Nature's 3D Printer: A Single-Cell Resolution Histology Atlas of the Oviduct Gland in Skate Okamejei kenojei

The oviduct gland of cartilaginous fish (sharks, skates, and chimeras) synthesizes a highly cross-linked biomaterial that forms a protective leathery egg case, commonly known as "mermaid's purses." Understanding the tissue structure of this organ is critical for elucidating the synthesis process of this promising material. In this study, we performed a histological investigation of the oviduct gland in an oviparous skate, Okamejei kenojei, using paraffin sectioning to generate three-dimensional morphological data. A total of 654 tissue sections were collected and made open source. Our results reveal that the oviduct gland of O. kenojei exhibits a typical zonal structure. The club and papillary zones account for 7.74% ± 7.58% of the gland, while the baffle zone, the largest region, comprises 25-30 layers of glandular ducts, including serous gland ducts (64.72% ± 5.86%) and mixed-type ducts (0.95% ± 1.74%), predominantly distributed along the lateral margins. The terminal zone represents the smallest region, accounting for 2.17% ± 0.54% of the gland. These findings enhance our understanding of the secretion mechanisms involved in the formation of this natural biomedical material. They also provide a foundation for further studies on marine biological histology and the development of biomimetic materials through comparisons with the oviduct glands of other cartilaginous fish.