George M. Malacinski

Axial structure development in ultraviolet-irradiated (notochord-defective) amphibian embryos.

Abstract Fertile Xenopus eggs were irradiated with low doses of ultraviolet irradiation (254 nm) and permitted to develop through neural morphogenesis. They were then examined with the scanning electron microscope. The effects of irradiation on the development of the notochord, neural tube, and somites were observed. Embryos which displayed defects in notochord structure frequently exhibited normal development of both neural tube and somites. Those embryos were examined in detail. The apparent normal character of axial structures in “notochord-defective” embryos prompted a discussion of the role of the notochord in normal axial structure morphogenesis.

Destruction of components of the neural induction system of the amphibian egg with ultraviolet irradiation.

Abstract Ultraviolet irradiation of the vegetal hemisphere of the fertilized amphibian (Xenopus laevis) egg prior to first cleavage results in the embryo developing an incomplete set of neural structures. The effects of irradiation on various morphogenetic processes, including cell division, formation of the dorsal lip, invagination at gastrulation, and neural induction by the primary organizer, were examined. A decrease in the capacity for invagination during gastrulation and a diminution in the neural inducing capacity of the primary organizer were found to account for defective neurulation in irradiated embryos. Consequently, irradiation of the uncleaved egg leads to interference with the events of both gastrulation and neurulation.

The origin of the mesoderm in an anuran, Xenopus laevis, and a urodele, Ambystoma mexicanum

We have investigated whether superficial cells of the blastula contribute to mesodermal structures in the anuran Xenopus laevis and the urodele Ambystoma mexicanum. The superficial cells alone of late blastulae of both embryos were labelled with Bolton-Hunter reagent and the embryos were allowed to develop. The progeny of the labelled cells were identified at later stages and the results demonstrate that superficial cells of Xenopus blastulae make no significant contribution to the mesoderm, whereas those of the axolotl Ambystoma always contribute.

Expression of myosin heavy chain transcripts during Xenopus laevis development

In amphibians, the composition and pattern of expression of the myosin heavy chain (myHC) gene family are mostly unknown. To understand better the regulation of this complex family, we screened cDNA libraries from swimming tadpole and adult leg muscle RNA and have isolated and partially sequenced clones for several myHCs. Two of these, E3 and E19, first appear in mesoderm at late gastrula stage and are coexpressed with muscle actin. They increase in abundance in trunk and tail myotome until metamorphosis, when they begin to decline. E3 and E19 are also both expressed in hind leg muscle at the beginning of metamorphosis, but decline to low levels in adult leg muscle. A new transcript, A7, first appears during early metamorphosis in both the tail and hind leg skeletal muscle. A7 transcripts then decline in degenerating tail but persist in hind leg through metamorphosis and in adults. Two other embryonic myHCs, E14 and E15, code for isoforms closely related to E19 and may be alleles or duplications. Although sequence comparisons with myHCs from other vertebrates could not correlate the transcripts with specific protein isoforms, from the pattern of expression E3 and E19 apparently code for embryonic fast skeletal myHC isoforms, whereas A7 codes for an adult skeletal isoform.

Establishment of the dorsal/ventral polarity of the amphibian embryo: Use of ultraviolet irradiation and egg rotation as probes

Abstract Ultraviolet irradiation and egg rotations were employed as probes for the study of the establishment of the dorsal/ventral polarity of the amphibian embryo. Ultraviolet irradiation was discovered to alter the natural position of the doral lip and to modify the pigmentation pattern of the early embryo. Rotation of the uncleaved egg was found to succeed in relocating the dorsal lip to a new site in the embryo. Also, rotation of the egg was capable of preventing the characteristic defects associated with irradiation of the uncleaved egg. A combination of these probes was employed, and the results were interpreted in terms of models for the role of the egg surface and the internal cytoplasm in the establishment of the dorsal/ventral polarity of the egg.

In vitro control of organogenesis and body patterning by activin during early amphibian development

In the process of amphibian development, an embryonic body plan is established through cell division, sequential gene expression, morphogenesis and cell differentiation. The mechanism of body patterning is complex and includes multiple induction events. Activin, a TGF-beta family protein, can induce several kinds of mesodermal and endodermal tissues in animal cap explants in a dose-dependent manner. In a recent study of the role of activin in organogenesis, we succeeded in raising a beating heart by treating animal caps with a high concentration of activin. Activin also participates in kidney organogenesis in combination with retinoic acid. An embryonic kidney induced by activin and retinoic acid in vitro can function in vivo when it is transplanted into a larva in which pronephros rudiments have already been removed. Further, the activin-treated animal caps clearly show organizer actions that are closely related to body patterning along the anteroposterior axis. These experiments will help to serve as a model system for understanding organogenesis and body patterning at the cellular and molecular levels.

Localization of Xenopus homoeo-box gene transcripts during embryogenesis and in the adult nervous system

Tissue distribution and localization of RNAs from the Xeb1 homoeo-box-containing gene were monitored with Northern blots and in situ hybridization. Xeb1 transcript distribution in larval stage embryos was established by blotting RNAs extracted from microdissected embryos. Those transcripts are restricted to a limited number of embryonic regions such as the dorsal trunk. The tissue/cell localization of Xeb1 transcripts was then monitored at several embryonic stages with in situ hybridization methods using [35S]RNA probes. These homoeo-box transcripts accumulated in a progressive and dynamic fashion. First localized in late gastrulae, they are distributed along the neural tube and in caudal mesoderm at later stages. By the swimming tadpole stage the spatial distribution of the homoeo-box transcripts is limited to specific regions of the central nervous system. Adult spinal cord shows the signal in specific neurons in the ventrolateral field of the gray matter.

Neural plate morphogenesis and axial stretching in “notochord-defective” Xenopus laevis embryos

Abstract The morphogenetic movements of neural ectoderm cells associated with neural plate development and neural fold fusion were examined in notochord-defective embryos. Those movements were apparently normal in embryos which displayed a notochord reduced in size or which completely lacked a notochord. Likewise, axial stretching in the anterior-posterior direction was also normal in “notochord-defective” embryos. A role for the anuran notochord in directing neural fold fusion and axial stretching can, therefore, be ruled out.

The association of primary embryonic organizer activity with the future dorsal side of amphibian eggs and early embryos

Abstract Contrary to previous reports, the capacity to induce a secondary set of axial structures (primary embryonic organizer activity) is not a property of the egg (zygote) cortex. Rather, as embryogenesis proceeds through cleavage division to blastulation, cells on the future dorsal side of the embryo progressively acquire primary organizer activity. That is, dorsal cells appear during embryogenesis to develop the ability to promote morphogenesis of a secondary set of axial structures. When implanted into the blastocoel of normal hosts, or cultured in the presence of competent ectoderm, dorsal cells from blastula- and early gastrula-stage embryos induced secondary axes.

Joint Loading Modality: Its Application to Bone Formation and Fracture Healing

Sports-related injuries such as impact and stress fractures often require a rehabilitation programme to stimulate bone formation and accelerate fracture healing. This review introduces a recently developed joint loading modality and evaluates its potential applications to bone formation and fracture healing in post-injury rehabilitation. Bone is a dynamic tissue whose structure is constantly altered in response to its mechanical environments. Indeed, many loading modalities can influence the bone remodelling process. The joint loading modality is, however, able to enhance anabolic responses and accelerate wound healing without inducing significant in situ strain at the site of bone formation or fracture healing. This review highlights the unique features of this loading modality and discusses its potential underlying mechanisms as well as possible clinical applications.