Pregnancy and Human Development

1
Chapter 28

• Pregnancy and Human Development

2
From Egg to Embryo

• Pregnancy events that occur from fertilization
  until the infant is born
• Conceptus the developing offspring
• Gestation period from the last menstrual period
  until birth
• Preembryo conceptus from fertilization until it
  is two weeks old
• Embryo conceptus during the third through the
  eighth week
• Fetus conceptus from the ninth week through
  birth

3
Accomplishing Fertilization

• The oocyte is viable for 12 to 24 hours
• Sperm is viable 24 to 72 hours
• For fertilization to occur, coitus must occur no
  more than
• Three days before ovulation
• 24 hours after ovulation
• Fertilization when a sperm fuses with an egg to
  form a zygote

4
Sperm Transport and Capacitation

• Fates of ejaculated sperm
• Destroyed by the acidic vaginal environment
• Fail to make it through the cervix
• Dispersed in the uterine cavity or destroyed by
  phagocytic leukocytes
• Reach the uterine tubes
• Sperm must undergo capacitation before they can
  penetrate the oocyte

5
Acrosomal Reaction and Sperm Penetration

• An ovulated oocyte is encapsulated by
• The corona radiata and zona pellucida
• Extracellular matrix
• Sperm binds to the zona pellucida and undergoes
  the acrosomal reaction
• Enzymes are released near the oocyte
• Hundreds of acrosomes release their enzymes to
  digest the zona pellucida
• Once a sperm makes contact with the oocytes
  membrane
• Beta protein finds and binds to receptors on the
  oocyte membrane
• Alpha protein causes it to insert into the
  membrane

6
Acrosomal Reaction and Sperm Penetration
7
Blocks to Polyspermy

• Only one sperm is allowed to penetrate the oocyte
• Two mechanisms ensure monospermy
• Fast block to polyspermy membrane
  depolarization prevents sperm from fusing with
  the oocyte membrane
• Slow block to polyspermy zonal inhibiting
  proteins (ZIPs)
• Destroy sperm receptors
• Cause sperm already bound to receptors to detach

8
Completion of Meiosis II and Fertilization

• Upon entry of sperm, the secondary oocyte
• Completes meiosis II
• Casts out the second polar body
• The ovum nucleus swells, and the two nuclei
  approach each other
• When fully swollen, the two nuclei are called
  pronuclei
• Fertilization when the pronuclei come together

9
Events Immediately Following Sperm Penetration
10
Preembryonic Development

• The first cleavage produces two daughter cells
  called blastomeres
• Morula the 16 or more cell stage (72 hours old)
• By the fourth or fifth day the preembryo consists
  of 100 or so cells (blastocyst)
• Blastocyst a fluid-filled hollow sphere
  composed of
• A single layer of trophoblasts
• An inner cell mass
• Trophoblasts take part in placenta formation
• The inner cell mass becomes the embryonic disc

11
Cleavage From Zygote to Blastocyst
Degenerating zona pellucida
Inner cell mass
Blastocyst cavity
Blastocyst cavity
Trophoblast
(a) Zygote(fertilized egg)
(b) 4-cell stage2 days
(c) Morula3 days
(e) Implanting blastocyst6 days
(d) Early blastocyst4 days
Fertilization(sperm meets egg)
(a)
(b)
(c)
Ovary
Uterine tube
(d)
Oocyte(egg)
(e)
Ovulation
Uterus
Endometrium
Cavity of uterus
12
Implantation

• Begins six to seven days after ovulation when the
  trophoblasts adhere to a properly prepared
  endometrium
• The trophoblasts then proliferate and form two
  distinct layers
• Cytotrophoblast cells of the inner layer that
  retain their cell boundaries
• Syncytiotrophoblast cells in the outer layer
  that lose their plasma membranes and invade the
  endometrium
• The implanted blastocyst is covered over by
  endometrial cells
• Implantation is completed by the fourteenth day
  after ovulation

13
Implantation of the Blastocyst
14
Implantation

• Viability of the corpus luteum is maintained by
  human chorionic gonadotropin (hCG) secreted by
  the trophoblasts
• hCG prompts the corpus luteum to continue to
  secrete progesterone and estrogen
• Chorion developed from trophoblasts after
  implantation, continues this hormonal stimulus
• Between the second and third month, the placenta
  
• Assumes the role of progesterone and estrogen
  production
• Is providing nutrients and removing wastes

15
Hormonal Changes During Pregnancy
16
Placentation

• Formation of the placenta from
• Embryonic trophoblastic tissues
• Maternal endometrial tissues
• The chorion develops fingerlike villi, which
• Become vascularized
• Extend to the embryo as umbilical arteries and
  veins
• Lie immersed in maternal blood
• Decidua basalis part of the endometrium that
  lies between the chorionic villi and the stratum
  basalis

17
Placentation

• Decidua capsularis part of the endometrium
  surrounding the uterine cavity face of the
  implanted embryo
• The placenta is fully formed and functional by
  the end of the third month
• Embryonic placental barriers include
• The chorionic villi
• The endothelium of embryonic capillaries
• The placenta also secretes other hormones human
  placental lactogen, human chorionic thyrotropin,
  and relaxin

18
Placentation
19
Placentation
20
Germ Layers

• The blastocyst develops into a gastrula with
  three primary germ layers
• ectoderm,
• endoderm, and
• mesoderm
• Before becoming three-layered, the inner cell
  mass subdivides into the upper epiblast and lower
  hypoblast
• These layers form two of the four embryonic
  membranes

21
Embryonic Membranes

• Amnion epiblast cells form a transparent
  membrane filled with amniotic fluid
• Provides a buoyant environment that protects the
  embryo
• Helps maintain a constant homeostatic temperature
• Amniotic fluid comes from maternal blood, and
  later, fetal urine
• Yolk sac hypoblast cells that form a sac on the
  ventral surface of the embryo
• Forms part of the digestive tube
• Produces earliest blood cells and vessels
• Is the source of primordial germ cells
• Allantois a small outpocketing at the caudal
  end of the yolk sac
• Structural base for the umbilical cord
• Becomes part of the urinary bladder
• Chorion helps form the placenta
• Encloses the embryonic body and all other
  membranes

22
Gastrulation

• During the 3rd week, the two-layered embryonic
  disc becomes a three-layered embryo
• The primary germ layers are ectoderm, mesoderm,
  and endoderm
• Primitive streak raised dorsal groove that
  establishes the longitudinal axis of the embryo
• As cells begin to migrate
• The first cells that enter the groove form the
  endoderm
• The cells that follow push laterally between the
  cells forming the mesoderm
• The cells that remain on the embryos dorsal
  surface form the ectoderm
• Notochord rod of mesodermal cells that serves
  as axial support

23
Primary Germ Layers

• Serve as primitive tissues from which all body
  organs will derive
• Ectoderm forms structures of the nervous system
  and skin epidermis
• Endoderm forms epithelial linings of the
  digestive, respiratory, and urogenital systems
• Mesoderm forms all other tissues
• Endoderm and ectoderm are securely joined and are
  considered epithelia

24
Primary Germ Layers
25
Primary Germ Layers
26
Organogenesis

• Gastrulation sets the stage for organogenesis,
  the formation of body organs
• By the 8th week all organ systems are recognizable

27
Specialization of Ectoderm

• Neurulation the first event of organogenesis
  gives rise to the brain and spinal cord
• Ectoderm over the notochord thickens, forming the
  neural plate
• The neural plate folds inward as a neural groove
  with prominent neural folds
• By the 22nd day, neural folds fuse into a neural
  tube, which pinches off into the body
• The anterior end becomes the brain the rest
  becomes the spinal cord
• Associated neural crest cells give rise to
  cranial, spinal, and sympathetic ganglia

28
Specialization of Ectoderm Neuralization
29
Specialization of Ectoderm Neuralization
30
Specialization of Endoderm

• Embryonic folding begins with lateral folds
• Next, head and tail folds appear
• An endoderm tube forms the epithelial lining of
  the GI tract
• Organs of the GI tract become apparent, and oral
  and anal openings perforate
• Endoderm forms epithelium linings of the hollow
  organs of the digestive and respiratory tracts

31
Folding of the Embryonic Body
32
Endodermal Differentiation
33
Specialization of the Mesoderm

• First evidence is the appearance of the notochord
  
• Three mesoderm aggregates appear lateral to the
  notochord
• Somites, intermediate mesoderm, and double sheets
  of lateral mesoderm
• The 40 pairs of somites have three functional
  parts
• Sclerotome produce the vertebrae and ribs
• Dermatome help form the dermis of the skin on
  the dorsal part of the body
• Myotome form the skeletal muscles of the neck,
  trunk, and limbs

34
Specialization of the Mesoderm

• Intermediate mesoderm forms the gonads and the
  kidneys
• Lateral mesoderm consists of somatic and
  splanchnic mesoderm
• Somatic mesoderm forms the
• Dermis of the skin in the ventral region
• Parietal serosa of the ventral body cavity
• Bones, ligaments, and dermis of the limbs
• Splanchnic mesoderm forms
• The heart and blood vessels
• Most connective tissues of the body

35
Specialization of the Mesoderm
36
Development of Fetal Circulation

• By the end of the 3rd week
• The embryo has a system of paired vessels
• The vessels forming the heart have fused
• Unique vascular modifications seen in prenatal
  development include umbilical arteries and veins,
  and three vascular shunts (occluded at birth)
• Ductus venosus venous shunt that bypasses the
  liver
• Foramen ovale opening in the interatrial septa
  to bypass pulmonary circulation
• Ductus arteriosus transfers blood from the
  right ventricle to the aorta

37
Effects of Pregnancy Anatomical Changes

• Chadwicks sign the vagina develops a purplish
  hue
• Breasts enlarge and their areolae darken
• The uterus expands, occupying most of the
  abdominal cavity
• Lordosis is common due to the change of the
  bodys center of gravity
• Relaxin causes pelvic ligaments and the pubic
  symphysis to relax
• Typical weight gain is about 29 pounds

38
Effects of Pregnancy Metabolic Changes

• The placenta secretes human placental lactogen
  (hPL), also called human chorionic
  somatomammotropin (hCS), which stimulates the
  maturation of the breasts
• hPL promotes growth of the fetus and exerts a
  maternal glucose-sparing effect
• Human chorionic thyrotropin (hCT) increases
  maternal metabolism
• Parathyroid hormone levels are high, ensuring a
  positive calcium balance

39
Effects of Pregnancy Physiological Changes

• GI tract morning sickness occurs due to
  elevated levels of estrogen and progesterone
• Urinary system urine production increases to
  handle the additional fetal wastes
• Respiratory system edematous and nasal
  congestion may occur
• Dyspnea (difficult breathing) may develop late in
  pregnancy
• Cardiovascular system blood volume increases
  25-40
• Venous pressure from lower limbs is impaired,
  resulting in varicose veins

40
Parturition Initiation of Labor

• Estrogen reaches a peak during the last weeks of
  pregnancy causing myometrial weakness and
  irritability
• Weak Braxton Hicks contractions may take place
• As birth nears, oxytocin and prostaglandins cause
  uterine contractions
• Emotional and physical stress
• Activates the hypothalamus
• Sets up a positive feedback mechanism, releasing
  more oxytocin

41
Parturition Initiation of Labor
42
Stages of Labor Dilation Stage

• From the onset of labor until the cervix is fully
  dilated (10 cm)
• Initial contractions are 1530 minutes apart and
  1030 seconds in duration
• The cervix effaces and dilates
• The amnion ruptures, releasing amniotic fluid
  (breaking of the water)
• Engagement occurs as the infants head enters the
  true pelvis

43
Stages of Labor Expulsion Stage

• From full dilation to delivery of the infant
• Strong contractions occur every 23 minutes and
  last about 1 minute
• The urge to push increases in labor without local
  anesthesia
• Crowning occurs when the largest dimension of the
  head is distending the vulva

44
Stages of Labor Expulsion Stage

• The delivery of the placenta is accomplished
  within 30 minutes of birth
• Afterbirth the placenta and its attached fetal
  membranes
• All placenta fragments must be removed to prevent
  postpartum bleeding

45
Extrauterine Life

• At 1-5 minutes after birth, the infants physical
  status is assessed based on five signs heart
  rate, respiration, color, muscle tone, and
  reflexes
• Each observation is given a score of 0 to 2
• Apgar score the total score of the above
  assessments
• 8-10 indicates a healthy baby
• Lower scores reveal problems

46
First Breath

• Once carbon dioxide is no longer removed by the
  placenta, central acidosis occurs
• This excites the respiratory centers to trigger
  the first inspiration
• This requires tremendous effort airways are
  tiny and the lungs are collapsed
• Once the lungs inflate, surfactant in alveolar
  fluid helps reduce surface tension

47
Occlusion of Fetal Blood Vessels

• Umbilical arteries and vein constrict and become
  fibrosed
• Fates of fetal vessels
• Proximal umbilical arteries become superior
  vesical arteries and distal parts become the
  medial umbilical ligaments
• The umbilical vein becomes the ligamentum teres
• The ductus venosus becomes the ligamentum venosum
• The foramen ovale becomes the fossa ovalis
• The ductus arteriosus becomes the ligamentum
  arteriosum

48
Transitional Period

• Unstable period lasting 6-8 hours after birth
• The first 30 minutes the baby is alert and active
• Heart rate increases (120-160 beats/min.)
• Respiration is rapid and irregular
• Temperature falls
• Activity then diminishes and the infant sleeps
  about three hours
• A second active stage follows in which the baby
  regurgitates mucus and debris
• After this, the infant sleeps, with waking
  periods occurring every 3-4 hours

49
Lactation

• The production of milk by the mammary glands
• Estrogens, progesterone, and lactogen stimulate
  the hypothalamus to release prolactin-releasing
  hormone (PRH)
• The anterior pituitary responds by releasing
  prolactin
• Colostrum
• Solution rich in vitamin A, protein, minerals,
  and IgA antibodies
• Is released the first 23 days
• Is followed by true milk production

50
Breast Milk

• Advantages of breast milk for the infant
• Fats and iron are better absorbed
• Its amino acids are metabolized more efficiently
  than those of cows milk
• Beneficial chemicals are present IgA, other
  immunoglobulins, complement, lysozyme,
  interferon, and lactoperoxidase
• Interleukins and prostaglandins are present,
  which prevent overzealous inflammatory responses
• Its natural laxatives help cleanse the bowels of
  meconium

51
Chapter 29

• Heredity

52
Genetics

• The study of the mechanism of heredity
• Nuclei of all human cells (except gametes)
  contain 46 chromosomes
• Sex chromosomes determine the genetic sex (XX
  female, XY male)
• Karyotype the diploid chromosomal complement
  displayed in homologous pairs
• Genome genetic (DNA) makeup represents two sets
  of genetic instructions one maternal and the
  other paternal

53
Alleles

• Matched genes at the same locus on homologous
  chromosomes
• Homozygous two alleles controlling a single
  trait are the same
• Heterozygous the two alleles for a trait are
  different
• Dominant an allele masks or suppresses the
  expression of its partner
• Recessive the allele that is masked or
  suppressed

Genotype and Phenotype

• Genotype the genetic makeup
• Phenotype the way ones genotype is expressed

54
Segregation and Independent Assortment

• Chromosomes are randomly distributed to daughter
  cells
• Members of the allele pair for each trait are
  segregated during meiosis
• Alleles on different pairs of homologous
  chromosomes are distributed independently
• The number of different types of gametes can be
  calculated by this formula
• 2n, where n is the number of homologous pairs
• In a mans testes, the number of gamete types
  that can be produced based on independent
  assortment is 223, which equals 8.5 million
  possibilities

55
Independent Assortment
56
Crossover

• Homologous chromosomes synapse in meiosis I
• One chromosome segment exchanges positions with
  its homologous counterpart
• Genetic information is exchanged between
  homologous chromosomes
• Two recombinant chromosomes are formed

57
Crossover
58
Random Fertilization

• A single egg is fertilized by a single sperm in a
  random manner
• Considering independent assortment and random
  fertilization, an offspring represents one out of
  72 trillion (8.5 million ? 8.5 million) zygote
  possibilities

Dominant-Recessive Inheritance

• Reflects the interaction of dominant and
  recessive alleles
• Punnett square diagram used to predict the
  probability of having a certain type of offspring
  with a particular genotype and phenotype
• Example probability of different offspring from
  mating two heterozygous parents
• T tongue roller and t cannot roll tongue

59
Dominant-Recessive Inheritance
60
Dominant-Recessive Inheritance

• Examples of dominant disorders achondroplasia
  (type of dwarfism) and Huntingtons disease
• Examples of recessive conditions albinism,
  cystic fibrosis, and Tay-Sachs disease
• Carriers heterozygotes who do not express a
  trait but can pass it on to their offspring

61
Incomplete Dominance

• Heterozygous individuals have a phenotype
  intermediate between homozygous dominant and
  homozygous recessive
• Sickling gene is a human example when aberrant
  hemoglobin (Hb) is made from the recessive allele
  (s)
• SS normal Hb is made
• Ss sickle-cell trait (both aberrant
  and normal Hb is made)
• ss sickle-cell anemia (only aberrant
  Hb is made)

62
Multiple-Allele Inheritance

• Genes that exhibit more than two alternate
  alleles
• ABO blood grouping is an example
• Three alleles (IA, IB, i) determine the ABO blood
  type in humans
• IA and IB are codominant (both are expressed if
  present), and i is recessive

63
Sex-Linked Inheritance

• Inherited traits determined by genes on the sex
  chromosomes
• X chromosomes bear over 2500 genes Y chromosomes
  carry about 15 genes
• X-linked genes are
• Found only on the X chromosome
• Typically passed from mothers to sons
• Never masked or damped in males since there is no
  Y counterpart

Polygene Inheritance

• Depends on several different gene pairs at
  different loci acting in tandem
• Results in continuous phenotypic variation
  between two extremes
• Examples skin color, eye color, and height

64
Environmental Influence on Gene Expression

• Phenocopies environmentally produced phenotypes
  that mimic mutations
• Environmental factors can influence genetic
  expression after birth
• Poor nutrition can effect brain growth, body
  development, and height
• Childhood hormonal deficits can lead to abnormal
  skeletal growth

65
Genomic Imprinting

• The same allele can have different effects
  depending upon the source parent
• Deletions in chromosome 15 result in
• Prader-Willi syndrome if inherited from the
  father
• Angelman syndrome if inherited from the mother
• During gametogenesis, certain genes are
  methylated and tagged as either maternal or
  paternal
• Developing embryos read these tags and express
  one version or the other

66
Extrachromosomal (Mitochondrial) Inheritance

• Some genes are in the mitochondria
• All mitochondrial genes are transmitted by the
  mother
• Unusual muscle disorders and neurological
  problems have been linked to these genes

67
Genetic Screening, Counseling, and Therapy

• Newborn infants are screened for a number of
  genetic disorders congenital hip dysplasia,
  imperforate anus, and PKU
• Genetic screening alerts new parents that
  treatment may be necessary for the well-being of
  their infant
• Example a woman pregnant for the first time at
  age 35 may want to know if her baby has
  trisomy-21 (Down syndrome)

68
Carrier Recognition

• Identification of the heterozygote state for a
  given trait
• Two major avenues are used to identify carriers
  pedigrees and blood tests
• Pedigrees trace a particular genetic trait
  through several generations helps to predict the
  future
• Blood tests and DNA probes can detect the
  presence of unexpressed recessive genes
• Sickling, Tay-Sachs, and cystic fibrosis genes
  can be identified by such tests

69
Pedigree Analysis
70
Fetal Testing

• Is used when there is a known risk of a genetic
  disorder
• Amniocentesis amniotic fluid is withdrawn after
  the 14th week and sloughed fetal cells are
  examined for genetic abnormalities
• Chorionic villi sampling (CVS) chorionic villi
  are sampled and karyotyped for genetic
  abnormalities

71
Fetal Testing
72
Human Gene Therapy

• Genetic engineering has the potential to replace
  a defective gene
• Defective cells can be infected with a
  genetically engineered virus containing a
  functional gene
• The patients cells can be directly injected with
  corrected DNA