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Development and Aging
Chapter 17
Stages of Development
Gamete formation
Fertilization
Cleavage
Gastrulation
Organ formation
Growth, tissue specialization
Cleavage
• Divides up cytoplasm
• Each blastomere ends
up with a portion
• Creates variation
among blastomeres
Three Primary Tissues
• Gastrulation rearranges blastomeres
and creates:
Endoderm
Mesoderm
Ectoderm
Organogenesis
• Process by which organs form
Cell determination
Cell differentiation
Morphogenesis
Fertilization
• Sperm penetrates to egg cytoplasm
• Secondary oocyte undergoes meiosis II;
forms mature egg
• Egg nucleus and sperm nucleus fuse to
form diploid zygote
Pregnancy
• Averages 38 weeks from fertilization
• Takes 2 weeks for blastocyst to form
• Weeks 3 to 8 are embryonic period
• Weeks 9 to birth are fetal period
Early Divisions
• Cleavage begins within 24 hours of
fertilization
Day 1
Day 2
Day 3
Day 4
(morula)
Day 5 - Blastocyst Forms
• Cell secretions produce a fluid-filled
cavity in center of ball of cells
• Layers of blastocyst
– Inner cell mass
– Trophoblast
– Blastocoel
inner cell mass
Implantation Begins
• Blastocyst
attaches to
endometrium;
begins to burrow
into maternal
tissues
Uterine cavity
blastocoel
inner cell
mass
trophoblast
Human Chorionic
Gonadotropin (HCG)
• Hormone secreted by the blastocyst
• Stimulates corpus luteum to keep making
progesterone and estrogens
• This maintains endometrium, prevents
menstruation
• Can be detected by week 3 with a home
pregnancy test
Extraembryonic Membranes
• The amniotic
membrane will
enclose embryo
• Yolk sac forms
• Chorion begins to
form fingerlike villi
chorionic
villi
chorion
amniotic
cavity
connecting
stalk
yolk sac
DAY 14
chorionic
cavity
The Placenta
• Interlocking fetal and
maternal tissues
• Performs digestive,
respiratory, and urinary
functions for the fetus
• Materials exchanged
across membrane that
separates
bloodstreams
Gastrulation - Day 15
• Primitive streak
forms along one
axis of the inner
cell mass
• Cells migrate
inward here to form
endoderm and
mesoderm
Vertebrate Body Plan
Emerges
paired
neural folds
pharyngeal
arches
somites
Morphogenesis
• Orderly changes result in
specialized tissues and early
organs
• Cells migrate
• Whole sheets of cells expand
and fold
• Programmed cell death
sculpts body parts
Cell migration
Embryonic Period
• Weeks 3 to 8
• By the close of embryo
period
– Appears human
– Primordial tissues of
all internal and
external structures
have formed
Week 4
Week 8
Fetal Period
• Weeks 9 to birth
• Fetus is initially about 1 inch long
• Fetus born before 22 weeks cannot
survive
• Survival is poor before 28 weeks
because lungs are not fully formed
• By 36 weeks, survival is 95 percent
Fetal Circulation
• Fetal hemoglobin
• Temporary bypass vessels form and
function until birth
• Umbilical arteries
• Foramen ovale and the arterial duct
• Venous duct
Birth (Labor)
• Cervical canal dilates
• Amniotic sac ruptures
• Uterine contractions
drive fetus from uterus
• Placenta is expelled as
afterbirth
Lactation
• During pregnancy,
progesterone and estrogen
stimulate gland
development
• After birth, prolactin induces
synthesis of enzymes for
milk production
• Oxytocin triggers
contractions
Cloning Embryos
• Biological carbon copy of an organism
• Use chemicals to spur unfertilized,
donated eggs to divide
OR
• Fuse enucleated egg with an adult cell
Fetal Nutrition
• All nutrients for fetal growth and
development must be delivered via
the placenta
• Mother’s diet affects fetal health
• Smoking may affect ability to absorb
nutrients and to pass them to fetus
Teratogens
• Drugs and environmental factors that
may induce deformities during
development
• Effect depends upon time of exposure
• Most have no effect until after second
week
• Thalidomide
Stages of Human
Development - Prenatal
• Zygote - Single cell
• Morula - Solid ball of cells
• Blastocyst - Ball with fluid-filled cavity
• Embryo - 2 weeks to 8 weeks
• Fetus - 9 weeks to birth
Stages of Human
Development - Postnatal
•
•
•
•
•
•
•
Newborn - First 2 weeks after birth
Infant - 2 weeks to 15 months
Child – To 10-12 years
Pubescent - At puberty
Adolescent - Puberty to maturation
Adult
Old age
Aging
• Programmed life-span hypothesis
• Cumulative-assaults hypothesis
Aging Skin and Hair
• Number of fibroblasts in the dermis
starts to decrease
• Elastin replaced by collagen
• Loss of sweat glands
• Hair follicles die or become less
active
• Pigment-producing cells die
Aging Muscle and Bone
• Fibers in skeletal muscle atrophy
• Lost muscle tends to be replaced by fat
and, with time, collagen
• Bones become weaker, more porous,
and brittle
• Cartilage in joints deteriorate
Aging Cardiovascular and
Respiratory Systems
• Walls of alveoli break down
• Heart muscle becomes slightly smaller
• Less blood and oxygen are delivered to
muscles and other tissues
• Blood vessels become narrowed and
less elastic
Aging Nervous System
• Brain neurons die steadily
throughout life
• Neurofibrillary tangles
• Beta amyloid plaques
• Memory loss
• Slowed reflexes
• Farsightedness
Aging Reproductive Systems
• Reduced secretion of estrogens and
progesterone triggers menopause
• Falling levels of testosterone reduce
male fertility
• Vascular changes impair ability to
achieve erection
• Prostate-gland enlargement
Aging Immunity, Nutrition, and
Urinary System
• Number of T cells falls and B cells
become less active
• Mucus-secreting glands of GI tract
degenerate
• Basal metabolic rate declines
• Weakening of muscles causes urinary
incontinence