51.1. Development Has Stages (p. 914)
A. Fertilization
1. Fertilization requires that sperm and egg interact to form a zygote.
a. The sperm has three parts. (Fig. 51.1)
1) Head contains haploid nucleus covered by an acrosome, a caplike covering that contains hydrolytic enzymes that allow a sperm to penetrate the egg.
2) The middle piece contains energy-producing mitochondria.
3) The tail is a flagellum that allows the sperm to swim.
2. Fertilization involves the following steps, some of which are species-specific. (Fig. 51.1)
a. Males release so many sperm that the egg is covered by them.
b. The egg has a plasma membrane, a vitelline envelope, and a jelly coat.
c. Acrosome enzymes digest away the zona pellucida around the egg as it extrudes a filament that attaches to a receptor on the vitelline jelly layer envelope.
d. This interaction between filament and receptor is a lock-and-key reaction that is species-specific.
e. The egg plasma membrane and the sperm nuclear membrane fuse, allowing the nucleus to enter.
f. Fusion takes place and the zygote begins development.
g. As soon as the plasma membranes of sperm and egg fuse, the plasma membrane and the vitelline envelope undergo changes that prevent entrance of any other sperm.
h. The vitelline envelope now becomes the fertilization envelope.
B. Early Developmental Stages
1. Development includes events and processes that occur as a single cell becomes a complex organism.
2. All chordate embryos go through same early developmental stages: zygote, morula, blastula, early and late gastrula.
3. The presence of yolk, dense nutrient material, affects how the embryonic cells complete the first three stages.
4. Following fertilization, a zygote undergoes cleavage, cell division without growth. (Fig. 51.2)
[transp. 294]
a. DNA replication and mitosis occur repeatedly; the cells get smaller each division.
b. As deuterostomes, lancelets have a radial and indeterminate pattern of cleavage.
1) In radial cleavage, any plane passing through will divide the embryo into symmetrical halves.
2) In indeterminate cleavage, cells have not differentiated; if separated, each one develops a complete organism.
5. Because the lancelet has little yolk, the cell divisions are equal in the resulting morula.
6. A cavity called the blastocoel develops forming the hollow ball called the blastula.
7. Gastrulation is invagination of some cells of the blastocyst into blastocoel to form three primary germ layers.
a. The outer layer of cells becomes the ectoderm.
b. The inner layer of cells becomes the endoderm.
c. The cavity created by the invagination becomes the primitive gut or archenteron.
d. The pore created by the invagination is the blastopore; in the lancelet, this becomes the anus.
e. The outer ectoderm gives rise to epidermis of skin, epithelial lining of mouth and rectum, and nervous system.
f. The inner endoderm gives rise to the epithelial lining of the digestive tract and respiratory tract, the associated glands of the digestive system and the respiratory system, and the lining of the urinary bladder.
g. Middle mesoderm gives rise to skeleton, dermis of skin, skeletal system, muscular system, excretory system, reproductive system (including most epithelial linings), and outer layers of respiratory and digestive systems.
8. The germ layers develop into future organs, a theory first proposed by Karl E. Von Baer in nineteenth century.
C. How Yolk Affects the Stages
1. The amount of yolk affects the manner in which animals complete the first three stages of development (cleavage, blastulation, and gastrulation). (Fig. 51.3) [transp. 295] (Table 51.1)
2. The lancelet and frog develop in water; they develop quickly into larvae that can feed themselves.
3. A chick provides substantial yolk inside a hard shell; development continues until chick can exist on its own.
4. Early stages of human development resemble the chick due to common evolutionary history.
5. In a frog embryo, cells at the animal pole have little yolk and those at the vegetal pole contain more yolk.
6. Presence of yolk causes cells to cleave more slowly.
7. In the chick, cleavage is incomplete; only cells lying on top of the yolk cleave and spread out over the yolk surface, in contrast to the ball-like morula of the lancelet.
8. In the frog, the blastocoel is formed at the animal pole only.
a. Cells containing yolk do not participate in gastrulation and do not invaginate.
b. Instead, a slitlike blastopore is formed when the animal pole cells begin to evaginate from above.
c. Other pole cells move down over the yolk; the blastopore then becomes rounded.
d. Yolk cells temporarily left in the region form the yolk plug.
e. Cells from the dorsal lip of the blastopore migrate between ectoderm and endoderm, forming a mesoderm.
f. Later, splitting of the mesoderm creates the coelom.
9. In the chick, a blastocoel is created when cells lift up from yolk and leave a space between cells and yolk. (Fig. 51.3b)
a. There is so much yolk that endoderm formation does not occur by invagination.
b. Instead, an upper layer of cells differentiates into ectoderm; a lower layer differentiates into endoderm.
c. Mesoderm arises by an invagination of cells along the edges of a longitudinal furrow in the midline of the embryo; it is called the primitive streak. (Fig. 51.3d)
d. Later, the newly formed mesoderm will split to form the coelomic cavity.
D. Neurulation Produces the Nervous System (Fig. 51.4) [transp. 295]
1. In chordate animals, newly formed mesoderm cells that lie along main axis of the animal coalesce to form a dorsal rod (notochord); it persists in lancelets but is replaced in frogs, chicks, and humans by vertebral column.
2. Nervous system develops from midline ectoderm located just above the notochord. (Fig. 51.4)
a. At first, cells on dorsal surface of the embryo thicken, forming the neural plate. (Fig. 51.4a)
b. Then, neural folds develop on either side of a neural groove, which become the neural tube when these folds fuse. (Fig. 51.4b,c)
c. At this point the embryo is called a neurula. (Fig. 51.4d)
d. Later, the anterior end of the neural tube develops into the brain.
3. Midline mesoderm cells that did not contribute to forming notochord now become 2 longitudinal masses of tissue.
a. The two tissue masses become blocked off into somites. (Fig. 51.5)
b. Somites give rise to segmental muscles in all chordates; in vertebrates, they also give rise to the vertebrae.
51.2. Cells Become Specialized and Embryos Take Shape (p. 918)
A. Development requires growth, differentiation, and morphogenesis.
1. Cellular differentiation occurs when cells become specialized in structure and function.
2. Morphogenesis is a change in shape and form of a body part.
B. How Cells Become Specialized
1. The process of differentiation starts long before we recognize types of cells.
2. The ectoderm, mesoderm, and endoderm of the gastrula look similar but develop into different organs.
3. Each cell of body contains full complement of chromosomes; so differentiation is not due to parceled out genes.
4. Cytoplasm is not uniform in content; each of the first few cells differs as to the cytoplasmic content.
a. After the first cleavage of a frog embryo, only a daughter cell that receives a portion of the gray crescent is able to develop into a complete embryo. (Fig. 51.6)
b. Hans Spemann received the Nobel Prize in 1935 for this embryo work; particular chemical signals within the gray crescent turn on genes that control development of the frog.
c. Genes are hypothesized to turn on/off due to ooplasmic segregation, the distribution of maternal cytoplasmic contents to various cells of the morula.
C. How Morphogenesis Occurs
1. As development proceeds, differentiation involves signals from neighboring cells.
2. Migration of cells occurs during gastrulation; one set of cells can influence the migratory path of another set.
3. Spemann showed a dorsal lip of a blastopore was necessary for development; it was the primary organizer.
a. The cells closest to the primary organizer become endoderm; those farthest away become ectoderm.
b. This suggests that a molecular concentration gradient acts as a signal to induce germ layer differentiation.
c. Induction is the ability of a chemical or tissue to influence development of another tissue.
d. The inducing chemical is called a signal.
4. Work by Jim Smith of the National Institute of Medical Research in London indicates a peptide growth factor called activin plays a role in signaling processes.
a. At low activin concentrations, animal pole cells become epidermis, normally an ectoderm-derived tissue.
b. At high concentrations, they become muscle and notochord, normally mesoderm-derived tissues.
5. Hans Spemann and Hilde Mangold worked on dorsal side of embryo where notochord and nervous system develop.
a. The presumptive notochord tissue induces the formation of the nervous system. (Fig. 51.7) [transp. 297]
b. Warren Lewis found reciprocal induction occurs between the lens and the optic vesicle in frog embryo eye formation.
6. The process of induction is thought to go on continuously with cells always influencing each other.
7. Either direct contact or production of a chemical acts as signal to activate gene expression and cause protein synthesis.
D. Genes That Control Pattern Formation
1. Homeotic genes control pattern formation in animal morphogenesis. (Fig. 51.8)
2. Homeotic genes have been found in many organisms; contain same sequence of nucleic acids called a homeobox.
3. Homeoboxes are derived from an original nucleic acid sequence that has been conserved because of its importance in regulation of animal development.
4. Homeotic genes are arranged in a definite order on a DNA molecule; those first in line determine the anterior portion of the embryo, while those later in the sequence determine the posterior portion of the embryo.
5. Homeotic gene codes for homeotic domain protein that stays in the nucleus and regulates transcription of genes.
6. Each has a homeodomain, a sequence of sixty amino acids that is found in other homeodomain proteins.
7. Research evidence seems to indicate that a homeodomain protein produced by one homeotic gene binds to and turns on the next homeotic gene, and this orderly process determines the overall pattern of the embryo.
8. It appears that homeotic genes also establish homeodomain protein gradients that affect the pattern development of specific parts (e.g., the retinoic acid gradient that determines wing formation in the chick).
51.3. Humans Are Embryos and Then Fetuses (p. 922)
A. Development encompasses those events from the time from conception (fertilization) to birth (parturition).
1. In humans, gestation period is 9 months; it is usually calculated by adding 280 days to start of last menstruation.
2. Only about 5% of babies arrive on the forecasted date due to many variables.
B. Human development is divided into embryonic and fetal development.
1. Embryonic development during months 1 and 2 is when the major organs are formed.
2. Fetal development during months 3-9 during which the organs systems grow and mature.
C. Extraembryonic membranes lie outside of the embryo and protect and nourish the embryo and, later, the fetus. (Fig. 51.9) [transp. 298]
1. The evolution of extraembryonic membranes in reptiles made development on land possible.
a. If an embryo develops in water, water supplies oxygen and takes away wastes.
b. Surrounding water prevents desiccation and provides a protective cushion.
c. For an embryo on land, these functions are performed by the extraembryonic membranes.
2. In the chick, extraembryonic membranes develop from extensions of germ layers, which spread out over yolk.
a. Chorion lies next to the shell and carries on gas exchange.
b. Amnion contains protective amniotic fluid that bathes the developing embryo.
c. Allantois collects nitrogenous wastes.
d. Yolk sac surrounds the remaining yolk that provides nourishment.
3. Humans have these membranes; their function is modified for internal development.
a. The chorion develops into the fetal half of the placenta.
b. The yolk sac is the first site of blood cell formation.
c. The allantoic blood vessels become the umbilical blood vessels.
d. The amnion surrounds the embryo and cushions it with amniotic fluid.
4. Therefore, all chordate animals develop in water, either in bodies of water or within amniotic fluid.
D. Human Embryos Don't Look Human
1. First Week (Fig. 51.10) [transp. 299]
a. Fertilization occurs in upper third of oviduct; cleavage begins as the embryo passes down this tube to the uterus.
b. By the time the embryo reaches the uterus on the third day, it is a morula.
c. By about the fifth day the morula is transformed into the blastocyst.
1) The blastocyst is a hollow ball of cells, resulting from cleavage.
2) The blastocoel is the fluid-filled cavity contained within a blastocyst.
3) The trophoblast is the outer single layer of cells, which later gives rise to the chorion.
4) The inner cell mass is the mass of cells from which the embryo, and eventually the fetus, will develop.
2. Second Week
a. At the end of the first week, the embryo begins the process of implantation.
b. The trophoblast secretes enzymes to digest away some of the tissue and blood vessels of the uterine wall.
c. Trophoblast begins to secrete human chorionic gonadotropin that causes corpus luteum to be maintained.
d. As the week progresses, the inner cell mass detaches itself from the trophoblast, and two more extraembryonic membranes form, the yolk sac and the amnion. (Fig. 51.11a)
e. Yolk sac forms below embryonic disk; with no nutritive function in humans, it is site of blood cell formation.
f. As in chick development, a human amnion and its cavity are where the embryo (and then fetus) develop.
g. In humans, amniotic fluid insulates against thermal changes; it cushions and protects the fetus from trauma.
h. Gastrulation occurs during this week, resulting in the inner cell mass flattening into the embryonic disc.
1) The embryonic disk is composed of two cell layers: ectoderm above and endoderm below.
2) Once the embryonic disk elongates to form the primitive streak (similar to that found in birds), the third germ layer, mesoderm, forms by invagination of cells along the streak.
i. The trophoblast is reinforced by mesoderm and becomes the chorion.
3. Third Week (Fig. 51.11b)
a. The nervous system is the first organ system to become visually evident.
1) It appears as a thickening along entire dorsal length of embryo; invagination occurs as neural folds appear.
2) When neural folds meet at the midline, the neural tube is formed. (Fig. 51.4)
b. The development of the heart begins in the third week and continues into the fourth.
1) Right and left heart tubes fuse; heart begins pumping blood, although chambers are not fully formed.
2) The veins enter this largely tubular heart posteriorly, and the arteries exit anteriorly.
3) Later the heart twists so that all major vessels are located anteriorly.
4. Fourth and Fifth Weeks (Fig. 51.11c,d)
a. A bridge of mesoderm (body stalk) connects caudal (tail) end of embryo with chorion, which has projections (chorionic villi).
b. A fourth extra embryonic membrane (allantois) is contained in this stalk; its blood vessels become umbilical vessels.
c. The head and the tail lift up, and the body stalk moves anteriorly by constriction.
d. Once this process is complete, the umbilical cord is fully formed. (Fig. 51.11e)
e. Limb buds appear from which will develop the arms and legs. (Fig. 51.12) [transp. 300]
f. The head enlarges and the sense organs become more prominent.
g. The rudiments of the eyes, ears, and nose are evident.
5. Sixth Through Eighth Weeks
a. The developing human becomes more humanlike in appearance.
b. Concurrent with brain development, head achieves its normal relationship with body as a neck region develops.
c. The nervous system is developed well enough to permit reflex actions (e.g., a startle response to touch).
d. At end of this period, embryo is 38 mm long and weighs no more than aspirin tablet; all organs are established.
E. The Placenta Fulfills Needs (Fig. 51.13) [transp. 300]
1. The placenta begins formation once the embryo is fully implanted.
2. The chorionic villi are treelike extensions of the chorion.
a. They project into maternal tissues. (Fig. 51.11c-e and 51.13)
b. Later, these disappear in all areas except where the placenta develops.
3. By tenth week, placenta is fully formed and has already begun to produce progesterone and estrogen.
a. These hormones have two effects.
1) Due to negative feedback control of hypothalamus and anterior pituitary, no new follicles mature.
2) They maintain the lining of the uterus; there is no menstruation during pregnancy.
4. Chorionic villi are surrounded by maternal blood sinuses; yet maternal and fetal blood normally do not mix.
5. Exchange of molecules between fetal and maternal blood takes place across the walls of the chorionic villi.
6. CO2 and wastes move from fetus; O2 and nutrients from maternal side.
7. The umbilical cord stretches between the placenta and the fetus. (Fig. 51.13) [transp. 301]
8. The umbilical arteries transport CO2 and other waste molecules to the placenta for disposal; the umbilical vein transports O2 and nutrient molecules from the placenta to the rest of the fetal circulatory system.
9. Harmful chemicals can cross the placenta.
a. This is of particular concern during the embryonic period, when various structures are first forming.
b. Each organ has a sensitive period during which a substance can alter its normal development.
F. Fetuses Look Human
1. Fetal development (months 3-9) involves an extreme increase in size; weight multiplies 600 times..
2. The genitalia appear in the third month so gender can be identified.
3. The fetus soon acquires features (hair, eyebrows, eyelashes, and nails) that contribute to its human appearance.
4. Fine, downy hair (lanugo) covers the limbs and trunk; it later disappears.
5. The skin grows so fast it wrinkles; a waxy vernix caseosa protects the skin from watery amniotic fluid.
6. A fetus at first only flexes its limbs; later it moves limbs vigorously; a mother feels movements from fourth month.
7. The fetus begins to suck its thumb, swallow amniotic fluid, and urinate.
8. After 16 weeks, the fetal heartbeat is heard through a stethoscope.
7. At 24 weeks, a born fetus may survive, although lungs are still immature and often cannot capture O2 adequately.
G. Birth Has Three Stages
1. Estrogen, prostaglandin, and oxytocin are hormones that cause the uterus to contract and expel the fetus.
2. The process of birth (parturition) has three stages: dilation of the cervix, birth of the baby, and expulsion of the afterbirth (the placenta and the extraembryonic membranes).