The ink gland in cephalopods is a modified part of the digestive tract that generates melanin used in defense activities against predators. In the present study, we document the histological structure of the ink gland of the cuttlefish Sepia pharaonis following light microscopy (LM) and transmission electron microscopy (TEM) examinations. Based on the observations, we infer the series of cellular events leading to the formation and secretion of melanin. LM observations revealed that the cuttlefish ink gland was cord‐like, and could be separated into two distinct regions with different functions, based on markedly different cellular and biochemical characteristics. The region with the function of secreting ink was the epithelial cell region (i.e., mature ink gland cells) and was mainly composed of melanosomes, rough endoplasmic reticulum, Golgi apparatus, and mitochondria. Conversely, the connective tissue region was the site of formation of cells and energy supply. In addition, the most probable sequence of activities from melanogenesis to the release, based on TEM observations, is as follows: (a) formation of a matrix of opaque electronic substances (melanin precursors); (b) melanization in the precursor matrix to form melanin‐like particles; (c) further melanin granule development and enlargement, and gradual intensifying of the hue; (d) production of melanin granules (melanosomes) in the precursor matrix; (e) binding of melanosomes to the cytoskeleton and migration to the cell surface; (f) fusion of melanosomes with the cell membrane at the apical pole, and discharge of melanin granules into intercellular spaces or lumen by exocytosis or cell fragmentation. We also investigated how the process of continuous ink release influences ink gland histology and ultrastructure. Inking stimulation induced the release of large amounts of ink by epithelial cells, and was associated with cell disorganization, and severe cellular vacuolization, in addition to severe organelle damage.

中文翻译：

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墨鱼乌贼墨鱼的墨腺组织学和超微结构与黑色素生成

头足类中的墨腺是消化道的修饰部分，产生黑色素，可用于抵御天敌的防御活动。在本研究中，我们记录了墨鱼乌贼墨鱼墨ink的组织学结构经过光学显微镜（LM）和透射电子显微镜（TEM）检查。基于这些观察，我们推断出一系列导致黑色素形成和分泌的细胞事件。LM观察表明，墨鱼墨腺呈绳状，可根据明显不同的细胞和生化特征分为功能不同的两个不同区域。具有分泌墨水功能的区域是上皮细胞区域（即成熟的墨水腺细胞），主要由黑素体，粗糙的内质网，高尔基体和线粒体组成。相反，结缔组织区域是细胞形成和能量供应的部位。此外，根据TEM观察，从黑色素生成到释放的最可能的活动顺序如下：（a）形成不透明电子物质（黑色素前体）的基质；（b）在前体基质中形成黑色素，形成黑色素样颗粒；（c）黑色素颗粒进一步发展和增大，色相逐渐增强；（d）在前体基质中产生黑色素颗粒（黑素体）；（e）黑素体结合至细胞骨架并迁移至细胞表面；（f）黑色素体与顶端的细胞膜融合，并通过胞吐作用或细胞破碎将黑色素颗粒排放到细胞间空间或内腔中。我们还研究了连续释放墨水的过程如何影响墨水腺的组织学和超微结构。上墨刺激引起上皮细胞释放大量墨水，并与细胞紊乱和严重的细胞空泡化有关，