Pregnancy

The process begins with the oocyte, where fertilization can occur.
Sperm is visualized at high power, showing multiple sperm competing to fertilize the egg.
- Typically, only one sperm should successfully enter the oocyte.

Fertilization Interaction: When the first sperm enters the egg, it must initiate mechanisms to prevent other sperm from entering.
- Fast Block:
- Triggered by a wave of calcium ions across the egg membrane.
- This wave alters the electrical properties of the cell membrane, momentarily stopping other sperm from entering.
- Slow Block:
- Involves the release of contents from cortical granules due to the calcium wave.
- Changes the physical properties of the zona pellucida, making it impenetrable to further sperm.
- Enzymatic actions result in cross-linked proteins that prevent sperm entry.

Fraternal Twins:
- Occurs when two oocytes are ovulated, each fertilized by different sperm, resulting in genetically unique siblings.
- Possible combinations: male-female, male-male, female-female.
Identical Twins:
- Results from a single fertilization event where the zygote splits into two cells, each developing into an independent fetus.
- All identical twins share the same genetic material.
- If the split occurs at an early stage, it can lead to more than two identical siblings (e.g., triplets).

Development begins from one or two cells capable of becoming an entire organism.
These stem cells have unlimited potential to differentiate into various cell types.

The process from oocyte to zygote and eventually to a blastocyst capable of implantation involves multiple cellular stages.
The remaining follicle transforms into the corpus luteum while the oocyte travels towards fertilization, approximately during the luteal phase.

Luteinizing Hormone (LH): Initiates ovulation and regulates the corpus luteum (CL).
Human Chorionic Gonadotropin (hCG): Released by the embryo post-fertilization.
- hCG binds to the CL, maintaining its function to produce progesterone.
- Maintains the uterine lining and prevents the normal termination of the luteal phase, resulting in a missed menstrual period.

Implantation typically occurs around days 8-10 of the luteal phase, in the context of a 14-day cycle.
The corpus luteum produces progesterone to maintain the endometrium readiness for the embryo.
After fertilization and implantation, the embryo signals its presence via hCG, which prevents the degradation of the endometrium.

The CL has a programmed lifespan of about 14 days without hCG.
In other species (e.g., pigs, horses), maintaining the CL is different; it relies on mechanisms that prevent its breakdown.

Adaptations occur in various organ systems during pregnancy:
- Increased appetite and nutrition absorption to support fetal growth.
- Water and sodium retention lead to swelling; however, these changes are crucial for improved blood flow in fetal development.

The embryo burrows into the endometrium, establishing a blood supply connection.
There are variations in placental structures among species:
- Discoidal: One large contact point (humans).
- Cotyledonary: Multiple contact points (cows, sheep).
- Diffuse: Equal distribution of connections (pigs, horses).

Key hormones influencing pregnancy include hCG, progesterone, relaxin, and others.
- hCG maintains CL function initially, followed by the placenta taking over progesterone production as pregnancy progresses.
- Prolactin supports breast tissue development in preparation for lactation.
- Cortisol assists in system development such as the fetal GI tract and lung surfactant production late in pregnancy.

Estrogen and cortisol levels rise toward the end of pregnancy, triggering various physiological changes preparing for labor.
- Estrogen increases oxytocin receptor expression and gap junction formation within the myometrium, allowing synchronized contractions during labor.
- Positive feedback loops involving oxytocin enhance uterine contractions, ultimately leading to delivery.

The cervix begins to soften and dilate in preparation for the fetus' passage. This dilation is critical to allowing the passage of the fetus during birth.

Multiple changes occur physiologically in maternal systems throughout pregnancy that support fetal development and prepare for labor, with a critical interplay of hormones signaling changes at various stages.
Exploration of birth processes across species highlights variations in the anatomy and physiology involved in gestation and labor.