Fertilization and Development Fertilization Fertilization of the Ovum

Fertilization and Development

Fertilization ¬Fertilization of the Ovum occurs in the oviduct – Only one sperm allowed • Electrical block • Surface of ovum altered • Binding sites for sperm altered

Development ¬Nuclei of sperm and ovum fuse to form the zygote – 1 st division (mitosis) Occurs in oviduct 30 hrs after fertilization

Development ¬Cell division continues as mass moves through the oviduct to the uterus – a central hollow forms and fills with fluid – Now called a blastocyst • Cells on inside develop into embryo • Outer cells develop into supportive cells – Identical Twins

Implantation ¬On 6 th day blastocyst breaks free of zona ¬Implantation blastocyst in uterine wall on day 7 - 8

Implantation

Implantation ¬ Endometrium is maintained – Human corionic gonadotropic hormone (HCG) sectreted by outer cells (Chorion) of blastocyst to maintain corpus luteum • LH levels remain high • HCG used in pregnancy tests – Corpus luteum maintains levels of estrogen and progesterone to maintain endometrium

The Embryo ¬During implantation, the outer layer of cells of the blastocyst produces two membranes that surround, protect, and nourish the embryo – Chorion forms the placenta – Amnion forms amniotic sac

Development ¬Considered an embryo after implantation is complete – Approx. 2 weeks from fertilization

Chorion

Amnion ¬Forms a fluid filled sac that insulates the embryo and fetus that protects from – Infection – Dehydration – Impact – Temperature changes

Umbilical cord ¬Contains one vein and two arteries – vein carries nutrient and oxygen rich blood to the fetus. – two arteries return blood containing waste from fetal circulation back to the placenta. • It takes about 30 seconds for a blood cell to make the round trip through the cord – cord can be as long as 4 feet at birth.

Prenatal Development The in’s and then outs

Prenatal development ¬ Pregnancy usually last for approximately nine months ¬ Divided into three trimesters ¬ First Trimester – Fertilization to the end of the 3 rd month – by second week – germ layers form • Ectoderm, endoderm, and mesoderm – At one month – size is only 7 mm

Prenatal development ¬First Trimester (Cont. ) – Heart and brain have formed • heart rate can be heard at 10 weeks with a special instrument called a Doppler – Limb buds with fingers and toes present – Placenta begins developing – Considered fetus after 8 th week – Movement occurs around 9 th week • Sucking reflex

st 1 Trimester

4 wks 8 wks 12 wks

nd 2 Trimester ¬Fetus is 57 mm – 350 mm long ¬recognizable body form – Arms, hands, fingers, legs, feet and toes are fully formed • Bone begins to replace cartilage ¬most of organs and tissues developed – Eyes are almost fully developed

nd 2 Trimester

16 wks 20 wks

nd 2 Trimester ¬ At the end of 16 wks: – Fetus is developing reflexes such as sucking and swallowing – Sex is identifiable – Skin is bright pink, transparent and covered with soft, downy hair ¬ At the end of 20 wks: – Mother begins to feel fetal movement – Internal organs are maturing – Eyebrows, eyelids and eyelashes appear

Human Sex Determination ¬Females have two XX chromosomes ¬Males have the Xy chromosomes (the “y” chromosome is much smaller)

¬ The male and female fetuses do not differ until week 6 or 7 ¬ At this time, the presence of SRY gene (Sexdetermining Region of the Y chromosome) will stimulate the developing gonad to become a testis (in males) ¬ The hormones subsequently produced by the testis will lead to the development of male characteristics ¬ Testes develop in the abdomen (like the ovaries) and gradually descend

¬The balance of hormones circulating in the maternal blood is crucial during fetal development ¬(note that each sex produces a small quantity of the opposite sexes primary hormone) ¬(eg) Too much estrogen could result in a genetic male showing outward female characteristics

nd 2 Trimester ¬At the end of 24 weeks: – Eyelids begin to part and eyes open occasionally for short periods of time – Skin is covered with protective coating called vernix – Fetus is able to hiccup

rd 3 Trimester ¬Rapid growth – Fetus is 350 mm – 530 mm long – Body mass increases from 0. 68 kg to 3. 4 kg – Organs are maturing

rd 3 Trimester ¬At the end of 7 months: – If born at this time, he/she will be considered a premature baby and require special care – Taste buds have developed – Fat layers are forming – Skin is still wrinkled and red

rd 3 Trimester

rd 3 Trimester ¬At the end of 8 months: – Tremendous brain growth occurs at this time – Most body organs are now developed with the exception of the lungs • Movements or "kicks" are strong enough to be visible from the outside • Kidneys are mature • Skin is less wrinkled • Fingernails now extend beyond fingertips

rd 3 Trimester ¬At the end of 9 months: – The lungs are mature – Baby is now fully developed and can survive outside the mother's body • Skin is pink and smooth • Baby settles down lower in the abdomen in preparation for birth and may seem less active • labour

Environmental Effects on Development ¬A teratogen is any substance that might interfere with normal development of a fetus or embryo ¬The first 9 weeks are especially critical because this is when organs of first differentiating and developing

Some examples ¬Cigarette smoke can constrict fetal blood vessels and prevent proper supply of O 2 ¬Babies tend to be underweight ¬Chance of stillbirths and premature birth increases ¬Possible behavioural or intellectual challenges

Some examples ¬Thalidomide ¬Prescribed to treat morning sickness throughout the 50’s and 60’s ¬Babies were born with missing or deformed limbs ¬Thalidomide specifically blocks blood vessels that lead to the limbs

Nutrition ¬Proper nutrition is also very important during embryonic/fetal development ¬(eg) A lack of folic acid may result in spina bifida ¬(eg) over-consumption of vitamin C may lead to symptoms of vitamin C deficiency in the baby

Parturition (Birth) ¬ Uterine contractions signal beginning of labor – Cervix thins and begins to dilate (max. 10 cm) – Labor starts when contractions are 10 -15 min ¬ Amniotic membrane is forced into birth canal – Often bursts and amniotic fluid lubricates the canal (water breaking) – Usually ruptures during delivery but may occur before ¬ Contractions start moving baby through birth canal

Parturition ¬birth

Hormones involved in birth ¬Relaxin – Produced by placenta causes ligaments in pelvis to loosen ¬Oxytocin – positive feedback loop enhances strength of contractions – Prostaglandins also involved

Hormones involved in birth ¬ Milk Production – Prolactin • Levels increase dramatically after birth occurs • estrogen & progesterone levels drop • Stimulates glands in the breasts to produce fluids 1. 5 L/day – Oxytocin • Stimulates contractions that move milk into ducts • Also causes contractions of sm. Muscle in uterus to return to normal size

Contraception ¬Any behavior or technology that prevents conception

Abstinence ¬Refraining from having sex ¬If you don’t have sex you can’t get pregnant!!!

Tubal Ligation ¬Cutting of fallopian tubes

Vasectomy ¬Cutting of vas deferens

Hormone Treatments ¬Contain estrogen and progesterone

Physical or chemical barriers ¬Condoms (male and female) ¬Diaphragms ¬spermicide

Natural Family Planning Uses natural functions of the body and menstrual cycle to calculate ovulation ¬Record body temperatures ¬Record changes in cervical mucus discharge ¬Requires periodic abstinence (7 -10 days) during ovulation OR barrier/withdrawl method