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