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 three factors that aid sperm during their movement towards the ovum.
sperm acrosome, prostaglandins, 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.
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 mem
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 are incorporated into th
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
What two events occur within the ovum after the male pronucleus enters?
1. The secondary o�cyte, 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.
2. The male (1n1c) and female (1n1c) pronuc
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 to
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
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, beco
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 clust
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
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
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.
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 tropho-blastic cells begin to grow rapidly and invade the soft, glycogen rich stratum functionalis. This eventually causes the e
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
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.
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.
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.
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 com
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.
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.
Discuss the placenta and umbilical cord.
see notes
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. organ systems complete development and become ready for normal functioning outside the uterus
2. 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
Name 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 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.
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.
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 hemor-rhage.
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 inspirati
Describe the changes in the cardiovascular system immediately after birth.
see notes
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 PR
In a stepwise fashion, describe the suckling (milk letdown) neuroendocrine reflex.
see notes