human embryology i
TRANSCRIPT
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Human Embryology I
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Prenatal period or embryogenesis
from fertilization till birth
lasts 280 days
40 weeks
10 lunar months
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Embryogenesis periods
Initial period – the 1st week of development
Embryonic period – from the 2nd week until the 8th weeks
Fetal period – from the 8th weeks up to the birth
an embryo
during the first two months
a fetus
from the third month until birth
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Embryonic development stages
Fertilization
Cleavage
Gastrulation
Germ layer initial differentiation
and axial organ formation
Histogenesis and organogenesis
Embryogenesis is accompanied by
- provisory organ development
- implantation
- placentation
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Initial period – the first week of development
Fertilization
Cleavage
Implantation
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Fertilization is the sequence of events by which a sperm
fuses with an ovum, forming a zygote
occurs in the oviduct ampulla
lasts about 24 h
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Ovum passes into the tubular ampulla by
beating action of cilia
muscular contractions
(stimulated by estrogens)
an ovum
(secondary oocyte)
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Spermatozoa pass into the tubular ampulla by
movements of sperm tails
uterus muscular contractions
(stimulated by seminal plasma prostaglandins)
chemotaxis and reotaxis
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Spermatozoa are reduced in number during the passage
through the female reproductive tract
due to the elimination of abnormal and poorly motile sperms
sperm selection mainly occurs in
the cervical canal
the oviduct isthmus
and
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Capacitation is the enzymatic removal of glycocalyx coat
from the sperm membrane over the acrosome
It takes about 7 hours to capacitate
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Acrosome reaction consists of structural changes
Acrosome membrane fuses at many places with the sperm head plasmalemma
Fused membranes rupture multiple perforations
Enzymes leave the acrosome through perforations
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Membrane fusion in acrosome reaction
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Acrosomal enzymes
facilitate the sperm passage through the ovum envelopes
Hyaluronidase Corona radiata
Acrosin Zona pellucida
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Penetration
Sperm head is attached to the ovum surface
Plasma membranes of the ovum and sperm fuse
and then break down
Sperm nucleus and sperm centriole enter the ovum
cytoplasm
Sperm plasma membrane and the tail
remain outside and degenerate
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Cortical reaction occurs when the first sperm passes
through the zona pellucida
Cortical granules open and release enzymes
Enzymes modify the zona pellucida
Zona pellucida becomes zona fertilization
Zona fertilization is impermeable to sperms
Cortical reaction prevents polyspermy
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Human zygote is synkaryon
because contains two pronuclei
Secondary oocyte completes the meiotic division
Ovum nucleus is the female pronucleus
Sperm nucleus enlarges to form the male pronucleus
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Fertilization completion
Male and female pronuclei
- approach each other
- come in contact
- lose their nuclear membranes
Maternal and paternal chromosomes intermingle
at the metaphase of the first division of cleavage
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Cleavage is a process of successive rapid mitotic divisions
without growth of the daughter cells called blastomeres
Cleavage mitotic divisions lack G1 phase
Cell cytoplasm volume does not increase
Blastomeres become progressively smaller
until they acquire the size of the somatic cells
The normal nucleus to the cytoplasm
volume ratio is restored
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Human cleavage characteristics
Holoblastic (total)
Unequal
Asynchronous
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Early blastomeres retain totipotentiality
blastomere separation development of identical twins
~30%
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Cleavage begins in the oviduct and is completed in the uterus
in the oviduct during the first 3 days
morula
in the uterus for 3 to 4 days
blastula
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Human morula
consists of from 8 to 16 blastomeres
enters the uterus as it is forming
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Human blastula is called blastocyst
Trophoblast
chorion
part of the placenta
Embryoblast
embryo proper
other provisional organs
Blastocele with fluid
Blastocyst is surrounded by the zona pellucida that prevents early implantation
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Duplication of the embryoblast results in twinning
~70%
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Implantation is the blastocyst invasion into the endometrium
begins on the 5th or 6th day after fertilization
Zona pellucida
- disappears
Trophoblast
- attaches itself to the uterine epithelium
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Beginning of implantation
hatching
adhesion
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Trophoblast differentiation
inner layer – cytotrophoblast
outer layer – syncytiotrophoblast
(symplastotrophoblast)
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Syncytiotrophoblast displays high metabolic activity
produces and releases enzymes facilitating the blastocyst invasion
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Syncytiotrophoblast processes invade the uterine mucosa
- epithelium
- stroma
- vessels
- glands
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Blastocyst goes deeper and deeper until the whole of it has
buried itself in the thickness of the endometrium
Invasion lasts about 40 hours
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Decidual cell layer restricts the blastocyst invasion
Invasion stimulates the decidual reaction
endometrial stromal cells
undergo transformation
decidual cells
- large
- pale
- rich in glycogen
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By the 10th day the blastocyst is completely embedded
in the endometrium
Interstitional type of implantation
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For about two days, the endometrium defect is closed
by a closing plug
Consisting of
- blood clot
- cellular debris
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Implantation continues and is completed in the second week
epithelium covers the endometrium defect
by the 12th day of development
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Implantation window is the period
when the uterus is receptive for blastocyst invasion
from the 6th day till the 10th day
after ovulation
Progesterone secreted by the corpus luteum prepares the endometrium for implantation
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Implantation site is an endometrial region
where the blastocyst invasion occurs
midportion of the uterine body
on the posterior wall
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Implantation in the lower uterine segment
results in the placenta previa (placental presentation)
and severe bleeding
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Ectopic (abnormal) implantation
is implantation in other organs
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Implantation in the uterine tube
Decidual reaction fails to occur in the oviduct
Tubular pregnancy ends with the oviduct rupture and bleeding
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Embryonic period - the second week of development
The 1st stage of gastrulation
Formation of the provisory organs
- amniotic vesicle
- yolk sac
- chorion
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The first stage of gastrulation
results in
- bilaminar embryonic disk formation
occurs
- on the 7th day of development
- concurrently with implantation
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Only embryoblast participates in the first stage of gastrulation
Embryoblast
delamination
bilaminar disk
- upper layer – epiblast
- lower layer – hypoblast
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Epiblast gives rise to all three germ layers
Hypoblast does not take part in the embryo body formation
Epiblast
ectoderm
mesoderm
endoderm
Hypoblast
is displaced to extraembryonic regions
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Provisory organ formation
Amniotic vesicle
Yolk sac
- are associated with the embryonic disk
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Epiblast forms the amniotic vesicle floor, hypoblast forms
the yolk sac roof
Amniotic wall
amniotic epithelium
Yolk sac wall
extraembryonic endoderm
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Extraembryonic mesoderm
surrounds the amnion and yolk sac
forms connecting stalk
underlies the trophoblast
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Chorion wall surrounds the chorionic cavity and forms villi
Primary chorionic villi
- cytotrophoblast
- syncytiotrophoblast
Secondary chorionic villi
- extraembryonic mesoderm
- cytotrophoblast
- syncytiotrophoblast
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Trophoblastic lacunar network
Syncytiotrophoblast produces enzymes
Enzymes erode the endometrium, forming the lacunae
Lacunae are filled with maternal blood from ruptured vessels
chorionic villi contact with maternal blood,
providing the mother-embryo exchange
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Embryo associated with the amnion and yolk sac is
suspended in the chorionic cavity by the connecting stalk
connecting stalk attaches the complex
to the inner surface of the chorionic sac
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Second week of development is called “the period of twos”
Embryonic disc
- epiblast
- hypoblast
Vesicles
- amnion
- yolk sac
Trophoblast
- cytotrophoblast
- syncytiotrophoblast
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Human embryology is characterized by
the early development of the provisory organs
Chorion
Amnion
Yolk sac
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Chorion appears in the 2nd week
forms the chorionic sac
consists of villi
- primary
- secondary
- tertiary
is in contact with maternal blood
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Chorion exists up to birth
Chorion functions
- mother-embryo exchange
- nutrient production
- enzyme secretion
(to erode the endometrium)
- immune defence
- hormone release
- placenta formation
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Human chorionic gonadotropin - hCG
Is secreted
- by the syncytiotrophoblast
- into maternal blood
Is excreted
- with maternal urine
Maintains
- corpus luteum activity
- progesterone secretion
• hCG detection in the woman’s urine is a simple, rapid, and an early test of pregnancy
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Amnion appears in the 2nd week of embryonic development
is filled with amniotic fluid
consists of
- amniotic epithelium
- extraembryonic mesoderm
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Amnion exists up to the birth
amnion grows with fetus development
and fills the chorionic cavity
by the 8th week
- surrounds the umbilical cord
- becomes a part of the fetal bladder
or amniochorionic membranes
- underlies the fetal placenta part
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Amnion functions
Amnion provides
- fetus watery environment
- fetus mechanical defence (water cushion)
- fetus body temperature control
- fetus free movements
- fetus muscular-skeletal development
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Yolk sac appears in the 2nd week of embryonic development
consists of
- yolk sac endoderm
- extraembryonic mesoderm
is filled with
- serous fluid
(yolk has been spent in the 1st day of development)
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Yolk sac exists for two months
Its remnant will be a part of the umbilical cord
Yolk sac functions
- primary hemopoiesis
- primary angiogenesis
- primary germ cell (gonoblast) localization
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Provisory organs of identical twins
~70%
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The End
Thank you for attention!