On Homology between the Neural Complex of Tunicates and the Hypophysis of Vertebrates

The organs of the tunicate neural complex and parts of the vertebrate hypophysis were compared on the basis of morphological, physiological, embryological, and endocrinological debatable data. The hypothesis of the homology between the tunicate neural gland and ciliary organ (together with the dorsal tube) and the vertebrate neurohypophysis and adenohypophysis, respectively, is substantiated. In contrast to the hypophysis, the neural complex is a multifunctional structure. The presence of hormonelike substances (typical of the adenohypophysis) in the neural gland testifies to the independent evolution of the neural complex and hypophysis, both of them originating from some “protohypophysis” of hypothetical protovertebrates.     

Structure,Ultrastructure and Function of the Neural Gland Complex of Ascidia interrupta (Chordata,Ascidiacea): Clarification of Hypotheses Regarding the Evolution of the Vertebrate Anterior Pituitary

Abstract The neural gland complex of Ascidia interrupta consists of three parts: dorsal tubercle, ciliated duct, and neural gland. The dorsal tubercle protrudes above the pharyngeal lining and bears a ciliated funnel. The funnel opens into a ciliated duct which opens into the neural gland, a blind sac in a blood sinus below the brain. Funnel and duct cells are joined by adhaerens junctions and, apically, by putative tight junctions. The neural gland wall is a loose, irregular, non-ciliated epithelium of phagocytes. Adhaerens, but not tight, junctions join the cells. Secretory cells were not observed. Tracers delivered onto the dorsal tubercle and dissolved in seawater around undissected animals are transported unidirectionally inward into the neural gland. The continuous ciliary incurrent moves the tracers across the wall of the neural gland and into the pharyngeal blood vessels. Small particulates and large macromolecules, however, are removed from the water stream by endocytosis on neural gland cells. Large particulates delivered onto the dorsal tubercle do not enter the system but rather are rejected by cilia on the surface of the tubercle. The neural gland complex is interpreted as an organ of blood volume regulation that consists of a pump (cilia) and coarse (tubercle) and fine (gland) filters. Analogous and homologous systems are discussed.     

Adrenocorticotropin-like immunoreactivity in the granules of neural complex cells of the ascidian Halocynthia roretzi

Immunohistochemical studies on the neural complex (neural gland, dorsal strand, and cerebral ganglion) of an ascidian, Halocynthia roretzi, were performed by using an antiserum against porcine ACTH. The antiserum recognized a considerable number of the cells scattered along the tubular structure of the dorsal strand and a few cells in the cerebral ganglion. Immunoelectron microscopic studies revealed that the ACTH-like substance resided within secretory granules with diameter of 300-500 nm. Furthermore, those ACTH-immunoreactive cells were demonstrated to be different from PRL-immunoreactive cells, the presence of which had previously been reported.     

Stomodeal and neurohypophysial placodes in Ciona intestinalis: insights into the origin of the pituitary gland

The ascidian larva has a central nervous system which shares basic characteristics with craniates, such as tripartite organisation and many developmental genes. One difference, at metamorphosis, is that this chordate-like nervous system regresses and the adult's neural complex, composed of the cerebral ganglion and associated neural gland, forms. It is known that neural complex differentiation involves two ectodermal structures, the neurohypophysial duct, derived from the embryonic neural tube, and the stomodeum, i.e. the rudiment of the oral siphon; nevertheless, their precise role remains to be clarified. We have shown that in Ciona intestinalis, the neural complex primordium is the neurohypophysial duct, which in the early larva is a short tube, blind anteriorly, with its lumen in continuity with that of the central nervous system, i.e. the sensory vesicle. The tube grows forwards and fuses with the posterior wall of the stomodeum, a dorsal ectodermal invagination of the larva. The duct then loses posterior communication with the sensory vesicle and begins to grow on the roof of the vesicle itself. The neurohypophysial duct differentiates into the neural gland rudiment; its dorsal wall begins to proliferate neuroblasts, which migrate and converge to build up the cerebral ganglion. The most anterior part of the neural gland organizes into the ciliated duct and funnel, whereas the most posterior part elongates and gives rise to the dorsal strand. The hypothesis that the neurohypophysial duct/stomodeum complex possesses cell populations homologous to the craniate olfactory and adenohypophysial placodes and hypothalamus is discussed.     

Progressive determination during formation of the anteroposterior axis in Xenopus laevis

The cement gland is an ectodermal organ in the head of frog embryos, lying anterior to any neural tissue. As analyzed by specific RNA expression, cement gland, like neural tissue, was induced by the dorsal mesoderm. Interestingly, mesoderm with the highest cement gland-inducing potential lay posterior to the ectoderm fated to form this organ, indicating that its induction occurred at a distance from the inducer source. Cement gland induction first occurred during early gastrulation. However, most initially induced cells did not contribute to the mature cement gland, but instead formed part of the neural plate. This change in fate could be reconstituted in vitro. These results suggest that determination of part of the anteroposterior axis occurs progressively, where future neural ectoderm is first induced to a cement glandlike state. As gastrulation proceeds, further induction by mesoderm may override this state, which persists only in the extreme anterior of the embryo.     

Structural changes of the guinea pig prostatic epithelial cells after gossypol treatment

The effect of gossypol acetic acid on the prostate gland of the guinea pig was assessed ultrastructurally. The three lobes of the gland showed differences in sensitivity and reacted differently to gossypol treatment. In the lateral prostate, there was a reduction in the profile of the granular endoplasmic reticulum and dense secretory granules with a concurrent appearance of smaller clear granules and an increase in cytoplasmic filamentous bundles. The latter feature was similar to that of the coagulating gland. In the dorsal prostate, the general reaction was similar to that in the lateral prostate except that there was no increase in filamentous content. In the coagulating gland, there was a reduction or total disappearance of apical secretory blebs and an increase in lysosomes, a feature not found in the lateral and dorsal lobes. Damages to mitochondria were observed in the dorsal prostate and coagulating gland but not in the lateral prostate. It is concluded that gossypol affects not only the testis, but also alters the structure and functions of the prostate gland.     

Xenopus galectin-VIa shows highly specific expression in cement glands and is regulated by canonical Wnt signaling

Anterior-posterior neural patterning of Xenopus embryo is determined during gastrulation and then followed by differentiation of neural structures including brain and eye. The cement gland is a mucus-secreting neural organ located in the anterior end of the neural plate. This study analyzed expression patterns of Xenopus galectin-VIa (Xgalectin-VIa) by whole-mount in situ hybridization, and found highly restricted expression of this gene in the cement gland region. These patterns were similar to those of XAG-1 and XCG, known cement gland-specific genes. In addition, Xgalectin-VIa was expressed in the dorsal edge of eye vesicles, the otic vesicle, and in part of the hatching gland at the tadpole stage. Although the spatial expression pattern was similar, the temporal expression of Xgalectin-VIa differed from that of XAG-1 and XCG. RT-PCR analysis showed only weak Xgalectin-VIa expression in early neurula embryos, whereas both XAG-1 and CGS were strongly expressed at that stage. We also showed that Xgalectin-VIa expression is repressed by enhancement of Wnt signaling and increased by its inhibition. Furthermore, Xgalectin-VIa expression was activated by neural-gene inducer Xotx2, as is the case for XAG-1 and CGS. Together, these results indicated that Xgalectin-VIa possesses different features from other cement gland genes and is a novel and useful marker of the cement gland in developing embryos.     

Regulation of ectodermal differentiation in Xenopus laevis animal caps treated with TPA and ammonium chloride

Animal caps isolated from Xenopus laevis embryos at the blastula stage were treated sequentially with NH₄Cl, a known cement gland inducer, and with 12-O-tetradecanoyl phorbol-13-acetate (TPA), a known neural inducer. The two artificial inducers were also used in reverse order to see if they can mimic the natural inducers acting during the progressive determination of the ectodermal organ. Immunofluorescence and whole-mount in situ hybridization were used to study the expression of tubulin, taken to indicate an early step on the pathway of cell elongation, and neural cell adhesion molecule (N-CAM) taken to indicate an early step in the determination of the nervous system. The expression of XCG-1, a marker of early specification of the cement gland, was also studied. The results showed that the two artificial inducers can mimic the effects of the natural inducers in animal cap explants. The TPA behaves like a neural inducer, reducing the number and the extension of the cement gland when added to the medium in addition to NH₄Cl, before or after NH₄Cl treatment. In the process of cement gland/neural induction, it is possible to redirect the ectoderm already specified as cement gland to neural tissue, but it does not seem possible to respecify the neural tissue as cement gland. Moreover, the animal caps were also cut into dorsal and ventral parts and the two halves were treated separately. The results were similar to those obtained with treatment of the entire animal cap, suggesting that a dorsal-ventral pattern is not yet established before the gastrula stage, and that in normal embryos there are boundaries between the effects of different inducers.     

Origin and evolution of the neural crest: a hypothetical reconstruction of its evolutionary history

The neural crest has long been regarded as one of the key novelties in vertebrate evolutionary history. Indeed, the vertebrate characteristic of a finely patterned craniofacial structure is intimately related to the neural crest. It has been thought that protochordates lacked neural crest counterparts. However, recent identification and characterization of protochordate genes such as Pax3/7, Dlx and BMP family members challenge this idea, because their expression patterns suggest remarkable similarity between the vertebrate neural crest and the ascidian dorsal midline epidermis, which gives rise to both epidermal cells and sensory neurons. The present paper proposes that the neural crest is not a novel vertebrate cell population, but may have originated from the protochordate dorsal midline epidermis. Therefore, the evolution of the vertebrate neural crest should be reconsidered in terms of new cell properties such as pluripotency, delamination-migration and the carriage of an anteroposterior positional value, key innovations leading to development of the complex craniofacial structure in vertebrates. Molecular evolutionary events involved in the acquisitions of these new cell properties are also discussed. Genome duplications during early vertebrate evolution may have played an important role in allowing delamination of the neural crest cells. The new regulatory mechanism of Hox genes in the neural crest is postulated to have developed through the acquisition of new roles by coactivators involved in retinoic acid signaling.     

Separation of an anterior inducing activity from development of dorsal axial mesoderm in large-headed frog embryos

The body of a vertebrate arises through a series of inductive interactions in the embryo. Macrocephaly is a distortion of the body in which a disproportionate amount of tissue is devoted to the head. This syndrome occurs in certain hybrids between frog species and appears to be due to an alteration of inductive relationships. Chimeric blastulae between normal and hybrid embryos developed macrocephaly when the marginal zone was derived from the hybrid. In these cases, a large cement gland, characteristic of the hybrid head, was induced to form from normal ectoderm. When hybrid zygotes were irradiated with ultraviolet (uv) light, all dorsoanterior structures, including notochord, somites, and central nervous system, were eliminated, but the most anterior-induced structure, the cement gland, remained. Embryos without dorsoanterior structures but with cement glands were also produced by injecting germinal vesicle extracts into the blastocoel of uv-irradiated nonhybrid embryos. These results demonstrate that an anterior inducing activity can be uncoupled from development of the neural tube and dorsal axial mesoderm.     

Morphological and functional heterogeneity in the rat prostatic gland.

Ductal morphogenesis and adult ductal branching patterns were examined in the rat prostate by a microdissection method. The rat prostate consists of paired (right and left) subdivisions which correspond in large part to the classically defined lobes: ventral prostate, lateral prostate, dorsal prostate, and coagulating gland. Of particular interest was the finding that the lateral prostate consists of two different ductal zones: (1) lateral type 1 prostate with 5-7 long main ducts (resembling miniature palm trees) that extend cranially towards both the seminal vesicle and dorsal prostate to arborize near the bladder neck, and (2) lateral type 2 prostate with 5-6 short main ducts that arborize caudal to the bladder neck and give rise to compact bushy glands. Both lateral prostatic groups had a ductal-acinar organization. The adult structure of the other rat prostatic lobes was also examined, and closely resembled their mouse counterparts. The ventral prostate, which had 2-3 pairs of slender main ducts per side, and the coagulating gland, which had 1 main duct per side, was completely ductal in structure. In contrast, the dorsal prostate, which had 5-6 pairs of main ducts per side, had a ductal-acinar structure. Ductal branching morphogenesis occurred at different rates in different lobes and was essentially complete in the prostate at the 30 days. Immunocytochemical studies with an antibody to DP-1, a major secretory protein of the rat dorsal prostate, revealed that secretory function was initiated at approximately 30 days after birth in the coagulating gland, the dorsal prostate, and lateral type 1 prostate. A consistent feature of the lateral type 2 prostate was the absence of DP-1. On Western blots, DP-1 was detected in the secretion of the coagulating gland, lateral type 1 and dorsal prostate, but not in the ventral and lateral type 2 prostate. Polyacrylamide gel electrophoresis confirmed this result and demonstrated that the lateral type 2 prostate expressed several low-molecular weight secretory proteins not found in the other lobes of the prostate. On the whole, the rat prostate exhibited considerable heterogeneity both between and within lobes in developmental processes, ductal patterning, histology, and functional expression.     

Fine Structure of the Esophagus of Males of Sarisodera hydrophila (Heteroderoidea)

The fine structure of the esophagus, including procorpus, metacorpus, isthmus, gland lobe, and esophago-intestinal junction, is examined in males of Sarisodera hydrophila. A cuticle-lined lumen extends most of the length of the esophagus, broadens to form a pump chamber in the metacorpus, and posteriorly is continuous with junctional complexes among four esophago-intestinal cells. These four cells are partially enveloped by the gland lobe which basically consists of three gland cells, one dorsal and two subventral. Each gland cell has an anterior process which opens into the lumen of the esophagus through a cuticle-lined duct. The dorsal gland joins the lumen in the anterior portion of the procorpus, whereas ducts of the subventral glands terminate at the base of the metacorpus pump chamber. The subventral glands are predominant in the posterior portion of the gland lobe and are partially ensheathed by a narrow portion of the dorsal gland which extends to within 5 μm of the posterior terminus of the gland lobe. Contents of the dorsal gland include primarily electron dense granules, although rough endoplasmic reticulum (RER) is predominant posteriorly. Secretory granules within the subventral glands vary in morphology and are evenly distributed throughout the two ceils among other organelles, including RER and a large Golgi apparatus. Innervation of the esophagus includes nerve processes which originate from several perikaryons (cell bodies) located in the anterior portion of the gland lobe. The esophagus of males of S. hydrophila is compared with that of other Heteroderoidea, Heterodera glycines and Meloidogyne incognita.     

Development of osteological structures in the sea bream: vertebral column and caudal fin complex

The development of cartilaginous structures in cultured sea bream Sparus aurata larvae and the timing of their ossification was studied. In cultivated sea bream larvae the first cartilaginous structure to be identified was hypural 1 at 4.1 mm notochord length ( L _N). By 5.3 mm L _N, prior to the onset of ossification, it was possible to distinguish the following cartilaginous structures: all 23 neural arches, all 13 haemal arches and two of the four pairs of parapophyses. The neural arches 1–4 and 15–23 were formed on the notochord and elongated dorsally, while neural arches 5–14 appeared on the dorsal side of the spinal cord and elongated ventrally. Initiation of ossification occurred at 5.7–6.0 mm standard length ( L _S) when the cartilaginous ontogeny of the vertebral column was completed. Ossification was coincident with dorsal flexion at the posterior end of the notochord and occurred in a sequential manner: (1) dorsoanteriorly, the cartilaginous neural arches and the centra were the first structures to ossify; (2) ventrad at the centre, at 7.0–7.5 mm L _S; (3) posteriorly at 7.1 mm L _S the hypural complex and urostyle (24th centrum) were ossified; and (4) dorsad at the centre (neural arches and spines).     

Ultrastructure of esophageal glands and their secretory granules in the root-knot nematodeMeloidogyne incognita

Summary The dorsal and subventral esophageal glands and their secretory granules in the root-knot nematodeMeloidogyne incognita changed during parasitism of plants. The subventral esophageal glands shrank and the dorsal gland enlarged with the onset of parasitism. While secretory granules formed by both types of glands were spherical, membrane-bound, and Golgi derived, the granules differed in morphology and size between the two types of glands. Subventral gland extensions in preparasitic second-stage juveniles were packed with secretory granules which varied in diameter from 700–1,100 nm and had a finely granular matrix. Within the matrix of each subventral gland granule was an electron-transparent core that contained minute spherical vesicles. The size and position of the core varied within different granules. Few granules were present in the dorsal gland extension in preparasitic juveniles. The matrix of dorsal gland secretory granules formed during parasitism was homogeneous and more electron-dense than the matrix of subventral gland granules. Subventral gland secretory granules of parasitic juveniles and adult females appeared degenerate.     

Zum Nachweis des Axialkomplexes bei Holothurien (Echinodermata)

On the basis of a histological investigation concerning 16 species of 4 holothurian orders, the homology of the so-called problematic canal with the axial sinus on the one side and the dorso-axial haemal strand with the axial gland on the other side is discussed. Combining these results with former investigations, the following conclusions are drawn. There is no axial complex as an organic unit represented within the class Holothuroidea. Nevertheless, this axial complex, formerly thought to be missing, is locally disintegrated into two parts, which are isolated from each other within the dorsal mesentery. One part is the stone canal—protocoelampulla—the other one the axial sinus—axial gland. Within the holothurians there is a tendency towards reduction of the protocoelampulla and the axial sinus. This recently aberrant structure of the axial complex can be deduced from an echinoid "plan".     

The postnatal development of the pelage and ventral gland of the male gerbil

The postnatal development of the pelage and ventral gland of male Mongolian gerbils ranging from newborn to 86 days of age was studied. The development of the gerbil pelage follows a pattern similar to that observed for other rodents. The length of the dorsal and ventral skin juvenile hair cycle was found to be 26 to 28 days with a 15 to 18 day anagen and a ten to 11 day catagen and telogen. Hair follicles in the ventral gland began growth ten days later than those of the general pelage and secondary follicles budded from the sides of primary follicles. The ventral gland area differed from the general pelage in that it lacked a panniculus carnosus. The ventral gland is a complex of pilosebaceous glands which, in the adult, fill the entire hypodermis. The length and width of the pilosebaceous canals of the gland units are greater than those of the dorsum. The period of telogen of the hair follicles in the ventral gland is very short. The mid-ventral gland of the male gerbil appears to be a secondary sexual characteristic.     

Ectodermal ablation of the third branchial arch in chick embryos and the morphogenesis of the parathyroid III gland.

The parathyroid glands have been classically considered to be derivatives of the third and fourth pharyngeal pouches in most species, including humans. Furthermore, the presence of neural crest-derived cells in the parathyroid glands connective tissue has been apparently established. However, our previous studies have provided a new hypothesis on the origin of these glands in human and chick embryos. To determine the origin of the parathyroid III (P3) gland, ectoderm of the third branchial arch was cauterized in chick embryos at Hamburger and Hamilton's stage 19 (embryonic day 3). Cauterization of the ventral half of the ectoderm was followed by the non-formation, on the same side, of the P3 gland. When the dorsal half of the ectoderm was cauterized, both the right and left P3 glands formed. Our observations suggest that the ectoderm of the ventral half of the third branchial arch is necessary for the organization of the P3 gland.     

A chemotactic model of trunk neural crest cell migration

Trunk neural crest cells follow a common ventral migratory pathway but are distributed into two distinct locations to form discrete sympathetic and dorsal root ganglia along the vertebrate axis. Although fluorescent cell labeling and time‐lapse studies have recorded complex trunk neural crest cell migratory behaviors, the signals that underlie this dynamic patterning remain unclear. The absence of molecular information has led to a number of mechanistic hypotheses for trunk neural crest cell migration. Here, we review recent data in support of three distinct mechanisms of trunk neural crest cell migration and develop and simulate a computational model based on chemotactic signaling. We show that by integrating the timing and spatial location of multiple chemotactic signals, trunk neural crest cells may be accurately positioned into two distinct targets that correspond to the sympathetic and dorsal root ganglia. In doing so, we honor the contributions of Wilhelm His to his identification of the neural crest and extend the observations of His and others to better understand a complex question in neural crest cell biology.