Reproductive System and Menstrual Cycle

Definition: Hemaphroditism is a reproductive strategy in which an organism has both male and female reproductive organs.
Reproductive Behavior:
- Although possessing both sexes, hemaphroditic animals usually seek out partners of the opposite sex to facilitate genetic diversity.
- In scenarios where a mate is unavailable, these organisms may resort to self-fertilization.

External Structures:
- External Urethra: The region responsible for expelling urine.
- Labia Majora and Labia Minora:
- Labia Majora: The outer fold which provides protection and contains glands for lubrication.
- Labia Minora: The inner fold that helps protect internal organs and directs urine flow.
- Both labia sets serve protective functions against bacteria and injury.

Protection: Shields internal reproductive organs and a developing fetus.
Lubrication: Provided primarily by the labia majora through associated glands.
Sensitivity: The labia minora is sensitive, often undergoing expansion during sexual activity due to the presence of terminal nerves.

Physical Structure: A small region with extensive internal structure, equipped with nerve endings.
Sensitivity: Contains approximately 10,000 nerve endings, primarily concentrated in the glans, making it highly sensitive during arousal.

Gonads:
- Function: The ovaries, which produce gametes (oocytes).
- Components: Each female possesses two ovaries producing oocytes and hormones crucial for the menstrual cycle.

Primary Function: Produce mature oocytes (eggs) and hormones.
- Hormones Produced: Estrogen (primarily estradiol) and progesterone, both vital for the menstrual cycle and secondary characteristics.

Role: Collect eggs from ovaries post-ovulation and serve as the site where fertilization occurs.
Fate of Egg: If fertilization occurs, a zygote is formed; if not, the egg disintegrates.

Function: The site where a fertilized egg may implant and develop.
Structure: Consists of the uterine cavity and endometrium (the uterine lining).
Menstrual Cycle Relation: The endometrium thickens and sheds during the menstrual cycle.

Role: Protects the uterus and vaginal passage, produces mucus with antimicrobial properties to support and lubricate.

Female Egg Production: Females generally produce one mature egg monthly, contrasting with males who produce sperm continuously.
- Oocyte Lifespan: Females are born with all the oocytes they will have throughout life, estimated at 0.5 to 1 million per ovary at birth, reducing to about 250,000 at puberty.
Hormone Functions:
- Estrogen aids in the development of primary and secondary sexual characteristics and helps regulate the menstrual cycle.
- Progesterone is crucial in preparing the body for potential pregnancy.

Oogenesis: The production of oocytes involves several phases, with oocytes undergoing meiotic division to eventually become the mature ovum.
Egg Development Cycle:
- Ovarian follicles containing oocytes mature through stages:
- Primary Follicle to Secondary Follicle
- During ovulation, the mature follicle ruptures to release the oocyte.
- If fertilized, this becomes the ovum.

Follicular Phase (Days 0-14):
- Development of follicles stimulated by FSH (Follicle Stimulating Hormone) leading to a rise in estrogen levels which prepares the endometrium.
- Menstruation occurs if fertilization does not take place, leading to the rebuilding of the uterine lining.
Ovulation: The surge of LH (Luteinizing Hormone) leads to the release of the mature egg.
Luteal Phase (Days 14-28):
- Post-ovulation stage where the corpus luteum forms and secretes progesterone, thickening the uterine lining in preparation for potential pregnancy.
- If fertilization does not occur, the corpus luteum degenerates into corpus albicans, leading to decreased progesterone and the eventual shedding of the uterine lining, thus starting a new menstrual cycle.

Definition and Duration: Menstruation is the shedding of the uterine lining, typically lasting 3 to 5 days.
Hormonal Changes: A drop in hormone levels leads to the shedding process.

Viability of the Oocyte: The mature egg is viable for 12 to 24 hours post-ovulation.
Sperm Lifespan: Sperm can survive in the female reproductive tract for 5 to 6 days, creating a ‘fertile window’ of 5 to 7 days around ovulation.

Fertilization: Occurs in the fallopian tubes when a sperm meets the egg; successful fertilization completes the final steps of oogenesis and transitions the oocyte to ovum.