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Graphical abstract Abstract While tissue form and function is highly dependent upon tissue-specific collagen composition and organization, little is known of the mechanisms controlling the bundling of collagen fibrils into fibers and larger structural designs that lead to the formation of bones, tendons and other tissues. Using the cornea as a model system, our previous 3 dimensional mapping of collagen fiber organization has demonstrated that macrostructural organization of collagen fibers involving interweaving, branching and anastomosing plays a critical role in controlling mechanical stiffness, corneal shape and refractive power. In this work, the cellular and mechanical mechanisms regulating critical events in the assembly of collagen macrostructure are analysed in the developing chicken cornea. We elucidated the temporal events leading to adult corneal structure and determined the effects of intraocular pressure (IOP) on the organization of the collagen macrostructure. Our findings indicate that the complex adult collagen organization begins to appear on embryonic day 10 (E10) after deposition of the primary stroma and full invasion of keratocytes. Importantly, organizational changes in keratocytes appearing at E9 preceded and predicted later changes in collagen organization. Corneal collagen organization remained unaffected when the development of IOP was blocked at E4. These findings support a primary role for keratocytes in controlling stromal organization, mechanical stiffness and corneal shape that are not regulated by the IOP. Our findings also suggest that the avian cornea represents an excellent experimental model for elucidating key regulatory steps and mechanisms controlling the collagen fiber organization that is critical to determining tissue form and function. Statement of significance This work by using an ex ovo model system, begins to investigate the potential mechanisms controlling collagen fibril macrostructure. In particular, this work highlights a convergent role for the corneal keratocytes in organizing the complex collagen macrostructure, necessary to support high visual acuity. Our data supports that the intraocular pressure does not influence collagen fibril macrostructure and suggest that the avian cornea represents an excellent experimental model for elucidating key regulatory steps and mechanisms controlling the collagen fiber organization that is critical to determining tissue form and function. Clearly understanding the cellular and molecular mechanisms that underlie collagen fibril macrostructure will be highly beneficial for future tissue engineering and regenerative medicine applications. [ABSTRACT FROM AUTHOR]