Download Primary yolk sac

Embryology
embryology - The branch of biology
that deals with the formation,
embryonic structure, early growth, and
development of living organisms.
• embryo - The initial product of conception from
implantation at about two weeks after
fertilization through the eighth week of
development.
• fetus - The unborn young from the end of the
eighth week after conception to the moment of
birth, as distinguished from the earlier embryo;
by this stage the developing unborn individual
has taken on a recognizable form with all parts in
place, tissues and organs differentiating.
Human prenatal development is divided into an
embryonic period and a fetal period
•
•
embryonic period - the embryonic period begins with fertilization and ends eight
weeks later; it involves the development of transient structures such as the three
primary embryonic tissues which establish body plan, organogenesis and placental
tissues, and their initial folding and form.
fetal period - from the ninth week to the fortieth week, approximately, or until
delivery, is a time of extensive growth in size and mass as well as ongoing
differentiation of organ systems established in the embryonic period.
PREGNANCY:
• Involves
• the transport of the
sperm and egg,
• fertilization,
• development of the
Zygote into a Blastocyst,
• implantation into
endometrium,
• development of the
placenta,
• development of the
fetus,
• parturition or birth at
the end of 9 months.
Germ cell and fertilization
ovum
fertilized ovum
sperm
Fertilization: The process in which the spermatozoon penetrates
into the ovum to form fertilized ovum.
In the ampulla of oviduct
Process of fertilization
① Fusion of the
membrane of
sperm and ovum
② The nucleus
of sperm
penetrate into
ovum
③ Formation of
pronuclei
Alteration of ovum
④ Fusion of
pronuclei
24 h
Spermatozoon
Capacitation
In female reproductive tract, the sperms were enabled to bind to the zona pellucida
receptors. Capacitation- removal of glycoproteins (decapacitation factor) which cover
the sperms
Process of fertilization
Sperm bind to sperm receptor ZP-3 induce
Acrosome reaction：release of acrosomal enzyme
Cortical reaction: Cortical granules→perivitelline
space →degrade ZP-3, alteration of zona pellucida →barrier
for sperm penetration
Ovum -
secondary oocyte arrested at
metaphase in the second meiotic division
The second meiotic
division of the
secondary oocyte is
rapidly lifted and the
second polar body is
released, leaving a
haploid female nucleus.
The female and male pronuclei
fuse, forming the diploid and 2N
nucleus of the fertilized zygote.
This is the zero time pointof
embryonic development. Ploidy
refers to the number of copies of
each chromosome present in the
nucleus. N number refers to the
number of copies of each unique
double-stranded DNA molecule
in the nucleus
Formation of blastocyst and implantation
Cleavage and
formation of
blastocyst
1. Cleavage： early
division of fertilized
egg
2. Blastomere：
daughter cells from
cleavage
3. Morula：12 to 16cell stage, enclosed in
the zona pellucida, like
morus
4. Blastocyst：about
100 blastmeres,
blastocoele, inner cell
mass and trophoblast
First days of development.
As the zygote travels down the oviduct it undergoes cleavage without increasing
its size. This subdivides the large zygote into many smaller daughter cells called
blastomeres. 24 Hours: First cleavage. 48 Hours: Second cleavage.
3 Days: Embryo consists of 6 to 12
cells, reorganization (compaction:
Uvomorulin, a glycoprotein is
involved in compaction) of the
blastomeres, starts at the 8 cell
stage. The centrally placed
blastomeres are now called the
inner cell mass (they give rise to
most of the embryo proper, which
is also called the embryoblast). The
blastomeres at the periphery
constitute the outer cell mass, they
are the primary source for the
membranes of the placenta. It is
also referred to as the trophoblast
4 Days: Embryo consists of 16 to 32 cells called
morula.
Phase of cleavage
Fertilized
egg
2-cell
stage
18~36h
4-cell
stage
36~48h
8-cell
stage
48~60h
Morula
5 Days: The embryo is now
called a blastocyst.
A large cavity called the
blastocyst cavity forms due
to the hydrostatic pressure.
The embryoblast cells form a
compact mass at one side of
the cavity (embryonic pole),
while the trophoblast is
organized into a thin, singlelayered epithelium
Appearance and inner structure of blastocyst
Appearance
Inner structure
Implantation (imbed)
The process by which the blastocyst settles
into endometrium. 6th day to 11th day
Fundus and body of uterus
zona pellucida disappear at the 5th day
1.Trophoblast：proliferate and differentiate into two layers
①Syncytiotrophoblast：outer layer, fused each other, cell
boundaries disappear
②Cytotrophoblast：inner layer， cell boundaries distinct, simple
cuboidal cells
• 6 Days: Blastocyst implants into the
uterine wall.
• The blastocyst hatches from the zona
pellucida before implanting.
• The trophoblast at the embryonic
pole differentiates to produce the
syncytiotrophoblast, and begins to
implant the blastocyst into the
uterine endometrium.
• Some of the proliferating trophoblast
cells lose their membranes and form
a syncytium. The trophoblast cells,
which form the wall of the
blastocyst, retain their cell
membranes and constitute the
cytotrophoblast.
Implantation at 7th day
Cytotrophoblast
Inner cell mass
Polar trophoblast
Endometrium
Decidua - endometrium implantation decidua
Decidua capsularis：
between the implantation
site and the uterine
lumen
Decidua parietalis：
remaining endometrium
Decidua basalis：under
the implantation site
Ectopic pregnancy results
when a blastocyst implants
in the peritoneal cavity on
the surface of the ovary,
within the oviduct, or at an
abnormal site in the uterus.
Because the blood vessels at
abnormal implantation sites
are apt to rupture, ectopic
pregnancy is often revealed
by symptoms of abdominal
pain and/or vaginal bleeding
Day 7
Formation of germ layer
Bilaminar germ disk：
Inner cell mass→2 layers of
cell in disc (7th day)
Epiblast: columnar cells
adjacent to trophoblast
Hypoblast: cuboidal cells
adjacent to blastocoele
• Day 8
• As implantation progresses, the
expanding syncytiotrophoblast
gradually envelops the
blastocyst.
• Day 9
• A layer of epiblast cells is
gradually displaced toward the
embryonic pole by
accumulating fluid. These cells
differentiate into amnioblasts,
which form the amniotic
membrane. The newly formed
cavity is called the amniotic
cavity
• hypoblast →extraembyronic
endoderm →primary yolk sac
Amnion: epiblast → amnioblast → aminiotic
membrane → amniotic cavity → amniotic fluid
Primary yolk sac： hypoblast →extraembyronic
endoderm →primary yolk sac
Bilaminar germ disc
Decidua
Syncytiotrophoblast
Epiblast
Hypoblast
Primary
yolk sac
Cytotrophoblast
Amniotic
cavity
• Day 12
• The growth and migration
of the extra embryonic
mesoderm gradually
separates the amnion from
the cytotrophoblast.
• On day 12, a second wave
of proliferation in the
hypoblast produces a new
membrane that migrates
out over the inside of the
extraembryonic
mesoderm, pushing the
primary yolk sac in front of
it. This new layer becomes
the endodermal lining of
the definitive (secondary)
yolk sac
Extraembryonic mesoderm:
10th-11th day
Extraembryonic cavity: 12th13th day
Extraembryonic mesoderm:
visceral layer
parietal layer
Secondary yolk sac: 13th day
Body stalk: 14th day
Significance of
primitive streak
Primitive
node
Determine
the
Primitive
pit
direction of the
Primitive
groove
embryo
Primitive
streak
notochord
notochord
• Induction of the neural plate.
• The axial mesoderm & notochord induce the overlying
ectoderm to form the neural plate.
• Neural Crest.
• During neurulation neural
crest cells begin to migrate
in a craniocaudal wave
• Neural crest cells are an
extremely important
population of cells that
migrate into the embryo to
form a variety of structures
Differentiation of mesoderm
•
•
•
•
17th day
Paraxial, intermediate, and lateral plate mesoderm.
The paraxial mesoderm gives rise somites.
Intermediate Mesoderm - urinary system and genital system
Lateral Plate Mesoderm
Somite
• Somites subdivide
into:
– 1.Dermatomes:
form the dermis of
the scalp, neck and
trunk.
– 2.Myotomes: form
the segmental
musculature of the
back and the
anterolateral body
wall.
– 3.Sclerotomes:
surround the
notochord and
neural tube and
eventually form the
vertebrae and ribs
Differentiation of lateral mesoderm
Lateral Plate Mesoderm splits into
1. the splanchnopleuric mesoderm(visceral mesoderm) becomes the mesothelial
covering of the visceral organs
2. and somatopleuric mesoderm(somatic mesoderm) and it gives rise
- to the lining of the body wall,
- parts of the limbs,
- and dermis
Parietal mesoderm
Intraembryonic
coelomic cavit
Visceral mesoderm
• The intermediate mesoderm is located on either side of the
somites and medial to the lateral plate mesoderm. Originally
organized as cords the first kidney that is established is the
pronephric kidney
Origin of the germ line.
Migration of the primordial germ cells:
1. Cells that give rise to the gametes originate within the primary ectoderm during
the second week of development. They then detach from the ectoderm and
migrate into the yolk sac.
2. During the fourth week, the primordial germ cells migrate into the posterior body
wall of the embryo from the yolk sac.
3. The germ cells then induce the adjacent coelonic epithelium and mesonephros to
proliferate and form the primitive sex cords. This creates a swelling (genital
ridges/primordial gonads) medial to each mesonephros on either side of the
vertebrae column.
Formation of primitive gut
Differentiation of endoderm
Epithelium of primitive gut → epithelium of digestive tract,
respiratory tract and lung
Primitive pharynx →5 pairs of pharyngeal pouch
A
B
digestive gland,
At the end of 8th week—
a small
human
Fetal membrane and placenta
Fetal membrane





Chorion
Amnion
Yolk sac
Allantois
Umbilical cord
Primary stem villus：at 2nd week，cytotrophoblast
Secondary stem villus：at 3rd week，extraembryonic mesoderm
Tertiary stem villus：at the end of 3rd week，blood vessels
Evolvement of chorion
Chorion laeve：adjacent to decidua capsularis
Chorion frondosum： adjacent to decidua basalis
6th weeks
Chorion laeve
Chorion frondosum
amniotic cavity - The amniotic fluid-filled cavity
surrounding the developing embryo/fetus within
the uterus.
Amnion：
amniotic membrane: 0.2-0.5mm
amniotic epithelium + extraembryonic mesoderm
amniotic fluid: 500-1000ml
Yolk sac
Yolk sac：5th-6th week, atresia
Abnormality:
① Meckel’s diverticulum
② Umbilical fistula
① Primitive blood cells
② Primordial germ cells
Umbilical cord - The flexible cordlike structure connecting a fetus at the abdomen
with the placenta and containing two umbilical arteries and one vein which transport
nourishment to the fetus and remove its wastes; it is derived from the chorion and
the allantois, the outer two extraembryonic membranes.
Umbilical cord：50 cm
• (CT, umbilical vein,
umbilical artery,
degenerated yolk sac and
allantois)
• covered with amniotic
membrane
• connect fetus with
placenta
Placenta
1.Placenta: Chorion frondosum + Decidua basalis
disk: 15-20cm
Sandwich: chorinic plate (fetus)
villi
basal plate (mother)
basal plate: decidua + cytotrophoblastic shell
placental septa → cotyledon (incomplete)
Umbilical vein
Umbilical artery
Decidua bsalis
Placenta septa
Uterus vein
Spiral artery
Placental membrane：
Structure between the blood of mother and fetus
①Syncytiotrophoblast；
②Cytotrophoblast and its basal lamina；
③Connective tissue；
④Endothelium and its basal lamina
protection
Placenta membrane
Syncytiotrophoblast
Cytotrophoblast
Blood of
mother
Nutriment
Metabolic product
胎
儿
血
Basal lamina
Connective tissue
Basal lamina
Endothelium
Function：
① Substance exchange
O2, nutriment,
CO2, waste
② Hormone
Human chorionic gonadotropin, HCG;
Human placental lactogen, HPL;
Human placental progesterone, HPP,
Human placental estrogen, HPE
Placenta
Function
① Substance exchange
Capillary of fetus
Blood of mother
Ｏ2
Nutriment
water、salt、others
water、salt、others
ＣＯ2
Metabolic waste
Placenta
Placenta
Function
② Barrier
Capillary of fetus
Blood of mother
s
Most of
microorganism
No way！
Placenta barrier
Placenta
1. Human chorionic gonadotropin - HCG;
2. Human placental lactogen – HPL or human
chorionic somatomammotropin (HCS), ;
3. Human placental progesterone - HPP,
4. Human placental estrogen - HPE
Function
③ Endocrine
Hormone
HCG
HCS
Function
Corpus luteum
Pregnancy
Mammary
gland
Appearance
Fastigium
3rd week
8th week
2nd month
8th month
HPE
Corpus luteum
4th month
8th month
HPP
Corpus luteum
4th month
8th month