Development of Animal Gametes: Videos & Practice Problems

Spermatogenesis is the process by which male gametes, or sperm, are produced in the seminiferous tubules of the testes. It begins with diploid primordial germ cells that undergo mitosis to form spermatogonia. These spermatogonia then undergo mitosis to form primary spermatocytes, which are also diploid. The primary spermatocytes undergo meiosis I to form haploid secondary spermatocytes, which then undergo meiosis II to form spermatids. Spermatids mature into motile sperm through spermiogenesis. In contrast, oogenesis is the process of forming female gametes, or eggs, in the ovaries. It starts with diploid primordial germ cells that form oogonia via mitosis. Oogonia then form primary oocytes, which begin meiosis I but halt until ovulation. Upon ovulation, meiosis I completes, forming a secondary oocyte and a polar body. The secondary oocyte only completes meiosis II if fertilized, resulting in one large ovum and additional polar bodies. Unlike spermatogenesis, which produces four sperm, oogenesis results in one ovum and several discarded polar bodies.

Spermatogenesis involves several stages:

1. **Primordial Germ Cells**: Diploid cells that undergo mitosis to form spermatogonia.

2. **Spermatogonia**: Diploid cells that divide by mitosis to produce primary spermatocytes.

3. **Primary Spermatocytes**: Diploid cells that undergo meiosis I to form haploid secondary spermatocytes.

4. **Secondary Spermatocytes**: Haploid cells that undergo meiosis II to produce spermatids.

5. **Spermatids**: Immature haploid cells that undergo spermiogenesis to become mature, motile sperm.

6. **Spermatozoa**: Fully mature sperm capable of fertilizing an egg.

Oogenesis and spermatogenesis differ significantly in timing and cell division. Oogenesis begins before birth, with primary oocytes formed during fetal development. These primary oocytes enter meiosis I but halt at prophase I until puberty. During each menstrual cycle, one primary oocyte completes meiosis I to form a secondary oocyte and a polar body. The secondary oocyte begins meiosis II but halts at metaphase II until fertilization. If fertilized, it completes meiosis II to form an ovum and another polar body. In contrast, spermatogenesis begins at puberty and continues throughout a male's life. It involves continuous mitotic and meiotic divisions, producing four sperm from each spermatogonium. Oogenesis results in one large ovum and polar bodies, while spermatogenesis produces four sperm.

Primary oocytes are diploid cells formed from oogonia via mitosis before birth. They begin meiosis I but halt at prophase I until puberty. During ovulation, a primary oocyte completes meiosis I to form a haploid secondary oocyte and a polar body. Secondary oocytes are haploid cells that begin meiosis II but halt at metaphase II until fertilization. If fertilized, the secondary oocyte completes meiosis II to form an ovum and another polar body. The primary oocyte is larger and contains more cytoplasm than the secondary oocyte, which is smaller and destined to be either fertilized or discarded if not fertilized.

Females produce polar bodies during oogenesis to ensure that the resulting ovum has sufficient cytoplasm and organelles to support early embryonic development. During meiosis I and II, the cytoplasm is unevenly divided, with most of it retained in the secondary oocyte and later the ovum. The smaller polar bodies, which contain minimal cytoplasm, are discarded. This asymmetrical division maximizes the resources available to the ovum, enhancing its chances of successful fertilization and subsequent development into a zygote. Polar bodies serve to discard the extra haploid sets of chromosomes, ensuring the ovum remains haploid.