An ultrastructural examination of the role of cell membrane surface coat material during neurulation.

Abstract

Data from neural crest cultures indicate that cell surface coat material (CSM) is directly involved in cellular migration and events surrounding differentiation. To investigate whether the CSM also has a morphogenetic role, embryos of the amphibian Arnbystorna maculatum were examined ultrastructurally throughout the stages of neurulation. Segments of the neural axis were fixed in glutaraldehydecontaining Alcian blue 8GX, which reportedly enhances preservation of CSM, and were postfixed in OsO4 containing 1% lanthanum nitrate, which stains the CSM. The medial groove formed by the appearance of the neural ridges contains a large amount of CSM and numerous vesicles coated with lanthanum-positive material. In contrast, the lateral ridge surfaces are covered by a small amount of uniformly distributed CSM and a paucity of vesicles. As the ridges begin to fold there is a progressive increase in the amount of CSM within the presumptive neural tube region. Further convergence of the neural folds is accompanied by an increase of CSM at their leading edges. As the folds approximate each other, lanthanum-positive material physically bridges the gap. However, as the apposing tissue actually abuts to form the neural tube, no CSM is observed in the remaining interspace. The specific distribution and sequential accumulation of cell CSM during the events of neurulation strongly suggest its direct participation in the morphogenetic process.The migratory and differentiative activities of cell populations during embryogenesis normally occur in a temporal and spatial pattern of awesome precision and complexity. Vogt's elegant fate-mapping experiments with amphibian embryos (30, 31) first demonstrated consistent directions of morphogenetic movement by various cell groups. Wilson's cellular reaggregation studies with sponges (37) introduced the concept of histogenetic specificity, i.e. a mixed-species population of cells will selectively reaggregate according to species. Galtsoff (10) extended this work by showing that mixed cells from various tissues of one species will reaggregate according to tissue.The relevance of such data to developing general principles of morphogenesis generated experimental interest in the location and nature of a control mechanism associated with this predictable cellular movement and interaction. The accumulated facts from the classic studies of Holtfreter (17,18)

Hyperglycemia-induced teratogenesis is mediated by a functional deficiency of arachidonic acid.

“…Similarly, the presence of arachidonic acid at 10 ,g/ml resulted in a neural tube fusion in 2 of 3 embryos (67%) incubated in the presence of D-glucOse (8 mg/ml). Thus, neural tube fusion was seen in 14 of 21 embryos cultured in the hyperglycemic (8 mg/ml) medium to which arachidonic acid had been added. This 67% rate of neural tube fusion was similar to what was observed in the control embryos (P = not significant) but was significantly different ( Table 3 also suggests that the B10 strain of mouse may be resistant to the teratogenic effect of glucose on neural tube closure in a manner similar to its resistance to phenytoin-or glucocorticoid-induced cleft palate.…”

“…Neural tube fusion and palatal shelf fusion are analogous in that both require embryonic cell movements and apposition of the two advancing layers (21,22), with cell death at that point leading to fusion and closure (23,24). In the malformation model of glucocorticoid-and phenytoin-induced cleft palate, this sequence of events has been experimentally delineated (11,25).…”

The arachidonic acid cascade is involved in the masculinizing action of testosterone on embryonic external genitalia in mice.

“…That blockade of testosterone synthesis or action causes failure of masculine anogenital development by affecting availability of arachidonic acid makes this embryonic malformation similar to what has been described for experimentally produced cleft palate and for hyperglycemia-induced-failure of neural tube fusion in experimental diabetic embryopathy. Penile and scrotal fusion may be analogous to palatal shelf and neural tube fusion, in that both latter embryonic processes require embryonic cell movements and apposition of the two advancing tissues (22,23). In the case of palatal and neural tube fusion, both processes involve cell death at the point leading to fusion and closure (24,25).…”

The Cause of Chiari II Malformation: A Unified Theory

“…One example of a genetic cause is the autosomal recessive Splotch mutation; cau dal neural tube defects occur in mice homozygous (Spd/Spd) for this gene [18,33,44], Our laboratory has observed notable differences in the time and type of expression of several glycoproteins in Splotch embryos with neural tube defects compared with normal littermates [19]. Complex carbohydrates, i.e., glycoconjugates, on the cell surface and within the extracellular matrix provide some of the required sig nals for cellular differentiation and maturation [3,12,38,42,45,56]. For example, the process of neuru lation in animals can be prevented by metabolic inhibitors (tunicamycin, an inhibitor of N-linked glycoprotein synthesis) [41,64], nutritional deficien cies, thermal irregularities, or excess vitamin A [6,8,19,42], Thus, a complex series of specific, timedependent cellular events is required in the induction of cells, their microenvironment, and overall organiza tion.…”