🩺chapter 24- growth, development and aging

Overview & Objectives

• Development: A continuous, biological process of modification and change — not isolated events.

• Prenatal period: Conception to birth.

• Postnatal period: Birth to death.

• Milestones like childhood or adolescence are convenient markers but are part of a seamless process.

• Chapter focus:

  1. Development from conception through prenatal period.

  2. Birth process.

  3. Postnatal developmental changes.

  4. Effects of aging on body systems.

Prenatal Period

• Begins at fertilization (union of sperm and ovum).

• Lasts ~ 39 weeks until birth.

• Study of prenatal development = Embryology.

• Describes transformation from a single cell to a complex human.

Fertilization to Implantation

1. Ovulation: Ovum released → pelvic cavity → uterine (fallopian) tube.

2. Fertilization:

   • Usually occurs in the outer one-third of the fallopian tube.

   • Produces a zygote: single cell with complete genetic info (sex, body build, skin color already determined).

3. Cell Stages:

   • Zygote undergoes mitosis.

   • ~3 days later → solid mass of cells = morula.

   • Morula hollows out into blastocyst.

4. Implantation:

   • Blastocyst embeds into uterine lining ~10 days after fertilization.

   • Before placenta forms, nutrients come from uterine fluids.

Amniotic Cavity & Placenta

• Blastocyst structure: Outer layer + inner cell mass → forms yolk sac & amniotic cavity.

• Yolk sac:

  • In humans, not for nutrition.

  • Functions: early blood cell production, stem cell formation for gonads.

• Amniotic cavity:

  • Fluid-filled sac (“bag of waters”) cushions & protects embryo.

• Chorion: Outer membrane → chorionic villi connect embryo’s blood supply to placenta.

• Placenta:

  • Exchange of nutrients, wastes, gases.

  • Acts as respiratory, excretory, endocrine organ.

  • Protective placental barrier — prevents most mixing of maternal & fetal blood.

  • Not foolproof: Alcohol, CMV, Zika, syphilis can cross.

Periods of Development

• Gestation period: ~39 weeks → divided into three trimesters.

• First trimester:

  • Zygote → morula → blastocyst → implantation.

  • Embryonic phase: Week 3–8 after fertilization.

  • By day 35: Heart beating, limbs forming.

• Fetal phase: Week 9–birth.

  • End of 1st trimester: Fetus ~7–8 cm, limbs complete, gender visible.

  • By month 4: All organ systems formed and functioning.

Formation of the Primary Germ Layers

• Early embryonic cells = stem cells (unspecialized, can become any tissue).

• Three primary germ layers:

  1. Endoderm (inside) → digestive tract lining, respiratory lining, glands.

  2. Mesoderm (middle) → muscles, bones, circulatory system, connective tissue.

  3. Ectoderm (outside) → skin, nervous tissue.

• These layers give rise to all specialized tissues and organs.

In Vitro Fertilization (IVF)

• Definition: Fertilization in lab dish; part of assistive reproductive technologies (ARTs).

• Classic method:

  • Ovum retrieved via laparoscopy.

  • Newer method: Needle guided by ultrasound through vaginal wall.

  • Ovum mixed with sperm → 2–5 days growth → implanted into uterus.

• Genetics:

  • Screening possible before implantation.

  • Ethical debates over gender selection.

Histogenesis & Organogenesis

• Histogenesis: Germ layers → tissues.

• Organogenesis: Tissues → organs.

• Development involves cell differentiation, multiplication, growth, apoptosis, rearrangement in set sequence.

• By 4 months: All organ systems in place; after that → growth dominates.

Table 24-1 — Primary Germ Layer Derivatives

• Ectoderm: Epidermis, tooth enamel, lens/cornea, outer ear, nasal cavity, brain/spinal cord, adrenal medulla, pituitary gland.

• Mesoderm: Dermis, circulatory system, kidneys, gonads, muscle, bones (except facial), adrenal cortex.

• Endoderm: GI tract lining, lung lining, hepatic/pancreatic ducts, bladder lining, thymus, thyroid, parathyroid, tonsils.

Birth — Parturition

• Transition from prenatal to postnatal life.

• Caused by uterine contractions + cervical dilation.

• Amniotic sac rupture = “water breaking”.

Breech Birth

• Fetus fails to rotate head-down before delivery → buttocks or feet present first.

• Can make vaginal delivery risky → often requires C-section.

Cesarean Section (C-section)

• Surgical delivery via abdominal & uterine incision.

• Indications: Breech birth, fetal/maternal complications, inability to safely deliver vaginally.

Stages of Labor

1. Stage One: Onset of contractions → full cervical dilation.

2. Stage Two: Full dilation → delivery of baby.

3. Stage Three: Delivery of placenta (~15 min after baby).

Pregnancy Length

• Average = 266 days from conception, 280 from last menstrual period.

• <37 weeks = premature; >42 weeks = postmature.

Figure 24-7 — Critical Periods

• Red bars = high risk of major congenital conditions.

• Yellow bars = risk of minor congenital conditions.

• Most vulnerable period: Early embryonic development.

Freezing Umbilical Cord Blood

• Cord blood rich in stem cells → can be stored for future treatments (e.g., leukemia).

• Safer and more available than bone marrow stem cells.

Stages of Parturition (Figure 24-8)

1. Fetus positioned.

2. Amniotic sac ruptures; cervix dilates.

3. Full cervical dilation; sac rupture widens.

4. Baby delivered through birth canal.

5. Placenta expelled.

Multiple Births

• Identical (monozygotic) twins:

  • One zygote splits early → genetically identical.

  • Usually share placenta, have separate cords.

  • Same genetic code, but can differ due to environment.

• Fraternal (dizygotic) twins:

  • Two ova fertilized by different sperm.

  • Genetically like siblings.

  • Separate placenta & amnion.

• Fertility treatments ↑ likelihood of multiple births.

Quick Check (page 674)

1. What is the postnatal period? The prenatal period?

   • Postnatal period: Begins at birth and continues until death.

   • Prenatal period: Begins at conception (fertilization) and ends at birth.

2. What is a zygote? How is it different from a morula or blastocyst?

   • Zygote: Single-celled organism formed at fertilization, containing the complete genetic code.

   • Morula: Solid mass of cells formed after several divisions of the zygote (~3 days).

   • Blastocyst: Hollow ball of cells formed from the morula before implantation.

3. Name and describe the three primary germ layers.

   • Endoderm: Inside layer; forms GI tract lining, lung lining, and some glands.

   • Mesoderm: Middle layer; forms muscles, bones, circulatory system, connective tissues.

   • Ectoderm: Outside layer; forms skin and nervous tissue.

4. What are stem cells?

   • Unspecialized cells capable of producing many different kinds of specialized cells.

5. What is meant by the term organogenesis?

   • The process by which tissues arrange themselves into organs.

Quick Check (page 677)

1. How does a breech birth differ from a typical birth?

   • In breech birth, the fetus is positioned so that the buttocks or feet are delivered first instead of the head.

2. Name and describe the three stages of labor.

   • Stage One: Onset of contractions to full cervical dilation.

   • Stage Two: Full dilation to delivery of the baby.

   • Stage Three: Delivery of the placenta.

3. How do multiple births occur?

   • Identical twins: One zygote splits early in development → genetically identical offspring.

   • Fraternal twins: Two separate ova are fertilized by different sperm during the same menstrual cycle.

   • Fertility treatments can increase the chance of multiple births.

4. Identify the primary difference between identical and fraternal twins.

   • Identical twins share the same genetic code from one zygote; fraternal twins are genetically like regular siblings from two separate fertilizations.

5. What is an Apgar score?

   • A scoring system used after birth to evaluate a newborn’s heart rate, respiration, muscle tone, skin color, and response to stimuli.

Conditions of Pregnancy

Implantation Conditions

• A successful pregnancy depends on proper implantation of the blastocyst in the uterine wall.

• Improper implantation can cause:

  • Ectopic pregnancy: Implantation outside the uterus (most often in a fallopian tube — tubal pregnancy). Tube can rupture → severe bleeding → life-threatening.

  • Placenta previa: Placenta grows too close to the cervical opening; may cause painless bleeding during late pregnancy, can endanger mother & baby.

  • Abruptio placentae: Complete separation of placenta from uterine wall before delivery → immediate fetal death and maternal hemorrhage.

• Ectopic pregnancies often require C-section delivery.

Preeclampsia

• Also called toxemia of pregnancy.

• Serious condition in ~1 in 20 pregnancies after 24 weeks.

• Characterized by:

  • High blood pressure.

  • Proteinuria.

  • Edema.

• Can cause abruptio placentae, stroke, hemorrhage, fetal malnutrition, low birth weight.

• May progress to eclampsia → seizures, coma, organ failure, possibly death.

Gestational Diabetes Mellitus (GDM)

• High blood glucose first detected during pregnancy.

• Caused by pregnancy hormones promoting insulin resistance.

• Can occur in women with undiagnosed type 1 or type 2 diabetes.

• Risks:

  • Large infant size → difficult delivery or need for C-section.

  • Increased risk of preeclampsia.

  • Can harm both mother and fetus if uncontrolled.

Fetal Death

• Miscarriage (spontaneous abortion): Loss before 20 weeks (<500 g or 1.1 lb).

• Stillbirth: Death after 20 weeks gestation.

• Causes: Hypertension, uterine conditions, hormonal imbalances, structural/genetic abnormalities.

Congenital Conditions

• Developmental problems present at birth — structural, functional, or behavioral.

• Causes:

  • Genetic factors (inherited mutations, chromosomal abnormalities).

  • Teratogens — environmental agents causing defects:

    • Radiation (x-rays).

    • Toxic chemicals (drugs, cigarette smoke, alcohol).

    • Infections in mother (herpes, Zika, CMV).

  • Nutritional deficiencies.

• Highest risk period = First trimester during organ differentiation.

• Severe damage can cause miscarriage.

Postpartum Conditions

• Puerperal fever (childbed fever):

  • Bacterial infection post-delivery.

  • Historically a major cause of maternal death; now rare with antibiotics.

• Lactation needs: Breastfeeding important for newborn nutrition.

• Mastitis: Inflammation/infection of breast tissue → possible milk contamination.

• Lactose intolerance in infants: Caused by lack of lactase enzyme → may need lactose-free formula.

Antenatal Diagnosis & Treatment

• Antenatal medicine: Diagnosis & treatment before birth.

• Includes:

  • Ultrasound (ultrasonogram) — 2D/3D imaging.

  • Prenatal surgery (e.g., bladder repair).

  • Monitoring fetal health.

  • Treating conditions like Rh incompatibility before birth.

Fetal Alcohol Syndrome (FAS)

• Caused by alcohol crossing placenta into fetal circulation.

• Effects:

  • Microcephaly (“small head”).

  • Low birth weight.

  • Developmental & learning disabilities.

  • Growth restrictions.

  • Facial abnormalities.

  • Possible fetal death.

• Even small amounts of alcohol in pregnancy can cause harm.

Quick Check (page 680)

1. What conditions can result from improper implantation in the uterine wall?

   • Ectopic pregnancy, placenta previa, abruptio placentae.

2. What complications may occur as a result of preeclampsia?

   • Abruptio placentae, stroke, hemorrhage, fetal malnutrition, low birth weight, and possible progression to eclampsia.

3. What is a teratogen? Give some examples.

   • An agent causing developmental defects; examples: radiation, toxic chemicals (drugs, alcohol), infections (herpes, Zika, CMV).

4. What is gestational diabetes?

   • Diabetes first diagnosed during pregnancy, caused by insulin resistance; can harm both mother and baby.

Postnatal Period – Growth, Development, and Aging

• Begins at birth, lasts until death.

• Divided into major periods: infancy, childhood, adolescence, adulthood, older adulthood.

• Growth & development involve continuous changes in body proportions and structure.

Body Proportion Changes

• Infant’s head ≈ 1/4 of total height; adult’s head ≈ 1/8.

• Face: infant’s face ≈ 1/8 of head; adult’s ≈ 1/2 of head.

• Legs grow proportionately longer, trunk proportionately shorter.

• Trunk shape changes from round → more elongated.

Infancy

• From birth to 18 months.

• First 4 weeks = neonatal period.

• Neonatology: medical specialty for newborns.

• Cardiovascular & respiratory system changes critical at birth (first breath triggers lung function).

• Birth weight doubles by 4 months, triples by 1 year.

• “Baby fat” peaks then declines; infant becomes leaner.

• Spinal curves: At birth only one curve; cervical curve appears by 3–4 months; lumbar curve appears by 12–18 months → toddler can stand/walk.

• Milestones:

  • Follow object with eyes (2 months)

  • Lift head/chest (3 months)

  • Sit (6 months)

  • Stand (12 months)

  • Run (18 months)

Childhood

• End of infancy to puberty (12–14 years girls, 14–16 boys).

• Rapid growth but less consistent month-to-month.

• Deciduous teeth lost ~age 6; permanent teeth erupt, including wisdom teeth by ~age 14.

Adolescence

• Teenage years (13–19).

• Physical growth → sexual maturity.

• Controlled by sex hormones; secondary sex characteristics develop.

• Girls: breast development first sign; menarche ~12–13 years.

• Boys: testicular enlargement 10–13 years; both sexes have height growth spurts (girls ~10–12, boys ~12–13).

Adulthood

• Maturation continues (bone closure, organ placement changes).

• Maintenance of body tissues is main process.

• With age, repair becomes harder → degeneration begins.

Older Adulthood

• Peak function early, then gradual decline in all systems.

• Gerontology = study of aging.

• Figure 24-15 shows % of function remaining in older adult organs vs. 20-year-old:

  • Brain weight: 85%

  • Cardiac output: 65%

  • Respiratory capacity: 55%

  • Kidney mass: 85%

  • Liver blood flow: 50%

Progeria

• Hutchinson-Gilford syndrome: rare, non-inherited genetic condition.

• Rapid aging due to excess progerin protein.

• Symptoms: thin skin, hair loss, stiff joints, loss of subcutaneous fat.

• Death usually from cardiovascular disease by ~14 years.

Aging – Mechanisms of Aging

• Senescence = degenerative changes with age.

• Hypotheses:

  • Telomere shortening: cells can only divide ~50 times.

  • Apoptosis: programmed cell death increases.

  • Free radical theory: reactive oxygen damages cells, antioxidants may help.

  • Autoimmunity: immune system attacks own tissues.

• Aging rates vary among individuals due to genetics & environment.

Quick Check (page 684)

1. Do the proportions of the human body change during postnatal development?

   • Yes; head becomes proportionately smaller, legs longer, trunk proportion changes.

2. What is the neonatal period of development?

   • The first 4 weeks of infancy after birth.

3. What biological changes happen during puberty?

   • Development of secondary sex characteristics, reproductive maturity, rapid physical growth.

4. What is senescence?

   • The process of aging marked by degenerative changes in body systems.

5. What are free radicals, and what role do they have in the aging process?

   • Reactive oxygen molecules that damage cells and contribute to aging.

Mechanisms of Aging (Senescence)

• Senescence = the degenerative changes of older adulthood.

• Exact causes aren’t fully known. Key hypotheses in the text:

  • Cell‑division limit / telomeres: human cells can’t divide indefinitely; telomeres at chromosome ends shorten with each division and eventually cells stop dividing.

  • Apoptosis: continual programmed cell death; in older adulthood, dead cells may not be replaced → tissue degeneration.

  • Autoimmunity: immune system sometimes attacks the body’s own tissues in aging.

  • Preprogrammed/“aging genes” idea: genes may make aging “preprogrammed.”

  • Free‑radical theory: reactive oxygen species from normal cellular activity damage cells and promote aging; antioxidants may offer some protection.

• Figure 24‑16: Loss of bone mineral density (BMD) in late adulthood → spinal curvature changes and shortened stature.

Effects of Aging (system by system)

Skin (Integument)

• Becomes dry, thin, and inelastic (“sags”); wrinkles and skin folds increase.

• Pigmentation changes; thinning or loss of hair are common.

Skeletal System

• Bones change in texture, degree of calcification, and shape with age.

• Edges of older bones look indistinct/shaggy with spurs—called lipping—which restricts movement by building up around joints.

• Reduced calcification → smaller bones, porous bones → fracture risk.

• Lower cervical & thoracic vertebrae fracture frequently → spine curvature + shorter stature (links to Fig. 24‑16).

• Degenerative joint disease (osteoarthritis) is common, but exercise started earlier in life lessens many changes.

• Exercise also helps offset bone mass loss when paired with good nutrition.

Muscular System

• Skeletal muscle mass declines with age: may start ~age 25, usually not reaching 10% loss until ~50.

• By ~80, many have lost ≈50% of skeletal muscle mass.

• Weight training before and during later years can increase remaining fiber size and counteract some loss in fiber number.

Central Nervous System

• Risk of dementia (loss of memory/other conscious thinking functions) increases.

• For most, memories remain mostly intact, with mature reasoning/decision‑making.

• Some experience depression (especially with illness or separation), but on average older adults are happier than in early/middle adulthood.

Special Senses

• Vision:

  • By ~65, lenses become hard and lose elasticity → presbyopia (can’t accommodate for near vision).

  • Many notice near‑vision difficulty at 40–45 → need bifocals/mixed‑focus lenses.

  • Lens loses transparency → cataract (clouding) requiring surgical removal.

  • Glaucoma incidence rises with age (↑ intraocular pressure); untreated, can cause blindness; risk of retinal degeneration/detachment also ↑ with age.

• Hearing:

  • Significant loss of hair cells in the spiral organ; eardrum/ossicles stiffen → poorer sound transmission; hearing impairment is nearly universal in older adults.

• Taste & Smell:

  • Both decrease; only ~40% of taste buds present at age 30 remain by 75.

Cardiovascular System

• Degenerative heart & vessel disease is common/serious.

• Atherosclerosis (fatty deposits in vessel walls) → narrowed lumens → coronary artery disease and myocardial infarction.

• Arteriosclerosis = hardening of arteries; rupture can cause stroke (CVA).

• Hypertension (HTN) more common; stiff arteries raise systolic pressure.

Respiratory System

• Costal cartilages harden/calcify → rib cage less able to expand/contract; may become fixed to sternum → “barrel chest.”

• Respiratory efficiency decreases.

• Atrophy of respiratory muscles + replacement by connective tissue → weaker inspiration/expiration.

Urinary System

• Functioning nephron units decrease by ~50% between ages 30 and 75.

• Renal blood flow decreases → ↓ renal function and excretory capacity (and ability to produce urine).

• Bladder muscle atrophies → incontinence & trouble emptying/voiding completely.

Reproductive Systems

• General: many remain sexually active; response patterns change, and fertility declines.

• Males: erection may be harder to achieve/maintain; urgency for sex may decline, possibly from reduced testosterone (“low T”). Some develop andropause when testosterone falls enough to cause infertility.

• Females: typical cessation of reproductive cycling 45–60 → menopause due to decreased estrogen.

  • Symptoms: hot flashes, sleep disturbances, dryness & thinning of the vaginal wall.

  • ↓ estrogen is tied to bone loss/osteoporosis (see Ch. 8).

  • Hormone replacement therapy (HRT/HT) used in the past; more cautious now due to increased risks (some cancers, stroke, blood clots), though still used individually.

  • Non‑estrogen meds also available to manage symptoms and other problems (e.g., bone loss, heart disease).

Research, Issues, & Trends — Extending the Human Life Span (p. 685)

• Better food, safer surroundings, and advanced medical care have extended quality living.

• Low‑tech lifestyle keys that most people can do:

  • Eat a healthful diet

  • Exercise (even light/moderate) — keeps skeletal/muscular systems fit and decreases aging’s effects on several systems

  • Manage stress (even meditation and tai chi help)

• Bottom line: we can often stay “young” longer if we eat right, exercise, and relax.

• CONNECT IT!: Notes the role of genes in longevity (see “Genes and Longevity”).

Science Applications — Embryology (p. 686)

• Rita Levi‑Montalcini (1909–2012):

  • Early lab work led to discovery of nerve growth factor (NGF); awarded the 1986 Nobel Prize.

  • Her work on growth regulators deepened understanding of nervous system development and other tissues/organs.

• Impact beyond prenatal care: insights from embryology now inform gerontology (study of aging) and geriatrics (care of the aged), including possibilities to repair/regenerate damaged tissues in older adults.

Quick Check (p. 687) — Answers

1. Skeletal changes with age?

   • Bone texture/calcification/shape change; lipping around joints; smaller, porous bones; frequent vertebral fractures → spinal curvature & shorter stature; osteoarthritis common.

2. Eyesight in late adulthood?

   • Presbyopia (stiff lens); need for bifocals; ↑ risk of cataracts and glaucoma; reduced lens transparency.

3. Cardiovascular changes in older adults?

   • Atherosclerosis/arteriosclerosis, ↑ risk of MI and stroke; hypertension more common as arteries stiffen.

4. Kidney function in old age?

   • ≈50% fewer nephrons (30→75 yrs) + ↓ renal blood flow → ↓ renal function and excretory capacity; bladder muscle atrophy → incomplete emptying/incontinence.