Overview of Sexual and Asexual Reproduction

Definition: The production of a new individual by joining two parents’ gametes (haploid reproductive cells).
Process: Male sperm and female ovum fuse to produce a zygote.
Offspring: Genetically different from both parents, which allows for greater genetic variation, enabling faster adaptation to environmental changes and facilitating the elimination of harmful alleles.
Prevalence: Used by most animal species.

Definition: Offspring are produced from a single parent.
Offspring: Genetically identical to the parent.
Three major forms:
- Budding: Portion of the parent organism pinches off to form a complete new individual.
- Regeneration: Complete organism is formed from a fragment of the parent's body.
- Parthenogenesis: Development of offspring from unfertilized eggs. An example includes some female reptiles laying eggs that hatch without being fertilized (e.g., Komodo dragon).

Where: Gametes are formed in gonads.
- Males: Testes
- Females: Ovaries
Germ Cells: Multiply by mitosis, resulting in spermatogonia (2n) in males and oogonia (2n) in females.
Development: Some spermatogonia become primary spermatocytes and some oogonia divide to become primary oocytes.
Meiosis: Primary spermatocytes (2n) and primary oocytes (2n) undergo meiosis to form haploid gametes (sperm and egg).

Process Description:
- A spermatogonium (diploid cell) replicates its chromosomes to become a primary spermatocyte.
- Primary spermatocyte undergoes meiosis I to form secondary spermatocytes (haploid).
- Secondary spermatocytes undergo meiosis II to produce spermatids (haploid), which mature into sperm cells.
Structure of Sperm:
- Components: Head (with acrosome), midpiece, and tail.

Process Description:
- Oogonia (diploid) divide by mitosis to produce primary oocytes, which enter Meiosis I.
- Each primary oocyte develops within a follicle.
- One mature gamete (ovum) is produced from this process.
- In mammals, oogenesis begins in the fetus before birth, where primary oocytes pause in meiosis until puberty.
Upon puberty, some primary oocytes resume development and complete meiosis I, yielding a secondary oocyte and a polar body.

Components:
- Penis and scrotum, where the scrotum holds testes at a temperature 2 degrees Celsius lower than core body temperature for optimal sperm development.
Testis Composition:
- Seminiferous tubules (site of spermatogenesis) and Leydig cells (which secrete testosterone).
- Sertoli cells provide nutrients and protection to developing sperm.

Overview: Sperm released into the lumen of seminiferous tubules, then move to epididymis to complete differentiation, becoming motile and capable of fertilization.
Pathway: Sperm travels through vas deferens to ejaculatory duct and urethra.
Semen Composition: Contains sperm (about 5% of volume) and fluids from seminal vesicles (fructose), bulbourethral glands (alkaline fluid), and prostate gland (alkaline fluid).

Testosterone:
- A hormone from the brain (GnRH from the hypothalamus) tells another part of the brain (anterior pituitary) to release two hormones called LH and FSH.
- LH tells special cells (Leydig cells) to make testosterone.
- Testosterone then:
  - Helps other cells (Sertoli cells and germ cells) make sperm.
  - Helps male reproductive parts grow.
  - Causes male features like facial hair or horns.
Controlling Hormone Levels (Negative Feedback Loop):
- FSH (along with testosterone) helps Sertoli cells and sperm production.
- When there's enough sperm and testosterone, special signals (inhibin and testosterone itself) tell the brain to slow down the release of GnRH, LH, and FSH. This helps keep sperm production at the right level.

Negative Feedback Loop:
- FSH (along with testosterone) stimulates Sertoli cells and spermatogenesis.
- Controlled by negative feedback through inhibin and testosterone, inhibiting the release of GnRH and LH/FSH to regulate sperm production.

External Genitalia:
- Labium majora, labium minora, and clitoris, leading to the vagina, cervix, and into the uterus.
Uterus:
- Has inner glandular lining (endometrium) and outer muscular layer (myometrium). Oocytes develop in ovaries.
Oogenesis:
- Most female mammals are born with all the primary oocytes they will ever have (~1 million at birth, about 200,000 remain at puberty).
- The ovarian cycle lasts approximately 28 days in humans, with one oocyte typically ovulated each cycle. Menopause occurs when oocytes become depleted, and ovulation stops.

External Fertilization:
- Eggs and sperm are released in close proximity, usually in aquatic environments to prevent gametes from drying out. Many fish and invertebrate species exhibit this behavior, usually releasing large numbers of eggs.
Internal Fertilization:
- Sperm deposited within the female reproductive tract during copulation, providing protection for gametes from environmental hazards.
- Indirect sperm deposition involves the production of spermatophores.

Begins with the release of IP3 from the plasma membrane, stimulating calcium ion release from the endoplasmic reticulum, which activates events that inactivate sperm-binding proteins and harden the outer egg coating to block further sperm entry.

The oviduct (fallopian tube) where oocyte is drawn into by fimbriae, facilitates fertilization, and transports the blastocyst to the uterus. The uterine cycle involves a series of phases that prepare the uterus for potential implantation of an embryo.