Focus: Like all animals, humans
begin life as a single cell from which
tissues and organs soon begin to
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
› Rearranges the morula’s cells to form…
Germ layers; three primary tissues
1. Ectoderm
2. Mesoderm
3. 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
› 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 →
› Two tissues
 Trophoblast
 Inner cell mass: forms the embryo
Identical twins
› Split usually during first cleavage or the inner cell
Fraternal twins
› 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)
› Development of the nervous system from
 Cells move from place to place with chemical
 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 allimportant placenta.
Certain cells of the embryonic disk give
rise to the extra-embryonic membranes
1. Yolk sac
2. Amnion
3. Allantois
 gives rise to blood vessels that invade the
umbilical cord
4. Chorion
› 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
› 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
Umbilical cord vessels
› Two arteries transport deoxygenated blood and
waste to placenta
› One vein brings oxygenated blood and nutrients to
Fetal lungs bypassed with blood moving
through the foramen ovale and the arterial
Fetal liver bypassed using the venous duct
Modifications occur at birth
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
› 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
Estrogen and progesterone during pregnancy
› Growth of mammary glands and ducts
› Colostrum produced initially
› Stimulates milk production
› lactation
› 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
 Folic acid important for neural tube
 spina bifida exposure of spinal cord in
› Gain between 20–35 pounds
Pregnant woman’s IgG antibodies cross the
› Protects fetus against many infections
› Agents that cause serious birth defects
Example: Rubella (German measles)
› Serious birth defects during first trimester
› Mother should be vaccinated to avoid this
Fetus most susceptible during the first trimester
› Thalidomide: 1960s; caused missing or deformed
Some sedatives and barbiturates
› Similar but less severe damage
Anti-acne drugs
› Increased risk of facial and cranial deformities
› Streptomycin
 Hearing problems; may affect the nervous system
› Tetracycline
 Child may develop yellowed teeth
› Fetal alcohol syndrome
 Many defects: smaller brain and head, facial
deformities, poor motor coordination, heart
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
Chorionic villus sampling (CVS)
› Removes sample of chorionic cells by
› Samples fluid from the amnion
Preimplantation diagnosis
› Uses recombinant DNA technology
› Uses sound waves
Focus: After a child enters the
world, a gene-dictated course of
further growth and development
leads to adulthood.
Newborn and
Childhood and
Puberty: arrival of
sexual maturity
Growth stops by
early twenties
Adult and old age
› Body parts and functioning begin to
deteriorate after age 40
Average life expectancy in the United
› 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
› 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
Aging: gradual loss of vitality
› Cells, tissues, and organs function less and less
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
› Walls of alveoli break down; less surface area for
gas exchange
› 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
› Beta amyloid forms: clotlike plaques between
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