Summary
Highlights
Gametogenesis is the process of forming mature sexual cells (gametes). In females, it's called oogenesis, and in males, spermatogenesis. They share common stages, chromosome number reduction, and morphological changes but also have distinct characteristics.
Both oogenesis and spermatogenesis involve three successive periods: proliferation (where primordial germ cells multiply), growth (where cells increase in size), and maturation (which involves meiosis, reducing chromosome number and promoting genetic variability).
Oogenesis starts prenatally with primordial germ cell proliferation, forming oogonia that develop into primary oocytes. In maturation, primary oocytes undergo meiosis I to form a secondary oocyte and a polar body. Meiosis II occurs only upon fertilization, leading to a mature ovum and a second polar body. If not fertilized, the oocyte degenerates.
Oocytes are surrounded by follicular cells, forming primordial follicles. During each ovarian cycle, several follicles mature, but typically only one develops into a mature follicle, restarting meiosis I. On day 14 of the cycle, a secondary oocyte is expelled during ovulation, arresting in meiosis II until fertilization. Oocytes are rounded, around 120 microns, with abundant cytoplasm, a zona pellucida, and a corona radiata.
Oogenesis begins prenatally, resumes at puberty, and ends at menopause. Females are born with all their primary oocytes (limited number). It’s a cyclic process, producing one oocyte every 28 days. Mature oocytes have 23 chromosomes, including an X chromosome. The process is lengthy, spanning several years.
Spermatogenesis begins at puberty. Primordial germ cells give rise to spermatogonia in the proliferation phase. During growth, spermatocytes are formed. In maturation, primary spermatocytes undergo meiosis I to produce two secondary spermatocytes, which then undergo meiosis II to form four spermatids. Spermiogenesis transforms spermatids into spermatozoa.
Spermiogenesis is an additional stage where spermatids undergo morphological changes to become spermatozoa. Spermatozoa are elongated, 60-80 microns, with a head (containing condensed nucleus and acrosome), neck, midpiece, and tail.
Spermatogenesis starts at puberty and continues until death, with an unlimited production of sexual cells (300-400 million spermatozoa per ejaculation). It’s a continuous process, with spermatozoa having 23 chromosomes; 50% carry an X chromosome and 50% a Y chromosome. The process duration is short, around 64 days.
Errors during meiosis, such as non-disjunction of homologous chromosomes or sister chromatids, can lead to gametes with abnormal chromosome numbers. Fertilization with such gametes can result in trisomies (e.g., Down syndrome, 47 chromosomes due to an extra chromosome 21) or monosomies (e.g., Turner syndrome, 45 chromosomes due to a missing sex chromosome X).