Effect of the notochord on the differentiation of a floor plate area in the neural tube of the chick embryo.

The role of a notochord fragment on the origin of an additional floor plate area in the neural tube is investigated by quantitative morphological methods. In 1.5 to 2 day chick embryos a notochordal fragment was implanted in close apposition to the lateral wall of the neural groove in the region between prospective wing and leg bud. At 4 days, adjacent to the implant a distinct area of the neural wall was present, which resembled the natural floor plate with respect to its thickness, the abluminal location of elongated nuclei and the absence of neuroblasts. The mitotic density of this area was reduced. This "additional floor plate" was distinct when the experiment was performed at 1.5 days but was hardly recognizable when it was carried out at 2 days. From these results it is concluded that a) the notochord induces floor plate like structures and diminishes proliferation, and b) that the period of floor plate induction by the notochord is very restricted.

Differentiation of gut endoderm in dependence of the notochord.

Presumptive intraembryonic endoderm, either isolated or together with adhering mesoderm, from 19-h chick embryos, was grafted to the coelom of 50-h host embryos. The viability of such grafts was low and endodermal differentiation was poor. In a second series the endoderm (with or without adhering mesoderm) was combined with a fragment of notochordal tissue from 48-60-h donor embryos. Then the recovery was much higher, notably after longer periods of in vivo culture. After 10 days of cultivation well-developed entero-endocrine (argyrophilic) cells were found among the regular enterocytes in both series.

Induction of an additional floor plate in the neural tube.

The role of the notochord in the morphogenesis of the neural tube was investigated by implanting a notochord fragment laterally to the neural wall of a 1.5 day chick embryo. Embryos were sacrificed at 4 days. In the basal part of the neural tube an additional floor plate was induced in the vicinity of the implant. This floor plate was characterized by a low proliferative activity, a thin wall, spindle-like nuclei crowded peripherally and some neuroblast-like cells. It was either blending with the natural floor plate or separated from it, depending on the exact position of the implant. In the latter case neuroblasts were observed in between both floor plates. The additional floor plate was present only when the implanted notochord was less than 25 micron apart from the neural tube; at larger distance an increase of the ventral horn neuroblast area could be seen. It is concluded that the implanted notochord is able to induce a floor plate at 1.5 days of incubation. The specific influence of the notochord on the morphogenesis of the neural tube, its inductive period as well as the presence of the neuroblast-like cells in the additional floor plate are discussed.

Effect of a notochordal implant on the early morphogenesis of the neural tube and neuroblasts: histometrical and histological results.

The role of the notochord on the early development of ventral horn neuroblasts was investigated in chick embryos by implanting an additional notochord fragment near the right side of the thoracic neural tube. When the implant was located directly lateral to the neural tube, an enlargement of the right half of the neural tube and of the area of neuroblasts occurred, and axons were found to pass through the outer membrane of the neural tube over a broad dorsoventral trajectory. When the notochord was located ventrolaterally a population of neuroblasts including their efferent axons was found at a more dorsal location. It is concluded that a notochordal implant is able to influence the differentiation of neuroblasts.

On the development of the spinal cord of the clawed frog, Xenopus laevis. I. Morphogenesis and histogenesis.

The morphogenesis and histogenesis of the spinal cord of Xenopus were examined. The study encompasses the developmental period between stage 41 and stage 66 (stages according to Nieuwkoop and Faber 1967). This period can roughly be divided into three phases. From stage 50 up to stage 53 strong proliferation and rapid growth are the most striking features. This developmental phase is preceded and followed by less dynamic periods. From stage 41 up to stage 50 the rate of proliferation is relatively low. The numbers of cells in the matrix and in the mantle layer are very small. In the mantle layer two classes of early differentiated transient neurons can be distinguished: primitive giant sensory or Rohon-Beard cells and primitive motor neurons. From stage 46 onward the originally tube-shaped spinal cord swells at the thoracic level into a thoracic enlargement. After stage 50 the proliferation strongly increases until a maximum at stage 53. Concomitantly a considerable acceleration of growth takes place. The major part of the mitoses are always concentrated in the dorsal part of the matrix. From stage 51 onward the cervical and lumbar regions show much more mitoses than the thoracic part. Distinct cervical and lumbar enlargements develop and are going to mask the thoracic swelling of the cord. From stage 54 on proliferation continues on an increasingly low level. The period between stage 54 and stage 66 is characterized by differentiation of the spinal neuronal elements.

On the development of the spinal cord of the clawed frog, Xenopus laevis. II. Experimental analysis of differentiation and migration.

In order to determine the time and site of origin and the final location of various cell groups in the spinal cord, tadpoles of Xenopus laevis, ranging from stage 48 to stage 56 were treated with tritiated thymidine and sacrificed at various stages from 49 to 66 (stages according to Nieuwkoop and Faber (1967). From the poorly developed matrix at stage 48-49 not only ventral horn cells, but also neuroblasts of the intermediate zone and the dorsal horn arise. Both the matrix and the ventricle expand in a dorsal direction. From the well-developed matrix at stage 54, in which the mitotic activity is almost exclusively confined to its dorsal part, mainly cells of the dorsal horn develop. However, this later-stage matrix also gives rise to a considerable number of neuroblasts, which become located in the central parts of the intermediate zone and the ventral horn. Generally the later-born cells come to lie dorsomedially to the older ones. The neuroblasts of the lateral motor column, however, migrate through and settle ventrolaterally to their predecessors. Our observations do not support the basal plate-alar plate concept of His (1893).

Topological analysis of the brain stem of the lungfish Lepidosiren paradoxa.

The ventricular sulcal pattern and the cellular structure of the brain stem of the lungfish Lepidosiren paradoxa have been studied in transversely cut Nissl and Bodian stained sections. Five longitudinal sulci, the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans, the sulcus intermedius dorsalis and the sulcus medianus superior could be distinguished. In the isthmus region a number of obliquely oriented sulci is present. One of these, designated here as sulcus d, continues as a longitudinal groove into the mesencephalon. In Lepidosiren most neuronal perikarya are contained within a diffuse periventricular gray. However, 24 separate cell masses could be delineated. Six of these are primary efferent nuclei, six are primary afferent center, six nuclei are considered to be components of the reticular formation and the remaining six may be interpreted as "relay" nuclei. The distribution of the cell masses and their relations to the ventricular sulci were studied with the aid of the graphical reconstruction procedure termed tolopogical analysis (cf. Nieuwenhuys, '74, and fig. 11). This analysis yielded the following results. In the rhombencephalon the gray matter is arranged in four longitudinal columns or areas which have been termed the area ventralis, area intermedioventralis, area intermediodorsalis and area dorsalis. In many places the sulcus intermedius ventralis, the sulcus limitans and the sulcus intermedius dorsalis mark the boundaries between these morphological entities. These longitudinal areas coincide largely, but not entirely, with so-called functional columns of Herrick and Johnston. The most obvious incongruity is that the area intermediodorsalis contains, in addition to the nucleus of the solitary tract, two non-visceral sensory cell masses, namely the magnocellular and parvocellular vestibular nuclei. The four longitudinal zones cannot be distinguished in the mesencephalon nor can the sulcus limitans be recognized here. Functionally, however, the medial part of the tegmentum mesencephali may be considered the rostral extreme of the somatic motor column, whereas the remainder of the midbrain contains a number of somatic sensory centers.