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Chapter 17

Development and Aging

What do you see in this picture? Let’s take a

closer look.

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Outline

• Fertilization• Pre-Embryonic and Embryonic

Development • Fetal Development• Pregnancy and Birth• Development after Birth (including Aging)

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Fertilization

• Steps of fertilization (Conception).– Several sperm penetrate corona radiata.– Several sperm attempt to penetrate zona

pellucida.– One sperm enters egg and nuclei fuse,

producing a zygote.Egg’s plasma membrane and zona

pellucida change to prevent polyspermy.

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Fertilization

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Occurrence of Pregnancy

• When a zygote begins dividing, it is termed an embryo (pre-embryo).

– Developing embryo travels down oviduct and eventually implants in endometrium. (Implantation / pregnancy)

Presence of human gonadotropic hormone (HCG) in the blood confirms pregnancy.

If implantation does not occur, a woman never knows fertilization took place.

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Human Development before Implantation

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• Following implantation, the placenta originates from maternal and fetal tissues.• Placenta then produces human chorionic gonadotropin (HCG) which maintains the corpus luteum in the ovary until the placenta begins its own production of progesterone and estrogen.• Physical signs of pregnancy include : no menstruation, increased urination, morning sickness, increased size of breasts, and darkening of areolae.

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Pre-Embryonic and Embryonic Development

• Processes of development.– Cleavage - Cell division without growth.– Growth - Increase in size of cells.– Morphogenesis - Shaping of embryo.– Differentiation - Cells take on specific

structure and function.

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Extraembryonic Membranes

• Membranes that extend out beyond the embryo.– Amnion - Provides fluid environment for

developing embryo and fetus.– Yolk sac - First site of red blood cell

formation.– Allantois - Contributes to cardiovascular

system.– Chorion – outermost membrane, develops

from the trophoblast, contributes to the placenta.

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Extraembryonic Membranes

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Pre-Embryonic and Embryonic Development

• Stages of development.– Morula - Solid mass of cells resulting from

cleavage.– Blastocyst (Blastula) - Ball of cells formed from

morula. Embryonic disk - Inner mass of cells of

blastocyst.– Gastrula - Embryo composed of three tissues.

Ectoderm, mesoderm, endoderm. These are the foundation of the body

systems (Germ Layer Theory).

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Early Developmental Stages

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Pre-Embryonic and Embryonic Development

• Stages of development.– Neurula - Nervous system develops from

ectoderm located just above the notochord.

Involves induction as one tissue influences the development of another tissue.

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Primitive Streak and Neurula

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Germ Layer Theory of Organ –System Development

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Embryonic Development

• Embryonic development occurs from the second week to the eighth week.

• Fetal development occurs from the third month through the ninth month.

• See Table 17.1 – Human Development (in text) for a summary of key events during the 9 months of pregnancy.

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Embryonic Development

• Immediately after fertilization, the embryo divides and develops into a blastocyst (Blastula).– Bounded by a layer of cells (trophoblast)

that becomes the chorion.• Implantation completed by the end of the

second week.• Embryo is a gastrula by the end of the third

week.• Placenta is forming by end of fourth week.

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Embryonic Development

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Embryonic Development

• By the end of the second month, all organs have appeared and the placenta is fully functioning.– Embryonic development complete.

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Five-Week-Old Embryo

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Fetal Circulation

• The umbilical cord stretches between the placenta and the fetus and contains the umbilical arteries and veins.

• Placenta functions:– Exchange of gases and nutrients between

maternal and fetal blood takes place in the umbilical arteries.

– Umbilical vein carries blood and oxygen away from the placenta to the fetus.

– Produce hormones to maintain pregnancy (estrogen, progesterone, HCG)

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Fetal Circulation and the Placenta

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Science FocusCloning Humans: Can It

Be Done?

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Fetal Development

• At the beginning of the third month, head growth begins to slow and the body increases in length.

• Ossification centers appear in bones.• Sex can be determined sometime in the

third month.

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Three-to-Four Month-Old Fetus

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Fifth through Seventh Months

• Mother begins to feel fetal movement.• Wrinkled skin covered by fine hair, lanugo,

is covered by a greasy substance vernix caseosa.

• Lungs lack surfactant so if baby is born prematurely it will have to be on a respirator (respiratory distress syndrome).

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Six-Month-Old-Fetus

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Eighth and Ninth Months

• Fetus usually rotates so head is pointed down toward cervix.

• Fetus is now about 530 mm in length and weighs about 3,400 g.

• Full-term babies have the best chance of survival.

• Breech birth (“rump first”) may require a cesarean section.

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Development of Male and Female Genitals

• Sex of an individual is determined at the moment of fertilization.– Gonads arise from indifferent tissue that

can develop into ovaries or testes, depending on the action of hormones.

In the absence of a Y chromosome and in the presence of two X chromosomes, ovaries develop instead of testes.

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Development of Male and Female Genitals

• Abnormal Development of Genitals • XY female syndrome / XX male syndrome• Due to misplaced SRY gene

(sex determining region of the Y).• Ambiguous Sex Determination• androgen insensitivity syndrome• male pseudohermaphroditism.

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HEALTH FOCUS

Preventing Birth Defects

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Pregnancy and Birth

• Major changes that take place in the mother’s body during pregnancy are due to placental hormones:

1. The uterus relaxes.

2. The pulmonary values increase.

3. Other effects – incontinence, edema, varicose veins, pregnancy-induced diabetes, striae gravidarum (“stretch marks”), increase in melanocyte activity.

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Birth

• False-labor (Braxton Hicks contractions) preliminary contractions that occur periodically but lasting less than 30 seconds.

• True labor is marked by uterine contractions that occur regularly every 15-20 minutes and last for 40 seconds or more.– Positive feedback control.

• Parturition is the technical term for giving birth to an offspring.

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Birth– Stage 1.

Mucous plug may be expelled from cervical canal – called “bloody show” before stage 1 technically begins.

Cervical canal slowly disappears called effacement (“taking up the cervix”).

“Breaking of the water”Cervix dilates completely.

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Birth

– Stage 2.Baby’s head descends into the vagina.Episiotomy?Baby is delivered.

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–Stage 3.Placenta delivered, usually within 15 minutes.Called the afterbirth.

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Stages of Parturition

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Female Breast and Lactation• Female breast contains 15-20 lobules, each

with a milk duct beginning at the nipple and ending in alveoli.– In pregnancy, breasts enlarge as ducts

and alveoli increase in number and size.Milk usually not produced during

pregnancy. Prolactin (lactogenic hormone)

suppressed due to increase in estrogen and progesterone.

Suckling stimulates release of oxytocin.

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Female Breast Anatomy

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Development after Birth

• Development is a lifelong process into adulthood, after which aging occurs.

• Gerontology is the study of aging.• Aging encompasses progressive changes

that contribute to an increased risk of infirmity, disease, and death.– Theories.

Genetic in Origin.Whole-Body Process.Extrinsic Factors.

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Effect of Age on Body Systems

• Skin.– Skin becomes less elastic due to changes

in elastic fibers.• Processing and transporting.

– Heart shrinks due to a reduction in cardiac muscle.

– Blood pressure gradually increases.– Liver not as efficient in metabolizing drugs.– Blood supply to kidneys reduced.

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Effect of Age on Body Systems

• Integration and coordination.– Few neural cells of the cerebral cortex are

lost during the aging process.– Reaction time slows.– Loss of skeletal muscle mass not

uncommon.• Reproductive system.

– Females undergo menopause (Ch. 16).– Male androgen levels fall between ages 50-

90, but sperm produced until death.

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BIOETHICAL FOCUS

Maternal Health Habits

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Outline

• Fertilization• Pre-Embryonic and Embryonic

Development • Fetal Development• Pregnancy and Birth• Development after Birth (including Aging)

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