Anatomy and Embryology

Neural crest cells make a substantial contribution to normal craniofacial development. Despite advances made in identifying migrating neural crest cells in avian embryos and, more recently, rodent embryos, knowledge of crest cell migration in primates has been limited to what was obtained by conventional morphological techniques. In order to determine the degree to which the nonhuman primate fits the mammalian pattern, we studied the features of putative neural crest cell migration in the hindbrain of the long-tailed monkey (Macaca fascicularis) embryo. Cranial crest cells were identified on the basis of reported distributional and morphological criteria as well as by immunocytochemical detection of the neural cell adhesion molecule (N-CAM) that labels a subpopulation of these cells. The persistent labeling of a sufficient number of crest cells with antibodies to N-CAM following their exit from the rostral, preotic and post-otic regions of the hindbrain facilitated tracking them along subectodermal pathways to their respective destinations in the first, second and third pharyngeal arches. Peroxidase immunocytochemistry was also employed to localize laminin and collagen-IV in neuroepithelial basement membranes. At stage 10 (8–11 somites), crest emigration occurred in areas of unfused neural folds through focal disruptions in the neuroepithelial basement membrane in both the rostral and pre-otic regions, although there was little evidence of crest migration in the post-otic hindbrain. By stage 11 (16–17 somites), the neural folds were fused (pre- and post-otic hindbrain) or in the process of fusing (rostral hindbrain), yet crest cell emigration was apparent in all three areas through discontinuities in the basement membrane. Emigration was essentially complete at stage 12 (21 somites) as indicated by nearly continuous cranial neural tube basement membranes. At this stage the pre-ganglia (trigeminal, facioacoustic and glossopharyngeal) were consistently stained with N-CAM. The current study has provided new information on mammalian neural crest in a well-established experimental model for normal and abnormal human development, including its use as a model for the retinoic acid syndrome. In this regard, the current results provide the basis for probing the mechanisms of retinoid embryopathy which may involve perturbation of hindbrain neural crest development.

The development of the rat organ of Corti was studied during the first postnatal weeks. The temporal and the spatial patterns of cochlear development were investigated between 4 and 24 days after birth by means of semi-thin sections at approx. ten equidistant positions along the entire cochlear duct. At all examined positions width, thickness and cross sectional area of basilar membrane, cross-sectional area of tectorial membrane, of cells of Hensen, Claudius and Boettcher and of the organ of Corti were quantitatively analyzed. The most conspicuous maturational changes occur between 8 and 12 days after birth. These are the detachment of the tectorial membrane, the first appearance of filaments within the basilar membrane, the formation of the tunnel of Corti and the opening of the inner spiral sulcus. Quantitative analysis revealed that structures of a given position along the cochlear duct do not develop synchronously. Width of the basilar membrane and cross-sectional area of the tectorial membrane are already mature at the onset of hearing (10–12 days after birth). Length, thickness and cross-sectional area of the basilar membrane as well as cross-sectional area of the organ of Corti and of the cells of Hensen, Claudius and Boettcher still develop after the onset of hearing (up to 20–24 days after birth). We suggest that basic cochlear function is established by structures which are mature before the onset of hearing. Cochlear structures which develop after the onset of hearing might be involved in this improvement during this period.

Retrograde axonal tracing studies were performed in combination with tritiated thymidine cell birthday analyses in order to determine whether or not any hodologicotemporal gradients exist in neuron genesis within the murine locus coeruleus. Following injections of retrograde tracers within the forebrain or cerebellum in mice exposed in utero to the radiolabeled nucleoside on embryonic days 9–11 (E9-11), combined histochemical and autoradiographic preparations revealed: 1) Locus coeruleus neurons that give rise to long distance axonal projections to the cortices are born exclusively on E9 (other studies indicate that these cells are noradrenergic); and 2) Locus coeruleus cells born on E10 and E11 are a class of smaller cells which were never observed to project to distant structures. The transmitters of these apparent local circuit neurons have not yet been determined, but gamma aminobutyric acid is one possible candidate. These findings support the interpretation that monoaminergic neurons tend to arise earlier during development than non-monoaminergic neurons within the locus coeruleus, and that distinctly different connectional arrangements exist for these monoaminergic and non-monoaminergic cells.

 Maternal stem arteries and arterioles of the endotheliochorial mink placenta have been shown to lack smooth muscle cells, suggesting a muscle-free attenuation of the maternal arterial pulse wave of the placenta. Since the endotheliochorial type of placenta by definition does not contain any maternal supportive tissue (e.g. connective tissue), except for the specialized interstitial layer, the aim of this study was to reveal cytoskeletal components able to compensate for the lack of conventional regulatory mechanisms of maternal placental blood flow. The study was undertaken on buffered formalin fixed tissues from 19 minks by immunohistochemistry and transmission electron microscopy to localize three major cytoskeletal filaments (desmin, vimentin and α-smooth muscle actin (α-sm-actin)) in non-pregnant uteri and placenta. The contractile α-sm-actin was immunodetected in the maternal subepithelial and periglandular connective tissue cells of the cyclic endometrium and during early gestation. During the transition from early- to mid gestation, maternal periendothelial cells appeared and showed α-sm-actin immuno-positivity; however, in late gestation, this activity could not be detected because the periendothelial cells had disappeared. Fetal endothelial cells displayed intense α-sm-actin immunoreactivity, which was in contrast to the α-sm-actin negative maternal endothelial cells. Allantochorionic mesenchymal cells also exhibited intense α-sm-actin immunostaining. Vimentin was immunohistochemically expressed in endothelial cells (maternal as well as fetal), maternal periendothelial cells, allantochorionic mesenchymal cells, and maternal connective tissue cells from early gestation. Desmin was not immunohistochemically detectable in cyclic endometrium and placental tissues. Transmission electron microscopy revealed the periendothelial cells to be enclosed by a thin interstitial layer. Additionally, the maternal endothelial cells displayed actin myofilament-like structures anchored basally. From our data we conclude that maternal periendothelial cells, immunoreactive for contractile actin, and maternal endothelial cells, possessing actin myofilament-like ultrastructures, act as supportive systems in the maternal vessel walls, probably influencing the regulation of the maternal blood flow.