Pig Embryo vs. Human Embryo: Similarities & Differences

An embryo is the earliest stage of development for a multicellular organism. In mammals like pigs and humans, the journey from a single cell to a fully formed individual is a complex process. While both species share a common mammalian heritage, their embryonic paths exhibit similarities and differences with implications for scientific understanding and medical advancements. Studying these embryos provides insights into development, genetics, and potential therapeutic applications.

Shared Beginnings: Early Embryonic Similarities

The embryonic journeys of pigs and humans begin on a similar path due to their shared mammalian ancestry. Both start as a zygote, a single cell formed at fertilization that undergoes cell divisions known as cleavage. This process leads to a solid ball of cells called a morula, which then becomes a blastocyst as a fluid-filled cavity forms. The blastocyst consists of an inner cell mass that will form the embryo and an outer layer called the trophoblast, which contributes to the placenta.

Next is gastrulation, where the inner cell mass organizes into three primary germ layers: ectoderm, mesoderm, and endoderm. The ectoderm gives rise to the nervous system and skin, the mesoderm forms muscles and the circulatory system, and the endoderm develops into the lining of the digestive and respiratory tracts. Research shows that genes involved in forming precursor egg and sperm cells are expressed similarly in both species during these early phases.

Pigs are considered a good model for studying human development partly because their embryos share a similar structure. Both human and pig embryos develop as flat discs, unlike the cylindrical shape of mouse embryos. This shared “flat-disc” structure means their cellular organization more closely mirrors human development, providing a window into events that are difficult to observe directly.

Divergence in Development: Timelines and Structures

While the initial blueprint is shared, the developmental pathways soon diverge, most notably in their timelines. A pig’s gestation period is approximately 114 days, a contrast to the 280 days for a human. This accelerated timeline means a pig embryo develops at a much faster rate, reaching structural milestones more quickly.

This difference in speed is evident during organogenesis, the formation of organs. The pig pancreas begins to form around embryonic day 18 (E18), with developmental markers appearing that are absent at a comparable stage in humans. Studies of pig embryos have identified different time windows for optimal organ growth. For instance, maximal liver growth potential is observed at E28, while the pancreas continues developing its potential toward E42.

As development progresses, distinct morphological features emerge. The pig embryo forms a snout, a tail, and specific digit characteristics. In contrast, the human embryo develops its own defining facial features, limb proportions, and unique hands and feet. These differences extend to internal structures; the pig uterus is bicornuate to accommodate litters, unlike the simplex human uterus, and the pig liver has five lobes compared to the human’s four.

The placenta, the embryo’s support system, also shows species-specific differences in its structure and interface with the maternal uterus. These anatomical and temporal divergences are controlled programs that ensure the embryo develops into the correct species. Studying these differences helps clarify the principles governing mammalian development.

Genetic Distinctions and Species Identity

Developmental differences between pig and human embryos are rooted in their genetic blueprints. The primary distinction is the chromosome number; humans have 46 chromosomes (23 pairs), while pigs have 38 (19 pairs). These chromosomes house the genes, the DNA sequences that provide instructions for building and operating the organism.

Beyond chromosome number, specific genes and their expression patterns dictate developmental pathways. While many developmental genes are conserved among mammals, variations in their sequence and regulation drive species-specific traits. For instance, different expression patterns in genes controlling craniofacial development lead to a pig’s snout versus a human’s nose and jaw. Similarly, genes in the HOX family are regulated differently to pattern the body axis and produce distinct body plans.

Genome comparisons show that while pigs and humans share many genes, there are differences in how they have evolved and are used. For example, pigs possess more active genes related to smell recognition, reflecting their developed sense of smell. This genetic program is the determinant of identity, ensuring a pig embryo develops into a pig and a human embryo into a human.

This genetic divergence also orchestrates the different rates of development. The faster maturation of pigs is the result of a genetic program that accelerates cell division, differentiation, and organ formation compared to the human timeline.

Relevance in Scientific Research and Ethical Discussions

The comparison between pig and human embryos extends into scientific research and ethical debates. Due to their anatomical and physiological similarities to humans, pigs serve as animal models for studying human health. Their comparable organ systems are valuable for research into conditions like diabetes and for testing new drugs and medical devices.

This utility is prominent in xenotransplantation—the transplantation of organs from one species to another. Facing a global shortage of human organs, researchers are investigating genetically engineered pig organs as a solution. Scientists use gene-editing technologies like CRISPR-Cas9 to modify pig embryos, removing pig genes that could cause immune rejection in humans and adding human genes to improve compatibility. In March 2024, a patient received the first successful transplant of a genetically edited pig kidney.

Another research frontier is creating interspecies chimeras by introducing human stem cells into early-stage pig embryos. A primary goal is to grow human organs, like kidneys or livers, inside a pig host for transplantation. This work could also provide insights into early human development and disease progression.

These endeavors are accompanied by ethical considerations. Animal welfare is a primary concern in genetic modification and chimera research. Creating human-animal chimeras raises questions about the moral status of such beings, especially if human cells contribute to the animal’s brain. Societal discussions focus on crossing species boundaries and the potential for unforeseen consequences, like transmitting animal viruses to humans.