Table of Contents
Overview: From Fertilization to Birth
Embryonic development in humans describes how a single fertilized egg cell (zygote) becomes a newborn baby. This process is usually divided into:
- Preimplantation period (1st week)
- Embryonic period (2nd–8th week): formation of major organs and body plan
- Fetal period (9th week to birth): growth and maturation
For beginners, it is helpful to focus on the sequence of development, the main stages, and why early development is especially sensitive to disturbances.
First Week: Fertilization, Cleavage, and Implantation
Fertilization and Formation of the Zygote
After sexual reproduction (covered in other chapters), a sperm cell and an egg cell meet in the ampulla of the uterine (fallopian) tube.
Key events:
- Penetration of the egg envelopes by a single sperm
- Fusion of the genetic material from sperm and egg, resulting in a zygote with the human chromosome number
- Determination of genetic sex (XX or XY) at this moment (detailed genetics are treated elsewhere)
The zygote is totipotent: at this very early stage, each cell still has the potential to give rise to a complete organism and supporting tissues.
Cleavage: Rapid Cell Divisions Without Growth
As the zygote travels along the uterine tube toward the uterus, it undergoes cleavage divisions:
- 2-cell stage → 4-cell stage → 8-cell stage → morula (a compact cluster of cells resembling a mulberry)
- There is no overall increase in size; the original volume of the egg is simply subdivided into more and more smaller cells (blastomeres)
- Early blastomeres remain totipotent; later, potential becomes gradually more restricted
Blastocyst Formation
Around day 4–5 after fertilization, a blastocyst forms:
- Outer cell layer: trophoblast
- Will later contribute mainly to the placenta and supporting structures
- Inner cell mass (embryoblast):
- Will form the actual embryo and some extraembryonic structures
- A fluid-filled cavity (blastocoel/blastocyst cavity) appears inside
At this stage, the embryo reaches the uterus.
Implantation (Nidation)
Around day 6–7, the blastocyst begins to implant into the uterine lining (endometrium):
- The trophoblast cells attach to the endometrium and differentiate into specialized layers that invade and anchor the blastocyst
- The uterine lining responds by thickening and increasing blood supply at the implantation site
- The embryo is now embedded and begins to draw nutrients from maternal tissues
This step marks the transition from a “free-floating” early embryo to one that is connected to the mother and can continue to develop.
Formation of Germ Layers and Early Body Plan
Bilaminar Disc (Beginning of Embryonic Organization)
Shortly after implantation, the inner cell mass becomes organized into two layers:
- Epiblast (upper layer)
- Hypoblast (lower layer)
Together, they form a bilaminar embryonic disc. Around the disc, early cavities form:
- Amniotic cavity above the epiblast
- Yolk sac (early nutrient and blood cell–forming site) below the hypoblast
These cavities and sacs do not feed the embryo directly as in egg-laying animals, but they are important for protection, early blood formation, and organization of the body plan.
Gastrulation: Formation of the Three Germ Layers
Gastrulation (starting in week 3) is a key event: the bilaminar disc becomes trilaminar, consisting of three germ layers:
- Ectoderm (outer layer)
- Mesoderm (middle layer)
- Endoderm (inner layer)
Through cell movements and ingrowth at a structure called the primitive streak, epiblast cells migrate and form these three layers.
Very simplified roles (details are covered in other chapters):
- Ectoderm → nervous system, skin surface, parts of sense organs
- Mesoderm → muscles, bones, blood, heart, kidneys, reproductive organs
- Endoderm → lining of digestive and respiratory tracts, liver, pancreas
By the end of gastrulation, the basic body axes are set:
- Head–tail (cranial–caudal)
- Back–belly (dorsal–ventral)
- Left–right
A disturbance at this time can have wide-ranging consequences, because all later tissues and organs arise from these germ layers.
Neurulation: Beginning of the Nervous System
Following gastrulation, part of the ectoderm forms the neural plate, which folds to create:
- Neural groove → neural tube:
- The neural tube becomes the brain and spinal cord
- Neural crest cells:
- A special population of migratory cells that give rise to parts of the peripheral nervous system, pigment cells, and other structures
Proper closure of the neural tube is crucial. Inadequate closure leads to neural tube defects (e.g., spina bifida, anencephaly). This is one reason why folic acid supplementation is recommended in early pregnancy.
Somites and Segmentation
From the mesoderm alongside the neural tube, paired blocks of tissue form called somites. These segments will develop into:
- Parts of the vertebral column
- Skeletal muscles
- Portions of the dermis (deep layer of skin)
Somites give the early embryo a segmented appearance and are used to roughly date embryonic age.
Extraembryonic Structures and Placenta
Amnion and Amniotic Fluid
The amnion is a thin, tough membrane that surrounds the embryo, forming the amniotic cavity filled with amniotic fluid.
Functions of amniotic fluid:
- Mechanical protection against shocks
- Allows the embryo/fetus to move, which is important for muscular and skeletal development
- Prevents adhesions between the fetus and surrounding tissues
- Helps maintain a relatively constant temperature
Later in pregnancy, amniotic fluid also reflects some aspects of fetal metabolism and can be analyzed for medical reasons.
Yolk Sac and Allantois
In humans:
- The yolk sac is an early site of:
- Blood cell formation
- Origin of germ cells (cells that later give rise to eggs or sperm)
- The allantois is a small outpouching involved in:
- Early blood vessel development
- Contributing to structures in the umbilical cord and urinary system
Unlike in egg-laying animals, these structures are not major nutrient sources, but they are essential for early development.
Chorion and Placenta
The trophoblast and associated extraembryonic mesoderm form the chorion, which develops finger-like projections called chorionic villi into the uterine lining.
The placenta develops from:
- Fetal tissues: primarily chorionic villi
- Maternal tissues: modified uterine lining (decidua)
Key properties of the placenta:
- It creates a very close contact between maternal and fetal blood without direct mixing (under normal conditions)
- Exchange functions:
- Oxygen and nutrients pass from maternal blood to the fetus
- Carbon dioxide and waste products pass from the fetus to the maternal circulation
- Barrier and filter:
- Many pathogens and large molecules are blocked
- However, some viruses, alcohol, certain drugs, and environmental toxins can cross and harm the embryo/fetus
- Hormone production:
- Produces hormones that help maintain pregnancy and adapt the maternal body (e.g., supporting the uterine lining, adjusting metabolism)
The placenta remains essential for fetal life until birth, when it is expelled as the afterbirth.
Umbilical Cord
The umbilical cord connects the fetus to the placenta. It typically contains:
- Two umbilical arteries:
- Carry blood from the fetus to the placenta, low in oxygen
- One umbilical vein:
- Carries oxygen- and nutrient-rich blood from the placenta toward the fetus
The cord is embedded in a gelatinous connective tissue (Wharton’s jelly), which protects the blood vessels from compression.
Organogenesis: Formation of Major Organs
Embryonic Period (Weeks 4–8): Critical Phase
From about week 4–8, the embryo rapidly develops a recognizable human form:
- The body folds from a flat disc into a three-dimensional shape
- Major organ systems begin to form (organogenesis)
- Limb buds appear (future arms and legs)
- The heart begins to beat very early and starts circulating blood
- Basic outlines of eyes, ears, and other facial structures appear
- The digestive tract forms from the endodermal tube, with budding of future organs like liver and pancreas
During this stage:
- The basic blueprint for the body and organs is established
- The embryo is highly sensitive to harmful influences (teratogens) such as:
- Certain medications
- Alcohol
- Tobacco smoke
- Ionizing radiation
- Some infections
- Damage can lead to congenital malformations of organs and limbs
By the end of the 8th week:
- The embryo has a more human-like appearance, though still small
- All major organ systems are present in a rudimentary form
- The term usually shifts from embryo to fetus
Fetal Period: Growth and Maturation
From the 9th week until birth, the developing human is called a fetus. The focus now shifts from building structures to growth and functional maturation.
First Trimester (Weeks 9–12)
Key changes:
- Rapid growth in length
- Further differentiation of organs formed during the embryonic period
- Development of external genitalia to a point where sex can often be distinguished (by imaging) later in this phase or in the second trimester
- Beginning of spontaneous movements (not yet usually felt by the mother)
At the end of the first trimester:
- The basic body plan is completed
- The risk of major structural malformations from new exposures decreases, though growth and functional disturbances are still possible
Second Trimester (Weeks 13–27)
Focus on growth and refinement:
- Continued growth of body and limbs; proportions change (head size becomes more balanced with the rest of the body)
- Development of fine structures:
- Hair on the skin (lanugo)
- Vernix caseosa (a protective, waxy coating)
- Increasing muscle strength and movement activity:
- The mother often first feels fetal movements (“quickening”) in this period
- Further maturation of organ systems, including the nervous system and senses
The fetus becomes progressively more viable (capable of survival) outside the uterus as the trimester progresses, but survival chances before the end of the second trimester are still low and heavily dependent on medical care.
Third Trimester (Weeks 28–Birth)
The third trimester is dominated by:
- Strong weight gain and further growth
- Rapid brain development:
- Expansion of cortical areas
- Increasing complexity of neural connections
- Maturation of the lungs:
- Production of surfactant (a substance that prevents lung collapse and is necessary for effective breathing after birth)
- Store-building:
- Fat deposits under the skin (important for temperature regulation after birth)
- Storage of iron and other nutrients
The fetus practices functions needed after birth:
- Breathing movements (without air, in amniotic fluid)
- Swallowing amniotic fluid
- Sucking and grasping movements
Near the end of pregnancy, the fetus usually adopts a head-down position in readiness for birth.
Critical Phases and Developmental Disturbances (Overview Only)
Detailed genetic and medical aspects are treated elsewhere, but some general points are specific to embryonic development in humans:
- Preimplantation period:
- Disturbances often lead either to no implantation or early loss, often unnoticed
- Embryonic period (weeks 3–8):
- Vulnerable to teratogens that can cause structural malformations (e.g., limb defects, heart malformations, neural tube defects)
- Fetal period:
- Main risk is impaired growth and functional disturbances (e.g., brain development, organ function), rather than complete absence of organs
Influences that may affect development include:
- Genetic factors (chromosomal abnormalities, gene mutations)
- Environmental factors (chemicals, radiation, infections)
- Maternal health (nutrition, chronic diseases, substance use)
Understanding the timing of development helps explain why some exposures have severe effects only in specific pregnancy windows.
Birth as the Endpoint of Intrauterine Development
Although the detailed process of birth itself belongs to other chapters, from the perspective of embryonic/fetal development:
- Birth marks the transition from dependence on the placenta to:
- Lung breathing instead of placental gas exchange
- Independent circulation (closure and remodeling of fetal blood shunts)
- Oral feeding instead of placental nutrient transfer
- Several circulatory changes occur immediately after the first breaths, adapting the newborn to life outside the uterus
From a single fertilized cell to a newborn, human development is a tightly choreographed sequence of cell divisions, movements, differentiation, and growth. The human embryo’s sensitivity and complexity during this time underline the importance of conditions within the uterus for lifelong health.