Human development begins when a sperm cell fertilizes an egg. From that single cell, a complex sequence of cell divisions, movements, and molecular signals produces the tissues and organs of the body. The process is highly coordinated, but it is not a simple matter of one cell gradually getting bigger. Cells repeatedly divide, change their behavior, communicate with neighboring cells, and specialize into different types.
In human biology, the term embryo generally refers to the developing human from fertilization through the end of the eighth week of development. After that point, the developing organism is called a fetus. Pregnancy is usually dated differently: clinicians count from the first day of the last menstrual period, so the pregnancy is described as about two weeks further along than the age of the embryo itself.
Fertilization creates the first cell
Fertilization usually occurs in a fallopian tube. A sperm penetrates the egg, and the genetic material from the two cells is combined. The resulting cell is called a zygote.
The zygote contains a complete set of chromosomes, with genetic material contributed by both parents. Its DNA provides instructions for making proteins and regulating when particular genes are active. But development depends on more than DNA alone. The egg also contains molecules that help control the earliest stages, and as development proceeds, cells receive signals from their surroundings that influence which genes they use.
The zygote begins dividing without first growing substantially larger. These early divisions produce progressively more cells within the same general volume. The cells are called blastomeres.
The embryo divides and forms a blastocyst
During the first several days, the developing embryo undergoes repeated cell divisions while traveling through the fallopian tube toward the uterus.
After several divisions, the cells become more tightly organized in a process called compaction. The embryo develops into a structure known as a blastocyst, which has a fluid-filled cavity and distinct groups of cells.
Two populations are especially important. The inner cell mass will give rise to the embryo itself, while the trophoblast, the outer layer of cells, contributes substantially to structures that support pregnancy, including much of the placenta.
This distinction illustrates an important principle of embryonic development: cells with the same original genome can acquire different identities because they receive different signals and activate different sets of genes.
Implantation connects the embryo to the uterus
Roughly a week after fertilization, the blastocyst begins attaching to the lining of the uterus, a process called implantation.
Cells of the trophoblast interact with and penetrate the uterine lining. Some trophoblast cells form structures that help establish an exchange system between maternal blood and the developing placenta. The placenta eventually serves as a major interface through which oxygen, nutrients, hormones, and waste products are exchanged between the pregnant person and the developing fetus, although maternal and fetal blood normally remain in separate circulatory systems.
Implantation also marks a major transition. The embryo is no longer developing largely on its own within the reproductive tract; it is becoming integrated with the tissues that will support pregnancy.
The embryo establishes its basic body plan
One of the most consequential periods of development occurs during the third week after fertilization, when the embryo undergoes gastrulation.
Gastrulation reorganizes a relatively simple sheet of cells into three fundamental layers:
- Ectoderm forms tissues including the nervous system and much of the skin.
- Mesoderm contributes to muscles, bones, connective tissues, blood and blood vessels, kidneys, and other structures.
- Endoderm contributes to the lining of the digestive and respiratory systems and to tissues of several associated organs.
These are called the three germ layers. They are not finished organs; rather, they are populations of cells with different developmental potentials that will undergo further specialization.
At about the same time, the embryo establishes important body axes: head-to-tail, back-to-front, and left-to-right. Molecular signals between cells help establish these patterns. This is why development is not simply a uniform expansion of tissue. Cells must acquire information about where they are and what structures should form around them.
The nervous system begins to take shape
After gastrulation, a region of ectoderm thickens to form the neural plate. The edges of this plate rise and fold toward each other, eventually forming the neural tube.
This process, called neurulation, establishes the early structure from which the brain and spinal cord develop. Specialized cells called neural crest cells also arise along the developing neural tube and migrate to other parts of the embryo. They contribute to a wide range of structures, including portions of the peripheral nervous system and several other tissues.
The neural tube does not immediately resemble a brain or spinal cord. Its cells continue dividing, specializing, and organizing into distinct regions. Development of the nervous system continues for many years after birth.
The heart and blood vessels develop early
The cardiovascular system is among the earliest organ systems to become functionally active.
Cells derived largely from mesoderm form early blood vessels and the primitive heart. The heart begins as a relatively simple tube that undergoes folding and remodeling as its chambers and associated structures develop.
The early heart begins beating during the embryonic period. Its structure is still immature, however, and continues to undergo extensive development throughout the remainder of pregnancy.
Blood-cell formation also changes locations during development. Early blood cells arise in embryonic and extraembryonic tissues before blood-forming activity becomes established in the developing liver and, later, primarily in the bone marrow.
Organs begin forming during organogenesis
During roughly the fourth through eighth weeks after fertilization, the embryo undergoes organogenesis, meaning the major organ systems begin taking shape.
The embryo’s body changes dramatically during this period. Tissue folds help transform a relatively flat arrangement of cells into a more three-dimensional body. Structures that will become the digestive tract form as parts of the embryo fold inward. The lungs, liver, pancreas, kidneys, and other organs begin through interactions between developing tissues.
The limbs also appear. Small limb buds emerge from the body wall and enlarge as cells proliferate and differentiate. Hands and feet initially have broad, paddle-like shapes. Later, programmed cell death removes tissue between developing digits, helping separate the fingers and toes.
The face develops through the growth and merging of several distinct embryonic structures. The eyes, ears, nose, mouth, and jaw all arise through precisely coordinated changes in position, growth, and tissue specialization.
At the same time, the brain enlarges rapidly and begins dividing into recognizable regions. The spinal cord and peripheral nervous system continue developing alongside it.
Development depends on cell communication
A central feature of embryology is cell signaling. Developing cells constantly respond to chemical signals from neighboring cells and from more distant tissues.
Some signaling molecules tell cells to multiply. Others influence where cells move, what type of cell they become, or whether they survive. Signals can also establish gradients across tissues, giving cells information about their position.
Genes called transcription factors help translate these signals into changes in gene activity. They can turn groups of genes on or off, causing a cell to follow a particular developmental pathway.
This system allows the embryo to construct complex structures from relatively simple starting materials. Development is therefore both genetic and environmental at the cellular level: DNA supplies the underlying instructions, while interactions among cells and tissues help determine how those instructions are used.
What happens by the end of the embryonic period?
By the end of the eighth week after fertilization, the embryo has developed recognizable versions of most of the major organ systems. The basic body plan is established, although the organs are far from fully mature.
The embryo has developed a distinct head and trunk, developing limbs with separated digits, a recognizable face, a functioning early cardiovascular system, and an increasingly complex nervous system. Many organs that will continue maturing throughout fetal development have already begun their initial formation.
This distinction between formation and maturation is important. An organ can be present without being fully developed or capable of performing all of its eventual functions.
How fetal development differs from embryonic development
At the beginning of the ninth week after fertilization, the developing human is generally described as a fetus rather than an embryo.
The fetal period is dominated more by growth and maturation than by the initial establishment of the body plan. Organs become more structurally complex and increasingly capable of functioning. The brain continues developing, bones become more developed, the lungs undergo important structural changes, and the digestive, urinary, cardiovascular, and other systems continue to mature.
Development does not end at birth. The nervous system, immune system, reproductive system, lungs, and many other organs continue changing after birth. Some aspects of human development extend through childhood and adolescence.
Why the timing of development matters
Embryonic development proceeds through overlapping processes rather than a sequence in which one organ system finishes before another begins. At any given time, many things are happening simultaneously: cells are dividing, tissues are moving, organs are forming, and signals are altering gene activity.
Because structures depend on earlier developmental events, disturbances during particular periods can have especially significant effects. Factors that can influence development include certain genetic changes, some infections, particular medications or other chemical exposures, nutritional deficiencies, and problems affecting the pregnancy environment. The effect depends heavily on the factor, its timing, its intensity, and the tissues involved.
At the same time, not every developmental difference results from an identifiable external cause. Embryonic development naturally involves substantial biological complexity, and genetic variation can influence how individuals develop.
The remarkable feature of human embryogenesis is therefore not simply that a single cell becomes a human body. It is that cells carrying essentially the same genome repeatedly acquire different identities, move into precise locations, communicate with one another, and assemble into tissues and organs whose development remains coordinated as the entire body changes.
