Solenocyte

Solenocytes are mesoderm-derived and morphologically diverse cells containing a cytoplasmic cap or enclosed cell body with a nucleus residing in its core. A long tubule is attached to the cell body, and within its intracellular lumen lies either one or two long flagella. The continuously moving vibratile flagella extend from a protein structure, referred to as the basal body, found at the base of the flagellar structure. Extending through the length of the tubule, the flagella are able to protrude into the protonephridium lumen rather designedly (see Figure 1). The tubule wall structure is composed of thin, pillar-like rods perforated by tiny openings. These pore spaces are likely the site of interstitial fluid filtration. == Function and mechanistic aspects ==

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. 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). Thousands of walking dynein motors are attached to the axoneme doublets, resulting in the hydrolysis of adenosine triphosphate (ATP), which fuels flagellar motility. In sum, flagellar motility enables solenocytes to waft excretory materials and coelomic fluid down the intracellular tubule lumen. 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. 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. == Implications of research ==

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. Hatschek's nephridium 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. 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. == References ==