Fertilization and Pregnancy Structure of the Ovum Structure

Fertilization and Pregnancy

Structure of the Ovum

Structure of the Sperm

Sexual Intercourse (Coitus) • Involves the insertion of the male’s erect penis into the female’s vagina with the release and deposit of sperm at or near the cervix. • The sperm are forced out of the penis by strong muscular contractions during orgasm.

• Sperm use their flagella to swim into the uterus and toward the fallopian tube containing the ovum (which have released strong chemicals to attract the sperm). • Muscular contractions of the fallopian tube, along with villi, sweep the ovum toward the uterus and the approaching sperm.

Fertilization • Only one sperm will fertilize an ovum to form a zygote (conception). • Digestive enzymes from the acrosome of the sperm try to dissolve the ovum’s outer layer in order to insert its genetic materials. • When one sperm has done so, none other can penetrate the outer layer.

Multiple Births – Fraternal Twins • When 2 or more ova are released and fertilized at the same time the result is fraternal twins (triplets, etc). • Each forms its own placenta • These twins are no more alike than any other siblings. • Can be boy/girl, boy/boy or girl/girl

Identical Twins • A single fertilized egg divides and separates completely into two cell masses. • Each cell mass will develop into a complete individual with identical genetic make up. • Share the same placenta • Both boys or both girls

Conjoined twins • When the cell mass begins to separate and develop on their own, but do not completely separate • Formerly called “Siamese twins” because the first medically recorded case was in Siam • Usually major organs are shared and they are very difficult to separate without losing one

Gestation • The time it takes for the fertilized zygote to grow and develop (ending at delivery or birth) • In humans this is about 9. 5 months or 40 weeks or 280 days. • This is divided into 3 trimesters of about 3 months each. • The gestation period of an elephant is 22 months!

Sources • Ovum & Sperm Picture: http: //www. pinkmonkey. com • Moving Sperm: www. nasa. gov/. . . /floating_fertility. html