Chapter 28F: Female Reproductive System: Uterus and Fertilization

For fertilization to occur, it must happen within $12$ to $24$ hours of the secondary oocyte entering the uterine tube (fallopian tube).
The ideal location for fertilization is in the area of the fallopian tube between the ampulla and the isthmus.
The uterine tube provides nutrients, including lipids and glycogen, to both the pre-embryo and sperm cells.
Secretions from PEG cells within the uterine tube contribute to nutrient provision and help complete capacitation of sperm.

The uterus is a primary structure of the female reproductive system.
Functions: Provides nutritional support, mechanical protection for the embryo and fetus, and waste removal.
Size and Shape: Surprisingly, it is not very big, measuring approximately $3$ inches by $2$ inches, and is pear-shaped.
Position: Typically bends anteriorly (towards the front) near its base.
- About $20\%$ of females have a retroflexed uterus, which bends backward; however, this has no clinical significance as it will bend forward during childbirth.
Suspensory Ligaments: Provide structure and support for the uterus.
- Uterosacral ligaments
- Round ligaments
- Cardinal ligaments

Fundus: The most superior, rounded portion of the uterine body, towards the head of the female (the 'big part' of an inverted pear).
Body: The main central portion of the uterus, extending inferiorly from the fundus.
Isthmus: The inferior portion of the uterine body, marking the transition to the cervix.
Cervix: The inferior portion of the uterus, extending from the isthmus to the vagina. Its distal end projects about $0.5$ inch into the vagina.
- External Os (External Orifice of the Uterus): The opening surrounded by the distal end of the cervix, leading into the cervical canal.
- Cervical Canal: A constricted passageway that opens to the uterine cavity of the body.

Blood Supply:
- Primarily delivered by the uterine arteries, which branch from the internal iliac artery.
- Also supplied by the ovarian artery, which originates from the abdominal aorta.
- The extensive blood flow is crucial to support embryo development.
Nerves:
- Provided by autonomic fibers from the hypogastric plexus (both superior and inferior).
- Also receives innervation from sacral segments $S3$ and $S4$ of the vertebrae.
- Segmental Block (Epidural): An anesthetic procedure common during labor to relieve pain (analgesic). It targets the lower thoracic vertebrae towards $L1$ by inserting a needle into the spinal column, effectively numbing the body from that point below. Bupivacaine is a common drug used in epidurals.

Perimetrium: The outermost layer, which is a serous layer continuous with the peritoneal lining of the abdomen. It covers the fundus and the posterior surface of the uterine body.
Myometrium: The middle and thickest portion of the uterine wall, constituting about $90\%$ of the uterus's mass.
- Composed of longitudinal, circular, and oblique layers of smooth muscle.
- This muscular layer provides the strength needed to contract during childbirth.
Endometrium: The inner glandular and vascular tissue layer, making up about $10\%$ of the uterine mass.
- It supports the physiological demands of a growing fetus.
- Uterine glands open onto the endometrial surface and extend deep into the tissue.
- Role of Estrogen: Estrogen causes periodic changes in the uterine glands, blood vessels, and epithelium of the endometrium, correlating with the phases of the monthly uterine cycle.

The uterine cycle is a repeating series of changes in the endometrium.
Average Duration: $28$ days, but can range from $21$ to $35$ days.
It responds to the hormones of the ovarian cycle.
Phases:
- Menses and Proliferative Phase: Occur during the ovarian follicular phase (pre-ovulation).
- Secretory Phase: Occurs during the ovarian luteal phase (post-ovulation).