Abstract The neurohypophysis of three species of the Osteoglossidae ( Osteoglossum bicirrhosum, Scleropages jardini and Heterotis niloticus ) was studied histologically. The rostral part of the neurohypophysis abuts on the pars distalis of the adenohypophysis, but does not interdigitate with it. The caudal part of the neurohypophysis is stained densely with aldehyde fuchsin and extensively interdigitates with the pars intermedia, forming a neurointermediate lobe. This organization of the neurohypophysis is intermediate between that of holosteans and that of non‐osteoglossomorph teleosts. The median eminence‐like features of the rostral part of the neurohypophysis are distinct in the Osteoglossidae in contrast to non‐osteoglossomorph teleosts. In Osteoglossum bicirrhosum , immunohistochemical study reveals the presence of somatotropin release‐inhibiting factor‐like substance in the rostral part of the neurohypophysis. Fibers immunoreactive to anti‐arginine vasopressin occur in the neurointermediate lobe almost exclusively. These observations also may support the assumption that the rostral part of the neurohypophysis corresponds functionally to the median eminence. For comparison, the occurrence and distribution of somatotropin release‐inhibiting factor‐like substance, luteinizing hormone‐releasing factor‐like substance and arginine vasopressin‐like substance in the neurohypophysis of Gymnarchus niloticus are described.
The fine structure of the kidney and the bladder of the bullfrog (Rana catesbeiana), the bullfrog tadpole, and the mudpuppy (Necturus maculosus) were studied with special attention to the innervation of renal tubule cells and bladder epithelial cells. In the bullfrog kidney, nerve terminals and varicosities were frequently associated with the tubule cells, apparently in an increasing order from the proximal tubule to the connecting tubule. Although these terminals and varicosities did not directly contact the tubular cell membrane, an aggregation of synaptic vesicles on the side facing the tubule was considered as morphological evidence that neurotransmitter can be released here and can affect the transport activity of the tubule cells. The association of nerve varicosities with canaliculi cells in the connecting tubule was also demonstrated. In the bullfrog tadpoles, renal tubule cells were occasionally innervated. In the mudpuppy, renal tubule cells were only poorly innervated. The epithelium of the bullfrog bladder was commonly innervated. Nerve terminals with synaptic vesicles were located very near basal cells and even contacted them directly on rare occasions. In the mudpuppy, the innervation of the bladder epithelium was observed infrequently. The bullfrog tadpoles did not possess an apparent bladder. In all materials studied, renal arterioles and bladder smooth muscle cells were innervated.
This study offers an example of the attempts to correlate the shape of an organ to the nucleus size. The pituitary glands of gobiid teleosts are partly embedded in the hypothalamus in contrast to those of non-gobiid teleosts. The pituitaries of the embryonic smelt, catfish, and guppy are partly embedded in the hypothalamus as in the embryonic and adult goby, Rhinogobius flumineus. The nucleus size of cerebellar granular cells, that may represent the genome size, is variable in gobies (16 species studied) as in non-gobiid perciforms (63 species studied). However, the ratio of the nucleus size of cerebellar Purkinje's cells to that of granular cells is consistently low in gobiids compared with non-gobiid perciforms. This ratio is a sort of peramorphic (morphologically advanced) index, and the low value may represent paedomorphosis (juvenile morphology). The gobiid pituitary gland partly embedded in the hypothalamus may be the result of paedomorphosis that could be reflected in the relatively poor increase rate in the size of Purkinje's cells as against the genome size.
Abstract The renal sacs of a diversity of cephalopod molluscs were examined to study the morphology of dicyemid mesozoans. Most of the dicyemid species studied were found to be host specific. Typically, two or more species of dicyemids were present in each host species or each host individual. When dicyemid species co‐occurred, their calotte shapes were distinctly different. The following variations in calotte shapes were usually detected within a given host individual: (1) when two species of dicyemids were present, two distinct calotte shapes, conical and discoidal, were observed; (2) when three species of dicyemids were present, three types of calotte configurations were observed, conical (two grades) and discoidal; (3) when more than four species of dicyemids were present, at least one species was characterized by its rare irregularly shaped calotte. As a rule, when more than two dicyemid species were present in a single host individual, calotte shapes were dissimilar. Calotte shapes in dicyemid species from different host species more closely resemble each other than those of dicyemids observed within the same host species. Dicyemids with conical or dome‐shaped calottes are found within the convolutions or folds of the renal appendages, whereas those with flat, discoidal calottes attach to the surface of the renal appendages. In the dicyemids, calotte morphology seems to result from adaptation to the structure of host renal tissues and helps to facilitate niche separation of coexisting species.
We reviewed recent advances of some aspects on the biology of dicyemid mesozoans. To date 42 species of dicyemids have been found in 19 species of cephalopod molluscs from Japanese waters. The body of dicyemids consists of 10–40 cells and is organized in a very simple fashion. There are three basic types of cell junction, septate junction, adherens junction, and gap junction. The presence of these junctions suggests not only cell-to-cell attachment, but also cell-to-cell communication. In the development of dicyemids, early stages and cell lineages are identical in vermiform embryos of four genera, Conocyema, Dicyema, Microcyema, and Pseudicyema. Species-specific differences appear during later stages of embryogenesis. In the process of postembryonic growth in some species, the shape of the calotte changes from conical to cap-shaped and discoidal. This calotte morphology appears to result from adaptation to the structure of host renal tissues and help to facilitate niche separation of coexisting species. In most dicyemids distinctly small numbers of sperms are produced in a hermaphroditic gonad (infusorigen). The number of eggs and sperms are roughly equal. An inverse proportional relationship exists between the number of infusorigens and that of gametes, suggesting a trade-off between them. Recent phylogenetic studies suggest dicyemids are a member of the Lophotrochozoa.
Patterns of cell division and cell lineages of the vermiform embryos of dicyemid mesozoans were studied in four species belonging to four genera: Conocyema polymorpha, Dicyema apalachiensis, Microcyema vespa, and Pseudicyema nakaoi. During early development, the following common features were apparent: (1) the first cell division produces prospective cells that generate the anterior peripheral region of the embryo; (2) the second cell division produces prospective cells that generate the posterior peripheral region plus the internal cells of the embryo; (3) in the lineage of prospective internal cells, several divisions ultimately result in cell death of one of the daughter cells. Early developmental processes are almost identical in the vermiform embryos of all four dicyemid genera. The cell lineages appear to be invariant among embryos and are highly conserved among species. Species-specific differences appear during later stages of embryogenesis. The number of terminal divisions determines variations in peripheral cell numbers among genera and species. Thus, the numbers of peripheral cells are fixed and hence species-specific.
Neurosecretory axo-axonic synapses were found in the median eminence of the turtle. Most of the presynaptic axons contain granules approximately 95 nm in diameter, and the postsynaptic elements have granules approximately 110 nm in diameter. The functional significance of axo-axonic synapses in the median eminence is discussed in relation to the discharge of releasing hormones from the axon terminals.
