Ultrastructural changes in the midgut of Ixodes ricinus nymphs across feeding and digestion stages

Ticks rely on host blood as their primary source of nutrients and energy. In hard ticks (Ixodidae), a single blood meal per life stage supports survival, metamorphosis, and, in females, egg production. The midgut, the major metabolic organ, is responsible for both digestion and nutrient storage. While previous studies have described tick midgut histology, many foundational works, often decades old, lacked modern histochemical, immunohistochemical, and high-resolution imaging techniques. Moreover, inconsistent nomenclature of midgut cells across species complicates comparative analyses. We herein chronologically examined structural changes in Ixodes ricinus nymphal midgut tissue before, during, and after feeding. Using immunofluorescence and immunoelectron microscopy, we monitored lipid droplet dynamics and the uptake of blood-derived proteins, including hemoglobin and serum albumin. To further investigate endocytic processes, we employed serial block-face scanning electron microscopy and TEM tomography, focusing on midgut samples from nymphs fed for 48 h. These approaches allowed us to visualize in 3D hemoglobin and albumin uptake from the gut lumen and their intracellular distribution within the nymphal caecum and midgut cells. Our findings enhance the understanding of tick gut cell biology, particularly nutrient processing and energy metabolism, providing a foundation for future research on tick physiology and vector competence.