🔬General Biology I Unit 43 – Animal Reproduction and Development

Key Concepts

• Reproduction is the biological process by which new individual organisms are produced from their parents
• Sexual reproduction involves the fusion of male and female gametes (sperm and egg) to form a zygote
• Asexual reproduction involves a single parent and produces offspring that are genetically identical to the parent (cloning, budding, fragmentation)
• Reproductive strategies vary among species and are influenced by environmental factors, resource availability, and evolutionary pressures
• Gametes are haploid cells containing half the genetic material of the parent organism
  • Sperm cells are produced in the testes through spermatogenesis
  • Egg cells (ova) are produced in the ovaries through oogenesis
• Fertilization is the fusion of male and female gametes to form a diploid zygote
• Embryonic development is the process by which a zygote develops into a multicellular organism with specialized tissues and organs
• Gestation is the period during which an embryo or fetus develops inside the mother's body (uterus in mammals, eggs in oviparous species)

Reproductive Systems

• Male reproductive system consists of testes, epididymis, vas deferens, seminal vesicles, prostate gland, and penis
  • Testes produce sperm and testosterone
  • Epididymis stores and matures sperm
  • Vas deferens transports sperm from the epididymis to the urethra
  • Seminal vesicles, prostate gland, and bulbourethral glands produce seminal fluid
• Female reproductive system consists of ovaries, fallopian tubes, uterus, cervix, and vagina
  • Ovaries produce eggs (ova) and hormones (estrogen and progesterone)
  • Fallopian tubes transport eggs from the ovaries to the uterus and serve as the site of fertilization
  • Uterus is where the embryo implants and develops during gestation
  • Cervix connects the uterus to the vagina and helps maintain pregnancy
  • Vagina is the birth canal and receives sperm during copulation
• Menstrual cycle in humans is a series of hormonal changes that prepare the uterus for pregnancy
  • Follicular phase: follicles in the ovaries mature, and estrogen levels rise
  • Ovulation: a mature egg is released from the ovary
  • Luteal phase: the uterine lining thickens in preparation for implantation, and progesterone levels rise
  • Menstruation: if fertilization does not occur, the uterine lining is shed

Mating and Fertilization

• Mating is the process by which male and female organisms come together to reproduce
• Mating systems vary among species and can be monogamous (one male and one female), polygynous (one male and multiple females), or polyandrous (one female and multiple males)
• Courtship behaviors are used to attract mates and can include visual displays, vocalizations, and chemical signals (pheromones)
• External fertilization occurs when sperm and eggs are released into the environment (aquatic species)
• Internal fertilization occurs when sperm is deposited inside the female reproductive tract (terrestrial species)
  • Copulation is the insertion of the male's penis into the female's vagina to deliver sperm
• Sperm competition occurs when sperm from multiple males compete to fertilize a single egg
  • Some species have evolved adaptations to increase the chances of their sperm fertilizing the egg (larger testes, sperm plugs, sperm removal)
• Cryptic female choice allows females to influence which male's sperm fertilizes their eggs (sperm storage, selective sperm retention)

Embryonic Development

• Cleavage is the rapid cell division of the zygote without an increase in size
  • Cleavage patterns vary among species (holoblastic, meroblastic)
• Blastulation is the formation of a hollow ball of cells called the blastula
  • Inner cell mass gives rise to the embryo
  • Trophoblast gives rise to extraembryonic membranes (amnion, chorion, yolk sac)
• Gastrulation is the formation of the three primary germ layers: ectoderm, mesoderm, and endoderm
  • Ectoderm gives rise to the nervous system, epidermis, and other external structures
  • Mesoderm gives rise to the musculoskeletal system, circulatory system, and other internal organs
  • Endoderm gives rise to the digestive system, respiratory system, and other internal organs
• Organogenesis is the development of organs from the primary germ layers
  • Neurulation is the formation of the neural tube, which gives rise to the brain and spinal cord
  • Somites give rise to the vertebrae, ribs, and skeletal muscles
• Extraembryonic membranes support the developing embryo
  • Amnion surrounds the embryo and provides a protective fluid environment
  • Chorion contributes to the formation of the placenta
  • Yolk sac provides nutrients to the embryo (in some species)
  • Allantois stores waste products and contributes to the formation of the umbilical cord (in some species)

Fetal Growth and Gestation

• Fetal period begins after organogenesis and continues until birth
• Fetal growth is rapid and involves the maturation of organs and systems
• Placenta is a temporary organ that connects the fetus to the mother's uterus
  • Allows for the exchange of nutrients, oxygen, and waste products between the mother and fetus
  • Produces hormones that support pregnancy (human chorionic gonadotropin, progesterone, estrogen)
• Umbilical cord connects the fetus to the placenta and contains blood vessels that transport nutrients and waste products
• Amniotic fluid surrounds the fetus and provides protection, temperature regulation, and allows for fetal movement
• Gestation length varies among species
  • Humans: approximately 40 weeks
  • Elephants: approximately 22 months
  • Mice: approximately 20 days
• Fetal development milestones include the formation of fingerprints, the ability to hear and respond to sounds, and the development of hair and nails

Birth and Parental Care

• Birth (parturition) is the process by which the fetus is expelled from the mother's body
  • Initiated by hormonal changes and uterine contractions
  • Stages of labor: dilation of the cervix, expulsion of the fetus, delivery of the placenta
• Mammary glands produce milk to nourish the newborn
  • Colostrum is the first milk produced and contains antibodies that provide passive immunity to the newborn
• Parental care varies among species
  • Altricial species have helpless young that require extensive parental care (birds, some mammals)
  • Precocial species have well-developed young that require minimal parental care (some mammals, reptiles, fish)
• Maternal-fetal conflict arises when the interests of the mother and fetus do not align
  • Fetus may manipulate the mother's physiology to obtain more resources
  • Mother may limit resource allocation to the fetus to ensure her own survival and future reproductive success
• Genomic imprinting is the differential expression of genes depending on whether they are inherited from the mother or father
  • Some imprinted genes are involved in fetal growth and development (IGF2, H19)

Evolutionary Perspectives

• Sexual reproduction provides genetic diversity through meiosis and fertilization
  • Increases the ability of populations to adapt to changing environments
• Asexual reproduction allows for rapid population growth and colonization of new habitats
• Parental investment theory predicts that the sex with the higher investment in offspring will be more selective in choosing mates
  • Females often have higher parental investment due to the costs of producing eggs and providing parental care
• Sexual selection occurs when traits that increase reproductive success are favored
  • Intrasexual selection involves competition among members of the same sex for access to mates (male-male combat, sperm competition)
  • Intersexual selection involves the preferences of one sex for certain traits in the opposite sex (female choice, male displays)
• Evolutionary arms race between males and females can lead to the development of complex reproductive strategies
  • Males may evolve adaptations to increase their chances of fertilization (larger testes, sperm plugs)
  • Females may evolve adaptations to control which males fertilize their eggs (cryptic female choice, sperm storage)

Practical Applications

• Assisted reproductive technologies (ART) help individuals with fertility issues conceive
  • In vitro fertilization (IVF) involves fertilizing an egg with sperm in a laboratory dish and implanting the resulting embryo into the uterus
  • Intracytoplasmic sperm injection (ICSI) involves injecting a single sperm directly into an egg
• Contraception prevents pregnancy by blocking ovulation, fertilization, or implantation
  • Hormonal methods include birth control pills, patches, and injections
  • Barrier methods include condoms, diaphragms, and cervical caps
  • Intrauterine devices (IUDs) prevent fertilization or implantation
• Prenatal testing allows for the detection of genetic disorders and developmental abnormalities
  • Amniocentesis involves sampling amniotic fluid to test for genetic disorders
  • Chorionic villus sampling (CVS) involves sampling placental tissue to test for genetic disorders
  • Ultrasound imaging allows for the visualization of fetal development and the detection of structural abnormalities
• Stem cell research holds promise for regenerative medicine and the treatment of developmental disorders
  • Embryonic stem cells are pluripotent and can give rise to any cell type in the body
  • Adult stem cells are multipotent and can give rise to specific cell types within a particular tissue or organ
• Understanding the mechanisms of reproductive biology and development can inform conservation efforts for endangered species
  • Captive breeding programs can help maintain genetic diversity and boost population numbers
  • Cryopreservation of gametes and embryos can preserve genetic material for future use