Growth and Development in Humans

Growth and Development in Humans

Growth and Development

  • Multicellular organisms, such as humans, experience growth as an increase in the number of cells through mitosis, leading to body enlargement.
  • Development encompasses growth and refers to the changes an individual undergoes from the prenatal to the postnatal period.

Fertilization and Pregnancy

  • Fertilization: This occurs when a sperm cell joins an oocyte, primarily in the Fallopian tubes.
  • Pregnancy starts when the embryo implants into the uterus wall.

Sperm and Egg

  • An ejaculate can contain 200–600 million sperm.
  • Sperm can fertilize an egg 24-48 hours post-ejaculation (can survive up to 6 days).
  • The egg survives for 12-24 hours post-ovulation, making fertilization possible around ovulation.

Prenatal Period

  • Spans the first 38 weeks post-conception and includes three stages:
    1. Cleavage Stage
    2. Embryonic Stage
    3. Fetal Stage

Cleavage Stage

  • Zygote Formation: A sperm and egg fuse to create a zygote.
  • Within 30 hours, the zygote undergoes mitosis, producing two cells (blastomeres).
  • This process continues exponentially (2, 4, 8, etc.), leading to the formation of a blastocyst around six days post-fertilization.
  • Implantation: The blastocyst embeds into the endometrium of the uterus.

Embryonic Stage

  • Lasts from the 2nd to the 8th week of development:
    • hCG Production: The developing embryo secretes human chorionic gonadotropin (hCG) which helps maintain the corpus luteum and supports fetal development.
    • Placenta Formation: Forms a vascular connection between the mother and embryo for nutrient exchange.
    • Germ Layer Development:
    • Ectoderm: Develops into nervous system and skin.
    • Endoderm: Becomes viscera.
    • Mesoderm: Forms musculoskeletal and cardiovascular systems.

Amnion and Amniotic Fluid

  • The amnion forms a protective fluid-filled sac around the embryo, providing a cushioning environment.

Birth Defects and Teratogens

  • Teratogens (e.g., alcohol, drugs, viruses) can cause developmental malformations during the embryonic stage.
  • Fetal Alcohol Syndrome: Caused by high alcohol intake during pregnancy, characterized by physical and intellectual disabilities.
  • Infections: Certain infections such as rubella and syphilis can lead to birth defects when contracted during specific prenatal periods.

Fetal Stage

  • Begins at the 9th week of gestation until birth, characterized by further body growth and organ development.
  • By the 12th week, external genitalia become distinguishable.
  • Survival chances in premature infants are lower, particularly due to the late maturing digestive and respiratory systems.

Birth Process

  • As the placenta ages, progesterone levels drop, triggering uterine contractions.
  • Oxytocin Release: Stretching of the uterus stimulates the brain to release oxytocin, which promotes contractions.
  • Afterbirth: Occurs after the fetus is delivered when the placenta separates from the uterus.

Lactation

  • Hormones from the placenta encourage mammary gland development during pregnancy.
  • Prolactin stimulates milk production post-birth, while oxytocin enables milk ejection during suckling, establishing the breastfeeding reflex.

Stages of Life

  1. Neonatal period: Birth to 4 weeks.
  2. Infancy: 4 weeks to 1 year.
  3. Childhood: 1 year to puberty.
  4. Adolescence: Puberty to adulthood.
  5. Adulthood: Post-puberty to about age 60.
  6. Senescence: Aging process characterized by degenerative changes.