Mature ovum (ovulated secondary oocyte)

corona radiata – follicle cell layer surrounding secondary oocyte

zona pellucida – glycoprotein layer surrounding ovum (only one sperm gets through)

Process of Fertilization• Sperm swim from vagina, up cervix, up

endometrial lining and into fallopian tube• If female is near the middle of her menstrual

cycle, then a secondary oocyte may be present• Typical location for fertilization is in the fallopian

tube; many sperm work to get through corona radiata

1. Sperm that make it to zona pellucida compete to get into ovum (only one gets in) by secreting enzymes held in their acrosomes

2. First sperm to get through has its plasma membrane fused with ovum’s plasma membrane (this initiates cortical reaction)

Cortical Reaction

• In cytoplasm of ovum (all around

plasma membrane) are small vesicles called cortical granules

• When the 2 gametes fuse their plasma membranes, the cortical granules fuse with the ovum’s membrane and secrete enzymes to the outside

• The enzymes cause a chemical change in zona pellucida making it impermeable to sperm

Note: this happens immediately after first sperm enters to ensure that only one sperm fertilizes egg

Pregnancy

I. Early Embryonic Development

• first mitotic division ~24 hrs after fertilization

• by the time the fertilized egg (zygote) reaches the uterus and is ready for implantation in the endometrium it is ~100 cells (~7 days later)

embryo at this stage is a blastocyst

- outer cells become the placenta

- inner cells will become the embryo

II. Early Pregnancy

• If implantation occurs then embryo will secrete its own hormone, HCG (human chorionic gonadotrophin)

• The HCG gets into mom’s bloodstream and targets the corpus luteum; it stimulates the corpus luteum to continue secreting progesterone so the endometrium stays in tact

Positive pregnancy test indicated by presence of HCG (this is only made by embryo)

III. Role of the Placenta

• After implantation, the blastocyst (outer layer) and mom’s endometrium form the placenta

• Growing embryo requires nutrients and therefore a connection between mom and fetus requires placenta and umbilical cord

• Later in pregnancy placenta secretes hormones

• Umbilical cord contains 3 blood vessels– 2 vessels carry blood from fetus to placenta (these

are deoxygenated and contain waste products)– 1 vessel carries blood from placenta to fetus

(oxygenated with nutrients)

Materials from fetus to mom via placenta

Materials from mom to fetus via placenta

Carbon dioxide Oxygen

Urea Nutrients (e.g., glucose, amino acids)

Water Water

Hormones (e.g., HCG) Hormones

Vitamins, minerals

Alcohol (why we don’t drink when pregnant)

Some viruses (measles, HIV)

Note: At no time does the blood of the fetus and the

blood of the mother actually mix…all

molecular exchanges occur via active/passive

transport in placenta

Note: Placenta also takes over job of secreting estrogen and progesterone to keep endometrium/blood supply available to fetus (corpus luteum is now gone)

• Some tissue of developing embryo is used to make amniotic sac

• Within the sac is amniotic fluid; the fetus floats in this fluid

• Functions of amniotic fluid:– Providing cushioning in case of

blunt force trauma to abdomen– Allows fetus free movement for

muscle and skeletal development

– Thermal stability

Note: this is the fluid sampled during amniocentesis

The Process of Birth1. Progesterone levels drop2. Oxytocin secreted from pituitary gland; levels

increases after each contraction which causes contractions to get more frequent and more intense (this is known as positive feedback and it only stops once the uterus

has nothing to contract on) (Pitocin – used to induce labor)

• Cervix dilates to 10cm• Babies typically born head first, face down

(otherwise called breech)• After baby, mom’s push out afterbirth (placenta)• Lactation (breast milk production) begins soon

after birth