Human Physiology, 14th edition (2016)

Reproduction 737

Figure 20.40 In vitro fertilization. A needle is used to
inject a single spermatozoon into a human oocyte in vitro.

Secondary

oocyte

Fertilization

Sperm cells

Sperm cell nucleus

Egg cell nucleus

Zygote

2-cell

stage

4-cell

stage

Cleavage

8-cell

Morula stage

Blastocyst

Implanted

blastocyst

Ovary

Corpus

luteum Maturing

follicle

Ovulation

Figure 20.41 Fertilization, cleavage, and the formation of a blastocyst. A diagram showing the ovarian cycle,
fertilization, and the events of the first week following fertilization. Implantation of the blastocyst begins between the fifth and seventh
day and is generally complete by the tenth day.

embryo. The paternal mitochondria, with their contents of mito-
chondrial DNA, are quickly eliminated in the zygote by a process
of autophagy (chapter 3, section 3.2). As a result, all of the mito-
chondrial DNA in a person is inherited from the mother’s oocyte.

A secondary oocyte that has been ovulated but not fertil-

ized does not complete its second meiotic division, but instead

disintegrates 12 to 24 hours after ovulation. Fertilization there-

fore cannot occur if intercourse takes place later than one day

following ovulation. Sperm, by contrast, can survive up to

three days in the female reproductive tract. Fertilization there-

fore can occur if intercourse takes place within a three-day

period prior to the day of ovulation.

Cleavage and Blastocyst Formation

At about 30 to 36 hours after fertilization, the zygote divides

by mitosis—a process called cleavage —into two smaller cells.

The rate of cleavage is thereafter accelerated. A second cleav-

age, which occurs about 40 hours after fertilization, produces

four cells. A third cleavage about 50 to 60 hours after fertiliza-

tion produces a ball of eight cells called a morula ( 5 mul-

berry). This very early embryo enters the uterus three days

after ovulation has occurred ( fig. 20.41 ).

Continued cleavage produces a morula consisting of 32 to

64 cells by the fourth day following fertilization. The embryo

remains unattached to the uterine wall for the next two days,