Study Notes on Reproduction & Embryonic Development
Reproduction & Embryonic Development
D. Weinbrenner (2025)
Chapter 26 - Part 1 Learning Objectives
LO1: Compare asexual, sexual reproduction and its advantages, hermaphroditism. (s3-9)
LO2: Describe evolutionary trends in vertebrate sexual reproduction. (s10-12)
LO3: Describe terminology, male reproductive system, spermatogenesis. (s13-16)
LO4: Describe female anatomy and the ovarian cycle. (s17-19)
LO5: Describe female hormones originations and functions. (s20-21)
LO6: Describe the human female reproductive cycle; compare ovarian to menstrual cycle. (s22-25)
Asexual & Sexual Reproduction
Asexual Reproduction: Results in the generation of genetically identical offspring.
Sexual Reproduction: Results in the generation of genetically unique offspring.
Types of Asexual Reproduction
Fission: Includes binary fission, where an organism splits into two.
Budding: A new organism grows on the parent and can detach.
Fragmentation & Regeneration: An organism can regrow missing parts, which can develop into a new organism if a fragment is detached.
Parthenogenesis: Development from an unfertilized egg.
Advantages of Asexual Reproduction
Normally faster than sexual reproduction.
Beneficial in stable, unchanging habitats.
Beneficial for sparse populations or where individuals do not move.
Sexual Reproduction in Animals
Involves the production of male gametes (sperm) and female gametes (eggs).
Hermaphroditism
Some organisms can reproduce both sexually and asexually.
Examples in plants: root suckers in aspen trees, runners in strawberries, and tubers in potatoes.
Examples in animals: tapeworms, snails, and earthworms.
Hermaphroditic Plants
Dioecious: Plants with male and female reproductive structures on separate individuals (e.g., holly, Ginkgo biloba).
Monoecious: Plants that carry both types on the same individual (e.g., pumpkin).
Hermaphroditic: Having both male and female reproductive organs in the same flower (e.g., peach flower).
Evolutionary Trends in Sexual Reproduction in Vertebrates
Fish, Amphibians, Reptiles, and Mammals:
External fertilization in fish, reliant on water.
Internal fertilization and amniotic egg in reptiles, allowed life on land.
Viviparous development in mammals, with young developing inside the female body and linked to the placenta, leading to live births.
Hormonal Control of the Testes
Hypothalamus: Secretes Gonadotropin Releasing Hormone (GnRH).
Anterior Pituitary Gland: Responds to GnRH producing Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH).
FSH: Stimulates spermatogenesis in testes.
LH: Stimulates androgen production.
Negative Feedback Mechanism: Maintains hormone levels and regulation.
The Human Male Reproductive System
Comprised of:
Testes
Epididymis
Vas deferens
Penis (glans, prepuce)
Bulbourethral gland
Prostate gland
Seminal vesicle
Spermatogenesis in the Human Male
Process: Occurs in the seminiferous tubules of the testes.
Spermatogonial Stem Cells: Diploid cells located near wall of tubules.
Meiosis:
Meiosis I: Produces secondary spermatocytes (haploid).
Meiosis II: Produces mature sperm cells.
Duration: Immature sperm released into the seminiferous tubule, moving to the epididymis for differentiation over a period of approximately 2.5-3 months.
The Human Female Reproductive System
Includes: Ovaries and structures that deliver gametes (oviduct, uterus, cervix, vagina).
Meiosis in the Human Female
Starts during fetal development and continues until ovulation.
Meiosis I completed at around 4 months fetal development.
Primary oocyte remains in prophase I until ovulation.
The Human Ovarian Cycle
Formation of Primary Follicle: Epithelial cells organize around primary oocyte.
Completion of Meiosis I forms secondary follicle containing secondary oocyte and first polar body.
Ovulation: Vesicular follicle bursts to release secondary oocyte, stopping meiosis at metaphase II.
Follicular development into corpus luteum.
Degeneration of corpus luteum if fertilization does not occur.
Hormones in the Human Female Reproductive Cycle
Major Hormones:
Releasing Hormone from the hypothalamus stimulates:
FSH (Follicle Stimulating Hormone) from the pituitary gland.
LH (Luteinizing Hormone).
Estrogen: Produced by ovarian follicles, regulates ovulation and endometrial growth.
Progesterone: Secreted by the corpus luteum, maintains the endometrium to support potential pregnancy.
Known dynamics:
High estrogen levels inhibit pituitary; low levels have the opposite effect.
Comparison: Ovarian Cycle vs Menstrual Cycle
The Ovarian Cycle consists of hormonal developments leading to ovulation.
The Menstrual Cycle involves the cyclical thickening and shedding of the uterine lining (endometrium).
Pregnancy and Implantation
HCG (Human Chorionic Gonadotropin): Produced by implanted embryo to maintain corpus luteum and prevent menstruation.
Chapter 26 - Part 2: Principles of Embryonic Development
Learning Objectives
LO7: Explain the process of fertilization (s29-30).
LO8: Explain cleavage, gastrulation, and tissue formation (s31-35).
LO9: Describe early embryonic development and homeotic gene pattern formation (s36-38).
LO10: Describe early human embryo development (s39-43).
LO11: Describe the 3 trimesters of human embryo development (s44-47).
LO12: Describe labor and birth of human baby (s48-49).
Fertilization Process
Key Elements of Sperm:
Flagellated Tail: For mobility.
Many Mitochondria: For energy production.
Acrosome: Contains enzymes to penetrate the egg.
Fertilization in Sea Urchins
Contact with Jelly Coating: Acrosome releases enzymes.
Acrosomal Enzymes: Digest a hole in the jelly, allowing sperm penetration.
Species-Specific Proteins: Bind to egg receptor proteins for fertilization.
Fusion of Membranes: Sperm nucleus enters egg cell.
Impenetrable Membrane Formation: Egg plasma membrane becomes impervious to additional sperm.
Hardening of Vitelline Layer: Provides further protection.
Nuclei Fusion: Results in a diploid zygote.
Cleavage Phase
Cleavage: Rapid division of the zygote.
Stages: Zygote → Morula → Blastula.
Timeframes: Approx. 10 hours in frogs; 4-5 days in humans.
Gastrulation Phase
Gastrulation: Forms three embryonic layers (ectoderm, mesoderm, endoderm).
Blastopore: Forms by invagination during gastrulation.
Organ Development Post-Gastrulation
Three Layers: Develop into organs:
Ectoderm: Skills, nervous system, etc.
Mesoderm: Muscle, bone, circulatory system, etc.
Endoderm: Digestive and respiratory systems.
Processes in Development
Various processes are crucial:
Embryonic Induction: Signals that direct cell differentiation.
Cell Migration: Movement of cells to their respective locations.
Apoptosis: Programmed cell death critical for shaping structures (e.g., digit separation in fetal hands).
Homeotic Genes in Development
Homeotic Genes: Regulatory genes that control the development of structures.
Code for transcription factors essential in determining organ identities and placements.
Early Human Embryo Development
Pregnancy Overview: Development timeline begins with ovulation and fertilization, resulting in live birth after several stages (three trimesters).
Embryonic Development Stages
First Month: Formation of the embryo and placenta.
Cleavage, blastocyst stage, and implantation in the endometrium.
**Formation of extra-embryonic membranes (yolk sac, chorion, amnion).
Trimester Overview
First Trimester (Week 1-12): Major organs form, is considered a fetus after week 8.
Second Trimester (Week 13-26): Continued development and growth of the fetus.
Third Trimester (Week 27-Birth): Rapid growth; fetal viability increases sharply; maturation of systems in preparation for birth.
Average fetal size around 20 inches and weight typically between 6-8 lbs.
Childbirth Process
Labor Phases:
Dilation of the Cervix: Widening of the birth canal.
Expulsion: Delivery of the infant.
Delivery of Placenta: Final stage post-birth.