Embryonic development of Girardia tigrina (Girard, 1850) (Platyhelminthes, Tricladida, Paludicola)

Abstract

The embryonic development of freshwater triclads is mainly known from studies of species of Dendrocoelum, Planaria, Polycelis, and, more recently, Schmidtea. The present study characterizes the development of Girardia tigrina (Girard, 1850) by means of optical microcopy using glycol methacrylate semi-thin sections. 94 cocoons were collected in the period from laying to hatching, with intervals of up to twenty-four hours. The sequence of morphological changes occurring in the embryo permitted the identification of nine embryonic stages. At the time of cocoon laying, numerous embryos were dispersed among many yolk cells, with a rigid capsule covering the entire cocoon. In the first stage (approx. up to 6 hours after cocoon laying), yolk cells and embryonic cells showed random distribution. Stage II (between 12 and 24 hours after cocoon laying) is characterized by aggregates of blastomeres, which later aggregate forming an enteroblastula. Approximately 2 days after cocoon laying (stage III), formation of the embryonic epidermis and embryonic digestive system took place, the latter degenerating during the subsequent stage. Stage V (until the fourth day) is characterized by the formation of the definitive epidermis. Between 4 and 6 days after laying, organogenesis of the definitive inner organs starts (stage VI). Approximately 14 days after laying (stage IX), formation of the nervous system is completed. At this stage, the embryo shows similar characteristics to those of newly hatched juveniles. The hatching of Girardia tigrina occurs in the period between twelve to twenty-two days after cocoon laying.Keywords: developmental stages, morphology, optical microscopy, Dugesiidae, freshwater planarians. , 1850) (Platyhelminthes, Tricladida, Paludicola) Desenvolvimento embrionário de Girardia tigrina (Girard ResumoO desenvolvimento embrionário dos tricladidos é conhecido, fundamentalmente, por estudos realizados em espécies de Dendrocoelum, Planaria, Polycelis e, mais recentemente, Schmidtea. O presente estudo descreve o desenvolvimento embrionário de Girardia tigrina (Girard, 1850), a partir de análises realizadas em cortes histológicos seriados e semifinos de glicol-metacrilato, ao microscópio óptico. Noventa e quatro casulos foram coletados no período entre a postura e a eclosão, em intervalos de até vinte e quatro horas. A seqüência das modificações morfológicas no embrião permitiu a identificação de nove estágios embrionários. Na postura dos casulos, envoltos por uma cápsula rígida, observam-se numerosos embriões dispersos entre grande quantidade de células vitelinas. No estágio I (aproximadamente até 6 horas após a postura), as células vitelinas e as embrionárias mostram uma distribuição aleatória. O estágio II (entre 12 e 24 horas após a postura) caracteriza-se pela formação de agrupamentos de blastômeros, os quais posteriormente formam uma enteroblástula. Aproximadamente dois dias após a postura (estágio III), ocorre a formação da epiderme e do sistema digestivo embrionário, sendo que este último degenera ...

Embryonic origin of adult stem cells required for tissue homeostasis and regeneration

“…In contrast to the synchronous, oriented blastomere cleavage patterns of Spiralian embryos ( Lambert, 2010 ), blastomeres in freshwater planarian embryos undergo dispersed cleavage among yolk cells: they divide asynchronously and are not in direct contact with one another ( Bardeen, 1902 ; Cardona et al, 2005 ; Hallez, 1887 ; Ijima, 1884 ; Le Moigne, 1963 ; Metschnikoff, 1883 ; Vara et al, 2008 ). During sphere formation, some blastomeres differentiate into temporary embryonic cell types that provide form and function to the embryo, including the primitive ectoderm, temporary embryonic pharynx and primitive gut ( Cardona et al, 2005 ; Hallez, 1887 ; Le Moigne, 1963 ; Sánchez Alvarado, 2003 ).…”

“…During sphere formation, some blastomeres differentiate into temporary embryonic cell types that provide form and function to the embryo, including the primitive ectoderm, temporary embryonic pharynx and primitive gut ( Cardona et al, 2005 ; Hallez, 1887 ; Le Moigne, 1963 ; Sánchez Alvarado, 2003 ). Temporary embryonic tissues are not thought to contribute to the juvenile body plan; they are thought to degenerate as the definitive organs form and morphogenesis proceeds ( Cardona et al, 2005 ; Le Moigne, 1963 ; Vara et al, 2008 ). In contrast, undifferentiated blastomeres remaining after sphere formation are thought to give rise to all definitive tissues found in juvenile worms ( Hallez, 1887 ; Hyman, 1951 ; Le Moigne, 1963 ; Sánchez Alvarado, 2003 ; Stevens, 1904 ).…”

Physiological evaluation of the behavior and epidermis of freshwater planarians (Girardia tigrina and Girardia sp.) exposed to stressors

“…Girardia sp. is a species native to Brazil and showed to be more resistant to higher temperatures than Girardia tigrina , which may be a local climatic adaptation characteristic of this tropical Brazilian species, since Girardia tigrina is a globalized species (Vara et al, 2008). The thermal adaptive capacity of Girardia tigrina was described by Claussen and Walters (1982) and later by Rivera and Perich (1994).…”

“…Images of the epidermis of the two species showed a great amount of active excretory pores (Figs 8 and 9), better structured organs and less presence of overlapping tissue with disorganized structures in Girardia tigrina . As a cosmopolitan species, commonly found in freshwater sources ranging from pH 4.0 to 9.0 in Europe, North America, and South America, and more rarely in other localities (Vara et al, 2008), Girardia tigrina may have developed wide adaptability at different pH values (Rivera and Perich, 1994). …”

“…This pH change occurs naturally according to each species of planarians, according to its physiological needs (Vara et al, 2008; Rink, 2013), in relation to the pH values of freshwater sources in its environment, which range between 4.0 and 9.0 around the world (Rivera and Perich, 1994). …”