Development

Gradual modification of anatomical structures and physiological characteristics from fertilization to maturity

Differentiation

Creation of different types of cells required in development

Occurs through selective changes in genetic activity

As development proceeds, some genes are turned off, others are turned on

Fertilization

Also called conception

When development begins

Embryological Development

Occurs during first 2 months after fertilization

Study of these events is called embryology

Fetal Development

Begins at start of ninth week

Continues until birth

Prenatal Development

Embryological and fetal development stages

Postnatal Development

Commences at birth

Continues to maturity when aging process begins

Inheritance

Transfer of genetically determined characteristics from generation to generation

Genetics

Study of mechanisms responsible for inheritance

Fertilization

Fertilization

Fusion of two haploidgametes, each containing 23 chromosomes

Produces zygote containing 46 chromosomes

Spermatozoon

Delivers paternal chromosomes to fertilization site

Travels relatively large distance

Is small, efficient, and highly streamlined

Gamete

Provides

Cellular organelles
Inclusions
Nourishment
Genetic programming necessary to support development of embryo for a week

Occurs in uterine tube within a day after ovulation

Secondary oocyte travels a few centimeters
Spermatozoa must cover distance between vagina and ampulla

Capacitation

Must occur before spermatozoa can fertilize secondary oocyte

Contact with secretions of seminal glands
Exposure to conditions in female reproductive tract

Hyaluronidase

Enzyme breaks down bonds between adjacent follicle cells

Allows spermatozoon to reach oocyte

Acrosin

Is a proteolytic enzyme

Is required to reach oocyte

Acrosomal Caps

Release hyaluronidase and acrosin

Penetrate corona radiata, zona pellucida, toward oocyte surface

Oocyte Activation

Contact and fusion of cell membranes of sperm and oocyte

Follows fertilization

Oocyte completes meiosis II, becomes mature ovum

Polyspermy

Fertilization by more than one sperm

Prevented by cortical reaction

Cortical Reaction

Releases enzymes that

Inactivate sperm receptors
Harden zona pellucida

Female Pronucleus

Nuclear material remaining in ovum after oocyte activation

Male Pronucleus

Swollen nucleus of spermatozoon

Migrates to center of cell

Amphimixis

Fusion of female pronucleus and male pronucleus

Moment of conception

Cell becomes a zygote with 46 chromosomes

Fertilization is complete

Cleavage

Series of cell divisions

Produces daughter cells

Differentiation

Involves changes in genetic activity of some cells but not others

Gestation

Induction

Cells release chemical substances that affect differentiation of other embryonic cells

Can control highly complex processes

Gestation

Time spent in prenatal development

Consists of three integrated trimesters, each 3 months long

Gestation

First Trimester

Period of embryological and early fetal development

Rudiments of all major organ systems appear

Second Trimester

Development of organs and organ systems

Body shape and proportions change

By end, fetus looks distinctively human

Third Trimester

Rapid fetal growth and deposition of adipose tissue

Most major organ systems are fully functional

The First Trimester

Cleavage

Sequence of cell divisions begins immediately after fertilization

Zygote becomes a pre-embryo, which develops into multicellular blastocyst

Ends when blastocyst contacts uterine wall

Implantation

Begins with attachment of blastocyst to endometrium of uterus

Sets stage for formation of vital embryonic structures

Placentation

Occurs as blood vessels form around periphery of blastocyst and placenta develops

Placenta

Complex organ permits exchange between maternal and embryonic circulatory systems

Supports fetus in second and third trimesters

Stops functioning and is ejected from uterus after birth

Embryogenesis

Formation of viable embryo

Establishes foundations for all major organ systems

Most dangerous period in prenatal life

40% of conceptions produce embryos that survive past first trimester

Blastomeres

Identical cells produced by cleavage divisions

Morula

Stage after 3 days of cleavage

Pre-embryo is solid ball of cells resembling mulberry

Reaches uterus on day 4

Blastocyst

Formed by blastomeres

Hollow ball with an inner cavity

Known as blastocoele

Trophoblast

Outer layer of cells separate outside world from blastocoele

Cells responsible for providing nutrients to developing embryo

Inner Cell Mass

Clustered at end of blastocyst

Exposed to blastocoele

Insulated from contact with outside environment by trophoblast

Will later form embryo

Implantation

Occurs 7 days after fertilization

Blastocyst adheres to uterine lining

Trophoblast cells divide rapidly, creating several layers

Cellular Trophoblast

Cells closest to interior of blastocyst

Syncytial Trophoblast

Outer layer

Erodes path through uterine epithelium by secreting hyaluronidase

Ectopic Pregnancy

Implantation occurs outside of uterus

Does not produce viable embryo

Can be life threatening

Lacunae

Trophoblastic channels carrying maternal blood

Villi

Extend away from trophoblast into endometrium

Increase in size and complexity until day 21

Amniotic Cavity

A fluid-filled chamber

Inner cell mass is organized into an oval sheet two layers thick

Superficial layer faces amniotic cavity
Deeper layer is exposed to fluid contents of blastocoele

Gastrulation

Formation of third layer of cells

Cells in specific areas of surface move toward central line

Known as primitive streak

Primitive Streak

Migrating cells leave surface and move between two layers

Creates three distinct embryonic layers, or germ layers

Ectoderm: consists of the superficial cells that did not migrate into interior of inner cell mass

Endoderm: consists of cells that face blastocoele

Mesoderm: consists of poorly organized layer of migrating cells between ectoderm and endoderm

Embryonic Disc

Oval, three-layered sheet

Produced by gastrulation

Will form body of embryo

Rest of blastocyst will be involved in forming extraembryonic membranes

Formation of the Extraembryonic Membranes

Support embryological and fetal development

Yolk sac

Amnion

Allantois

Chorion

Yolk Sac

Begins as layer of cells spread out around outer edges of blastocoele to form complete pouch

Important site of blood cell formation

Amnion

Combination of mesoderm and ectoderm

Ectodermal layer enlarges and cells spread over inner surface of amniotic cavity

Mesodermal cells create outer layer

Continues to enlarge through development

Amniotic Fluid

Contained in amniotic cavity

Surrounds and cushions developing embryo or fetus

Allantois

Sac of endoderm and mesoderm

Base later gives rise to urinary bladder

Chorion

Combination of mesoderm and trophoblast

Blood vessels develop within mesoderm

Rapid-transit system for nutrients that links embryo with trophoblast

First step in creation of functional placenta

Chorionic Villi

In contact with maternal tissues

Create intricate network within endometrium carrying maternal blood

Body Stalk

Connection between embryo and chorion

Contains distal portions of allantois and blood vessels that carry blood to and from placenta

Yolk Stalk

Narrow connection between endoderm of embryo and yolk sac

Decidua Capsularis

Thin portion of endometrium

No longer participates in nutrient exchange and chorionic villi in region disappear

Decidua Basalis

Disc-shaped area in deepest portion of endometrium

Where placental functions are concentrated

Decidua Parietalis

Rest of the uterine endometrium

No contact with chorion

Umbilical Cord

Connects fetus and placenta

Contains allantois, placental blood vessels, and yolk stalk

Blood Flow to Placenta

Through paired umbilical arteries

Returns in single umbilical vein

The Endocrine Placenta

Synthesized by syncytial trophoblast, released into maternal bloodstream

Human chorionic gonadotropin (hCG)

Human placental lactogen (hPL)

Placental prolactin

Relaxin

Progesterone

Estrogens

Human Chorionic Gonadotropin (hCG)

Appears in maternal bloodstream soon after implantation

Provides reliable indication of pregnancy

Pregnancy ends if absent

Helps prepare mammary glands for milk production

Stimulatory effect on other tissues comparable to growth hormone (GH)

Placental Prolactin

Helps convert mammary glands to active status

Relaxin

A peptide hormone secreted by placenta and corpus luteum during pregnancy

Increases flexibility of pubic symphysis, permitting pelvis to expand during delivery

Causes dilation of cervix

Suppresses release of oxytocin by hypothalamus and delays labor contractions

Embryogenesis

Body of embryo begins to separate from embryonic disc

Body of embryo and internal organs start to form

Folding, differential growth of embryonic disc produces bulge that projects into amniotic cavity

Projections are head fold and tail fold

Organogenesis

Process of organ formation

The Second and Third Trimesters

Second Trimester

Fetus grows faster than surrounding placenta

Third Trimester

Most of the organ systems become ready

Growth rate starts to slow

Largest weight gain

Fetus and enlarged uterus displace many of mother’s abdominal organs

Pregnancy and Maternal Systems

Developing fetus is totally dependent on maternal organ systems for nourishment, respiration, and waste removal

Maternal adaptations include increases in

Respiratory rate and tidal volume

Blood volume

Nutrient and vitamin intake

Glomerular filtration rate

Uterus and mammary glands increase in size

The Second and Third Trimesters

Progesterone

Released by placenta

Has inhibitory effect on uterine smooth muscle

Prevents extensive, powerful contractions

Opposition to Progesterone

Three major factors

Rising estrogen levels

Rising oxytocin levels

Prostaglandin production

The Second and Third Trimesters

False Labor

Occasional spasms in uterine musculature

Contractions not regular or persistent

True Labor

Results from biochemical and mechanical factors

Continues due to positive feedback

Labor Contractions

Begin in myometrium

Parturition is forcible expulsion of fetus

Contractions

Begin near top of uterus, sweep in wave toward cervix

Strong, occur at regular intervals, increase in force and frequency

Change position of fetus, move it toward cervical canal

Labor

Dilation Stage

Begins with onset of true labor

Cervix dilates

Fetus begins to shift toward cervical canal

Highly variable in length, but typically lasts over 8 hours

Frequency of contractions steadily increases

Amniochorionicmembrane ruptures (water breaks)

Expulsion Stage

Begins as cervix completes dilation

Contractions reach maximum intensity

Continues until fetus has emerged from vagina

Typically less than 2 hours

Delivery

Arrival of newborn infant into outside world

Placental Stage

Muscle tension builds in walls of partially empty uterus

Tears connections between endometrium and placenta

Ends within an hour of delivery with ejection of placenta, or afterbirth

Accompanied by a loss of blood

Episiotomy

Incision through perineal musculature

Needed if vaginal canal is too small to pass fetus

Repaired with sutures after delivery

Cesarean Section (C-section)

Removal of infant by incision made through abdominal wall

Opens uterus just enough to pass infant’s head

Needed if complications arise during dilation or expulsion stages

Premature Labor

Occurs when true labor begins before fetus has completed normal development

Newborn’s chances of surviving are directly related to body weight at delivery

Immature Delivery

Refers to fetuses born at 25–27 weeks of gestation

Most die despite intensive neonatal care

Survivors have high risk of developmental abnormalities

Premature Delivery

Refers to birth at 28–36 weeks

Newborns have a good chance of surviving and developing normally

Forceps Delivery

Needed when fetus faces mother’s pubis instead of sacrum

Risks to infant and mother are reduced if forceps are used

Forceps resemble large, curved salad tongs

Used to grasp head of fetus

Breech Birth

Legs or buttocks of fetus enter vaginal canal first instead of head

Umbilical cord can become constricted, cutting off placental blood flow

Cervix may not dilate enough to pass head

Prolongs delivery

Subjects fetus to severe distress and potential injury

Dizygotic Twins

Also called fraternal twins

Develop when two separate oocytes were ovulated and subsequently fertilized

Genetic makeup not identical

70% of twins

Monozygotic Twins

Identical twins

Result either from

Separation of blastomeres early in cleavage

Splitting of inner cell mass before gastrulation

Genetic makeup is identical because both formed from same pair of gametes

Conjoined Twins

Siamese twins

Genetically identical twins

Occurs when splitting of blastomeres or of embryonic disc is not completed

Rates of Multiple Births

Twins in 1 of every 89 births

Triplets in 1 of every 892 (7921) births

Quadruplets in 1 of every 893 (704,969) births

Postnatal Life

Five Life Stages

Neonatal period

Infancy

Childhood

Adolescence

Maturity

Neonatal Period: extends from birth to 1 month

Infancy: 1 month to 2 years of age

Childhood: 2 years until adolescence

Adolescence: period of sexual and physical maturation

Senescence: process of aging that begins at end of development (maturity)

Developmental Stages

Neonatal period, infancy, childhood, and adolescence

 Two major events occur

Organ systems become fully operational

Individual grows rapidly and body proportions change significantly

Pediatrics

Medical specialty focusing on postnatal development from infancy to adolescence

Neonate

Newborn

Neonatal Period

Transition from fetus to neonate

Systems begin functioning independently

Respiratory

Circulatory

Digestive

Urinary

Colostrum

Secretion from mammary glands

Ingested by infant during first 2–3 days

Contains more proteins and less fat than breast milk

Many proteins are antibodies that help ward off infections until immune system is functional

Mucins present inhibit replication of rotaviruses

As production drops, mammary glands convert to milk production

Breast Milk

Consists of water, proteins, amino acids, lipids, sugars, and salts

Also contains large quantities of lysozymes—enzymes with antibiotic properties

Milk Let-Down Reflex

Mammary gland secretion triggered when infant sucks on nipple

Continues to function until weaning, typically 1–2 years

Infancy and Childhood

Growth occurs under direction of circulating hormones

Growth hormone

Suprarenal steroids

Thyroid hormones

Growth does not occur uniformly

Body proportions gradually change

Puberty is a period of sexual maturation and marks the beginning of adolescence

Generally starts at age 12 in boys, age 11 in girls

Three major hormonal events interact

Hypothalamus increases production of GnRH

Circulating levels of FSH and LH rise rapidly

Ovarian or testicular cells become more sensitive to FSH and LH

Hormonal changes produce sex-specific differences in structure and function of many systems

Adolescence

Begins at puberty

Continues until growth is completed

Maturity (Senescence )

Aging

Reduces functional capabilities of individual

Affects homeostatic mechanisms

Sex hormone levels decline at menopause or male climacteric

Geriatrics

Medical specialty dealing with problems associated with aging

Trained physicians, or geriatricians

Inheritance

Nucleated Somatic Cells

Carry copies of original 46 chromosomes present in zygote

Genotype

Chromosomes and their component genes

Contain unique instructions that determine anatomical and physiological characteristics

Derived from genotypes of parents

Phenotype

Physical expression of genotype

Anatomical and physiological characteristics

Inheritance

Homologous Chromosomes

Members of each pair of chromosomes

23 pairs carried in every somatic cell

At amphimixis, one member of each pair is contributed by spermatozoon, other by ovum

Autosomal Chromosomes

22 pairs of homologous chromosomes

Most affect somatic characteristics

Each chromosome in pair has same structure and carries genes that affect same traits

Sex Chromosomes

Last pair of chromosomes

Determine whether individual is genetically male or female

Karyotype

Entire set of chromosomes

Locus

Gene’s position on chromosome

Alleles are various forms of given gene

Alternate forms determine precise effect of gene on phenotype

Homozygous

Both homologous chromosomes carry same allele of particular gene

Simple Inheritance

Phenotype determined by interactions between single pair of alleles

Heterozygous

Homologous chromosomes carry different allele of particular gene

Resulting phenotype depends on nature of interaction between alleles

Strict Dominance

Dominant allele expressed in phenotype, regardless of conflicting instructions carried by other allele

Recessive Allele

Expressed in phenotype only if same allele is present on both chromosomes of homologous pair

Incomplete Dominance

Heterozygous alleles produce unique phenotype

Codominance

Exhibits both dominant and recessive phenotypes for traits

Penetrance

Percentage of individuals with particular genotype that show “expected” phenotype

Expressivity

Extent to which particular allele is expressed

Teratogens

Factors that result in abnormal development

Punnett Square

Simple box diagram used to predict characteristics of offspring

Polygenic Inheritance

Involves interactions among alleles on several genes

Cannot predict phenotypic characteristics using Punnett square

Linked to risks of developing several important adult disorders

Suppression

One gene suppresses other

Second gene has no effect on phenotype

Complementary Gene Action

Dominant alleles on two genes interact to produce phenotype different from that seen when one gene contains recessive alleles

Sources of Individual Variation

During meiosis, maternal and paternal chromosomes are randomly distributed

Each gamete has unique combination of maternal and paternal chromosomes

Genetic Recombination

During meiosis, various changes can occur in chromosome structure, producing gametes with chromosomes that differ from those of each parent

Greatly increases range of possible variation among gametes

Can complicate tracing of inheritance of genetic disorders

Crossing Over

Parts of chromosomes become rearranged during synapsis

When tetrads form, adjacent chromatids may overlap

Translocation

Reshuffling process

Chromatids may break, overlapping segments trade places

Genomic Imprinting

During recombination, portions of chromosomes may break away and be deleted

Effects depend on whether abnormal gamete is produced through oogenesis or spermatogenesis

Chromosomal Abnormalities

Damaged, broken, missing, or extra copies of chromosomes