Germ layer differentiation during early hindgut and cloaca formation in rabbit and pig embryos

Abstract

Relative to recent advances in understanding molecular requirements for endoderm differentiation, the dynamics of germ layer morphology and the topographical distribution of molecular factors involved in endoderm formation at the caudal pole of the embryonic disc are still poorly defined. To discover common principles of mammalian germ layer development, pig and rabbit embryos at late gastrulation and early neurulation stages were analysed as species with a human-like embryonic disc morphology, using correlative light and electron microscopy. Close intercellular contact but no direct structural evidence of endoderm formation such as mesenchymal-epithelial transition between posterior primitive streak mesoderm and the emerging posterior endoderm were found. However, a two-step process closely related to posterior germ layer differentiation emerged for the formation of the cloacal membrane: (i) a continuous mesoderm layer and numerous patches of electron-dense flocculent extracellular matrix mark the prospective region of cloacal membrane formation; and (ii) mesoderm cells and all extracellular matrix including the basement membrane are lost locally and close intercellular contact between the endoderm and ectoderm is established. The latter process involves single cells at first and then gradually spreads to form a longitudinally oriented seam-like cloacal membrane. These gradual changes were found from gastrulation to early somite stages in the pig, whereas they were found from early somite to mid-somite stages in the rabbit; in both species cloacal membrane formation is complete prior to secondary neurulation. The results highlight the structural requirements for endoderm formation during development of the hindgut and suggest new mechanisms for the pathogenesis of common urogenital and anorectal malformations.

On the origin of the human germline

“…However, a recent study of cynomolgus monkey embryos has indicated that nascent PGCs can emerge from the early amnion, which is an extra-embryonic membrane structure that subsequently surrounds the fetus (Sasaki et al, 2016). In primates, including humans, the amnion is formed from the pluripotent epiblast soon after implantation, albeit at a different time; this is not the case in pig and other mammalian embryos, in which the amnion develops after the initiation of gastrulation (Hassoun et al, 2010). In monkey embryos, WNT is expressed in the cytotrophoblast layer relatively close to the amnion, whereas the amnion itself is a source of BMP, indicating that there is an appropriate environment in this tissue for PGC specification (Sasaki et al, 2016).…”

A correlative study of the allantois in pig and rabbit highlighting the diversity of extraembryonic tissues in four mammalian species, including mouse and man

“…Staging was carried out on dark‐field photographs after 2–3 hr of primary fixation. Early‐stage denominations of rabbit and pig embryos (Table ; Hassoun, Schwartz, Rath, Viebahn, & Männer, ) with their main developmental features (s. Blum et al, ; Viebahn, ) were used as a basis for scheduling allantois development in both species. Stage numbers were supplemented with + or ‐ to indicate advanced (i.e., Stage 8+) or early (i.e., Stage 9‐) substages.…”

Epithelial–mesenchymal transformation and apoptosis in rat urethra development

“…Presence of the cloacal membrane at the bottom of the urogenital sinus has been shown in rabbits, pigs, and human embryos (29)(30)(31)(32)(33)(34). Kluth et al (35) also documented the presence of cloacal membrane at the ventral surface of the urethra in rats under SEM; however, they did not observe the fusion of urethral folds.…”