Morphogenesis of the neural crest

Macrophage-like cells in the presumptive optic pathways in the floor of the diencephalon of the chick embryo

In the suboptic necrotic centres (SONCs) of the chick embryo diencephalon floor, large numbers of cells die in Hamburger and Hamilton's (HH) developmental stages 14-23. Until recently, it was thought that in these centres, the fragments of dead cells were phagocytosed exclusively by neighboring healthy cells but not by specialized macrophages. We now report morphological evidence of macrophage-like cells within the SONCs of the chick embryo. The distinctive features of these cells are their more or less spherical shape, a nucleus with a thin band of heterochromatin just beneath the nuclear envelope, and cytoplasm showing abundant digestive vacuoles and mitochondria with an electron-lucent matrix. These cells are capable of undergoing mitosis, and selectively stain with the histochemical technique for acid phosphatase. The macrophage-like cells are rare in SONCs in stages HH14-20 and become much more abundant in developmental stages just before the disappearance of these necrotic centres, suggesting that they phagocytose debris from the last cells to die in the SONCs. Acid phosphatase-positive mesenchymal cells with morphological features similar to those of macrophage-like cells are seen in intimate relationship with the basal surface of the SONCs in places where the basal lamina is sometimes missing. These observations suggest that macrophage-like cells in the SONCs arise from the underlying mesenchyme. Free macrophage-like cells with mitotic capacity are also seen in the ventricular lumen adjacent to the apical surface of the diencephalon floor in zones related to the presumptive optic pathways. These cells phagocytose cell debris shed from both the SONCs and from the partially disorganized areas in the neuroepithelium. In these latter we have identified mesenchymal cells with morphological features similar to the macrophage-like cells in the process of traversing the neuroepithelium from the mesenchymal compartment toward the ventricular lumen, thus suggesting that the intraventricular macrophage-like cells arise from the mesenchyme underlying the diencephalon floor.

Study of the origin of connective tissue sheaths of peripheral nerves in the limb of avian embryos

Quail-chick and chick-quail chimeras were constructed by grafting, isotopically, the limb bud of quail embryos into a chick of the same developmental stage and vice versa, prior to the entry of nerve fibres into the limb. After 5-14 days reincubation of the embryos, the components of the connective tissue sheaths of the peripheral nerves were observed by using Feulgen-Rossenbeck staining and light microscopy, in order to distinguish quail cells and chick cells. In all the chimeras studied, the connective tissue sheaths of peripheral nerves (the epineurium, perineurium, perineural septa and endoneural fibroblasts) were formed from the mesenchyme of the limb bud, while Schwann cells were of host origin. Also the outer and inner capsule of muscle spindles originated from the limb bud mesenchyme. These experiments suggest that the connective tissue sheaths of peripheral nerves (at least in the limb region of avian embryos) are not of neural crest origin, but are formed from limb bud mesenchyme.

Directional cell movement during early development of the teleost Blennius pholis: I. Formation of epithelial cell clusters and their pattern and mechanism of movement

Embryos of the teleost Blennius pholis provide exceptional material for observation of the formation and movement of cell clusters in vivo because the clusters are packed with melanosomes and migrate beneath the transparent enveloping layer. These clusters arise from two pigmented cell masses (PCM) which appear precociously on either side of the embryonic axis at 3/5 epiboly, at the future level of somites 1 and 2. As development proceeds, each PCM enlarges and spreads on its lateral margins to form an epithelial sheet. As spreading continues, the sheet fragments, forming small cell clusters that move in a distad direction in the yolk sac. The highly motile lateral marginal cells of the spreading PCM, as well as those of the marginal cells of each moving cluster, invariably protrude highly flattened lamellipodia, which terminate in long, fine, often branched filopodia. As cell clusters leave the PCM, they form long, taut retraction fibers. The rate of spreading of both the lateral edge of the PCM and the initial phase of cluster movement, is higher (1.0 micron/min or greater) than the later rate of cluster movement, apparently because at this phase, motile activity is confined to the distal borders of each. This directional migration ceases in 24 h at 16 degrees - 18 degrees C, when the farthest clusters have reached the ventral region of the yolk sac. By then, all clusters are spaced more or less evenly, apparently due to cessation of all cluster movement at about the same time. Once movement ceases, the clusters remain immobile for 2-4 days, depending on the temperature.