A scanning electron microscopic study of the developing epithelial scleral papillae in the eye of the embryonic chick

Eyes of early embryonic chicks possess 14 scleral papillae, derived from the conjuctival epithelium and present as transient structures between seven and 11 days of incubation. These papillae induce the formation of the 14 scleral ossicles, which develop in the adjacent, neural crest-derived ectomesenchyme. Each papilla undergoes a predictable series of developmental changes, divided by Murrary ('43) into six morphological stages (M stages 1–6). We have confirmed his staging, and provide a scanning electron microscopic (SEM) evaluation of papilla development. The earliest stage that can be visualized with the S.E.M. is M stage 2. We describe the initial modifications of the surface of papilla cells, the presence of large microvilli and the asymmetrical morphogenesis and growth of the papillae. Papillae are shed by a mechanism that involves elongation of the cells at the base of the papilla. Such moribund papillae consist of necrotic cells coated with fibers.     

Modulation of gap junction-mediated intercellular communication in embryonic chick mesenchyme during tissue remodeling in vitro

Gap junction-mediated intercellular communication was analyzed in a model system in which tissue necrosis and remodeling could be modulated. This in vitro system, previously used for analysis of epithelial-mesenchymal tissue interaction, was modified to permit analysis of the presence and extent of intercellular communition by monitoring intercellular transfer of the micro-injected fluorescent dye, Lucifer Yellow. Light and transmission electronmicroscopy were employed to correlate the presence and degree of gap junctional communication (coupling) with tissue morphology. Digital image analysis was used to determine cell density and mitotic indices within the outgrowths of explants. Our results indicated that cell communication in outgrowths adjacent to necrotic foci within an explant was minimal or absent. Cell-coupling in outgrowths adjacent to a compartment of viable mesenchyme was significantly higher-equivalent to unseparated control cultures. A time-course study demonstrated correlation of increased levels of cell-coupling in outgrowths with the level of tissue remodeling within an explant. Our conclusions from these studies are that embryonic mesenchymal cell populations may be selectively uncoupled as a result of alterations in the microenvironment produced by a proximate impaired cell population. It is proposed that endogenous factors in the microenvironment (wound signals), emanating from impaired cell populations, regulate gap junction-mediated intercellular communication in adjacent viable tissue. Normal, unimpaired populations of cells surrounding an area of injury are thereby isolated from the effects of a potentially toxic environment. This could serve as a protective function in development and may represent, in a more general sense, part of the repertoire of events associated with tissue repair and remodeling.     

Transfilter analysis of the inductive influence of proventricular mesenchyme on stomach epithelial differentiation of chick embryos

Summary To clarify the precise conditions under which chick embryonic proventricular mesenchyme can induce proventricular epithelial differentiation, transfilter experiments were carried out. Six-day proventricular epithelium formed glands and expressed pepsinogen when a Nucleopore filter with a pore size of more than 0.6 m, but not 0.2 m, was inserted between the epithelium and the proventricular mesenchyme. The larger the pore size of the filter, the more elongated the glands and the more pepsinogen was induced in the explants. The quail nuclear marker and scanning electron microscopy were used to examine penetration of mesenchymal cells through the Nuclepore filter. The filter of more than 0.2 m pore size allowed cell processes of mesenchymal cells to pass through. However, only the filter with a pore size of more than 0.6 m allowed actual migration of mesenchymal cells through the filter, and the larger the pore size of the filter, the more mesenchymal cells passed through. Under the same conditions 6-day and 4.5-day gizzard epithelium formed glands and expressed pepsinogen. These results indicate that a flow of diffusible substances through a Nuclepore filter and even direct contact of a few short cell processes of mesenchymal cells with epithelial cells are not sufficient for induction, and that direct contact of mesenchymal cell processes and/or mesenchymal cells with epithelial cells over a considerably wide area may be prerequisite for the induction.     

Rescue of an in vitro palate nonfusion model using interposed embryonic mesenchyme

The authors previously established an in vitro palate nonfusion model on the basis of a spatial separation between prefusion embryonic day 13.5 mouse palates (term gestation, 19.5 days). They found that an interpalatal separation distance of 0.48 mm or greater would consistently result in nonfusion after 4 days in organ culture. In the present study, they interposed embryonic palatal mesenchymal tissue between embryonic day 13.5 mouse palatal shelves with interpalatal separation distances greater than 0.48 mm in an attempt to "rescue" this in vitro palate nonfusion phenotype. Because no medial epithelial bilayer (i.e., medial epithelial seam) could potentially form, palatal fusion in vitro was defined as intershelf mesenchymal continuity with resolution of the medial edge epithelia bilaterally. Forty-two (n = 42) palatal shelf pairs from embryonic day 13.5 CD-1 mouse embryos were isolated and placed on cell culture inserts at precisely graded distances (0, 0.67, and 0.95 mm). Positive controls consisted of shelves placed in contact (n = 6). Negative controls consisted of shelves placed at interpalatal separation distances of 0.67 mm (n = 6) and 0.95 mm (n = 7) with no interposed mesenchyme. Experimental groups consisted of embryonic day 13.5 palatal shelves separated by 0.67 mm (n = 11) and 0.95 mm (n = 12) with interposed lateral palatal mesenchyme isolated at the time of palatal shelf harvest. Specimens were cultured for 4 days (n = 19) or 10 days (n = 23), harvested, and evaluated histologically. All positive controls at 4 and 10 days in culture showed complete histologic palatal fusion. All negative controls at 4 days and 10 days in culture remained unfused. Five of six palatal shelves separated at 0.67 mm interpalatal separation distance with interposed mesenchyme were fused at 4 days, and all five were fused at 10 days. At an interpalatal separation distance of 0.95 mm with interposed mesenchyme (n = 12), no palates (zero of four) were fused at 4 days, but seven of eight were fused at 10 days. These data suggest that nonfused palatal shelves can be "rescued" with an interposed graft of endogenous embryonic mesenchyme to induce fusion in vitro.     

Establishment of the scleral cartilage in the chick.

This study was undertaken to investigate the establishment of the scleral cartilage in the chick embryo. Johnston et al. (1974) has demonstrated that most of the cells of the scleral cartilage originate in the cranial neural crest. By means of a series of chorioallantoic grafts of pigmented retina, and its adherent periocular mesenchyme from stage 11 to 25, the present experiments show that the cranial neural crest cells arrive at the eye in sufficient numbers to form cartilage by stage 14. Pigmented retina, denuded of mesenchyme, from stage 16 embryos implanted into the head of stage 13 embryos induces cartilage formation in head mesenchyme. However, neither pigmented retina nor spinal cord could induce cartilage formation in chorioallantoic mesenchyme. Combination grafts of cranial neural crest and presumptive optic vesicle developed neural tissue, pigmented retina, and in some cases sclera-like cartilage. Thus, periorbital mesenchyme, derived largely from cranial neural crest, at about stage 14 develops the scleral cartilage in response to induction by the pigmented retina.     

Chondrogenesis of mandibular mesenchyme from the embryonic chick is inhibited by mandibular epithelium and by epidermal growth factor

This study documents the role of mandibular epithelium and epidermal growth factor (EGF) in the initiation, maturation and maintenance of Meckel's cartilage using percent 3H-thymidine-labelled cells as an index of proliferative activity and distribution of labelled cells, chondrocyte size and relative amount of extracellular matrix as indices of chondrogenesis. Mandibular mesenchyme from embryos of H.H. stages 18, 22, 25 was cultured for 2 to 10 days (a) unseparated from mandibular epithelium, (b) in isolation, or (c) after recombination with mandibular epithelium in the presence or absence of 5-40 ng/ml EGF. Epithelium delayed both initiation of chondrogenesis and maturation of already formed cartilage. The 3H-thymidine-labelling index was reduced in cartilage that differentiated in the presence of mandibular epithelium. Epithelium influenced the timing of mesenchymal differentiation (a) by delaying cytodifferentiation through prolonging high levels of proliferation, and (b) by directly affecting differentiation itself. EGF, especially at 10-20 ng/ml, affected both proliferation of mesenchyme and chondrogenesis in mesenchyme cultured with or without epithelium. All observed effects of epithelium on intact tissues could be duplicated by exposing isolated mesenchyme to EGF at 10 ng/ml, i.e. a role for EGF in chondrogenesis is suggested.     

Toward an understanding of the structural basis of translation

The recently solved X-ray crystal structures of the ribosome have provided opportunities for studying the molecular basis of translation with a variety of methods including cryo-electron microscopy - where maps give the first glimpses of ribosomal evolution - and fluorescence spectroscopy techniques.     

Relative growth rates of maxillary mesenchyme in the chick embryo

Chick embryos were injected with [3H]-thymidine at days 3-7 of incubation and were fixed and embedded in plastic. The embryos were divided into three stage groupings of development [Hamburger and Hamilton: J Morphol 88:49-92, 1951], and labeling indices were determined for each of the following delineated regions within the maxillary process at each stage: region 1, subepithelial mesenchyme located at the medial side of the maxillary process adjacent to the roof of the stomodeum; region 2, subepithelial mesenchyme at the ventral tip of the maxillary process (as seen on cross section); region 3, subepithelial mesenchyme at the lateral portion of the maxillary process below the eye; and region 4, interior mesenchyme defined as the central portion of the maxillary process and separated from the epithelium by the three other regions. Results indicated that differences exist among the regions examined and that these differences were stage specific. At stages 19-21 and stages 24-25 1/2, growth rates were higher in subepithelial regions than interiorly. At stages 28-29, however, a statistically significant difference among the regions was not found. These results suggested that there is an association between growth rates in the maxillary process mesenchyme and its proximity to the overlying epithelium and that these effects are related to the stage of development.     

Toward an understanding of the brain substrates of reward in humans

A network of brain regions has been implicated in food-reward processing. now provide evidence that this network is differentially modulated by anticipation versus receipt of a food reward and suggest an additional effect of valence of the stimulus.