Conception and Fetal Development

1. Conception and Fetal Development Class 41 April 3, 2003 NURS1228

2. Terms Acrosomal Reaction Capacitation Decidua Basalis Ductos Arteriosis Endoderm Gametogenesis Mitosis Trophoblast Amnion Chorion Decidua Capsularis Ductus Venosus Fertilization Haploid Number of chromosomes Morula Umbilical Cord Amniotic Fluid Chromosomes Decidua Vera (perietalis) Ectodoerm Fetus Lanugo

3. Terms, continued Placenta Vernix Caseosa Bag of Waters (BOW) Cleavage Diploid number of chromosomes Embryo Foramen Ovale Meiosis Postconception Age Periods Whartons’s Jelly Blastocyst Cortyledon Ductos Arteriosis Embryonic membranes Gametes Mesoderm Teratogen Zygote

4. Where It All StartsAfter the DNA and Chromosomes Mitosis Results in diploid body Exact copies of original cell Happens all the time with our cells Meiosis Cell division leading to development of eggs and sperm Has only a “haploid” number of chromosomes Only half the DNA of the “normal” cell

5. Gametogenesis Oogenesis Female gamete Divides into two “half cells” Secondary oocyte Polar body Spermatogenesis Primary and Secondary spermatocyte Nucleus compacted in to head of sperm (acoro- some cap) with tail

6. Fertilization When sperm and ovum fuse together Usually takes place in the outer third of the fallopian tube. High estrogen levels propel the ovum through the tube by peristalsis Thin cervical mucus allows the sperm to move up into the uterus and fallopian tube. Read about the membrane of the ovum – p. 154 of text

7. Fertilization continued Ova fertile for 24 hours after ovulation Sperm survive for 48-72 hours after ejaculation, but best bets are for 24 hours for healthy sperm. Transit time between ejaculation and reaching the fallopian tube can be as short as 5 minutes, but usually takes 4-5 hours Fallopian tubes facilitate movement of both the sperm toward the ovum and ovum toward the sperm.

8. Fertilization continued Sperm Changes for Fertilization 1: Capacitation – removal of plasma membrane overlying the acrosomal area 2. Loss of semimal plasma proteins Capacitation – within female reproductive tract within about 7 hours. Now they are ready for “the final act!” Many, many sperm at once cause the acrosomal reaction so that the outer layer of the ovum is broken down. Only one sperm penetrates!!

9. Fertilization continued Block to polyspermy occurs and no further sperm can penetrate. At the moment of fertilization: Secondary oocyte completes meiosis, the polar body is ejected and the nuclei of the ovum and sperm approach one another and unite. Chromosomes pair to form a diploid zygote – 46 – the complete number forms the genetic code for the new human being. The sex is determined at that time on the 23rd pair of chromosomes: XX (female) or XY (male). It is the sperm which may carry the “Y” chromosome and determine whether the zygote will be male or not. The ova only carry the “X” chromosome.

10. Twins 1 in 80 pregnancies Fraternal: dizygotic (2 separate ova and sperm). 2 placentas, 2 chorions, 2 amnions; same or different sex Increases with maternal age until about 35 Genetic Identical: monozygotic – single, fertilized ovum with common placenta. Originate from same fertilized ovum Number of amnions and chorions depends on timing of division. (see p. 157 of text) A random event, not dependent on genetics; survival rate is 10% lower than disygotic twins. Congenital abnormalities more prevalent

11. Intrauterine Development Cellular Multiplication Begins as the zygote moves toward the uterine cavity Rapid mitosis called clevage occurs: blastomeres morula Inner mass: blasotcyst Develops into the embryo Outer layer: trophoblast Develops into the chorion Cellular Differentiation (embryonic membrane) Primary Germ layer Ectoderm, mesoderm, endoderm All tissues, organs and organ systems will develop! Embryonic Membranes Form at time of implantation Chorion/chorionic villi will form and eventually form the placenta Amnion: from the ectoderm, contains the amniotic fluid Surrounds the embryo and yolk sac except where the cord will attach to placenta; forms the “bag of waters.”

12. Implantation Occurs in between cellular multiplication and cellular differentiation. Blastocyst nourished by uterine glands and trophoblast attaches to the endometrium for nourishment Upper part of the posterior uterine wall Blastocyst burrows into endometrium (toward maternal capillaries) until covered. Villi are then called; The endometrium is now called the “decidua.” Decidua capsularis Decidua basalis Decidua vera (parietalis

13. Why is it important to know which tissues and organs arise from each of the germ layers?

14. Amniotic Fluid Cushion protecting from injury Temperature control Goes from 30 – 350 mL to 700 to 1000 mL Moves back and forth across placenta Fetal urine increases volume (fetus also swallows fluid) Fetus “breathes” fluid: 400 mL

15. Amniotic Fluid continued Oligohydramnios: and fetal problems Hydramnios/Polyhydramnios: and fetal problems

16. Placenta Begins with chorionic villi Begins and develops from the third week of development to 20 weeks. Maternal portion: decidua basalis and circulation Fetal portion: chorionic villi and circulation. Covered by amnion and is shiny from amniotic membranes. Exchange is limited during first 3-4 months

17. Placental Functions “Placental exchange functions occur only in those fetal vessels which are in intimate contact with the covering synctytial membrane. Fetal respiration, nutrition, excretion Metabolic and transfer activities Endocrine functions hCG (human chorionic gonadotropin), hPL (human placental lactogen), and estrogen and progesterone.

18. Placental Endocrine Functions continued hCG: keeps the corpus luteum functioning until after the 11th week of pregnancy. The corpus luteum secretes increased estrogen and progesterone Then the placenta itself will produce the hormones and the corpus luteum can fade out Male fetus: help with testosterone production hCG Used for the basis of pregnancy tests (available in maternal serum for 8-10 days after fertilization and detectable in maternal urine at the time of missed menses) hPL: responsible for maternal changes which support pregnancy

19. Umbilical Cord One large vein and two smaller arteries Surrounded and protected by Wharten’s jelly Prevents compression of cord in utero (along with high blood volume) Can be knotted (“true knot”) due to a long cord. May cause problems in fetal oxygenation. Nuchal cord: fetal activity causes the cord to encircle the neck

20. Fetal Circulation Blood supply mostly passes the fetal lungs due to no gas exchange there Ductus Veosus Foramen ovale Ductus arteriosis

21. Diagram of placental circulation

22. Diagram of fetal circulation (see also p. 167 of text)

23. Fetal Heart Variability: beat-to-beat Increase Decrease

24. Development and Organ Formation Pre-embryonic: first 14 days. Rapid cellular multiplication Embryonic stage: day 15 to 8 weeks. Embryo is about 3 cm long Tissue differentiates into essential organs and main external features develop. Most vulnerable to teratogens at this time!! By the end of 28 days, heart is beating and beginning circulation. Brain is differentiated into 5 areas and 10 cranial nerve pairs recognizable

25. Further development See text for specifics on pp. 168-169 Fetal stage: At the end of the 8th week. Every organ system now developed Just needs refining and developing Milestones: week 16 – sex can be seen; 17-18 hard tissue for teeth FHT by doppler 8-12 weeks (20 weeks for fetoscope) 20 weeks: quickening 24 weeks: resp. movements begin 28 weeks: surfactant needed for breathing. Eyes open and close and can breathe outside uterus 32 weeks: fingernails and toenails, fat is being laid down 38-4- weeks: antibodies from mother. Fills total uterus