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– 1st 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 6th 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 trimestersFirst Trimester

– Fertilization to the end of the 3rd month– by second week – germ layers form

• Ectoderm, endoderm, and mesoderm

– At one month – size is only 7mm

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 8th week– Movement occurs around 9th week

• Sucking reflex

1st Trimester

4 wks

8 wks

12 wks

2nd 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

2nd Trimester

16 wks 20 wks

2nd 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 hairAt the end of 20 wks:

– Mother begins to feel fetal movement – Internal organs are maturing – Eyebrows, eyelids and eyelashes appear

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

2nd Trimester

3rd Trimester

Rapid growth– Fetus is 350 mm – 530 mm long– Body mass increases from 0.68 kg to 3.4 kg– Organs are maturing

3rd 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

3rd Trimester

3rd 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

3rd 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

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

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