Microstructure and Ultrastructure of the Oviducal Gland of Sepioteuthis lessoniana

Cephalopods, which have a rapid growth rate and short life cycle, are regarded as an important marine fishing resource. Sepioteuthis lessoniana is among the most widely distributed species in the Loliginidae family in the Indo-Western Pacific Ocean. Its meat is delicious and nutritious. It is widespread in the East China Sea, South China Sea, and other marine areas in China, and it is considered an important local marine resource. In this study, the ultrastructure of the oviducal gland of S. lessoniana was investigated for the first time via anatomical dissection, tissue sectioning, and electron microscope projection. The external morphology and internal structure of the gland were clearly described, as well as the role of the oviduct gland in the reproductive activities of cephalopods. In this experiment, all the samples were collected from the open ocean, with the samples of wild S. lessoniana coming from marine areas in Fujian Province. The body surfaces of the samples were healthy and undamaged, with an average mantle length of (17.5±6.4) cm and an average body weight of (392.0±76.0) g. The samples were dissected using standard anatomical methods, and the glandular characteristics were recorded. The oviducal gland was dissected with a scalpel for tissue sectioning and preserved for electron microscope projection and observation. The experimental results showed that S. lessoniana had a single oviducal gland, which was located on the right side of its abdominal cavity. The sexually mature gland was milky white overall, with brownish-yellow pigmentation near the inner shell. The oviducal gland was enveloped in a transparent membrane, and regular gaps were visible. The gland consisted of three parts: the proximal oviducal gland, the distal oviducal gland, and the transparent valve. The proximal oviducal gland was infundibular and connected with the hyaline oviduct. The distal oviducal gland was cylindrically tapering, and a single mature egg was observed inside. The transparent valve was at the end of the gland, also known as the valve. The microstructure of the oviducal gland was observed via a microscope. The gland was composed of a glandular wall, lamellar, and muscle tissue. The glandular wall tissue was composed of adventitia, loose connective tissue, and a small amount of muscle tissue, blood vessels, and ducts that were scattered in the connective tissue. The transparent valve was composed of columnar epithelial cells and muscle tissue. A small number of water-droplet goblet cells were dispersed between the epithelial cells, while a large number of cilia were generated on the exterior. The lamellar was attached to the glandular wall tissue and distributed in layers within the gland; it was mainly composed of ciliated columnar epithelium and support cells. When the oviducal gland was at various development stages, the cell types of the lamellar and the size of the intercellular space within the leaflet were different. When the gland was immature, the lobe had a regular shape, with more connective tissue in the center and a single layer of columnar epithelial cells on each side. When the gland was about to mature, the connective tissue increased, the number of columnar epithelial cells decreased, and a large number of mucous acinus were simultaneously generated. After the glands matured and spawned, the amount of connective tissue decreased, the columnar epithelial cells disappeared, the mucous acinus ruptured, and the secretory leaflets were filled with secretory substances. The individual oviduct gland secretory cells of S. lessoniana were large and contained many closely arranged mucus granules, which were round or oval in shape. The cytoplasm contained many organelles, such as the endoplasmic reticulum, mitochondria, and Golgi apparatus. In addition, secretory cells were continuously distributed outside the cell with cilia and numerous secretory granules. The nucleus of the muscle cells in the hyaline valve was irregular, and a substantial number of myofilaments, collagen fibers, and capillaries could be seen around it. The oviducal gland is an important gland in the reproductive system of female cephalopods. The morphological characteristics of oviducal glands, such as the number, shape, size, and presence or absence of pigmentation, are some of the distinguishing characteristics between different cephalopod species. The tissue structure and cell types of the cephalopod oviducal glands changed with the growth of individuals. The changes in oviducal glands in S. lessoniana were similar to those in Loligo forbesi. The secretory lobes successively generated an increasing number of mucous acini and secretory substances as cell types shifted. These secreted substances had multiple functions, such as attracting sperm during the sperm-egg hatching process, expanding the chorionic membrane, forming the vitelline space, and regulating the osmotic pressure between the fertilized egg and the ambient seawater. However, the oviducal glands of the orders Cuttlefish and Liliformes differ from those of the order Occarpus, whose oviducal glands had the function of storing sperm. It is concluded that the oviducal gland primarily plays a secretory function in the reproductive activities of cephalopods and that its secretory material forms the second layer of the egg membrane of fertilized eggs, ensuring the normal hatching of fertilized eggs.