Pregnancy, Growth and
Development
Chapter 23
Conception
• A secondary oocyte can be fertilized for
about 24 hours after ovulation
• Sperm remain viable for up to 48 hours
within the female reproductive tract
• This gives a three day “window” for
intercourse to result in fertilization: two
days before to one day after ovulation
• Fertilization usually takes place in the
outer one-third of the uterine tube, but can
take place in the abdominal cavity
• Sperm swim up the female reproductive
tract, aided by muscular contractions of
the uterus stimulated by prostaglandins in
the semen.
• The oocyte may also secrete a chemical
that attracts sperm
• Sperm undergo a functional change in the
female tract – called capacitation
• During this process the membrane around
the acrosome becomes fragile, and its
enzymes are released.
• It requires the combined action of many
sperm to allow one sperm to penetrate the
oocyte.
• When the first sperm enters the egg, the
cell depolarizes causing the release of
calcium ions inside the cell.
• This stimulates the release of granules
that cause changes in the zona pellucida
to prevent entry of other sperm.
• Secondary oocyte completes division, and
nuclei of ovum and sperm unite to form a
zygote.
Twins
• Dizygotic or fraternal twins occur when
two separate eggs are ovulated. May be of
different sexes.
• Monozygotic or identical twins occur when
a single egg is fertilized but dividing cells
break into two groups and develop into
two individuals. Genetically identical
(clones)
• Zygote undergoes rapid mitotic cell
division, but these do not increase the size
of the zygote – called cleavage divisions
• Cleavage produces a solid sphere of cells,
still surrounded by zona pellucida – now
called a morula.
• At 4.5 to 5 days, cells have developed into
a hollow ball of cells – blastocyst.
• It is at this stage that it enters the uterus.
• Blastocyst has an outer layer of cells
called the trophoblast, an inner cell mass,
and a fluid filled cavity called the
blastocele.
• The trophoblast and part of the inner cell
mass will form the membranes of the fetal
portion of the placenta, the rest of the
inner mass forms the embryo.
Implantation
• The blastocyst remains free in the uterus a
short time, during which the zona pellucida
disintegrates.
• Blastocyst nourished by glycogen from glands
of the endometrium.
• At about 6 days after ovulation blastocyst
implants – orients cell mass toward
endometrium, and secretes enzymes which
allow it to penetrate (digest) the endometrial
wall. This nourishes the blastocyst for about a
week after implantation.
• Implantation can also occur in uterine
tube, cervix, or the abdominal cavity.
• Implantation anywhere outside the
uterus is called an ectopic pregnancy.
• It is possible for fetus to grow in the
abdominal cavity, but growth inside the
uterine tube causes the tube to rupture,
resulting in severe bleeding.
• As early as 8 -12 days after fertilization,
the blastocyst begins to secrete human
chorionic gonadotropin or hCG.
• hCG keeps the corpus luteum active until
the placenta can produce estrogens and
progesterone.
• The presence of hCG is the basis for
pregnancy tests.
• Inner cell mass forms two cavities:
– The yolk sac
– Amniotic cavity
• In humans the yolk sac produces blood
cells and future sex cells
• The amniotic cavity becomes the cavity in
which the embryo floats. Fluid is produced
from fetal urine, and secretions from the
skin, respiratory tract, and amniotic
membranes.
Primary germ layers
• In between the yolk sac and the amniotic
cavity is the embryonic disc, which gives
rise to the primary germ layers:
– Endoderm
– Mesoderm
– Ectoderm
Gestation period
• Divided into three trimesters.
• During first trimester individual starts out
as a zygote, then morula, blastocyst, and
after implantation, is called an embryo.
• Embryonic phase of development lasts
from fertilization until the 8th week of
gestation, when it becomes a fetus.
• By day 35 the heart is beating, and eye
and limb buds are present.
• By month four, the rudiments of all organ
systems are formed and functioning, and
from then on, fetal development is
primarily a matter of growth.
• By the end of the third month the placenta
is functioning.
The placenta
• The chorion develops into the fetal part of the
placenta.
• The chorionic villi connect the fetal circulation
to the placenta
• Composed of both fetal and maternal tissues
Functions of the placenta:
1 Transfer gasses
2 Transport nutrients
3 Excretion of wastes
4 Hormone production – temporary endocrine
organ – estrogen and progesterone
5 Formation of a barrier – incomplete,
nonselective – alcohol, steroids, narcotics,
anesthetics, some antibiotics and some
organisms can cross
Quickening
• The first movement of the fetus felt by the
mother, usually occurring during the fourth
or fifth month of pregnancy
• By month seven the fetus is quite active
• During the last month the fetus becomes
less active (usually due to space
considerations.)
• At the end of pregnancy both the mother
and the uterus become “irritable”
• The uterus undergoes Braxton-Hicks
contractions: intermittent, painless
contractions which can come 10 to 20
minutes apart.
• Become more frequent as gestation
progresses, and can be mistaken for onset
of labor
• Cervix begins to thin and dilate
Labor (parturition)
• Stage one – the period from the onset of
true labor contractions until the cervix is
completely dilated at 10 cm.
• The uterine contractions cause the cervix
to dilate, and the amniotic sac may
rupture.
• Usually lasts 6 – 24 hours depending on
the number of previous deliveries.
Stage 2
• Period from maximal cervical dilation until
the birth of the baby
• Lasts minutes to an hour
• Contractions become more intense and
frequent.
Stage 3
• The expulsion of the placenta
• Usually occurs within 15 minutes after the
birth of the baby, but can range from 5 to
60 minutes.
The End !!
•
•
•
•
That’s it !
You’ve made it !
Study well !
Good luck !