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Journal ArticleDOI

Fetal tooth development and adult replacement in Dermophis mexicanus (Amphibia: Gymnophiona): Fields versus clones

01 Nov 1980-Journal of Morphology (Wiley Subscription Services, Inc., A Wiley Company)-Vol. 166, Iss: 2, pp 203-216
TL;DR: It is suggested that substances regulating differentiation mediate early development, and hormones later development, including inception of adult teeth, and are comparable to “field substances” influencing primordia that originate according to clone theory.
Abstract: Teeth of fetuses of a caecilian, Dermophis mexicanus (Amphibia: Gymnophiona), show ontogenetic variation in crown structure from small, multidenticulate, and non-pedicellate to larger, spoon-shaped, pedicellate teeth with a single apical spike. Number of denticles decreases as enamel-secreting cells mature. Numbers of teeth and of tooth rows increase ontogenetically. A fetal vomeropalatine set of teeth is present in D. mexicanus but absent in species previously examined. Teeth transitional to the adult shape and arrangement appear shortly before birth. The transition is correlated with birth, not fetal size. There is relatively little increase in numbers of teeth during the juvenile period. The pattern of development does not fully agree with either morphogenetic field theory or with clone theory, both as defined by Osborn ('78). Sequence of initiation is appropriate to either. Tooth shape changes agree with aspects of clone theory. Multiple rows of fetal teeth and the transition to adult follow field theory. Clone theory holds that patterns of development and shape are self-regulated, field theory that they are controlled extrinsically. I suggest that substances regulating differentiation mediate early development, and hormones later development, including inception of adult teeth, and are comparable to "field substances" influencing primordia that originate according to clone theory. Components of both theories are appropriate to analyzing tooth development phenomena.
Citations
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Journal ArticleDOI
TL;DR: Phylogenetic analyses indicate that viviparity has originated independently in more than 150 vertebrate lineages, including a minimum of 115 clades of extant squamate reptiles, and substantial matrotrophy has arisen at least 33 times in these v Viviparous clades.
Abstract: Phylogenetic analyses indicate that viviparity (live-bearing reproduction) has originated independently in more than 150 vertebrate lineages, including a minimum of 115 clades of extant squamate reptiles. Other evolutionary origins of viviparity include 13 origins among bony fishes, nine among chondrichthyans, eight in amphibians, one in Paleozoic placoderms, six among extinct reptiles, and one in mammals. The origins of viviparity range geologically from the mid-Paleozoic through the Mesozoic to the Pleistocene. Substantial matrotrophy (maternal provision of nutrients to embryos during pregnancy) has arisen at least 33 times in these viviparous clades, with most (26) of these origins having occurred among fishes and amphibians. Convergent evolution in patterns of matrotrophy is widespread, as reflected by multiple independent origins of placentotrophy, histotrophy, oophagy, and embryophagy. Specializations for nutrient transfer to embryos are discontinuously distributed, reflecting the roles of phylogenetic inertia, exaptation (preadaptation), and constraint. Ancestral features that function in gas exchange and nutrition repeatedly and convergently have been co-opted for nutrient transfer, often through minor modification of their components and changes in the timing of their expression (heterochrony). Studies on functional and evolutionary morphology continue to play a central role in our attempts to understand viviparity and mechanisms of fetal nutrition.

222 citations

Journal ArticleDOI
TL;DR: It is concluded that stegokrotaphy (complete skull roofing) in caecilians is a derived condition, correlated with fossoriality, and does not indicate a direct relationship of caECilians to any known early amphibian taxon.
Abstract: The development of the skull of Dermophis mexicanus (Caeciliidae) is described and compared to that of other caecilians. The chondrocranium is well developed in embryos of 25 mm total length (TL); ossification begins in the quadrate and the articular element of the lower jaw at about 30 mm TL. The occipital arch then ossifies, followed by the dorsal and lateral dermal bones, then the ventral endochondral and dermal elements. The stapes ossifies at 55 mm TL. The amount of skull roofing increases during ontogeny, although the anterior rims of the nasal capsules, the anterior part of the mesethmoid, and the hyoid apparatus remain cartilaginous throughout life. Dermophis mexicanus lacks many primary embryonic ossification centers reported by Marcus et al. ('35) for Hypogeophis; presence of these ossification centers has been presumed to be indicative of a primitive skull morphology derived with little modification from archaic amphibians (“stegocephalians”). The fetal skull is highly kinetic, and some kinesis is retained in adults. We suggest that fetal skull kinesis and early ossification of jaw suspension elements are functionally related to the intraoviducal feeding mode of this viviparous species. Based on this evidence, together with the observed ossification pattern and bone homologies, we conclude that stegokrotaphy (complete skull roofing) in caecilians is a derived condition, correlated with fossoriality, and does not indicate a direct relationship of caecilians to any known early amphibian taxon.

114 citations

Journal ArticleDOI
TL;DR: A comparison of oviduct morphology, function, endocrinology, ecology and phylogeny in amphibians with diverse reproductive modes suggests a number of highly productive avenues of investigation.
Abstract: The structure and function of the oviducts of members of the three Orders of the Class Amphibia (Anura, frogs and toads; Urodela, salamanders and newts; Gymnophiona, caecilians) are well described for only a few species. Further, the majority of such descriptions relate only to temperate species that breed in water, lay their eggs there, and have free-living larvae, the presumed ancestral condition of oviparity. Many species of amphibians have derived reproductive modes. Such modes include breeding terrestrially and arboreally, making foam nests, parental transport of eggs and/or tadpoles, direct development (copulating on land, laying the eggs in terrestrial sites, fully metamorphosed juveniles hatching, obviating the free-living larval stage). Other derived modes are ovoviviparity (developing embryos retained in the oviducts, born at a diversity stages of development, no maternal nutrition in addition to yolk) and viviparity (oviductal retention of developing young, maternal nutrition after yolk is resorbed, young born as fully metamorphosed juveniles). The amphibian oviduct is regionally differentiated to secrete varying numbers of layers of material around each egg, which function in fertilization, etc.; it is responsive to endocrine output and environmental mediation during the reproductive cycle; and it maintains developing embryos in some members of all three orders, some with oviductal epithelial secretion of nutrients. However, little is known of the structure-function relationships of the oviduct in species with derived reproductive modes. A comparison of oviduct morphology, function, endocrinology, ecology and phylogeny in amphibians with diverse reproductive modes suggests a number of highly productive avenues of investigation.

101 citations

Journal ArticleDOI
TL;DR: Given that the plesiomorphic state in vertebrates is a short embryonic development, the generalized Type 1 first‐generation tooth is considered to represent an ancestral character for gnathostomes and hypothesize that an extended embryonic development leads to the suppression of tooth generations in the development of dentition.
Abstract: The present study focuses on the main characteristics of first-generation teeth (i.e., the first teeth of the dentition to develop in a given position and to become functional) in representatives of the major lineages of nonmammalian vertebrates (chondrichthyans, actinopterygians, and sarcopterygians: dipnoans, urodeles, squamates, and crocodiles). Comparative investigations on the LM and TEM level reveal the existence of two major types of first-generation teeth. One type (generalized Type 1) is characterized by its small size, conical shape, atubular dentine, and small pulp cavity without capillaries and blood vessels. This type is found in actinopterygians, dipnoans, and urodeles and coincides with the occurrence of short embryonic periods in these species. The other type assembles a variety of first-generation teeth, which have in common that they represent miniature versions of adult teeth. They are generally larger than the first type, have more complex shapes, tubular dentine, and a large pulp cavity containing blood vessels. These teeth are found in chondrichtyans, squamates, and crocodiles, taxa which all share an extended embryonic period. The presence in certain taxa of a particular type of first-generation teeth is neither linked to their phylogenetic relationships nor to adult body size or tooth structure, but relates to the duration of embryonic development. Given that the plesiomorphic state in vertebrates is a short embryonic development, we consider the generalized Type 1 first-generation tooth to represent an ancestral character for gnathostomes. We hypothesize that an extended embryonic development leads to the suppression of tooth generations in the development of dentition. These may still be present in the form of rudimentary germs in the embryonic period. In our view, this generalized Type 1 first-generation teeth has been conserved through evolution because it represents a very economic and efficient way of building small and simple teeth adapted to larval life. The highly adapted adult dentition characteristic for each lineage has been possible only through polyphyodonty.

93 citations

Journal ArticleDOI
TL;DR: It is proposed that the presence of a basilar papilla is a synapomorphy of tetrapods and Latimeria, that the translocation of the papilla neglecta is related to the unique course of the amphibian periotic canal, and that regressive changes in the inner ear arerelated to the primitive absence of a tympanic ear.
Abstract: The inner ears of representatives of all six gymnophionan families, as well as an ontogenetic series of one species, were studied in order to understand the origin and changes of the amphibian and basilar papillae. The amphibian papilla is in close proximity to the papilla neglecta in some adult gymnophionans. During ontogeny, both epithelia are adherent before they are separated by the formation of the utriculosaccular foramen. The nerve fibers to both epithelia run together, and both epithelia show a comparable variation in size and position among amphibians (amphibian papilla) and among vertebrates (papilla neglecta). Based on these arguments we propose that the amphibian papilla is a translocation of a part of the papilla neglecta specific to amphibians. Present in all primitive gymnophionans, the basilar papilla is lost in all derived gymnophionans. In contrast to anurans, but similar to some urodeles, amniotes, and Latimeria, the basilar papilla rests partly on a basilar membrane. Because of similarities in structure, topology, and innervation, the basilar papilla is suggested to be homologous in Latimeria and tetrapods. The structural differences of most amphibian basilar papillae, compared to those of amniotes and Latimeria, may be due to the different course of the periotic system and the formation of a basilar papillar recess rather than to a separate evolution of this epithelium. In addition to loss of the basilar papilla, some derived gymnophionans have lost the lagena, presumably independently, and the amphibian papilla is extremely reduced in the only genus without a stapes (Scolecomorphus). The papilla neglecta is, for unknown functional reasons, relatively large in aquatic gymnophionans, whereas it is almost lost in some thoroughly terrestrial gymnophionans. The regressive changes in the inner ear are not reflected in obvious changes in the pattern of eighth nerve projection. However, there is a rearrangement of cell masses in the rhombencephalic alar plate of derived gymnophionans, which may be related to the partial or complete loss of lateral line afferents. We propose that the presence of a basilar papilla is a synapomorphy of tetrapods and Latimeria, that the translocation of the papilla neglecta is related to the unique course of the amphibian periotic canal, and that regressive changes in the inner ear are related to the primitive absence of a tympanic ear.

85 citations

References
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Journal ArticleDOI
TL;DR: These concepts provide a unifying framework within which a wide variety of patterns formed from fields may be discussed, and give new meaning to classical concepts such as induction, dominance and field.

2,800 citations

Journal ArticleDOI
TL;DR: This paper describes a modification of the Simons and Van Horn (1971) procedure for rendering cartilage blue, bone red, and soft tissue translucent or transparent in whole vertebrate specimens, using formalin as a fixative.
Abstract: This paper describes a modification of the Simons and Van Horn (1971) procedure for rendering cartilage blue, bone red, and soft tissue translucent or transparent in whole vertebrate specimens. Alcian blue and alizarin red S are used to stain cartilage and bone respectively. In our procedure formalin is used as a fixative. This is a significant modification because formalin is the common fixative for museum specimens. This clearing and staining procedure is thus readily applicable to comparative studies in anatomy, embryology and systematic zoology.

428 citations

Book ChapterDOI
01 Jan 1974

289 citations

Journal ArticleDOI
TL;DR: The crown pattern of the tooth is essentially that of the surface of the dentine (dentine‐enamel junction), modified by the deposition of enamel which may be uneven in thickness.
Abstract: SUMMARY The crown pattern of the tooth is essentially that of the surface of the dentine (dentine-enamel junction), modified by the deposition of enamel which may be uneven in thickness. The dentine-enamel junction preserves in the completed tooth the form of the membrana praeformativa, the basement membrane of the inner enamel epithelium of the enamel organ. Folding of this membrane creates the crown pattern. The inner enamel epithelium is subjected to pressure from both sides. On the basal side there is the rapidly growing mesenchyme of the dental papilla, and on the occlusal side there is the stellate reticulum, which swells by the accumulation of fluid. The stellate reticulum prevents distortion of the epithelium by growth of the papilla, and thus ensures that folding of the epithelium is due to its intrinsic growth pattern. This makes for more accurate control of the crown pattern, the details of which are of importance in the function of chewing. The enamel knot is a region of the inner enamel epithelium from which cells are contributed to the stellate reticulum. It represents the tip of the primary cusp. The enamel cord (‘enamel septum’) which consists of cells which are in process of transforming into stellate reticulum, has been confused with two other structures that develop later: a cleavage septum, preparatory to the formation of crown cementum, and an epithelial septum, found in marsupials and crocodilians. The epithelial nodules of monotremes are probably degenerate relics of an epithelial septum. The inner enamel epithelium is a diaphragm passing across the interior of the dental follicle, and folding to adapt its increased area to a confined space. The cement organ, which in some mammals develops from the follicle, probably plays no part in the deformation of the epithelium, but the follicle as a whole may be subject to compression by adjacent follicles. A cusp is a centre of precocious maturation of the cells of the inner enamel epithelium. Here growth ceases (perhaps after a transitory burst of mitosis) and eventually the hard tissues are deposited. The process starts at the tip of the cusp and extends basally, so that growth continues longest in the valleys, intensifying the crown relief. Dens in dente is due to retarded maturation of an area of the enamel epithelium. Throughout the development of the crown there is a marginal zona cingularis, where growth continues. The crown pattern depends upon the position and the stage of growth in which cusps are differentiated from the zona cingularis, by accelerated maturation of groups of cells. Cusps which appear late in development stand low on the crown and frequently form part of a cingulum. Changes in the timing of cusp formation play an important part in serial modifications of pattern, as well as in phylogeny. Ridges are probably produced by tensions set up in the epithelium by the relative movement of cusps, owing to unequal growth or to changes in the shape of the follicle. They form in areas where growth has slowed down but the apposition of hard tissues has not begun. The lobes of the basal outline of the tooth are produced by growth centres in the papilla, which cause evagination of the follicle. Each growth centre is supplied by a bundle of blood vessels. The roots form in relation to these blood vessels, and so reflect the organization of the papilla. Hertwig's epithelial sheath grows between the follicle and the base of the papilla, the direction of its growth being controlled by the surrounding tissues owing to the absence of stellate reticulum on the basal portion of the tooth. There is no constant relation between cusps and roots, although marginal cusps frequently arise in association with lobes of the basal outline. The crown pattern results from an interaction between the growth pattern of the inner enamel epithelium and that of the papilla, the latter controlling the shape of the margin which limits the folding epithelium.

286 citations

Journal ArticleDOI
Osborn Jw1
TL;DR: It is argued that the wave replacement of alternate teeth is an automatic sequel to this and is of only secondary functional significance and a new model to explain the sequence of tooth initiation in reptiles is proposed.
Abstract: Edmund (1960) has shown that in the dentitions of almost all non-mammalian vertebrates, teeth are replaced in waves which regularly sweep through alternate tooth positions. He explained the ontogeny of these patterns of tooth replacement in terms of biological units called Zahnreihen whose existence has been accepted by nearly all workers studying tooth replacement. In the present paper it is argued that there is no unequivocal evidence, either during development or in adult animals, that Zahnreihen have any biological significance. Reconstructions were made from serial sections of the developing dentitions in the lower jaws of 15 embryos of Lacerta vivipara. It was evident that Zahnreihen have no significance in this animal. Rudimentary teeth were produced with varying frequency in positions 3, 5, 6, 8, 10 and 13. Contrary to the predictions of all previous theories explaining the ontogeny of tooth development in reptiles it was in these apparently random positions that the first teeth were produced. Furthermore, apart from during the first few days of embryonic dental development, it was clear that the development of a row of alternating teeth was initiated in sequence from the back to the front of the jaw to be followed by a similar sequence of development of the intervening teeth. On the basis of this evidence a new model to explain the sequence of tooth initiation in reptiles is proposed. The following assumptions have been made. (A) Ectomesenchymal cells migrate anteriorly through the developing jaws initiating a reaction from the oral ectoderm. (B) The oral ectoderm develops competence to react to the ectomesenchyme in three stages. First it generates abortive clumps of ectodermal cells; second it becomes capable of inducing the adjacent ectomesenchymal cells to form dentine and third it becomes capable of laying down enamel. (C) At all times the dental lamina has the potential of taking part in tooth development according to the regional competence achieved. (D) Developing tooth germs produce a condition which inhibits tooth development around them. Using these assumptions it is possible to explain all stages in the development of the wave replacement of alternate teeth in L. vivipara. It is also possible to explain previous observations on the ontogeny of reptilian dentitions. The sphere of inhibition which surrounds developing teeth is particularly important because it ensures that developing teeth are evenly spaced through the jaw. It is argued that the wave replacement of alternate teeth is an automatic sequel to this and is of only secondary functional significance.

128 citations