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Prenatal:
◦ Pre-Implantation
◦ Embryonic
◦ Fetal
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An oocyte is normally fertilized by a single sperm
12-24 hours after it’s released.
When a sperm penetrates an oocyte conception has
occurred (now 46 chromosomes)
This initial cell is called
a zygote
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About 36 hours after fertilization occurs, the
oocyte divides into 2 cells (cell division
process called cleavage).
By day 4, it has multiplied to 12-16 cells
◦ At this stage it is known as a morula
◦ The morula travels down the oviduct, and on day
7 it enters the cavity of the uterus
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Each cell of the morula is capable of forming
a complete individual
When the morula fails to reach the uterine
cavity, but continues to develop, it may
implant on in the wall of the fallopian tube
(ectopic pregnancy)
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The morula continues to develop. Some fluid
(the blastocoel) appears inside it.
◦ It is now called a blastocyst (~day 5)
◦ Blastulation: the sprouting of a hollow sphere of cells
formed during the early embryonic development
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The blastocyst implants into the endometrium
◦ Progesterone and estrogen secreted from the corpus
luteum have influenced the glands and blood vessels in
the endometrium to develop and be ready for the
blastocyst
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When the blastocyst attaches, it has an inner
mass of cells which develops into the embryo
and an outer mass of cells that are called
trophoblasts.
The trophoblast cells begin to devlop their
own blood supply and this outer cell mass
becomes known as the chorion (which later
develops into the placenta).
◦ Also produced the hormone hCG (human chorionic
gonadotrophin) which is the hormone detected by
pregnancy tests.
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The shaping of an organism by embryological
processes of differentiation of cells, tissues,
and organs and the development of organ
systems.
The development of the body form following
the 3rd week of development
The inner cell mass becomes a platelike
structure called the embryonic disk
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By day 14:
◦ Trophoblast is completely embedded and
implantation site in the endometrium heals over
◦ The inner cells begin to line up as 2 layers of
different cells
◦ Pregnancy now suspected due to missed period
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Through the process of gastrulation, the
single layered blastula is reorganized into a
structure known as a gastrula. It now possess
the 3 germ layers
◦ All tissues and organs develop from these 3 layers
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During this period the embryo is called a
gastrula.
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Cell differentiation continues, and (as early as
5 weeks from last period), the inner mass of
cells begins to change into tissues and
organs)
ECTODERM
 Top cell layer of the embryonic disc. Will later form skin,
hair, lenses of the eyes, lining of the internal and external
ear, nose, sinues, mouth, anus, tooth enamel, pituitary and
mammary glands, and all parts of the nervous system.
MEDODERM
 Middle cell layer of the embryonic disc. The precursor to
the muscles, bones, lymphatic tissue, spleen, blood cells,
heart, lungs, reproductive and excretory systems.
ENDODERM
 Inner cell layer of the embryonic disc. Will later form the
lining of the lungs, the tongue, tonsils, urethra and
associated glands, bladder and digestive tract.
Endoderm
Mesoderm
Ectoderm
Respiratory
Tract
Dermis and
Muscles
Epidermis, Hair,
and Nails
Digestive Tract,
Bladder and
Thyroid
Liver and
Pancreas
Bone,
connective and
lymph tissue
Reproductive &
Cardiovascular
System
Lens of Eye and
Skin Glands
Central and
Peripheral
Nervous System
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Using prgnancy ultrasound, the blastocyst is
seen as a small circle measuring 2-4mm
within the uterus.
The first pulse of the heartbeat can been seen
as early as 36 days after the last period.
Yolk sac is seen close to the embryo, and
measures ~10mm
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Gestational period from week 5 – 9 is called
the embryonic period.
This is a vital time for organs (most
abnormalities or defects that are found after
birth have happened during this time frame)
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Beginning at ~10 weeks gestation, the outline
of the baby becomes recognisable
◦ Now called a fetus
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Organ formation almost complete, fetus now
starts to mature and prepare for delivery (at
~40 weeks from last period)
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Skin initially appears red and transparent, but
progressively thicken in the last few weeks
The biological sex is recognisable on external
appearance by 14 weeks
◦ Ultrasound scans at this time are rather early to be
certain of sec, but usually at the time of the 20-week
scan it is possible depending on fetus position
Fetal weight and length changes
Weeks of pregnancy
Fetal weight (grams)
Crown–rump length (cm)
10 weeks
10 g
3 cm
22 weeks
500 g
20 cm
28 weeks
1,000 g
25 cm
40 weeks
3,500 g
35 cm
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The fetus becomes viable at 24 weeks
(meaning it is possible for it to survive if
delivered after this time)
However the organs and tissues are still very
immature, and survival rate is low
Most babies survive after 30 weeks
 Human embryo, aged 2 weeks
seen in uterus. Embryo measures
~0.2mm and has no organs or
nervous system
Human embryo, aged 3 weeks
seen in uterus. Embryo measures
~3mm in length
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Amnion
Yolk Sac
Allantois
Chorion
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Lines the amniotic cavity
◦ Provides diffusion area
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Amniotic fluid
◦ Allows movement
◦ Provides cushion
◦ Cells lost in fluid can be collected
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Allantois
◦ Becomes the blood vessels inside the umbilical cord
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Yolk Sac
◦ Becomes part of the digestive and respiratory tracts
◦ Produces fetal blood cells until bone marrow takes
over
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Chorion
◦ Outermost membrane
◦ Exchange portion of the placenta
◦ Produces hCG