INTERCOURSE AND SPERM MOVEMENT
While the previous chapter described the human sexual organs, this chapter explains how these organs interact to allow fertilization of a secondary oocyte, which can then develop into a fully formed individual. You will also read about how this new individual develops within the female’s reproductive tract.
The interaction between the male’s and female’s reproductive systems involves a process called intercourse, also called coitus (KŌ-i-tus, “sexual intercourse”) or copulation (kop-ū-LĀ- shun, “joining”). This process involves the insertion of the male’s penis into the female’s vagina and thrusting motions that prompt ejaculation of semen into the female reproductive tract. It is important for semen to remain near the cervix long enough for sperm to enter the uterine cavity. Se- men coagulation accomplishes this. When semen enters the vagina, it quickly coagulates due to the interaction of fibrinogen and clotting factors in the semen. This prevents the semen from leaking out of the vagina and keeps it near the cervix. Within a short time, plasminogen in the semen becomes plasmin and dissolves the fibrin threads. This process, called liquefaction (lik-wih-FAK-shun), lessens the semen’s viscosity.
Prostaglandins in the semen stimulate the uterus to contract, creating a negative pressure in the uterine cavity, which helps pull the semen through the cervical canal. An orgasm experienced by the female also causes uterine contractions. The spermatozoa use their waving flagella to move toward the oviducts, and chemicals secreted from the secondary (2o) oocyte help direct the sperm towards it (this is an example of chemotaxis). In most cases, less than 1% of the 200-300 million ejaculated sperm reach the oocyte near the infundibulum.
As a sperm cell moves toward the oocyte, the membrane around its acrosome breaks down, enabling the sperm to release its acrosomal enzymes at an appropriate time to wear away the protective coverings of the 2o oocyte. The wearing away of the cholesterol during the sperm’s journey is called sperm capacitation (ka-pas-i-TĀ-shun, “become capable”). If capacitation occurs too soon within the female reproductive tract, the sperm releases its acrosome enzymes prematurely. If capacitation occurs too late, the sperm’s acrosome enzymes will not be available to wear down the oocyte’s covering.
BARRIERS TO FERTILIZATION
When one considers all the factors that might prevent fertilization, it’s a wonder that fertilization ever occurs. Some of the barriers to fertilization include the following:
- Acidity of the vagina (kills the sperm)
- Mucus plug in the cervix blocks sperm’s entrance into the uterus
- Phagocytosis in the female’s reproductive tract (WBCs attack and destroy sperm)
- Great length of the female reproductive tract (many sperm die of starvation along the way)
- Premature capacitation (releasing acrosomal enzymes prematurely prevent sperm from breaking through oocyte’s protective coverings)
- Insufficient number of sperm produced (since most die along the way, there needs to be a large number of sperm at the beginning of the trip)
- Abnormal sperm shape and nonfunctional flagella (nonmotile sperm cannot propel themselves toward the oocyte)
- Oocyte may not enter the oviduct (instead, it enters the abdominal cavity)
- Ovaries alternate ovulation each month; therefore, half of the sperm may enter an empty oviduct
- Short lifespan of the 20 oocyte (oocyte is viable for about one day, so even if numerous sperm arrive, the oocyte may have already begun degeneration)
- Cilia of an oviduct produce a current moving toward the uterus (sperm must swim against this current like salmon swimming upstream against a swift current)
Sperm may penetrate the oviduct’s mucosa (sperm waste energy trying to fertilize some- thing other than the oocyte)
FERTILIZATION
Fertilization or syngamy (SIN-ga-mē; syn, togeth-er; gamy, gametes) in humans refers to the union of a single sperm cell with a secondary oocyte (not an ovum). When sperm cells reach the oocyte, they must penetrate its protective coverings (the corona radiata and zona pellucida). A sperm that arrives “late” is more likely to penetrate the oocyte’s cell membrane because other sperm have already been working on penetrating the oocyte’s coverings.
When a sperm cell fuses with the oocyte’s cell membrane, Na+ ion channels in the membrane open. As Na+ ions diffuse into the cytoplasm, the oocyte’s membrane depolarizes. This electrical change, called the fast-block to polyspermy (pol-ē-SPER-mē), prevents other sperm cells from fusing with the oocyte. At this time, the sperm’s nucleus (not the entire sperm cell) enters the oocyte’s cytosol. During the fast-block period, the oocyte’s endoplasmic reticulum releases Ca2+ ions into the cytosol, which cause vesicles in the oocyte’s cortex region to release a variety of chemicals into the extracellular space (located between the oocyte’s plasma membrane and the zona pellucida). Enzymes in this released fluid destroy sperm receptors on the oocyte’s plasma membrane and increase the fluid’s osmotic pressure, which causes water to enter the space. Consequently, no more sperm can attach to the oocyte. The swelling of this thin channel is analogous to filling a “moat” around a castle, and it pushes all other sperm away from the oocyte. This event is the slow-block to polyspermy.
The inflow of Ca2+ ions during the slow-block period stimulates the 20 oocyte to complete meiosis II, while the sperm nucleus waits inside the oocyte’s cytosol adjacent to the plasma membrane. After completing meiosis II, the oocyte undergoes an unequal cytokinesis. The large cell (ootid) contains the sperm’s nucleus and the small cell is a polar body. The sperm’s nucleus and the ovum’s nucleus fuse to form a single diploid nucleus, and the cell is now a zygote (ZĪ-gōt, “yolked”). The following sections describe various stages of development and Figure 33-1 can give you an idea of where the earliest stages occur.
EMBRYO DEVELOPMENT
The embryo (EM-brē-ō; “in-swelling”) is the stage from the zygote up to 8 weeks of development. Almost immediately after the sperm and ovum nuclei unite, the zygote’s 46 DNA molecules replicate. Within about two days, the cell divides to form two identical 2N daughter cells. Each of these cells undergoes cell division to form more daughter cells. Within about 3 days, the zygote develops into a “solid ball” of 16-32 cells called a morula (MŌR-ū-la; “mulberry”). As cell divisions continue, the morula becomes a fluid-filled hollow ball of cells called a blastocyst (BLAS-tō-sist; blast, bud; cyst, bladder). The hollow cavity within the blastocyst is the blastocoel (BLAS-tō-sēl; coel, cavity), and the layer of cells surrounding it is the trophoblast (TRŌ-fō-blast; troph, feeding). Continued cell division at one end of the blastocyst produces a mass called the inner cell mass.
In rare instances, the two daughter cells formed by the first cytokinesis separate and each cell produces a blastocyst. The two blastocysts then develop into monozygotic (mon-ō-zi-GOT-ik) twins (or identical twins). If twins originate from two separate oocytes, they are dizygotic (dī-zī-GOT-ik) twins (or fraternal twins), who are no more similar than are siblings born years apart.
IMPLANTATION OF BLASTOCYST
The blastocyst usually makes contact with the endometrium 5-7 days after fertilization. When it attaches to the endometrium, the trophoblast cells secrete enzymes that digest a small region of the stratum functionalis. The liquefied functionalis cells provide nourishment for the developing embryo. Eventually, the entire blastocyst embeds itself in the endometrium in a process called implantation. The endometrium quickly regenerates cells over the implantation site. Shortly after implantation, the trophoblast begins to secrete a hormone called human chorionic gonadotropin (HCG) that prevents menstruation (see previous chapter).
ORGANOGENESIS
Following implantation, a cavity begins to form in the inner cell mass and eventually becomes the amniotic (am-nē-OT-ik; amnion, membrane) cavity. A single layer of cells, called ectoderm (EK-tō-derm; “outer skin”), separates the amniotic cavity from the blastocoel. The ectoderm cells soon divide to form a second layer of cells, called the endoderm (EN-dō-derm; “inner skin”), which lies next to the blastocoel. Together the ectoderm and endoderm layers represent germ layers, so-named because they “germinate,” or give rise to, tissues and organs. The ectoderm will give rise to nervous tissue, epidermis, ears, lens, cornea, and epithelia of the oral and nasal cavities. The endoderm will give rise to epithelia in the digestive, urinary, respiratory, and reproductive systems. In this early stage, the ectoderm and endoderm layers make up the embryonic disc.
By day 16, an event called gastrulation (gas- trū-LĀ-shun; gastr, belly) occurs; this stage is so- named because it marks the beginning of stomach and intestinal development. A crease called the primitive streak develops in the ectoderm and will become the longitudinal axis for the embryo. A bulb-like enlargement, called the primitive knot, develops at one end of the primitive streak and will develop into the head of the embryo. Some of the ectoderm cells migrate through the streak and spread out between the ectoderm and endoderm. This new middle germ layer is the mesoderm (MĒ-sō-derm; “middle skin”). The mesoderm cells quickly forms a rod-like structure called the notochord (NŌ-tō-kord; noto, back) that serves to orient the development of the spinal cord and backbone. The mesoderm gives rise to connective and muscle tissues and certain membranes. Refer to Figure 33-3 to see how the different germ layers and other structures develop.
Figure 33-1. Earliest stages of development
FETAL PERIOD
The fetus (FĒ-tus; “offspring”) is the time of development from the ninth week until birth. All organs develop during this time, but must undergo significant development to become fully functional. Structures that serve to nourish or protect the fetus during the fetal period include the placenta, umbilical cord, and amnion, yolk sac, and allantois.
The placenta (pla-SEN-ta; “cake”) is a highly vascularized tissue where nutrient and waste exchange occurs between the mother and fetus. The portion of the placenta made by the fetus is the chorion (KOR-ē-on; “membrane”), whereas, the part made by the mother is the decidua basalis (de-SID-ū-a, “fall off”; ba-SAL-is, “base”).
The chorion consists of trophoblast cells and mesoderm cells, and it lines a cavity called the extraembryonic coelom (SĒ-lōm; “cavity”). The villus (VIL-us) chorion has fingerlike projections that penetrate into the endometrium and secretes HCG. Under normal conditions, blood of the mother and fetus do not mix.
The umbilical (ūm-BIL-i-kal) cord attaches the fetus to the placenta, and it contains a mesenchyme tissue called Wharton’s (HOR-tonz) jelly. It contains one umbilical vein and two umbilical arteries. The umbilical arteries branch from the fetus’s internal iliac arteries and deliver deoxygenated blood and wastes to the placenta. The umbilical vein delivers oxygenated blood and nutrients from the placenta to the fetus.
The amnion (or amniotic sac) is a membranous sac that surrounds the amniotic cavity and contains a watery fluid called amniotic fluid (AF). The AF contains fetal urine and maternal substances filtered through the placenta. The AF cushions the embryo, maintains its homeostatic temperature, and protects it against abrasion.
The yolk sac is a baglike structure formed by repeated divisions of endoderm cells (within the first two weeks of development). It produces blood for the embryo.
The allantois (a-LAN-toyz; “sausage”) is a sausage-shaped projection of the yolk sac that produces blood cells for the early embryo and gives rise to the umbilical blood vessels.
APPROACHING BIRTH
As birth approaches, the mother begins to feel fetal movement known as quickening. During this time, fine silky hair called lanugo (lan-Ū-gō, “wool”) develops on the head and neck of the fetus. In addition, sebaceous glands of the fetus secrete a fatty, pasty material called vernix caseosa (VER- niks, “varnish”; kas-ē-Ō-sa, “cheesy”) that covers the fetus’s skin and protects it from AF. As birth approaches, the placenta increases secretion of a hormone called relaxin that loosens the mother’s pelvic ligaments and her pubic symphysis. This allows expansion of the pelvic cavity and more room for fetal growth. During this time, the fetus performs fetal respiration, during which it “inhales” AF into the lungs then exhales it. The fetus also swallows some of the AF.
LABOR AND PARTURITION
Labor refers to the events leading up to parturition (par-tur-ISH-un) or birth. During false labor, uterine contractions produce pain at irregular intervals. During true labor, contractions and pain occur at regular intervals and progressively shorten as the time for birth approaches. During true labor, the vagina discharges a fluid called show, which consists of blood and mucus.
The initiation of labor begins as the mother’s blood estrogen level becomes very high, possibly in response to chemicals released from the fetus. The high level of estrogen stimulates the smooth myofibers in the myometrium to up-regulate their oxytocin receptors, which makes the uterus becomes more “excitable.” The uterus then begins to experience weak, irregular contractions (false labor), which eventually leads to true labor. True labor involves a dilation stage, expulsion stage, and ends with the placental stage.
1. Dilation Stage
The dilation stage is the time from the beginning of true labor until complete dilation of the cervix. Cervical dilation occurs when the baby’s head moves into the cervical canal. The contracting uterus and dilating cervix initiates impulses to the hypothalamus causing the release of oxytocin (OT; ok-sē-TŌ-sin; “swift birth”) from the posterior pituitary gland. The OT causes vigorous uterine contractions that push the fetus farther into the cervical canal. As the cervix experiences more dilation, it sends more impulses to the brain, which, in turn, releases more oxytocin. This positive feedback continues until cervical dilation is complete and the amniotic sac ruptures (Figure 33-2). When this happens, it is said that the “water has broken.”
Figure 33-2. The oxytocin-feedback cycle
2. Expulsion Stage
The expulsion stage is the time from complete dilation to delivery of the child.
3. Placental Stage
The placental stage is the time between the delivery of the child and delivery of the placenta. The delivered placenta is the afterbirth. Continued uterine contractions (due to oxytocin) constrict uterine blood vessels and reduce the amount of bleeding from the uterine wall. During this time in the heart of a normal fetus, the foramen ovale, ductus arteriosus, and ductus venosus close. Shortly after birth, the infant may excrete a greenish fecal material called meconium (mē-KŌ-nē-um, “poppy”), which includes ingested AF, mucus, bile, and swallowed skin cells.
Within 1-5 minutes after delivery, clinicians assign the infant an APGAR score, which is an objective ranking of its health based on five characteristics. Each characteristic can receive a score ranging from 0 and 2, where 0 is below normal, 1 is acceptable, and 2 is very good. Interestingly, APGAR is the name of a person, but each letter represents a certain trait: A is appearance (bluish → pink); P is pulse rate (>100 good); G is grimace (facial grimace in response to plantar stroking); A is activity (strong kick, motion); and R is respiration (strong cry is good). Very healthy babies have an APGAR score of 8-10.
Figure 33-3. Later stages of development
LACTATION
Mammary glands produce milk in response to the hormone prolactin (PRL). The anterior pituitary gland secretes PRL throughout pregnancy, but high levels of estrogen and progesterone secreted from the placenta during this time inhibit milk production. However, very high levels of estrogen and progesterone near the end of pregnancy cause the hypothalamus to release large amounts of prolactin-releasing hormone (PRH), causing the anterior pituitary to secrete more PRL.
After delivery of the placenta, estrogen and progesterone levels decrease and the mammary glands begin to secrete liquid called colostrum (kō-LŌS-trum, “foremilk”). This fluid does not contain much lactose or fat, but it contains a high concentration of vitamins, minerals, and protein (including antibodies). The passage of antibodies from the mother to the infant is an example of passive immunity in the fetus. A few days after delivery, the mammary glands begin to produce true milk.
The sucking action of the infant on the mother’s nipple sends impulses to the hypothalamus, causing the release of more OT from the posterior pituitary. OT stimulates contraction of smooth muscle around the alveoli of the mammary gland causing the ejection of milk into the lactiferous ducts. At the end of the lactiferous duct, milk oozes from the nipple in a process called milk letdown. OT released at this time also stimulates uterine contractions, which may cause the mother to experience abdominal pain when the infant is nursing. The uterine contractions can help minimize blood loss from the uterus. Milk production can continue for several years, as long as the infant is still nursing.
TOPICS TO KNOW FOR CHAPTER 33
(Development)
afterbirth
allantois
amnion
amniotic cavity
amniotic fluid
amniotic sac
APGAR score
approaching birth
barriers to fertilization
birth
blastocoel
blastocyst
cervical plug
chemotaxis
chorion
coitus
colostrum
copulation
corona radiata
daughter cells
decidua basalis
dilation stage
diploid zygote
dizygotic twins
ductus arteriosus
ductus venosus
ectoderm
embryo
embryo development
embryonic disc
endoderm
expulsion stage
extraembryonic coelom
false labor
fast-block to polyspermy
fertilization
fetal period
fetal respiration
fetus
foramen ovale
fraternal twins
gastrulation
germ layers
HCG
human chorionic gonadotropin
human development
identical twins
implantation
implantation of blastocyst
inner cell mass
intercourse
labor
lactation
lactiferous ducts
lanugo
mammary glands
meconium
mesoderm
milk letdown
monozygotic twins
morula
notochord
organogenesis
orgasm
OT
oxytocin
oxytocin receptors
parturition
passive immunity
placenta
placental stage
positive feedback
PRH
primitive knot
primitive streak
PRL
prolactin
prolactin-releasing hormone
prostaglandins in semen
quickening
relaxin
secondary oocyte
semen liquefaction
slow-block to polyspermy
sperm capacitation
sperm movement
syngamy
trophoblast
true labor
umbilical arteries
umbilical cord
umbilical vein
vernix caseosa
villus chorion
Wharton’s jelly
yolk sac
zona pellucida
EOC Questions
