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Características Morfológicas De La Gástrula En Ceratophrys Stolzmanni (Anura: Ceratophryidae)

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Características Morfológicas De La Gástrula En Ceratophrys Stolzmanni (Anura: Ceratophryidae) 50 Juan Diego Santillana Ortiz y Eugenia M. del Pino Características morfológicas de la gástrula en Ceratophrys stolzmanni (Anura: Ceratophryidae). Juan Diego Santillana Ortiz y Eugenia M. del Pino Pontificia Universidad Católica del Ecuador, Escuela de Ciencias Biológicas, Laboratorio de Biología del Desarrollo, 12 de Octubre 1076 y Roca, Quito, Ecuador [email protected] RESUMEN: Se estudió la morfología de la gástrula y néurula en embriones de Cer- atophrys stolzmanni (el sapo bocón de la costa). Los huevos, cuyo diámetro al comienzo de la gastrulación es de 2,2 mm, se desarrollan rápidamente. El proceso de gastrulación dura aproximadamente cinco horas. Los embriones fueron fijados y analizados en monta- je entero, bisecciones y secciones de vibrátomo. Se encontró que los huevos tienen pig- mento oscuro al igual que en Xenopus laevis, pero tienen un tamaño mayor. La morfología de la gástrula de C. stolzmanni es similar a la descrita para Ceratophrys ornata (el sapo cornudo argentino) por otros autores. Los estadios de gástrula tardía y néurula temprana, estudiados mediante secciones con tinción fluorescente para núcleos celulares, revelan ausencia de endodermo que recubra al notocordio y a los somitas en la parte dorsal poste- rior del embrión. Esta región se denomina GRP (Gastrocoel roof plate, por sus siglas en inglés) y tiene importancia en la determinación del eje izquierdo-derecho en anfibios. En este estudio se caracterizó el GRP de C. stolzmanni al nivel morfológico. PALABRAS CLAVE: Ceratophrys ornata, Ceratophrys stolzmanni, Gastrulación, GRP, Xenopus laevis. ABSTRACT: The gastrula morphology of Ceratophrys stolzmanni (the Pac-man frog) was studied. Its eggs, whose diameter at the beginning of the gastrulation process is 2.2 mm in diameter, develop rapidly. The gastrulation process requires five hours approxi- mately. The embryos were fixed and analyzed in whole-mount, in bisections, and vibratome sections. The eggs of C. stolzmanni were pigmented, as in Xenopus laevis, but their size was bigger. The gastrula morphology of C. stolzmanni is similar to the one described for Ceratophrys ornata (the argentine horned-frog) by other authors. The late gastrula and early neurula stages, studied with vibratome sections stained for cell nuclei, revealed lack of endoderm covering both the posterior notochord and the somites found on this region. This region is called the GRP (Gastrocoel roof plate) and it plays a major role in the development of the left-right axis in amphibians. In this work the GRP of C. stolzmanni was characterized at the morphological level. KEYWORDS: Ceratophrys ornata, Ceratophrys stolzmanni, Gastrulation, GRP, Xenopus laevis. Revista Ecuatoriana de Medicina y Ciencias Biológicas - Vol. XXX Números 1 y 2: 50-56, octubre 2009 Características morfológicas de la gástrula en Ceratophrys stolzmanni 51 INTRODUCCIÓN Una rana lejanamente relacionada con las anteriores especies es Ceratophrys Las diferencias en el desarrollo tem- stolzmanni (Steindachner, 1882) (el sapo prano de las ranas, tales como el tamaño bocón de la costa). Esta rana vive bajo el de los embriones, el proceso de gastru- suelo en ambientes xéricos del sur del lación y la velocidad del desarrollo, son Ecuador y norte del Perú y deposita sus aspectos necesarios de estudio para la huevos en charcas temporales cuando comprensión del desarrollo embrionario. comienza la estación lluviosa. Ésta es una En particular, el proceso de gastrulación especie interesante para ser estudiada en diversas especies de anfibios muestra debido a su diferente estrategia reproducti- la existencia de tres patrones distintos. va. Si bien el desarrollo embrionario de C. Estos patrones se relacionan con la veloci- stolzmanni no se había estudiado, sí se dad del desarrollo de las especies. Por un conocen aspectos del desarrollo en una lado, las ranas de desarrollo rápido, como especie cercana a ésta: la rana cornuda argentina Ceratophrys ornata. El anuro C. Xenopus laevis (Daudin, 1802), extienden ornata tiene un modo de vida terrestre pero el arquenterón y alargan el notocordio deposita sus huevos, de 1,5 - 2,0 mm de durante la gastrulación; mientras que diámetro, en el agua. Su desarrollo desde la ranas de desarrollo lento, como Gas- fertilización hasta convertirse en renacuajo trotheca riobambae (Fowler, 1913), es sumamente rápido, entre 24-48 horas. posponen el alargamiento del notocordio a De su desarrollo temprano se conoce que la post gástrula (1). Esto sugiere que el presenta mesodermo superficial en el techo proceso de gastrulación es modular, es del gastrocele el mismo que ingresa a las decir que la expansión del arquenterón y capas mesodermales profundas durante la el cierre del blastoporo son procesos regu- neurulación (2). lados de forma independiente y a distinto Este mesodermo superficial forma una tiempo (1). Los estudios comparativos estructura en el techo del arquenterón que entre ranas que difieren en sus modos de está expuesta a la cavidad del gastrocele reproducción, tales como Epipedobates (3). En los anfibios a esta estructura se la machalilla (Coloma, 1995) y Epipedo- conoce como placa del techo del gastro- bates tricolor (Boulenger, 1899b) (repro- cele, GRP, gastrocoel roof plate, por sus ducción terrestre), G. riobambae siglas en inglés (3). El GRP es una región (incubación en una bolsa materna) y la con células ciliadas que promueven un rana acuática X. laevis sugieren que las flujo de sustancias no determinadas en sen- diferencias en el desarrollo embrionario tido izquierdo. Este flujo es el evento que temprano están relacionadas con distintas permite el establecimiento de la cascada de estrategias reproductivas (1). Es impor- la señal molecular Nodal para la especifi- tante entonces ampliar los estudios com- cación asimétrica del cuerpo. El GRP está parativos y describir los procesos de bordeado por las crestas laterales endoder- desarrollo para especies con diversas males (4) las que gradualmente lo cubren estrategias reproductivas. (1). El GRP es análogo a estructuras simi- lares encontradas en otros vertebrados (3). Revista Ecuatoriana de Medicina y Ciencias Biológicas - Vol. XXX Números 1 y 2: 50-56, octubre 2009 52 Juan Diego Santillana Ortiz y Eugenia M. del Pino En X. laevis y Xenopus tropicalis la metodología de tinción fluorescente (Wagler 1827) el GRP mide aproximada- para núcleos celulares (tinción Hoechst mente 260 µm y 150 µm de largo respec- 33258). Se documentaron las observa- tivamente. Los cilios tienen una longitud ciones mediante microfotografías usando promedio de 5 µm (5). Por el significado una cámara digital acoplada a un esté- del GRP en el establecimiento del eje reomicroscopio o a un microscopio de izquierdo-derecho es importante demos- fluorescencia. Todas estas técnicas se trar su presencia en embriones de ranas enmarcan en metodologías estándar del con modos divergentes de gastrulación Laboratorio de Biología del Desarrollo de como es C. stolzmanni. la PUCE. Los estadios de los embriones se determinaron de acuerdo a la tabla ge- neral para el desarrollo de anuros (6) y MATERIALES Y MÉTODOS para C. ornata (2). Los embriones con los que se realizó este estudio fueron donados por el Labo- RESULTADOS ratorio de Herpetología de la Pontificia Universidad Católica del Ecuador Morfología de la gástrula de C. (PUCE). Estos embriones provinieron de stolzmanni.- El proceso de gastrulación dos posturas: La primera de ranas colec- duró aproximadamente 5 horas. La tadas en la provincia de El Oro, Ecuador; gástrula temprana (estadio 10,5; 2,2 mm y la segunda de ranas criadas en cautive- de diámetro) se caracterizó por la forma- rio dentro del programa de investigación ción del labio dorsal del blastoporo, en “Balsa de los sapos”. forma de hendidura, en la región sub-ecu- Los embriones se cultivaron en reci- atorial del embrión. Se detectaron células pientes de plástico. La temperatura en forma de botella en dicha hendidura. ambiente fue de 30 °C y 26 °C la del Se observó pigmento ocre oscuro en el agua. Los acuarios fueron artificialmente polo animal del embrión así como un pig- oxigenados. mento blanco claro en el polo vegetal. Se estudió la morfología externa de la La gástrula media (estadio 11) se carac- gástrula y néurula mediante montajes terizó por la extensión lateral y ventral del enteros de embriones fijados en el fijativo labio del blastoporo, el mismo que tuvo de Smith. La estructura interna de los forma de herradura (Fig. 1A). Internamente embriones fue descrita mediante bisec- el blastocele ocupó la mayor parte de la ciones y secciones. Para realizar las sec- región animal. El arquenterón inició su for- ciones los embriones fueron primero mación pero no se expandió. embebidos en gelatina, luego montados en cubos de agarosa y, mediante el uso del vibrátomo, se obtuvieron secciones de 40-50 µm de grosor. Algunas de estas sec- ciones fueron también estudiadas usando Revista Ecuatoriana de Medicina y Ciencias Biológicas - Vol. XXX Números 1 y 2: 50-56, octubre 2009 Características morfológicas de la gástrula en Ceratophrys stolzmanni 53 Fig. 1. Desarrollo temprano de C. stolzmanni: gástrula y néurula. (A) Gástrula media. (B) Néurula media. (C) Estadio de cerramiento del tubo neural. Abreviaturas: lb, labio del blastoporo; pn, pliegues neu- rales; bp, blastoporo; tb, tubo neural; ab, arco branquial; ah, arco hioideo; am, arco mandibular. Todas las bar- ras corresponden a 250 µm. La gástrula tardía (estadio 12) exter- el techo del arquenterón. En el estadio 14,5 namente mostró una disminución gradual (2,4 mm de largo) los pliegues neurales del diámetro del tapón de yema. Interna- fueron más prominentes (Fig. 1B). mente los movimientos de convergencia y Durante el estadio medio de pliegues neu- extensión dorsales permitieron el rales (estadio 15; 2,7 mm de largo) los alargamiento y expansión del arquenterón. pliegues neurales se acercaron pero el tubo En el estadio 12,5 se observó que el tapón neural permaneció abierto. En el estadio de yema continuaba su gradual reducción. de cerramiento del tubo neural (estadio En una sección sagital de un embrión de 16), el tubo neural se cerró pero per- este estadio, teñido para núcleos celulares, maneció abierto en la región rostral.
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