The egg, a staple in diets worldwide, offers a concentrated source of nutrients. While its outward appearance is simple, the creation of its central, nutrient-rich yolk is a complex biological process.
The Hen’s Reproductive System
Yolk production begins within the hen’s reproductive system, specifically in the single functional left ovary. This ovary contains thousands of tiny ova, which are the precursors to yolks, each encased within a protective structure called a follicle. These follicles exist in a hierarchical arrangement based on their size and developmental stage.
Follicles are categorized into different groups, including small white follicles (less than 1 mm), large white follicles (2–4 mm), and small yellow follicles (4–8 mm). As a follicle is selected to mature, it enters the preovulatory hierarchy, rapidly increasing in size and accumulating yolk material.
Yolk Formation (Vitellogenesis)
The process of yolk accumulation, known as vitellogenesis, is a period of rapid growth where the follicle increases in size. During this phase, the hen’s liver plays a central role, synthesizing the primary components of the yolk from absorbed nutrients. These precursors include vitellogenin (VTG) and very-low-density lipoproteins (VLDLs).
Once synthesized in the liver, these yolk precursors are transported through the bloodstream to the ovary. The developing follicle cells then actively take up these substances via receptor-mediated endocytosis, depositing them within the oocyte. This continuous deposition forms the characteristic concentric layers observed in a yolk, with alternating light and dark rings. These rings represent daily deposition cycles.
Maturation and Ovulation
As the yolk accumulates sufficient material, it reaches its full size and completes its maturation within the follicle. This mature yolk, now ready for release, is approximately 30 to 40 mm in diameter. Hormonal signals, particularly a surge in luteinizing hormone (LH), trigger the final stage of this process.
Ovulation then occurs, where the mature yolk ruptures from the follicle. This rupture happens along a specialized, non-vascularized line on the follicle called the stigma, which minimizes blood loss. The newly released yolk is then captured by the infundibulum, the funnel-shaped opening of the oviduct. At this point, the yolk is fully formed and ready to proceed through the rest of the oviduct where other egg components, such as albumen and shell, will be added around it.