Chapter 29 Flashcards
Prenatal development
Is the time from fertilization to birth and includes both the embryonic and fetal periods. Prenatal development is divided into periods of three calendar months each, called trimesters.
Describe the three trimesters of pregnancy
First trimester: the most critical stage of development; all of the major organ-systems begin to form; is the period when the developing organism is most vulnerable to the effects of drugs, radiation, and microbes.
Second trimester: the nearly complete development of organ systems; by the end of this stage, the fetus assumes distinctively human features.
Third trimester: a period of rapid fetal growth in which the weight of the fetus doubles; during the early stages of this period, most of the organ systems become fully functional.
What is the approximate size and weight and the representative changes in weeks 1-4 of the embryonic period?
Approximate size and weight: 0.6 cm (3/16 in.)
Representative changes: primary germ layers and notochord develop. Neurulation occurs. Primary brain vesicles, somites, and intraembryonic coelom develop. Blood vessel formation begins and blood forms in yolk sac, allantois, and chorion. Heart forms and begins to beat. Chorionic villi develop and placental formation begins. The embryo folds. The primitive gut, pharyngeal arches, and limb buds develop. Eyes and ears begin to develop, tail forms, and body systems begin to form.
What is the approximate size and weight and the representative changes in weeks 5-8 of the embryonic period?
Approximate size and weight: 3 cm (1.25 in.) 1 g (1/30 oz)
Representative changes: limbs become distinct and digits appear. Heart becomes four-chambered. Eyes are far apart and eyelids are fused. Nose develops and is flat. Face is more humanlike. Bone formation begins. Blood cells start to form in liver. External genitals begin to differentiate. Tail disappears. Major blood vessels form. Many internal organs continue to develop.
What are the four principal events of the first week of development?
- Fertilization
- Cleavage of the zygote
- Blastocyst formation
- Implantation
Fertilization
During this event, the genetic material from a haploid sperm cell (spermatozoon) and a haploid secondary oocyte merges into a single diploid nucleus. Occurs in the uterine (fallopian) tube.
Capacitiation
A series of functional changes that sperm undergo, that cause the sperm’s tail to beat even more vigorously and prepare its plasma membrane to fuse with the oocyte’s plasma membrane. Occurs mostly in the uterine (fallopian) tube.
What two layers must a sperm penetrate in order for fertilization to occur? Describe them:
- Corona radiata: the granulosa cells that surround the secondary oocyte.
- Zona pellucida: the clear glycoprotein layer between the corona radiata and the oocyte’s plasma membrane.
Acrosome
A helmet-like structure that covers the head of a sperm and contains several enzymes. Along with the strong tail movements, help the sperm to penetrate the cells of the corona radiata and come in contact with the zona pellucida.
Acrosomal reaction
Triggered by the binding of ZP3 (one of the glycoproteins in the zona pellucida) to specific membrane proteins in the sperm head. Release of the contents of the acrosome occurs. The acrosomal enzymes digest a path through the zona pellucida as the lashing sperm tail pushes the sperm cell onward.
Polyspermy
Fertilization by more than one sperm cell.
Male pronucleus
What the nucleus in the head of the sperm develops into.
Female pronucleus
What the nucleus of the fertilized ovum develops into.
Syngamy
Process in which the male and female pronuclei fuse and produce a single diploid nucleus; the fusion of the haploid (n) pronuclei restores the diploid number (2n) of 46 chromosomes.
Zygote
A fertilized ovum.
Dizygotic (fraternal) twins
Produced from the independent release of two secondary oocytes and the subsequent fertilization of each by different sperm. They are the same age and in the uterus at the same time, but genetically they are as dissimilar as any other siblings. Dizygotic twins may or may not be the same sex.
Monozygotic (identical) twins
Develop from a single fertilized ovum, they contain exactly the same genetic material and are always the same sex. Monozygotic twins arise from separation of the developing cells into two embryos, which in 99% of the cases occurs before 8 days have passed.
Conjoined twins
A situation in which twins are joined together and share some body structures. Arise from separation of the developing cells into two embryos after 8 days have passed.
Cleavage
The rapid mitotic cell divisions of the zygote after fertilization.
Blastomeres
The progressively smaller cells
produced by cleavage.
Morula
A solid sphere of cells that successive cleavages eventually produce. Is still surrounded by the zona pellucida and is about the same size as the original zygote.
Uterine milk
A glycogen-rich secretion from the glands of the endometrium of the uterus that passes into the uterine cavity and enters the morula through the zona pellucida. Fluid that, along with nutrients stored in the cytoplasm of the blastomeres of the morula, provides nourishment for the developing morula.
Blastocyst cavity
A large, fluid-filled cavity that the morula reorganizes around.
Blastocyst
The developing mass that forms once the blastocyst cavity is formed.
During the formation of a blastocyte, two distinct cell populations arise. What are they? Describe them:
- Embryoblast (inner cell mass): located internally and develops into the embryo.
- Trophoblast: located externally and forms the spherelike wall of the blastocyte. 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.
Implantation
Process in which the blastocyst loosely attaches to the endometrium. Occurs about 6 days after fertilization.
Decidua
A modified portion of the endometrium that develops after implantation.
What are the three different regions of the decidua?
- Decidua basalis: the portion of the endometrium between the embryo and the stratum basale of the uterus; it provides large amounts of glycogen and lipids for the developing embryo and fetus and later becomes the maternal part of the placenta.
- Decidua capsularis: the portion of the endometrium located between the embryo and the uterine cavity.
- Decidua parietalis: the remaining modified endometrium that lines the noninvolved areas of the rest of the uterus.
What are the two principal events of the second week of development?
- The trophoblast develops into two layers in the region of contact between the blastocyte and endometrium.
- The embryoblast develops into two layers.
Amniotic cavity
A large cavity within the epiblast.
Amnion
A single layer of squamous cells that forms a dome-like roof above the epiblast cells. Forms as the amniotic cavity enlarges.
Yolk sac
The former blastocyst cavity during earlier development. Is relatively empty and small, and decreases in size as development progresses. Has several important functions in humans: supplies nutrients to the embryo during the second and third weeks of development; is the source of blood cells from the third through sixth weeks; contains the first cells (primordial germ cells) that will eventually migrate into the developing gonads, differentiate into the primitive germ cells, and form gametes; forms part of the gut (gastrointestinal tract); functions as a shock absorber; and helps prevent drying out of the embryo.
Lacunae
Small spaces that develop in one of the layers of a trophoblast.
Chorion
Surrounds the embryo and later the fetus. Eventually it becomes the principal embryonic part of the placenta, the structure for exchange of materials between mother and fetus. Also protects the embryo and fetus from the immune responses of the mother.
Connecting (body) stalk
The future umbilical cord.
Gastrulation
Involves the rearrangement and migration of cells from the epiblast.
Primative node
A rounded structure formed by a small group of epiblast cells.
Endoderm
Epithelia composed of tightly packed cells. As the embryo develops, the endoderm ultimately becomes the epithelial lining of the gastrointestinal tract, respiratory tract, and several other organs.
Mesoderm
A loosely organized connective tissue (mesenchyme). Gives rise to muscles, bones, and other connective tissues, and the peritoneum.
Ectoderm
Epithelia composed of tightly packed cells. Develops into the epidermis of the skin and the nervous system.
Notochordal process
A hollow tube of cells in the midline. Formed by mesodermal cells from the primitive node that have migrated towards the head end of the embryo.
Notochord
A solid cylinder of cells. Formed by the notochordal process. Plays an extremely important role in induction, specifically of mesodermal cells into vertebral bodies and the neural plate.
Neural plate
Formed by the induction of mesodermal cells.
Neural fold
What the lateral edges of the neural plate forms.
Neural groove
What the depressed midregion of the neural plate forms.
Neural tube
What the neural plate becomes when the neural folds fuse.
Neurulation
The process by which the neural plate, neural folds, and neural tube form.
Neural crest
Several layers of cells that some of the ectodermal cells from the tube migrate to form. Gives rise to all sensory neurons and postganglionic neurons of the peripheral nerves, the adrenal medullae, melanocytes (pigment cells) of the skin, arachnoid mater, and pia mater of the brain and spinal cord, and almost all of the skeletal and connective tissue components of the head.
Chorionic villi
Fingerlike projections that consist of chorion that projects into the endometrial wall of the uterus. By the end of the third week, blood capillaries develop in the chorionic villi. Blood vessels in the chorionic villi connect to the embryonic heart by way of the umbilical arteries and umbilical vein through the connecting (body) stalk, which will eventually become the umbilical cord.
Placenta
Is formed by the chorionic villi of the embryo and the decidua basalis of the endometrium of the mother.The site of exchange of nutrients and wastes between the mother and fetus. Also produces hormones needed to sustain the pregnancy.
Umbilical cord
The actual connection between the placenta and embryo, and later the fetus. Develops from the connecting stalk.
Fetal period
Is the time from the ninth week until birth. Development during the fetal period is mostly concerned with the growth and differentiation of tissues and organs formed during the embryonic period.
Human chorionic gonadotropin (hCG)
Secreted by the chorion of the placenta. 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.
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.
Human chorionic somatomammotropin (human placental lactogen (hPL))
Secreted by the chorion of the placenta. Helps prepare the mammary glands for lactation, enhances maternal growth by increasing protein synthesis, and regulate certain aspects of metabolism in both mother and fetus.
Corticotropin-releasing hormone (CRH)
Produced by the placenta. Is 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. It also increases secretion of cortisol, which is needed for maturation of the fetal lungs and the production of surfactant.
What are the three stages of true labor? Describe them:
- Stage of dilation: the time from the onset of labor to the complete dilation of the cervix; 6-12 hours.
- Stage of expulsion: the time from complete cervical dilation to delivery of the baby; ~10 minutes- several hours.
- Placental stage: the time after delivery until the placenta or “afterbirth” is expelled by powerful uterine contractions; ~5-30 minutes or more.
Milk ejection reflex
Oxytocin stimulates contraction of myoepithelial cells in the breasts, which squeezes the glandular and duct cells and causes milk ejection.
