1 Fertilization, Development, and Birth adapted by S. Jackson ‘08 What do you see in this picture? Let’s take a closer look.

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3 Outline Fertilization Embryonic Development (before 8 weeks = 56 days) Miscarriages Fetal Development (9 weeks to 40 weeks – birth) Birth

4 Fertilization An ovum (egg cell) can be fertilized for about 24 hours after ovulation Sperm remain viable for up to 48 hours within the female reproductive tract This gives a three day “window” for intercourse to result in fertilization: two days before to one day after ovulation Fertilization usually takes place in the oviduct.

5 Fertilization Steps of fertilization (Conception). – Several sperm penetrate corona radiata. – Several sperm attempt to penetrate zona pellucida. – One sperm enters egg and nuclei fuse, producing a zygote.  Egg’s plasma membrane and zona pellucida change to prevent polyspermy.

6 Fertilization

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8 Human Development before Implantation

9 Processes of development Zygote undergoes rapid mitotic cell division, but these do not increase the size of the zygote – called cleavage divisions Cleavage produces a solid sphere of cells, still surrounded by zona pellucida – now called a morula. At 4.5 to 5 days, cells have developed into a hollow ball of cells – blastocyst. It is at this stage that it enters the uterus.

10 Implantation At about 6 days after ovulation blastocyst implants into endometrium of the uterus.

11 The blastocyst starts to secrete HCG - human chorionic gonatotropin Stimulates estrogen and progesterone to prevent menstrual flow Causes "morning sickness" in some women... Pregnancy test measures the amount of this hormone

12 After Implantation Days 7-10: Gastrulation: major cellular reorganization into 2 or 3 tissue (germ) layers: -Ectoderm: skin, nervous system -Endoderm: lining of gut and internal organs -Mesoderm: muscles, bones, heart Gastrula: Early embryo with 3 tissue layers. All cells have the same DNA; however, different cells now begin to "turn on" (or "express") different genes to become different organs. (cells begin to differentiate)

13 Gastrulation : The formation of germ layers from a (1) blastula to a (2) gastrula. Some of the ectoderm cells (orange) move inward forming the endoderm (red).

14 Early Developmental Stages

15 If the embryo makes it this far...it is usually off to a good start! The first 3 weeks are the most hazardous periods in your life. Roughly one third to one half of all fertilized zygotes never make it beyond this point. Sadly, many women will have at least one miscarriage in their childbearing years. Some of the critical errors that can cause a miscarriage are :

16 Inheritance of a defective set of chromosomes. Errors in meiosis (called nondisjunctions) can produce an egg or sperm that has an abnormal number of chromosomes or broken chromosomes. This is almost always lethal. About half of the early miscarriages in humans are afflicted with this kind of random chromosomal defect.

17 Errors in mitosis after fertilization. A nondisjunction in a dividing blastula may produce one abnormal cell -- but since the blastula has so few cells, that means a significant fraction of the embryo is defective, preventing further development.

18 Implantation errors. Human embryos have to nestle down in a good home, in the uterus. If the mother's hormones are not just right, that can prevent implantation, and the otherwise healthy blastocyst may be sloughed away. In addition, % of all pregnancies are ectopic: the blastocyst tries to implant in the wrong place, most often in the fallopian tubes. This is always fatal for the embryo, and has the potential to be fatal for the mother.

19 Week 4: How you looked at 1 month past conception.... Placenta is forming by the 4 th week

20 Week 4-8 is when all the major organ systems of the body are formed and when most teratogens (substances that can cause birth defects) have their greatest effect By day 35 (5 weeks) the heart is beating, and eye and limb buds are present.

21 Week 9-40 it is now a fetus. After 12 weeks or so, the baby's development is largely "finished" - except brain and lung development. Gender can be determined during the 3 rd month. The fetus just spends much of the 2nd and 3rd trimesters just growing (and doing various flip-turns and kicks inside the amniotic fluid)

22 Three-to-Four Month-Old Fetus

23 Fifth through Seventh Months Mother begins to feel fetal movement. Wrinkled skin covered by fine hair – lanugo and is covered by a greasy substance - vernix caseosa. Lungs lack surfactant so if baby is born prematurely it will have to be on a respirator (respiratory distress syndrome).

24 Six-Month-Old-Fetus

25 Eighth and Ninth Months Fetus usually rotates so head is pointed down toward cervix. Fetus is now about 20 in. in length and weighs about 6.5 lbs. Full-term babies have the best chance of survival.

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27 Birth True labor is marked by uterine contractions that occur regularly every minutes and last for 40 seconds or more. Labor (parturition) Stage 1. The period from the onset of true labor contractions until the cervix is completely dilated at 10 cm. The uterine contractions cause the cervix to dilate, and the amniotic sac may rupture. Usually lasts 6 – 24 hours depending on the number of previous deliveries.

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29 Stage 2. Period from maximal cervical dilation until the birth of the baby Lasts minutes to an hour Contractions become more intense and frequent.

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31 Stage 3.  Placenta delivered.  Called the afterbirth.