Cresta neural, cuarta hoja, germinativa embrionaria / Neural crest, fourth embryonary germinative leaf

RESUMEN Las células de la cresta neural son pluripotenciales y son llamadas la cuarta hoja germinativa del embrión. Con el objetivo de estructurar los referentes teóricos actualizados que sustenten la afirmación precedente y que constituirá material de estudio para los estudiantes de las Ciencias Médicas, se realizó la revisión de 28 referencias bibliográficas, de ellas 89% actualizadas. Estas células aparecen durante la neurulación y pasado este proceso transitan de epitelial a mesenquimatosa; migran siguiendo señales de la matriz extracelular a todo el cuerpo del embrión diferenciándose en tejidos disimiles. Muy vinculados en su evolución a mecanismos epigenéticos, hacen a esta población celular vulnerables a ser dañadas invocándose en la etiología de diferentes defectos congénitos y enfermedades crónicas no trasmisibles como cáncer. Como conclusión por su pluripotencialidad y por los mecanismos moleculares que distinguen su evolución son consideradas por muchos autores la cuarta hoja germinativa del embrión (AU).

SUMMARY Neural crest cells are pluripotentials, and are called the fourth germinative leaf of the embryo. With the objective of structuring the updated theoretical referents backing up the precedent affirmation that will be study material for the students of Medical Sciences, the authors reviewed 28 bibliographic references, 89 % of them updated. These cells appear during neurulation and after this process they transit from epithelial to mesenchymal; following extracellular matrix signals, they migrate to the whole embryo body differentiating themselves in dissimilar tissues. Tightly related in their evolution to epigenetic mechanisms, this cell population is very likely to be damaged and so they are invoked in the etiology of different congenital defects and noncommunicable chronic diseases like cancer. In conclusion, due to their pluripotentiality and the molecular mechanisms distinguishing their evolution, many authors consider them the embryo´s fourth germinative leaf (AU).

Omissions in the synthetic theory of evolution

The Synthetic Theory of Evolution is the most unifying theory of life science. This theory has dominated scientific thought in explaining the mechanisms involved in speciation. However, there are some omissions that have delayed the understanding of some aspects of the mechanisms of organic evolution, principally: 1) the bridge between somatic and germinal cells, especially in some phylum of invertebrates and vertebrates; 2) horizontal genetic transferences and the importance of viruses in host adaptation and evolution; 3) the role of non-coding DNA and non-transcriptional genes; 4) homeotic evolution and the limitations of gradual evolution; and 5) excessive emphasis on extrinsic barriers to animal speciation. This paper reviews each of these topics in an effort to contribute to a better comprehension of organic evolution. Molecular findings suggest the need for a new evolutionary synthesis.

GATA family of transcription factors of vertebrates: phylogenetics and chromosomal synteny.

GATA genes are an evolutionarily conserved family, which encode a group of important transcription factors involved in the regulation of diverse processes including the development of the heart, haematopoietic system and sex gonads. However, the evolutionary history of the GATA family has not been completely understood. We constructed a complete phylogenetic tree with functional domain information of the GATA genes of both vertebrates and several invertebrates,and mapped the GATA genes onto relevant chromosomes. Conserved synteny was observed around the GATA loci on the chromosomes. GATAs have a tendency to segregate onto different chromosomes during evolution. The phylogenetic tree is consistent with the relevant functions of GATA members. Analysis of the zinc finger domain showed that the domain tends to be duplicated during evolution from invertebrates to vertebrates. We propose that the balance between duplications of zinc finger domains and GATA members should be maintained to exert their physiological roles in each evolutionary stage. Therefore,evolutionary pressure on the GATAs must exist to maintain the balance during evolution from invertebrates to vertebrates. These results reveal the evolutionary characteristics of the GATA family and contribute to a better understanding of the relationship between evolution and biological functions of the gene family, which will help to uncover the GATAs' biological roles,evolution and their relationship with associated diseases.

Cell biology, molecular embryology, Lamarckian and Darwinian selection as evolvability

The evolvability of vertebrate systems involves various mechanisms that eventually generate cooperative and nonlethal functional variation on which Darwinian selection can operate. It is a truism that to get vertebrate animals to develop a coherent machine they first had to inherit the right multicellular ontogeny. The ontogeny of a metazoan involves cell lineages that progressively deny their own capacity for increase and for totipotency in benefit of the collective interest of the individual. To achieve such cell altruism Darwinian dynamics rescinded its original unicellular mandate to reproduce. The distinction between heritability at the level of the cell lineage and at the level of the individual is crucial. However, its implications have seldom been explored in depth. While all out reproduction is the Darwinian measure of success among unicellular organisms, a high replication rate of cell lineages within the organism may be deleterious to the individual as a functional unit. If a harmoniously functioning unit is to evolve, mechanisms must have evolved whereby variants that increase their own replication rate by failing to accept their own somatic duties are controlled. For questions involving organelle origins, see Godelle and Reboud, 1995 and Hoekstra, 1990. In other words, modifiers of conflict that control cell lineages with conflicting genes and new mutant replication rates that deviate from their somatic duties had to evolve. Our thesis is that selection at the level of the (multicellular) individual must have opposed selection at the level of the cell lineage. The metazoan embryo is not immune to this conflict especially with the evolution of set-aside cells and other modes of self-policing modifiers (Blackstone and Ellison, 1998; Ransick et al., 1996. In fact, the conflict between the two selection processes permitted a Lamarckian soma-to-germline feedback loop. This new element in metazoan ontogeny became the evolvability of the vertebrate adaptive immune system and life as we know it now. We offer the hypothesis that metazoan evolution solved this ancient conflict by evolving an immunogenetic mechanism that responds with rapid Lamarckian efficiency by retaining the ancient reverse transcriptase enzyme (RNACopyright DNA copying discovered by Temin in 1959 (see Temin, 1989) and found in 1970 in RNA tumor viruses by Temin and Baltimore), which can produce cDNA from the genome of an RNA virus that infects the cells. It seems that molecular

Diferenciaçäo sexual dos vertebrados: contribuiçäo de fatores somáticos e ambientais / Sexual differentiation of vertebrates: contribution of somatic and environmental factors

Há vários séculos a questäo da determinaçäo sexual dos animais é debatida. No entanto, um verdadeiro avanço no entendimento de como e quais fatores estäo ou podem estar envolvidos neste evento foi alcançado somente há algumas décadas. O presente trabalho descreve e discute os fatores genéticos, ambientais (temperatura, concentraçäo de íons, pH, dentre outros) e hormonais que estabelecem o padräo sexual do embriäo indiferenciado de animais pertencentes às várias classes de vertebrados.

La matriz extracelular y los diferentes tipos de colágena / The extracellular matrix and the different collagen types

Las cilágenas son una familia de proteínas que forman parte de la matriz extracelular. Se han descrito hasta ahora, 14 tipos genéticos que tienen en común, ser proteínas estructurales que contienen uno o más dominios con una estructura de triple hélice. Todas las moléculas participan en agregados supramoleculares y forman estructuras muy variadas que se han intentado agrupar en diferentes clases. 1) las colágenas fibrilares que se encuentran en la mayoría de los tejidos conjuntivos y que constituyen las fibras con bandas características que se observan al microscopio electrónico (colágenas tipo I, II, III V y XI); 2) Las colágenas, con triple hélice interrumpida, asociadas a fibras que parecen conectar las fibrillas a otros elementos de la matriz (colágenas tipo IX, XII y XIV); 3) Las colágenas que forman láminas basales o membranas proteicas que rodean tejidos u organismos (colágenas tipo IV y VIII); 4) Colágenas que forman fibrillas de encaje entre el epitelio basal y las fibras de colágenas (colágena tipo VII); 5) Colágenas que forman filamentos en rosario que pueden interactuar entre las fibras y las células (colágenas tipo VI)