Chapter 29: Development

1. Chapter 29: Development BIO 211 Lecture Instructor: Dr. Gollwitzer

2. Today in class we will: • Define and describe development • Trace the general processes from ovulation through fertilization and formation of a zygote • List the stages of development • List the 3 stages of gestation and briefly describe the major events associated with each • Distinguish between embryo and fetus • Discuss the two major roles of the placenta • Discuss the basic structural and functional changes in the uterus during gestation • Briefly describe the events that occur during labor and delivery • Describe lactation and milk let-down reflex

3. Development • Begins with fertilization (conception) = • When male and female gametes (sperm and egg) unite to form single-cell zygote • Occurs in uterine tube 12-24 hr after ovulation • Is the gradual modification from fertilization to maturity of: • Anatomical structures • Physiological characteristics

4. Fertilization Figure 29–1a, b

5. Amphimixis • Fusion of female and male pronuclei • Moment of conception • Cell becomes zygote (46 chromosomes) • Fertilization complete

6. Stages of Development • Prenatal – before birth • Embryological development • Occurs during first 2 mo after fertilization • Organs established • Fetal development • Begins at 9th wk and continues to birth • Organs develop • Postnatal – after birth • Neonate = newborn

7. Gestation • Time spent in prenatal development • Consists of 3 trimesters, each 3 months long • First trimester • Second trimester • Third trimester

8. First Trimester • Cell cleavage (division) and blastocyst formation • Blastocyst implantation = burrowing into uterine wall • Placentation = formation of placenta • Temporary structure in uterine wall • Permits diffusion between fetal and maternal circulatory systems • Embryogenesis – all organ systems begin to be established; but nonfunctional • Embryo = organism in the developmental stage beginning at fertilization and ending at the start of the third developmental month (weeks 1 – 8)

9. First Trimester • Most dangerous period in prenatal life • Only 40% of conceptions produce embryos that survive past first trimester

10. Cleavage and Blastocyst Formation Figure 29–2

11. Stages in Implantation Figure 29–3

12. Placenta • Complex organ that permits exchange between maternal and embryonic circulatory systems • Supports fetus in second and third trimesters • Stops functioning and is ejected from uterus after birth

13. Extraembryonic Membranes and Placenta Formation Figure 29–5 (1, 2 of 5)

14. Extraembryonic Membranes and Placenta Formation Figure 29–5 (3, 4 of 5)

15. Extraembryonic Membranes and Placenta Formation Figure 29–5 (5 of 5)

16. First Trimester Figure 29–7a, b

17. First Trimester Figure 29–7c, d

18. First Trimester: hCG • hCG = human chorionic gonadotropin • Produced by placenta • Appears in maternal bloodstream soon after implantation • Used as pregnancy test/kit • Maintains CL for 3-4 months • So CL  P (until placenta takes over P production)

19. Second Trimester • Fetal stage = development of all organ systems (organogenesis) • Rapid growth of fetus • Fetus = organism in the developmental stage lasting from the start of the third developmental month to delivery (week 9 through delivery) • Body proportions change • Progesterone levels increase

20. Third Trimester • Organ systems fully functional • Fetal growth rate slows • Largest weight gain

21. Growth of Fetus and Uterus Figure 29–9a, b

22. Growth of Fetus and Uterus Figure 29–9c, d

23. Overview of Prenatal Development Table 29–2 (1 of 4)

24. Overview of Prenatal Development Table 29–2 (3 of 4)

25. Overview of Prenatal Development Table 29–2 (2 of 4)

26. Overview of Prenatal Development Table 29–2 (4 of 4)

27. Third Trimester Hormones • P (placental) • Until 3rd trimester, “calms” myometrium so no contractions • E (placental) • Increases  myometrial contractions • Sensitizes uterus to oxytocin (maternal and fetal)  prostaglandins (PGs)  initiate labor

28. Third Trimester Hormones • Human placental lactogen (hPL) • Helps prepare mammary glands for milk production • Effects on other tissues comparable to GH • Prolactin (placental) • Helps convert mammary glands to active status • Relaxin (CL and placental)  • Increased flexibility of pubic symphysis  pelvis expands •  Dilation of uterine cervix  so fetus can enter vagina

29. Factors Involved in Initiation of Labor and Delivery Figure 29–10

30. Labor • False • Occasional spasms in uterine musculature • Contractions not regular or persistent • True • Results from biochemical and mechanical factors • Continues due to positive feedback • Premature • When labor begins before fetal development complete; survival related to BW

31. Labor and Delivery • Goal: parturition = forcible expulsion of fetus • Stages of labor • Dilation • Explusion • Placental

32. Labor and Delivery • Dilation stage • Begins with onset of true labor • Cervix dilates • Fetus moves toward cervical canal • Frequency of contractions increases • Amniochorionic membrane ruptures (“water breaks”)

33. Stages of Labor Figure 29–11 (1,2 of 4)

34. Labor and Delivery • Expulsion stage • Cervix completely dilated • Maximum intensity of contractions • Continues until fetus emerges from vagina = delivery/birth

35. Labor and Delivery • Placental stage • Uterine contractions tear connection between endometrium and placenta • Placenta (afterbirth) ejected • Accompanied by loss of blood, usually tolerated without difficulty

36. Stages of Labor Figure 29–11 (3,4 of 4)

37. Milk Let-Down Reflex Figure 29–12