Neurosecretory cells found along the dorsal strand of ascidians

脑垂体在所有脊椎动物中都具有维持体内平衡的功能，但在低等脊椎动物中却突然以完整的形式出现。也就是说，脑垂体是脊椎动物特有的器官，而在无脊椎动物中是不存在的。从系统发育的观点来看，原脊索动物中可能存在任何与脊椎动物腺垂体相似的结构。在海鞘中，神经腺被认为是与脊椎动物腺垂体同源的，因为它们的形态相似。然而，尽管近年来使用各种方法进行了广泛的研究，但表明其同源性的证据非常少。在本研究项目中，我们获得了一些重要的细胞化学和发育证据，揭示了asacidian背链与脊椎动物腺垂体的同源性。它们是：(1)沿真丝链存在催乳素（PRL）免疫反应细胞。免疫反应性物质在这些细胞分泌颗粒中的定位已通过免疫电镜确定。(2)存在促性腺激素（GTH）免疫反应细胞。(3)促肾上腺皮质激素（ACTH）和pomc相关的β -内啡肽免疫反应细胞的存在。此外，许多产生促性腺激素释放激素（GnRH）的神经元位于背束附近。海鞘的这些结构可能与脊椎动物的下丘脑-垂体系统相对应。

Hypophysis, which functions to maintain the homeostasis in all vertebrate species, appears suddenly in its complete form in lower vertebrates. That is, the hypophysis is the organ unique for vertebrates and is absent in invertebrates. From a phylogenetic point of view, the possibility that any structure homologous to the vertebrate adenohypophysis may exist in protochordates has long been pointed out. In ascidians, the neural gland was considered to be homologous to the vertebrate adenohypophysis because of its morphological similarity. However, despite of recent extensive studies using various methods, the evidence that indicates its homology is very scarce.In this research project, we obtained some important cytochemical and developmental evidences revealing the homology of asacidian dorsal strand to the vertebrate adenohypophysis. They are : (1) presence of prolactin (PRL) -immunoreactive cells along the doesal strand. The localization of the immunoreactive material in the secretory granules of these cells has been ascertained by immunoelectron microscopy. (2) presence of gonadotropin (GTH) -immunoreactive cells. (3) presence of adrenocorticotropin (ACTH) -and POMC-related beta-endorphin immunoreactive cells. In adition, numerous gonadotropin-releasing hormone (GnRH) -producing neurons are localized in the vicinity of the dorsal strand. These structures in ascidians may correspond to the hypothalamus-pituitary system of vertebrate.