Animal Reproduction and Human Reproductive Biology

Asexual Reproduction: Produces genetically identical clones; advantageous in stable environments or when finding a mate is difficult, but carries extinction risks if environmental conditions change.
- Fragmentation: An organism breaks into pieces, with each piece regenerating a complete organism (e.g., planarians, sea stars with a portion of the central disc, airplane plants, cactuses, chicken and hens).
- Budding: New individuals develop as buds from an original body portion or bulb.
- Parthenogenesis: An unfertilized egg develops into a complete, genetically identical individual without sperm.
Hermaphroditism:
- Organisms possess both male and female reproductive systems.
- Options include self-fertilization (offering no genetic variability) or reciprocal gamete exchange with another hermaphrodite.
Sex Determination Systems:
- Chromosomal: Standard male ( $XY$ ) and female ( $XX$ ) systems in most organisms; birds and reptiles utilize the $ZZ$ (male) and $ZW$ (female) systems.
- Temperature-Dependent: Nesting temperatures determine offspring sex ratios in certain species (e.g., crocodiles, sea turtles); deeper, cooler nest positions shift male versus female proportions.
- Sequential Hermaphroditism: Intermittent sex switching throughout a lifetime (e.g., oysters start as male, transition to female, hatch an egg clutch, and die).

Reproductive Strategies:
- External Fertilization: Gametes are released into a liquid environment without mate selection or courtship; produces large numbers of offspring with minimal parental investment to offset high predation in hostile environments.
- Internal Fertilization: Sperm fertilizes the egg inside the female body; involves varying levels of species-specific parental investment.
Modes of Development:
- Oviparity: Fertilized eggs enclosed in rigid (calcium carbonate) or leathery shells are laid outside the body, nourished entirely by yolk (e.g., birds, amphibians, reptiles, fish).
- Ovoviviparity: Fertilized eggs are retained inside the female body, where embryos feed on yolk before birth (e.g., sharks, vipers, bony fish).
- Viviparity: Live offspring develop inside the womb and receive direct nourishment via a placenta (e.g., placental mammals).
Anatomical Structures:
- Placenta & Umbilical Cord: The placenta performs exchange of oxygen, nutrients, and waste products; the umbilical cord connects the placenta directly to the fetal bloodstream.
- Cloaca: A single shared opening for digestive, excretory, and reproductive functions in non-mammalian vertebrates.
- Mammalian Structure: Separate vaginal, urethral, and anal openings. Uterine structures vary from a single uterus (humans) to multichambered bicornuate uteruses (rabbits).

Gametogenesis:
- Spermatogenesis: Production of sperm inside testicles; held externally in the scrotum in many mammals for temperature regulation below normal body temperature. Sperm consist of a head with an acrosome, a midpiece containing mitochondria, and a flagellum tail.
- Oogenesis: Production of ova in ovaries; female germ cell pool is fixed during embryonic development and declines over time until menopause.
Female Reproductive Cycles:
- Ovarian Cycle: Maturation and ejection of an ovum from an ovarian follicle; the remaining follicular structure forms the corpus luteum, secreting progesterone.
- Menstrual Cycle: Controlled by $FSH$ , $LH$ , estrogen, and progesterone. Day 1 is marked by the onset of menses (sloughing of the functional uterine layer), followed by tissue regrowth and vascularization.
Hormonal Aging and Decline:
- Menopause & Perimenopause: Perimenopause starts around age 40 to 45 and lasts 10 to 15 years, leading to complete cessation of female fertility.
- Testosterone Decline: Male testosterone levels regulate brain development, muscle mass, secondary sex traits, and libido; levels gradually decrease with age during the male climacteric.

Stages of Early Development: Fertilization in the fallopian tube $\rightarrow$ Cleavage $\rightarrow$ Morula $\rightarrow$ Blastocyst $\rightarrow$ Uterine Implantation.
- Ectopic & Intra-Abdominal Pregnancy: Implantation occurring outside the uterine cavity (such as in fallopian tubes or abdominal cavity).
Trimesters & Teratogens:
- First Trimester: Fundamental body structures form; highly vulnerable to teratogens (e.g., thalidomide exposure during limb formation causes flipper-like limb defects).
- Second & Third Trimesters: Organ system maturation and rapid fetal growth.
Labor and Delivery: Driven by oxytocin in a positive-feedback loop, causing uterine contractions that dilate the cervix to $10\,\text{cm}$ , culminating in delivery of the fetus and afterbirth (placenta).
Contraception & Sexually Transmitted Infections (STIs):
- Abstinence is the only 100% effective contraceptive method; surgical methods (vasectomy, tubal ligation) are highly effective but can occasionally revert.
- Protist and bacterial STIs are treatable with antibiotics, whereas viral STIs are permanent and incurable.

Role of the Umbilical Cord:
- Connects the placenta directly to the fetal bloodstream (entering near the aorta) to supply oxygen and nutrients while eliminating carbon dioxide and waste.
Hormonal Birth Control & Cycle Suppression:
- Hormonal contraceptives keep the uterine lining at a thin basilar layer without growth or sloughing, preventing pregnancy and menses.
Mammography Experience:
- Involves diagnostic compression on breast tissue; recommended starting around age 40 to screen for breast cancer risks (which increase to 1 in 8 by age 75).
Pregnancy Risk from Pre-Ejaculatory Fluid:
- Pre-ejaculatory fluid can contain viable sperm remaining from previous ejaculations (which can survive up to three days in the male ductal system), making pregnancy possible without full ejaculation.