HUMAN BIOLOGY

Development anD aging 333

FOCUS ON HEALTH

after cleavage, What next developmental

steps lead to implantation of the embryo?

• A blastocyst develops from the morula.


• About 7 days after fertilization, the blastocyst implants as cells
  of its outer layer of epithelium invade the endometrium of the
  uterus.


• Implantation usually is complete by 2 weeks after the original
  oocyte was ovulated.

taKe-Home message

1 7. 3

in the uterus. As time passes it will sink into the connective

tissue of the uterus, and the endometrium will close over it.

Occasionally a blastocyst implants in the wrong place—

in the oviduct or in the outer surface of the ovary or in the

abdominal wall. The result is an ectopic (tubal) pregnancy. It

can’t go to full term and must be terminated by surgery.

Implantation is complete 2 weeks after the original

oocyte was ovulated. Menstruation, which would begin

at this time if the woman were not pregnant, does not

occur because the implanted blastocyst secretes HCG

(human chorionic gonadotropin). HCG stimulates the

corpus luteum to continue secreting estrogen and proges-

terone, which prevent the uterus lining from being shed. By

the third week of pregnancy, HCG can be detected in the

endometrium

actual

size

inner cell mass

E Days 6–7.

Some of the blastocyst’s

surface cells attach

themselves to the

endometrium and start to

burrow into it. Implantation

has started.

D Day 5.

A fluid-filled cavity forms in

the morula. By the 32-cell

stage, differentiation is

occurring in an inner cell

mass that will give rise to

the embryo. This embryonic

stage is the blastocyst.

C Day 4.

By 96 hours there is a ball

of 16 to 32 cells. This is the

morula. Cells of the surface

layer will function in

implantation and will give

rise to a membrane, the

chorion.

inner cell mass

fertilization

implantation

blastocoel

fluid

endometrium

uterine

cavity

B Day 3.

After the third

cleavage, cells

form a compact

ball.

trophoblast (surface

layer of cells of the

blastocyst)

A Days 1–2.
The first cleavage
furrow extends
between the two
polar bodies.

a Baby times two

as Section 17.2 mentioned, fraternal twins result when two

eggs are fertilized at once, and identical twins result when a

very early stage embryo splits into two. We now know more

about some patterns of twinning, such as twins “running in

families” and cases of conjoined twins.

Conjoined twins form when an embryo partially splits after

day 12 of development. the twins remain joined, typically

at the chest or abdomen, although other configurations also

are possible (Figure 17.6a). Doctors usually try to surgically

separate conjoined twins early in life so that each twin can

develop as normally as possible.

a high level of the hormone FSh, which stimulates the

maturation of a woman’s eggs (Section 16.2), increases

the likelihood of fraternal twins. genetic quirks explain

why twinning runs in some families and is more common

in some ethnic groups. a woman who herself is a fraternal

twin has double the average chance of giving birth to

fraternal twins, and once she does her odds triple for

having a second set. the Yoruba people of africa have

the world’s highest fraternal twinning rate. Yoruba women

have unusually high levels of FSh. Unfortunately, many

Yoruba mothers lack access to good medical care and half

of twins die soon after birth (Figure 17.6B).

F i g u r e 17. 4 A blastocyst implants in the endometrium of the

uterus. In this image the blastocyst is colored gold.

© Yorgos Nikas/Wellcome Images

F i g u r e 17. 5 A home pregnancy test measures HCG.

test area; line

here indicates

that HCG is

control area; line present in urine

here indicates

that the test is

working

© iStockphoto.com/RonTech2000

Figure 17.6 Twinning may bring health issues. A These

Guatemalan sisters were joined at the head until doctors

at UCLA’s Mattel Hospital separated them. B This grieving

Yoruba mother of twins carries a doll as a ritual point of

contact for one of her infants, who died.

A B

Top left: © Amy Waddell/UCLA’s Mattel Children’s Hospital; Bottom left: Jorge Silva/Reuters/Landov; Right: © Marilyn Houlberg

mother’s blood or urine. Home pregnancy tests use chemi-

cals that change color when the woman’s urine contains

HCG (Figure 17.5).

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).