Download Lecture Notes 27- Development

DEVELOPMENT
Once spermatozoa and ova have developed through meiosis and maturation, and the
sperm have been deposited into the vagina, pregnancy can occur.
What is pregnancy?
Pregnancy is the sequence of events that normally includes fertilization,
implantation into the uterus, embryonic and fetal growth, and 266 days after
fertilization (the gestation period), birth (parturition).
A.
DEVELOPMENT DURING PREGNANCY
1.
FERTILIZATION
What is fertilization?
Fertilization is the series of events from initial contact of sperm with
the barriers surrounding the egg to the union of the genetic
materials of the sperm and ovum into a single nucleus, forming the
zygote.
How many sperm actually reach the vicinity of the egg?
Of the 300 million or so ejaculated into the vagina, less than
0.0001% (about 2,000 - 3,000) reaches the egg.
When and where does fertilization take place?
A successful pregnancy usually begins in the ampulla of the
oviduct, some 12 - 24 hours after ovulation.
Name and then briefly describe three factors that aid sperm during their
movement towards the ovum.
sperm acrosome -- Once the sperm is within the female reproductive fluids, the acrosome of the sperm releases the enzyme
acrosin. This enzyme works to increase sperm motility.
prostaglandins -- Prostaglandins in semen stimulate uterine contractions that draw sperm towards the uterine openings of
the oviducts.
the ovum -- The ovum itself is thought to secrete a chemical
attractant for the sperm, giving the sperm a chemotactic
gradient to follow.
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Define capacitation.
Capacitation refers to the functional changes that occur in sperm
once inside the female reproductive tract that allow them to fertilize
the ovum.
What is the approximate minimum time that sperm must reside in the
female reproductive fluids for capacitation to occur?
about 10 hours
What happens during the capacitation process?
The membrane of the sperm head contains substantial amounts of
cholesterol, making the membrane tough to prevent accidental
leakage of the acrosomal enzymes. The female fluids “wash away”
this extra cholesterol and other inhibitory factors, making the
membrane more fragile, thus allowing the acrosome to be released.
List and define the barriers to penetration through which a sperm passes
to enter an ovum.
Sperm, using acrosomal enzymes and flagellar movement, push
through the following barriers, from the outside in:
1.
corona radiata
2.
zona pellucida
3.
ovum cell membrane
Describe the corona radiata.
The corona radiata consists of the several rows of follicular
cells immediately around the ovum when it was ovulated.
They provide protection and probably nourishment for the
ovum during its journey down the oviduct.
Describe the zona pellucida.
The zona pellucida is a gelatinous material surrounding the
ovum cell membrane. On reaching the zona pellucida,
sperm bind in a species-specific manner to receptors there
and greatly increase their flagellar activity.
Describe the ovum cell membrane and its role in fertilization.
When the first sperm touches the egg cell membrane, it
begins to push itself into the egg. As the sperm and egg cell
membranes merge they ultimately fuse together and the two
cells become one. The sperm cell membrane and cytoplasm
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are incorporated into that of the ovum. The male genetic
material, inside the ovum, is known as the male pronucleus.
What are blocks to polyspermy and when are they initiated? What is their
purpose?
When the first sperm touches the egg membrane, a series of
events known as the blocks to polyspermy are initiated. There is a
change in egg cell membrane polarity (more positive) and a
pushing away of the zona pellucida. These changes prevent other
sperm from entering the egg, thus preserving genetic integrity of
the zygote.
What two events occur within the ovum after the male pronucleus enters?
1.
2.
The secondary oöcyte, which was arrested in meiosis II,
resumes its meiotic division. This gives rise to the female
pronucleus, with its proper number of chromosomes (1n1c),
and a polar body that is expelled.
The male (1n1c) and female (1n1c) pronuclei fuse and the
chromosomes (now 2n), line up at metaphase for their first
mitotic cell division.
Fertilization is completed and the fertilized egg is now called a
zygote.
What is the fast block to polyspermy?
When the first sperm membrane touches the egg cell membrane, a
wave of depolarization sweeps across the cell, much like the
depolarization that occurs in muscle and nerve cells. This change
to a positive membrane potential prevents any other sperm from
touching the ovum.
What is the slow block to polyspermy?
The wave of depolarization also causes the release of stored
materials from the egg into the space between cell membrane and
zona pellucida. This physically forces the zona pellucida and all
attached sperm away from the egg. Furthermore, the egg secretes
hydrogen peroxide into this newly-formed space, effectively killing
any sperm in the space.
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2.
3.
FORMATION OF THE MORULA
DEVELOPMENT OF THE BLOASTOCYST
Describe each of the following:
cleavage -- After fertilization, the one-cell zygote undergoes a
series of rapid mitotic divisions in a process called cleavage.
During this time the total number of cells of the zygote
increases, but not its overall size as individual cells, called
blastomeres, become smaller and smaller.
morula -- The first cleavage occurs 36 hours after fertilization, with
each successive one occurring about every 24 hours. At
about 96 hours (4 days), the cluster of 16 - 32 blastomeres
forms a solid ball of tightly-packed cells that resemble a
mulberry. This cluster, still in the oviduct, is known as a
morula.
blastocyst -- By the fifth day, the morula, still moving through the
oviduct towards the uterine cavity, continues to divide, giving
rise to a hollow ball of 64 - 128 cells called the blastocyst.
The blastocyst consists of two distinct groups of cells, the
trophoblast and the inner cell mass. There is an internal
cavity called blastocoel.
trophoblast -- The trophoblast forms a single-cell layer around the
blastocyst, separating its contents from the surrounding
environment. The cells of the trophoblast are used in
implantation into the stratum functionalis of the uterus and
will form the fetal portion of the placenta.
inner cell mass -- The inner cell mass is a cluster of cells tucked
into one side of the blastocyst. Opposite it is the cavity
known as the blastocoel. The cells of the inner cell mass
give rise to the embryo proper and the extraembryonic
membranes.
4.
IMPLANTATION
Describe implantation.
On the 6th – 7th day, the blastocyst is moved into the uterine cavity
where it begins to attach itself to the endometrium. The trophoblastic cells begin to grow rapidly and invade the soft, glycogen rich
stratum functionalis. This eventually causes the embryo to be
completely embedded in the wall of the uterus.
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What is accomplished by implantation?
Three things are accomplished at implantation:
1.
The blastocyst establishes itself in an environment rich in
nutrients.
2.
The trophoblast orients itself for formation of the fetal portion
of the placenta.
3.
It induces formation of the maternal portion of the placenta.
B.
EMBRYONIC DEVELOPMENT
1.
BEGINNINGS OF ORGAN SYSTEMS
When is the embryonic period of development?
The first two weeks of development are given over to the events of
cleavage and implantation.
Weeks 3-8 mark the embryonic period.
Name the three major events of the embryonic period.
The three major events of the embryonic period are:
1.
gastrulation
2.
formation of the extraembryonic membranes
3.
formation of the placenta
What is gastrulation?
During the embryonic period the inner cell mass of the blastocyst
differentiates into the 3 primary germ layers by the process of cell
migration collectively known as gastrulation.
Name the primary germ layers.
The three primary germ layers are the:
1.
ectoderm
2.
endoderm
3.
mesoderm
What is the ectoderm and what are its major derivatives in the
adult?
Ectoderm, the outermost germ layer, gives rise to the
nervous system and all of its derivatives and the epidermis,
and all of its derivatives.
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What is the endoderm and what are its major derivatives in the
adult?
Endoderm, the innermost germ layer, gives rise to the
epithelial linings of the digestive, respiratory, urinary, and
reproductive systems.
What is the mesoderm and what are its major derivatives in the
adult?
Mesoderm, the middle germ layer, gives rise to the
connective tissues, muscles, cardiovascular system, and
blood of the body.
2.
EMBRYONIC MEMBRANES
What are the extraembryonic membranes?
The second major event of the embryonic period is the formation of
the extraembryonic membranes. These membranes lie outside the
embryo proper and serve to protect and nourish it.
Name the extraembryonic membranes.
The extraembryonic membranes are the:
1.
yolk sac
2.
amnion
3.
chorion
4.
allantois
Describe the yolk sac.
The yolk sac is an endodermal structure that gives rise to the
original blood cells and germ cells of the individual. Its
function as a yolk sac in mammals is vestigial (a bodily part
or organ that is small and degenerative or imperfectly
developed in comparison to one or more fully developed in
an earlier stage of the individual, in a past generation, or in
closely related forms), since mammalian ova have no yolk.
Describe the amnion.
The amnion is an ectodermal sac that surrounds the
embryo/fetus and secretes the amniotic fluid that bathes the
embryo/fetus. It serves to absorb shocks, help regulate
temperature, and prevent adhesion during growth.
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Describe the chorion.
The chorion is a mesodermal structure derived from the
trophoblast. It serves to form the fetal contribution to the
placenta.
Describe the allantois.
The allantois is a vestigial endodermal structure in humans.
In lower animals it is used to store liquid wastes (urine) prior
to hatching. In mammals the allantoic blood vessels are
used to help make the placenta.
3.
PLACENTA AND UMBILICAL CORD
Discuss the placenta and umbilical cord.
The third major event of the embryonic period is formation of the
placenta, a structure formed by trophoblastic cells of the embryo
and stratum functionalis cells of the uterus. What is the role of the
placenta in development?
The fetal placenta, containing fetal blood, lies within the maternal
placenta, which contains maternal blood. This arrangement allows
free exchange by diffusion of gases, nutrients, and wastes between
the two compartments.
The umbilical cord is the vascular connection between the fetus
and the placenta. Describe the blood vessels and connective
tissue components of the cord.
The umbilical cord contains two umbilical arteries, each derived
from the internal iliac arteries, which carry relatively nutrient-poor
and oxygen-poor blood to the placenta.
The umbilical cord contains one umbilical vein that brings fresh
oxygen and nutrients into the fetus, opening into the inferior vena
cava of the fetus.
Surrounding the vessels is a layer of amnion filled with a supporting
mucous connective tissue known as Wharton’s jelly.
At birth, the placenta detaches from the uterus and becomes the
afterbirth. The umbilical cord, after it’s tied and cut, eventually dies
and leaves a scar called the umbilicus (navel, belly button).
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C.
FETAL GROWTH
What is the extent of the fetal period of development?
At the end of the 8 weeks, the germ layers have formed the rudiments of
all the future tissues and organs of the body. It is at this time that the fetal
period begins, extending from weeks 9 - 38 (birth).
What are the primary events of the fetal period?
1.
2.
organ systems complete development and become ready for
normal functioning outside the uterus
individual grows from 1 inch and 1/30th ounce to about 20 inches
and 7 pounds.
Distinguish the ovarian phase from the placental phase of pregnancy.
During the first 10 weeks of pregnancy, the corpus luteum continues
secreting estrogen and progesterone to maintain the uterus. This is the
ovarian phase of the pregnancy.
After this time, the placenta becomes the source of the hormones of
pregnancy. This is the placental phase.
D.
HORMONES OF PREGNANCY
Name and describe the hormones of pregnancy.
The hormones of pregnancy are:
1.
human chorionic gonadotropin (HCG)
2.
estrogen
3.
progesterone
4.
human somatomammotropin (human placental lactogen)
5.
relaxin
6.
inhibin
Describe the role of HCG in pregnancy.
HCG is secreted by the trophoblast and fetal placenta for about 12
weeks following fertilization. It stimulates the continued secretion of
estrogen and progesterone from the corpus luteum to ensure
maintenance of the endometrium in the secretory phase.
Describe the role of estrogen and progesterone in pregnancy.
As corpus luteum activity tapers off, the placenta becomes the
source of estrogen and progesterone. These hormones continue to
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maintain the uterus and begin preparing the mammary gland for
milk synthesis and secretion.
Describe the role of human somatomammotropin in pregnancy.
Human chorionic somatomammotropin (human placental lactogen)
is secreted during the last trimester. It aids in breast development
for lactation and enhances fetal growth by causing mother to
conserve glucose, thus making it available for the fetus.
Describe the role of relaxin in the pregnancy.
Relaxin works to relax the pubic symphysis and the ligaments of
the sacroiliac and sacrococcygeal joints, thus widening the pelvis
and therefore the bony birth canal. It also helps to dilate the cervix.
Describe the role of inhibin in pregnancy.
The role of inhibin is unknown, but it is believed to inhibit the
secretion of FSH during pregnancy.
E.
PARTUITION OF LABOR
What is the process of labor?
After approximately 266 days of gestation, the process of parturition (birth)
begins.
Labor is the process by which the fetus is expelled from the uterus through
the birth canal to the outside.
Name the three stages of labor.
The three stages of labor are
1.
stage of dilation
2.
stage of expulsion
3.
placental stage
What are the events of the stage of dilation?
The stage of dilation is from the onset of labor to the complete
dilation of the cervix (10 cm). The regular uterine contractions that
occur during this time are in response to oxytocin. Also during this
time is the rupture of the amniotic membrane.
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What occurs during the stage of expulsion?
The stage of expulsion extends from complete cervical dilation to
delivery of the baby.
What occurs during the placental stage of birth?
The placental stage is the time after delivery until the placenta is
expelled. It is characterized by very powerful uterine contractions
that aid in constriction of uterine blood vessels to prevent hemorrhage.
F.
ADJUSTMENTS OF THE INFANT AT BIRTH
Immediately after birth, the neonate must make certain anatomical and
physiological adjustments to ensure continued life outside of the uterus.
1.
RESPIRATORY SYSTEM
Describe the changes that occur in the respiratory system.
When the umbilical cord is cut, fetal blood oxygen drops rapidly, as
carbon dioxide, and therefore hydrogen ion, concentration rises.
The rising hydrogen ions stimulate the inspiratory neurons of the
medullary rhythmicity area. Because the first inspiration is so deep,
exhalation is vigorous and the baby emits crying sounds as air
rushes past the vocal cords.
2.
CARDIOVASCULAR SYSTEM
Describe the changes in the cardiovascular system immediately after birth.
Following the first inspiration, the cardiovascular system must also
make immediate changes. Name and discuss the five differences
between the fetus and the adult.
1.
Increased pressure in the left atrium due to the increased
venous return from the lungs causes the foramen ovale to
close, thus closing the interatrial septum functionally The
septum grows closed, forming the fossa ovalis.
2.
The ductus arteriosus, a vascular connection found between
the pulmonary trunk and the aortic arch, collapses and
grows together to form the ligamentum arteriosum.
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G.
3.
The ductus venosus, a fetal bypass of the liver that allowed
blood to move from the umbilical vein directly to the inferior
vena cava, collapses and grows together to form the
ligamentum venosum of the adult liver.
4.
The umbilical vein collapses and fuses to form the
ligamentum teres (round ligament of the adult liver).
5.
The right and left umbilical arteries collapse and grow
together to form the median umbilical ligaments. The
proximal parts of each vessel persist as vessels to the
urinary bladder.
PHYSIOLOGY OF LACTATION
Now that we have a baby, we have to feed her. What is lactation?
Lactation refers to the synthesis, secretion, and ejection of milk from the
mammary glands.
How are the mammary glands prepared for milk synthesis during pregnancy?
Estrogen, progesterone, and human chorionic somatomammotropin
stimulate the milk-producing cells of the breast to increase their numbers
by mitosis.
If the breasts are prepared to make milk during the pregnancy, why does the
mother not make milk during this time? In fact, why does it take a couple of days
after delivery for milk letdown to occur?
Milk synthesis is dependent upon prolactin (PRL) synthesis by the anterior
pituitary gland. PRL secretion is controlled by PRL-RF and PRL-IF from
the hypothalamus. High blood levels of estrogen and progesterone during
pregnancy prevent the secretion of PRL by stimulating the secretion of
PRL-IF. Low blood levels of estrogen and progesterone allow the secretion of PRL by stimulating the secretion of PRL-RF. After birth, when
estrogen and progesterone levels fall, PRL-RF and therefore PRL is
secreted, allowing milk synthesis to begin.
In a stepwise fashion, describe the suckling (milk letdown) neuroendocrine reflex.
The initial stimulus for milk let down is …?
tactile stimulation of the nipple and areola by the sucking reflex.
Sensory input from the tactile receptors is to the ...?
hypothalamus
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Motor output is in the form of what 2 hormones?
oxytocin – Oxytocin stimulates the myoepithelial cells surrounding
the ducts of the mammary glands to contract, thus ejecting
milk into the lactiferous sinuses where it can be suckled.
prolactin – Prolactin stimulates the milk-producing cells of the
breasts to synthesize more milk.
One often hears, or reads in books that are supposed to contain correct information, that while nursing your baby you don’t have to worry about birth control
because you cannot ovulate. Why might this be so?
High PRL levels TEND to inhibit FSH secretion. No FSH means that the
ovarian cycle cannot begin to produce a new egg for ovulation.
THIS IS A VERY UNRELIABLE BIRTH CONTROL!!!!!!!!!!!!
If you use this method of birth control, you will most likely be having
another child in 11 months or so.
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