Carmen Roldán

ULTRASTRUCTURAL OBSERVATIONS ON THE PHORONID NERVOUS SYSTEM

Standard transmission electron microscopy (TEM) techniques have been applied to three species of phoronids to describe cytological features of their nervous systems. Four types of neurons have been distinguished by morphological and ultrastructural criteria, mainly involving their kinds of vesicles. Glial cells with characteristic polymorphic dense granules are abundant in the plexus, with possible roles in neuronal nourishment, mechanical support, and neurosecretion. Synaptic contacts and neuromuscular junctions across the connective tissue are described. The different structural features of the trunk giant axons in the three species mainly concern the enveloping sheath, and are described and discussed. Data on the innervation of internal organs, such as gut and nephridia, are also given. A detailed description of the structure, arrangement, and relationships of tentacular sensory cells, probably mechanoreceptors, is included, with a tentative hypothesis concerning the functions of these cells.

Ultrastructural study of two glandular systems in the proboscidial glandular epithelium of Riseriellus occultus (Nemertea, Heteronemertea)

Abstract Two different types of glandular system in the proboscidial epithelium of Riseriellus occultus have been investigated by transmission electron microscopy. As expected, most of the epithelial cells are glandular in nature. With regard to differences in the ultrastructure of these gland cells and in the formation and morphology of their secretory granules, we have categorized and described four types of gland cell, indicated as G1, G2, G3, and G4. Each gland cell has a completely intraepithelial body characterized by a prominent nucleus, developed rough endoplasmic reticulum, Golgi complexes, and numerous secretory granules at different stages of maturation. These four types of gland cell appear associated in pairs forming numerous glandular systems of two types (A, B). These glandular systems are restricted to the ventral surface of the proboscis and are scattered irregularly throughout its length. Each glandular system consists of two gland cells of different types. The gland cell necks in each glandular system extend together to the epithelial surface; they protrude onto this and form a papilla where they open in a common area. The epithelial supportive cells adjacent to the glandular systems have long, stout microvilli which have a core of tonofilaments. These tonofilaments gather into dense bundles which pass vertically through the supportive cells and attach to the extracellular matrix underlaying the cells by hemidesmosomes. Moreover, a single sensory process stands close to each papilla. The ultrastructural morphology of the type A glandular systems suggests that they have an adhesive function operating in a similar way to that of the duo-gland adhesive systems in other invertebrate groups, although they are not homologous with these. The spatial arrangement of the secreted products of the type B glandular systems suggests that these may contribute to increasing the grip of the proboscis on the prey. The secretory granules (=pseudocnids) of the type G3 gland cells are very likely an autapomorphy of the Anopla, providing a character by which the relationships within the Nemertea can be evaluated.

Ultrastructural study of sensory cells of the proboscidial glandular epithelium of Riseriellus occultus (Nemertea, Heteronemertea)

Only one sensory cell type has been observed within the glandular epithelium of the proboscis in the heteronemertine Riseriellus occultus. These bipolar cells are abundant and scattered singly throughout the proboscis length. The apical surface of each dendrite bears a single cilium enclosed by a ring of six to eight prominent microvilli. The cilium has the typical 9×2 + 2 axoneme arrangement and is equipped with a cross‐striated vertical rootlet extending from the basal body. No accessory centriole or horizontal rootlet was observed. Large, modified microvilli (stereovilli) surrounding the cilium are joined together by a system of fine filaments derived from the glycocalyx. Each microvillus contains a bundle of actin‐like filaments which anchor on the indented inner surface of a dense, apical ring situated beneath the level of the ciliary basal body. The tip of the cilium is expanded and modified to form a bulb‐like structure which lies above the level where the surrounding microvilli terminate. In the region where the cilium emerges from the microvillar cone, the membrane of the microvillar apices makes contact with a corresponding portion of the ciliary membrane. At this level microvilli and cilium are apparently firmly linked by junctional systems resembling adherens junctions. The results suggest that these sensory cells may be mechanoreceptors.

Ultrastructural study of the proboscis endothelium of Riseriellus occultus (Nemertea, Heteronemertea)

We have examined with transmission electron microscopythe epithelial layer exposed to the rhynchocoel fluidof the proboscis in the heteronemertine Riseriellus occultus. This epithelium is organized asa monociliated, pseudostratified myoepitheliumconsisting of two cell types: apically situatedmonociliated supportive cells and subapical myocyteslacking cilia. The low supportive cells form acontinuous adluminal sheet and reach with numerouscytoplasmic processes into the extracellular matrix;these cells are characterized by numerous, irregularlyshaped, apical folds projecting into the rhynchocoelfluid, delimiting broad extracellular spaces. Theauthors suppose that both apical and basal folds couldaccommodate stretching of the endothelium when theproboscis is everted. The apical folds of thesupportive cells increase the interface of these withthe rhynchocoel fluid; this feature, together with thepresence of pinocytotic vesicles in such cells,suggest that they could be involved in the exchange ofsubstances between the rhynchocoel fluid and theproboscis. The myocytes are scattered singly withinthe monociliated pseudostratified myoepithelium. Theyare situated between the supportive cells and thesubjacent extracellular matrix. Basement membraneseparating both cells types is lacking. Myofibrillarparts protrude basally from the myocyte somata. Themyofibrillar parts lie in direct apposition to theextracellular matrix, and are oriented circular to thelongitudinal axis of the proboscis. We consider themyocytes to be intra-epithelial, myoepithelial cells.

A place for nourishment or a slaughterhouse? Elucidating the role of spermathecae in the terrestrial annelid Hormogaster elisae (Clitellata: Opisthopora: Hormogastridae)

The capacity of storing sperm within the female reproductive tract occurs widely across vertebrate and invertebrate species. Although the type and position of spermathecae have been commonly used as a taxonomic character in Opisthopora, few studies have focused on the ultrastructural description of these interesting storage organs. This study is the first to report on the ultrastructure of the spermathecae and spermatozoa of Hormogaster elisae, an endemism of the central area of the Iberian Peninsula that presents two pairs of tubular spermathecae. Light and electron microscopy showed that the spermathecae are full of highly packed spermatozoa embedded in an electron-dense substance. Two layers constitute the spermathecal wall. The outer layer consists of peritoneal cells, collagenous basal laminae at different levels, several layers of striated muscle, and numerous blood vessels. The inner layer is a monostratified epithelium of prismatic cells presenting long and abundant microvilli probably for the maintenance of a favorable environment for the spermatozoa. The epithelial cells show high activity, and three different types of secretions were detected: holocrine, merocrine, and apocrine, whose hypothetical function on nourishment and/or causing quiescence is discussed here. Although no phagocytotic processes were detected, some sperm cells were observed in digestive vesicles within the cytoplasm of the epithelial cells, and there was also evidence of active sperm entrance into the epithelium. A place for nourishment or a slaughterhouse? Probably both.