Feb 1, 2011 AIM: How does a zygote develop from embryo to fetus to full-term baby? DN: On slips, check true, false, or idk Quiz FRI on Human Reproduction, Regents-style questions HW #32 – Exam corrections due THURS Feb 3. Explain what the question wants, what you need to know, the correct answer, and your mistake.
Embryonic Development
Development of Embryo Fertilization Cleavage Differentiation/Gastrulation Ectoderm Mesoderm Endoderm
1. Fertilization Combination of egg and sperm Fertilized egg = zygote External fertilization - occurs outside the female’s body (ex: fish and amphibians) Internal fertilization – occurs inside the female’s body (ex: birds and mammals)
Internal Fertilization
2. Cleavage The zygote goes through a series of rapid cell divisions (mitosis) called cleavage
A scanning electron micrograph of a human embryo at the eight-cell stage (day three)
3. Gastrulation- formation of 3 cell layers
Parts of the Embryo Embryo Placenta Amnion Umbilical cord
External Development – OUTSIDE of the female's body Egg laying High number of eggs Low survival rate of offspring Offspring are less protected In fish, amphibians, reptiles, birds
Internal Development – INSIDE of the female's body Low number of eggs produced High survival rate of offspring Offspring are protected inside mother because the embryo grows within the mother’s body In mammals
During Internal Fertilization in Placental Mammals Egg matures in the ovary Egg leaves ovary and enters Fallopian tube Egg is fertilized in Fallopian tube The fertilized egg (zygote) travels down Fallopian tube, dividing After several divisions, the zygote has become an embryo. The embryo attaches itself to the uterus, where it will continue to grow.
Feb 2, 2011 AIM: How does a developing mammalian baby receive needed substances and eliminate waste? DN: To hand in: draw and label what you think a fetus, umbilical cord, and placenta are arranged within the uterus (womb). Quiz FRI on Human Reproduction, Regents-style questions HW #32 – Exam corrections due THURS Feb 3. Explain what the question wants, what you need to know, the correct answer, and your mistake.
Placenta
Placental Mammals (Internal Development) Animals such as humans that feed the embryo using a placenta Placenta - where wastes, oxygen, carbon dioxide and nutrients are exchanged between mother and embryo The mother’s blood and the baby's blood DO NOT MIX Gas exchange at the placenta occurs due to diffusion and active transport
Placental Mammals (cont’d) Umbilical cord - attaches the embryo to the placenta (carries blood between embryo and placenta) Uterus - site of embryonic development (baby grows here), also known as the womb
Human Embryo After the zygote attaches to the wall of the uterus it is known as an embryo (~7 days) Remember: the uterus has been thickening in preparation to receive the zygote Receives nutrients from fluids in the uterus until the placenta develops Eventually, an umbilical cord develops that connects the embryo to the placenta
Placenta Materials diffuse between the mother’s blood and the embryo’s blood (their bloods do NOT mix) Blood vessels in the umbilical cord carry nutrients and oxygen from mother’s blood through placenta to embryo Can also carry drugs, toxins, and other disease organisms
Amnion Fluid filled sac that surrounds embryo Filled with amniotic fluid which cushions the embryo and keeps it moist Forms during 3rd week of pregnancy
Fetal Development Pregnancy lasts about 38-39 weeks! At 5 weeks, embryo has head with eyes, nose, and mouth features Weeks 6 and 7 - fingers and toes During 1st two months, major organs form and heart structure begins to beat (called fetus after first 2 months)
Embryo: 4, 5, 6, 7, & 8 weeks
12 Weeks
14 Weeks
3rd Trimester
Twins Fraternal twins – develop from two separate eggs, each fertilized by different sperm Identical twins – develop from one zygote that splits in half during early cleavage
Birth Begins with muscular contractions of the uterus The amniotic sac usually breaks and releases its fluid Over a period of hours, the contractions cause the opening of the uterus to widen More powerful and frequent contractions push the baby out through the vagina to its new environment