Faculdade de Zootecnia, Campus Experimental de Dracena, Sa˜ o Paulo State University, Rod. Cmte. Joa˜ o Ribeiro de Barros, SP 294, Km 651, 17900-000, Dracena, Sao Paulo State, Brazil Initial Development of Bovine Placentation (Bos indicus) from the Point of View of the Allantois and Amnion A. C. Assis Neto*, E. C. C. Santos, F. T. V. Pereira and M. A. Miglino Address of authors: Faculdade de Zootecnia, Campus Experimental de Dracena, Sa˜o Paulo State University, Rod. Cmte. Joa˜o Ribeiro de Barros, SP 294, Km 651, 17900-000, Dracena, Sao Paulo, Brazil; *Corresponding author: Tel.: 55 18 3822 1861; e-mail: antonioassis@dracena.unesp.br With 2 figures and 1 table Received August 2008; accepted for publication May 2009 Summary The aim of this study was to perform a morphological charac- terization of the initial bovine placental development, between 20 and 70 days post-insemination (p.i.), with emphasis on the differentiation of the allantois and amnion. After collection, the conceptuses were dissected, macroscopically measured and photographically documented. The extraembryonic membranes were cut into fragments measuring 5 cm 2 , and then fixed in 4% paraformaldehyde for analysis using light microscopy, and in 2.5% glutaraldehyde for use in scanning and transmission electron microscopy. The extraembryonic and fetal membranes presented variable degrees of development throughout the periods analysed. The macroscopic appearance of vasculariza- tion of the allantois, its attempt to merge with the chorium and the effective appearance of the first cotyledons in development were the events observed from 30 to 40 days of pregnancy. The measurements of the amnion increased gradually as gestation developed. The allantoic epithelia presented cellular dimor- phism from 20 to 25 days of pregnancy, but was shown to be immature from 60 to 70 days of pregnancy. Introduction The bovine herd is the largest group of ruminants extensively exploited in Brazil. Emphasis is laid on the importance of embryonic and fetal attachments for implantation and survival of embryos and fetuses, as a considerable percentage of gestations at the initial stage do not reach the end, because of complications in placental development. The phylogeny of the allantois in mammals has been well documented; however, not much is known of the bovine allantoic development after artificial insemination (Melton et al., 1951; Latshaw, 1987; Wooding and Flint, 1994). The allantoic epithelium is clearly involved in the transplacental exchange process (Leiser, 1975). Nevertheless, there is very little information available about its growth and development in the bovine species. The development of embryonic membranes, with special reference to tissue morphology, has recently been the object of research in dogs and cats to establishing procedures for cultivating amniotic cells and extracting haemangioblasts as the origin of stem cells coming from the placenta (Miglino et al., 2006). Remarkable differences have, however, been observed in other species at ultrastructural level, mainly using scanning electron microscopy. In swine and cats, the allantois exhibits an impressive specificity, whereas the amniotic epithelium presents a characteristic similar to that of other mammals (Tiedemann, 1979a,b). In bovines the morphological charac- teristics of allantois were defined by scanning electron micro- scopy in fetal stages of gestation at 38 cm of crown-rump- length (Tiedemann, 1982); however, the maturation of allantoic epithelium was not treated clearly. Some authors reported problems with regard to the development of embryonic and fetal membranes in animals fertilized in vitro and cloned, such as for example, underde- velopment and abnormalities of the allantois (Thompson and Peterson, 2000; De Sousa et al., 2001), chorium (Stice et al., 1996; Hill et al., 2000; Wrenzycki et al., 2001; Bertolini and Anderson, 2002; Bertolini et al., 2002; Edwards et al., 2003; Arnold et al., 2008), amnion (Miglino et al., 2007; Kohan- Ghadr et al., 2008) and yolk sac (De Sousa et al., 2001) during embryonic and fetal development. Mechanisms that result in embryonic losses in the first 6 months of gestation and the possible failures in the development of bovine fetal membranes are not clearly understood. Therefore, the intention of this study is to contribute to future studies the morphological data related to the initial development of gestation, both in gestations by natural inseminations, as well as in those that followed specialized reproduction techniques such as in vitro fertilization and animal cloning. Materials and Methods Procedures for collection and preparation of material for study Uteri from pregnant bovines of Nelore breed (Bos indicus) were collected in Cold Storage Depots and Abattoirs in the Region of Dracena – SP, Brazil. For analyses, the uteri were opened at the Cold Storage Depot, and the membranes were fixed in the following solutions: 2.5% glutaraldehyde, 4% paraformaldehyde and 10% buffered formol. All the fixed materials were placed in appropriate boxes and then trans- ported to the laboratory. The uteri were collected and the uterine horns opened as well as dissected at their dorso-antimesometrial line in order to analyse the embryonal/foetal membranes. For the small Ó 2009 Blackwell Verlag GmbH Anat. Histol. Embryol. 38, 341–347 (2009) doi: 10.1111/j.1439-0264.2009.00949.x ISSN 0340–2096