Download Bilaminar germ disc Second week of development

Foundation
Embryology
Development of bilaminar and trilaminar germ discs (ID#7070)
(Second and third week of development)
Dr. Narayana Kilarkaje
Department of Anatomy
Figures or photographs used in this presentation are originally
reproduced from Langman’s Medical Embryology by T. W. Sadler,
10th Edition, Lippincott Williams & Wilkins, for teaching purpose
only.
Objectives
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Describe the processes involved in formation
of inner cell mass and trophoblast
Describe the beginning of formation of germ
layers from the inner cell mass
Describe the formation of ectoderm, endoderm
and mesoderm
Describe the formation of notochord and body
axis establishment
Describe the growth of the embryonic disc
Background- What happens in the first week?
Uterus
Ovary
Ampulla
of uterine
tube
Implantation
Second week (day 8-14)
Day 8
• The blastocyst is partially embedded in the endometrium.
• The trophoblast forms 2 layers-inner layer-cytotrophoblast;
and an outer layer of cells without cell boundariessyncytiotrophoblast.
• Two cavities appear- blastocyst cavity (forms the primitive
yolk sac) and amniotic cavity.
• The inner cell mass forms 2 layers-lower hypoblast in
relation with the yolk sac.
The upper layer-in relation with the amniotic cavity-epiblast.
• The endometrial stroma is edematous and highly vascular
and filled with glycogen and lipids.
• This process is called decidual reaction and the
endometrium is called decidua.
Days 9 and 10
Days 9 and 10
• The blastocyst is more deeply embedded in the
endometrium-interstitial implantation
• The penetration point on the endometrium is
closed by fibrin coagulum
• The vacuoles appear in the syncytiotrophoblast,
and large spaces appear called lacunae
• This stage of development of trophoblast is known
as lacunar stage
• A layer of cells probably derived from the
hypoblast form a lining on the yolk sac called
exocoelomic membrane (Heuser’s)
Days 11 and 12
Days 11 and 12
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The blastocyst now protrudes into the uterine lumen
The trophoblast is characterized by lacunar spaces
The trophoblast erodes the maternal capillaries (sinusoids)
The maternal blood enters the lacunae, thus establishing
utero-placental circulation
A new cell layer is formed between the yolk sac and
cytotrophoblast layer known as extra-embryonic mesoderm
Spaces appear in the extra-embryonic mesoderm called
extra-embryonic celom
The extra-embryonic celom divides the extra-embryonic
mesoderm into 2 layers
The layer surrounding the yolk sac is called splanchnic
mesoderm
The layer with the trophoblast is called somatic mesoderm
The bilaminar disc remains very small
Day 13
Day 13
• By 13th day, the site of implantation is healed, but bleeding
takes place-implantation bleeding (coinciding with 27th/28th
day of an otherwise menstrual cycle)
• This bleeding could be mistaken for last menstrual period,
thus making it difficult to predict the expected delivery date
• The trophoblast has villous like structures
• The cytotrophoblast penetrates into the syncytiotrophoblast
(primary villi)
• The primary yolk sac transforms into secondary yolk sac,
which is smaller
• The extra-embryonic coelom expands and forms the
chorionic cavity
• The connecting stalk (later becomes the umbilical cord)
suspends the embryo
0
4
No fertilization;
therefore,
menstruation
Ovulation
14 days before
next menstrual
14
28
period
Menstrual period
No pregnancy
MP
Secretory
Phase
Proliferative
Phase
Ovulation Implantation
20/ 21 day
Menstrual period
with pregnancy
0
4
MP
MP- Menstrual period
14
MP
Implantation
bleeding 13 day
28
Secretory
No MP
Proliferative
Phase
Phase
Fertilization
Missed MP
Abnormal implantation
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Implantation at a site other than at the normal site-most common site is the uterine tube (90%)
The sites of abnormal implantation are1) In the recto-uterine cavity, 2) ampulla of the uterine tube, 3) tubal, 4) interstitial, and 5) at
internal os of uterus
Implantation at internal os results in placenta previa-leading to bleeding during late stage of
pregnancy
Ectopic pregnancy- Outside the uterus
Syncytiotrophoblast produces human chorionic gonadotropin (hCG); by the end of second week, it
can be detected by radioimmunoassay (pregnancy test)
Third week of development - Gastrulation (Trilaminar germ disc)
• Important event occurring during 3rd week is gastrulation
• The gastrulation is a process by which 3 germ layers-ectoderm,
mesoderm, and endoderm-are formed from epiblast
• It begins with the formation of primitive streak; the latter establishes the
body axis (Right-left; cranial-caudal)
• It is clearly formed by 15-16 days as a narrow groove
• The cephalic end of the streak is called primitive node in which a small
pit is present-primitive pit
• The epiblast cells migrate through the primitive pit by invagination
• The cell migration is controlled by fibroblast growth factor 8 (FGF8)
• The migrating cells move between the epiblast and hypoblast
• The prechordal plate is formed in the hypoblast at cephalic end; no
mesoderm is present at this point
• This prechordal plate forms the buccopharyngeal (oropharyngeal)
membrane
Third week of development - Gastrulation
Cranial
(Prechordal plate)
Left
Right
Caudal
Primitive pit
Formation of the notochord
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Amniotic cavity
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LS
Yolk sac
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TS
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TS
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The cells from the primitive pit
migrate cranially in the midline up
to
the
prechordal
plate
(buccopharyngeal membrane).
These cells become intercalated
with hypoblast for a short time, then
they get separated to form a solid
cord of cells called definitive
notochord.
The notochord lies under (ventral)
the neural tube.
It forms the basis of axial skeleton.
Because the migrating cells
intermingle with hypoblast cells,
there is a continuity for a while
between the amniotic cavity and
yolk sac through the neurenteric
canal.
The cloacal membrane is formed at
the caudal end of the embryo.
At both prechordal plate and
cloacal membrane, the mesoderm
is absent.
The notochord disappears, but
remains of it form the nucleus
pulposus of the intervertebral disc.
Growth of the embryonic disc
Cephalic end
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Caudal end
Initially flat and almost round, gradually
becomes elongated.
The cephalic end is broader and
caudal end is narrower.
Expansion takes place at cephalic
region.
Growth and elongation of the
embryonic disc are caused by
continuous migration of cells from the
primitive streak to the cephalic region.
The cell migration takes place up to 4th
week, then it stops because of
disappearance of primitive streak.
Differentiation of germ layers begins at
the middle of 3rd week in cephalic
region, but at 4th week in caudal
region.
Teratogenesis associated with gastrulation
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Caudal dysgenesis (Sirenomelia)
When the gastrulaton is initiated
during 3rd week, the embryo is
highly sensitive for teratogenic insult
(critical period).
Alcohol consumption at this stage
causes holoprosencephaly (fusion
of lateral ventricles). Anterior part of
germ disc affected.
Because the gastrulation takes
place 2 weeks after the fertilization
or 4 weeks after the last menses,
pregnant woman may not know that
she is pregnant- prone to risk
Caudal dysgenesis (Sirenomelia):
-Due to insufficient formation of the
mesoderm at the caudal region.
-Abnormalities are many-fusion of
lower limbs, renal defects, and
vertebral abnormalities.
-Maternal diabetes also causes this
condition in humans.
Tumors associated with gastrulation
• Sometimes, remnants of
primitive streak or primordial
germ cells (Pluripotent)
persist in the
sacrococcygeal region
• Those cells form tumors
known as sacrococcygeal
teratomas
• This is the most common
tumor in newborns
(1/37,000)
• These teratomas contain
tissues derived from all
three germ layers
Sacrococcygeal teratoma
Further Reading
• T. W. Sadler. Langman’s Medical Embryology 12th Edition,
Lippincott Williams & Wilkins. Pages 43-62.