Zona pellucida

Zona pellucida is a thick, gel-like glycoprotein membrane that coats the oocyte and must be penetrated by sperm before fertilization can occur.

Zona Pellucida

Ovarian follicle development

• An ovarian follicle consists of a developing oocyte surrounded by one or more layers of cells called follicular cells.
• At the same time that the oocyte is progressing through meiosis, corresponding changes are taking place in the follicular cells.
• Primordial follicles, which consist of a primary oocyte surrounded by a single layer of flattened cells, develop in the fetus and are the stage that is present in the ovaries at birth and throughout childhood.

Zona pellucida

Structure of an ovary

• Beginning at puberty, follicle-stimulating hormone stimulates changes in the primordial follicles.
• The follicular cells become cuboidal, the primary oocyte enlarges, and it is now a primary follicle.
• The follicles continue to grow under the influence of follicle-stimulating hormone, and the follicular cells proliferate to form several layers of granulose cells around the primary oocyte.
• Most of these primary follicles degenerate along with the primary oocytes within them, but usually one continues to develop each month.
• The granulosa cells start secreting estrogen and a cavity, or antrum, forms within the follicle.
• When the antrum starts to develop, the follicle becomes a secondary follicle.
• The granulose cells also secrete a glycoprotein substance that forms a clear membrane, the zona pellucida, around the oocyte.
• After about 10 days of growth the follicle is a mature vesicular (graafian) follicle, which forms a "blister" on the surface of the ovary and contains a secondary oocyte ready for ovulation.

Oocyte with Zona pellucida

Ovulation

• Ovulation, prompted by luteinizing hormone(LH) from the anterior pituitary, occurs when the mature follicle at the surface of the ovary ruptures and releases the secondary oocyte into the peritoneal cavity.
• The ovulated secondary oocyte, ready for fertilization is still surrounded by the zona pellucida and a few layers of cells called the corona radiata.
• If it is not fertilized, the secondary oocyte degenerates in a couple of days.
• If a sperm passes through the corona radiata and zona pellucida and enters the cytoplasm of the secondary oocyte, the second meiotic division resumes to form a polar body and a mature ovum

Corpus luteum

• After ovulation and in response to luteinizing hormone, the portion of the follicle that remains in the ovary enlarges and is transformed into a corpus luteum.
• The corpus luteum is a glandular structure that secretes progesterone and some estrogen.
• Its fate depends on whether fertilization occurs.
• If fertilization does not take place, the corpus luteum remains functional for about 10 days; then it begins to degenerate into a corpus albicans, which is primarily scar tissue, and its hormone output ceases.
• If fertilization occurs, the corpus luteum persists and continues its hormone functions until the placenta develops sufficiently to secrete the necessary hormones.
• The corpus luteum ultimately degenerates into corpus albicans, but it remains functional for a longer period of time.

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