In the viviparous lizard Trachylepis ivensi (Scincidae) of central Africa, reproducing females ovulate tiny ∼1 mm eggs and supply the nutrients for development by placental means. Histological study shows that this species has evolved an extraordinary placental pattern long thought to be confined to mammals, in which fetal tissues invade the uterine lining to contact maternal blood vessels. The vestigial shell membrane disappears very early in development, allowing the egg to absorb uterine secretions. The yolk is enveloped precocially by the trilaminar yolk sac and no isolated yolk mass or yolk cleft develops. Early placentas are formed from the chorion and choriovitelline membranes during the neurula through pharyngula stages. During implantation, cells of the chorionic ectoderm penetrate between uterine epithelial cells. The penetrating tissue undergoes hypertrophy and hyperplasia, giving rise to sheets of epithelial tissue that invade beneath the uterine epithelium, stripping it away. As a result, fetal epithelium entirely replaces the uterine epithelium, and lies in direct contact with maternal capillaries and connective tissue. Placentation is endotheliochorial and fundamentally different from that of all other viviparous reptiles known. Further, the pattern of fetal membrane development (with successive loss and re-establishment of an extensive choriovitelline membrane) is unique among vertebrates. T. ivensi represents a new extreme in placental specializations of reptiles, and is the most striking case of convergence on the developmental features of viviparous mammals known.
Or in plain English:
In central Africa, an unassuming little lizard has evolved a spectacular and oddly human feature of gestation: a complex placenta. It is the first time that scientists have observed such an advanced version of this organ connecting the fetus to the womb in nonmammals.
Biologist Alexander Flemming made the anatomical find, announced late last year, while sorting through specimens at the Port Elizabeth natural history museum in South Africa. Flemming and his collaborator, Daniel Blackburn, knew that about 20 percent of lizards give birth to live young, but finding the placenta came as a shock.
Whereas virtually all cold-blooded reptiles supply embryos with nutrients from a large egg yolk, five-inch-long Trachylepis ivensi females ovulate small, yolk-poor eggs that implant in the uterus. As the fetus develops, its tissues become intimately entangled with the blood vessels of its mother, providing ready access to nutrients and oxygen in the mother’s blood. Sound familiar? “The fetal tissues actually invade the uterine ones, much like in humans,” Blackburn says. “It’s totally unexpected.”
A lizard with a human-like reproduction system? It is yet another example in the long list of evolutionary expectations gone wrong. We must believe that random mutations just happened to replicate a complex, mammalian-like placental pattern in a lizard, though it had a perfectly good reproductive system to begin with.
Nothing in biology makes sense in the light of evolution.