Hamlett W C, Allen D J, Stribling M D, Schwartz F J, Didio L J
J Submicrosc Cytol. 1985 Jan;17(1):31-40.
External gill filaments of sharks are purely transient embryonic structures. They contain a single vascular sinusoidal loop that is continuous with the afferent and efferent branchial arteries. Each filament is comprised of a squamous epithelial bilayer that rests upon a prominent basal lamina. A collagenous stroma separates the epithelium from the underlying endothelium. The epithelium, from an embryo 4.5 cm in total length, is characterized by microvilli with smooth walled vesicles at their bases, a luminal glycocalyx, prominent tubular and vesicular elements, rough endoplasmic reticulum, a Golgi complex, a flattened nucleus, coated vesicles, lipid-like inclusion bodies and sparse cytoplasmic fibrils. Adjacent epithelial cells are joined by a zonula occludens, a zonula adherens and up to five maculae adherentes. The endothelium possesses mitochondria, rough endoplasmic reticulum, a Golgi complex, coated vesicles and many micropinocytotic vesicles on both the adluminal and abluminal surfaces. The endothelium has no basal lamina and is not associated with smooth muscle. After exposure to horseradish peroxidase (HRP) for 10 min, reaction product nearly occludes the cytoplasm of some surface epithelial cells. The deeper epithelial cells have reaction product in smooth walled vesicular and tubular elements. Reaction product is also present in smooth walled endothelial vesicles. Gill filaments from a 10 cm embryo show marked changes from earlier stages. In the epithelial cells, there is an increase in the number of cytoplasmic filaments and the formation of a dense terminal web. Fewer vesicles, tubules, rough endoplasmic reticulum and mitochondria, and a less elaborate Golgi complex characterize the epithelium. The endothelium remains unchanged. The amount of collagen increases and fibroblasts are observed in the stroma. These modifications contribute to the strength of the filaments and allow the gills to withstand increased abrasion by the developing skin denticles. These experiments establish the capability of external gill filaments to take up a macromolecular tracer in the form of horseradish peroxidase. Later in development, the yolk sac of R. terraenovae becomes modified as a yolk sac placenta which functions both in respiration and hematrophic nutrition. In viviparous sharks, the uterus elaborates nutrient-rich secretions. External gill filaments may thus serve as a nutrient absorptive membrane before the establishment of the yolk sac placenta as well as perform its respiratory function.
鲨鱼的外部鳃丝纯粹是短暂的胚胎结构。它们包含一个单一的血管窦状环，该环与入鳃动脉和出鳃动脉相连。每根鳃丝由位于突出基膜上的鳞状上皮双层组成。一层胶原质基质将上皮与下面的内皮分隔开。对于一个全长4.5厘米的胚胎，其上皮的特征为微绒毛，微绒毛基部有壁光滑的囊泡、腔内糖萼、明显的管状和囊泡状结构、粗面内质网、高尔基体、扁平的细胞核、有被小泡、类脂包涵体和稀疏的细胞质纤维。相邻的上皮细胞通过紧密连接、黏着连接和多达五个黏着斑连接在一起。内皮细胞含有线粒体、粗面内质网、高尔基体、有被小泡，在管腔面和基底面都有许多微饮小泡。内皮细胞没有基膜，也不与平滑肌相关。用辣根过氧化物酶（HRP）处理10分钟后，反应产物几乎占据了一些表面上皮细胞的细胞质。较深层的上皮细胞在壁光滑的囊泡状和管状结构中有反应产物。反应产物也存在于壁光滑的内皮小泡中。来自10厘米胚胎的鳃丝与早期阶段相比有明显变化。在上皮细胞中，细胞质纤维数量增加，形成了致密的终末网。上皮细胞的囊泡、小管、粗面内质网和线粒体数量减少，高尔基体也不那么复杂。内皮细胞保持不变。基质中胶原的量增加，并观察到成纤维细胞。这些变化增强了鳃丝的强度，使鳃能够承受发育中的皮肤小齿增加的磨损。这些实验证实了外部鳃丝能够摄取辣根过氧化物酶形式的大分子示踪剂。在发育后期，新地鳐的卵黄囊演变成卵黄囊胎盘，兼具呼吸和造血营养功能。在胎生鲨鱼中，子宫会分泌富含营养的物质。因此，外部鳃丝在卵黄囊胎盘形成之前可能作为营养吸收膜，并履行其呼吸功能。
