The endoplasmic reticulum (ER) is a large, dynamic organelle involved in calcium storage, protein synthesis, lipid metabolism, and protein folding. It consists of tubules, sheets, and the nuclear envelope, with distinct domains performing specific functions. The ER's structure and dynamics are regulated by proteins and cellular signals, including calcium levels, cell cycle state, and developmental cues. The ER is crucial for protein synthesis, folding, and transport, as well as lipid biogenesis and calcium signaling. ER sheets and tubules have different functions, with sheets involved in protein synthesis and tubules in lipid transport and calcium signaling. The ER's shape and organization are influenced by proteins such as reticulons, DP1/Yop1, and Atlastin, which promote tubule formation and junction stability. ER dynamics are also regulated by phosphorylation and interactions with microtubules. During mitosis, the ER undergoes significant structural changes, with sheets becoming dominant, and proteins involved in ER shaping being redistributed. In oocyte maturation and fertilization, the ER reorganizes in response to cellular signals, including calcium release and microtubule dynamics. The ER's ability to respond to cellular signals is essential for maintaining cellular function and homeostasis. Understanding the ER's structure, function, and regulation is critical for elucidating its role in various cellular processes and diseases.The endoplasmic reticulum (ER) is a large, dynamic organelle involved in calcium storage, protein synthesis, lipid metabolism, and protein folding. It consists of tubules, sheets, and the nuclear envelope, with distinct domains performing specific functions. The ER's structure and dynamics are regulated by proteins and cellular signals, including calcium levels, cell cycle state, and developmental cues. The ER is crucial for protein synthesis, folding, and transport, as well as lipid biogenesis and calcium signaling. ER sheets and tubules have different functions, with sheets involved in protein synthesis and tubules in lipid transport and calcium signaling. The ER's shape and organization are influenced by proteins such as reticulons, DP1/Yop1, and Atlastin, which promote tubule formation and junction stability. ER dynamics are also regulated by phosphorylation and interactions with microtubules. During mitosis, the ER undergoes significant structural changes, with sheets becoming dominant, and proteins involved in ER shaping being redistributed. In oocyte maturation and fertilization, the ER reorganizes in response to cellular signals, including calcium release and microtubule dynamics. The ER's ability to respond to cellular signals is essential for maintaining cellular function and homeostasis. Understanding the ER's structure, function, and regulation is critical for elucidating its role in various cellular processes and diseases.