During follicle maturation, which occurs within female ovaries, both the follicle and its enclosed oocyte undergo significant changes. Here are the key changes during follicle and oocyte maturation:
Follicular Development: Primordial follicles in the ovary mature and develop into primary, secondary, and then tertiary (Graafian) follicles. The follicles are composed of an oocyte surrounded by granulosa cells, which are supported by a layer of theca cells.
Oocyte Growth: Within the follicle, the oocyte grows and enlarges. This process is known as the oocyte or oogenesis. The oocyte increases in size due to the accumulation of cytoplasmic organelles, nutrients, and macromolecules necessary for embryonic development.
Granulosa Cell Proliferation: Granulosa cells surrounding the oocyte undergo proliferation and expansion. They form multiple layers around the oocyte, and the innermost layer is called the cumulus oophorus. The granulosa cells support oocyte growth and development.
Zona Pellucida Formation: The oocyte secretes a glycoprotein-rich extracellular matrix called the zona pellucida. The zona pellucida surrounds the oocyte and plays a critical role in fertilization and early embryonic development. It provides protection and serves as a barrier to sperm penetration.
Nuclear Maturation: The oocyte undergoes nuclear maturation, also known as meiosis. In humans, the oocyte is arrested in phase I of the meiotic process until it receives a maturation signal. With the maturation signal, the oocyte resumes meiosis and completes the first meiotic division (phase I). This results in the formation of two haploid cells: a secondary oocyte and the first polar body.
Cytoplasmic Changes: The oocyte undergoes various cytoplasmic changes, including the rearrangement and redistribution of organelles, such as mitochondria and the endoplasmic reticulum. These changes are necessary for the oocyte's metabolic activity and future embryonic development.
Cumulus Expansion: The cumulus cells surrounding the oocyte undergo expansion, forming a gel-like matrix. This expansion helps maintain the structural integrity of the follicle and facilitates the transfer of nutrients and signaling molecules between the oocyte and granulosa cells.
Ovulation: Once the follicle and oocyte have matured, a surge in luteinizing hormone (LH) triggers ovulation. The mature follicle ruptures, releasing the secondary oocyte along with the surrounding cumulus cells into the fallopian tube. The released oocyte is then available for fertilization by a sperm.
These changes are essential for the successful development and release of a mature oocyte. This is a critical step in female reproductive physiology.