Youson J H, Freeman P A
J Morphol. 1976 May;149(1):73-103. doi: 10.1002/jmor.1051490105.
The general morphology of the gills is similar in larval (ammocoetes) and parasitic adult sea lampreys, Petromyzon marinus, despite different methods of ventilation necessitated by their feeding habits. The gill lamellae are supported by randomly-distributed pillar cells which enclose blood spaces and collagen columns. The distribution of these cells in lampreys is different from that of higher fishes and it may be inefficient for respiratory exchange. The presence of cytoplasmic microfilaments suggests that these cells have the ability to reduce the lamellar blood spaces through contraction. Marginal channels at the tips of the lamellae are lined only by endothelial cells. The thickness of the water-blood pathway in lampreys falls within the range described for higher fishes, with the most efficient gas exchange likely occuring at the lamellar tips where only a single layer of epithelial cells is present. The abrupt increase in height of the epithelium near the lamellar bases in adults, compared to the gradual transition in height along the lamellae in ammocoetes, is perhaps reflective of higher oxygen requirements during the parasitic stage. The consistent appearance of wide, lateral intercellular spaces within the respiratory epithelium of lampreys indicates possible involvement of these spaces in transport. Mucous secretion appears to be an important function of the superficial platelet cells in ammocoetes. "Mitochondria-rich" and "mitochondria-poor" superficial cells are observed in both ammocoetes and adults, with the mitochondria-rich cells more prevalent toward the lamellar bases. The possibility that at least some of these cells may be involved in absorption is discussed. Mitochondria-rich cells in the interlamellar region are morphologically different in ammocoetes and adults but all possess an abundance of smooth endoplasmic reticulum and hence resemble "chloride cells" of higher fishes. The similarity of these cells in the parasitic adult lamprey to chloride cells of marine fishes may reflect the potential of the adult lamprey to osmoregulate in salt water. A scarcity of these cells in ammocoetes and their resemblance to chloride cells in freshwater fishes may reflect the restriction of larval lampreys to a freshwater habitat.
七鳃鳗幼体（沙隐虫）和成体寄生海七鳃鳗（海七鳃鳗）的鳃的总体形态相似，尽管它们的摄食习性决定了不同的呼吸方式。鳃小片由随机分布的柱状细胞支撑，这些柱状细胞包围着血腔和胶原柱。这些细胞在七鳃鳗中的分布与高等鱼类不同，可能对呼吸交换效率不高。细胞质微丝的存在表明这些细胞有通过收缩来减少鳃小片血腔的能力。鳃小片尖端的边缘通道仅由内皮细胞排列。七鳃鳗中水 - 血途径的厚度在高等鱼类描述的范围内，最有效的气体交换可能发生在仅存在单层上皮细胞的鳃小片尖端。与沙隐虫鳃小片高度沿鳃小片逐渐过渡相比，成体鳃小片基部附近上皮高度的突然增加可能反映了寄生阶段更高的氧气需求。七鳃鳗呼吸上皮内始终出现宽阔的侧向细胞间隙，表明这些间隙可能参与运输。黏液分泌似乎是沙隐虫中表面血小板细胞的一项重要功能。在沙隐虫和成体中都观察到了“富含线粒体”和“线粒体贫乏”的表面细胞，富含线粒体的细胞在鳃小片基部更普遍。讨论了这些细胞中至少一些可能参与吸收的可能性。沙隐虫和成虫的鳃小片间区域中富含线粒体的细胞在形态上有所不同，但都拥有大量的光滑内质网，因此类似于高等鱼类的“氯细胞”。寄生性成年七鳃鳗中的这些细胞与海水鱼类氯细胞的相似性可能反映了成年七鳃鳗在盐水中进行渗透调节的潜力。沙隐虫中这些细胞的稀缺以及它们与淡水鱼类氯细胞的相似性可能反映了幼体七鳃鳗对淡水栖息地的限制。
