Download Chapter 17

Chapter 17
Development and Aging
Mader, Sylvia S. Human Biology. 13th Edition. McGraw-Hill, 2014.
Points to Ponder
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What is fertilization?
Describe the steps in fertilization.
What is cleavage? Growth? Morphogenesis? Differentiation?
What are the 4 extraembryonic membranes?
Be familiar with what happens during pre-embryonic development, fetal
development and development after birth.
Follow the path of fetal circulation.
What determines the sex of an individual? Be sure to understand the 3
hormones involved and the SRY gene.
What are two conditions in which sex determination is ambiguous and
2 conditions in which there are abnormal development of the sex
organs?
What are the 3 stages of birth?
What can you do to help prevent birth defects?
What are the 3 hypotheses of aging?
What is the effect of aging on the body?
17.1 Fertilization
Fertilization
• Fertilization is the union of the sperm and
egg to form zygote
• Egg is surrounded by an outer matrix
called the zona of pellucida
• Outside this matrix has a few layers of
follicular cells collectively called the corona
radiata
– These cells had nourished oocyte within the
ovarian follicle
Steps of Fertilization
• Steps of fertilization:
1. Several sperm penetrate the corona radiata
2. Acrosomal enzymes digests a portion of the
zona pellucida
3. The sperm binds to and fuses with the egg’s
plasma membrane
4. Sperm nucleus enters the egg
5. Sperm and egg nucleus will fuse
17.1 Fertilization
Fertilization
17.1 Fertilization
What prevents more than one sperm
from entering?
• The egg’s plasma membrane changes to
prevent other sperm from binding
– depolarizes
• Vesicles within the egg
– release enzymes that cause the zona pellucida to
become impenetrable and sperm cannot bind
17.2 Pre-embryonic and embryonic development
What are the main processes of development?
• Cleavage
– cells undergo division (2, 4, 8 etc) without the
embryo increasing in size
• Growth
– cells undergo division as well as increase in size
• Morphogenesis
– the embryo begins to take shape as cells migrate
• Differentiation
– when cells take on specific structure and function
(the nervous system is the first visible system)
Functions of the extraembryonic membranes
(Outside of the embryo)
• Chorion: Gas Exchange
– fetal half of the placenta, the organ that provides the
embryo with nourishment and gets rid of wastes
• Allantois: Collects Nitrogenous Wastes
– gives rise to the bladder
– blood vessels of umbilical cord carry blood to/from the fetus
• Yolk sac: Provides Nourishment
– contains many blood vessels
– blood cells first form
• Amnion: Provides Protection
– contains amniotic fluid that cushions and protects the
embryo
17.2 Pre-embryonic and embryonic development
Extraembryonic membranes
What are the stages of development?
1. Pre-embryonic development
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1st week of development after fertilization
2. Embryonic development
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2nd week after fertilization until the end of the
2nd month
3. Fetal development
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the 3rd through the 9th months of development
4. Development after birth
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stages of life including infancy, childhood,
adolescence and adulthood
1. Pre-embryonic development
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Cleavage
– cell division that increases the number of cells
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Morula
– compact ball of embryonic cells that becomes a blastocyte
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Early blastocyst
– inner cell mass  becomes the embryo
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All inner cells have capabilty to become any cell type
– Inner mass covered by layer of cells  becomes chorion
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Implantation
– embryo embeds into the uterus around day 6
2. Embryonic development: week 2
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Pregnancy begins after
implantation
Human chorionic
gonadotropin (HCG)
secreted
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maintain corpus luteum and
endometrium
HCG is the basis for a
pregnancy test
The inner cell mass
detaches itself and becomes
the embryonic disk
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goes through gastrulation to
become 3 primary germ
layers
3 Primary Germ Layers
• Ectoderm: outer layer
– Epidermis of skin
– Epithelial lining of oral cavity and rectum
– Nervous system
• Mesoderm: middle layer
– Skeleton, muscular system, dermis of skin
– Cardiovascular system, urinary system
– Reproductive system, out layers of respiratory and
digestive system
• Endoderm: inner layer
– Epithelial lining of digestive tract and respiratory tract and
associated glands
– Epithelial lining of urinary bladder
17.2 Pre-embryonic and embryonic development
2. Embryonic development: week 3
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Nervous system
begins to develop
The posterior neural
tube will become the
spinal cord and brain
Development of the
heart begins
17.2 Pre-embryonic and embryonic development
2. Embryonic development: weeks 4 & 5
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4th week:
– Chorionic villi form
– Umbilical cord forms
– Limb buds form (later develop into legs and
arms)
– Hands and feet are apparent
• 5th week:
– Head enlarges
– Eyes, ears and nose become prominent
2. Embryonic development: weeks 4 & 5
17.2 Pre-embryonic and embryonic development
The embryo at week 5
17.2 Pre-embryonic and embryonic development
2. Embryonic development: weeks 6-8
• Embryo begins to look like a human being
• Reflex actions occur
– Startle response to touch
• All organ systems have been established
• 38mm by the end of this period and weight
~ the same as an aspirin tablet
3. Fetal development
• Placenta
– Source of progesterone and estrogen during
pregnancy
– Functions of hormones:
• Negative feedback on hypothalamus and anterior
pituitary prevent new follicles from maturing
• Maintain endometrium so menstruation does not
occur
– Gasses and wastes are exchanged between
the maternal and fetal sides of the placenta via
diffusion
Fetal circulation
• Umbilical artery
– Carries O2-poor blood to the placenta towards the maternal
side
• Umbilical vein
– Carries O2-rich blood, rich in nutrients from the placenta to the
fetus
• O2-rich and O2-poor blood mix in the heart of the fetus
• Path of O2-rich blood
– Umbilical vein Liver  join venous duct  merge with inferior
vena cava  return blood to right atrium  enter the heart 
shunted to left atrium via oval opening  left ventricle pump
blood into the aorta
• Path of O2-poor blood
– Enter right atrium  pumped into pulmonary trunk  joins
aorta via arterial duct
Fetal circulation
Note: Fetal and maternal blood do not mix because exchange of materials between
the two occurs at the chorionic villi.
17.3 Fetal development
3. Fetal development: months 3 & 4
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Hair develops
The head slows in growth so that the body size
can catch up
Cartilage begins to be replaced by bone
– Fontanels “soft spots” remain in the skull for birthing and
brain growth
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Able to distinguish female from male (month 3)
– Presence of SRY gene on Y chromosome leads to the
development of testes and male genital
– Lack of SYR gene allows ovaries and female genital to
form
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Can hear heartbeat with the stethoscope (month 4)
By the end of this period the fetus is ~6 inches and
~6 ounces
17.3 Fetal development
3. Fetal development: months 5-7
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Fetal movement can
be felt by the mother
Fetus is in fetal
position
Eyelids are fully
open
Fetus size has
increased to ~12
inches and ~3
pounds
17.3 Fetal development
3. Fetal development: months 8 & 9
• Weight gain is about a pound per week
• Fetus rotates so that the head is pointed
towards the cervix
• At the end of fetal development the fetus
weighs ~7.5 pounds and ~20.5 inches
Development of the sex organs
Sex of an individual is determined at conception
(XX is female and XY is male)
- Six weeks: both male and females have the
same types of ducts
If the SRY (the sex determining region on the Y
chromosome) gene is present
– at ~ week 6 testes develop and testosterone is produced
1. Testosterone stimulates Wolffian ducts to become male
genital ducts
2. Wolffian ducts enter the urethra, which belongs to both the
urinary and reproductive systems in male
3. Anti-mullerian hormone causes Mullerian ducts to regress
Development of the sex organs
If the SRY (the sex determining region on
the Y chromosome) gene is absent
1. Ovaries develop instead of testes
2. Wolffian ducts regress
3. Estrogen from ovaries causes Mullerian
ducts to develop into the uterus and
oviducts
Development of the sex organs
17.3 Fetal development
Development of the sex organs
• At 14 weeks primitive testes and ovaries
with gametes are already developing
• The development of the external organs is
dependent on the presence or absence of
dihydrotestosterone (DHT) produced by the
testes
Hormones produce by the Testes
• Testosterone:
– Stimulate development of epididymides, vasa
deferentia, seminal vesicles, etc
• Anti-Mullerian hormone:
– prevents development of female structures
• Dihydrotestosterone:
– directs development of the urethra, prostate
gland, penis and scrotum
Abnormal development of the sex organs
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XY female syndrome
– an individual develops into a female
– piece of Y chromosome containing the SRY gene
is missing
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XX male syndrome
– an individual develops into a male
– same small piece of the Y containing the SRY
gene is present on an X chromosome
– SRY gene allows formation of the testes
– Testes produce 3 important hormones involved in
the development of the male genital
17.3 Fetal development
Ambiguous sex determination
• Results from the absence of testosterone,
Anti-Mullerian hormone and/or DHT
• Androgen insensitivity syndrome:
– all hormones are made
– testosterone receptors on cells are ineffective
– individual has testes that do not descend and
outwardly appear to be female
• Male pseudo-hermaphroditism:
– an individual appears female until puberty when
anti-Mullerian hormone is produced but the testes
never produce testosterone or DTH
What are the good and bad changes in
the mother’s body?
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Nausea and vomiting are common symptoms early
on (morning sickness)
Some mothers report an overall increase in energy
levels and sense of well-being
Acid reflux and constipation are common problems
There is an increase in vital capacity
Incontinence is not uncommon
The placenta produces peptide hormones that makes
cells resistant to insulin so diabetes can result
Melanocyte activity increases in some areas
Pregnancy: Uterus Relaxes
• Placenta produce progesterone which:
– Decrease uterine motility by relaxing smooth
muscle
• Arteries Dilate  leads to low blood pressure
– Stimulate renin-angiotension-aldosterone results in
sodium and water retention
– Increase in blood cells
• Gastrointestinal effects of pregnancy
• Heartburn: relaxation of esophageal sphincter
• Constipation: caused by a decrease in intestinal tract
motility
Pregnancy: Pulmonary Values Increase
• Bronchial tubes relax
• Increase vital capacity and tidal volume
• Maternal oxygen level changes little, but
blood carbon dioxide levels fall by 20%
– creates concentration gradient favorable to
flow of carbon dioxide from fetal blood to
maternal blood at the placenta
Birth
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True labor is characterized by uterine contraction that occur
every 15-20 minutes and last for at least 40 seconds
3 stages:
– 1st: Effacement occurs; Cervical Dilation
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cervical canal slowly disappears
babies head acts as a wedge to cause cervical dilation
– 2nd: Uterine contractions; Birthing
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uterine contractions every 1-2 minutes lasting for ~1 minute e
an incision is made to the opening to help the baby as its
head reaches the exterior
once the baby is born the umbilical cord is cut and tied
– 3rd: Afterbirth; release of placenta
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Afterbirth is delivered about 15 minutes after birth of baby
17.4 Pregnancy and birth
Birth
17.4 Pregnancy and birth
Health focus: preventing birth defects
• Get physical exams by a trained doctor
• Have good health habits:
– proper nutrition and adequate sleep and
exercise
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Avoid smoking, alcohol and drug abuse
Avoid having x-rays
Avoid certain medications and supplements
Avoid sexually transmitted diseases or
know if you have one
17.5 Development after birth
Aging
• Stages of life: infancy, childhood, adolescence
and adulthood
• 3 hypotheses of aging:
– Genetic origin:
• suggests mitochondrial activity is involved with aging
– Whole body processes
• suggests aging may be a result from body systems or
type of tissue
– such as loss of hormonal activities or cross-linking in support
tissues
– Extrinsic factors:
• suggests that aging may be due to years of poor health
habits such as a poor diet and lack of exercise
17.5 Development after birth
What are the effects of age on body systems?
• Skin becomes thinner, less elastic and dry
• Less adipose in the skin so one feels colder
more easily
• Decrease in melanocytes leading to gray hair
while some of the remaining cells are larger
leaving “age spots” (dark spots on the skin)
• Heart shrinks and arteries become more rigid
• Reaction time slows and senses are muted
What are the effects of age on body systems?
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Lens in the eye loses ability to accommodate
Blood pressure usually increases
Bone density declines
Muscle mass decreases
Weight gain results from a decrease in
metabolism and an increase in inactivity
• Females undergo menopause and males
andropause