Neurosecretory cells found along the dorsal strand of ascidians

在所有脊椎动物物种中起维持体内平衡作用的垂体，在低等脊椎动物中突然以其完整的形式出现。也就是说，脑垂体是脊椎动物特有的器官，在无脊椎动物中不存在。从系统发生的观点来看，任何与脊椎动物腺垂体同源的结构都可能存在于原索动物中的可能性早已被指出。海鞘类的神经腺与脊椎动物的腺垂体在形态上相似，因此被认为是同源的。然而，尽管最近采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缺乏。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，揭示了无囊体背链与脊椎动物腺垂体的同源性。它们是：（1）沿雌鼠股沿着存在催乳素（PRL）免疫反应细胞。这些细胞的分泌颗粒中的免疫反应物质的定位已被确定通过免疫电镜。(2)促性腺激素（GTH）免疫反应细胞的存在。(3)存在促肾上腺皮质激素（ACTH）和POMC相关β-内啡肽免疫反应细胞。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.