HG - week 5
Development and Sex Determination
Human Reproductive Systems
- Gamete: Unfertilized sex cell.
- Gonads: Organs that produce gametes - testes (male) and ovaries (female).
- Oocyte: Immature egg cell.
- Spermatocyte: Immature sperm cell.
- Zygote: Fertilized egg.
Male Reproductive System
- Structures:
- Testes: Produce sperm.
- Scrotum: External pouch containing testes.
- Duct system: Transports sperm.
- Glands: 3 types (seminal vesicles, prostate gland, bulbourethral gland) aid sperm motility and viability.
- Penis: Used for sexual intercourse.
Male Reproductive System - Structure
- Side View Overview:
- Bladder, Anus, Bulbourethral gland, Prostate gland, Urethra, Erectile tissue, Penis, Testis, Scrotum, Vas deferens, Epididymis.
- Front View Overview:
- Includes seminal vesicles and other components as listed above.
Spermatogenesis
- Occurs in the seminiferous tubules of the testes.
- Initiated at puberty; ongoing with 100’s of millions produced daily.
- Process:
- Primary spermatocytes undergo meiosis to form secondary spermatocytes.
- Second meiosis results in haploid spermatids, which then mature into spermatozoa via spermiogenesis.
Spermiogenesis
- Transformation of spermatid into spermatozoa:
- Golgi apparatus packages proteins.
- Centrioles and mitochondria organize for tail formation.
- DNA condenses, acrosome forms.
- Excess cytoplasm is absorbed by Sertoli cells.
Male Sperm Passage
- Sperm travels through the epididymis (where maturation occurs) via peristalsis, taking about 2 weeks to exit.
- Mature sperm passes through the vas deferens, where components from seminal vesicles and glands combine to aid sperm functionality.
Female Reproductive System
- Structures:
- Ovaries: Produce eggs (oocytes).
- Fallopian tubes: Connect ovaries to the uterus.
- Uterus: Nourishes the fertilized embryo.
- Cervix: The lower neck of the uterus.
- Vagina: External opening and birth canal.
Female Reproductive System - Structure
- Includes the uterus, bladder, ovaries, and fallopian tubes among other components.
Oogenesis
- Ceases at fertilization; starts before birth.
- Uneven cell division leads to 1 large egg and multiple smaller polar bodies (2-3).
- Oocyte begins meiosis I during fetal development but pauses until puberty.
- Upon ovulation, primary oocyte completes meiosis I, forming a secondary oocyte that is released.
Oogenesis Process
- Involves primary oocytes, follicle maturation, and oocyte release.
- Follicle develops into a corpus luteum post-ovulation.
Comparison: Spermatogenesis vs Oogenesis
| Characteristic | Spermatogenesis | Oogenesis |
|---|---|---|
| Start Point | Puberty | Before birth |
| Time to Complete | 48 days | 12-50 years |
| Production | Constant production (billions) | One oocyte per month |
| Outcome | 4 functional sperm | 1 large egg, 2-3 polar bodies |
| Total Lifetime | Billions of sperm | 400-500 oocytes lifetime |
Fertilisation to Birth
- Fertilization: Fusion of oocyte and sperm; occurs in upper fallopian tube.
- Zygote formation begins at fertilization.
- Mitosis occurs before the zygote reaches the uterus; becomes a blastocyst 5 days after fertilization and implants into the uterus.
Implantation
- Begins 6-7 days after fertilization with the blastocyst embedding into the endometrium.
- Chorionic villi form the placenta, providing nutrients and waste exchange between mother and embryo.
Development Stages
- First Trimester (Weeks 1-12): Organ systems develop; heartbeat detectable.
- Second Trimester (Weeks 13-26): Bone formation; genitals identifiable.
- Third Trimester (Weeks 27-40): Rapid growth; maturing systems.
Risks to Fetal Development
- Genetic Factors: May cause birth defects.
- Environmental Agents (teratogens) like chemicals, radiation, and viruses can disrupt normal development.
- Alcohol: Leading preventable cause of birth defects (FASD).
Sexual Development
- Genetic sex determined at fertilization.
- Gonadal sex develops primarily due to the presence of the Y chromosome.
- SRY gene: Initiates male development; absence results in female development.
Phenotypic Sex Differences
- Hormonal influence leads to divergence in sexual traits and development of genitalia based on the presence or absence of dihydrotestosterone (DHT).
Conclusion
- Cases of genetic and phenotypic sex divergence exist (e.g., androgen insensitivity, pseudohermaphroditism). An understanding of these factors is crucial in appreciating sexual development and disorders.