Single palatal processes or homotypic paired palatal processes from 12-day mouse embryos (strain ... more Single palatal processes or homotypic paired palatal processes from 12-day mouse embryos (strain C57Bl) were grown in organ culture on Millipore filters. The epithelium along the medial edge of each palatal process became disrupted, and, by 72 hours of culture, paired palatal processes achieved mesenchymal fusion at the site of disruption. The nasal epithelium of paired palatal processes became a recognizable pseudostratified ciliated columnar epithelium by 96 hours of culture, and the oral epithelium became a recognizable stratified squamous epithelium by 120 hours of culture. These events correspond temporally to the differentiation of palatal tissue in vivo. Differentiative changes of single palatal processes followed the same schedule as double processes. Observations also suggest that the epithelium at the tip of the vertically oriented young palatal process corresponds to the medial epithelium of the more advanced horizontal process-these observations have bearing on the mecha...
Journal of embryology and experimental morphology, 1977
The epithelium of the secondary palate from embryonic mice (12-14 days of gestation) was enzymati... more The epithelium of the secondary palate from embryonic mice (12-14 days of gestation) was enzymatically separated from the palatal mesenchyme and was cultured in association with mesenchyme from the embryonic tongue, incisor, and salivary gland. The histological differentiation of the epithelium in each type of heterotypic recombination was always palatal. Tongue mesenchyme supported full histodifferentiation of the palatal epithelium, whereas incisor mesenchyme supported only a limited histodifferentiation in the epithelium. Salivary gland mesenchyme in direct combination with palatal epithelium influenced the epithelium to become organized into a system of ducts and acini typical of glandular morphogenesis; histodifferentiation of the epithelium, however, was palatal. Salivary gland mesenchyme grown in isolation did not undergo glandular morphogenesis.
Intact mandibular processes and the enzymatically separated mesenchymal and epithelial components... more Intact mandibular processes and the enzymatically separated mesenchymal and epithelial components of the mandible from embryonic chicks of 2.5-to 5day incubation (Hamburger and Hamilton, '51: stages 1 6 2 5 ) were grown individually, either in organ culture or as grafts to the chorioallantoic membranes of host embryos. The differentiation of cultured and grafted intact mandibular processes was histologically normal, but the time of histodifferentiation differed from that in vivo. The histodifferentiation of cultured and grafted mandibular mesenchyme grown isolated from its epithelium depended upon the age of the embryo from which the mesenchyme had been obtained. Intramembranous ossification producing membrane bones of the mandible occurred i n mesenchyme isolated from 4.5-to 5-day embryos (HH 24-25), but did not occur in mesenchyme isolated from younger embryos. Cartilage (Meckel's) and subperichondrial bone in the articular process of Meckel's cartilage differentiated in mesenchyme isolated from embryos of all age groups tested (HH 16-25). Mandibular mesenchyme, therefore, requires the presence of epithelium until 4.5 days of incubation if the membrane bones of the mandible are to differentiate; if epithelial influences are required for Meckel's cartilage and subperichondrial bone formation, they are not required beyond 2.5 days of incubation. Mandibular epithelium isolated from its mesenchyme became layers of squamous cells in culture; but when grafted onto the chorioallantoic membrane, the epithelium became underlain by host fibroblasts and differentiated into a stratified squamous epithelium. Mandibular epithelium, therefore, is capable of differentiation in the presence of foreign fibroblasts derived from the chorioallantoic membrane.
In light of previous evidence t h a t epithelial influences are required for membrane bone format... more In light of previous evidence t h a t epithelial influences are required for membrane bone formation in certain instances (mandible, Tyler and Hall, '77 a,b; cranium, Benoit and Schowing, '701, t h e possibility t h a t maxillary epithelium influences membrane bone formation within the maxillary mesenchyme was investigated. Intact maxillary processes, and enzymatically separated epithelial and mesenchymal components of t h e maxilla were obtained from embryonic chicks of three t o five days of incubation (Hamburger and Hamilton "511 stages (HH) 18-25) were grown individually a s grafts on the chorioallantoic membranes of host chick embryos. The histodifferentiation of grafted intact maxillary processes was similar to t h a t in vivo. The histodifferentiation of grafted maxillary mesenchyme depended upon the embryonic age a t which t h e mesenchyme was isolated from its epithelium. In grafts of maxillary mesenchyme isolated from its epithelium at HH 22 or earlier stages, cartilage differentiated, but membrane bone did not form. In grafts of maxillary mesenchyme isolated from its epithelium at HH 23 or later stages, membrane bone formed in addition to cartilage. The results indicate t h a t maxillary mesenchyme requires the presence of epithelium through HH 22 in order for membrane bone to form within t h e mesenchyme. Cartilage formation within t h e mesenchyme does not require epithelial influences during t h e embryonic period tested . Maxillary epithelium isolated from its mesenchyme
The frontal region of the embryonic chick was studied to determine whether epithelial influences ... more The frontal region of the embryonic chick was studied to determine whether epithelial influences are necessary for frontal bone development. The frontal bone is a membrane bone, of neural crest and head mesodermal origin, which develops within mesenchyme sandwiched between two epithelia, neural ectoderm and epidermis. Rudiments were treated enzymatically to separate epithelial and mesenchymal tissues. Frontal mesenchyme then was grown as chorioallantoic membrane grafts either in the presence or absence of neural ectoderm and/or epidermis. The results indicate that neural ectoderm, though required during early stages of development to induce frontal bone development (Schowing, 1968), is not required during later stages (HH 22-30, the stages tested in this study) for osteogenesis. Epidermis, however, was shown to be required for frontal bone development during the stages tested. Frontal mesenchyme formed bone when epidermis was present on the outer aspect of the mesenchyme, and did no...
Embryonic chick mandibles (Hamburger and Hamilton [HH] stages 17-25) were cultured in the presenc... more Embryonic chick mandibles (Hamburger and Hamilton [HH] stages 17-25) were cultured in the presence of various concentrations of vitamin A to determine the effect of hypervitaminosis A on membrane bone formation. In normal development, the mandible differentiates a centrally located Meckel's cartilage surrounded by membrane bones. Mandibles cultured without added vitamin A differentiated normally, though the timing of differentiation was retarded from that in ovo. Treatment with vitamin A interfered with skeletogenesis to varying degrees depending upon the initial age of the explant and the concentration of vitamin A. At low concentrations of vitamin A (1 microgram/ml), neither cartilage nor membrane bone formed in young explants (HH stage 17), whereas cartilage formed in 78% and membrane bone in 11% of older explants (HH stage 25). Higher concentrations of vitamin A (2-4 micrograms/ml) inhibited membrane bone formation in all explants, and 4 micrograms/ml of vitamin A inhibited ...
Proceedings of the National Academy of Sciences, 2008
The sea slug Elysia chlorotica acquires plastids by ingestion of its algal food source Vaucheria ... more The sea slug Elysia chlorotica acquires plastids by ingestion of its algal food source Vaucheria litorea. Organelles are sequestered in the mollusc's digestive epithelium, where they photosynthesize for months in the absence of algal nucleocytoplasm. This is perplexing because plastid metabolism depends on the nuclear genome for >90% of the needed proteins. Two possible explanations for the persistence of photosynthesis in the sea slug are (i) the ability of V. litorea plastids to retain genetic autonomy and/or (ii) more likely, the mollusc provides the essential plastid proteins. Under the latter scenario, genes supporting photosynthesis have been acquired by the animal via horizontal gene transfer and the encoded proteins are retargeted to the plastid. We sequenced the plastid genome and confirmed that it lacks the full complement of genes required for photosynthesis. In support of the second scenario, we demonstrated that a nuclear gene of oxygenic photosynthesis, psbO, is expressed in the sea slug and has integrated into the germline. The source of psbO in the sea slug is V. litorea because this sequence is identical from the predator and prey genomes. Evidence that the transferred gene has integrated into sea slug nuclear DNA comes from the finding of a highly diverged psbO 3 flanking sequence in the algal and mollusc nuclear homologues and gene absence from the mitochondrial genome of E. chlorotica. We demonstrate that foreign organelle retention generates metabolic novelty (''green animals'') and is explained by anastomosis of distinct branches of the tree of life driven by predation and horizontal gene transfer.
The epidermis of members of the major parasitic taxon Neodermata is distinctive among flatworms, ... more The epidermis of members of the major parasitic taxon Neodermata is distinctive among flatworms, being a syncytial, insunk, non-ciliated epidermis that develops through a wholesale replacement of larval epidermis at metamorphosis when the larva attacks a host. How it arose in evolution from what must have been a turbellarian-like ancestor is not immediately evident. While many turbellarian flatworms have also adopted a symbiotic way of life, the literature on ultrastructure of epidermis in these symbionts shows quite a variety of morphologies, many not so different from that of their free-living relatives. Various turbellarians do have syncytial or insunk epidermises or reduction of epidermal ciliation as is characteristic of the Neodermata, but co-occurrence in a single turbellarian of all features common to neodermatans has not been reported. Urastoma cyprinae, for example, which is ectosymbiotic on bivalves, has a ciliated cellular epidermis that is little different from what is known of epidermises of its free-living relatives. The endoparasitic Anoplodium hymanae, from the coelom of sea cucumbers, also bears a ciliated cellular epidermis, as is typical of many other rhabdocoels, but it shows marked phagocytic activity as well as incorporation of endosymbiotic bacteria. The closest similarity to neodermatan epidermis is that of the turbellarian Genostoma kozloffi, an ectosymbiont of the crustacean Nebalia: covering the bulk of the body is a non-ciliated syncytium with multiple branching connections to insunk nucleated portions, much as in epidermis of adult neodermatans and, on its ventral surface, is a field of ciliated cellular insunk epidermis resembling the epidermis of some larval neodermatans. Developmental clues to the origin of the neodermatan epidermis can be seen in turbellarian embryos. Before hatching, embryos of proseriate and triclad embryos go through 3 generations of epidermis, each replacing the next; 2 generations of epidermis are reported in the literature on rhabdocoel embryos. This process of replacement parallels the epidermal replacement that larval neodermatans undergo at metamorphosis. Ultrastructural study of developing acoel, polyclad and macrostomid embryos shows that they, too, have epidermal replacement and growth through immigration of deeper-lying cells, comparable to the processes seen in higher flatworms. Succession of distinct generations of epidermis in such animals as the proseriates, triclads and rhabdocoels is probably an adaptation to development of ectolecithal eggs, providing the means for the embryo to use yolk that resides in vitellocytes, outside its blastomeres. We propose that the Neodermata has taken advantage of this developmental mechanism, producing successive generations of epidermal cells even in its larval stages, to counter the defenses of hosts.
The avian secondary palate is normally cleft and does not show a rise in intracellular cyclic AMP... more The avian secondary palate is normally cleft and does not show a rise in intracellular cyclic AMP like that of the fused mammalian palate. The secondary palate of the embryonic chick (HH stages 25534) was exposed in vitro to dibutyryl cyclic AMP and theophylline to determine whether raising mtracellular cyclic-AMP levels would alter medial-epithelial development. Differentiation of the medial cpithelium was not altered but there was mucous-cell hyperplasia in nasal epithelium and inhibition of membrane-bone formation in the mesenchyme. Thus the developmental factors that cause the avian palate to be cleft and the mammalian palate to be fused are more complex than differences in intracellular cyclic-AMP levels 26, 427443.
Mandibles were cultured in the presence of dibutyryl cyclic AMP (1 mM) and theophylline (I mM) an... more Mandibles were cultured in the presence of dibutyryl cyclic AMP (1 mM) and theophylline (I mM) and in control medium. Controls differentiated normally but the timing of events differed from that in ore and feathers did not form. With elevated intracellular cyclic-AMP levels, membrane bone did not form in the earlier stages tested and was inhibited or reduced in older ones; chondrogenesis was inhibited only in young explants (HH stage 20) and. in certain instances, it was enhanced. There was precocious and hyperplastic differentiation of mucous cells within oral epithelium but the aboral epithelium was unchanged.
Maxillary mesenchyme (HH stages 1%21), separated from its epithelium and grown as a graft on the ... more Maxillary mesenchyme (HH stages 1%21), separated from its epithelium and grown as a graft on the chorioallantoic membranes of host chick embryos, did not form membrane bone. However, when isolated maxillary mesenchyme was recombined directly with chorioallantoic endoderm or chorioallantoic ectoderm and then grafted, it did form membrane bone in 87.5 and 70% of cases, respectively. This novel finding of extra-embryonic epithelia eliciting an osteogenic response from membrane bone-forming mesenchyme indicates that the germ-layer origin of an epithelium does not necessarily restrict its potential to promote membrane bone formation.
Intramembranous osteogenesis in the neural crest-derived maxilla and mandible depends upon intera... more Intramembranous osteogenesis in the neural crest-derived maxilla and mandible depends upon interactions between the bone-forming mesenchyme and the overlying epithelium. Whether or not an epithelium is required for the formation of the neural crest-derived palatine bone was examined. Palatal shelf mesenchyme, removed from embryonic chicks (5-7 days of incubation) prior to in-uiuo palatal osteogenesis (10 days of incubation), was grafted with and without its epithelium to the chorioallantoic membrane of host chick embryos. During early secondary palatal development, removal of the palatal epithelium resulted in the failure of osteogenesis within the palatal mesenchyme. Removal of the palatal epithelium at subsequent developmental stages did not interfere with palatal osteogenesis. Epithelial influences, therefore, are required during early palatal development to promote palatal osteogenesis but cease to be required several days prior to the actual onset of ossification,
Various regions of the mandibular process were tested for these potentials to determine whether r... more Various regions of the mandibular process were tested for these potentials to determine whether regional differences exist and vary with embryonic age . Mandibular processes from HH stages 17-21 .5 were cultured and grafted intact, or were subdivided into medial, mediolateral and lateral fragments and the separate regions cultured or grafted . The intact mandible from all these stages can form cartilage and membrane bone, but the 3 regions are not equally osteogenic and chondrogenic. The lateral region from all stages could form cartilage and membrane bone ; the mediolateral region could form cartilage and membrane bone but, in mediolateral fragments from HH stage 17, membrane bone was formed only in scant amounts. The medial region from HH stages 17 and 18 formed cartilage in only 50 per cent of cases and never formed membrane bone . By HH stage 20, the medial region could form membrane bone, but only in scant amounts . Medial fragments from HH stage 21 .5 formed extensive membrane bone and cartilage . The acquisition of these potentials, therefore, proceeds in a lateral-tomedial sequence, and the acquisition of an osteogenic potential lags slightly behind that of a chondrogenic potential. These findings do not indicate the mechanisms by which the two subpopulations of chondrogenic and osteogenic cells are distinguished from one another, but they give the temporal and spatial sequence in which this determination must occur .
Many institutions require candidates for faculty positions to present a teaching demonstration as... more Many institutions require candidates for faculty positions to present a teaching demonstration as part of the interview process. To help job candidates prepare for this and to assist departments in planning how to structure this portion of the interview, we surveyed biology faculty from community and liberal arts colleges and master's- and PhD-granting institutions concerning how their departments assess teaching potential. We also asked survey respondents to share advice on how candidates can prepare for teaching demonstrations. Here we report on the survey results and offer suggestions based on comments from respondents.
Single palatal processes or homotypic paired palatal processes from 12-day mouse embryos (strain ... more Single palatal processes or homotypic paired palatal processes from 12-day mouse embryos (strain C57Bl) were grown in organ culture on Millipore filters. The epithelium along the medial edge of each palatal process became disrupted, and, by 72 hours of culture, paired palatal processes achieved mesenchymal fusion at the site of disruption. The nasal epithelium of paired palatal processes became a recognizable pseudostratified ciliated columnar epithelium by 96 hours of culture, and the oral epithelium became a recognizable stratified squamous epithelium by 120 hours of culture. These events correspond temporally to the differentiation of palatal tissue in vivo. Differentiative changes of single palatal processes followed the same schedule as double processes. Observations also suggest that the epithelium at the tip of the vertically oriented young palatal process corresponds to the medial epithelium of the more advanced horizontal process-these observations have bearing on the mecha...
Journal of embryology and experimental morphology, 1977
The epithelium of the secondary palate from embryonic mice (12-14 days of gestation) was enzymati... more The epithelium of the secondary palate from embryonic mice (12-14 days of gestation) was enzymatically separated from the palatal mesenchyme and was cultured in association with mesenchyme from the embryonic tongue, incisor, and salivary gland. The histological differentiation of the epithelium in each type of heterotypic recombination was always palatal. Tongue mesenchyme supported full histodifferentiation of the palatal epithelium, whereas incisor mesenchyme supported only a limited histodifferentiation in the epithelium. Salivary gland mesenchyme in direct combination with palatal epithelium influenced the epithelium to become organized into a system of ducts and acini typical of glandular morphogenesis; histodifferentiation of the epithelium, however, was palatal. Salivary gland mesenchyme grown in isolation did not undergo glandular morphogenesis.
Intact mandibular processes and the enzymatically separated mesenchymal and epithelial components... more Intact mandibular processes and the enzymatically separated mesenchymal and epithelial components of the mandible from embryonic chicks of 2.5-to 5day incubation (Hamburger and Hamilton, '51: stages 1 6 2 5 ) were grown individually, either in organ culture or as grafts to the chorioallantoic membranes of host embryos. The differentiation of cultured and grafted intact mandibular processes was histologically normal, but the time of histodifferentiation differed from that in vivo. The histodifferentiation of cultured and grafted mandibular mesenchyme grown isolated from its epithelium depended upon the age of the embryo from which the mesenchyme had been obtained. Intramembranous ossification producing membrane bones of the mandible occurred i n mesenchyme isolated from 4.5-to 5-day embryos (HH 24-25), but did not occur in mesenchyme isolated from younger embryos. Cartilage (Meckel's) and subperichondrial bone in the articular process of Meckel's cartilage differentiated in mesenchyme isolated from embryos of all age groups tested (HH 16-25). Mandibular mesenchyme, therefore, requires the presence of epithelium until 4.5 days of incubation if the membrane bones of the mandible are to differentiate; if epithelial influences are required for Meckel's cartilage and subperichondrial bone formation, they are not required beyond 2.5 days of incubation. Mandibular epithelium isolated from its mesenchyme became layers of squamous cells in culture; but when grafted onto the chorioallantoic membrane, the epithelium became underlain by host fibroblasts and differentiated into a stratified squamous epithelium. Mandibular epithelium, therefore, is capable of differentiation in the presence of foreign fibroblasts derived from the chorioallantoic membrane.
In light of previous evidence t h a t epithelial influences are required for membrane bone format... more In light of previous evidence t h a t epithelial influences are required for membrane bone formation in certain instances (mandible, Tyler and Hall, '77 a,b; cranium, Benoit and Schowing, '701, t h e possibility t h a t maxillary epithelium influences membrane bone formation within the maxillary mesenchyme was investigated. Intact maxillary processes, and enzymatically separated epithelial and mesenchymal components of t h e maxilla were obtained from embryonic chicks of three t o five days of incubation (Hamburger and Hamilton "511 stages (HH) 18-25) were grown individually a s grafts on the chorioallantoic membranes of host chick embryos. The histodifferentiation of grafted intact maxillary processes was similar to t h a t in vivo. The histodifferentiation of grafted maxillary mesenchyme depended upon the embryonic age a t which t h e mesenchyme was isolated from its epithelium. In grafts of maxillary mesenchyme isolated from its epithelium at HH 22 or earlier stages, cartilage differentiated, but membrane bone did not form. In grafts of maxillary mesenchyme isolated from its epithelium at HH 23 or later stages, membrane bone formed in addition to cartilage. The results indicate t h a t maxillary mesenchyme requires the presence of epithelium through HH 22 in order for membrane bone to form within t h e mesenchyme. Cartilage formation within t h e mesenchyme does not require epithelial influences during t h e embryonic period tested . Maxillary epithelium isolated from its mesenchyme
The frontal region of the embryonic chick was studied to determine whether epithelial influences ... more The frontal region of the embryonic chick was studied to determine whether epithelial influences are necessary for frontal bone development. The frontal bone is a membrane bone, of neural crest and head mesodermal origin, which develops within mesenchyme sandwiched between two epithelia, neural ectoderm and epidermis. Rudiments were treated enzymatically to separate epithelial and mesenchymal tissues. Frontal mesenchyme then was grown as chorioallantoic membrane grafts either in the presence or absence of neural ectoderm and/or epidermis. The results indicate that neural ectoderm, though required during early stages of development to induce frontal bone development (Schowing, 1968), is not required during later stages (HH 22-30, the stages tested in this study) for osteogenesis. Epidermis, however, was shown to be required for frontal bone development during the stages tested. Frontal mesenchyme formed bone when epidermis was present on the outer aspect of the mesenchyme, and did no...
Embryonic chick mandibles (Hamburger and Hamilton [HH] stages 17-25) were cultured in the presenc... more Embryonic chick mandibles (Hamburger and Hamilton [HH] stages 17-25) were cultured in the presence of various concentrations of vitamin A to determine the effect of hypervitaminosis A on membrane bone formation. In normal development, the mandible differentiates a centrally located Meckel's cartilage surrounded by membrane bones. Mandibles cultured without added vitamin A differentiated normally, though the timing of differentiation was retarded from that in ovo. Treatment with vitamin A interfered with skeletogenesis to varying degrees depending upon the initial age of the explant and the concentration of vitamin A. At low concentrations of vitamin A (1 microgram/ml), neither cartilage nor membrane bone formed in young explants (HH stage 17), whereas cartilage formed in 78% and membrane bone in 11% of older explants (HH stage 25). Higher concentrations of vitamin A (2-4 micrograms/ml) inhibited membrane bone formation in all explants, and 4 micrograms/ml of vitamin A inhibited ...
Proceedings of the National Academy of Sciences, 2008
The sea slug Elysia chlorotica acquires plastids by ingestion of its algal food source Vaucheria ... more The sea slug Elysia chlorotica acquires plastids by ingestion of its algal food source Vaucheria litorea. Organelles are sequestered in the mollusc's digestive epithelium, where they photosynthesize for months in the absence of algal nucleocytoplasm. This is perplexing because plastid metabolism depends on the nuclear genome for >90% of the needed proteins. Two possible explanations for the persistence of photosynthesis in the sea slug are (i) the ability of V. litorea plastids to retain genetic autonomy and/or (ii) more likely, the mollusc provides the essential plastid proteins. Under the latter scenario, genes supporting photosynthesis have been acquired by the animal via horizontal gene transfer and the encoded proteins are retargeted to the plastid. We sequenced the plastid genome and confirmed that it lacks the full complement of genes required for photosynthesis. In support of the second scenario, we demonstrated that a nuclear gene of oxygenic photosynthesis, psbO, is expressed in the sea slug and has integrated into the germline. The source of psbO in the sea slug is V. litorea because this sequence is identical from the predator and prey genomes. Evidence that the transferred gene has integrated into sea slug nuclear DNA comes from the finding of a highly diverged psbO 3 flanking sequence in the algal and mollusc nuclear homologues and gene absence from the mitochondrial genome of E. chlorotica. We demonstrate that foreign organelle retention generates metabolic novelty (''green animals'') and is explained by anastomosis of distinct branches of the tree of life driven by predation and horizontal gene transfer.
The epidermis of members of the major parasitic taxon Neodermata is distinctive among flatworms, ... more The epidermis of members of the major parasitic taxon Neodermata is distinctive among flatworms, being a syncytial, insunk, non-ciliated epidermis that develops through a wholesale replacement of larval epidermis at metamorphosis when the larva attacks a host. How it arose in evolution from what must have been a turbellarian-like ancestor is not immediately evident. While many turbellarian flatworms have also adopted a symbiotic way of life, the literature on ultrastructure of epidermis in these symbionts shows quite a variety of morphologies, many not so different from that of their free-living relatives. Various turbellarians do have syncytial or insunk epidermises or reduction of epidermal ciliation as is characteristic of the Neodermata, but co-occurrence in a single turbellarian of all features common to neodermatans has not been reported. Urastoma cyprinae, for example, which is ectosymbiotic on bivalves, has a ciliated cellular epidermis that is little different from what is known of epidermises of its free-living relatives. The endoparasitic Anoplodium hymanae, from the coelom of sea cucumbers, also bears a ciliated cellular epidermis, as is typical of many other rhabdocoels, but it shows marked phagocytic activity as well as incorporation of endosymbiotic bacteria. The closest similarity to neodermatan epidermis is that of the turbellarian Genostoma kozloffi, an ectosymbiont of the crustacean Nebalia: covering the bulk of the body is a non-ciliated syncytium with multiple branching connections to insunk nucleated portions, much as in epidermis of adult neodermatans and, on its ventral surface, is a field of ciliated cellular insunk epidermis resembling the epidermis of some larval neodermatans. Developmental clues to the origin of the neodermatan epidermis can be seen in turbellarian embryos. Before hatching, embryos of proseriate and triclad embryos go through 3 generations of epidermis, each replacing the next; 2 generations of epidermis are reported in the literature on rhabdocoel embryos. This process of replacement parallels the epidermal replacement that larval neodermatans undergo at metamorphosis. Ultrastructural study of developing acoel, polyclad and macrostomid embryos shows that they, too, have epidermal replacement and growth through immigration of deeper-lying cells, comparable to the processes seen in higher flatworms. Succession of distinct generations of epidermis in such animals as the proseriates, triclads and rhabdocoels is probably an adaptation to development of ectolecithal eggs, providing the means for the embryo to use yolk that resides in vitellocytes, outside its blastomeres. We propose that the Neodermata has taken advantage of this developmental mechanism, producing successive generations of epidermal cells even in its larval stages, to counter the defenses of hosts.
The avian secondary palate is normally cleft and does not show a rise in intracellular cyclic AMP... more The avian secondary palate is normally cleft and does not show a rise in intracellular cyclic AMP like that of the fused mammalian palate. The secondary palate of the embryonic chick (HH stages 25534) was exposed in vitro to dibutyryl cyclic AMP and theophylline to determine whether raising mtracellular cyclic-AMP levels would alter medial-epithelial development. Differentiation of the medial cpithelium was not altered but there was mucous-cell hyperplasia in nasal epithelium and inhibition of membrane-bone formation in the mesenchyme. Thus the developmental factors that cause the avian palate to be cleft and the mammalian palate to be fused are more complex than differences in intracellular cyclic-AMP levels 26, 427443.
Mandibles were cultured in the presence of dibutyryl cyclic AMP (1 mM) and theophylline (I mM) an... more Mandibles were cultured in the presence of dibutyryl cyclic AMP (1 mM) and theophylline (I mM) and in control medium. Controls differentiated normally but the timing of events differed from that in ore and feathers did not form. With elevated intracellular cyclic-AMP levels, membrane bone did not form in the earlier stages tested and was inhibited or reduced in older ones; chondrogenesis was inhibited only in young explants (HH stage 20) and. in certain instances, it was enhanced. There was precocious and hyperplastic differentiation of mucous cells within oral epithelium but the aboral epithelium was unchanged.
Maxillary mesenchyme (HH stages 1%21), separated from its epithelium and grown as a graft on the ... more Maxillary mesenchyme (HH stages 1%21), separated from its epithelium and grown as a graft on the chorioallantoic membranes of host chick embryos, did not form membrane bone. However, when isolated maxillary mesenchyme was recombined directly with chorioallantoic endoderm or chorioallantoic ectoderm and then grafted, it did form membrane bone in 87.5 and 70% of cases, respectively. This novel finding of extra-embryonic epithelia eliciting an osteogenic response from membrane bone-forming mesenchyme indicates that the germ-layer origin of an epithelium does not necessarily restrict its potential to promote membrane bone formation.
Intramembranous osteogenesis in the neural crest-derived maxilla and mandible depends upon intera... more Intramembranous osteogenesis in the neural crest-derived maxilla and mandible depends upon interactions between the bone-forming mesenchyme and the overlying epithelium. Whether or not an epithelium is required for the formation of the neural crest-derived palatine bone was examined. Palatal shelf mesenchyme, removed from embryonic chicks (5-7 days of incubation) prior to in-uiuo palatal osteogenesis (10 days of incubation), was grafted with and without its epithelium to the chorioallantoic membrane of host chick embryos. During early secondary palatal development, removal of the palatal epithelium resulted in the failure of osteogenesis within the palatal mesenchyme. Removal of the palatal epithelium at subsequent developmental stages did not interfere with palatal osteogenesis. Epithelial influences, therefore, are required during early palatal development to promote palatal osteogenesis but cease to be required several days prior to the actual onset of ossification,
Various regions of the mandibular process were tested for these potentials to determine whether r... more Various regions of the mandibular process were tested for these potentials to determine whether regional differences exist and vary with embryonic age . Mandibular processes from HH stages 17-21 .5 were cultured and grafted intact, or were subdivided into medial, mediolateral and lateral fragments and the separate regions cultured or grafted . The intact mandible from all these stages can form cartilage and membrane bone, but the 3 regions are not equally osteogenic and chondrogenic. The lateral region from all stages could form cartilage and membrane bone ; the mediolateral region could form cartilage and membrane bone but, in mediolateral fragments from HH stage 17, membrane bone was formed only in scant amounts. The medial region from HH stages 17 and 18 formed cartilage in only 50 per cent of cases and never formed membrane bone . By HH stage 20, the medial region could form membrane bone, but only in scant amounts . Medial fragments from HH stage 21 .5 formed extensive membrane bone and cartilage . The acquisition of these potentials, therefore, proceeds in a lateral-tomedial sequence, and the acquisition of an osteogenic potential lags slightly behind that of a chondrogenic potential. These findings do not indicate the mechanisms by which the two subpopulations of chondrogenic and osteogenic cells are distinguished from one another, but they give the temporal and spatial sequence in which this determination must occur .
Many institutions require candidates for faculty positions to present a teaching demonstration as... more Many institutions require candidates for faculty positions to present a teaching demonstration as part of the interview process. To help job candidates prepare for this and to assist departments in planning how to structure this portion of the interview, we surveyed biology faculty from community and liberal arts colleges and master's- and PhD-granting institutions concerning how their departments assess teaching potential. We also asked survey respondents to share advice on how candidates can prepare for teaching demonstrations. Here we report on the survey results and offer suggestions based on comments from respondents.
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Papers by Mary Tyler