Morphogen Gradients and Positional Information

In sexual reproduction, an animal develops from a fertilized egg. Repeated divisions produce cells that later specialize as skin, muscle, nerves and other tissues. Chemical communication helps direct these choices. Signals affect which genes a cell activates, producing differential gene expression.

Development begins with cleavage, divisions that turn the egg into many smaller cells. During gastrulation, cells rearrange into layers that contribute different body parts. Organogenesis then forms organs as groups of cells respond to signals and specialize.

Most cells share the same DNA, yet a liver cell and a brain cell use different sets of genes. Their roles depend largely on which genes are active. Some signals form concentration gradients, strongest near their source and weaker farther away. Cells respond to these concentrations as positional information, helping them adopt roles appropriate to their location.

Most body cells contain the same genetic information. Nevertheless, eye and liver cells differ in appearance, function and the proteins they produce. They achieve this by activating different combinations of genes rather than using every gene alike.

Gene regulation controls this activity. A cell's expression pattern helps establish its identity. During development, chemical signals from nearby surroundings can activate some genes and suppress others. After specialization, a cell can maintain its expression pattern through division. These regulated changes allow one starting cell to produce a whole organism.

Morphogens participate in this process. Cells respond differently to different morphogen concentrations, changing gene expression according to their position within a gradient.

When a sperm and an egg join, they form a single cell called a zygote. That one cell has everything needed to grow into a whole organism. It divides into two cells, then four, then eight, and keeps going. At first the cells are all alike. Then something remarkable happens: they begin to differentiate. Differentiation means a cell specializes, becoming one particular type, like a muscle cell or a nerve cell. Cells decide what to become based on cues. Some cues come from inside the cell, and some come from neighboring cells sending chemical messages. A message received by a cell can flip switches in its DNA, turning certain genes on and others off. The genes that stay on decide what the cell becomes. Growth and differentiation work together as the embryo develops. This is why signals that vary from place to place, like a morphogen gradient, are so powerful. They give different cells different instructions, so a smooth change in signal strength can produce sharply different cell types side by side.

Animals are multicellular, obtain energy from other organisms and usually move during some stage of life. Many share developmental processes: a fertilized egg divides, cells form layers, and those layers rearrange and specialize to produce a body plan.

In many animals, Hox genes help specify regional identity along the head-to-tail axis. Their activity helps structures develop in suitable locations. Misplaced Hox activity can produce a body part in an inappropriate region, demonstrating the effects of positional information.

Other signals help regulate these genes. Morphogen concentrations can vary across an embryo, exposing regions to different signal levels. Responses involving Hox genes can contribute to different cell fates. Together, gradients and gene regulation help organise distinct regions along the body.