Ch.29 Growth and Development

cytotrophoblast

which is the inner cellular layer of the trophoblast

syncytiotrophoblast

which is the outer, thick layer of the trophoblast where no plasma membranes are visible

afterbirth

Immediately after the baby is born, the placenta is also expelled from the uterus. The expelled placenta is often called the afterbirth

Nulliparous women

@Dilation Stage

effaced

@Dilation Stage

parous women

@Dilation Stage

crowning

@Expulsion Stage

episiotomy

@Expulsion Stage

Premature labor

@Labor

parturition

@Labor

bilaminar germinal disc, or blastodisc.

@embryoblast

blastodisc

@embryoblast

epiblast

@embryoblast

hypoblast

@embryoblast

teratogens

@organogenesis

Labor

Labor is also known as parturition and is the physical expulsion of the fetus and placenta from the uterus. True labor typically occurs at 38 weeks for a full-term pregnancy, but not all uterine contractions lead to true labor. We first consider factors that initiate labor, and then compare the contractions of false labor with those of true labor; we conclude with a description of the three stages of labor ■ All of these factors result in the uterine myometrium becoming more sensitive and "irritable" in the later stages of pregnancy, and contractions begin to occur. These contractions typically are weak and irregular, but as levels of estrogen and oxytocin continue to rise in the later stages, they become more intense and frequent. Thus, weak contractions may occur and be noticed as soon as the second trimester of pregnancy. ■ Premature labor refers to labor that occurs prior to 38 weeks. Premature labor (and giving birth to a premature infant) is not desirable because the infant's body systems, especially the lungs, may not be fully developed. Very premature infants are at greater risk for morbidity and mortality due to their underdeveloped organ systems. Thus, the ideal outcome is for labor to begin as close as possible to term.

Zona Pellucida Penetration

After the sperm have made a pathway through the corona radiata, they release digestive enzymes from their acrosomes to penetrate the zona pellucida. This release of enzymes (primarily hyaluronidase and acrosin) from the acrosome is known as the acrosome reaction. When one sperm successfully penetrates the zona pellucida and its nucleus enters the secondary oocyte, immediate changes occur to both the zona pellucida and the oocyte, so that no other sperm can enter. In essence, the zona pellucida hardens, preventing other sperm from binding to and ultimately digesting their way through this layer. This process is necessary to ensure that only one sperm fertilizes the oocyte.

cleavage

After the zygote divides once and reaches the 2-cell stage, a series of mitotic divisions called cleavage results in an increase in cell number, but not an increase in the overall size of the structure

embryoblast

By day 8, the cells of the embryoblast begin to differentiate into two layers. A layer of small, cuboidal cells adjacent to the blastocyst cavity is termed the hypoblast layer, and a layer of columnar cells adjacent to the amniotic cavity is called the epiblast layer (figure 29.5). Together, these layers form a flat disc termed a bilaminar germinal disc, or blastodisc.

organogenesis

Subsequent interactions and rearrangements among the cells of the three layers prepare for the formation of specific tissues and organs

amnion

The amnion (am′nē-on; amnios = lamb) is a thin membrane that is formed from and continuous with the epiblast layer. The amnion eventually encloses the entire embryo in a fluid-filled sac called the amniotic cavity to protect the embryo from desiccation (drying out). The amniotic membrane is specialized to secrete the amniotic fluid that bathes the embryo.

extraembryonic membranes

The bilaminar germinal disc and trophoblast also produce extraembryonic membranes to mediate between them and the environment (figure 29.7). They first appear during the second week of development and continue to develop during the embryonic and fetal periods. They protect the embryo and assist in vital functions such as nutrition, gas exchange, and removal and storage of waste materials. These extraembryonic membranes are as follows

chorion

The chorion (kō′rē-on; membrane covering the fetus) is the outermost extraembryonic membrane and is formed from both the rapidly growing cytotrophoblast cells and syncytiotrophoblast. These cells blend with the functional layer of the endometrium and eventually form the placenta, the site of gas and nutrient exchange between the embryo and the mother.

Dilation Stage

The dilation stage begins with the onset of regular uterine contractions and ends when the cervix is *effaced* (thinned) and dilated (expanded) to 10 centimeters in diameter. This is the longest of the three stages and is the stage marked by the greatest variability. *Nulliparous women* (those who have never given birth before) generally experience a longer dilation stage—on average, 8 to 24 hours—than do *parous women* (those who have given birth previously); the dilation stage in parous women may range from 4 to about 12 hours. The dilation stage starts with regularly spaced uterine contractions that increase in intensity and frequency. Each time the uterus contracts, the baby's head pushes against the cervix and causes it to efface and dilate slightly, until the cervix dilates from 1 cm to 10 cm in diameter. This stage is also marked by the rupture of the amniotic sac and the release of amniotic fluid, also commonly known as the "water breaking." If the amniotic sac doesn't rupture on its own by the end of this stage, the physician or other birth practitioner will manually rupture the sac in preparation for the expulsion stage.

pluripotent

The embryoblast will form the embryo proper. These early cells are pluripotent, which means they have the power to differentiate into any cell or tissue type in the body

Embryonic Period

The embryonic period begins in week 3 with the establishment of the three primary germ layers through the process of gastrulation. Subsequent interactions and rearrangements among the cells of the three layers prepare for the formation of specific tissues and organs, a process called organogenesis. By week 4 the embryo has a beating heart, and by the end of the embryonic period (week 8), the main organ systems have been established, and the major features of the external body form are recognizable. Table 29.2 summarizes the events that occur during the embryonic period.

Expulsion Stage

The expulsion stage begins with the complete dilation of the cervix and ends with the expulsion of the fetus from the mother's body. This stage may last as little as several minutes but typically takes 30 minutes to several hours. As with the dilation stage, nulliparous women typically have a longer expulsion stage than parous women. The uterine contractions help push the fetus through the vagina and may be facilitated if the woman "bears down" (i.e., uses the Valsalva maneuver to increase abdominal pressure as she pushes) with each contraction. When the first part of the baby's calvarium distends the vagina, this is referred to as crowning. The head is then followed by the rest of the body. If there is difficulty in expelling the baby from the vagina, then an episiotomy may be performed, which is where the perineal muscles are surgically incised to create a wider opening for the baby to pass through. (This cut is sutured after the birth.) When the baby's body is fully expelled, the umbilical cord is clamped and tied off.

pronuclei

The nucleus of the sperm and the nucleus of the ovum are called pronuclei

placenta

The placenta begins to form during the second week of development

Placental Stage

The placental stage occurs after the baby is expelled. The uterus continues to contract, and these contractions help compress uterine blood vessels and also help displace the placenta from the uterine wall. The placenta and remaining fetal membranes (e.g., the amnion) collectively are referred to as the afterbirth. The expulsion of the afterbirth typically is completed within 30 minutes. The obstetrician or other birth practitioner carefully examines the afterbirth to make sure all portions of the placenta have been expelled from the uterus, because fragments of placenta left in the uterus can lead to extensive bleeding or other postpartum complications.

compaction

The process by which contact between cells is increased to the maximum

second week

The second week of development may be thought of as the "period of twos," because many paired structures develop. A two-layered (epiblast and hypoblast) germinal disc forms. Two membranes (the yolk sac and the amnion) develop on either side of the bilaminar germinal disc. The placenta develops from two components that merge (the chorion and the functional layer of the endometrium of the uterus).

Corona Radiata Penetration

The sperm that successfully reach the secondary oocyte are initially prevented entry by both the corona radiata and the zona pellucida. When sperm reach the corona radiata, their motility allows the sperm to push through the cell layers. Once the sperm have penetrated the corona radiata, they then encounter the more solid zona pellucida

human chorionic gonadotropin (hCG).

The syncytiotrophoblast is responsible for producing a hormone

organogenesis

The upper and lower limbs attain their adult shapes, and the rudimentary forms of most organ systems have developed by week 8 of development ■ By the end of the embryonic period, the embryo is slightly longer than 2.5 centimeters (1 inch), and yet it already has the outward appearance of a human (see figure 29.1). During the embryonic period, the embryo is particularly sensitive to teratogens (ter′ă-tō-jen, te-ră′tō-jen; teras = monster, gen = producing), substances that can cause birth defects or the death of the embryo. Teratogens include alcohol, tobacco smoke, drugs, some viruses, and even some seemingly benign medications, such as aspirin. Because the embryonic period includes organogenesis, exposure to teratogens at this time can result in the malformation of some or all organ systems. ■ Although rudimentary versions of most organ systems have formed during the embryonic period, different organ systems undergo "peak development" periods at different times. For example, the peak development for limb maturation is weeks 4-8, whereas peak development of the external genitalia begins in the late embryonic period and continues through the early fetal period. Teratogens cause the most harm to an organ system during its peak development period. So, a drug such as thalidomide (which causes limb defects) causes the most limb development damage if taken by the mother during weeks 4-8

yolk sac

The yolk sac is the first extraembryonic membrane to develop. It is formed from and continuous with the hypoblast layer. In humans, it does not store yolk as it does in eggs of birds and reptiles, but it is an important site for early blood cell and blood vessel formation.

acrosome reaction

This release of enzymes (primarily hyaluronidase and acrosin) from the acrosome is known as the acrosome reaction

ovum

When the nucleus of the sperm enters the secondary oocyte, the secondary oocyte completes the second meiotic division and forms an ovum

Fusion of Sperm and Oocyte Plasma Membranes and Nuclei

When the sperm and oocyte plasma membranes come into contact, they immediately fuse. Only the nucleus of the sperm enters the cytosol of the secondary oocyte. The midpiece and flagellum of the sperm degenerate shortly thereafter, and typically never enter the fertilized cell. When the nucleus of the sperm enters the secondary oocyte, the secondary oocyte completes the second meiotic division and forms an ovum (see section 28.3b). The nucleus of the sperm and the nucleus of the ovum are called pronuclei (pro = before, precursor of) because they have a haploid number of chromosomes. These pronuclei come together and fuse, forming a single nucleus that contains a diploid number (23 pairs) of chromosomes. The single diploid cell formed is the zygote.

blastocyst cavity

fluid begins to leak through the degenerating zona pellucida into the morula. As a result, a fluid-filled cavity

morula

forming a 16-cell stage

fetal period

includes the remaining 30 weeks of development prior to birth, when the organism is called a fetus (fē′tus; offspring). During the fetal period, the fetus continues to grow, and its organs increase in complexity.

embryonic period

includes the third through eighth weeks of development. It is a remarkably active time during which rudimentary versions of the major organ systems appear in the body, which is now called an embryo

trophoblast

is an outer ring of cells surrounding the fluid-filled cavity. These cells will form the chorion, one of the extraembryonic membranes discussed later in this section.

pre-embryonic period

is the first 2 weeks of development (the first 2 weeks after fertilization), when the single cell produced by fertilization (the zygote) becomes a spherical, multicellular structure (a blastocyst). This period ends when the blastocyst implants in the lining of the uterus

Gastrulation

occurs during the third week of development immediately after implantation, and is one of the most critical periods in the development of the embryo. Gastrulation is a process by which the cells of the epiblast migrate and form the three primary germ layers, which are the cells from which all body tissues develop. The three primary germ layers are called ectoderm, mesoderm, and endoderm. Once these three layers have formed, the developing trilaminar (three-layered) structure may be called an embryo.

embryoblast

or inner cell mass, is a tightly packed group of cells located only within one side of the blastocyst. The embryoblast will form the embryo proper. These early cells are pluripotent (plū-rip′ō-tent; pluris = multi, potentia = power), which means they have the power to differentiate into any cell or tissue type in the body. (For more information about pluripotent cells

pregnancy length

subdivided into trimesters: • The first trimester encompasses the first 3 months of pregnancy (or the first 12 weeks of development of the embryo and fetus). During this time period, the zygote develops into an embryo and then into an early fetus. • The second trimester includes months 4 to 6 of pregnancy and is marked by growth of the fetus and expansion of maternal tissues. • The third trimester encompasses months 7 to 9 of pregnancy. During this time period, the fetus grows most rapidly and gains weight, and the mother's body prepares for the eventual labor and delivery.

trophoblast

subdivides into two layers: a cytotrophoblast (sī′-tō-trō′fō-blast; kytos = cell), which is the inner cellular layer of the trophoblast, and a syncytiotrophoblast (sin-sish′ē-ō-trō′fō-blast), which is the outer, thick layer of the trophoblast where no plasma membranes are visible

ampulla

the widest part of the uterine tube (Fertilization occurs in )

polyspermy

two or more sperm cell nuclei simultaneously enter the secondary oocyte, a phenomenon called polyspermy

prenatal period

• begins with fertilization, when a secondary oocyte and sperm unite, and it ends approximately 38 weeks later with birth. • The general term for the new organism is a conceptus, but you will see that different terms are used during specific parts of the prenatal period ■ broken down into three shorter periods: • pre-embryonic period • embryonic period • fetal period

pre-embryo

■ During the first 2 weeks, the zygote undergoes mitotic cell divisions, and the number of cells increases, forming a pre-embryo ■ spans the time from fertilization in the uterine tube through completion of implantation

placenta function :

■ Exchange of nutrients, waste products, and respiratory gases between the maternal and fetal blood ■ Transmission of maternal antibodies to the developing embryo or fetus (see section 22.8c) ■ Production of estrogen and progesterone to maintain and build the uterine lining

Implantation

■ Implantation is the process by which the blastocyst burrows into and embeds within the endometrium ■ By the end of the first week after fertilization, the blastocyst enters the lumen of the uterus. The zona pellucida around the blastocyst begins to break down as the blastocyst prepares to invade the functional layer of the uterus. Implantation is the process by which the blastocyst burrows into and embeds within the endometrium. ■ The blastocyst begins the implantation process by about day 7 (the end of the first week of development), when trophoblast cells begin to invade the functional layer of the endometrium (figure 29.5). Simultaneously, the trophoblast subdivides into two layers: a cytotrophoblast (sī′-tō-trō′fō-blast; kytos = cell), which is the inner cellular layer of the trophoblast, and a syncytiotrophoblast (sin-sish′ē-ō-trō′fō-blast), which is the outer, thick layer of the trophoblast where no plasma membranes are visible. Over the next few days, the syncytiotrophoblast cells burrow into the functional layer of the endometrium and bring with them the rest of the blastocyst. ■ By day 9, the blastocyst has completely burrowed into the uterine wall and makes contact with the pools of nutrients in the uterine glands. Thus, implantation begins during the first week of development and is not complete until the second week. ■ The syncytiotrophoblast is responsible for producing a hormone called human chorionic gonadotropin (hCG). Recall from section 28.3b that hCG signals the corpus luteum that fertilization and implantation have occurred. Thus, the corpus luteum does not degenerate but rather persists for another 3 months, producing large amounts of progesterone and estrogen that thicken and maintain the uterine lining. ■ By the end of the second week of development, sufficient quantities of hCG are produced to be detected in a woman's urine (when it was filtered from the blood within the kidneys) (figure 29.6). The presence of hCG in urine indicates a woman is pregnant, and thus hCG is the basis for most modern-day pregnancy tests. For the first 3 months of pregnancy, hCG levels remain high, but after that they decline. When hCG declines, the corpus luteum degenerates as well. However, by this time, the corpus luteum is no longer needed because the placenta is producing its own estrogen to maintain the pregnancy.

Capacitation

■ Sperm must undergo capacitation, which is a physiologic conditioning, before they can accomplish fertilization ■ occurs in the female reproductive tract, and typically takes several hours ■glycoprotein coat and some proteins are removed from the sperm plasma membrane that overlies the acrosomal region of the sperm.

Fertilization

■ fertilization restores the diploid number of chromosomes, determines the sex of the organism, and initiates cleavage\ ■ Fertilization typically occurs in the widest part of the uterine tube, called the ampulla. ■ Following ovulation, the *secondary oocyte remains viable in the female reproductive tract for no more than 24 hours, whereas sperm remain viable for an average of 3 to 4 days after ejaculation from the male* ■millions of sperm are deposited in the vagina tract ■only a few hundred have a chance at fertilization. • milions leak out of vagina • acid environment kill em • uterine con tracks • some are not motile( able to swim ) The oocyte cytosol and the cumulus cells release chemotaxic signals (chemicals) to attract the sperm to its location

blastocyst

■ has two distinct components • trophoblast • embryoblast ■ approximately the same size as the initial zygote, but the blastocyst contains many more cells than the zygote

zygote

■ when the male's sperm and the female's secondary oocyte unite to form a single diploid cell called the zygote ■ The zygote is the same size as the secondary oocyte, which typically is between 100 and 120 micrometers (µm) in diameter