Anatomy Chap 29 Human Development and Aging

Egg must be fertilized within 12 to 24 hours of ovulation, if it is to survive

• Sperm must encounter the egg somewhere in the distal 1/3 of uterine tube (____________)

Infundibulum

Vast majority of sperm do not make it to egg

• Destroyed by vaginal acid or drain out of vagina

• Fail to penetrate the mucus of the cervical canal

• Destroyed by leukocytes in the uterus

• Half go up wrong uterine tube (50/50 chance)

• Of the 300 million that were ejaculated, about 200 spermatozoa reach the vicinity of the egg

Sperm move by lashing of tail as they crawl along the female mucosa

• May be assisted by female physiology

• Strands of cervical mucus guide them through the cervical canal

• Uterine contractions may suck semen from vagina and spread it throughout the uterus

• Chemical attractant molecules released by egg may attract sperm from a short distance

Spermatozoa reach distal uterine tube in half an hour or less but cannot fertilize the egg for 10 hours

• ____________: process that migrating sperm must undergo to make it possible to penetrate an egg

Capacitation

When sperm encounters an egg, it undergoes an ______________—exocytosis of the acrosome, releases enzymes needed to penetrate egg

• Enzymes of many sperm are released to clear a path for the one that will penetrate the egg

Acrosomal reaction

___________________ enter egg

Egg destroys ____________, only passes on maternal to offspring

Sperm head and midpiece

Sperm mitochondria

_______—fertilization by two or more sperm which would produce a doomed fertilized egg

Polyspermy

As soon as a sperm binds with the _____________ of egg, _____________ release secretion to form fertilization barrier to prevent other sperm from fertilizing and prevent polyspermy

Zona pellucida

Cortical granules (Granulosa cells)

Gamete migration and implantation sequence

Secondary oocyte gives oof first polar body, sperm cell fertilizes, sperm and egg pronuclei fuse to form a zygote (second polar body is given off), Zygote begins cleavage (forms blastomeres), forming a morula once it reaches the uterus, and then develops into a blastocyst before implantation into the uterine lining.

from fertilization through 12 weeks

• More than half of all embryos die due to chromosomal defects

• Conceptus is most vulnerable to stress, drugs, and nutritional deficiencies during this time

First trimester

weeks 13 through 24

• Organs complete most of their development

• Fetus looks distinctly human

• Chance of survival (with intensive care) if born near end of this trimester

Second trimester

week 25 to birth

• Fetus grows rapidly and organs achieve enough cellular differentiation to support life outside of womb

• At 35 weeks & 5.5 lbs, fetus is considered mature

Third trimester

• About two-thirds of twins

• Two eggs are ovulated and both are fertilized by

separate sperm forming two zygotes

• No more or less genetically similar than any other siblings

• Implant separately in the uterine wall and each forms its own placenta

Dizygotic twins (fraternal twins)

• One egg is fertilized (one zygote) but embryoblast divides into two up to 14 days after fertilization

• Genetically identical (or nearly so), of the same sex, and very similar in appearance

Monozygotic twins (identical twins)

• The body cavity becomes divided into the __________ and ________ by the diaphragm

• By end of Week 5, the thoracic cavity further subdivides into _______________

• Appearance of ________ that will become brain & spinal cord

• Appearance of ___________—segmentation of the mesoderm into blocks of tissue that will give rise to the vertebral column, trunk muscles, & dermis of the skin

Thoracic cavity, Peritoneal cavity

Pleural and pericardial cavities

Neural tube

Somites

Formation of organs from primary germ layers

• At _________, all organs are present in fetus (3 cm long)

8 weeks

Derivatives of ectoderm (outmost germ layer)

Epidermis, nervous system, lens and cornea, internal ear

Derivatives of the mesoderm (middle germ layer)

Skeleton, muscle, cartilage, blood, lymphoid tissue, gonads and ducts, kidneys and ureters

Derivatives of the endoderm (innermost germ layer)

Gut and respiratory epithelium and glands, bladder, and urethra

Accessory organs develop with embryo

placenta, umbilical cord, and four embryonic membranes: amnion, yolk sac, allantois, & chorion

transparent sac that grows to completely enclose the embryo:

• Penetrated only by the umbilical cord

• Fills with amniotic fluid which protects embryo from trauma, infections, & temperature fluctuations

• Allows freedom of movement important to muscle development

• Enables embryo to develop symmetrically

• Prevents body parts from adhering to each other

• Stimulates lung development as fetus “breathes” fluid

Amnion

small sac suspended from ventral side of embryo

Contributes to formation of digestive tract, blood cells, and future egg or sperm cells

Yolk sac

Begins as an out pocketing of the yolk sac

Forms to foundation for the umbilical cord, becomes part of the urinary bladder

Allantois

outermost membrane enclosing all the rest of the membranes and the embryo

Has shaggy outgrowths: chorionic villi around entire surface

• Forms fetal portion of placenta

Chorion

By the end of 8 weeks of fetal development

• All organ systems are present

• Individual is about 3 cm long & now considered a fetus

• Bones have begun to calcify

• Skeletal muscles exhibit spontaneous contractions but too weak to be felt by the mother

• Heart, beating since Week 4, now circulates blood

• Heart and liver are very large, forming the prominent ventral bulge

• Head is nearly half the total body length

Week 9 until birth

Fetus is in final stage of prenatal development

Organs mature to support life outside of the mother

Adapting to Life Outside the Uterus – Respiratory Adaptations

Neonate begins breathing spontaneously

CO2 accumulation stimulates respiratory chemoreceptors

Great effort to inflate lungs (collapsed alveoli) for first few breaths

First 2 weeks: 45 breaths per minute

Stabilized at about 12 breaths per minute

• Proximal parts become superior vesical artery

• Distal parts become median umbilical ligaments of abdominal wall

Umbilical arteries

becomes round ligament (ligamentum teres) of the liver

Umbilical vein

becomes ligamentum venosum on inferior surface of the liver

Ductus venosus

Flaps of __________ fuse to close shunt and leave a depression, the fossa ovalis, in the interatrial septum of the heart

Foramen ovale

collapses and forms the ligamentum arteriosum between aorta and pulmonary trunk

Ductus arteriosus

___________ appears in early fetal development but still weak

Infant born with near adult levels of ____ from mother through placenta (Breaks down rapidly after birth)

Remains high enough for 6 months to protect against measles, diphtheria, polio, and most infectious diseases (NOT whooping cough)

Breast-fed neonate acquires protection from gastroenteritis from the ___ present in the colostrum (breast milk)

Cellular immunity

IgG

IgA

Neonate adaptations for thermoregulations bc infant has a larger ratio of SA to volume then an adult and loses heat more regularly

Brown fat (heat generating tissue) deposited during weeks 17 to 20 as fetus

Baby grows and increases metabolic rate, producing heat

Accumulates subcutaneous fat, retaining heat

Neonate adaptations for fluid balance

Since kidneys not fully developed at birth, neonates have a high rate of water loss and require more fluid intake, relative to body weight

Other organ systems still require significant maturation in the neonate

Liver, joints, myelination of NS

• Infants born before 37 weeks suffer from:

• ____________________________: insufficient surfactant causing alveolar collapse with exhalation

• Thermoregulatory problems due to undeveloped ______________

• Digestive issues with small stomach volume, and undeveloped sucking & swallowing reflexes; fed through nasogastric or nasoduodenal tubes

• Immature liver

•Leads to failure to synthesize adequate proteins results in edema (lack of albumins), & clotting deficiency

•__________ is common in neonates, especially premature babies since liver cannot dispose of bile pigments (bilirubin)

Infant respiratory distress syndrome (IRDS)

Hypothalamus

Jaundice

abnormal structures or positions of organs at birth resulting from a defect in prenatal development

Birth defects or congenital abnormalities

____________—agents that cause anatomical deformities in the fetus

• Fall into 3 major classes

Teratogen exposure during first 2 weeks may cause spontaneous abortion

• Period of greatest vulnerability is ______________

Teratogens

1. Drugs and medications

2. infectious diseases

3. Radiation (X Rays)

Weeks 3 thru 8

_____________—drug that caused birth defects

• Taken by mothers for morning sickness or insomnia, was not approved by FDA

• Caused babies to be born with unformed arms & legs, and often with defects of ears, heart, & intestines (in 1957, 10-20K affected)

Thalidomide

________ causes more birth defects than any other drug

• _______________________: characterized by small head, malformed facial features, cardiac and central nervous system defects, stunted growth, and behavioral signs such as hyperactivity, nervousness, and poor attention span

Alcohol

Fetal alcohol syndrome (FAS)

__________ smoking contributes to fetal and infant mortality, ectopic pregnancy, anencephaly (failure for the cerebrum to develop), cleft palate and lip, and cardiac anomalies

• ________________ during pregnancy can be teratogenic

Cigarette smoking

Diagnostic X rays

Several microorganisms can cross placenta and cause serious anomalies, stillbirth, neonatal death

Can produce blindness, hydrocephalus, cerebral palsy, seizures, and profound physical and mental retardation

• Viral infections: herpes simplex, rubella, cytomegalovirus, and HIV (zika virus causes microcephaly)

• Bacterial infections: gonorrhea and syphilis, Toxoplasma (cats/little boxes)

Genetic anomalies appear to account for 1/3 of all birth defects

• ____________—changes in DNA structure

• Can cause achondroplastic dwarfism, microcephaly, stillbirth, & childhood cancer

• Can occur with errors in DNA replication during cell cycle

• ___________—environmental agents (chemicals, viruses, radiation) that cause mutations

• Some genetic disorders result from ______________: failure of homologous chromosomes to separate in meiosis

• Both go to same daughter cell, so one gets 24 chromosomes while the other receives 22

Mutations

Mutagens

Nondisjunction

__________—the presence of an extra chromosome or the lack of one

• Most common cause of miscarriage

• ____________: the lack of a chromosome leaves one without a match

• ____________: the extra chromosome produces a triple set

• Can be detected prior to birth

• ____________—examination of cells in amniotic fluid

• ________________—biopsy of cells from the chorion

Aneuploidy

Monosomy

Trisomy
Amniocentesis
Chorionic villus sampling

Nondisjunction of sex chromosomes

Triple X syndrome, Klinefelter syndrome, Turner syndrome

egg receiving 2 X chromosomes fertilized by X- carrying sperm

• Infertile female with mild intellectual impairment

Triple X syndrome (XXX)

egg receiving 2 X chromosomes fertilized by Y-carrying sperm

• Sterile males with average intelligence (undeveloped testes)

Klinefelter syndrome (XXY)

egg contains no X chromosome but is fertilized by X- carrying sperm

• 97% die before birth; survivors show no serious impairment as children

• Webbed necks, widely spaced nipples, secondary sexual characteristics fail to develop at puberty, sterile, & usually short stature

Turner syndrome (XO)

Only three autosomal trisomies are survivable

Involve chromosomes 13, 18, and 21.

Nearly all die before birth

• Infants born are severely deformed, and fewer than 5% survive 1 year

Patau syndrome (trisomy-13) and Edward syndrome (trisomy-18)

most survivable trisomy

• Impaired physical development—short stature, relatively flat face with flat nasal bridge, low-set ears, epicanthal folds at medial corners of the eye, enlarged protruding tongue, stubby fingers, short broad hand with one palmar crease

• Outgoing, affectionate personalities

• Mental retardation common

• 1 in 700 to 800 live births in the United States

• 75% die before birth; 20% of those born die before age 10

• Survivors beyond age 10 have life expectancy of 60 years

• Aging eggs have less and less ability to separate their chromosomes

Down syndrome (trisomy-21)