccular otoconia. In 12-week specimens, the maculae were covered by otoconial membranes with clearly defined crystalline and gelatinous layers. Near the end of the first trimester, newly-formed otoconia were present on the inferior utricular wall. X-ray analysis showed that a marked increase in otoconial calcium content occurs during the period from 7 to 12 weeks gestational age.

Development of otoconia

The development of the majority of otoconia in the macula utriculi occurs on the fifteenth and sixteenth gestational days (CBA/CBA strain of mice). The epithelial cell layer shows extensive protrusion of some cells, cytoplasmic blebbing from the surface of both hair cells and supporting cells, and electron dense inclusion bodies and vesicles surrounded by a double membrane in the apical part of supporting cells. This may reflect the formation of otoconia of varying sizes. Otoconia have an organic matrix that often shows a fibrillar substructure early in development. The majority of the organic aggregations that probably become the future otoconia are surrounded by a limiting membrane. The hexagonal shape of the otoconia can be observed as early as the sixteenth to seventeenth gestational days. Some otoconia show a nucleus-like focus in the center of the organic substance. Elemental mapping of calcium on the sixteenth gestational day shows an accumulation of calcium not only in the otoconia but also in the apical part of the epithelia cell layer below the otoconia. The endolymphatic space does not contain much calcium. These findings indicate that during the incorporation of calcium into otoconia, calcium is likely to emanate largely from the surface cells of the utricle.