Question

If embryos form as balls of dividing cells that then form a pocket or fold, how do we get shapes?

Answer 1

Cells have the power to organise themselves to form complex and stable structures, to create a fully shaped baby from a single egg, or to allow adult salamanders to grow a new limb after losing a leg.Researchers have looked at the chemical messages exchanged by cells, or they have recorded the different shapes an embryo goes through during development.

Morphogenesis (beginning of the shape) is the biological process that causes an orgamism to develop its shape.

Morphogenesis arises because of cellular proliferation and motility.Morphogenesis involves changes in the cellular structure or how cells interact in tissues. These changes can result in tissue elongation, thinning, folding, invasion or separation of one tissue into distinct layers. This is called cell sorting. Cell sorting out consists of cells moving so as to sort into clusters that maximize contact between cells of the same type.

Tissues change their shape and separate into distinct layers via cell contractility.

Myosin can contract different parts of the cytoplasm to change its shape or structure. Myosin-driven contractility in embryonic tissue morphogenesis is seen during the separation of germ layers.

As embryo develops, sheets of cells fold and unfold to create complex 3D shapes, like the tubes that make our lungs. Moulding sheets into tubes relies on interactions between cells that are not the same in all directions. There are two types of asymmetry (or polarity) that guide these interactions. Apical-basal polarity runs across a sheet of cells, that is top surface of the sheet differs from the bottom. Planar cell polarity runs along the sheet and distinguishes one end from the other. Apical-basal polarity marks the inner and outer surfaces of our skin, while planar cell polarity controls the direction in which our hair grows.

Simulations of cells with only apical-basal polarity were able to generate different shapes in the tissues produced in living organisms. External conditions like how cells were arranged to start with, determined the resulting shape. With both apical-basal and planar cell polarities, the simulations reproduced an important change that occurs during early development. They also replicated how the tubes that transport nutrients and oxygen form.

Simple properties of individual cells, such as polarities, can produce different shapes in developing tissues and organs.

Gastrulation is the stage in which morphogenetic movements start to form the tri-layered embryo.A cell shape change occurs to cause a pocket to form.