Capacitation and Fertilization Timeline

Capacitation is the process that sperm undergo to gain the ability to fertilize an egg.

Overlay of embryological and fetal periods on a timeline.
- Gestational period is from the last menstrual period until parturition.
- Pregnancy is defined from fertilization of the egg until parturition, referred to as carrying the conceptus.
- Gestation is approximately 280 days.
- Average pregnancy duration is approximately 266 days, often cited as 270 days in texts.
- Calculated by adding 14 days of the follicular phase to pregnancy duration.
Breakdown of pregnancy into trimesters:
- First trimester: Weeks 1 to 12.
- Second trimester: Weeks 13 to 26.
- Third trimester: Weeks 27 to 40.
- Embryonic period: First eight weeks, subdivided into pre-embryonic (0-2 weeks) and embryonic (2-8 weeks).
Fetal period: Duration of the pregnancy after the embryonic period.

Gestational Age vs. Fertilization Age:
- Gestational age includes the two weeks before fertilization, while fertilization age starts counting from the day of fertilization.

Estrogen levels rise leading to physiological changes:
- High levels of estrogen from the ovaries lead to the degradation of the cervical mucus plug.
- Vaginal canal pH becomes less acidic, increasing in alkalinity.
Menstrual phase leading to the proliferative phase of the uterine lining.
Ovulation: Occurs when the egg is released, having approximately 24 hours to be fertilized.

Production of sperm occurs continuously; they are stored in the tail of the epididymis.
Sperm lifespan post-ejaculation: typically 48 hours up to 6 days depending on conditions.
- Ejaculate contains approximately 200 to 400 million sperm per milliliter, with a typical volume ranging from 1 to 5 milliliters.
Sperm can fertilize if intercourse occurs:
- Up to 2 days prior to egg ovulation or within 24 hours after.
- Sperm can reach the egg within 30 minutes of ejaculation.

Fertilization window spans:
- Two days before ovulation to 24 hours post-ovulation.
Must achieve a coordinated event of sperm viability and egg availability.

Capacitation involves the following steps:
- The sperm’s acrosomal cap releases enzymes necessary to penetrate the egg's barriers (zona pellucida and corona radiata).
- Alkaline secretions from the female during ovulation help to facilitate sperm function by weakening the sperm's membrane.
- Cholesterol present in the sperm's plasma membrane makes it resilient; during capacitation, this is degraded to enable successful penetration of the egg.
Physiological changes and challenges for sperm:
- Sperm must navigate through mucus and face a hostile environment (e.g., immune response).
- Sperm experience physical agitation in the uterus, promoting activation and mobility through the release of chemicals from the egg, enhancing chemotaxis.
Only a fraction of the sperm (200-1000 out of billions) reach the egg, illustrating the challenges faced in natural conception.
Importance of timing for fertilization with respect to the female’s menstrual cycle and sperm viability indicates challenges in fertility and assisted reproductive technologies.