Pregnancy Stages and Development

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Pregnancy

• Pregnancy is a sequence of events that begins with
fertilization; proceeds to implantation, embryonic
development, and fetal development; and ideally ends
with birth about 38 weeks later, or 40 weeks after the last
menstrual period.

Fertilization
• The genetic material from a haploid sperm cell
(spermatozoon) and a haploid secondary oocyte
merges into a single diploid nucleus.
• Fertilization normally occurs in the uterine
(fallopian) tube within 12 to 24 hours after
ovulation.
• Sperm can remain viable for about 48 hours
after deposition in the vagina, although a
secondary oocyte is viable for only about 24
hours after ovulation.

Sperm
• Each day about 300 million sperm complete the process
of spermatogenesis.
• A sperm is about 60 m long and contains several
structures that are highly adapted for reaching and
penetrating a secondary oocyte.
• The major parts of a sperm are the head and the tail.
• The flattened, pointed head of the sperm is about 4–5 m
long.
• It contains a nucleus with 23 highly condensed
chromosomes.

• Covering the anterior two-
thirds of the nucleus is
the acrosome.
• a caplike vesicle filled
with enzymes that help a
sperm to penetrate a
secondary oocyte to bring
about fertilization.
• Among the enzymes are
hyaluronidase and
proteases.

• Sperm swim from the vagina into the cervical canal by
the whiplike movements of their tails (flagella).
• The passage of sperm through the rest of the uterus and
then into the uterine tube results mainly from
contractions of the walls of these organs.
• Prostaglandins in semen are believed to stimulate
uterine motility at the time of intercourse and to aid in the
movement of sperm through the uterus and into the
uterine tube.
• Sperm that reach the vicinity of the oocyte within minutes
after ejaculation are not capable of fertilizing it until about
seven hours later.

• During this time in the female reproductive tract, mostly
in the uterine tube, sperm undergo capacitation.
• A series of functional changes that cause the sperm’s
tail to beat even more vigorously and prepare its plasma
membrane to fuse with the oocyte’s plasma membrane.
• During capacitation, sperm are acted on by secretions in
the female reproductive tract that result in the removal of
cholesterol,glycoproteins, and proteins from the plasma
membrane around the head of the sperm cell.
• Only capacitated sperm are capable of being attracted
by and responding to chemical factors produced by the
surrounding cells of the ovulated oocyte.

• For fertilization to occur, a sperm cell first must penetrate
two layers:
• the corona radiata , the granulosa cells that surround
the secondary oocyte
• the zona pellucida, the clear glycoprotein layer between
the corona radiata and the oocyte’s plasma membrane
• The acrosome,a helmet like structure that covers the
head of a sperm , contains several enzymes.
• Acrosomal enzymes and strong tail movements by the
sperm help it penetrate the cells of the corona radiata
and come in contact with the zona pellucida.

• One of the glycoproteins in the zona pellucida, called
ZP3, acts as a sperm receptor.
• Its binding to specific membrane proteins in the sperm
head triggers the acrosomal reaction, the release of
the contents of the acrosome.
• The acrosomal enzymes digest a path through the zona
pellucida as the lashing sperm tail pushes the sperm cell
onward.
• Although many sperm bind to ZP3 molecules and
undergo acrosomal reactions, only the first sperm cell to
penetrate the entire zona pellucida and reach the
oocyte’s plasma membrane fuses with the oocyte.

• The fusion of a sperm cell with a secondary oocyte sets
in motion events that block polyspermy, fertilization by
more than one sperm cell.
• Within a few seconds, the cell membrane of the oocyte
depolarizes, which acts as a fast block to polyspermy
• A depolarized oocyte cannot fuse with another sperm.
• Depolarization also triggers the intracellular release of
calcium ions, which stimulate exocytosis of secretory
vesicles from the oocyte.
• The molecules released by exocytosis inactivate ZP3
and harden the entire zona pellucida, events called the
slow block to polyspermy.

• Once a sperm cell enters
a secondary oocyte, the
oocyte first must complete
meiosis II.
•It divides into a larger
ovum (mature egg) and a
smaller second polar body
that fragments and
disintegrates.

• The nucleus in the head of the sperm develops into the
male pronucleus, and the nucleus of the fertilized ovum
develops into the female pronucleus.
• After the male and female pronuclei form, they fuse,
producing a single diploid nucleus, a process known as
syngamy .
• Thus, the fusion of the haploid (n) pronuclei restores
thediploid number (2n) of 46 chromosomes.
• The fertilized ovum now is called a zygote.

Cleavage of the Zygote
• After fertilization, rapid mitotic cell divisions of
the zygote called cleavage take place .
• The first division of the zygote begins about 24
hours after fertilization and is completed about 6
hours later.
• The progressively smaller cells produced by
cleavage are called blastomeres.
• Successive cleavages eventually produce a
solid sphere of cells called the morula.
• The morula is still surrounded by the zona
pellucida and is about the same size as the
original zygote.

Blastocyst Formation
• The morula enters the uterine cavity on day 4 or 5
• a glycogen-rich secretion from the glands of the
endometrium, called uterine milk, provides nourishment
for the developing morula.
• At the 32-cell stage, the fluid enters the morula, collects
between the blastomeres, and reorganizes them around
a large fluid-filled cavity called the blastocyst cavity
• also called the blastocoels
• the developing mass is called the blastocyst.
• Though it now has hundreds of cells, the blastocyst is
still about the same size as the original zygote.

• During the formation of the blastocyst two distinct
cell populations arise: the embryoblast and
trophoblast.
• The embryoblast or inner cell mass, is located
internally and eventually develops into the
embryo.
• The trophoblast is the outer superficial layer of
cells that forms the spherelike wall of the
blastocyst.
• It will ultimately develop into the outer chorionic
sac that surrounds the fetus and the fetal portion of
the placenta, the site of exchange of nutrients and
wastes between the mother and fetus.

• On about the fifth day after fertilization, the
blastocyst “hatches” from the zona
pellucida by digesting a hole in it with an
enzyme, and then squeezing through the
hole.
• This shedding of the zona pellucida is
necessary in order to permit the next step,
implantation (attachment) into the
vascular, glandular endometrial lining of
the uterus.

Implantation
• The blastocyst remains free within the
uterine cavity for about 2 days before it
attaches to the uterine wall.
• At this time the endometrium is in its
secretory phase.
• About 6 days after fertilization, the
blastocyst loosely attaches to the
endometrium in a process called
implantation

• As the blastocyst implants, usually in either the
posterior portion of the fundus or the body of the
uterus, it orients with the inner cell mass toward
the endometrium .
• About 7 days after fertilization, the blastocyst
attaches to the endometrium more firmly,
endometrial glands in the vicinity enlarge, and
the endometrium becomes more vascularized
(forms new blood vessels).
• The blastocyst eventually secretes enzymes and
burrows into the endometrium and becomes
surrounded by it.

• Prenatal development is the time from
fertilization to birth and is divided into three
periods of three calendar months each, called
trimesters.
1. The first trimester is the most critical stage
of development, during which the rudiments
of all the major organ systems appear
• also during which the developing organism is
the most vulnerable to the effects of drugs,
radiation, and microbes.

2. The second trimester is characterized
by the nearly complete development of
organ systems.
• By the end of this stage, the fetus
assumes distinctively human features.
3. The third trimester represents a period
of rapid fetal growth.
• During the early stages of this period,
most of the organ systems are becoming
fully functional.

MATERNAL CHANGES
DURING PREGNANCY
• During the first 3 to 4 months of pregnancy, the
corpus luteum in the ovary continues to secrete
progesterone and estrogens, which maintain
the lining of the uterus during pregnancy and
prepare the mammary glands to secrete milk.
• The amounts secreted by the corpus luteum,
however, are only slightly more than those
produced after ovulation in a normal menstrual
cycle.

• From the third month through the remainder
of the pregnancy, the placenta itself provides
the high levels of progesterone and
estrogens required.
• As noted previously, the chorion secretes
human chorionic gonadotropin (hCG) into
the blood From the third month through the
remainder of the pregnancy, the placenta
itself provides the high levels of progesterone
and estrogens required. As noted previously,
the chorion secretes human chorionic
gonadotropin (hCG) into the blood

• In turn, hCG stimulates the corpus luteum to
continue production of progesterone and
estrogens—an activity required to prevent
menstruation and for the continued attachment of
the embryo and fetus to the lining of the uterus
• By the eighth day after fertilization, hCG can be
detected in the blood and urine of a pregnant
woman.
• Peak secretion of hCG occurs at about the ninth
week of pregnancy
• During the fourth and fifth months the hCG level
decreases sharply and then levels off until
childbirth.

• The chorion begins to secrete estrogens after the
first 3 or 4 weeks of pregnancy and progesterone by
the sixth week.
• These hormones are secreted in increasing
quantities until the time of birth
• By the fourth month, when the placenta is fully
established, the secretion of hCG is greatly reduced,
and the secretions of the corpus luteum are no
longer essential.

• A high level of progesterone ensures that
the uterine myometrium is relaxed and that
the cervix is tightly closed.
• After delivery, estrogens and progesterone
in the blood decrease to normal levels.

• Relaxin, a hormone produced first by the
corpus luteum of the ovary and later by the
placenta
• increases the flexibility of the pubic
symphysis and ligaments of the sacroiliac
and sacrococcygeal joints and helps dilate
the uterine cervix during labor.
• Both of these actions ease delivery of the
baby.

• A third hormone produced by the chorion of the
placenta is human chorionic
somatomammotropin (hCS) also known as
human placental lactogen (hPL).
• The rate of secretion of hCS increases in
proportion to placental mass, reaching
maximum levels after 32 weeks and remaining
relatively constant after that

• It is thought to help prepare the mammary
glands for lactation, enhance maternal
growth by increasing protein synthesis,
and regulate certain aspects of
metabolism in both mother and fetus.
• For example, hCS decreases the use of
glucose by the mother and promotes the
release of fatty acids from her adipose
tissue, making more glucose available to
• the fetus.

• The hormone most recently found to be
produced by the placenta is
corticotropin-releasing hormone (CRH)
• which in nonpregnant people is secreted
only by neurosecretory cells in the
hypothalamus.
• CRH is now thought to be part of the
“clock” that establishes the timing of birth.

• Secretion of CRH by the placenta begins at
about 12 weeks and increases
• enormously toward the end of pregnancy.
Women who have higher levels of CRH
earlier in pregnancy are more likely to deliver
prematurely; those who have low levels are
more likely to deliver after their due date.
• CRH from the placenta has a second
important effect: It increases secretion of
cortisol, which is needed for maturation of
the fetal lungs and the production of
surfactant

Pregnancy Stages and Development

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