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Abstract：
Spicules are mineral-based biocomposites skeletal structures that are widely distributed among phylogenetically distant groups of invertebrates (Porifera, Cnidaria, Mollusca, Echinodermata). Subepidermal spicules are formed under the ectodermal epithelium and are characterized for all groups except mollusks (Aplacophora, Polyplacophora, Bivalvia), their spicules are located on the surface of the body. However, one group of mollusks (Gastropoda: Heterobranchia) have unique subepidermal spicules that have never been detected above the ectodermal epithelium and similarly to those characterized for Porifera, Cnidaria and Echinodermata. Understanding subepidermal spicule formation in mollusks could help solve the question on the origin of spicules. Spicules in nudibranchs have been described for more than 150 years, yet ontogenetic dynamics of spicules have never been studied and the full mechanism of their formation remains unknown. Herein we investigate the spicule formation in different stages of postlarval development of the nudibranch Onchidoris muricata (O.F. Müller, 1776). For the first time, ontogenetic transformations of the spicule complex are described using experiments and different morphological methods. Our studies demonstrate that spicules of O. muricata form in the subepidermal space in early developmental stages immediately after veliger settlement. A single spicule forms inside a huge vacuole within a sclerocyte and remains there throughout the entire life of the specimen. Signs of spicule or sclerocyte migration under the epithelium in postlarval development was not found. Spicules only form during larval settlement, increasing only in size as development furthers. For the first time, spicule mineralization zones were detected at the tips of the spicules as well as the presence of collagen I in the overall composition of the spicules. Thus, our findings suggest that spicules form by an ectodermal cell that emerged under the ectodermal epithelium during the earliest stages of postlarval development.
摘要：
针状体是基于矿物的生物复合材料骨骼结构，广泛分布在无脊椎动物的系统发育远距群体中（Porifera，Cnidaria,软体动物,棘皮动物）。表皮下针状体在外胚层上皮下形成，除软体动物外，所有组都具有特征（Acplacophora，胎盘息肉,Bivalvia)，它们的针状体位于身体的表面。然而,一组软体动物（腹足纲：异型鳞茎）具有独特的表皮下针状体，从未在外胚层上皮上方检测到，与Porifera的特征相似，雀巢和棘皮虫。了解软体动物表皮下针状体的形成可以帮助解决针状体起源的问题。裸鱼中的针状体已经被描述了150多年，然而，针状体的个体发育动力学从未被研究过，其形成的全部机制仍然未知。在此，我们研究了裸枝Onchidorismuricata（O.F.Müller，1776).第一次,使用实验和不同的形态学方法描述了针状复合体的个体发育转化。我们的研究表明，O.muricata的针状物在veliger沉降后立即在早期发育阶段在表皮下空间形成。单个针状物在巩膜细胞内的巨大液泡内形成，并在标本的整个生命周期中都保留在那里。在幼虫后发育中未发现针状体或巩膜细胞在上皮下迁移的迹象。针状体仅在幼虫沉降期间形成，随着发展的进一步发展，规模只会增加。第一次,在针状体的尖端检测到针状体矿化区，并且在针状体的整体组成中存在胶原蛋白I。因此，我们的发现表明，针状体是由一个外胚层细胞形成的，该细胞在幼虫后发育的最早阶段出现在外胚层上皮下。
