Ultrastructure of the body cavities in juveniles and adults of the appendicularian Oikopleura gracilis (Tunicata, Chordata) suggests how the heart may have evolved in tunicatesстатья

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[1] Savelieva A., Temereva E. Ultrastructure of the body cavities in juveniles and adults of the appendicularian oikopleura gracilis (tunicata, chordata) suggests how the heart may have evolved in tunicates // Invertebrate Biology. — 2020. — Vol. 139, no. 2. — P. accepted. The organization of the body cavities is an important morphological trait that can be used for establishing the phylogenetic relationships between different groups of animals. In the present study, the hemocoel and coelomic system of 10-h-old juvenile and adult hermaphroditic oikopleurids, Oikopleura gracilis, are examined using light and transmission electron microscopy. The trunk hemocoel in 10-h-old juveniles represents small clefts containing layers of extracellular matrix of adjacent tissues or interstices with migrating primordial germ syncytium (mPGS). The wide hemocoel is present only in tail which contains extracellular strands, subdividing the hemocoel into hemal sinuses. In adults, the large hemocoel appears in the trunk and tail also containing extracellular strands. The hermaphroditic gonad is surrounded by own lining, separating it from the hemocoel. The gamete-filled cavity in the ovary and testis appears only at a late stage of gonadogenesis, when the pre-spawning reduction of syncytium occurs in the gonads. The coelomic origin of the gonads in appendicularians is discussed. The true coelom in 10-h-old juveniles and adults is represented by pericardium. The lining of the pericardium consists of myoepithelial and peritoneal cells. In the myoepithelial cells of 10-h-old juveniles, myofibrils are formed. The myoepithelial cells of adults have several parallel rows of completely differentiated myofibrils. The substantial reduction of the coelomic and circulatory systems in O. gracilis evidently results from the extreme shortening of appendicularians ontogeny. Development of O. gracilis from the early juvenile to the adult involves the following steps that suggest how the tunicate heart may have evolved: a single-layered coelomic sac gives rise to a grooved pericardium with an open hemal sinus (simple heart). In ascidians, this simple heart in turn gives rise to a closed tubular double-layered heart-pericardial complex with a separate pericardial cavity and a closed heart, whose wall is formed by specialized myocardium. [ DOI ]

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