Development and Organogenesis

Core Concepts

Cleavage: dividing without growing

The zygote begins with a series of rapid mitotic divisions called cleavage. The key feature of cleavage is that the embryo divides the existing cytoplasm into smaller and smaller cells — blastomeres — without an overall increase in size. This restores a normal cell-size-to-nucleus ratio, which is essential for the gene-regulation machinery to work properly. In many animals the cleavage-stage embryo looks like a mulberry; this solid ball is called the morula. With continued division, cells arrange into a hollow sphere surrounding a fluid-filled cavity, the blastocoel — this stage is the blastula (in mammals, the equivalent structure is called the blastocyst).

Gastrulation: building the three germ layers

Gastrulation is the most dramatic event of early development: the ball of cells reorganizes into a three-layered embryo. Cells migrate inward through an opening (the blastopore) in a choreographed movement, transforming the single-layered blastula into the gastrula with three distinct layers, the germ layers:

• Ectoderm (outer layer) gives rise to the skin (epidermis) and its derivatives (hair, nails, glands), and to the entire nervous system — brain, spinal cord, and nerves.
• Mesoderm (middle layer) gives rise to muscle, bone and cartilage, blood and blood vessels, kidneys, gonads, and connective tissues.
• Endoderm (inner layer) gives rise to the lining of the digestive tract and respiratory tract and to organs derived from them, such as the liver and pancreas.

Gastrulation also establishes the basic body axes — head-to-tail and back-to-belly — that all later structures are organized along. It is during and just after gastrulation that the embryo is most sensitive to disruptive influences, because the fundamental architecture of the body is being laid down.

Organogenesis: germ layers become organs

Organogenesis is the formation of organs from the germ layers, driven by cell movements, cell–cell signaling, and programmed cell death (apoptosis) that sculpts structures. A signature example in vertebrates is neurulation, the formation of the nervous system: the dorsal ectoderm thickens into a neural plate, its edges rise and fold together to form the neural tube, which develops into the brain and spinal cord. Meanwhile, blocks of mesoderm called somites form along the back and give rise to the vertebrae, ribs, and skeletal muscles; the endoderm rolls into the gut tube from which the digestive organs bud. Organogenesis does not happen all at once — organs form on a schedule, with the nervous system and heart among the earliest, which is why early embryos are especially vulnerable to teratogens.

Differentiation: how identical cells become different

All the cells of an animal's body (with few exceptions) contain the same genome — the same DNA sequence. What makes a muscle cell different from a neuron is not which genes it has, but which genes it uses. Differentiation is the process by which cells become specialized by expressing different subsets of genes, often irreversibly. Cells coordinate this through signals from their neighbors and through transcription factors — proteins that turn specific genes on or off. The developmental potential of a cell narrows as development proceeds: a zygote is totipotent (can form every cell type plus extraembryonic tissues), cells of the early embryo are pluripotent (can form all cell types of the body), and cells of the adult are increasingly multipotent or unipotent (restricted to particular lineages, such as blood-forming stem cells). This hierarchy of potential is the basis of stem cell biology.

Supporting the embryo: extraembryonic membranes and the placenta

Animals that develop in eggs (reptiles, birds, and mammals' relatives) and mammals themselves use extraembryonic membranes — tissues that develop alongside the embryo but are not part of the embryo's body. In birds and reptiles, four membranes support development inside the shell: the amnion (encloses the embryo in fluid, cushioning it), the chorion (lies just inside the shell and mediates gas exchange), the yolk sac (digests and delivers yolk nutrients), and the allantois (stores waste and also contributes to gas exchange). Because of the amnion, these animals are called amniotes.

In placental mammals, the same membranes are repurposed: the chorion and allantois contribute to the placenta, the organ where maternal and fetal blood vessels come close enough for exchange of oxygen, nutrients, and wastes without the two blood supplies actually mixing. The amnion still provides the fluid-filled sac around the embryo, and the yolk sac is reduced (human eggs contain little yolk). The placenta makes it possible for the embryo to develop inside the mother's body — the arrangement studied in detail in the next topic.

Common Confusions