Human embryonic development transforms a single fertilized cell into a fully structured organism. This period of rapid, precise organization spans the first eight weeks following conception, establishing the fundamental body plan and forming the initial structures of all major organ systems. The orderly progression of these stages defines the embryonic period, after which the developing human is referred to as a fetus until birth. The sequence starts with cell division, moves through the establishment of basic body axes, and culminates in the formation of recognizable body features.
From Fertilization to Implantation: Weeks One and Two
The developmental journey begins in the fallopian tube, where a sperm and an egg unite to form a single-celled zygote. The zygote immediately initiates cleavage, undergoing rapid mitotic cell divisions without increasing in overall size. This process results in progressively smaller cells called blastomeres, encased within the protective layer, the zona pellucida.
By about the fourth day, the dividing cells form a compact ball of 16 or more cells known as a morula. The morula travels toward the uterus, reorganizing internally to form a blastocyst around day five or six. This blastocyst is a fluid-filled sphere with two distinct cell populations: the inner cell mass (embryoblast), which gives rise to the embryo itself, and the outer layer (trophoblast), which forms the placenta and supporting structures.
To prepare for implantation, the blastocyst sheds its surrounding zona pellucida in a process called “hatching.” Implantation begins around day six, as the trophoblast cells adhere to the endometrium, the lining of the uterus. The trophoblast actively penetrates the uterine wall, embedding the blastocyst fully into the maternal tissue, a process that is typically complete by day 10 to 12. Successful implantation establishes the connection for the embryo to receive nutrients and marks the transition into the next organizational phase of development.
Establishing the Body Plan: Gastrulation and Neurulation
Following implantation, the next major sequential event is gastrulation, which occurs during the third week of development. This reorganization transforms the two-layered structure of the embryoblast into a three-layered, or trilaminar, embryonic disc. Cells of the epiblast layer migrate inward through the primitive streak, displacing existing cells to form the three primary germ layers.
The three newly formed layers are the body’s foundational blueprint, with each layer destined to form specific tissues and organs.
Germ Layers and Their Derivatives
- The ectoderm gives rise to the central and peripheral nervous systems, the epidermis of the skin, and sensory organs.
- The mesoderm develops into muscle, bone, connective tissue, the circulatory system, and the kidneys.
- The endoderm forms the lining of the digestive and respiratory systems, as well as organs such as the liver and pancreas.
Immediately following the establishment of the germ layers, the process of neurulation begins. The notochord, a rod-like structure formed from the mesoderm, signals the overlying ectoderm to thicken and form the neural plate. This neural plate folds in on itself, creating the neural groove, and the edges fuse to form the neural tube. The neural tube is the precursor to the entire central nervous system, including the brain and spinal cord, and its closure is a defining event of the fourth week of development.
Completing Embryogenesis: Organ Development (Weeks 4-8)
With the body plan established, the embryo enters organogenesis, spanning weeks four through eight. This period is characterized by the rapid differentiation of the germ layers into specific organs and systems. A significant early event in week four is the folding of the embryonic disc, which gives the embryo its definitive cylindrical body shape. This folding incorporates part of the endoderm to form the primitive gut, the beginning of the digestive tract.
The cardiovascular system is the first to become functional. The primitive heart tube forms and begins to beat and pump blood around day 22 to 23. Shortly after, small outgrowths called limb buds, which develop into the arms and legs, become visible by the middle of the fourth week.
During weeks five through eight, the head grows disproportionately large as the brain undergoes rapid development, dividing into its major regions. By week eight, the embryo possesses all major internal organs and external features in a rudimentary form, taking on a distinctly human appearance. Structures like the eyes, ears, nose, and jaw are identifiable, and the webbed fingers and toes have begun to separate. The completion of this intensive structural formation marks the end of the embryonic stage and the beginning of the fetal period.