D3.1 Reproduction

Sexual vs Asexual Reproduction

Reproduction is essential for the continuation of a species. It occurs in two primary forms: sexual and asexual reproduction.

• Sexual Reproduction: Involves the fusion of male and female gametes to form a zygote. It results in offspring that are genetically distinct from their parents. Promotes genetic variation, which is crucial for adaptation and evolution in changing environments.
• Asexual Reproduction: Involves a single parent producing offspring that are genetically identical to itself (clones). It is advantageous in stable environments as it allows for rapid population growth without the need to find a mate. Examples include binary fission in bacteria, budding in yeast, and vegetative propagation in plants.

Interactive Vocabulary

Human Male Reproductive System

The male reproductive system is designed to produce, maintain, and transport sperm (the male reproductive cells) and protective fluid (semen).

• Testes: Produce sperm and testosterone. They are housed in the scrotum to keep them at a slightly lower temperature than the body, which is optimal for spermatogenesis.
• Epididymis: A coiled tube where sperm mature and are stored until ejaculation.
• Vas Deferens: Transports mature sperm to the urethra.
• Seminal Vesicles, Prostate Gland, Bulbourethral Gland: Add fluids to the sperm to create semen. These fluids provide nutrients (like fructose), neutralize the acidity of the vaginal tract, and facilitate sperm motility.

Human Female Reproductive System

The female reproductive system is designed to produce eggs (ova), receive sperm, provide an environment for fertilization, and support the development of a fetus.

• Ovaries: Produce eggs and the female sex hormones estrogen and progesterone.
• Fallopian Tubes (Oviducts): Transport the egg from the ovary to the uterus. This is typically where fertilization by a sperm cell occurs.
• Uterus (Womb): A hollow, pear-shaped organ where a fertilized egg implants and develops into a fetus. The inner lining, the endometrium, thickens each month in preparation for a potential pregnancy.
• Cervix & Vagina: The cervix is the lower part of the uterus that opens into the vagina, the canal that receives the penis during intercourse and serves as the birth canal.

Gametogenesis (HL)

Gametogenesis is the process by which diploid precursor cells undergo meiotic division to become haploid gametes (sperm and egg).

Spermatogenesis:

• Occurs in the seminiferous tubules of the testes.
• Starts at puberty and continues throughout a male's life.
• Spermatogonia (diploid) divide by mitosis to maintain a stem cell population. Some differentiate into primary spermatocytes.
• Meiosis I produces two secondary spermatocytes (haploid).
• Meiosis II produces four spermatids, which differentiate into mature spermatozoa.

Oogenesis (HL)

Oogenesis:

• Occurs in the ovaries.
• Starts during fetal development. Oogonia divide by mitosis and begin meiosis I, but arrest in prophase I as primary oocytes within primordial follicles.
• At puberty, under the influence of FSH, a few follicles resume development each month. One completes meiosis I just before ovulation, forming a secondary oocyte and a small polar body.
• The secondary oocyte arrests in metaphase II. Meiosis II is only completed if fertilization occurs, yielding a mature ovum and a second polar body.

Hormonal Control of Reproduction

Reproduction is tightly regulated by hormones, primarily governed by the Hypothalamic-Pituitary-Gonadal (HPG) axis.

• GnRH (Gonadotropin-Releasing Hormone): Secreted by the hypothalamus; stimulates the anterior pituitary.
• FSH (Follicle-Stimulating Hormone) & LH (Luteinizing Hormone): Secreted by the anterior pituitary; regulate the gonads.

The Menstrual Cycle

A complex interaction of hormones regulates the roughly 28-day cycle in females.

• Follicular Phase: FSH stimulates follicle development in the ovary. The growing follicle secretes estrogen.
• Ovulation: A surge in estrogen causes a positive feedback loop leading to an LH surge, which triggers the release of the secondary oocyte.
• Luteal Phase: The ruptured follicle becomes the corpus luteum, which secretes progesterone and some estrogen. Progesterone maintains the endometrial lining.
• Menstruation: If fertilization does not occur, the corpus luteum degenerates, hormone levels fall, and the endometrium is shed.

Fertilization and Pregnancy (HL)

Fertilization occurs when a sperm fuses with an egg, initiating development.

• Acrosome Reaction: Enzymes released from the sperm head digest the zona pellucida of the egg.
• Cortical Reaction: Once one sperm fuses, cortical granules in the egg release enzymes that harden the zona pellucida, preventing polyspermy.
• hCG (Human Chorionic Gonadotropin): Early embryo secretes hCG, which maintains the corpus luteum so it continues producing progesterone, maintaining the pregnancy until the placenta takes over.
• Placenta: Facilitates the exchange of materials between maternal and fetal blood (oxygen, nutrients, waste). Produces estrogen and progesterone in later pregnancy.

Birth and Positive Feedback (HL)

The process of birth (parturition) is mediated by positive feedback mechanisms.

• As the fetus grows and the head pushes against the cervix, stretch receptors are stimulated.
• This signals the posterior pituitary to release oxytocin.
• Oxytocin stimulates contractions of the uterine smooth muscle.
• More contractions cause more pressure on the cervix, releasing more oxytocin. This cycle continues until birth.