Morphology and Morphometry of the Human Lens in the Embryonic and Early Fetal Period

The three-dimensional structural distribution of lens fiber cells, which form an elaborate lens during the late embryonic and early fetal periods (first trimester), remains unresolved. Therefore, this study aimed to assess the sequential three-dimensional morphology, morphometry, and inner structure of human embryonic (N = 10) and fetal (N = 27) lenses in the first trimester using magnetic resonance imaging and tractography. The T1-weighted magnetic resonance imaging intensity and fractional anisotropy values were high at the embryonic lens, where at least two layers, the inner and outer layers, became distinguishable during the late embryonic period (Carnegie stage 21-23) and the subsequent early fetal period. A comparison with histological findings indicated that the unique spherically oriented eigenvectors in the lens correspond to the lens fiber cell orientations. Furthermore, tractography demonstrated that the eigenvectors were oriented concentrically from the anterior to the posterior pole. Conversely, the eigenvectors around the centroid of the coronal section were irregular and exhibited low fractional anisotropy. Three-dimensional reconstructions, morphometry, and tractography revealed no apparent quadrant-specific deviations, as determined by the z-axis (anterior-posterior axis, AP axis), either morphologically or structurally, throughout the observation period. During the embryonic and early fetal periods, human lens cells may exhibit an exquisite hyperbolic arrangement. The ratio of transverse-to-AP length was < 1 in the late embryonic period when the lens cavity was obliterated. This ratio decreased linearly from approximately 0.9 to 0.7 as the crown-rump length increased. The three-dimensional morphometry of the lens remained unaffected, irrespective of its histological structure.