Gametes and Fertilization

• Gametes are reproductive cells (sex cells) containing half the normal number of chromosomes.
• They are haploid cells, possessing a single set of chromosomes (n).
• In humans, the gametes are sperm cells (male) and oocytes (female).
• Gametes are produced through meiosis, a type of cell division that reduces the chromosome number.
• During fertilization, a sperm cell and an oocyte fuse to form a diploid zygote (2n), restoring the normal chromosome number.

Male Gametes (Sperm)

• Spermatozoa (sperm cells) are the male gametes.
• Sperm production (spermatogenesis) occurs in the seminiferous tubules of the testes.
• Spermatogenesis begins at puberty and continues throughout a male's life.
• A mature sperm cell consists of a head, midpiece, and tail.
• The head contains the nucleus with the haploid set of chromosomes and the acrosome.
• The acrosome contains enzymes that help the sperm penetrate the oocyte during fertilization.
• The midpiece contains mitochondria that provide energy for the sperm's movement.
• The tail (flagellum) is a whip-like structure that propels the sperm.
• Sperm cells are highly motile to reach the oocyte in the female reproductive tract.

Female Gametes (Oocytes)

• Oocytes (egg cells) are the female gametes.
• Oogenesis (oocyte production) occurs in the ovaries.
• Unlike spermatogenesis, oogenesis begins before birth.
• A female is born with a finite number of primary oocytes arrested in meiosis I.
• At puberty, some primary oocytes resume meiosis I, resulting in a secondary oocyte and a polar body.
• The secondary oocyte is released during ovulation.
• Meiosis II is completed only if fertilization occurs.
• A mature oocyte is a large, non-motile cell containing the haploid set of chromosomes.
• It is surrounded by the zona pellucida (a glycoprotein layer) and the corona radiata (follicle cells).

Gametogenesis - Spermatogenesis

• Spermatogenesis is the process by which sperm cells are produced from spermatogonia.
• It occurs in seminiferous tubules of the testes.
• Process includes:
  • Spermatogonia undergo mitosis to produce more spermatogonia.
  • Spermatogonia develop into primary spermatocytes.
  • Primary spermatocytes undergo meiosis I to produce secondary spermatocytes.
  • Secondary spermatocytes undergo meiosis II to produce spermatids.
  • Spermatids undergo spermiogenesis to become mature spermatozoa (sperm cells).
• Spermiogenesis involves the differentiation of spermatids into sperm, including the formation of the acrosome and flagellum.
• Sertoli cells support and nourish developing sperm cells.

Gametogenesis - Oogenesis

• Oogenesis is the process by which oocytes are produced from oogonia.
• It occurs in the ovaries.
• Process includes:
  • Oogonia undergo mitosis to produce more oogonia before birth.
  • Oogonia develop into primary oocytes before birth and enter meiosis I (arrested stage).
  • At puberty, some primary oocytes resume meiosis I, producing a secondary oocyte and a polar body.
  • The secondary oocyte is ovulated.
  • If fertilization occurs, the secondary oocyte completes meiosis II, producing a mature oocyte and another polar body.
• Polar bodies are small cells that contain very little cytoplasm and eventually degenerate.
• Primary oocytes are contained within follicles in the ovary.
• Follicle development supports and nourishes developing oocytes.

Hormonal Control of Gametogenesis

• Gametogenesis is regulated by hormones from the hypothalamus, pituitary gland, and gonads.
• Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland.
• In males, FSH stimulates Sertoli cells to support spermatogenesis, and LH stimulates Leydig cells to produce testosterone, which is also essential for spermatogenesis.
• In females, FSH stimulates follicle development and estrogen production, and LH triggers ovulation and the formation of the corpus luteum, which produces progesterone and estrogen.

Differences Between Spermatogenesis and Oogenesis

• Timing: Spermatogenesis begins at puberty and continues throughout life; oogenesis begins before birth and is arrested until puberty.
• Number of Gametes: Spermatogenesis produces four functional sperm cells from each primary spermatocyte; oogenesis produces one functional oocyte and polar bodies from each primary oocyte.
• Meiosis: In spermatogenesis, meiosis occurs continuously; in oogenesis, meiosis is arrested at different stages and is completed only if fertilization occurs.
• Gamete Size: Sperm cells are small and motile; oocytes are large and non-motile.
• Duration: Spermatogenesis takes approximately 64-72 days; oogenesis takes years to complete.

Genetic Variation

• Meiosis introduces genetic variation through crossing over and independent assortment.
• Crossing over occurs during prophase I of meiosis, where homologous chromosomes exchange genetic material.
• Independent assortment occurs during metaphase I, where homologous chromosomes align randomly along the metaphase plate.
• Genetic variation is important for evolution and adaptation.

Gamete Transport

• After being produced in the testes, sperm cells are stored in the epididymis and travel through the vas deferens during ejaculation.
• Semen, the fluid ejaculated from the penis, contains sperm cells and secretions from the seminal vesicles, prostate gland, and bulbourethral glands.
• During ovulation, the oocyte is released from the ovary and enters the fallopian tube (oviduct).
• Cilia lining the fallopian tube and muscular contractions help transport the oocyte towards the uterus.

Fertilization

• Fertilization is the fusion of a sperm cell and an oocyte to form a zygote.
• It typically occurs in the fallopian tube.
• The sperm cell must penetrate the corona radiata and zona pellucida to reach the oocyte's plasma membrane.
• The acrosomal reaction releases enzymes that help the sperm penetrate the zona pellucida.
• Once the sperm enters the oocyte, the oocyte completes meiosis II, and the sperm and egg nuclei fuse.
• The resulting zygote is diploid and contains genetic material from both parents.

Consequences and Applications

• Infertility can result from problems with gamete production, transport, or fertilization in both males and females.
• Assisted reproductive technologies (ART), such as in vitro fertilization (IVF), can help couples with infertility conceive.
• Understanding the process of gametogenesis is essential for developing new contraceptive methods.
• Gamete cryopreservation (freezing) can be used to preserve fertility for medical or social reasons.

Gametes are reproductive cells with half the normal chromosome number. In humans, these are sperm and oocytes, produced via meiosis. During fertilization, sperm and oocyte fuse to form a diploid zygote, restoring the normal chromosome number.