Solenocyte
In biology, solenocytes are elongated, flagellated cells commonly found in lower invertebrates, such as flatworms, chordates and several other animal species. In terms of function, solenocytes play a significant role in the excretory systems of their host organism. For example, the lancelets, also referred to as amphioxus, utilize solenocytic protonephridia to perform excretion. In addition to excretion, these cells contribute to ion regulation and osmoregulation. With this in mind, solenocytes form subtypes of protonephridium and are often compared to another specialized excretory cell type, i.e., flame cells. Solenocytes have flagella, while flame cells are generally ciliated.
Cellular structure and configuration
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.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.
A nephridium contains approximately 500 solenocytes, each of which is roughly 50 microns in length. The excretory organ of Amphioxus belcheri contains clusters of solenocytes. These clusters are composed at patterned intervals, generating groups amongst the renal tubules of B. belcheri, which in a way, resemble mesothelial cells surrounding the human body's internal organs. Additional studies indicate a resemblance to vertebrate podocytes, as vascular fluid within the ligamentum denticulatum may travel into the coelom through the narrow network of solenocyte gaps or foot processes.
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, 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. Situated at the center of each flagellum is the highly conserved axoneme, which contains nine doublet microtubules encircling a pair of singlet microtubules. Thousands of walking dynein motors are attached to the axoneme doublets, resulting in the hydrolysis of adenosine triphosphate, which fuels flagellar motility. 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. In sum, flagellar motility enables solenocytes to waft excretory materials and coelomic fluid down the intracellular tubule lumen.In several lower invertebrates, solenocyte clusters project directly into coelomic canals, where they are submerged in coelomic fluid. This fluid contains a variety of materials, including salts, proteins, and corpuscles. 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.