[HD 201] E01-T06-Fertilization and Implantation_compressed

• The topic is fertilization and implantation in human development.

Transfer of Gametes

• Oocyte transport involves a secondary oocyte arrested at metaphase II during gametogenesis.
• Follicle-stimulating hormone (FSH) causes follicles to undergo development.
• The dominant follicle, with the highest sensitivity to FSH, becomes a secondary oocyte while the rest undergo atresia.
• The macula pellucida or stigma forms as an avascular area on the ovary due to cessation of blood flow, indicating pending ovulation.
• The dominant follicle has the lowest threshold for FSH and develops into a secondary oocyte.
• During ovulation, fimbriae of the fallopian tube contract to catch the released secondary oocyte.
• The oocyte travels via cilia movement to the ampulla for fertilization.
• Secondary oocytes survive for 24 hours.
• The oocyte is enveloped by the corona radiata and zona pellucida.
• The egg is released from the Graafian follicle.
• The luteinizing hormone (LH) surge activates collagenase production.
• Collagenase breaks down collagen fibers in the tunica albuginea of the ovary.
• Smooth muscles contract to aid in the breakdown of the ovarian surface and expulsion of the oocyte.
• Females have a defined number of oocytes at birth.
• Follicles develop due to FSH.
• Developing follicles produce estrogen and inhibin.
• High estrogen and inhibin levels have a negative feedback effect on FSH.

Sperm Transport

• Sperm pathway includes seminiferous tubules, epididymis, vas deferens, seminal vesicle, ejaculatory duct, prostatic urethra, membranous urethra, spongy urethra, to the external urethral orifice.
• Seminiferous tubules are the site of sperm production.
• The epididymis is for sperm maturation.
• Seminal vesicles contribute 70% of semen.
• Ejaculation has 2 phases:
  • Emission of semen to the prostatic urethra.
  • Expulsion of semen from the spongy urethra to the external urethral orifice.
• Emission and ejaculation are sympathetic responses; erection and lubrication are parasympathetic.
• Sperm motility upon deposition in the vagina is affected by the flagellum which makes forward propulsions, prostaglandins secreted by the seminal gland which contract the uterine cavity, and pH.
• Sperm moves faster in a basic environment.
• Sperm head contains the condensed nucleus, acrosome, and plasma membrane.
• The acrosome has hydrolytic enzymes before fertilization.
• Chemoattractants guide sperm to the correct fallopian tube.
• Sperm must penetrate the corona radiata and zona pellucida.

Fertilization - Sperm Activation Events

• Cannot occur until capacitation and acrosome reaction have taken place.

Capacitation

• Occurs in sperm in the uterus and uterine tubes.
• Takes 7 hours, involves biochemical changes.
• Sperm membrane destabilizes by removing glycoprotein and seminal proteins and reorganizing lipids and proteins.
• This results in sperm being more active.

Acrosome Reaction

• Occurs when sperm contact the zona pellucida.
• Sperm attach to the ZP3 receptor.
• Perforations occur in the acrosomal membrane, releasing enzymes to digest the zona pellucida and facilitate fertilization.

Phases of Fertilization

• Cortical reaction happens before fertilization.
• Oocyte releases granules, releasing lysosomal enzymes that kill other penetrating sperms.
• Lysosomal enzymes change the zona pellucida to prevent polyspermy by hardening the zona pellucida.
• Fusion of plasma membranes of the oocyte and sperm occurs.
• The head and tail of the sperm enters.
• Oocyte completes 2nd meiotic division.
• Male and female pronuclei fuse to create a zygote, restoring the full set of 46 chromosomes, and determines the sex of the fetus.

Assisted Techniques in Fertilization

• Include in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI), and gamete intrafallopian transfer (GIFT).

In Vitro Fertilization

• Step 1: Ovaries are stimulated by injected gonadotropins.
• Step 2: Mature oocytes are extracted vaginally.
• Step 3: Oocytes are placed in a petri dish, capacitated sperm are introduced for fertilization, and the zygote is cultured.
• Step 4: A 4-8 cell stage embryo is transferred through the cervix into the uterine cavities.

Intracytoplasmic Sperm Injection

• An egg is directly injected with a single sperm to form an embryo which is then implanted.

Gamete Intrafallopian Transfer

• Several oocytes and sperms are placed into the fallopian tube for fertilization to occur inside the tube.