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Conception & Fetal
Development
Lecture 2
REVIEW
Gametogenesis: production of gametes.
Male gamete (sperm) produced in seminiferous tubules of
testes during spermatogenesis. 200-600 mil.@
ejacula.
Female gamete (ovum) produced in graafian follicule of
ovary during oogenesis. At birth, each ovary has 2 mil.
immature oocytes, occurs 1st 5 mos. of development.
Chromosomes divide (meiosis) from 46 → 23 before
fertilization. 2 meiotic divisions in both sperm & ovum
1 spermatogonium >> 4 spermatids (approximately1000
sperm per second or ~ 30 billion/year)
1 oogonium >> 1 mature ovum & 3 polar bodies
Ovum: 1st meiotic division completed before ovulation & 2nd
meiotic division completed at fertilization.
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Conception: fertilization of sperm & ovum
in ampulla [upper 1/3rd ] of fallopian tube.
Now “zygote”.
½ genetic material comes from each
parent cell. Head of mature sperm
contains chromosomes.
Zona pellucida (ovum) changes
chemical composition so multiple sperm
cannot enter.
Fertilized ovum begins mitotic cell division.
B. Cellular Multiplication
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Zygote undergoes mitosis (cleavage) - rapid series
of cell divisions. Forms morula; solid ball of cells.
Outer shell of cells with an attached inner group of
cells forms, changing morula into “blastocyst”.
Blastocyst consists of inner cell mass and
trophoblast.
Outer group of cells become membranes that
nourish & protect inner group of cells (embryo).
Blastocyst reaches uterus ~ 5th day.
Inside blastocyst, inner cell mass generates 3 major
layers inside the sphere: ectoderm, mesoderm,
endoderm.
Inner cell mass develops into fetus.
C. Implantation
 Outer wall of blastocyst (trophoblast) attaches to
endometrium (anterior or posterior fundal
region)
7 - 9 days > fertilization.
 Occurs 2-4 days > entering uterus.
 Blastocyst receives nourishment via mother's
bloodstream.
 Embryo fully implanted by day 10.
 During time between implantation & 8th week,
cellular differentiation occurs.. (blood cells,
kidney cells, nerve cells, etc.).
 From 8th week until birth - “fetus”.
Yolk Sac
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Attached to the embryo
Continuous with intestinal cavity of embryo
Supplied with blood vessels which transport
nutritive yolk products to developing embryo
Degenerates by week 12 when placenta
takes over
D. Placentation: ongoing process of fetal &
maternal placental formation.
 Begins immediately after implantation.
 Endometrium now “decidua”.
 3 parts: basalis, capsularis, & vera.
 Basalis unites with chorion to form maternal
side of placenta.
 Capsularis surrounds chorionic sac.
 Decidua Vera is mucous membrane lining main
cavity of pregnant uterus other than at site of
implantation.
Pregnancy Hormones
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Developing embryo begins to produce hCG (human
chorionic gonadotropin) - enables corpus luteum to
continue to secrete progesterone/estrogen.
In early pregnancy, steroid hormones are
responsible for maintaining endometrium [uterus]
rich with blood vessels so zygote can develop.
> 7th week shift to placental production of hormones
begins.
By 12th week, hormone production entirely from
placenta.
E. Fetal Membranes
 Chorion: outer fetal membrane closest
to uterine wall
 Fingerlike projections “chorionic villi”
form & invade endometruim; becomes
fetal part of future placenta 2-3 weeks >
fertilization.
 Intervillous space: space between the
chorionic villi where maternal blood
circulates within placenta
Complex network of embryonic vessels
allows diffusion of nutrients, oxygen, &
wastes bet. mother & fetus.
 Amnion: inner fetal membrane
 Chorion & amnion fuse to become one
membrane, amniotic sac.
 Holds fetus & amniotic fluid.
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F. Umbilical Cord:
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50-55 cm length; 2 cm diameter
2 arteries & 1 vein – “AVA”
Made of Wharton’s jelly (gelatinous)
Blood flows thru cord @ rate of ~ 400 ml/min.
Vessels remain patent
Develops @ 5 wks.gestation; lifeline between
placenta & fetus.
Carries waste & O2 poor blood away from
fetus to placenta.
Carries O2 rich blood back to fetus.
G. Placenta
Structure: 15-20 subdivisions “cotyledons”.
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Fetal surface: smooth, shiny, covered by
amnion.
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Maternal surface: red, flesh-like, 15-20
subdivisions or cotyledons.
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"Dirty Duncan" and "Shiny Schultz"
Physiology:
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Function: transport mechanism between
mom & fetus.
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Lifespan depends on O2 consumption.
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Function depends on maternal circulation.
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Circulation best when mother in lateral
position.
Functions of Placenta
 Receives maternal 02 via diffusion.
 Produces all hormones to sustain
endometrium thus the pregnancy
(HPL, estrogen, progesterone, relaxin)
 Supplies fetus with CHO, water, fats,
proteins, minerals & inorganic salts.
 Carries end products of fetal metabolism into
maternal circulation for excretion.
 Transfers passive immunity via maternal
antibodies.
H. Amniotic Fluid
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800-1200ml. Clear, yellow fluid
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Contains albumin, lanugo & urea.
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Replaced every 3 hours; swallowed by fetus.
Functions
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Prevents heat loss; preserves constant
fetal body temperatures.
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Cushions fetus.
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Acts as excretion – collection system.
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Facilitates fetal growth & development
First Trimester - fetus most susceptible to
damage from external sources including:
 teratogens (causing birth defects … ie.
alcohol, some Rx & recreational drugs)
 infections (ie. rubella or cytomegalovirus)
 radiation (x-rays, radiation therapy, or
accidental exposure to radiation)
 nutritional deficiencies
Summary of Fetal Development
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4th wk: Fetal heart begins to beat.
(smaller than grain of rice).
8 wks: All body organs formed. Weighs
less than grape. 1/3 baby is head. Arms
& legs are tiny buds. L: 30mm ( 1in.)
Wt. = 2 g.
12 wks: Fetal heart tones heard by
Doppler. Baby can kick. Sex organs
formed. Fingers & toes developed. Eyelids
fused. L: 4 in or 11cm. W: 45 g.
Fetal Development
16 wks: Sex can be seen. Thin; looks like
baby. Uterus size of grapefruit. Fetus as
large as orange. Starting to suck/swallow.
Nails on fingers/toes. L: 5 in or 15 cm.
W: 200 g. Actively swallows amniotic fluid.
Lanugo forms.
 20 wks: Heartbeat heard with fetoscope.
Develops regular schedule: sleeping, sucks
thumb, kicking. Hands grasp.Vernix caseosa
begins to form. Assumes favorite position in
utero. Lanugo keeps oil on skin. L: 25 cm.
W: 400 g. + fetal movement (quickening)
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Fetal Development
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24 wks: L: 30 cm. W: 750 g. Weighs about
1.3 lb. Increased activity. Respiratory
movement begins. Skin is thin. No fat.
Regular sleep time.
28 wks: Eyes open & close. Baby makes
breathing motions. Surfactant begins
forming. Testes descend. More fat forms.
Can hiccup, cry, hear your voice. 14” long;
2.2 lbs. Baby 2/3rd final size. L: 35 cm.
W: 1200g.
Fetal Development
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32 weeks: More subcutaneous fat laid down.
Appears less red & wrinkled; 14” long; 4 lbs.
brain growing. Lungs immature. Gains 1/2 lb/wk.
L:35-38 cm. W: 2000 g. Skin pink; covered with
vernix caseosa; lanugo begins to disappear.
Braxton Hicks are felt.
38-40 wks: Full term @ beg. of 37th wk. Fills
uterus. Gets IgA ab from mother. Gains 2 ½ lbs;
mostly fat. L: 48-52 cm. or 20” long. W: 30004000g, or ~ 8 lbs.
Fetal Circulation
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Placenta [O2 rich blood >> maternal circulation]
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Enters Umbilical vein (O2 rich blood)
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ductus venous (2/3rd bypasses liver)
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hepatic vein (small amt. blood flow)
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Enters inferior vena cava [IVC]
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30-35% enters R. atrium >> passes through
foramen ovale (shunt in fetal heart) >> L. Atrium >>
L.Ventricle >> Ascending aorta to Head & upper
extremities (to oxygenate where needed most)
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65-70%of blood in Right atrium mixes with O2
poor blood returning from SVC >> Right
Ventricle >> enters ductus arteriosis to be
shunted away from lungs >> enters
descending aorta >> lower extremities/trunk
Only ~ 8 % enters pulmonary arterial bed
through right & left pulmonary arteries and
returns from lungs to left atrium via 4
pulmonary veins.
Eventually all O2 poor blood leaves thru
aorta >> 2 umbilical arteries >> placenta to
re-oxygenate.
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Fetal circulation: Preferential shunting of blood
with highest O2 saturation to L side of heart.
Assures adequate oxygenated blood flow to
coronary & cerebral circulations—tissues with
greatest need.
Circulation > birth: With first breaths, larger amount of
blood sent to lungs to pick up O2. Ductus arteriosus
no longer needed; begins to wither & close off in 1-2
days. PDA may be heard 1st 24-48 hours of life.
Circulation in lungs ^ & more blood flows into L
atrium. This ^ pressure causes foramen ovale to
close & blood circulates normally.
Umbilical arteries/vein degenerate. Shunts &
vessels > ligaments & supporting structures.
http://www.indiana.edu/~anat550/cvanim/fetcirc/fetcirc.htm
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