Focus: Like all animals, humans begin life as a single cell from which tissues and organs soon
begin to develop.
Cleavage: zygote undergoing cell divisions
Morula: cluster of sixteen cells› Each cell called a blastomere
Gets a different portion of the cytoplasm Helps determine the fate of the cells to come
later
Gastrulation› Rearranges the morula’s cells to form…
Germ layers; three primary tissues1.Ectoderm2.Mesoderm3.Endoderm
Cell differentiation› Process of cell specialization› Begins in the germ layers› Different genetic instructions (genes) operate
in different cells
Adults have ~200 differentiated cell types
Morphogenesis:› Tissues and organs formation› Cell division in certain areas of gastrula› Movement of cells and tissues within
gastrula› Folding of tissues into organs› Death of certain cells
Focus: A newly formed embryo cannot survive unless it
implants in the mother’s uterus.
Fluid-filled cavity forms in the morula → blastocyst› Two tissues
Trophoblast Inner cell mass: forms the embryo
Identical twins › Split usually during first cleavage or the inner cell
mass
Fraternal twins
Implantation› Begins ~one week after fertilization;
completed two weeks after ovulation› Blastocyst breaks out of zona pellucida› Epithelial cells invade endometrium →
cross into connective tissue of uterus› Endometrium covers blastocyst› Blastocyst secretes HCG
human chorionic gonadotropin
Focus: The embryonic period lasts for eight weeks. During that time, the basic body plan of the embryo takes shape.
Embryonic disk› Primitive streak along axis and thickening of
cells marks gastrulation
Ectoderm thickens and forms a neural tube› Forerunner of brain and spinal cord› Notochord; vertebral column forms around it
Mesoderm gives rise to somites› Gives rise to most bones and muscles› Coelom forms from spaces (body cavities)
Neurulation:› Development of the nervous system from
ectoderm Cells move from place to place with chemical
cues Adhesive cues stop their movement
Apoptosis: programmed cell death
Focus: During implantation and over the next few weeks, four specialized membranes form outside the embryo. These membranes include the all-important placenta.
Certain cells of the embryonic disk give rise to the extra-embryonic membranes1. Yolk sac2. Amnion3. Allantois
gives rise to blood vessels that invade the umbilical cord
4. Chorion
Placenta› Organ formed from chorion and upper cells
of the endometrium› Chorionic villi contain small blood vessels› Site of exchange of nutrients and wastes
What other maternal substances can cross the placenta?› Alcohol, caffeine, drugs, pesticide residues,
toxins in cigarette smoke, and HIV
Focus: By the end of four weeks, the embryo has grown to 500 times its original size. Over the next several weeks it will develop recognizable
human features.
Rapid growth and cell specialization
Umbilical cord forms; head region grows faster; gonads develop
After eight weeks, embryo → fetus› Heart beating; genitals well-formed
Miscarriage› Spontaneous expulsion of embryo or fetus
Focus: In the second and third trimesters, organs and organ systems gradually mature in
preparation for birth.
Fetus at three months› 4.5 inches long› Reddish skin
Fetus at four to six months› Facial muscles produce frowning,
squinting, and sucking movements› Movement of fetus felt by mother› Eyelids and eyelashes form
Fetus from seven months to birth› 11 inches at seven months› Organ systems maturing› Fetuses born between 23–25 weeks will
need intensive care
Respiratory distress syndrome› Premature infants at risk› Lungs lack surfactant; can’t expand
adequately
Umbilical cord vessels› Two arteries transport deoxygenated blood and
waste to placenta› One vein brings oxygenated blood and nutrients
to fetus
Fetal lungs bypassed with blood moving through the foramen ovale and the arterial duct
Fetal liver bypassed using the venous duct
Modifications occur at birth
Normal
Focus: Birth, or parturition, takes place about 39 weeks after
fertilization—about 280 days from the start of the women’s last
menstrual period.
Fetal organs (hypothalamus, pituitary, and adrenal glands) produce hormones› Indirectly cause mother’s uterus to contract
Hormones cause placenta to produce more estrogen› Triggers oxytocin and prostaglandins to be
produced› Stronger uterine contractions
Labor length varies
First stage› Contractions push fetus against cervix, which
dilates to 10 cm; usually amniotic sac ruptures
Second stage› Very strong contractions; baby delivers head
first› “Bottom-first” or breech delivery; complications
Third Stage› Placenta forced out of the mother› Baby’s first breath
Video
Estrogen and progesterone during pregnancy› Growth of mammary glands and ducts› Colostrum produced initially
Prolactin› Stimulates milk production
› lactation
Oxytocin› Regulates flow of milk› Reflex; positive feedback control
Focus: From fertilization until birth, a woman’s future child is at the mercy of her diet and lifestyle.
Pregnant women should:› Follow a healthy diet› Take supplemental vitamins and
minerals Folic acid important for neural
tube development: spina bifida exposure of spinal cord in
utero
› Gain between 20–35 pounds
Pregnant woman’s IgG antibodies cross the placenta› Protects fetus against many infections
Teratogens› Agents that cause serious birth defects
Example: Rubella (German measles)› Serious birth defects during first trimester› Mother should be vaccinated to avoid this
threat
Fetus most susceptible during the first trimester
Tranquilizers› Thalidomide: 1960s; caused missing or deformed
limbs
Some sedatives and barbiturates› Similar but less severe damage
Anti-acne drugs› Increased risk of facial and cranial deformities
Antibiotics› Streptomycin
Hearing problems; may affect the nervous system
› Tetracycline Child may develop yellowed teeth
Alcohol› Fetal alcohol syndrome
Many defects: smaller brain and head, facial deformities, poor motor coordination, heart defects
Cocaine, especially crack› Improper nervous system development
Tobacco smoke› Great risk of miscarriage, stillbirth, and
premature delivery› Underweight› Less vitamin C in blood› Effect of secondhand smoke still unknown
1. Chorionic villus sampling (CVS)› Removes sample of chorionic cells by
suction
2. Amniocentesis› Samples fluid from the amnion
3. Preimplantation diagnosis › Uses recombinant DNA technology
4. Fetoscopy › Uses sound waves
Focus: After a child enters the world, a gene-dictated course of further growth and development
leads to adulthood.
1. Newborn and infancy
2. Childhood and adolescence› Puberty: arrival of
sexual maturity› Growth stops by
early twenties
3. Adult and old age
Senescence› Body parts and functioning begin to
deteriorate after age 40
Average life expectancy in the United States› Males: 74› Females: 79
Focus: Time takes a toll on body tissues and organs. To some extent, our genes determine how long each of us will live.
Consistency of lifespan within a species
Most human cells divide between 80–90 times:› Chromosomes capped with segments of DNA
called telomeres These shorten with each cell division
Cancer cells and cells in gonads that give rise to sperm and oocytes:› Produce an enzyme that lengthens the telomere› These cells can divide over and over
Aging probably involves many factors including:› Genes› Free radical damage› Decline in DNA repair
mechanisms
Aging: gradual loss of vitality› Cells, tissues, and organs function less and
less efficiently
Skin: thinner, collagen more rigid, less elasticity, drier, less hair, graying of hair
Muscles: loss mass and strength; lost muscle replaced with fat
Bones: osteoporosis and osteoarthritis
Lungs› Walls of alveoli break down; less surface area
for gas exchange
Heart› Slightly enlarged; less-efficient pump
Blood transport› Vessels become stiffer› Cholesterol plaques narrow the vessels
Immune system› T cell numbers fall; B cells become less active› Autoimmune response increase
Digestive tract› Mucus glands in the lining break down› Pancreas secretes fewer digestive enzymes› Basal metabolic rate decreases
Hormones: most keep steady levels; sex hormones are the exception
Brain neurons die throughout life brain shrinks
Aged brain› Neurofibrillary tangles: may disrupt normal
cell operations› Beta amyloid forms: clotlike plaques
between neurons
Sensory organs and taste buds› Less efficient over time
Alzheimer’s disease › Physiology
Masses of neurofibrillary tangles riddled with beta amyloid plaques
› Symptoms Progressive memory loss Disruptive changes to personality
› Causes Some are genetic
› Treatments Limited; may slow progression