The heart is the first functional organ in a human embryo, beginning to beat around 21 to 23 days after fertilization. But before the heart starts pumping, the groundwork for the brain, spinal cord, and circulatory system is already underway. Organ development follows a remarkably tight schedule during the first several weeks of pregnancy, with most major organs at least beginning to form by the end of the eighth week.
How Organs Begin From Three Cell Layers
Before any organ can form, the embryo organizes itself into three foundational layers of cells during a process called gastrulation, which takes place around the third week after fertilization. Each layer gives rise to specific organs. The outer layer produces the skin, hair, nails, and the entire nervous system, including the brain. The middle layer becomes the heart, muscles, bones, kidneys, blood vessels, and blood cells. The inner layer develops into the lining of the digestive tract, the liver, pancreas, thyroid, and the inner portions of the lungs.
These layers don’t produce finished organs right away. Instead, small clusters of cells begin budding off or folding in specific patterns to create what are called organ primordia: the earliest recognizable precursors of each organ. Over the following weeks, those tiny buds grow, specialize, and eventually start to function.
The Heart: First to Beat
The cardiovascular system holds the distinction of being the first organ system to function in the human embryo. Around day 21 to 23 after fertilization (roughly the beginning of the sixth gestational week, counting from the last menstrual period), the embryonic heart begins its first contractions. At this point, the heart doesn’t look anything like the four-chambered organ you’d recognize. It’s a short, straight tube running along the front of the embryo, resembling a simple blood vessel more than a heart.
This tube-like heart quickly starts looping and folding to create the chambers that will eventually separate oxygen-rich and oxygen-poor blood. But even in its primitive form, it’s already pumping. The embryo needs a working circulatory system this early because it’s growing too large for nutrients and oxygen to simply diffuse through its tissues. Blood flow is essential for delivering what the rapidly dividing cells need to keep building every other organ.
The Brain and Spinal Cord Start Even Earlier
While the heart is the first organ to function, the nervous system actually begins forming a few days before the heart starts beating. Between days 17 and 21 after fertilization, a flat sheet of cells called the neural plate folds inward to create the neural tube, the structure that becomes the brain and spinal cord. By day 25, the front opening of the tube closes, and by day 28, the back end seals shut. Failure of the tube to close properly at either end is what causes conditions like spina bifida or anencephaly.
Once the neural tube is sealed, it rapidly begins to differentiate. The front end swells into three distinct bulges that will become the forebrain, midbrain, and hindbrain. These early brain vesicles are visible by the end of the fourth week. The spinal cord extends from the lower portion. Although the nervous system starts forming early, it takes far longer than the heart to become functional. Meaningful nerve signaling doesn’t begin for several more weeks.
Blood Cells Form Before the Heart Beats
Even before the heart tube begins pumping, the embryo is already producing its first blood cells. This happens in the yolk sac, a temporary structure attached to the embryo that serves as its earliest source of nutrition. Clusters of cells in the yolk sac called blood islands begin generating primitive red blood cells, white blood cells, and platelets.
The yolk sac also establishes the embryo’s first circulatory network. Blood flows back and forth between the yolk sac and the embryo through a set of early blood vessels, absorbing nutrients and oxygen that sustain growth until the placenta is fully established. This early blood-making system is temporary. Over the coming weeks, blood cell production migrates first to the liver and later, around the 14th week, to the bone marrow, where it remains for life.
Eyes and Ears Begin in Week Three
The earliest sign of eye development appears as early as week three after fertilization, when small grooves form on the surface of the developing forebrain. These optic grooves eventually push outward to form optic vesicles, the precursors to the retinas. The lens, iris, and other structures develop over the following weeks through a complex exchange of signals between the brain tissue and the surface cells of the embryo’s face.
The inner ear structures begin forming around the same period, arising from thickened patches of surface cells near the hindbrain. These patches eventually fold inward to create the fluid-filled chambers responsible for both hearing and balance. Like the nervous system, the sensory organs start forming early but take months to become functional.
Liver, Lungs, and Kidneys Emerge in Weeks Four and Five
The liver is one of the earliest internal organs to appear after the heart. Around the end of the fourth week (using mouse embryology timelines that closely parallel human development), a small pouch of tissue buds off from the wall of the primitive gut tube near the developing heart. This is the hepatic diverticulum, the liver’s first recognizable precursor. Within days, cells from this pouch invade the surrounding tissue to form the liver bud, which then grows rapidly. The liver quickly takes on a critical role beyond digestion: it becomes the embryo’s primary blood-making organ, a job it holds through much of fetal life before the bone marrow takes over.
The lungs first appear during the fourth week as a small outgrowth from the front wall of the foregut, the same tube of tissue that forms the esophagus and stomach. This respiratory diverticulum gradually separates from the esophagus as ridges of tissue pinch the two apart. The lung buds then begin branching repeatedly, a process that continues well into late pregnancy and even after birth. Functional gas exchange doesn’t happen until the final weeks before delivery, making the lungs one of the last organs to mature despite being one of the earlier ones to start forming.
The kidneys go through three distinct versions during embryonic development, progressing from head to tail along the embryo. The first version, the pronephros, appears in the fourth week but never functions in humans. The second version, the mesonephros, serves as a temporary kidney. The third and final version, the metanephros, begins forming during the fifth week and eventually becomes the permanent kidneys. This sequential replacement is one of the more unusual features of human development, essentially a draft system where each version is more sophisticated than the last.
The Timeline at a Glance
- Days 17 to 21: The neural plate folds to begin forming the brain and spinal cord
- Day 21 to 23: The heart tube begins beating
- Day 25: The front of the neural tube closes; the earliest sign of eye development is visible
- Day 28: The back of the neural tube closes; three primitive brain regions are distinguishable
- Week 4: The liver bud, lung bud, and first kidney system appear; blood cells are being produced in the yolk sac
- Week 5: The permanent kidneys begin forming
- Week 8: The embryo is reclassified as a fetus, and all major organ systems have at least begun development
By the end of the eighth week after fertilization, the embryo transitions to being called a fetus. At that point, every major organ system has been established in at least a rudimentary form. The remaining months of pregnancy are devoted to growth, maturation, and refinement, particularly for the brain, lungs, and immune system, which need the longest runway to reach full function.