PowerLecture:Chapter 17 Development and Aging
Learning Objectives • Describe early embryonic development and distinguish each of the following: oogenesis, fertilization, cleavage, gastrulation, and organ formation. • Correlate the three germ layers—ectoderm, mesoderm, and endoderm—with the tissues that eventually form from each. • Outline the principal events of prenatal development.
Learning Objectives (cont’d) • Describe some of the risks to the early development of the fetus. • Describe the events of aging.
Impacts/Issues Fertility Factors and Mind-Boggling Births
Fertility Factors and Mind-Boggling Births • Multiple births are becoming more common; twins, triplets, quads, and so on are usually the result of the administration of fertility drugs to the prospective mother.
Fertility Factors and Mind-Boggling Births • The rise in higher order multiple births worries some doctors. • The risk of miscarriage, premature delivery, and delivery complications is increased. • Multiples’ birth weights are lower and mortality rates higher. • Parents face more physical, emotional, and financial burdens.
How Would You Vote? To conduct an instant in-class survey using a classroom response system, access “JoinIn Clicker Content” from the PowerLecture main menu. • Should we restrict the use of fertility drugs to conditions that could limit the number of embryos that form? • a. Yes, multiple pregnancies are too risky and can lead to serious disabilities or death for infants. • b. No, reproductive decisions belong to individuals. There are other ways to reduce multiple births.
Section 1 The Six Stages of Early Development: An Overview
The Six Stages of Early Development • In the first three stages, gametes form, an egg is fertilized, and cleavage occurs. • Development begins when gametes (sperm and eggs) form and mature in the prospective child’s parents. • Fertilization occurs when a sperm penetrates an egg; after a series of steps, fertilization produces a zygote. • Cleavage converts the zygote into a ball of cells called a morula.
Fig. 17.1, p. 314 zygote after first cleavage beginning of the ball of cells called a morula
The Six Stages of Early Development • The number of cells increases but not individual cell size. • Each new cell (blastomere) contains a particular portion of the egg’s cytoplasm, which will determine its developmental fate. • In stage four, three primary tissues form. • Gastrulation lays out the organizational framework for the body as the cells are arranged into three primary germ layers.
The Six Stages of Early Development • Ectoderm is the outer layer; it gives rise to the nervous system and the outer layers of the integument. • Mesoderm is the middle layer; muscles as well as organs of circulation, reproduction, excretion, and the skeleton are derived from it. • Endoderm is the inner layer; it gives rise to the lining of the digestive tube and organs derived from it. • Each layer will split into subgroups to give rise to the body’s various tissues and organs.
The Six Stages of Early Development • In stages five and six, organs begin to form, then grow and become specialized. • Organogenesis begins as germ layers subdivide into populations of cells destined to become organs and tissues that are unique in structure and function. • Growth and tissue specializationallow organs to grow and acquire functional capabilities.
The Six Stages of Early Development • During the first several weeks of development three key processes are at work: • During cell determination, the eventual developmental path is established. • In cell differentiation, cells come to have specific structures, products, and functions associated with a specific purpose in the body. • Morphogenesis is the organization of differentiated cells into tissues and organs by means of localized cell division, movements of tissues, folding, and the like.
Fig. 17.2, p. 315 Gamete Formation top view a Eggs form and mature in female reproductive organs. Sperm form and mature in male reproductive organs. Organ Formation Gastrulation Growth, Tissue Specialization Fertilization Cleavage e Subpopulations of cells are sculpted into specialized organs and tissues in spatial patterns at prescribed times. d Cell divisions, migrations, and rearrangements produce two or three primary tissues, the start of specialized tissues and organs. f Organs increase in size and gradually assume their specialized functions. b A sperm and an egg fuse at their plasma membrane. Then the nucleus of one fuses with the nucleus of the other to form the zygote c Cell divisions carve up different regions of egg cytoplasm for daughter cells.
Fig. 17.2, p. 315 Gamete Formation Organ Formation Gastrulation Growth, Tissue Specialization Fertilization Cleavage top view a Eggs form and mature in female reproductive organs. Sperm form and mature in male reproductive organs. e Subpopulations of cells are sculpted into specialized organs and tissues in spatial patterns at prescribed times. d Cell divisions, migrations, and rearrangements produce two or three primary tissues, the start of specialized tissues and organs. f Organs increase in size and gradually assume their specialized functions. b A sperm and an egg fuse at their plasma membrane. Then the nucleus of one fuses with the nucleus of the other to form the zygote c Cell divisions carve up different regions of egg cytoplasm for daughter cells. Stepped Art
Section 2 The Beginnings of You—Fertilization to Implantation
The Beginnings of You – Fertilization to Implantation • Fertilization unites sperm and oocyte. • Of the millions of sperm deposited in the vagina during coitus, only a few hundred ever reach the upper region of the oviduct where fertilization occurs. • The acrosome of the sperm becomes structurally unstable in a process called capacitation. • Many sperm will bind to the zona pellucida of the egg.
The Beginnings of You – Fertilization to Implantation • Only one sperm will successfully enter the cytoplasm of the secondary oocyte because of changes to the egg’s membrane that prevent entry by additional sperm. • The arrival of that sperm inside stimulates the completion of meiosis II in the secondary oocyte, which yields a mature ovum and a second polar body. • The sperm nucleus fuses with the egg nucleus to restore the human diploid chromosome number of 46.
Fig. 17.3a-d, p. 316 FERTILIZATION oviduct ovary uterus OVULATION follicle cell opening of cervix egg nucleus vagina zona pellucida sperm enter vagina a b fusion of sperm nucleus with egg nucleus nuclei fuse c d
The Beginnings of You – Fertilization to Implantation • Cleavage produces a multicellular embryo. • Repeated divisions of the zygote produce the morula; the cells are not necessarily larger but differ in size, shape, and activity. • When the morula reaches the uterus, it transforms into a blastocyst, consisting of a surface layer of cells—the trophoblast—and an inner cell mass, from which the embryo develops. • Identical twins are the result of a separation of the two cells produced by the first cleavage; fraternal twins are not identical because they are the result of two separate fertilizations.
The Beginnings of You – Fertilization to Implantation • Implantation gives a foothold in the uterus. • Implantation into the wall of the uterus takes place about a week after fertilization. • The blastocyst contacts and invades the endometrium; eventually the endometrium will close over it. • Sometimes an ectopic (tubal) pregnancy occurs; this is where the fertilized egg implants outside of the uterus, often in the oviduct, and must be surgically removed. Figure 17.25
The Beginnings of You – Fertilization to Implantation • The implanted embryo releases HCG (human chorionic gonadotropin), which stimulates the corpus luteum to continue secreting estrogen and progesterone to maintain the uterine lining; the presence of HCG in the mother’s urine is the basis for home pregnancy tests.
Fig. 17.4, p. 317 trophoblast (surface layer of cells of the blastocyst) endometrium fertilization implantation endometrium blastocoel inner cell mass fluid inner cell mass uterine cavity a Days 1-2 b Day 3 c Day 4 d Day 5 e Days 6-7 A fluid-filled cavity forms in the morula. By the 32-cell stage, differentiation is occurring in an inner cell mass that will give rise to the embryo. This embryonic stage is the blastocyst. By 96 hours there is a ball of 16 to 32 cells. This is the morula. Cells of the surface layer will function in implantation and will give rise to a membrane, the chorion. Some of the blastocyst’s surface cells attach themselves to the endometrium and start to burrow into it. Implantation has started. After the third cleavage, cells form a compact ball The first cleavage furrow extends between the two polar bodies.
Fig. 17.4, p. 317 trophoblast (surface layer of cells of the blastocyst) endometrium fertilization implantation endometrium blastocoel inner cell mass fluid inner cell mass uterine cavity a Days 1-2 b Day 3 c Day 4 d Day 5 e Days 6-7 Stepped Art
Section 3 How the Early Embryo Takes Shape
How the Early Embryo Takes Shape • First, the basic body plan is established. • By the time of implantation, the inner cell mass has transformed into a pancake-shaped embryonic disk. • Gastrulation rearranges the cells into the three germ layers and the primitive streak; ectoderm thickens around the streak to establish the neural tube and notochord, which eventually forms the brain, spinal cord, and vertebral column.
gut cavity epidermis peritoneum lined body cavity (coelom); lining also holds internal organs in place Fig. 17.5a, p. 318
How the Early Embryo Takes Shape • By week three, blocks of mesoderm called somites form and will give rise to connective tissues, bones, and muscles; pharyngeal arches (face, neck, and associated parts) and the coelom (body cavity) also begin to form.
Fig. 17.5b, p. 318 yolk sac pharyngeal arches chorionic cavity future brain embryonic disk amniotic cavity primitive streak neural tube somites a DAY 15. A primitive streak appears along the axis of the embryonic disk. This thickened band of cells marks the onset of gastrulation. b DAYS 19-23. Cell migrations, tissue folding, and other morphogenic events lead to the formation of a hollow neural tube and to somites (bumps of mesoderm). The neural tube gives rise to the brain and spinal cord. Somites give rise to most of the axial skeleton, skeletal muscles, and much of the dermis. c DAYS 24-25. By now, some cells have given rise to pharyngeal arches, which contribute to the face, neck, mouth, nasal cavities, larynx, and pharynx.
How the Early Embryo Takes Shape • Next, organs develop and take on the proper shape and proportions. • Neurulation is the first stage in the development of the nervous system. • Ectodermal cells at the midline of the embryo elongate to form a neural plate. • Cells of the neural plate fold over and meet at the midline to form a neural tube that will eventually form the spinal cord and brain.
How the Early Embryo Takes Shape • The folding of sheets of cells is an important part of morphogenesis. • Cells migrate from one place to another by sending out pseudopods that guide them along prescribed routes using adhesive and chemical cues. • Body parts are sculpted by apoptosis, a mechanism of genetically programmed cell death. Figures 17.6b and 17.7
Fig. 17.6, p. 319 ectoderm at gastrula stage “climbing” nerve cell neural plate formation b a neural tube
Section 4 Vital Membranes Outside the Embryo
Vital Membranes Outside the Embryo • Four extraembryonic membranes form. • The inner cell mass becomes the embryonic disk; some cells will give rise to the embryo, others to the extraembryonic membranes. • The yolk sac gives rise to the digestive tube and is a source of early blood cells. • The amnion is a fluid-filled sac that keeps the embryo from drying out and acts as a shock absorber; the fluid is amniotic fluid.
Vital Membranes Outside the Embryo • The allantois gives rise to the blood vessels that will become enclosed in the umbilical cord, linking the embryo to the placenta. • The chorion, a protective membrane around the embryo, secretes HCG to maintain the uterine lining after implantation.
Fig. 17.8, p. 320 chorionic cavity chorionic villi start of amniotic cavity start of embryonic disk blood-filled spaces chorion amniotic cavity yolk sac start of chorionic cavity start of yolk sac connecting stalk a DAYS 10-11. The yolk sac, embryonic disk, and amniotic cavity have started to form from parts of the blastocyst. b DAY. 12 Blood filled spaces form in maternal tissue. The chorionic cavity starts to form. c Day 14 A connecting stalk has formed between the embryonic disk and chorion. Chorionic villi which will be features of a placenta start to form.
Vital Membranes Outside the Embryo • The placenta is a pipeline for oxygen, nutrients, and other substances. • The placenta is a combination of endometrial tissue and embryonic chorion. • The maternal tissue consists of tissues rich in arterioles and venules. • The embryo’s chorion extends into the maternal tissue as tiny chorionic villi.
Vital Membranes Outside the Embryo • Materials are exchanged between the blood capillaries of mother and fetus where these vessels associate in the blood-filled spaces of the endometrium; exchange is by diffusion. • Maternal and fetal bloods do not mix. • Harmful substances, such as alcohol, caffeine, drugs, and even infectious agents such as HIV can also cross the placenta.
Fig. 17.9 (1), p. 321 4 weeks MATERNAL CIRCULATION FETAL CIRCULATION embryonic blood vessels mother’s blood vessels 8 weeks blood passes to and from mother’s vessels umbilical cord space between chorionic villi 12 weeks chorionic villus AMNIOTIC FLUID tissues of uterus appearance of the placenta at full term fused amniotic and chorionic membranes
Section 5 The First Eight Weeks—Human Features Emerge
The First Eight Weeks – Human Features Emerge • The embryonic stage ends as the eighth week draws to a close; by this time morphogenesis has begun to form the features that show us to be human. Figure 17.10
WEEK 4 yolk sac embryo connecting stalk future lens pharyngeal arches forebrain developing heart upper limb bud somites neural tube forming lower limb bud a tail Fig. 17.10a, p. 322
Fig. 17.10b, p. 322 WEEKS 5–6 head growth exceeds growth of other regions future external ear retinal pigment umbilical cord forms between weeks 4 and 8 (amnion expands, forms tube that encloses the connecting stalk and a duct for blood vessels) upper limb differentiation (hand plates develop, then digital rays of future fingers;wrist, elbow start forming) foot plate b
WEEK 8 final week of embryonic period; embryo looks distinctly human compared to other vertebrate embryos upper and lower limbs well formed; fingers and then toes have separated early tissues of all internal, external structures now developed tail has become stubby Fig. 17.10c, p. 322
The First Eight Weeks – Human Features Emerge • Gonad development begins by the second half of the first trimester. • An embryo with a Y chromosome will have a sex-determining region on the chromosome that triggers the development of testes; testes will produce male hormones that will influence further sex differentiation. • An XX embryo will become a female because of the absence of testosterone; no other hormones are necessary at this point.
Fig. 17.11, p. 323 7 weeks Y chromosome present Y chromosome absent 10 weeks penis vaginal opening birth approaching birth approaching
Fig. 17.11, p. 323 penis vaginal opening 7 weeks Y chromosome present Y chromosome absent 10 weeks Stepped Art birth approaching birth approaching
The First Eight Weeks – Human Features Emerge • At the end of eight weeks of development, the embryo is designated a fetus; a heart monitor at this point can detect the fetal heartbeat. • Miscarriage is the spontaneous expulsion of an embryo or fetus. • This occurs in about 20% of all conceptions, usually during the first trimester. • More than half of all spontaneous abortions occur because of genetic disorders in the embryo/fetus.