Desarrollo embrionario del pargo colorado Lutjanus colorado (Jordan & Gilbert, 1882) / Embryonic development of pargo colorado Lutjanus colorado (Jordan & Gilbert, 1882)

El pargo colorado (Lutjanus colorado) es una especie con un alto valor comercial en el mercado mexicano, con potencial para su cultivo. Hasta la fecha no existen estudios sobre su reproducción, cultivo larvario y engorda en cautiverio. El presente trabajo es el primer reporte sobre la descripción a detalle del desarrollo embrionario de la especie bajo condiciones de cultivo. Los huevos fertilizados viables del pargo colorado son pelágicos, esféricos, transparentes y con una sola gota de aceite. Midieron 0,77±0,09 mm de diámetro y la gota de aceite 0,14±0,01 mm. La primera división ocurrió a las 0,05 horas post fertilización (HPF). La eclosión se llevó a cabo a las 17,22 HPF bajo las condiciones del presente estudio. Las larvas recién eclosionadas midieron 1,8±0,1 mm de longitud total (LT). El desarrollo embrionario de esta especie fue similar a la descrita para especies de la misma familia. Los resultados del presente estudio aportan información básica para iniciar el desarrollo de la biotecnología para la producción de semilla de esta especie a escala comercial.

The Colorado snapper (Lutjanus colorado) is one of the most commercially important fish species in México and it is considered a suitable candidate for culture. Until now, no research has been carried out on its reproduction, larviculture and fattening in captivity. This study is the first description of embryonic development of this species under controlled conditions. Fertilized eggs of Colorado snapper are pelagic, spherical and transparent and contain one drop of oil. Eggs measured 0.77±0.09 mm and the drop of oil 0.14±0.01 mm. First cell division occurred at 0.05 h post-fertilization (HPF), hatching at 17.22 HPF under the above described conditions. Larvae total length (LT) was 1.8±0.1 mm. Embryonic development of this species was similar to other lutjanidae species. These results provide basic information for developing the necessary biotechnology for commercial seed production of the Colorado snapper.

Histopatologia da interação de Bacillus thuringiensis e extratos vegetais no intestino médio de Spodoptera frugiperda (Lepidoptera: Noctuidae) / Histopathology of the interaction of Bacillus thuringiensis and plant extracts in the midgut of Spodoptera frugiperda (Lepidoptera: Noctuidae)

Plantas são fontes naturais de substâncias inseticidas, já que podem ser produzidas pelo vegetal em resposta a ataques de insetos podendo representar uma alternativa no manejo de insetos-praga, como Spodoptera frugiperda. Dessa forma, o presente trabalho objetivou a análise histopatológica do intestino médio de lagartas de S. frugiperda, após a ingestão dos extratos obtidos por maceração e infusão de Petiveria alliacea. Zingiber officinale. Cymbopogon citratus. Malva silvestris. Baccharis genistelloides e Ruta graveolens, assim como a associação desses extratos com Bacillus thuringiensis aizawai. As lagartas foram tratadas in vivo com cada extrato e a bactéria, mais a associação desses extratos com B. thuringiensis e após, uma reação de cinética entre 3 e 27 horas, foram fixadas para o preparo de cortes histológicos, os quais foram corados e analisados comparativamente às testemunhas em microscopia óptica. Os resultados mostraram mudanças na histologia do intestino médio das lagartas de S. frugiperda, 3 horas após a aplicação dos tratamentos à base de alliacea. Z. officinale. C. citratus e M. silvestris, enquanto que para B. genistelloides e R. graveolens só foram observados alterações após 6h. Na interação dos extratos com B. thuringiensis observou-se alterações nas microvilosidades, desorganização do intestino médio e a hipertrofia das células epiteliais que projetaram-se para o lúmen. Os resultados desse trabalho mostram que o efeito histopatológico de Z. officinale. M. silvestris. R. graveolens e B. genistelloides, foram mais ativos quando comparados aos extratos de P. alliacea e C. citratus, os quais apresentaram uma interação positiva com B. thuringiensis.

The natural insecticidal substances produced by plants in response to insect attack may represent an alternative for the management of pest insects such as Spodoptera frugiperda. This study was therefore aimed to histopathologically analyze the midgut of larvae of S. frugiperda, after ingestion of extracts obtained by maceration and infusion of Petiveria alliacea. Zingiber officinale. Cymbopogon citratus. Malva silvestris. Baccharis genistelloides and Ruta graveolens, and the association of these extracts with Bacillus thuringiensis aizawai. The larvae were treated in vivo with each extract, B. thuringiensis, and the combination of these extracts with the bacteria, and after a reaction kinetics of between 3 to 27 hours they were fixed for the preparation of histological sections, which were stained and analyzed in comparison to the controls with light microscopy. The results showed changes in the histology of the midgut of the larvae of S. frugiperda, 3 hours after application of treatments based on P. alliacea. Z. officinale. C. citratus and M. silvestris, while for B. genistelloides and R. graveolens changes were observed only after 6h. In the interaction of extracts with B. thuringiensis the analysis revealed changes in microvilli, disorganization of the midgut and hypertrophy of epithelial cells that projected into the lumen. The findings show that the histopathological effects of Z. officinale. M. silvestris. R. graveolens and B. genistelloides were more active when compared to extracts of P. alliacea and C. citratus, which showed a positive interaction with B. thuringiensis.

Tailgut Cyst in a Neonate

Tailgut cysts are rare congenital lesions thought to arise from vestiges of the tailgut/postanal gut, which is a primitive gut temporarily present at the most caudal portion of the embryo. The lesions are usually multilocular cysts lined by various types of epithelium in the retrorectal space. We experienced a tailgut cyst in an 18-day-old girl initially presented as an epidermoid cyst like skin lesion in the coccygeal region. MRI showed a well-defined multiloculated cystic mass between the rectum and coccyx, measuring 12x23 mm and markedly hyperintense on T2 weighted images and hypointense on T1-weighted images. The cyst was surgically removed via a posterior sagittal approach and microscopically lined by ciliated columnar, transitional and squamous cells. Tailgut cysts are usually found in adult females, and extremely rarely in neonates. We report a tailgut cyst in a neonate with a review of the literature.

Development beyond the gastrula stage and digestive organogenesis in the apple-snail Pomacea canaliculata (Architaenioglossa, Ampullariidae)

Development of Pomacea canaliculata from the gastrula stage until the first day after hatching is described. Trochophore embryos are developed after gastrulation, showing the prototroch as a crown of ciliated orange-brownish cells. However, no true veliger embryos are formed, since the prototroch does not fully develop into a velum. Afterward, the connection between the fore- and midgut is permeated and the midgut becomes full of the pink-reddish albumen, which is stored into a central archenteron's lake, from where it is accumulated into the large cells forming the midgut wall ("giant cells"). Electron microscopy of giant cells in late embryos showed that albumen is engulfed by large endocytic vesicles formed between the irregular microvilli at the top of these cells. By the end of intracapsular development, giant cells become gradually replaced by two new epithelial cell types which are similar to those found in the adult midgut gland: the pre-columnar and the pre-pyramidal cells. Pre-columnar cells have inconspicuous basal nuclei and are crowned by stereocilia, between which small endocytic vesicles are formed. Pre-pyramidal cells have large nuclei with 2-3 nucleoli and show a striking development of the rough endoplasmic reticulum. The genesis of the three cell lineages (giant, pre-columnar and pre-pyramidal cells) is hypothetically attributed to epithelial streaks that occur at both sides of the midgut since early stages of development.

Descripción del desarrollo embrionario de zigotos híbridos obtenidos por el cruce de machos de Cachama Blanca (Piaractus brachypomus) y hembras de Cachama Negra (Colossoma macropomum) / Embrionary development of hybrid Zygote obtained from males of Piaractus brachypomus and females of Colossoma macropomum

Fueron seleccionados tres machos de Piaractus brachypomus y 3 hembras de Colossoma macropomum de 2.5 a 3 años de edad. Los reproductores se trataron con EPC, según el grado de madurez sexual; la fertilización se realizó en seco y se usaron incubadoras de flujo ascendente. Cada 10-15 minutos se observó una muestra del 0.1 por ciento y se determinaron sus características. El huevo inicia su hidratación, el corion se separa del vitelo, a los 00:08 minutos comienza el período de “cigote”, se forma el blastodisco, y aparece el espacio perivitelino, a los 00:10 minutos comienza el período de “clivaje”, donde se observa el paso de dos a 64 células entre los 10 minutos y 1:45 horas pos inseminación. El período de “Blástula” y “Gástrula”se caracterizan por la formación del epiboly y el tubo ectodermal (diferenciación de cabeza y cola). A las 10:05 horas se da el engolfamiento del vitelo. A las 11:00 horas se inicia el período de “Segmentación” continúa con el de faringulación correspondientes al desarrollo de somitas (rudimentos de la columna vertebral), diferenciación de órganos básicos, vesícula óptica y los segmentos del mesodermo; a las 17:10 horas se diferenciaclaramente el saco vitelino y las somitas. Alrededor de las 19:00 horas posinseminación se da la eclosión de las larvas.

Intestinal Rotation, Luminal Obstruction and Recanalization in the Human Embryos and Fetuses / 체질인류학회지

The development of intestine can be characterized by the following two traits. First, at the beginning of its development, the intestine is temporarily herniated into the umbilical cord, then reduced into the abdominal cavity, and finally reaches its adult location, rotating along the way. Nevertheless, no time table is yet available as to show, in a precise manner, when the intestine is herniated, when it is reduced, or to what extent the rotation occurs. Secondly, it is known that in the course of intestinal development, the lumen is temporarily obstructed by epithelial proliferation and then undergoes the recanalization. However, it is not yet certain whether there is practically a complete luminal obstruction, even by supposing that there is a complete obstruction, no theory has yet been established to determine the time, location, or the extent of the obstruction, and the time of the recanalization. In this study, 2 ~12 week old human embryos and fetuses have been used which were microdissected under a surgical microscope and were subsequently observed in an ordinary tissue slide. We have obtained the following results. 1. The intestine was first formed at Carnegie stage 5. Together with the folds of the embryos, three parts of the primitive gut were observed for the first time at stage 10. At stage 12, the primitive gut was located on the midline. 2. At stage 15, the intestine rotated to 90 counterclockwise, and at the next stage, it started herniation. There was another 90 counterclockwise rotation at stage 20, and at the beginning of the 9th week, with the rapid reduction into the abdominal cavity, the cecum was located in the proper adult position, and the rotation completed. 3. Although epithelial proliferation was recognizable in the esophagus, duodenum and other parts of the intestine of the developing embryo, we could not find any cases in which the lumen was completely obstructed.

A Catalogue of Gene Expression Difference in Biliary Cirrhosis due to Biliary Atresia Using Differential Expressed Sequence Tags(EST) Screening

PURPOSE: Extrahepatic biliary atresia is the most common indication for liver transplantation in children, but the etiology of this disorders remains unknown. It would be very signficant to identify genes that are specifically expressed in pathologic liver tissue of biliary atresia and analyze the pattern of expression in those genes. METHODS: We made dot blot panels consisting of 1,730 different EST (expressed sequence tags) clones which were isolated from human hair dermal papilla cell cDNA library. Liver tissues were taken from a recipient with biliary atresia and a normal donor during living-related liver transplantation. Total RNA was extracted from each sample and reversely transcribed to make cDNA. Then radiolabelled cDNA probe pools were made by random primed DNA labeling method and used for screening differentially expressed genes using EST dot blot panel. RESULTS: Among the total of 1,730 EST clones, 26 cDNA clones were overexpressed in biliary cirrhosis. They revealed homology to genes encoding bcl-w, laminin binding protein, hepatocyte growth factor-regulated tyrosine kinase substrate (HRS), thymosin beta-4, 10; transforming growth factor (TGF)-beta, tissue inhibitor of metalloproteinase (TIMP)-1, signal recognition particle (SRP)4, eukaryotic initiation factor (eIF)-2alpha kinase, lysyl oxidase, aldolase A, gamma-glutamylcystein synthetase, collagen type I alpha1, 2, collagen type III, fibronectin, osteonectin, insulin-like growth factor binding protein (IGFBP)-2, 3, and more. In addition, the expression of 2 clones showed that gastrula zinc finger protein and one novel gene were decreased in biliary atresia. CONCLUSOIN: This study identified differentially expressed genes in biliary cirrhosis from progressive biliary atresia using differential EST screening technique.

Umbilical Shape and Diseases Related to Age and Sex / 대한피부과학회지

BACKGROUND: The umbilicus is a simple scar that obliterates the portal through which the omphalomesenteric duct connected the primitive gut to the yolk sac; through which the intestine was expelled prior to its rotation; through which passed the umbilical arteries and vein; at which the allantosis attached to the early urinary bladder; and at which the umbilical cord that carried these structures was fused to the body wall. The umbilicus is situated at the level of L3 to L4 in the midline of the abdomen. It's shape, size, and configuration vary greatly related to age and sex. Deramtoses of the umbilicus are umbilical hernia, congenital abnormalities, granulomas, omphalith, infections, eczema and tumors. OBJECTIVE: The purpose of this study was to examine the umbilical shape and size related to age and sex and diseases of the umbilicus. METHODS: During a 6-month-period from May to October, 1998, 420 person's umbilical shapes and size were examined with regard to age and sex. The diseases of the umbilicus were evaluated, too. RESULTS & CONCLUSIONS: 1. The mean vertical diameter of the umbilicus is 1.89+/-0.65cm and the mean transverse diameter is 1.98+/-0.86cm. The mean diameter of umbilicus in women(vertical: 1.98+/-0.73cm, transverse: 2.07+/-0.99cm) is larger than the mean diameter of umbilicus in men(vertical: 1.80+-0.53cm, transverse: 1.89+/-0.66cm)(pvertical diameter) is more than the elliptical type(vertical diameter>transverse diameter) in men, and the elliptical type(vertical diameter>transverse diameter) is more than the elliptical type(transverse diameter>vertical diameter) in women. 4. The diseases of the umbilicus are allergic contact dermatitis(6 cases), pityriasis rosea(5 cases), psoriasis(3 cases), scabies(3 cases), rhus dermatitis(2 cases), pemphigus(2 cases), drug eruption(1 case), and insect bite(1 case).

Effect of Dexamethasone on Experimental Candida Endophthalmitis

The use of intravitreal corticosteroids in the management of endophthalmitis remains controversial. This study was performed to evaluate the effectiveness of dexamethasone(DXM) in Candida endophthalmitis. In rabbits, Candida albicans(3,000 blastospore: Group A, 30,000 blastopore: Group B) was inoculated intrabitreally. At 24, 48, and 72 hours postinoculation, single intravitreal dose of amphotericin B(AMP, 5microgram/0.1ml) with DXM(400microgram/0.1ml) or AMP alone was given. Clinical examination, protein analysis, electrophysiological and histopathologic measures were utilized to rate DXM effectiveness. Dosage of 5microgram AMP resulted in no culture in both Group A and B, On examinations including the electrophysiologic and histopathologic studies, eyes treated with AMP and DXM did not differ from eyes treated with AMP alone in the Group A(p>0.05). On protein analysis for vitreous in the Group B, however, eyes treated with AMP alone at 24 and 48 hours postinoculation(p<0.05). On electrophysiologic study the decrease rate of the amplitude showed the difference between at 24 hours postinoculation and at 48 or 72 hours postinoculation(p<0.05). Histologic preparations showed a mild or moderate cellular infilteration in the vitreous and swelling in the sensory retina, but small abscess in the vitreous observed when treatment was delayed more than 72 hours following inoculation in both Group A and B. DXM in the experimental Candida endophthalmitis had the beneficial effect in eyes with 30,000 blastspore inoculation and in eyes treated with AMP and DXM at 24 hours postinoculation.

Anomalia anorretal associada a regressäo caudal / Retal and anal anomalies associate with caudal regression

Os autores apresentam um caso de sirenomelia (síndrome da sereia) em uma criança de dois anos de idade, branca, com múltiplas malformaçöes associadas, sem história materna de doenças , ingestäo de drogas ou medicamentos durante a gravidez. As malformaçöes associadas säo: anomalia anorretal alta sem fístula urinária; deformidades de várias vertebras, com arcos alongados e displásicos; deformidades vertebrais importantes, com cifoescoliose em S da coluna torácica, com convexidade para a direita; arcos posteriores em nível lombossacro com fusäo parcial e displásicos; dextrocardia; deformidades de costelas, ausência do membro inferior esquerdo; deformidade importante do membro superior esquerdo; assimetria das dimensöes renais, com hipoplasia direita; aumento do diâmetro cerebral, com alargamento significativo do sistema ventricular supratentorial, em especial em nível dos ventrículos laterais e dos espaço subdural, nas regiöes frontotemporais, bilateral; malformaçäo da coluna lombossacra, com agenesia e presença de hemipélvis direita; testículo direito localizado no canal inguinal e testículo esquerdo intra-abdominal. Discutem-se o tipo de cariótipo da criança, os exames pré-operatorios a que foi submetida e o tratamento cirúrgico. Os autores relatam os aspectos embriológicos da malformaçäo, bem como fazem um levantamento da literatura existente, revelando um número pequeno de casos de síndrome de regressäo caudal em seu grau extremo, que é a sirenomelia (síndrome da sereia).

The Effects of Retinoic Acid on Gastrulation of Hatched Mouse Embroyos / 대한산부인과학회잡지

Retinoic acid(RA), formed in vivo by oxidation of retinol, is known as morphogenic signal. RA plays an active role in normal embryonic development at physiological concentration, but excess RA can be a powerful teratogen in human and animals. The present study was designed to examine the direct effect of RA on murine embryogenesis(gastrulation) and to define the specific development processes perturbed by RA. Five to fifteen blastocysts were randomly assigned to separate culture dishes of the experimental group. Various concentrations of RA(10(-9) M, 10(-7) M, and 10(-5) M) were used in culturing blastocysts. In the effect of RA on the normal grouwth of embryo, the rates of development to the stages of attachment, early egg cylinder(EEC), late egg cylinder(LEC), and early somite(ES)were significantly(p < 0.01) decreased as the RA concentration increased. Stil in the yolk sac formation rate, there was a significant, dose-dependent difference(p < 0.01) according to the RA concentration. In the degeneration of embryos by RA, the effect was more apparent as the concentration of Ra increased. The production rates of embryos devoid of egg cylinder region and embryos with abnormal egg cylinder region were increased (p < 0.01)in a dose-dependent manner according to RA concentration. In conclusion, RA probably act as teratogen at gastrula stage embryos in high concentration and effect of teratogenesis is dose-dependent.

Alteration of morphogenetic cell interactions by 1-beta-D-arabinofuranosylcytosine in Bufo arenarum embryo

Amphibian gastrulation was used as a model system to study the action of the nucleoside 1-beta-D-arabinofuranosylcytosine (Ara-C) on the early events of amphibian morphogenesis. Ara-C inhibits both glycoprotein and glycolipid synthesis and interferes with DNA synthesis. Thus, it is useful to investigate the importance of the cell surface and the nucleous during Bufo arenarum morphogenesis. Living embryos were incubated with Ara-C at blastula and gastrula stages. Treated-embryos undergo abnormal gastrulation, most of the embryos exogastrulate, although some do not gastrulate at all. This antimetabolite did not interfere with neural induction, as partial exogastrulae developed a small neural tube. We have proven that Area-C disturbs the typical intercellular organization and inhibits the radial intercalation of the blastocoelic roof. The mesodermal migration is the most affected morphogenetic process. The results described in this paper demonstrate that the timing of gastrulation movements strongly involves the participation of surface and extracellular molecules in cell recognition and cell interaction but does not involve a significant increase in cell division rate and can also occur in the absence of the cell division.

Differential Expression of Transforming Growth Factor-α and Epidermal Growth Factor in Developing Mouse Embryo / 체질인류학회지

The distribution of transforming growth factor-α (TGF-α) and epidermal growth factor (EGF) in developing mouse embryos of gestational age 8 to 15 days was immunohistochemically (ABC method) studied to investigate the differential expression of these growth factors. Paraffin embedded sections were immunostained with antibodies for TGF-α and EGF. Staining of TGF-α was observed in several organs derived from endoderm, mesoderm and ectoderm in 9-day-old mouse embryos, such as in the heart, optic pit, head mesenchyme, neural tube and primitive gut, and the staining became more intense in 10 to 15-day-old mouse embryos. The staining of EGF was seen in the heart and primitive gut derived from mesoderm and ectoderm respectively, in 9-day-old mouse embryos, but it was observed in other organs as well in 10 to 15-day-old embryos although the intensity was weaker. In the development of heart, immunoreactivity for TGF-α was more intense than EGF, which suggests more active involvement of TGF-α. In the lung, TGF-α staining was observed both in the bronchus and lung bud, whereas EGF staining was seen only the bronchus. In the nervous system, TGF-α was expressed more extensively and more intensively than EGF. In the developing skeletal system, TGF-α staining was stronger and the expression was observed at earlier stage compared with EGF. These results indicate that the activity of TGF-α is more potent than EGF in the development of mouse embryo in general, especially, in the development of mouse heart, nervous system, mesenchyme and skeletal system.

Multiple neural inductions in area opaca by grafts of the Hensen's node do not retard host chick embryo development.

Four hundred and forty four full primitive streak stage chick embryos were cultured in vitro and 261 were transplanted with 1, 3 or 5 Hensen's nodes in the area opaca. Irrespective of the number of grafts, neural induction was observed in 90% cases. The development of control and grafted embryos and the size of blastoderm area were monitored at the time of grafting and after 20 hr. We find that the induced neural tissue and differentiated tissue of graft-origin neither fuse with the host embryonic axis, nor retard its development.

Analysis of the amphibian gastrula with the ammoniacal silver reaction

Se analizó la composición celular de la gástrula del sapo Bufo arenarum mediante microscopía de luz utilizando la reacción de plata amoniacal. Al comienzo de la gastrulación, las células del epiblasto, de la zona marginal y del hemisferio vegetativo mostraron distintas formas de coloración. La capa epiblástica presentó una tinción oscura debido a la presencia de depósitos de plata relativamente densos en el área cubierta por gránulos de pigmento. En el hemisferio vegetativo la coloración fue entre amarillenta clara y parda. Las células localizadas en la zona marginal se caracterizaron por la presencia de depóstios argénticos difusamente distribuídos en el citoplasma y por sus límites intensamente teñidos. En algunos ejemplares se detectaron también diferenciais tintoriales entre los sectores dorsal y ventral de esta zona. Una particularidad notable de algunas células vitelinas aisladas, probablemetne las células germinales primordiales, fue la acumulación de depósitos densos de plata alrededdor del núcleo. Diferencias en la colcoración de las polaquetas citelinas nos permitieron reconocer áreas bien delimitadas en la región más periférica de la mitad subecuatorial del hueve. En las zonas mencionadas se detectó también una diferente afinidad de los núcleos para la reacción de plata amoniacal. En la gástrula mas avanzada, en el estadio 12, la mayoría de los territorios celulares identificados en la gástrula inicial fueron encontados en las áreas previstas como resultado de los movimientos morfogenéticos ocurridos durante este período del desarrollo

Mechanisms underlying the organizer formation in Bufo arenarum embryos

El mesodermo prospectivo en la gástula temprana de Bufo arenarum fue previamente identificado ocmo una banda marginal de células grises. Para analizar su capacidad de diferenciación, explantos de estas células, fueron cultivados en el interior de vesículas ectodermales, en aislamiento y en combinación con componentes vegetativos. Cuando se cultivaron en aislamiento, fragmentos dorsales y ventrales de la zona marginal profunda se comportaron diferentemente. Mientras que los explantos ventrales produjeron células sanguíneas, los explantos dorsales fallaron en diferenciar permenecidendo como masas de células ricas en vitelo. Por otra parte, ambos cultivos fueron drásticamente modificados cuando se asociaron con células superficiales de la zona blastoporal, las que causaron los siguiente s efectos. a) Promoción de diferenciación en exploantos marginales dorsales, capaces ahora de generar estructuras notocordales y somáticas en adición a células mesenquimáticas. b) Promoción de dorsalización en explantos marginales ventrales, los cuales cambiaron su destino esperando desarrollando componentes axiales, similares a aquellos producidos por explantos dorso marginales "activados". Por el contrario, cultivos combinados de piezas animales y vegetativas fueron incapaces de generar estructuras mesodermales. Estos estudios sugieren que el mesodermo axial, identificado como el "organizador", se desarrolla a partir de un sustrato marginal de células mesodérmicas genuinas, mediante un estímulo inductivo dorsalizante originado en células periblastoporales superficiais

Analysis of the mechanisms involved in neural induction

La determinación del área neural presuntiva en el embrión de vertebrado tiene lugar durante la gastrullación bajo la influencia inductora del cordomesoblasto en el curso de su invaginación. A pesar de los extensivos estudios destinados a esclarecer este complejo proceso, el problema no ha llegado a ser resuelto. En el presente artículo se consiera principalmente la competencia de la célula efectora y la naturaleza de la señal mediadora en este proceso de interación celular. Ambos puntos son discutidos en base a datos experimentales obtenidos principalmente de embriones de anfibios