The concept of a "biostructural unit" is presented as the combination of biological and structural building blocks to create scaffolds or constructs via a bottom-up approach. Three types of biostructural units were constructed using the process of fiber formation by interfacial polyelectrolyte complexation: protein-encapsulated fiber, ligand-immobilized fiber, and cell-encapsulated fiber units. Water-soluble chitin (WSC) and alginate were used as the polyelectrolyte combination to form fiber. Encapsulation and sustained release of bovine serum albumin from the fiber could be achieved, release profiles being dependent on the WSC/alginate concentration ratio. Released nerve growth factor (NGF) retained its bioactivity, as demonstrated on PC12 cells. Biotinylated fiber could be fabricated by biotinylating alginate before drawing fiber with WSC, enabling biotinylated NGF to be immobilized to fiber via an avidin bridge. The immobilized NGF induced the differentiation of PC12 cells seeded on the fiber. Bovine pulmonary endothelial cells, human dermal fibroblasts, and human mesenchymal stem cells were encapsulated, demonstrating good viability as determined by Live/Dead and WST-1 assays. The assembly of biostructural units into constructs was illustrated by using human mesenchymal stem cell-encapsulated fiber units. Cells in the resulting constructs could be induced to differentiate along chondrogenic and osteogenic lineages.