Oogenesis: Female Gamete Formation

• Oogenesis is the process of female gamete formation, which involves the production of ova (eggs) within the ovaries
• This process begins during female embryonic development but is not completed until fertilization

Stages of Oogenesis

• Oogenesis can be divided into prenatal and postnatal stages

Prenatal Oogenesis

• Begins with primordial germ cells (PGCs) migrating to the developing ovaries
• PGCs differentiate into oogonia, which undergo mitosis to increase their numbers
• Oogonia start meiosis I but arrest at the prophase I stage, becoming primary oocytes
• Each primary oocyte is surrounded by a layer of follicular cells, forming a primordial follicle
• The number of primary oocytes peaks during fetal development and then declines through atresia (programmed cell death)
• By birth, a female has a limited number of primary oocytes in primordial follicles

Postnatal Oogenesis

• Starts at puberty when some primordial follicles develop further each menstrual cycle
• The follicular cells proliferate and become cuboidal, transforming the primordial follicle into a primary follicle
• The primary oocyte grows in size, and zona pellucida, a glycoprotein layer, forms around it
• The follicular cells continue to proliferate, forming multiple layers and turning the primary follicle into a secondary follicle
• The theca folliculi, connective tissue around the follicle, differentiates into theca interna and theca externa
• Fluid-filled spaces appear between the follicular cells, which coalesce to form the antrum, thus the secondary follicle becomes an antral follicle or tertiary follicle
• Before ovulation, the primary oocyte completes meiosis I, forming two haploid cells: a secondary oocyte and a first polar body
• The secondary oocyte receives most of the cytoplasm, while the first polar body is small and eventually degenerates
• The secondary oocyte starts meiosis II but arrests at metaphase II until fertilization
• Ovulation involves the release of the secondary oocyte from the ovary
• If fertilization occurs, the secondary oocyte completes meiosis II, producing an ootid and a second polar body
• The ootid then matures into an ovum (egg)
• The first polar body may also divide into two polar bodies, which will degenerate along with the second polar body

Hormonal Control

• Follicle-stimulating hormone (FSH) stimulates the growth and development of follicles in the ovary
• Luteinizing hormone (LH) triggers ovulation
• Estrogen, produced by the follicular cells, promotes the growth and maintenance of the female reproductive system

Comparison with Spermatogenesis

• Oogenesis produces one functional ovum from each primary oocyte that completes meiosis, whereas spermatogenesis produces four functional spermatozoa from each primary spermatocyte
• Oogenesis involves unequal cytoplasmic division, resulting in one large ovum and small polar bodies, while spermatogenesis involves equal division, yielding four spermatozoa of equal size
• Oogenesis has long interruptions, while spermatogenesis is a continuous process once it begins at puberty

Key Terms

• Oogonium: A diploid germ cell in the ovary that undergoes mitosis to produce primary oocytes
• Primary oocyte: A diploid cell that has started meiosis I and is arrested in prophase I
• Secondary oocyte: A haploid cell that is produced after the first meiotic division and is arrested in metaphase II
• Ovum: A mature haploid egg cell that is capable of being fertilized
• Polar body: A small, non-functional cell that is produced during oogenesis
• Follicle: A structure in the ovary that contains a developing oocyte surrounded by follicular cells
• Primordial follicle: A primary oocyte surrounded by a single layer of flattened follicular cells
• Primary follicle: A primary oocyte surrounded by a single layer of cuboidal follicular cells
• Secondary follicle: A primary oocyte surrounded by multiple layers of follicular cells and beginning of antrum formation
• Antral follicle: A follicle with a well-formed antrum (fluid-filled cavity)
• Ovulation: The release of the secondary oocyte from the ovary

Clinical Significance

• Oogenesis is essential for female fertility
• Disorders in oogenesis can lead to infertility, such as polycystic ovary syndrome (PCOS) and premature ovarian failure
• The risk of chromosomal abnormalities, such as Down syndrome, increases with maternal age due to errors during meiosis I in oogenesis
• Assisted reproductive technologies (ART) such as in vitro fertilization (IVF) involve stimulating oogenesis and retrieving oocytes for fertilization

Oogenesis is the development of female gametes (ova) in the ovaries, beginning in embryonic development but completing upon fertilization. Prenatal oogenesis involves primordial germ cells differentiating into oogonia, which become primary oocytes arrested in meiosis I. Postnatal oogenesis starts at puberty with follicle development during each menstrual cycle.