For the first time in the world, scientists contribute to carrying engineered embryos in female monkeys!

Scientists have developed embryo-like models using monkey stem cells for the first time that, when transplanted into surrogate monkey 'mothers', appear to progress through the early stages of pregnancy.

Human versions of these stem cell models have already been created, but until recently the 14-day limit applied to scientists cultivating them only for research purposes. Moreover, transplanting them into a human surrogate is currently out of the question, making it difficult to use the models to study embryonic development under real-world conditions.

In search of an alternative approach, a team of Chinese researchers turned to the crab-eating macaque (Macaca fascicularis).

The scientists exposed the embryonic stem cells of macaques in a cell culture to carefully select for growth factors. The cells proceeded to form embryo-like structures known as blastoids (a term that distinguishes these stem cell-derived structures from naturally occurring blastocysts).

A quarter of the trial's stem cells advanced into blastoids, a success rate equal to that of previous experiments using human embryonic stem cells. It was then cultured in the laboratory until it began to form three distinct layers in a process known as gastrulation, a key moment in embryonic development that sets the basic blueprint for the body. Many of these blastomas present to form a yolk sac and an amniotic fluid (a fluid-filled membrane covering the developing embryo). Under a microscope, the scientists observed that blastoids have similar features to seventh-stage embryos (in humans, this stage occurs at about 18 to 21 days after fertilization).

This monkey embryo model tends to collapse at about 18 days in which it develops outside the body, although in future experiments the researchers plan to explore other systems that successfully culture blaster mice.

The scientists chose blastoids that had been cultured for seven days, and which had an inner and outer cell mass and a visible cavity, because these are key features of normal embryos. Each surrogate monkey was 'impregnated' with 8 to 10 blastoids and monitored using ultrasound and hormone testing over the next 20 days.

The embryo-like structures were successfully implanted in three of the surrogate mothers, forming early gestational sacs (the membrane filled with amniotic and amniotic fluid) that persisted a week or more after implantation. The surrogate mothers also had increased levels of progesterone and chorionic gonadotropin, hormones associated with pregnancy.

All of the embryonic structures completely disappeared after about a week, but the fact that they implanted these hormones and triggered their release indicates that the monkeys were in fact pregnant for a brief moment.

Conducting similar experiments using human cells and their substitutes could tell us a great deal about our biology and their potential to cause disease and disorder, but at an ethical cost it would be comforting to pay so little. And the use of macaques comes with its own controversy: primate experiments like this one are banned in many places around the world.

The paper's co-author, neuroscientist Qian San, is known as the director of the non-human primate facility at the Chinese Academy of Sciences (CAS), where this experiment took place.

In his career, he created a pair of cloned macaques, Zhong Zhong and Hua Hua, and discovered a short path to sexual maturity by grafting testicular tissue from young macaques onto mice.

San believes that generating a large number of transgenic monkey models quickly and efficiently "will promote the development of relevant animal models of human disorders and deepen our mechanistic understanding in the life sciences."

Others say such experiments cannot be justified when animal studies cannot reliably predict human outcomes and rarely translate into human treatments in the real world.

This research paper has been published in Cell Stem Cell.