Chapter 21

 

Chapter 21: Development and Ageing

Fertilization

• Fertilization creates an entire ______________________________
• Sperm DNA fuses with egg DNA

 

21.1 The Production of Embryonic Stem Cells

• A ___________________________ is an undifferentiated cell that gives rise to many different kinds of cells
• Embryonic cells typically go through the process of _____________________________ when their function becomes specified and fixed
• Embryonic stem cells are undifferentiated and can develop into any cell in the body

 

• There are three types of stem cells: ___________________________, ___________________, and _________________________________
• Adult stem cells may be useful in regenerative medicine, but they are uncommon and produce a limited number of daughter cells

 

• Fetal and embryonic stems cells are produced before birth
  • _______________________________________ are produced during the first weeks of development
  • _______________________________________ are produced during the fetal stage (9^th week until birth)
    • Embryonic and fetal stem cells divide indefinitely, are easy to obtain and give rise to a large number of cell types
  • The ultimate stem cell is the newly fertilized egg
    • Every cell in the body, including the membranes that will surround the developing organism start here!

 

• Researcher are interested in stem cells that are isolated from pre-embryos, a few days after fertilization, which are left over from fertility treatments

 

• _________________________________________ is a common fertility program that generates embryonic stem cells
  • IVF use began in 1977
  • IVF begins with harvesting eggs from a woman’s ovaries after drug therapy, which tends to generate multiple eggs
• The eggs are mixed with sperm

 

• Fertilization in vitro is a given because there are so many sperm, and because the environment is much more hospitable than the high acidity of the vagina and the detours found in the folds of the cervix, vagina, uterus, and length of the oviduct
• Once sperm makes contact with the egg, the process is the same whether in a glass petri dish or in an oviduct

 

• When the fertility treatments end, the remaining pre-embryos may be discarded, donated to other infertile couples or used for stem cell research
• In natural and assisted reproduction, the remainder of _________________________ takes place in a woman’s uterus and continues after birth

 

Prenatal Development

• There are three distinct phases in development that produces a multicellular embryo ready to produce adult tissues:
  • _____________________________________________________
    • The ovum is fertilized by sperm
    • Is quickly implanted into the uterus
    • This phase lasts roughly two weeks and ends with the formation of the embryo
  • ____________________________________________________
    • Weeks 3-8
    • The embryo develops
  • ___________________________________________________
    • Weeks 9 through birth
    • Now referred to as the fetus

 

Cellular Development

• Throughout pregnancy, the developing infant undergoes three basic kinds of cellular division and development:
• ____________________________________
  • A series of rapid cell divisions in the first 4 days after fertilization
  • Does not increase the size of the zygote
• ____________________________________
  • The cells of the developing infant begin to specialize and take on different forms and functions

 

• ____________________________________
  • Major changes in shape and functions of the embryo and fetus, and their organs and tissues
• Both differentiation and morphogenesis results in growth
  • The increasing number of cells causes an increase in size and mass

 

21.1 The Production of Embryonic Stem Cells

• Pre-embryonic development produces a multicellular embryo ready to produce adult tissues
• During the pre-embryonic stage, there are many cell divisions, but the embryo does not grow – although it does change its form (_______________________________________)
• The first step is a series of mitotic divisions, _______________________________, that occur while the zygote is still in the oviduct

 

• The __________________________________ occurs about 24 hours after fertilization and the 2^nd and 3^rd take place the next day
• During the first cleavage or two, it is possible for the daughter cells to break apart forming _____________________________________________ (identical)

 

• The 16-cell stage is called the ____________________________, which continues to divide resulting in a fluid-filled sphere of about 250 cells called a _______________________________________

 

• At this point, some 30-34 cells inside the blastocyst form the ________________________________, which will give rise to the embryo
• The rest of the blastocyst is called the ____________________________, which includes the ___________________________ one of the extraembryonic membranes
  • The chorion, along with tissues from the mother, will eventually make up the placenta

 

• IVF embryos are placed in the uterus at the blastocyst stage
  • The same stage that cells of the inner cell mass are harvested to create stem cells
  • Harvesting stem cells destroys the pre-embryo creating the moral dilemma of stem cell research
    • These cell lines are colonies of cells that given hormones and nutrients can replicate themselves

 

• About a week after fertilization, the blastocyst enters the uterus and begins the process of __________________________________
  • Up to half of the embryos fail to implant
  • 20% of pregnancies end in ________________________________ (spontaneous abortion) because of a genetic or developmental abnormality of the embryo
  • Only about 15% of IVF embryos implant, which is the reason multiple eggs are prepared

 

21.2 Early Embryonic Development

• During implantation (week 2 of pregnancy), the embryo secretes enzymes to digest part of the _______________________________, the lining of the uterus
• Within a few days, the embryo becomes engulfed by the endometrium

 

• Implantation marks the beginning of ___________________________________________, the time at which the pregnancy can be detected by the presence of _________________________________________________ (HCG)
  • Pregnancy tests detect HCG in the urine of the pregnant female

 

• HCG inhibits the ovary’s corpus luteum from degrading, so it continues to produce estrogen and progesterone, which maintain the endometrium and inhibit menstruation
• Estrogen and progesterone levels from the corpus luteum increase during the first 5 weeks of pregnancy
  • Causing the nausea of morning sickness

 

• There are four extraembryonic membranes that form: the _______________________, the ____________________________, the _______________________, and the __________________________________
  • Develop into essential carriers of nutrients
  • Each provides vital support to the embryo and fetus

 

• The _________________________ is closest to the embryo and lines the amniotic cavity providing a diffusion area for the amniotic fluid
  • _______________________________ is derived from maternal interstitial fluid and removes embryonic and fetal waste products
  • Protects the developing embryo and fetus from outside injury
  • Allows free fetal movements so the muscular system can grow
  • Maintains a constant temperature
  • Permits proper lung development

 

• The __________________________ provides the starting material for the blood vessels of the umbilical cord
  • These vessels will transport fetal blood so that oxygen, nutrients and waste material are diffused
  • Once the vessels form, the allantois degenerates
• The ______________________________ will become part of the digestive and respiratory tracts
  • Also produces fetal blood cells until the bone marrow takes over

 

• The ___________________________ is the outermost layer
  • The exchange portion of the placenta
  • Produces HCG, which maintains pregnancy until the placenta is fully functional
  • 99% accurate in identifying pregnancy
    • A positive test is an accurate indication of an implantation and a developing chorion

 

• The inner cell mass separates from the trophoblast and is called the __________________________________
• It makes some conformational changes to produce the _________________________, a structure with three germ layers

 

• ______________________________ - the outermost germ layer; gives rise to the skin, nervous system, and sensory organs
• _____________________________ - the middle layer; gives rise to the muscles, heart, blood vessels, kidneys, sex organs, and skeletal system
• _____________________________ - the innermost layer; gives rise to the digestive system and respiratory system

 

• ____________________________________ of ectoderm, endoderm, and mesoderm results from the distribution of chemicals in the cytoplasm
  • These chemicals are unevenly distributed so that cells in different regions of the body end up with varying chemical signals

 

21.3 Organ Formation

• Cells differentiate to form tissues, which form organs (____________________________________)
• By the end of the embryonic period, organogenesis is nearly completed and the embryo is distinctly human
• This period of organ formation is risky because environmental exposure to toxic compounds can easily disrupt the process
  • The nervous system is the first to develop and it is the most sensitive

 

• Organogenesis results not only from differentiation, but _______________________ and _____________________ (programmed cell death) are also involved
• Chemical signals along the route guide migrating cells to find their final destination

 

• Environmental toxins can disrupt _________________________
  • Nicotine interferes with the migration of bone marrow cells from the fetal liver – resulting in cardiovascular and immunological problems in the babies of smokers
• Embryonic stem cell research might lead to a better understanding of cell migration

 

• _________________________ is poorly understood
  • Must occur in order for fingers and toes to form from the early paddle-like embryonic hands and feet

 

• The central nervous system begins to form in the gastrula when the ________________________________ forms along the midline
• By the 3^rd week, it thickens forming the _____________________, which folds causing a depression with ridges on either side
  • These ridges fuse to form the hollow _________________________ covered with a layer of ectoderm

 

• The front of the neural tube will become the brain and the rest will become the spinal cord
  • The neural tube will separate and connecting cells will either die or migrate to other areas to form the peripheral nerves
• The mesoderm on either side of the neural tube will form ______________________, which will become muscular, skeletal, and dermal tissues

 

• ______________________________ is a birth defect in which the neural tube doesn’t close properly
  • The result is that part of the spinal cord may be enclosed in a cyst outside the spine
• Likely due to a _____________________ vitamin deficiency in the mother’s diet
  • As folic acid has been added to flour and processed foods, neural tube defects have declined

 

• Prior to week 7, male and female embryos look the same
  • Both contain embryonic gonads, which will produce gametes and an appropriate set of ducts
• Differentiation of these embryonic structures into male or female organs is determined by the expression of sex-specific genes, some of which are found on the sex chromosomes

 

• Gonad development begins as cells migrate to the abdominal region to form the _______________________________
• Once there, the cells copy themselves to form two ____________________________, which can form either testes or ovaries
  • Inside the indifferent gonads are germ cells that can become either sperm or eggs

 

• Both sexes contain two different sets of ducts:
  • The _________________________________, which develops into male structures
  • The _________________________________, which develops into female structures
• The sex hormones that the embryo will be exposed to around the 7^th week of development will determine which of these structures remains and develops

 

21.4 Fetal Development and Birth

• The ______________________ is fully developed by the beginning of the fetal period
• The placenta works like a large diffusion filter, allowing the exchange of nutrients, gases, and antibodies ____________________________________________________
  • In addition to providing nutrition and oxygen, the placenta also produces a range of hormones responsible for maintaining pregnancy

 

• The placenta is unique!
  • Consists of both _________________ and _____________________ tissue
  • Begins forming at implantation when the trophoblast digests part of the endometrium, causing some blood to pool, in which the trophoblast will sit
    • Oxygen, nutrients, and wastes diffuse through the pool of blood between mother and embryo

 

• As the embryo develops blood vessels, the chorion develops ___________________ into the surrounding uterine tissue to increase the surface area for diffusion between mother and embryo

 

• The placenta produces progesterone and estrogen taking over the corpus luteum’s job of maintaining the endometrium and preventing egg formation
  • Progesterone relaxes the uterine smooth muscle, allowing the uterus to expand
  • The smooth muscle of the digestive tract also relaxes
    • This leads to heart burn and constipation

 

• ____________________________ relaxes arteries, causing blood pressure to drop, which triggers the kidneys to release hormones that increase blood volume
• ____________________________ promotes water retention, so these two effects result in a 40% increase in blood volume in pregnant women
• Estrogen and progesterone also cause some areas of the skin to darken
  • Areola around the nipples
  • Face (freckles)
  • Line running from the navel to the pubic region (linea nigra)

 

• The placenta produces several other hormones
  • One hormone makes the mother’s cells resistant to insulin, perhaps to increase blood sugar levels to supply the fetus
  • The placenta synthesizes ___________________________________, which is involved in stimulating mammary gland development

 

• In a typical pregnancy, the combined weight of the placenta, fetus, enlarged breasts and uterus as well as the increase of blood and amniotic fluid results in an increased weight of 20-30 lbs

 

• The placenta is required for fetal survival because it is the exchange site for food, gasses, and wastes
• The exchange of materials across the placenta requires some differences from the adult circulation at the placenta surface and within the fetus

 

• Dissolved substances (oxygen, carbon dioxide, wastes) move across the placental membrane by diffusion
  • Efficient movement of materials to benefit the fetus requires the appropriate __________________________________ between mother and fetus
• The high levels of maternal glucose ensures a steady delivery to the fetus

 

• Oxygen flows to the fetal blood because fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin
• Fetal hemoglobin is produced a few weeks before the fetal period
  • Levels decline in the newborn until replaced by adult hemoglobin at about one year of age

 

• Carbon dioxide flows from the fetal blood to the mother’s blood
  • As the uterus grows, it widens the thoracic cavity leading to a 40% increase in the mother’s lung capacity and a 20% decline in maternal blood carbon dioxide levels
• The fetus has a rapid heart rate, which ensures the rapid transfer of dissolved gases across the placenta

 

• The placenta has receptors for different molecules, but some materials are able to cross without receptors
  • Alcohol easily crosses the placenta and leads to skull deformities and reduced mental capacity

 

• The ______________________________ contains blood vessels connecting the blood stream of the fetus to the placenta
• The fetal heart pumps blood throughout the body to the _______________________, which carry blood to the placenta
• Blood returns to the fetus from the placenta via the _____________________________, which eventually merges with the inferior vena cava

 

• Nutrients passing into the fetal bloodstream have been processed by the maternal liver, so that most of the returning blood bypasses the fetal liver
• Fetal lungs are not needed until birth, so a gap called the _______________________, located between the two atria allows some blood to bypass the lungs and enter the left atrium

 

• The left ventricle pumps blood into the systemic circulation
• Blood reaching the right ventricle bypasses the lungs and empties into the aorta
• After birth, blood flows to and from the lungs as in the adult

 

Prenatal Development

• From weeks 5-8, the embryo becomes increasingly human in appearance
  • The tail (appeared 1^st month) regresses
  • The head enlarges
  • Limb buds (appeared 1^st month) are forming structures that look very much like arms and legs, and hands and feet
  • The nose is flat
  • Gonads are formed
  • Eyes are widely spaced and open
• By 8^th eek, all the ____________________________________________________________________, though not fully functional

 

• By the end of the embryonic period (week 8), the newly formed individual has a recognizably human form
  • All the internal and external structures are present at the end of this phase
  • The placenta is mature and functioning
• Fetal development begins at ________________________ after conception, and is a stage of rapid ______________________________ and ________________________________
  • The fetus begins this stage 1-inch long and weighing ~ 1 gram
  • Within several months, the fetus will grow to an average of 20 inches and weigh 8 lbs

 

• Prenatal development and pregnancy is divided into trimesters
• The ___________________________ includes all embryonic development and the first month of fetal development

 

• The _______________________________ includes months 4, 5, and 6
  • At month 4, the face begins to resemble its final form at birth
  • Blood cells are produced by the liver and bone marrow, and ovarian follicles are forming in the ovaries
  • The fetus has grown to 6 inches and weighs 165 grams
  • By the 5^th month, the nervous and muscular systems have developed enough that movements may begin

 

• The heartbeat can be heard
• The fetal skin is covered in soft hair, called ________________________
• By the end of the 6^th month, the fetus weighs 450 grams (1 lb)
  • The lungs begin secreting surfactant, allowing the alveoli to inflate and deflate without the walls sticking together
  • With excellent and immediate medical care, the fetus can survive outside the womb

 

• The _________________________________ is characterized by continued rapid growth and maturation
  • Eyes open and close
  • Sucking response develops
  • Loud noises initiate a startle reaction
  • Fetus moves regularly
  • Lanugo is lost and a layer of protective fat begins to develop
  • Testes descend into the scrotal sac

 

• By the end of week 38, the fetus is prepared for life outside the uterus
  • It has maneuvered to a had-downward position and “dropped”
    • The fetus is now resting on the cervix rather than filling the center of the uterus

 

21.4 Fetal Development and Birth

• _________________________________________ initiates the end of pregnancy
• Labor begins with hormonal triggers
  • The placenta increases estrogen secretion and decreases progesterone secretion
  • Estrogen stimulates _______________________ release and increases placental prostaglandin production
  • Oxytocin then initiates ____________________________________ in the uterus

 

• Delivery has three stages: dilation, expulsion, and the placental stage

 

• The first stage in _________________________                
  • Fetal head presses on the cervix
  • This pressure, combined with uterine contractions stretches the cervical opening, which increases with each uterine contraction
    • Going from slightly less than 2 cm to over 10 cm (10 inches)
  • As the cervix enlarges, the mucus plug that was created by placental hormones drops out
  • The thin amnion is all that remains between the fetus and the external environment
    • This fragile membrane ruptures under increasing pressure, releasing a rush of amniotic fluid (“water breaking”)
  • After the amniotic fluid is lost, labor begins

 

• The second stage of delivery, ______________________________, is relatively short usually lasting less than an hour
  • The time from cervical dilation to delivery
  • Uterine contractions gain strength and the mother assists in the birth by pushing with voluntary muscles
    • The baby moves through the cervix and out the vagina

 

• The placenta and umbilical cord are expelled in the final stage, ____________________________________________ (afterbirth)
  • The placenta is still in the uterus after the baby is born
  • Strong uterine contractions continue and they tear the placenta from the walls of the now shrinking uterus

 

21.5 Development After Birth

• During the first month of life, the _________________________ period, the baby’s digestive system begins to function and fat begins to deposit under the skin
  • This fat allows the baby to be more efficient in temperature control
• The brain is also maturing and the neonate is adjusting better to loud stimuli

 

• _______________________________ is the key to the neonate’s health
  • Breast milk is the primary source (or formula)
• The first few days after birth, the mammary glands produce __________________, a yellow fluid low in fat, but rich in proteins, antibodies, minerals, and vitamin A
• _____________________________ stimulates __________________________ (milk production) and sucking on the nipple triggers oxytocin release, which stimulates the release of milk into the milk ducts

 

• __________________________________: 1-15 months after birth
• __________________________________: 15 months to 15 years
• __________________________________ (puberty): 10-16 years in girls; 13-16 years in boys
• __________________________________: ages 19-21
• Development continues during __________________________ in many areas, but bone growth and brain maturation reach completion by age 25

 

• The overall growth rate continues through childhood, but at the onset of puberty, there is a growth spurt after which little growth occurs
• After birth, the pattern of growth is irregular (_______________________________), with some body parts growing fast and other growing slowly
  • Notice how short an infants legs are relative to its body length
  • The relationship changes as the limbs grow faster than the trunk during childhood and puberty

 

• Puberty in girls
  • Puberty begins with development of breasts around age 10-11 and with pubic hair appearing a bit later
  • _______________________________, the first menstruation, occurs about two years after breast development and it takes about two years for the menstrual periods to become regular
  • Body hair under/on the arms, legs, and upper lip is also part of puberty
  • The lower pelvis increases in size and fat storage leads to larger hips
  • Growth in female height is complete in most women about two years after menarche

 

• Puberty in boys
  • The enlargement of the penis and testes are the first signs of puberty
  • Libido increases and erections become common
    • Early in puberty, ejaculations are possible but few (if any) sperm are present
  • The larynx grows causing the voice to deepen
  • Voice change is followed by facial and body hair changes
  • The growth spurt in boys lasts longer than in girls, resulting in a 4-inch height difference between adult men and women

 

21.6 Ageing

• __________________________________ involves the progressive changes that contribute to an increased risk in disease, disability, and death
  • There are many habits and behaviors that contribute to ageing, but for most people, begins around age 40

 

• In the U.S., the average lifespan for men is 72 and for women is 79
• There are several hypotheses that explain what limits life span
  • Coded in our genes, and idea supported by the observation that cells divide a certain number of times before dying
    • This limit is due to ______________________________, DNA repeats on the ends of chromosomes, which get shaved down a bit with each cell division

 

• Some cells such as sperm generating cell, cancer cells, fetal and embryonic stem cells produce and enzyme called ________________________________ that restores the telomere after each division
• Some organisms have life span genes, which if mutated, greatly affect life span in both directions depending on the mutation
  • Humans have similar genes, but their function is much less understood

 

• Genetic and environmental factors determine how a person ages
• Cells gradually have a loss of function and death resulting in organ system failure
• Free radicals often damage DNA
  • Free radicals are produced by aerobic respiration, but caloric restriction suppresses them
• Ageing is a positive feedback system because damage to one system affects other systems
  • Once the immune system is affected, we become more susceptible to illness, cancer, and autoimmune diseases as we often see in the elderly

 

• ___________________________________ is the field that studies ageing and hopes of finding ways to prolong a healthy life span
• Gerentologists promote exercise programs and a healthy diet to prevent (or slow) the negative effects of ageing

 

• Memory function is associated with connections between nerons
• ________________________________ begins with the formation of tangled clumps of fibers inside brain neurons, which cause neuronal death and loss of function for entire brain regions
  • Affects 3% of people over 65 and nearly half of those 85 and older
  • As of now, there are few treatments available

 

• ___________________________________ is a degenerative disease of the CNS that affects motor skills
  • Occurs when the brain cells that produce dopamine cease to function
  • Most cases occur after the age of 60
• Perhaps stem cells could be used to replace those cells
  • Mouse embryonic stem cells have been used to cure Parkinson’s-disease in rats