Dynamic Transport and Cementation of Skeletal Elements Build Up the Pole-and-Beam Structured Skeleton of Sponges

Abstract

Animal bodies are shaped by skeletons, which are built inside the body by biomineralization of condensed mesenchymal cells in vertebrates [1, 2] and echinoderms [3, 4], or outside the body by apical secretion of extracellular matrices by epidermal cell layers in arthropods [5]. In each case, the skeletons' shapes are a direct reflection of the pattern of skeleton-producing cells [6]. Here we report a newly discovered mode of skeleton formation: assembly of sponges' mineralized skeletal elements (spicules) in locations distant from where they were produced. Although it was known that internal skeletons of sponges consist of spicules assembled into large pole-and-beam structures with a variety of morphologies [7-10], the spicule assembly process (i.e., how spicules become held up and connected basically in staggered tandem) and what types of cells act in this process remained unexplored. Here we found that mature spicules are dynamically transported from where they were produced and then pierce through outer epithelia, and their basal ends become fixed to substrate or connected with such fixed spicules. Newly discovered "transport cells" mediate spicule movement and the "pierce" step, and collagen-secreting basal-epithelial cells fix spicules to the substratum, suggesting that the processes of spiculous skeleton construction are mediated separately by specialized cells. Division of labor by manufacturer, transporter, and cementer cells, and iteration of the sequential mechanical reactions of "transport," "pierce," "raise up," and "cementation," allows construction of the spiculous skeleton spicule by spicule as a self-organized biological structure, with the great plasticity in size and shape required for indeterminate growth, and generating the great morphological diversity of individual sponges.

Skeleton construction upon local regression of the sponge body

“…Thus, that study and our more recent study (Sugano‐Yasunaga et al , in preparation) showed that as a consequence of multiple rounds of five sequential reactions of player cells (namely, (a) spicule “transport”, (b) “pierce outer epithelia”, (c) “raise up front end of the spicule”, (d) “cement the basal end of the spicule”, and the recently revealed (e) “the outer epithelia eventually move up to the top of the pierced spicule”), the spiculous skeleton is self‐organized spicule by spicule (Figure a,b) (Nakayama et al, , Sugano‐Yasunaga et al , in preparation). In contrast, the previously known skeleton construction mechanisms of animals can be summarized as being “pre‐patterning‐based”, and as using skeletal elements produced in situ.…”

“…Since it had been shown that transport cells can change the direction of spicule transport as the back‐going spicule transport in growing juvenile sponges (Nakayama et al, ), we then more precisely examined whether back‐going spicule transport and local retraction of the basopinacoderm are correlated (Figure a). We quantified the direction of spicule transport and the extension/retraction of the basopinacoderm as follows.…”

“…These back‐going spicules were carried until they happened to pierce outer epithelia, and then they were held up (Figure d–f’). The following facts suggested that where the carried spicules can pierce outer epithelia is dependent on the 3‐dimensional curvature of ENCM (Nakayama et al, , W. Yasunga‐Sugano et al, in preparation): (a) a group of transport cells attach around the middle of a spicule, (b) spicules are relatively long (150–250 μm), (c) transport cells stochastically migrate on basopinacoderm or ENCM, and (d) carried spicules tend to pierce outer epithelia near the edge of the body, where the angle between basopinacoderm and ENCM is narrow. Actually, back‐going spicules carried on the basopinacoderm in an inward direction were thus carried for a relatively long time and distance until they would pierce the outer epithelia in the periphery.…”

“…Our previous studies clearly showed that the spicule “pierce” step (Figure bii) is a key step to hold up spicules in accord with the plastic nature of the body growth of sponges (Nakayama et al, , Sugano‐Yasunaga et al , in preparation). Once a spicule becomes held up, the outer epithelia move up to the top of the spicule (Sugano‐Yasunaga et al , in preparation).…”

“…Archaeocytes migrate out from the gemmule coat, proliferate and differentiate to form a functional juvenile sponge in about 7 days. Spicule‐producing cells (sclerocytes) and spicule‐carrying cells (transport cells) can be detected from an early developmental stage (stage 2, according to Funayama, Nakatsukasa, Hayashi, et al, 2005) (Figure a, developmental stage 2) (Funayama, Nakatsukasa, Kuraku, et al, 2005 and Nakayama et al, , respectively).…”