Animal Reproduction and Development Flashcards

Pregnancy and Birth in Mammals

• Pregnancy (gestation): Embryo or fetus grows within the uterus.
• Gestation length varies among species.
  • Mice: 16 to 21 days
  • Humans: 268 days
  • Elephants: 660 days
• Placenta: Connects fetus to the uterine wall for nutrient and gas transfer.

Implantation

• Blastocyst embeds in uterine endometrium.
• Placenta develops from embryonic tissues and endometrium.

Placenta

• Connects fetus and mother, providing nutrients.
• Maternal and fetal portions (chorion).
• Fetal blood flows in chorionic villi; substances are exchanged via diffusion.

Birth or Parturition

• Initiated by hormones and other factors.
• Estrogen promotes gap junction formation and enhances uterine sensitivity to oxytocin.
• Oxytocin stimulates uterine contractions (positive feedback loop).

Hormonal Control of Parturition

• Estradiol readies uterus for oxytocin.
• Fetus's head against cervix activates sensory neurons, stimulating hypothalamus.
• Hypothalamus secretes oxytocin from posterior pituitary.
• Oxytocin stimulates stronger contractions.
• Prostaglandins enhance contractions.

Birth (Labor)

• Dilation and thinning of the cervix.
• Uterine contractions push fetus through cervix and vagina.
• Placenta detaches after birth.

Embryonic Development

• Fertilized egg transforms into an animal with distinct systems and body parts.
• Stages: Fertilization, Cleavage, Gastrulation, Neurulation, Organogenesis

Cleavage

• Initial cell cycles occur without cell growth.
• Time of cell cycles varies.
  • Amphibians = 20 minutes
  • Mammals = 12 hours

Early Divisions

• Cleavage begins within 24 hours of fertilization.

Blastocyst Forms

• Trophectoderm: outer layer, develops into the placenta.
• Inner cell mass: develops into the embryo.

Cleavage and Implantation

• Blastocyst hatches from zona pellucida upon reaching the uterus to prevent premature adhesion in the oviduct.
• Hatched blastocysts then implant into the uterus.

Implantation Begins

• Blastocyst attaches to the uterine lining and begins to sink into it.
• Releases HCG (human chorionic gonadotropin); maintains corpus luteum.

Gastrulation

• Blastocyst reorganizes into a gastrula.
• Formation of germ layers:
  • Ectoderm (outer layer)
  • Mesoderm (middle layer)
  • Endoderm (inner layer)

Cell Layers in the Gastrula

• Ectoderm: forms epidermis and nervous system.
• Mesoderm: forms heart, limbs, muscles, kidneys, blood, connective tissues, and notochord.
• Endoderm: forms epithelial lining of gut, liver, pancreas, thyroid, lungs, and bladder.

Neurulation

• Formation of the central nervous system (CNS).
• Neural tube (derived from ectoderm):
  • All neurons and their supporting cells in the CNS are derived from the neural tube
• Neural crest:
  • All neurons and their supporting cells in the peripheral nervous system (PNS) are derived from neural crest

Organogenesis

• Process of organ formation.
• Most organs contain all four tissue types.
• Organs develop during or after neurulation and become functional at different times.
• Heart is first functional organ.
• Lungs function shortly after birth.

Infertility

• 5 to 10% of US adults are infertile.
• Male infertility: issues with sperm count or quality.
• Female infertility: inability to become pregnant or maintain a pregnancy.

Causes of Infertility

• Sexually transmitted diseases (STDs)
• Developmental disorders
• Polycystic ovary syndrome (PCOS)
• Malnutrition
• Unexplained infertility in 25% of cases

Infertility Treatments

• Assisted reproductive technologies (ART)
• Intrauterine insemination (IUI)
• In vitro fertilization (IVF): Sperm and egg are harvested, fertilized in a culture, and the embryo is inserted into the uterus.

Contraception

• Blocks fertilization or implantation of egg.
• Permanent methods: vasectomy, tubal ligation.
• Temporary methods: diaphragm, condoms, oral contraception, IUD.