Department of Palaeontology, Faculty of Earth Sciences, Geography and Astronomy, Geozentrum, University of Vienna, Althanstraße 14, A-1090, Vienna, Austria.
Front Zool. 2012 Oct 25;9(1):29. doi: 10.1186/1742-9994-9-29.
Matrotrophy or extraembryonic nutrition - transfer of nutrients from mother to embryo during gestation - is well known and thoroughly studied among vertebrates, but still poorly understood in invertebrates. The current paper focuses on the anatomy and ultrastructure of the oogenesis and placentotrophy as well as formation of the brood chamber (ovicell) in the cheilostome bryozoan Bicellariella ciliata (Linnaeus, 1758). Our research aimed to combine these aspects of the sexual reproduction into an integral picture, highlighting the role of the primitive placenta-like system in the evolution of bryozoan reproductive patterns.
Follicular and nutrimentary provisioning of the oocyte occur during oogenesis. Small macrolecithal oocytes are produced, and embryos are nourished in the ovicell via a simple placental analogue (embryophore). Every brooding episode is accompanied by the hypertrophy of the embryophore, which collapses after larval release. Nutrients are released and uptaken by exocytosis (embryophore) and endocytosis (embryo). Embryos lack specialized area for nutrient uptake, which occurs through the whole epidermal surface. The volume increase between the ripe oocyte and the larva is ca. 10-fold.
The ovicell is a complex organ (not a special polymorph as often thought) consisting of an ooecium (protective capsule) and an ooecial vesicle (plugging the entrance to the brooding cavity) that develop from the distal and the fertile zooid correspondingly. Combination of macrolecithal oogenesis and extraembryonic nutrition allows attributing B. ciliata to species with reproductive pattern IV. However, since its oocytes are small, this species represents a previously undescribed variant of this pattern, which appears to represent a transitional state from the insipient matrotrophy (with large macrolecithal eggs) to substantial one (with small microlecithal ones). Altogether, our results substantially added and corrected the data obtained by the previous authors, providing a new insight in our understanding of the evolution of matrotrophy in invertebrates.
胎生或胚外营养——在妊娠期间从母体向胚胎输送营养物质——在脊椎动物中是众所周知且研究透彻的，但在无脊椎动物中仍知之甚少。本文重点介绍了瓣鳃类苔藓虫 Bicellariella ciliata（Linnaeus，1758）的卵子发生和胎盘营养以及孵育室（卵囊）形成的解剖结构和超微结构。我们的研究旨在将有性生殖的这些方面结合成一个整体，强调原始胎盘样系统在苔藓虫生殖模式进化中的作用。
卵母细胞的滤泡和营养供应发生在卵子发生过程中。产生小的巨卵母细胞，胚胎通过简单的胎盘类似物（胚胎营养体）在卵囊中得到滋养。每次孵育都会伴随着胚胎营养体的肥大，在幼虫释放后会崩溃。营养物质通过胞吐作用（胚胎营养体）和胞吞作用（胚胎）释放和摄取。胚胎缺乏专门的营养吸收区域，这是通过整个表皮表面进行的。从成熟卵到幼虫的体积增加约为 10 倍。
卵囊是一个复杂的器官（不像通常认为的那样是一种特殊的多态），由卵囊（保护胶囊）和卵囊泡（堵塞孵育腔的入口）组成，分别由远端和有生殖力的个体发育而来。大卵母细胞发生和胚外营养的结合使得 B. ciliata 被归为具有生殖模式 IV 的物种。然而，由于其卵母细胞较小，因此该物种代表了以前未描述的这种模式的变体，这种模式似乎代表了从初期胎生（具有大的巨卵母细胞）到实质性胎生（具有小的微卵母细胞）的过渡状态。总的来说，我们的结果大大补充和纠正了以前作者获得的数据，为我们理解无脊椎动物胎生的进化提供了新的认识。
