*Department of Biological Sciences, University of Alberta, Edmonton, AB, Canada T6G 2E9; Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA.
Integr Comp Biol. 2009 Aug;49(2):167-77. doi: 10.1093/icb/icp038. Epub 2009 Jun 22.
An epithelium is important for integrity, homeostasis, communication and co-ordination, and its development must have been a fundamental step in the evolution of modern metazoan body plans. Sponges are metazoans that are often said to lack a true epithelium. We assess the properties of epithelia, and review the history of studies on sponge epithelia, focusing on their homology to bilaterian epithelia, their ultrastructure, and on their ability to seal. Electron micrographs show that adherens-type junctions are present in sponges but they can appear much slighter than equivalent junctions in other metazoans. Fine septae are seen in junctions of all sponge groups, but distinct septate junctions are only known from Calcarea. Similarly, all sponges can have collagenous sheets underlying their epithelia, but only homoscleromorphs are established to have a distinct basal lamina. The presence of most, but not all, gene families known to be involved in epithelial development and function also suggests that sponge epithelia function like, and are homologous to, bilaterian epithelia. However, physiological evidence that sponge epithelia regulate their internal environment is so far lacking. Given that up to six differentiated epithelia can be recognized in sponges, distinct physiological roles are expected. Recognition that sponges have epithelia challenges the perception that sponges are only loose associations of cells, and helps to relate the biology and physiology of the body plan of the adult sponge to the biology of other metazoans.
上皮组织对于完整性、动态平衡、通讯和协调至关重要，其发育必然是现代后生动物体模式演化的基本步骤。海绵是后生动物，通常被认为缺乏真正的上皮组织。我们评估了上皮组织的特性，并回顾了有关海绵上皮组织的研究历史，重点关注其与两侧对称动物上皮组织的同源性、它们的超微结构以及它们的密封能力。电子显微镜照片显示，海绵中存在着黏着连接，但它们可能比其他后生动物的等效连接更轻微。在所有海绵群体的连接处都可以看到精细的隔膜，但只有在石灰质海绵中才知道有明显的有隔连接。同样，所有海绵都可以在其上皮组织下有胶原片，但只有同骨海绵被确定有明显的基底膜。大多数（但不是全部）已知参与上皮组织发育和功能的基因家族的存在也表明，海绵上皮组织的功能类似于两侧对称动物的上皮组织，并且与两侧对称动物的上皮组织具有同源性。然而，到目前为止，还没有生理证据表明海绵上皮组织可以调节其内部环境。鉴于在海绵中可以识别多达六种分化的上皮组织，预计它们具有不同的生理功能。认识到海绵具有上皮组织，这挑战了海绵仅仅是松散的细胞集合的观念，并有助于将成年海绵体模式的生物学和生理学与其他后生动物的生物学联系起来。
