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 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

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 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

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.
2.
Newborn and
infancy
Childhood and
adolescence
›
›
3.
Puberty: arrival of
sexual maturity
Growth stops by
early twenties
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