Embryonic Development

1. Embryonic Development • Development: series of orderly, precise steps that transform a zygote into a multicellular embryo ~early stage of development of multicellular organism • Includes: 1. cell division 2. cell growth 3. cell differentiation ~changing of unspecialized embryonic cells into specialized cells, tissues,& organs

2. Early Embryonic Development • Cleavage is the first major phase of embryonic development • It is the rapid succession of cell divisions (Mitotic) • It creates a multicellular embryo from the zygote • NO growth • Stages: • 1. Morula~solid ball of cells • 2.Blastula~ single layer of cells surrounding a fluid-filled cavity called the blastocoel ZYGOTE 2 cells 4 cells 8 cells Blastocoel Many cells(solid ball) Cross sectionof blastula BLASTULA(hollow ball)

3. Embryonic Development • Gastrulation is the second major phase of embryonic development • The cells at one end of the blastula move inward, • It adds more cells to the embryo • It sorts all cells into three distinct cell layers • The embryo is transformed from the blastula into the gastrula

4. Embryonic Development • The three layers produced in gastrulation • Ectoderm, the outer layer • Endoderm, an embryonic digestive tract • Mesoderm, which partly fills the space between the ectoderm and endoderm

5. Embryonic Development • The tissues and organs of a tadpole emerge from cells of the ectoderm, mesoderm, and endoderm

6. Embryonic Development • ectoderm cells~ eventually they develop into the skin and nervous tissue of the animal • endoderm cells ~develop into the lining of the animal’s digestive tract and into organs associated with digestion. • mesoderm cells ~develop into the muscles, circulatory system, excretory system, and, in some animals, the respiratory system.

7. Embryonic Development • Organs start to form after gastrulation • Embryonic tissue layers begin to differentiate into specific tissues and organ systems

8. Changes in cell shape, cell migration, and programmed cell death give form to the developing animal • Tissues and organs take shape in a developing embryo as a result of • cell shape changes • cell migration Ectoderm

9. Cellsuicide Dead cellengulfed anddigested byadjacentcell Development • programmed cell death (apoptosis)

10. Embryonic induction initiates organ formation • Induction is the mechanism by which one group of cells influences the development of tissues and organs from ectoderm, endoderm, and mesoderm • Adjacent cells and cell layers use chemical signals to influence differentiation • Chemical signals turn on a set of genes whose expression makes the receiving cells differentiate into a specific tissue

11. Pattern formation organizes the animal body • Pattern formation is the emergence of a body form with structures in their correct relative positions • It involves the response of genes to spatial variations of chemicals in the embryo

12. HUMAN DEVELOPMENT • The embryo and placenta take shape during the first month of pregnancy • Gestation is pregnancy • It begins at conception and continues until birth • Human gestation is 266 days (38 weeks or 9 months) • Mouse gestation is 1 month • Elephant gestation is 22 months

13. HUMAN DEVELOPMENT • Human development begins with fertilization in the oviduct Cleavage starts Fertilizationof ovum Ovary Oviduct Blastocyst(implanted Ovulation Endometrium Uterus

14. HUMAN DEVELOPMENT • Cleavage produces a blastocyst • A blastocyst is a fluid-filled cavity • The inner cells of the blastocyst form the baby • The outer cells form the embryo trophoblast ENDOMETRIUM ENDOMETRIUM Inner cell mass Cavity Trophoblast

15. HUMAN DEVELOPMENT • The trophoblast secretes enzymes to enable the blastocyst to implant in the uterine wall ENDOMETRIUM Blood vessel(maternal Future embryo Multiplying cellsof trophoblast Futureyolk sac Trophoblast UTERINE CAVITY

16. HUMAN DEVELOPMENT • Gastrulation occurs and organs develop from the ectoderm, endoderm, and mesoderm Amnion Mesodermcells Chorion Yolk sac

17. HUMAN DEVELOPMENT • Meanwhile, the four embryonic membranes develop • Amnion~fluid filled sac for protection • Chorion ~ will form the embryo’s part of the placenta • Yolk sac ~ produces first blood cells &germ cells • Allantois ~ will form the umbilical cord (ropelike structure that attaches embryo to uterus) Chorion Amnion Allantois Yolk sac

18. HUMAN DEVELOPMENT • The embryo floats in the fluid-filled amniotic cavity, • The placenta’schorionic villi absorb food and oxygen from the mother’s blood Mother’s bloodvessels Placenta Yolk sac Amnioticcavity Amnion Embryo Chorion Chorionicvilli

19. Placenta • Food & gases diffuse across blood vessels

20. HUMAN DEVELOPMENT • Embryonic development of essential organs occur in early pregnancy • The embryo may encounter risks from faults in its genes & from mother’s exposure to environmental factors • The placenta allows for a variety of substances to pass from mother to fetus • Protective antibodies • German measles virus • HIV • Drugs (prescription and nonprescription) • Alcohol • Chemicals in tobacco smoke

21. Human development from conception to birth is divided into three trimesters • First trimester • First three months • The most rapid changes occur during the first trimester • 7 weeks 10 weeks 4 weeks

22. Human development from conception to birth is divided into three trimesters • Second trimester • Increase in size of fetus • General refinement of human features 12 weeks

23. Human fetal development • The fetus just spends much of the 2nd & 3rd trimesters just growing …and doing various flip-turns & kicks inside amniotic fluid Week 20

24. Human fetal development • 24 weeks (6 months; 2nd trimester) fetus is covered with fine, downy hair called lanugo. Its skin is protected by a waxy material called vernix

25. Human fetal development • 30 weeks (7.5 months) umbilical cord

26. Getting crowded in there!! • 32 weeks (8 months) The fetus sleeps 90-95% of the day & sometimes experiences REM sleep, an indication of dreaming

27. Human development from conception to birth is divided into three trimesters • Third trimester • Growth and preparation for birth

28. Childbirth is hormonally induced and occurs in three stages • Hormonal changes induce birth • Labor is controlled by a positive feedback mechanism • Estrogen released from the ovaries increases the sensitivity of the uterus to oxytocin

29. Positive feedback Mechanism • Oxytocin is a powerful stimulant for the smooth muscles of the uterus • Oxytocin also stimulates the placenta to make prostoglandins that stimulate the uterine muscles to contract even more • Uterine contractions stimulate the release of more and more oxytocin and prostoglandins

30. Birth positive feedback

31. Three stages of labor • 1.Dilation of the cervix is the first stage -Cervix reaches full dilation at 10cm • Longest stage of labor (6-12 hours or longer) Dilation of the cervix

32. Three stages of labor • Expulsion is the second stage • Period from full dilation of the cervix to delivery of the infant • Uterine contractions occur every 2-3 minutes • Mother feels urge to push down with her abdominal muscles • Infant is forced down and out of uterus and vagina within a period of 20 minutes

33. Three stages of labor • The delivery of theplacenta is the final stage of labor • Usually occurs within 15 minutes after the birth of the baby

34. The end of the journey! And you think 9 months of Biology is hard!

35. Childbirth is hormonally induced and occurs in three stages • Hormones continue to be important after the baby and placenta are delivered • Decreasing progesterone and estrogen levels allow the uterus to return to its pre-pregnancy state • Oxytocin and prolactin stimulate milk secretion

36. Reproductive technology increases our reproductive options • Reproductive technology • Hormone therapy can increase sperm or egg production • Surgery can correct blocked oviducts • Assisted reproductive technology • In vitro fertilization (IVF) • Surrogate motherhood