Rhesus macaques (Macaca mulatta) are Old World monkeys and primates, sharing many complex physiological processes with humans. Regarding the placenta, the answer is unequivocally yes; the similarities are so profound that the Rhesus macaque is considered one of the most informative models for human pregnancy. The placenta is a temporary organ that forms during gestation, acting as the interface between the mother and the developing fetus. This shared biology makes the species valuable in biomedical research, particularly for understanding common complications of human pregnancy.
Defining the Mammalian Placenta
The placenta is a transient organ that develops within the uterus shortly after implantation of the fertilized egg. It sustains the growing fetus throughout gestation until birth. This composite structure is formed from both maternal uterine tissue and fetal membranes.
The primary function of the placenta is to facilitate the exchange of gases, nutrients, and waste products. It transports oxygen and essential substances like glucose and amino acids from the maternal circulation to the fetal circulation. Simultaneously, the placenta removes metabolic waste products, such as carbon dioxide and urea, passing them back into the mother’s bloodstream for excretion.
The placenta also functions as a powerful endocrine gland. It synthesizes and secretes hormones necessary to maintain the pregnancy. These hormones regulate maternal metabolism, prepare the mother’s body for birth and lactation, and influence fetal growth and development.
The placenta acts as a selective barrier, regulating what passes between the mother and the fetus. It allows for the transfer of maternal antibodies, providing passive immunity to the fetus. It works to prevent the direct mixing of maternal and fetal blood. This separation is important for protecting the fetus from the mother’s immune system.
The Rhesus Placenta: Structure and Type
The Rhesus macaque placenta is structurally very similar to the human placenta, sharing the hemochorial classification. This type describes the intimate relationship between maternal and fetal tissues at the organ’s interface. In a hemochorial placenta, the fetal tissue—specifically the outermost layer of cells called the trophoblast—is in direct contact with the mother’s blood.
This direct contact occurs because the fetal tissue invades the uterine wall, breaking down maternal blood vessels to create pools of blood called intervillous spaces. Fetal blood vessels, housed within tiny projections known as villi, float within these maternal blood pools. They are separated only by a thin layer of fetal cells, allowing for highly efficient nutrient, gas, and waste transfer.
The Rhesus placenta is typically characterized as bidiscoidal, forming with two distinct, disc-shaped regions of contact on the uterine wall. In approximately 80% of Rhesus pregnancies, the placenta appears as two separate discs, though some individuals may present with a single disc. While the human placenta is generally a single discoid structure, both species share the overall discoidal shape common among higher primates.
Despite the high degree of similarity, subtle differences exist, such as the depth of trophoblast invasion into the uterine lining. The human placenta exhibits a slightly deeper and more extensive invasion of the uterine wall compared to the Rhesus macaque. However, the fundamental villous architecture and the cell layers separating the two blood supplies remain analogous, which is the basis for their utility in research.
Why Rhesus Monkeys are Key Models in Placental Research
The close structural and functional resemblance between the Rhesus macaque and human placentas makes this species an indispensable model for translational research. They are the preferred non-human primate for studying pregnancy-related conditions unique to humans and other higher primates. The analogous mechanisms of placental transfer allow scientists to investigate how substances move across the maternal-fetal barrier, which is crucial for drug safety testing.
Researchers use the Rhesus model to study the pathology of pregnancy complications like preeclampsia, a disorder characterized by high blood pressure and organ damage. The Rhesus model helps to explore the underlying molecular and cellular events that contribute to the disease. This research provides insights into new ways to manage or prevent this life-threatening condition.
The Rhesus placenta model is also widely used to understand the vertical transmission of infectious diseases from mother to fetus. Because the placental barrier functions similarly to the human barrier, it is an ideal system for tracking how viruses like Zika and Cytomegalovirus (CMV) cross into the fetal circulation. This work helps in the development of vaccines and therapeutic strategies aimed at protecting the unborn child from congenital infections.
The model is also employed to study how environmental toxins or therapeutic drugs might affect fetal development. By observing the pharmacodynamics and permeability of the Rhesus placenta, scientists can predict the potential risks of medication use during human pregnancy. The insights gained from these studies directly inform clinical guidelines and improve outcomes for both mother and child.