Departamento de Microscopia, Instituto de Ciências Biomédicas Abel Salazar (ICBAS), Universidade do Porto, Porto, Portugal.
Centro Interdisciplinar de Investigação Marinha e Ambiental (CIIMAR), Matosinhos, Portugal.
J Morphol. 2023 Mar;284(3):e21562. doi: 10.1002/jmor.21562.
The gill of Aplysia depilans consists of several wedge-shaped pinnules with a highly folded structure, differing from the typical ctenidial gills of mollusks. Light microscopy and transmission electron microscopy were used to investigate this organ in juveniles and adults. In this species, the gill epithelium comprised ciliated, unciliated, and secretory cells. The ultrastructural analysis suggests other functions for the gill besides respiration. The deep cell membrane invaginations associated with mitochondria in the basal region of epithelium point to a role in ion regulation. Endocytosis and intracellular digestion were other activities detected in epithelial cells. In juveniles, an intranuclear crystalline structure was seen in some ciliated cells. The presence of an intranuclear crystalline structure was frequently associated with chromatin decondensation, swelling of the nuclear envelope and endoplasmic reticulum cisternae, and abundance of Golgi stacks. As these intranuclear inclusions were not found in the gill of the adult specimens, their occurrence in the two juveniles seems likely to be an anomalous condition whose cause cannot be established at the moment. Mucous cells were the most abundant secretory cells in the epithelium, but a few epithelial serous cells were also found. In addition, large protein-secreting subepithelial cells had the main cell body inserted in the connective tissue and a long thin neck crossing the epithelium. Mucous cells can be considered responsible for the production of the mucus layer that protects the epithelium, but the specific functions of the epithelial and subepithelial protein-secreting cells remain elusive. Below the epithelium, a layer of connective tissue with muscle cells lined the narrow hemolymph space. The connective tissue included cells with a large amount of rough endoplasmic reticulum cisternae. Bacteria were found on the surface of the gill, and the most abundant had a thin stalk for attachment to the epithelial cells.
海兔的鳃由几个楔形的羽鳃组成，具有高度折叠的结构，与软体动物典型的栉齿鳃不同。使用光镜和透射电镜观察了幼体和成体的这种器官。在该物种中，鳃上皮由纤毛细胞、无纤毛细胞和分泌细胞组成。超微结构分析表明，鳃除了呼吸之外还有其他功能。上皮基底部与线粒体相关的深细胞膜内陷表明其在离子调节中起作用。内吞作用和细胞内消化也是上皮细胞中检测到的其他活动。在幼体中，一些纤毛细胞中可见核内结晶结构。核内结晶结构的存在常与染色质解聚、核膜和内质网池肿胀以及高尔基体堆叠丰富有关。由于这些核内包含物在成年标本的鳃中未被发现，因此它们在两个幼体中的存在似乎很可能是一种异常情况，目前无法确定其原因。上皮中的粘液细胞是最丰富的分泌细胞，但也发现了一些上皮浆液细胞。此外，大的分泌蛋白的上皮下细胞的主要细胞体插入结缔组织中，并穿过上皮有一个细长的颈部。粘液细胞可以被认为是产生保护上皮的粘液层的原因，但上皮和上皮下分泌蛋白细胞的具体功能仍然难以捉摸。在上皮下方，一层富含内质网池的含有肌肉细胞的结缔组织排列在狭窄的血腔中。结缔组织包括含有大量内质网池的细胞。在鳃的表面发现了细菌，其中最丰富的细菌有一个用于附着在上皮细胞上的细柄。
