Lecture Outline - Chapter 22

22.1. Early Developmental Stages (p. 416)

1. Most animal embryos go through developmental stages of: zygote, morula, blastula, early and late gastrula.
2. Fertilization (Fig. 22.1)
   • a. Sperm has flagellar tail for swimming, mitochondria in mid-piece for energy-production, and head with haploid nucleus and acrosome with enzymes for egg penetration.
   • b. Sperm acrosome digests the jelly coat and exudes a filament that attaches to receptors on egg's vitelline membrane.
   • c. Sperm nuclear membrane and egg plasma membrane fuse, allowing sperm nucleus to enter.
   • d. Egg vitelline membrane undergoes changes immediately; resulting fertilization membrane prevents entrance of other sperm.
3. Zygote
   • a. Undergoes cleavage or cell division without growth. (Fig. 22.2)
   • b. DNA replication occurs repeatedly; cells become smaller with each division.
   • c. Cells divisions are equal making cells uniform size in morula.
   • d. Forms hollow ball called a blastula; cavity is blastocoel.
4. Gastrula
   • a. Cells invaginate into blastocoel, first forming a double layer of cells.
     • i. Ectoderm is outer layer.
     • ii. Endoderm is inner layer.
     • iii. Space formed by invagination is archenteron; later becomes gut.
     • iv. Pore or hole created by invagination is blastopore.
   • b. Mesoderm is middle layer formed from outpocketing of endoderm in two places.
   • c. Space formed between the mesoderm growth becomes coelom or body cavity containing internal organs.
   • d. Germ layer theory states organ development can be traced from embryonic germ layers of ectoderm, mesoderm, and endoderm.
5. How Yolk Affects the Stages
   • a. Embryos vary in speed of development and amount of yolk. (Table 22.1)
   • b. Aquatic lancelet and frog develop rapidly to feeding larvae; need little yolk.
   • c. Chick needs much yolk for development isolated inside hard shell.
   • d. Human egg has little yolk but has development similar to chick due to common evolutionary history.
   • e. Frog embryo cells at animal pole have little yolk and develop fast; cells at vegetal pole contain more yolk but develop slowly. (Fig. 22.3)
   • f. Morula forms ball of cells in lancelet and frog; chick morula sees cells spread out on surface of yolk.
   • g. Yolk cells of frog do not invaginate; animal pole cells encircle from above and below in gastrulation.
   • h. Chick endoderm/ectoderm forms on top/ bottom layers without invagination.
   • i. Chick mesoderm layer arises as invagination of cells at edge of furrow in midline of embryo called primitive streak.
6. Neurulation Produces the Nervous System (p. 419)
   • a. Mesoderm cells on longitudinal axis form dorsal supporting rod called the notochord.
   • b. In frogs, chicks, and humans, notochord is replaced by vertebral column.
   • c. Midline mesoderm just above notochord thickens into neural plate; neural folds rise at either side of neural groove; and fuse to form neural tube.
   • d. Other midline mesoderm cells become somites, block portions that form muscle and vertebral bones. (Fig. 22.5)

1. Differentiation occurs when cells become specialized in structure and function.
2. Morphogenesis refers to the additional form taken on by these specialized tissues.
3. How Cells Become Specialized
   • a. Genes are not parceled out into various embryonic cells; each embryonic cell contains a full set of chromosomes.
   • b. It is hypothesized that genes are turned on or off due to ooplasmic segregation, the distribution of maternal cytoplasmic contents to various cells of the morula; if a frog's egg is divided so that only one daughter cell receives gray crescent, only that cell can become a tadpole. (Fig. 22.6)
4. How Morphogenesis Occurs (p. 421)
   • a. Cell differentiation is influenced by chemical signals given off by neighboring cells.
   • b. One set of cells can influence migration of other cells by producing extracellular fibrils.
   • c. Neurulation, development of nervous system, involves induction, the ability of one tissue to influence development of another tissue. (Fig. 22.7)
     • i. If potential notochord tissue is cut out and transplanted beneath what would be belly ectoderm, this ectoderm differentiates into a neural plate.
     • ii. Optic vesicles induce ectoderm to thicken and become a lens, which in turn induces the optic vesicle to form the optic cup.
5. Genes that Control Pattern Formation
   • a. Drosophila morphogenesis involves homeotic genes that determine how segments develop.
   • b. Homeotic genes containing the same sequence of nucleic acids occur in many organisms; this sequence used to determine developmental pattern is a homeobox.
   • c. Since homeoboxes are found in most eukaryotic organisms, it appears that developmental "problems" are solved by evolution only once.
   • d. Each homeodomain gene codes for a protein sequence of 60 amino acids.
   • e. Researchers anticipate that each homeotic gene product turns on next homeotic gene, etc.
   • f. Homeotic mutations include extra wings, legs where antennae should be, etc., and are due to turning on homeotic genes at wrong time. (Fig. 22.8)

1. Extraembryonic Membranes (Table 22.2 and Fig. 22.10)
   • a. Chorion is next to shell and serves for gas exchange in chick, part of placenta in humans.
   • b. Amnion surrounds and cushions embryo; prevents drying out.
   • c. Allantois collects nitrogenous wastes of chick; provides blood vessels that become umbilical vessels.
   • d. Yolk sac surrounds yolk in chick for nourishment; is first site of blood formation.
2. Embryos Don't Look Human (p. 424)
   • a. First Week
     • i. Fertilization occurs in upper third of oviduct. (Fig. 22.11)
     • ii. Embryo reaches uterus on third day as morula.
     • iii. By fifth day, morula is transformed into blastocyst.
     • iv. Blastocyst (hollow ball of cells) consists of the trophoblast (outer layer of cells that give rise to chorion) and inner cell mass (gives rise to fetus).
   • b. Second Week
     • i. Embryo begins to implant at end of first week.
     • ii. Trophoblast secretes enzymes to digest away some of the uterine wall, and secretes HCG to maintain corpus luteum..
     • iii. Inner cell mass detaches from trophoblast; forms yolk sac and amnion.
     • iv. Gastrulation involves embryonic disk with upper ectoderm and lower endoderm layers.
     • v. Trophoblast is reinforced by mesoderm; becomes chorion.
   • c. Third Week (p. 425)
     • i. Nervous system appears first with thickening on dorsal side, then invagination for form neural tube.
     • ii. Heart forms when right and left heart tubes fuse.
   • d. Fourth and Fifth Weeks
     • i. Tail of embryo bridges by body stalk to chorion; projections of chorion are chorionic villi. (Fig. 22.14)
     • ii. Allantois is fourth extraembryonic membrane; contained within body stalk, it becomes umbilical blood vessels.
     • iii. Umbilical cord connecting developing embryo to placenta is fully formed.
     • iv. Limb buds appear; head begins to develop sense organs.
   • e. Sixth Through Eighth Weeks
     • i. By end of eight weeks, only 38 mm (1.5 inches) long.
     • ii. Embryo becomes easily recognized as human.
     • iii. All organ systems are established.
   • f. Placenta Fulfills Needs
     • i. Upon implantation, chorion formed chorionic villi that project into maternal tissue and become placenta. (Fig. 22.14)
     • ii. Placenta Structure
       - Contributed by both fetal chorionic villi, maternal endometrium.
       - Fully formed by tenth week.

       iii. Placental Functions

       - Produces progesterone and estrogen to feedback, to pituitary and hypothalamus to prevent additional egg production and to uterus to maintain uterine lining.

       - Exchanges nutrients, gases (carbon dioxide and oxygen), and wastes (urea) between fetus and mother.

       - Blood of mother and fetus never mix.

       iv. Umbilical cord consists of fetal arteries and vein carrying diffused molecules between placenta and fetal circulatory system.
3. Fetuses Look Human (p. 430)
   • a. Fetal development extends from third through ninth month.
   • b. Third and Fourth Months: Can Tell Sex
     • i. Head growth slows down as rest of body increases in length.
     • ii. Hair, fingernails, and nipples appear.
     • iii. Cartilage is being replaced by bone.
     • iv. Six membranous areas called fontanels permit flexibility during childbirth and future brain expansion in skull; fontanels disappear by age 16 months.
     • v. Can distinguish males from females due to gene called the testis determining factor on Y chromosome.
     • vi. Once gonads differentiate into testes or ovaries, they produce sex hormones that stimulate growth of the respective genitals.
     • vii. Testes should descend into scrotum during last trimester; failure to descend requires corrective surgery.
     • viii. Fetal heartbeat is loud enough to be heard with stethoscope.
   • c. Fifth through Seventh Months: Fetus Moves
     • i. Mother feels movements of fetus.
     • ii. Covered by lanugo (fine down) and coated with vernix caseosa (white, greasy, cheeselike substance for protection from amniotic fluid).
     • iii. At end of seventh month, fetus is about 300 mm (12 inches) long and 1,380 g (three pounds); now has chance to survive if born.
     • iv. Fetal circulation has four features not present in the adult. (Fig. 22.16)
       - Oval opening (foramen ovale) between the two atria allows blood to by-pass lungs; after birth, it seals off due to pressure changes. If it does not seal off, a "blue baby" results, which can be corrected by surgery.
       - Arterial duct (ductus arteriosus) connects pulmonary artery to aorta and allows blood to by- pass lungs in the fetus; constricts later.
       - Umbilical arteries carry fetus waste products to placenta; umbilical vein brings in oxygen and nutrients to fetus.
       - Venous duct (ductus venosus) connects the umbilical vein with vena cava, allowing most of incoming blood to by-pass liver.
   • d. Birth: Occurs in Stages (p. 432)
     • i. Fetus rotates with head pointed toward cervix.
     • ii. If not in position, breech birth (rump first) may require cesarean section.
     • iii. End of ninth month, fetus averages 525 mm (20 inches) and 3,380 grams (7.5 pounds).
   • e. Birth: Occurs in Stages (p. 432)
     • i. Mild indiscernible contractions occur throughout pregnancy; become stronger and more frequent near end of pregnancy; true labor involves contractions lasting over 40 seconds occurring every 15 - 20 minutes.
     • ii. Trigger of childbirth involves prostaglandins and perhaps oxytocin from mother's pituitary; both hormones can induce birth.
     • iii. Parturition includes labor and expulsion of fetus; involves three stages.
       - Stage 1: Cervix dilates (effacement); mucus plug from cervical canal is expelled; amniotic membrane ruptures to release amniotic fluid; stage ends when cervix is fully dilated. (Fig. 22.17)
       - Stage 2: Baby emerges from uterine contractions that occur every 1 - 2 minutes, lasting one minute each; if vagina cannot expand enough, an episiotomy is performed and baby is born. Umbilical cord is cut, shriveling and leaving scar that becomes navel. (Fig. 22.18)
       - Stage 3: Placenta (afterbirth) is expelled from uterus about 15 minutes after delivery of baby. (Fig. 22.19)

1. Development continues:
   • a. Through infancy (age 2) with tremendous growth, sensorimotor development.
   • b. Childhood with more growth and change in body proportions.
   • c. Adolescence, beginning at puberty till growth stops.
   • d. Adulthood with gradual loss of certain body abilities. (Fig. 22.20)
2. Aging involves progressive changes that contribute to increased risk of infirmity, disease and death; gerontology is study of aging and is of greater interest with current surge in older populations.
3. The maximum human life span is about 110 - 115 years.
4. Why We Age
   • a. Genetic in Origin (p. 434)
     • i. Number of times a cell divides is species-specific; for humans, maximum number of divisions is around 50.
     • ii. Some cell lines become nonfunctional long before maximal number of divisions has occurred; suggests accumulation of DNA mutations and nonfunctional proteins.
     • iii. Children of long-lived parents tend to live longer than those of short-lived parents; individuals producing antioxidant enzymes may detoxify free radicals that are normally produced and destroy cell components.
   • b. Whole-Body Processes (p. 434)
     • i. Decrease in hormones and immune system function occurs with aging.
     • ii. Increased amount of cross-linking in collagen (advanced glycosylation end products or AGEs) may account for loss of elasticity, atherosclerosis, etc.
   • c. Extrinsic Factors
     • i. Poor health habits (lack of calcium in diet, heavy alcohol intake, smoking, lack of exercise) may contribute to aging.
     • ii. Osteoporosis is common in elderly; involves bone loss that may be accelerated by poor diet in youth and above habits.
     • iii. Because of different lifestyles across age groups, comparisons within same age cohort may detect extrinsic factors promoting health.
5. How Aging Affects Body Systems (p. 435)
   • a. Skin
     • i. Becomes thinner, less elastic, fewer sweat glands, and has decreased number of melanocytes.
     • ii. Less adipose tissue means more bony appearance and sensitivity to cold.
   • b. Processing and Transporting
     • i. Heart size shrinks from reduction in cardiac muscle cell size; healthy heart can still meet demands of activity.
     • ii. Arteries become more rigid from cross-linkage of elastic fibers; internal size reduced due to plaque buildup; blood pressure rises.
     • iii. Liver does not metabolize drugs as efficiently as before.
     • iv. Lungs become more inelastic; unnoticed unless under high demand of oxygen; problem when combined with circulatory changes.
     • v. Kidneys become smaller, less efficient at filtration; may combine with prostate enlargement in men to make urination difficult.
     • vi. Loss of teeth is likely caused by neglect, not aging; adds to problem of decline in efficiency of digestive tract.
   • c. Integration and Coordination
     • i. No new nerve or muscle cells are formed in adults.
     • ii. Few neural cells in cerebral cortex are lost with age; however loss of short term memory is common.
     • iii. Learning new material and remembering still possible if more time is given to respond.
     • iv. Reaction time slows down.
     • v. Hearing decreases after age 50, especially of high frequencies.
     • vi. Eye lens may fail to accomodate well, develop cataract; glaucoma results from reduction in size of anterior cavity of eye.
     • vii. Skeletal muscle mass loss controlled by regular exercise.
     • viii. Other symptoms of aging include: decline in bone mass, arthritis of joints, weight gain from lower basal metabolism and replacing muscle with stored fat and water.
   • d. The Reproductive System (p. 436)
     • i. Females Undergo Menopause
       - Walls of oviducts and vagina narrow.
       - Female sex hormones fall dramatically.

       ii. Males

       - Continue to produce sperm until death.

       - Level of androgens falls gradually.

       iii. Longevity of Females Over Males

       - Estrogen may protect against circulatory disorders; females see increase is after menopause.
   • e. How To Age Well
     • i. Effects of aging are not all necessary. (Fig. 22.21)
     • ii. Extrinsic factors mean habits developed while young may assist in longer life.