Chapter 24
Development &
Inheritance
Copyright 2010, John Wiley & Sons, Inc.
Fertilization





Genetic material from haploid sperm &
ovum merge into single diploid nucleus
Occurs in Uterine tube= 12-24 hours after
ovulation
Sperm last ~48 hours
Contraction of uterus & uterine tube walls
help move sperm
Prostaglandins in semen help stimulate
Fertilization





Sperm Capacitation- ready for penetration
Acrosomal enzymes penetrate outer cell
layers and one sperm fuses with ovum
meiosis II is completed- polar body
discarded
Fusion of DNA’s  diploid = zygote
Rapid cell division = cleavage
Fertilization
Fertilization
Early Development







Continual division ball of small cells
=Morula- enters uterine cavity ~ day 4-5
Reorganizes around fluid filled cavity
=blastocyst- about same size as zygote
Rearrangement Inner cell mass embryo
+ Trophoblast fetal part of Placenta
Implants after ~2 days
 ~6 days after fertilization
Early
Development
Early Development
nd
2




Week of Development
Trophoblast synctiotrophoblast +
cytotrophoblast  chorion
 Aids in implantation & secretes hCH
Inner cell mass hypoblast & epiblast
Cavity in epiblast  amniotic cavity
 Thin membrane develops to surround embryo
 = amnion filled with amniotic fluid
Hypoblast ceils –cover inner surface= wall of
yolk sac (from blastocyst cavity)
2nd Week of Development






Yolk sac –nutrients for weeks 2 & 3
+ source of blood cells (week 3-6)
Day 9 – blastocyst embedded in endometrium
Lacunae develop in trophoblast
 Maternal blood and secretions fill
 Serve as exchange point for embryonic
nutrient and waste removal
Day 12 - cells from yolk sac + two layers of
trophoblast  chorion
 surrounds embryo & becomes fetal part of
placenta
2nd Week of Development
2nd Week of Development
rd
3




Week
Gastrulation= transformation of disc from 2 -> 3
layers=
Primary ectoderm, mesoderm & endoderm
 Cells of epiblast move inward & detach
 Push out cells of hypoblast endoderm
 Become mesoderm
 Rest of epiblast  ectoderm
Mesoderm notocord vertebrae
Ectoderm neural plate neural tube
 neurulation
rd
3
Week
rd
3
Week
Allantois, Chorionic Villi & Placenta




Outpouch of yolk sac  allantois
Chorionic villi develop with capillaries
end 3rd week - capillaries connect to
embyronic heart via umbilical arteries &
veins
Placenta: chorionic villi & maternal
endometrium


Exchange organ.
Some viruses can get through
Allantois, Chorionic Villi & Placenta
Allantois, Chorionic Villi & Placenta
Allantois, Chorionic Villi & Placenta
th
4 


th
8
Weeks
All major organs appear at this time
Converted from disc to cylinder
22 days
Ear- otic placode & eyes- lens placode

4th week- upper & lower limb buds & heart
prominence on surface
5th week- rapid growth of brain & head

Neck & trunk straighten, 4 chambered heart,
digits appear
End of 8th week looks human fetus


Fetal Birth



Continual growth & differentiation
Few new structures added
Last 2 1/2 months – half of weight added
Fetal Birth
Fetal Birth
Fetal Birth
Fetal Birth
Fetal Birth
Fetal Birth
Fetal Birth
Fetal Birth
Hormones

hCG- maintains corpus lutem
progesterone & estrogen


Chorion secretes several hormones:
estrogens after 3-4 wks & progesterone by
wk 6


Peaks at week 9
Rise through pregnancy
Progesterone helps keep myometrium
relaxes & cervix closed
Hormones (cont.)


Relaxin- increases flexibility of pubic
symphysis & ligaments & relaxes cervix for
birth
Human placental lactogen (hPL)




Rises with fetal mass
Prepare mammary glands
Support metabolic capacity
Corticotrophin-Releasing Hormone (CRH)

Helps support birth timing & cortisol production
for lung maturation
Endocrine Support
Trophoblast cells secrete hCG
hCG maintains corpus luteum
corpus luteum continues to secrete relaxin,
progesterone and estrogen & Chorion adds hPL,
progesterone + estrogen & CRH
Estrogen & progesterone promote growth, development
and maintenance of the uterine wall hPl supports
maternal growth and changes CRH corticoids lung
maturation
Changes During Pregnancy


Uterus fills up pelvic cavity-3rd month
Further growth into abdominal cavity




Pushes on intestines, stomach, liver
Skin changes: areola, pigmentation
around eyes, stretch marks
Increased: maternal C.O., blood volume,
nutrient consumption appetite , O2
needs ventilation , renal activity
Pressure: diaphragm  dyspnea


Stomach acid reflux & nausea
Bladder  frequent urination
Exercise During Pregnancy





Major issues: postural changes with weight
gain
extra Heat production may affect
development early on
Care to avoid dehydration
Moderate exercise doesn’t hurt
May be beneficial
Labor & Delivery






Labor = expelling fetus from uterus
through vagina= parturition
Progesterone inhibits contractions
Estrogen rises toward end  overcome
inhibition
+  prostaglandin production
+  oxytocin receptors on myometrium
Oxytocin contractions
Stages of Labor

Dilation: onset to complete dilation of cervix

Expulsion- dilation to complete delivery

Placental- powerful uterine contractions expel
placenta
Positive Feedback


Contractions baby’s head against cervix
Neural signals  hypothalamus more
oxytocin  more pushing


Stops after birth when cervix is not stretched
True labor shows rhythmic pulses of
contraction
Negative Feedback Systems
Interactions Animation

Positive Feedback Control of Labor
You must be connected to the internet to run this animation.
Lactation

Prolactin(PRL)  milk production






PRL increases as pregnancy proceeds
Progesterone inhibits milk production
Progesterone decreases after delivery
Oxytocin milk letdown
feedback loop for maintenance
Suckling  nerve input to
hypothalamus maintain PRL & release
oxytocin
Colostrum





Formed during first few days after birth
Less lactose & no fat + antibodies
Protect infant during first few months of life
Good nutrition for infant
Oxytocin also speeds maternal recovery
Inheritance


Passage of hereditary traits- genetics
23 pairs of chromosomes






1 maternal and 1 paternal
Each homolog (one of pair) has gene for
same traits
variant genes for a trait = alleles
Genotype= genetic makeup
Homozygous = have 2 of same allele
Heterozygous= 2 different alleles
Phenotype= trait that is visible (brown
eyes)
Inheritance




Dominant allele  shows product no matter
what the other allele is
Recessive allele  masked by other allele
Can have incomplete dominance
phenotype mixture of allele products
Inheritance can be simple (one gene) or
multiple allele.
Inheritance
Autosomes & Sex Chromosomes




Autosomes: chromosomes where members
of pair look alike - 22 pairs
Sex chromosomes - look different
Females have 2 X chromosomes
Males have X and Y chromosomes


Maleness from SRY of Y chromosome
Traits on X but not Y sex linked inheritance
Autosomes & Sex Chromosomes
Autosomes & Sex Chromosomes
End of Chapter 24

Copyright 2010 John Wiley & Sons, Inc.
All rights reserved. Reproduction or translation of this
work beyond that permitted in section 117 of the 1976
United States Copyright Act without express permission
of the copyright owner is unlawful. Request for further
information should be addressed to the Permission
Department, John Wiley & Sons, Inc. The purchaser may
make back-up copies for his/her own use only and not
for distribution or resale. The Publishers assumes no
responsibility for errors, omissions, or damages caused
by the use of these programs or from the use of the
information herein.