Placenta

Lore & Background

The protein syncytin, found in the outer barrier of the placenta, has a certain RNA signature in its genome that has led to the hypothesis that it originated from an ancient retrovirus, essentially a virus that helped pave the transition from egg-laying to live-birth. The placenta, as a temporary organ, has acted as a testing ground for newly acquired genes or mutations, with limited consequences, because the mother is protected from harm once she sheds the placenta after birth.

Reader's Guide

The placenta's significance lies in its role as a rapidly evolving organ that has enabled the vast variation of reproductive strategies in mammals. Its temporary nature and lower levels of gene regulation allowed for the accumulation of mutations and integration of viral genes, such as syncytin, which became beneficial for placentation. This placenta-driven evolution hypothesis explains why mammalian placentas, though sharing the same functions, differ greatly in structure and function across species—for example, human, bovine, equine, and canine placentas are very different at both gross and microscopic levels. The placenta also differs in its ability to provide maternal immunoglobulins to the fetus. Understanding placental biology is crucial for comprehending pregnancy complications, as the fetoplacental circulation is vulnerable to hypoxia and reoxygenation, which can contribute to pre-eclampsia and other issues.

Did You Know?

• The word 'placenta' comes from the Latin word for a type of cake, referring to its round, flat appearance in humans.
• The placenta occasionally takes a form with multiple distinct parts connected by blood vessels, called bilobed, trilobed, or succenturiate placenta.
• The syncytiotrophoblast is a multinucleated continuous cell layer that forms as a result of differentiation and fusion of underlying cytotrophoblasts.

Evolutionary Origins and the Retroviral Legacy

The placenta's story stretches back roughly 150 to 166 million years, when the first mammalian versions of this organ emerged. What makes its origin particularly striking is the hypothesis surrounding syncytin, a protein embedded in the syncytiotrophoblast—the outer barrier separating mother from fetus. Syncytin carries an RNA signature in its genome that points to an ancient retroviral ancestor. In other words, a virus that once threatened to destroy its host may have been co-opted to build the very structure that made live birth possible in mammals. This viral legacy helped pave the transition from egg-laying to viviparity. Placentation did not arise only in mammals; it evolved independently multiple times, including in certain fish such as the genus Poeciliopsis and in some reptiles. Among mammals, the placenta is the defining trait of placental (Eutherian) species, though marsupials and some non-mammals also possess versions with varying degrees of development.

The Placenta as an Evolutionary Laboratory

Because the placenta is a temporary organ that the mother sheds after birth, it occupies a unique niche in evolutionary biology. New genes or mutations that would be strongly selected against in long-lived organs can accumulate here with relatively limited consequences. The mother is effectively protected once the placenta is expelled, so harmful phenotypes do not persist. Additionally, the placenta exhibits substantially lower levels of gene regulation than other tissues, permitting what researchers call "leaky" expression of viral genes that would normally be silenced by methylation. Once integrated into the genome and expressed in this permissive environment, these genes can gather further mutations until, by random chance, some acquire beneficial functions for placentation. This mechanism, known as the placenta-driven evolution hypothesis, explains why the placenta has evolved so rapidly and why mammalian reproductive strategies vary so enormously. All mammalian placentas share the same core functions, yet their structures differ dramatically—human, bovine, equine, and canine placentas are distinct at both gross and microscopic levels, and they differ in how they transfer maternal immunoglobulins to the developing fetus.

Architecture and Physical Form

In humans, the mature placenta is a disc-shaped organ averaging about 22 centimeters in length and 2 to 2.5 centimeters in thickness, with the center being the thickest and the edges the thinnest. It typically weighs around 500 grams and presents a dark reddish-blue or crimson hue. The umbilical cord, roughly 55 to 60 centimeters long, connects the fetus to the chorionic plate in an eccentric attachment and contains two arteries and one vein. Vessels branch across the placental surface, subdividing into a network covered by a thin cellular layer, ultimately forming villous tree structures. On the maternal side, these trees cluster into lobules known as cotyledons. Occasionally, the placenta takes a multi-lobed form—bilobed, trilobed, or even more complex—with a clearly discernible main lobe and auxiliary lobe termed a succenturiate placenta. In such cases, the connecting blood vessels can sometimes obstruct fetal presentation during labor, a condition called vasa previa. After birth, a thin layer of maternal decidual tissue is expelled along with the organ, sometimes mislabeled as the "maternal part" of the placenta.

Molecular Identity and Gene Expression

The placenta is not merely a passive exchange surface; it is a remarkably active endocrine and molecular organ. Of the roughly 20,000 protein-coding genes expressed in human cells, approximately 70 percent are active in the normal mature placenta. Around 350 of these are more specifically expressed there, and fewer than 100 are highly placenta-specific. The proteins they encode are predominantly found in trophoblast cells and serve pregnancy-related functions. Specific examples include PEG10 and the cancer-testis antigen PAGE4, both expressed in cytotrophoblasts; CSH1 and KISS1 in syncytiotrophoblasts; and PAPPA2 and PRG2 in extravillous trophoblasts. The syncytiotrophoblast itself is a multinucleated continuous cell layer formed by the differentiation and fusion of underlying cytotrophoblasts, a process that continues throughout development. This layered architecture establishes the critical barrier between maternal and fetal circulations while still permitting the exchange of nutrients, gases, and waste products. The word "placenta" itself traces back to Latin, ultimately from Greek plakóeis, meaning "flat, slab-like," a nod to the organ's round, disc-like appearance.