A polar body is a small cell that forms alongside an egg cell, also known as an oocyte, during its development in the ovary. They are a natural byproduct of the complex process that creates a mature egg. A polar body is a haploid cell, meaning it contains half the number of chromosomes found in a typical body cell, similar to the egg cell itself.
How Polar Bodies Form
The formation of polar bodies is linked to oogenesis, the process by which female gametes, or eggs, develop. This process involves two specialized cell divisions called meiosis I and meiosis II. Before birth, immature egg cells, called primary oocytes, begin meiosis I but then pause. Upon puberty, a primary oocyte resumes meiosis I, resulting in two cells with an unequal division of cytoplasm.
This first division produces a large secondary oocyte and a much smaller first polar body. The first polar body typically contains duplicated chromosomes but very little cytoplasm. The secondary oocyte then proceeds to meiosis II but arrests at a later stage, completing this division only if fertilized. If fertilized, it divides again, yielding a mature egg (ovum) and a second polar body. The first polar body may also divide into two smaller polar bodies, leading to a potential total of three polar bodies alongside one mature egg.
Why Polar Bodies Form and Their Purpose
Polar body formation results from unequal cytokinesis, the uneven distribution of cellular material during cell division. During oogenesis, the primary oocyte divides so that almost all cytoplasm, organelles, and nutrients concentrate into one large cell, the egg. This ensures the egg receives sufficient resources to support early embryonic development after fertilization.
The polar bodies, receiving only a small amount of cytoplasm, discard extra chromosomes while preserving most maternal cytoplasm for the future embryo. These smaller cells are generally considered non-functional in contributing to an embryo. They degenerate and are reabsorbed by the body within about 24 hours.
Polar Bodies and Reproductive Science
Polar bodies are important in modern reproductive science, particularly for genetic analysis. Since polar bodies mirror the egg’s genetic material, their analysis provides insights into the egg’s genetic makeup. This is relevant in preimplantation genetic testing (PGT), a technique used with in vitro fertilization (IVF). A small sample (biopsy) of the polar body can be safely taken without affecting the potential embryo.
This analysis allows for the screening of genetic abnormalities in the egg before fertilization or embryo transfer. For instance, PGT-A (Preimplantation Genetic Testing for Aneuploidies) can detect abnormal chromosome numbers, a common cause of implantation failure or miscarriage. PGT-P (Preimplantation Genetic Testing for Monogenic Diseases) can identify specific gene mutations that cause inherited conditions. While polar body biopsy offers the advantage of being less invasive to the developing embryo, it primarily provides information about maternal genetic contributions and cannot detect paternal genetic issues or errors arising after fertilization.