The mesodermal layer of the embryo develops into a loose collection of cells known as parenchyma tissue. This tissue occupies the entire space between the outer body wall and the endoderm of the gut.
|lungs||alveoli, respiratory bronchiole, alveolar duct and terminal bronchiole|
|spleen||white pulp and red pulp|
Parenchyma cells are thin-walled cells of the ground tissue that make up the bulk of most nonwoody structures, yet sometimes their cell walls can be lignified. Parenchyma cells in between the epidermis and pericycle in a root or shoot constitute the cortex, and are used for storage of food. Parenchyma cells within the center of the root or shoot constitute the pith. Parenchyma cells in the ovary constitutes the nucellus and are brick-like in formation. Parenchyma cells in the leaf constitute the mesophyll; they are responsible for photosynthesis and they allow for the interchange of gases.