In this Thesis, we theoretically study the mechanical properties of epithelial tissues using a two-dimensional vertex-based model building on apicobasal polarization. Within this model, which focuses on tissue cross-section, we investigate the formation of lengthwise folds in tubular epithelia, which are found, e.g., in the gastrointestinal tract and which are to a good approximation characteristic of the early stages of development in the embryo of the fruit fly as an important model organism. We first compute the energy-minimizing shapes as a function of the mechanical apico\-basal polarity defined as the difference between the apical and the basal surface tension, and as a function of the radius of the external rigid cylindrical constraint. In the phase diagram, we find two qualitatively different regions. If the radius of the constraint exceeds a critical, polarity-dependent value, the number of folds in the tissue increases with differential tension in steps of 1. Below the critical radius, separate few- and many-lobes regions appear. We also focus on tissue folding due to growth caused by cell division. We analyze two models: Quasi-stationary growth, in which the tissue takes the energy-minimizing shape for each separate number of cells, and dynamic growth, in which energy relaxation is determined by the overdamped equation of motion. In the quasi-stationary growth mode, the number of lobes in the tissue increases quickly with the difference between the apical and the basal surface tension as well as with the number of cells. Conversely, in the dynamic growth mode the number of lobes does not change with time to a good approximation, and three regimes of tissue growth exist. In the first one, the number of lobes depends solely on the rate of cell division; in the second one it depends solely on the difference between the apical and the basal surface tension; and in the intermediate regime the effects of both factors are comparable. These results provide a fresh theoretical insight into the morphogenesis of tubular epithelia such as the intestinal epithelium in the chick embryo.