Fertilization and Early Embryonic Development

Core Concepts

Cleavage: Dividing Without Growing

Cleavage is the series of rapid mitotic divisions that follows fertilization. The zygote divides again and again, but the embryo does not increase in overall size: the original cytoplasm is simply partitioned among smaller and smaller cells called blastomeres. Because no growth phase occurs between divisions, cleavage runs much faster than ordinary mitosis. In many animals the pattern of cleavage depends on how much yolk the egg contains — eggs with little yolk divide evenly, while yolky eggs (such as bird eggs) divide only in a cap of cytoplasm at the animal pole. After several rounds of division, the embryo is a solid ball of cells called the morula.

The Blastula and the Mammalian Blastocyst

As cleavage continues, cells secrete fluid into the interior, creating a hollow, fluid-filled cavity called the blastocoel; the resulting stage is the blastula. In mammals the blastula is called a blastocyst, and it is special in two ways. First, its cells have already become functionally different: the outer layer, the trophoblast, will contribute to the placenta, while the inner cell mass, a cluster of cells at one end, will form the embryo itself. Second, the mammalian blastocyst must escape the zona pellucida — the glycoprotein coat left over from the egg — before it can implant. Implantation typically begins about a week after fertilization (a commonly taught reference timing) as the trophoblast invades the uterine lining and begins secreting human chorionic gonadotropin (hCG), the hormone detected by pregnancy tests, which signals the corpus luteum to keep producing progesterone.

Gastrulation: Rearranging Cells Into Three Layers

Gastrulation is the dramatic reorganization that follows cleavage. Cells migrate, fold, and stream into the interior of the embryo, converting the single-layered blastula into a three-layered structure. The three germ layers are the ectoderm (outer), mesoderm (middle), and endoderm (inner). In humans, gastrulation begins in the third week after fertilization with the appearance of the primitive streak, a groove along the embryo's midline through which cells move inward (a commonly taught reference timeline). No new genes are switched on just to create the layers; rather, cells that have already received different molecular signals move to different positions, where new signals will instruct their fates.

What the Germ Layers Become

Each germ layer gives rise to characteristic structures. The ectoderm forms the epidermis of the skin, the nervous system (brain, spinal cord, and nerves), and sensory organs such as the eyes and inner ear. The mesoderm forms muscle, bone and cartilage, blood and blood vessels, connective tissue, kidneys, and gonads. The endoderm lines the digestive tract and respiratory passages and gives rise to the liver, pancreas, and thyroid. The germ layers are therefore not just an embryology curiosity: mapping a tissue back to its germ layer predicts what it can become, which is why teratomas — tumors that can contain hair, teeth, and muscle — reveal how broad the developmental potential of early cells can be.

Common Confusions