Journal of embryology and experimental morphology最新文献

The cellular events that take place during reconstruction of larval forms from dissociated embryonic cells of the starfish are investigated by thick and thin sections. Dissociated cells reaggregate, form an external epithelium (ectoderm), internal epithelial vesicles (endoderm), the blastocoel and the mesenchyme. The internal vesicles continue to fuse until there is only one large one suspended in the centre of the blastocoel. Eventually, the ectoderm invaginates at one or more sites and fuses with the endoderm to form blastopore(s). Special emphasis is placed on the differences in cell behaviour during endoderm-to-endoderm and endoderm-to-ectoderm fusion.

Three lectins, wheat germ agglutinin (WGA), soybean agglutinin (SBA) and Ricinis communis agglutinin I (RCA), were used to study the basement membrane of developing chick lungs. Thinning of the basement membrane at the tips of newly formed bronchi was visualized with all three lectins, but was particularly evident using SBA. Control sections established the ability of the lectins to stain hyaluronic acid and chondroitin sulphate. Neuraminidase, bovine testes hyaluronidase and Streptomyces hyaluronidase removed some of the staining, but none were able to affect the staining of the basement membrane. Possible explanations for this are discussed in the text. Incorporation of [3H]thymidine is enhanced at the tips relative to the interbud area in stage-30 lungs, extending previous studies on stage-26 lungs. Evidence has been presented here which demonstrates that mechanisms of morphogenesis used in avian embryos are similar to those already elucidated in work on mammalian embryos.

In the present work, we have analysed the possible involvement of ectodermal tissue in the control of interdigital mesenchymal cell death. Two types of experiments were performed in the stages previous to the onset of interdigital cell death: removal of the AER of the interdigit; removal of the dorsal ectoderm of the interdigit. After the operation embryos were sacrificed at 10-12 h intervals and the leg buds were studied by whole-mount cartilage staining, vital staining with neutral red and scanning electron microscopy. Between stages 27 and 30, ridge removal caused a local inhibition of the growth of the interdigit. In a high percentage of the cases, ridge removal at these stages was followed 30-40 h later by the formation of ectopic nodules of cartilage in the interdigit. The incidence of ectopic cartilage formation was maximum at stage 29 (60%). In all cases, cell death took place on schedule although the intensity and extent of necrosis appeared diminished in relation to the intensity of inhibition of interdigital growth and to the presence of interdigital cartilages. Ridge removal at stage 31 did not cause inhibition of the growth of the interdigit and ectopic chondrogenesis was only detected in 3 out of 35 operated embryos. Dorsal ectoderm removal from the proximal zone of the interdigit at stage 29 caused the chondrogenesis of the proximal interdigital mesenchyme in 6 out of 18 operated embryos. The pattern of neutral red vital staining was consistent with these results revealing a partial inhibition of interdigital cell death in the proximal zone of the interdigit. It is proposed that under the present experimental conditions the mesenchymal cells are diverted from the death programme by a primary transformation into cartilage.

Testes of foetal pigs between 26 to 35 days post coitum (p.c.) were investigated histochemically and ultrastructurally. Diaphorase and delta 5-3 beta-hydroxysteroid dehydrogenase activities were studied using, respectively, NADH and pregnenolone and dihydroxy androsterone as substrates. Ultrastructurally, attention was focused on the development of mesenchymal cells and on the sustentacular cells in the primitive sex cords in an attempt to detect the origin of Leydig cells. Histochemically there is a concentration of activity toward the interstitium with increasing age. Also the reactions increase in intensity. Ultrastructurally no evidence for Leydig cell development from Sertoli cells could be observed. Mesenchymal cells between the sex cords show a development toward Leydig cells. This is absent in mesenchymal cells in the future tunica albuginea. Before 30 days p.c. no 'true' Leydig cells can be observed morphologically. The role of the rough endoplasmic reticulum/mitochondrial complex, which is present in many mesenchymal and sustentacular cells, is discussed.

The genital ridges of Xenopus laevis tadpoles reared from eggs kept in an inverted position contain less than 40% of the number of primordial germ cells (PGCs) of controls (Cleine & Dixon, 1985). It has been suggested that this reduction is caused by the germ cells' ectopic position in the anterior endoderm of larvae from inverted eggs, from where they may be unable to migrate into the genital ridges (Cleine & Dixon, 1985). This hypothesis is tested here by interchanging anterior and posterior endodermal grafts between pairs of inverted embryos at the early tailbud stage. Replacement of anterior by posterior endoderm has no effect but replacement of posterior by anterior endoderm increases the number of PGCs in the genital ridges and significantly reduces the proportion of sterile embryos. In a control series, in which the same type of grafting was done with normal embryos, replacement of posterior by anterior endoderm reduced the number of germ cells to almost zero, but replacement of anterior by posterior endoderm nearly doubled it. These findings are explained in terms of the distribution of the germ cells in the endoderm at the time of grafting. The results firstly show that the position of the germ cells is crucial to successful migration and secondly they support the notion that germ plasm has a determinative role during early germ cell differentiation.

Monoclonal antibodies, specific against cell surface differentiation antigens of human corneal epithelial cells, were developed using epithelial cells resected from human corneas as the immunogens. One of these antibodies reacted specifically with corneal epithelial cells and not with epithelial cells of other tissues when tested by an indirect immunoperoxidase technique. Nonidet P-40 extracts of different subcellular fractions of human corneal epithelial cells were tested for their reactivity against this antibody using an enzyme-linked immunosorbent assay. The results indicated that the antigen recognized by this antibody is associated with the plasma membrane. This was further verified by immuno-electron-microscopic analysis using ferritin-conjugated anti-mouse IgG antibody. This antigen was not detectable in the corneal epithelial cells in primary cultures nor in the epithelial cells from early stages of developing cornea (12 to 18 weeks in utero) but was present in the epithelial cells in the corneas of an 8-month-old infant. Therefore, this surface-associated antigen identified in the present study is a developmentally regulated marker of human corneal epithelium.

The regulation of the cell cycle during the transition from prophase to metaphase I was studied by analysing protein changes and introducing protein blocks during the transition phase. The results show that the progression to metaphase in ovine oocytes is totally dependent on new protein synthesis. By delaying the addition of the inhibitor, cycloheximide, for progressively longer periods after the resumption of meiosis it was established that the required synthesis occurs in the 1-2 h immediately preceding germinal vesicle breakdown (GVBD). The action of cycloheximide was fully reversible: removal of the drug resulted in GVBD between 3 and 4 h later. The synthesis and modification of proteins during these first few hours of maturation were studied by short-term radiolabelling of oocytes with [35S]methionine and [32P]phosphate followed by rapid assessment of their precise nuclear configuration. Changes in phosphorylation of two polypeptides were detected 4-5 h after the beginning of culture, but these changes were not dependent upon protein synthesis. The earliest change in synthesis was the appearance of a new polypeptide 6-8 h after explantation, immediately before GVBD. This polypeptide (Mr 47 X 10(3), pI 5.8) was not significantly phosphorylated and was relatively stable. Oocytes released from cycloheximide treatment began to synthesize this molecule 3-4 h later, again coinciding with GVBD. Synthesis of the polypeptide was suppressed by inhibition of transcription with alpha-amanitin.

Hensen's node from stage 4 to stage 10 shows polarizing activity when grafted to the anterior margin of the chick limb bud. It can specify additional digits though its action is somewhat attenuated when compared with the effect of a grafted polarizing region. At stage 10 the activity disappears from the node and is found both posterior to the node and in the future wing region of the flank. The ability of Hensen's node to generate a positional signal suggests that the signal in the limb and early embryo may be similar. The results support the view of the polarizing region as a discrete signalling region.

The effect of protein synthesis inhibitors on compaction of the 8-cell mouse embryo has been investigated. The effects observed depended upon the duration and time of drug application and on the features of compaction scored. Continuous application from the late 2-cell or early 4-cell stages allowed cell flattening and surface polarization to occur in most embryos and advanced development of these features in many of them. Cell coupling developed only when drug addition was delayed until the mid 4-cell stage, and cytoplasmic polarization developed only when drug addition was delayed until the late 4-cell stage. We suggest that control over the timing of compaction is achieved at a post-translational level via a global permissive change within the blastomeres of the embryo.

To what extent do motor and sensory nerve fibres depend on one another for guidance during the development of peripheral nerve patterns? This question has been examined by looking at the paths taken by sensory nerve fibres growing into the embryonic chick wing in the absence of motor axons. The precursors of the motoneurones were destroyed by irradiating the appropriate part of the neural tube with a focused beam of ultraviolet light, before axons had grown out. The limb nerve patterns seen 5 to 7 days later revealed that sensory fibres followed the usual paths of main nerve trunks and formed cutaneous nerve branches in an almost normal way. However, the sensory fibres did not take the paths where muscle nerve branches are normally seen. Apparently, sensory axons for the most part do not depend on motor axons for guidance, except in the case of proprioceptive fibres, which require guidance from motor axons over the final steps of their path into muscle.