Uncovering mitotic ultrastructure in the native hair follicle using volume electron microscopy

The hair follicle undergoes repeated cycles, with anagen representing the active growth phase. During anagen, transit amplifying cells within the germinative matrix at the follicle bulb drive rapid proliferation for hair growth. This region exhibits some of the highest mitotic rates observed in any tissue, offering a rare opportunity to study mitosis in its native epithelial context, previously studied only in cultured cell lines. We applied volume electron microscopy to intact chemically fixed hair follicles enabling exceptional ultrastructural preservation of the entire mini-organ. Morphometric analysis revealed stage-specific changes in chromosomal and organelle volume and spatial distribution, highlighting coordinated roles for the mitochondria, vesicles and endoplasmic reticulum, and enabled, to our knowledge, the first ultrastructure-based karyotype of ovine chromosomes. This work advances understanding of mitosis by resolving ultrastructure in a highly proliferative, spatially constrained epithelial microenvironment, demonstrating the power of serial block face scanning electron microscopy to bridge in vitro models and native tissue architecture.