Prenatal Development: A Summary By Dr Salim Khan

1. 1 Prenatal Development: A Summary By Dr Salim Khan

2. 2 Prenatal Development Learning Outcomes By the end of today�s session, you should be able to: Identify the 3 periods of prenatal development and the main events that occur within each of these periods Describe what is meant by and identify critical periods of development for particular organ systems Describe or illustrate neural tube closure.

3. 3 The duration of pregnancy is divided into three equal segments called trimesters. The prenatal development is sometimes separated into three development periods.

4. 4 The first trimester (months 1-3) Essential to the proper development of the infant and encompasses both the ovum and embryonic period of prenatal development. This is when all organs, nerve cells and brain cells develop. This is when most spontaneous abortions (miscarriages) occur. They generally are caused by abnormal development of the feotus and are nature�s way of eliminating a chromosomal abnormality. It is vital that all necessary nutrients be available to the feotus in order to develop properly.

5. 5 The second trimester (months 4-6) Often referred to as the �Golden trimester�. This is when the mother generally feels the best. Morning sickness and nausea have generally disappeared and the mother is quite comfortable. The third trimester comprises (months 7-9) These are important months for the baby as its organs and body systems mature and prepare to function on their own. The fat accumulated during this time will give the baby a �head start� on life.

6. 6 The 3 periods of Prenatal Development The Germinal period (period of zygote) This stage begins at conception and lasts until the zygote is implanted in the mother�s uterus. It lasts for about 10-14 days. The zygote grows to be about the size of a pinhead. Roots grow from the zygote into the wall of the uterus where they can receive nutrients from the mother�s blood.

7. 7

8. 8

9. 9 The Embryonic period (period of embryo) 2 to 8 weeks post-fertilisation Development of most organ systems occurs at this time. The embryo is attached to the mother by the umbilical cord (20 inches long) which reaches from the embryo�s stomach to the wall of the uterus. The umbilical cord contains arteries which carry the embryo�s waste products away from the embryo to the mother�s blood system to be purified. It also brings oxygenated and nutrient-rich blood back to the embryo to keep it alive. The umbilical cord is connected to the placenta.

10. 10

11. 11 The placenta is an organ which serves as a medium for the exchange of nutrients and waste between the mother and the feotus. Throughout this period, the embryo is inside the amniotic sac (a bag filled with watery substance called amniotic fluid). The fluid will protect the developing baby against bumps, bruises and temperature changes. During this period all of the organs that will be present at birth are formed.

12. 12

13. 13

14. 14 Placenta Recipes (These are from 'Mothering Magazine' - September 1983: ) - Roast Placenta - Placenta Cocktail- Placenta Lasagne - Placenta Spaghetti Bolognaise - Dehydrating your placenta Roast Placenta 1-3lb fresh placenta (must be no more than 3 days old) 1 onion 1 green or red pepper (green will add colour) 1 cup tomato sauce 1 sleeve saltine crackers 1 tspn bay leaves 1 tspn black pepper 1 tspn white pepper 1 clove garlic (roasted and minced)

15. 15

16. 16 The Foetal period (period of foetus) From 8 weeks up to birth During this stage, the developing baby is referred to as a foetus. The body parts, organs and systems which were formed during the embryo period will become much more developed and begin to function. The foetus will begin will begin to resemble a human being and features will increase in clarity. During the foetal period the baby may increase in length as much as twelve inches.

17. 17 Events in the Germinal Period Fertilisation results in a zygote. Following a series of mitotic cell divisions, a morula is formed (solid mass of 8-16 cells). A fluid-filled cavity develops within the morula as cell division continues. This forms a blastocyst. The inner cell mass of the blastocyst will give rise to the foetus The outer trophoblast forms the placenta

18. 18 Implantation of the blastocyst in the endometrium results in the formation of an embryonic disc in which the 3 germinal layers develop: - ectoderm - mesoderm - endoderm

19. 19 The 3 germinal layers serve as the tissues from which all body organs will derive. Ectoderm: forms structures of nervous system and skin Endoderm: forms epithelial linings of digestive, respiratory, and uro-genital systems and associated glands Mesoderm: forms everything else

20. 20 Organogenesis Formation of body organs and organ systems Cells of embryo rearrange themselves Form clusters, rods or membranes that differentiate into definitive tissues and organs By end of embryonic period (8 weeks), embryo is around 22mm in length (1 inch)

21. 21 Critical Periods of development and Teratogens For most organs systems, the most critical period of development is during the embryonic period. This is the time when differentiation is taking place Teratogens are infections, drugs, or chemicals that affect prenatal development. - Rubella virus � causes cataracts and deafness - Streptomycin - causes auditory nerve damage and hearing loss - Alcohol � causes growth retardation and microcephaly (under development of the brain)

22. 22 1. Environmental teratogens Infectious agents. Maternal Infections - Torch - Rubella - Cytomegalovirus (CMV) - Toxoplasmosis - Herpes Simplex Virus (HSV) - Varicella - Mumps - Measles - AIDS - Fifth Disease (erythema infectiosum)

23. 23 b) Radiation absorbed by the human feotus can produce damage to the CNS. c) Chemical and pharmacological teratogens. Many substances are toxic to embryos, but most are not teratogenic. Among those environmental agents that are well-established teratogens are organophosphorus insecticides such as malathion, anti-tumor agents, methylmercury 2. Chromosomal and genetic factors Roughly 1% of all newborns have a CD of genetic etiology (Epstein, Experientia 42:1117, 1986).

24. 24

25. 25

26. 26

27. 27 Development of the Neural Tube The neural tube is the precursor of the brain and spinal cord At 16-18 days post-fertilisation, the ectoderm develops into the neural plate Mitotic cell divisions in the neural plate result in the edges of the neural plate folding and joining to form a closed neural tube (see PAC I notes). Consequences of failure of neural tube closure.

28. 28 Homework Q1. What is a Neural Tube Defect (NTD) Q2. When do NTD�s occur Q3. Identify and briefly describe the main types of NTD�s Q4. Which medical intervention can reduce NTD�s Q5. What prenatal screening detects NTD�s

29. 29 �TIMELINE OF PRENATAL DEVELOPMENT� Day 1 - conception takes place. 7 days - tiny human implants in mother�s uterus. 10 days - mother�s menses stop. 18 days - heart begins to beat. 21 days - pumps own blood through separate closed circulatory system with own blood type. 28 days - eye, ear and respiratory system begin to form. 42 days - brain waves recorded, skeleton complete, reflexes present. 7 weeks - photo of thumbsucking. 8 weeks - all body systems present.

30. 30 9 weeks - squints, swallows, moves tongue, makes fist. 11 weeks - spontaneous breathing movements, has fingernails, all body systems working. 12 weeks - weighs one ounce. 16 weeks - genital organs clearly differentiated, grasps with hands, swims, kicks, turns, somersaults, (still not felt by the mother.) 18 weeks - vocal cords work � can cry. 20 weeks - has hair on head, weighs one pound, 12 inches long. 23 weeks - 15% chance of viability outside of womb if birth premature.* 24 weeks - 56% of babies survive premature birth.* 25 weeks - 79% of babies survive premature birth.*

31. 31 Testing for Problems in Prenatal Development Miscarriages due to abnormality of the uterus or genetic disorder are possible between conception and about 13 weeks. Genetic abnormalities identified via amniocentesis, Procedure in which cells in the amniotic fluid are examined for chromosomal damage. Another prenatal test, called chorionic villi sampling (CVS), analyzes a sample of embryonic tissue for defects. This procedure can be safely used during the first trimester of pregnancy.

32. 32

33. 33

34. 34

35. 35

36. 36

37. 37