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Lecture Outline
Human Development and Birth
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Fertilization
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The zygote is formed when the
ovum and sperm unite
Sperm Anatomy
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Head – contains the nucleus
Middle piece – contains energyproducing mitochondria
Tail (flagellum)
Ovum Anatomy
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Zona pellucida
Corona radiata
Fertilization
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Steps of Fertilization
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Several sperm penetrate the corona
radiata
Several sperm attempt to penetrate
the zona pellucida
One sperm enters the ovum
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Acrosome releases digestive enzymes
When a sperm binds to the ovum, their
plasma membranes fuse
Accidental entry of more than one
sperm will halt development of the
zygote
Development
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Processes of Development
Cleavage
Growth
Morphogenesis
Differentiation
Extraembryonic Membranes
Are not part of the embryo and fetus
Functions in humans:
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Chorion – develops into the fetal half of the
placenta
Yolk sac – first site of blood cell formation
Allantois – the allantois blood vessels become the
umbilical blood vessels
Amnion – contains fluid to cushion and protect the
embryo
Development
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Stages of Development
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Pre-embryonic development
Events of the first week
After fertilization, the zygote divides
repeatedly
A morula becomes a blastocyst
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Inner cell mass surrounded by the trophoblast
The trophoblast will become the chorion
Each cell has the genetic capability of becoming
any tissue
Development
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Embryonic development
Second week
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Embryo usually begins the process of implantation
If implantation is successful, the female is
clinically pregnant
An ectopic pregnancy occurs if the embryo
implants in the uterine tube
The trophoblast begins to secrete HCG
Acts like LH
Stimulates corpus luteum to secrete
progesterone and the endometrium is
maintained
The inner cell mass separates from the trophoblast
Gastrulation occurs
Inner cell mass becomes the embryonic disk
Primary germ layers (ectoderm, mesoderm, and
endoderm) form
Development
Third week
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The nervous system becomes evident
Neural plate
Neural groove
Neural folds
Neural tube forms – later becomes the brain
and spinal cord
If the neural tube fails to close completely,
a neural tube defect will result
Development of the heart begins
Fig
18.5
Development
Fourth and fifth weeks
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Exchange of gases, nutrients, and wastes can
take place across the capillary network within
the chorionic villi
Blood vessels of the allantois become the
umbilical blood vessels
The umbilical cord completes formation
Limb buds appear
The head enlarges and the sense organs
become more prominent
Sixth through eighth weeks
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Embryo is easily recognized as human
The head achieves its normal relationship with
the body
Nervous system has developed enough to allow
reflex actions
Development
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Placenta
Usually fully functional by the end of the
embryonic period
Two portions:
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Fetal portion composed of chorionic tissue
Maternal portion composed of uterine tissue
Placental membrane
Placenta supplies the fetus with its
nutritional and excretory needs
The umbilical cord transports fetal blood
to and from the placenta
The placenta begins to produce
progesterone and estrogen
Development
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Fetal development and birth
Third and fourth months
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Head growth slows and the rest of the body
increases in length
Epidermal refinements appear
Cartilage begins to be replaced by bone
Possible to distinguish males from females
During the fourth month, the fetal heartbeat can be
auscultated
Fifth through seventh months
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Fetal movement can be felt by the mother
Lanugo and vernix caseosa
Eyelids are fully open
Eighth through ninth months
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Fetus usually rotates so that the head is pointed
towards the cervix
Weight gain due to an accumulation of fat
Development
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Development of male and female sex organs
Gender is determined at the moment of fertilization
Gonads start developing in the seventh week
Genes on the Y chromosome cause:
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The production of testosterone
Causes testes to develop
Stimulates the wolffian ducts to become male genital
ducts
If there is no Y chromosome present
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Ovaries develop instead of testes
Müllerian ducts develop into the uterus and uterine tubes
External genitalia
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At six weeks – a small bud of indifferent tissue is seen
between the legs
At nine weeks – urogenital groove appears
By fourteen weeks
The groove has disappeared in males and the scrotum
has formed
In females the groove becomes the vaginal opening
Birth
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Labor is marked by regular, long-lasting
contractions
A positive feedback mechanism causes
the onset and continuation of labor
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Stretching of cervix stimulates oxytocin
release
Oxytocin stimulates the uterine muscles
Uterine contractions push the fetus forward
causing the cervix to stretch more
Expulsion of the mucous plug is usually
the first sign that birth is imminent
Birth
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Stage 1
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Effacement
Amniotic membrane will probably
ruptured if it has not already
Ends when the cervix has dilated
completely
Birth
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Stage 2
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Uterine contractions occur every 1-2
minutes and last about 1 minute each
Baby’s head descends into the vagina
The baby is delivered
Stage 3
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The placenta is delivered
About 15 minutes after delivery of the
baby, the placenta is dislodged from
the uterus and expelled into the
vagina
Birth
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Effects of Pregnancy on the Mother
Early pregnancy
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Weight gain
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Nausea and vomiting
Loss of appetite and fatigue
Breast and uterine enlargement
Weight of the fetus
Amount of amniotic fluid
Size of the placenta
Increase in her own body fluid
Storage of proteins, fats, and minerals
Physiological changes due to placental hormones
Increase in pulmonary values
Stress incontinence
Edema and varicose veins
Possibility of pregnancy-induced diabetes
Striae gravidarum (stretch marks)