Solenocyte

In regards to function, flagella play a significant role in the excretory nature of solenocytes. These motile appendages extend from the solenocyte membrane and utilize the support of an axial filament (or axoneme), basal body, as well as numerous microtubules.^[5] That said, the stability of the flagellum is crucial to its motility. The basal body, composed of nine triplet microtubules, functions to anchor the flagella in place (acting as a modified centriole). Situated at the center of each flagellum is the highly conserved axoneme, which contains nine doublet microtubules encircling a pair of singlet microtubules (generating a 9+2 pattern).^[6][7] Thousands of walking dynein motors are attached to the axoneme doublets, resulting in the hydrolysis of adenosine triphosphate (ATP), which fuels flagellar motility.^[7] More specifically, the dyneins anchor onto one doublet within the outer microtubule ring, and as they "walk" towards an adjacent doublet, the entire flagellar structure is able to bend and beat (see Figure 2).^[8] In sum, flagellar motility enables solenocytes to waft excretory materials and coelomic fluid down the intracellular tubule lumen.^[4]

In several lower invertebrates, solenocyte clusters project directly into coelomic canals, where they are submerged in coelomic fluid.^[9] This fluid contains a variety of materials, including salts, proteins, and corpuscles (e.g., leucocytes, phagocytes, eleocytes, mucocytes, etc.). In that respect, solenocytes play a major role in osmoregulation, ion regulation, and homeostasis through the movement of coelomic fluid.^[10]

Branchiostoma nephridia also have tiny blood vessels, and the protonephridia function to absorb nitrogenous waste from coelomic fluid, as well as the blood sinuses via diffusion.^[9]

In addition to a greater understanding of excretory organs within other invertebrates, further research on solenocyte composition and function can advance current knowledge on renal function, human health, and even certain genetic diseases within the vertebrate world. The cephalochordate amphioxus (see Figure 3) can contribute to this research as a close relative to vertebrates.^[4][11][12]

Compared to paired series of protonephridia, Hatschek's nephridium is a large unpaired excretory structure found within Branchiostoma virginiae. The nephridium, along with its collection tubule, is located to the left of the notochord and beside the left anterior aorta.^[11] Hatschek's nephridium is like a protonephridium with a single, bent branch consisting of numerous solenocytes. The anterior end of this structure sits directly in front of Hatschek's pit, while the posterior end (at the rear of the velum) opens into the endodermal pharynx. Flagellated filtration cells called cyrtopodocytes occupy the length of the collection tubule. These filtration cells closely resemble solenocyte structure and function.^[11]

Research suggests that coelomic myoepithelial cells in amphioxus (Branchiostoma) may have significance in renal function.^[4] Located along the coelom, myoepithelial cells have both thick (18–25 nm in diameter) and thin (5–7 nm in diameter) microfilaments. That said, these microfilaments appear to be more abundant in myoepithelial cells that are in close proximity to solenocytes attached to the ligamentum denticulatum coelomic surface.^[4] The beating of solenocyte flagella to propel coelomic fluid throughout excretory tubules leads to the idea that myoepithelial cells near solenocyte clusters can impact renal function by regulating fluid motility within the coelomic cavity.^[4]

Within the vertebrate lineage, significant genome duplications took place after the divergence of Branchiostoma, thus making it a potentially valuable model for gaining insight into vertebrate biological mechanisms.^[12] Branchiostoma has use for investigating human health and genetic disease. Along with signaling pathways, numerous homologs of vertebrate organs share cellular, developmental, and physiological parameters with their vertebrate equivalents.^[12] On that premise, solenocyte function within Branchiostoma could provide insight into metabolic diseases, such as renal cell carcinoma (RCC).^[13]