Fertilization and Early Gestation

Fertilization and Early Gestation Overview

• Topics of discussion include fertilization, oocyte development, species-specific fertility characteristics, and embryonic development stages.

Key Concepts and Definitions

Timeframes of Gestation

• Equine - Gestation lasts approx. 80 days.

• Elephant - Fetal development noted around 16 weeks (approximately 4 months).

Mammalian vs. Chick Egg Structure

• Mammalian Egg

  • Largest single cell in the body.

  • Contains limited yolk; nourished internally by the mother.

• Chick Egg

  • Also a single cell, primarily composed of yolk (vitellus).

  • Contains all nutrients necessary to support embryonic development outside the mother.

Oocyte Development

• The process occurs within a recruited follicle.

Primary Oocyte Characteristics

• Represents the condition most mammals are born with.

• Structure:

  • Comprises cytoplasm and a large nucleus (germinal vesicle).

  • Encased by a single layer of flat follicle cells in primordial and primary follicles.

Structural Components of the Mammalian Egg

• Zona Pellucida

  • A thick glycoprotein coat surrounding the egg's membrane.

  • Functions to protect the egg, enabling sperm selection and preventing polyspermy.

  • Note: Not found in chick eggs which contain only the vitelline membrane.

Final Maturation Process

• Initiated after the LH surge.

• Changes in granulosa cells occur.

• Meiosis resumes in the egg, leading to ovulation.

• Results in the expulsion of the 1st polar body and formation of a secondary oocyte.

Fertilization

• The sperm penetrates cumulus and corona cells, binds to the zona pellucida.

• Acrosome reaction occurs enabling penetration into the egg, blocking further sperm entry.

• Second polar body expelled as the sperm binds to the vitelline membrane.

Formation of a New Organism

• The onset of a new organism is marked by the fusion of the pronuclei, forming one diploid cell.

Fertilization Timing and Mechanism

• Female mammals ovulate after estrus onset and LH surge.

• Eggs typically transported from ovaries after ovulation.

• Egg Lifespan - Varies by species; critical for effective fertilization:

  • Egg lifespan may only last 12-24 hours.

  • Artificial insemination (AI) requires precise timing regarding ovulation to maximize chances of fertilization.

Natural Service vs. Artificial Insemination

• Natural Service

  • Larger ejaculation volume containing many sperm.

  • Some species (e.g., stallion, boar) produce a gelatin plug post-ejaculation to block other mating attempts.

• Natural Service Sites

  • Vagina: cattle, sheep, primates, rodents, horses.

  • Cervix: pigs.

  • Cloaca: in birds.

• Artificial Insemination Sites

  • Cervix to uterus for pigs.

  • Uterus for cattle, sheep, humans.

  • Laparoscopic AI directly into the oviduct for sheep.

Sperm Survival Estimates Post Natural Service

• Sperm viability varies:

  • Human: up to 5 days; average survival 1.4 days for sperm, 0.7 days for eggs.

  • Dogs: 9-11 days.

  • Mares: 4-5 days.

  • Pigs: 2 days.

  • Cattle: 1-2 days.

  • Birds: can last weeks to months.

Reproductive Fidelity and Hybridization

• In species with multiple sires (e.g., litter-bearing species), multiple males can contribute genetically, strengthening litter viability.

• Factors influencing interspecies hybrids include sperm transport compatibility and chromosomal assembly.

• Many interspecies hybrids are often sterile due to incomplete meiosis and degradation of embryos post-fertilization.

Examples of Hybrid Animals

• Livestock Examples:

  • Male donkey (31 chromosomes) mated with a horse (32 chromosomes) = mules.

  • Sheep (54 chromosomes) and goats (60 chromosomes) yield geep.

  • Cattle (60 chromosomes) crossed with bison = beefalo.

• Companion Animal Examples:

  • Dogs (78 chromosomes) can breed with wolves (39 chromosomes) yielding wolfdogs.

• Exotic Species:

  • Lions and tigers (both 38 chromosomes) may produce hybrids.

Gestational Stages and Development

• Divided into three trimesters:

  • 1st Trimester: Embryo stage; organs start developing without identifiable limbs.

  • 2nd Trimester: Fetal stage; recognizable species forms.

  • 3rd Trimester: Growth stage; significant development and growth.

Cleavage Stage of Embryo Development

• Timeframe and processes:

  • 2-cell stage: 18 hours.

  • 4-cell stage: 32 hours.

  • 8-cell stage: 54 hours.

  • Morula: 4 days.

  • Blastocyst: 7 days.

  • Hatching period begins at 8 days.

  • Every 18 hours, cells divide. May reach the uterus in 48-72 hours.

Characteristics of the Blastocyst Stage

• Formed from blastomeres:

  • Inner Cell Mass (ICM): Forms internal organs, glands, skeleton, muscle; involves gap junctions.

  • Trophoblast (trophectoderm): Supports placental membranes, includes neural, sensory organs, and skin; utilizes tight junctions.

  • Contains a Blastocoele Cavity.

Cloning Potential of Blastomeres

• Blastomeres from the 2-8 cell stage are described as totipotent, meaning they can develop into any type of tissue and can be transferred to an empty zona.

Hatching of Blastocyst

• Enzymatic action from trophoblast weakens the zona, allowing hatching:

  • Specific hatching days vary by species:

  • Pig and human: Day 6-7.

  • Sheep and horse: Day 8.

  • Cow: Day 9.

  • Intact zona protects the blastocyst; after hatching, it becomes more vulnerable.

Embryo Expansion Post-Hatching

• After hatching, blastocysts undergo expansion:

  • Shape changes from spherical to elongated, species-specific timing:

  • Elongated blastocysts in pigs, sheep, and cattle from days 11-21.

  • Horses display spherical but larger embryos.

  • Humans and other primates do not undergo PGF2α release.

Pregnancy Signals by Species

• Pregnancy signals prevent prostaglandin release, maintaining corpus luteum production of progesterone.

  • Some species do not release any signals post-conception.

Attachment, Implantation, and Placentation

• Embryo ceases locomotion, the trophoblast binds to the endometrium, initiating placentation.

  • Implantation times vary by species:

  • Rodents: 4-5 days.

  • Rabbit: 6-7 days.

  • Pig: 13-14 days.

  • Cow: 19-30 days.

  • Sheep: 15-16 days.

  • Human and other primates: 8-20 days.

  • Horse: 30-37 days.

  • Placentation increases nutrient and gas exchange.

Embryo and Pregnancy Losses

• Notable losses occur in early gestation:

  • Pigs: 30% of embryos lost.

  • Cattle: 15%.

  • Women: 15% miscarriage rate.

Conceptus Definition

• The conceptus includes the embryo/fetus along with the extra-embryonic membranes, serving functions of protection, hydration, and space for growth.

Extra-Embryonic Membranes

• Amnion: Innermost membrane; fluid-filled.

• Allantois: Often fuses with chorion; fills with fluid to assist in waste and gas exchange.

• Chorion: The outer membrane attaching to the uterus.