chapter 17 development and aging mader, sylvia s. human biology. 13 th edition. mcgraw-hill, 2014

Chapter 17

Development and Aging

Mader, Sylvia S. Human Biology. 13th Edition. McGraw-Hill, 2014.

Points to Ponder• 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?

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

17.1 Fertilization

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

Fertilization17.1 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

17.1 Fertilization

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)

17.2 Pre-embryonic and embryonic development

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

Extraembryonic membranes


What are the stages of development?

1. Pre-embryonic development - 1st week of development after fertilization

2. Embryonic development - 2nd week after fertilization until the end of the 2nd

month

3. Fetal development - the 3rd through the 9th months of development

4. Development after birth - stages of life including infancy, childhood,

adolescence and adulthood

1. Pre-embryonic development

• Cleavage – cell division that increases the number of cells

• Morula – compact ball of embryonic cells that becomes a blastocyte

• Early blastocyst – inner cell mass becomes the embryo

• All inner cells have capabilty to become any cell type

– Inner mass covered by layer of cells becomes chorion

• Implantation – embryo embeds into the uterus around day 6

2. Embryonic development: week 2• Pregnancy begins after

implantation• Human chorionic

gonadotropin (HCG) secreted – maintain corpus luteum and

endometrium

• HCG is the basis for a pregnancy test

• The inner cell mass detaches itself and becomes the embryonic disk – 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

2. Embryonic development: week 3

• Nervous system begins to develop

• The posterior neural tube will become the spinal cord and brain

• Development of the heart begins


2. Embryonic development: weeks 4 & 5

• 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

The embryo at week 5


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.

3. Fetal development: months 3 & 4

• 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

• 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• 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

• 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


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


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



• 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

• XY female syndrome – an individual develops into a female– piece of Y chromosome containing the SRY gene

is missing

• 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

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?

• 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• 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• cervical canal slowly disappears

• babies head acts as a wedge to cause cervical dilation

– 2nd: Uterine contractions; Birthing• 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• Afterbirth is delivered about 15 minutes after birth of baby

Birth17.4 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

• 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

17.4 Pregnancy and 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

17.5 Development after birth

What are the effects of age on body systems?

• 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

chapter 17 development and aging mader, sylvia s. human biology. 13 th edition. mcgraw-hill, 2014

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