Human Embryology I

Prenatal period or embryogenesis

from fertilization till birth

lasts 280 days

40 weeks

10 lunar months

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

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

Initial period – the first week of development

Fertilization

Cleavage

Implantation

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

Ovum passes into the tubular ampulla by

beating action of cilia

muscular contractions

(stimulated by estrogens)

an ovum

(secondary oocyte)

Spermatozoa pass into the tubular ampulla by

movements of sperm tails

uterus muscular contractions

(stimulated by seminal plasma prostaglandins)

chemotaxis and reotaxis

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

Capacitation is the enzymatic removal of glycocalyx coat

from the sperm membrane over the acrosome

It takes about 7 hours to capacitate

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

Membrane fusion in acrosome reaction

Acrosomal enzymes

facilitate the sperm passage through the ovum envelopes

Hyaluronidase Corona radiata

Acrosin Zona pellucida

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

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

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

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

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

Human cleavage characteristics

Holoblastic (total)

Unequal

Asynchronous

Early blastomeres retain totipotentiality

blastomere separation development of identical twins

~30%

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

Human morula

consists of from 8 to 16 blastomeres

enters the uterus as it is forming

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

Duplication of the embryoblast results in twinning

~70%

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

Beginning of implantation

hatching

adhesion

Trophoblast differentiation

inner layer – cytotrophoblast

outer layer – syncytiotrophoblast

(symplastotrophoblast)

Syncytiotrophoblast displays high metabolic activity

produces and releases enzymes facilitating the blastocyst invasion

Syncytiotrophoblast processes invade the uterine mucosa

- epithelium

- stroma

- vessels

- glands

Blastocyst goes deeper and deeper until the whole of it has

buried itself in the thickness of the endometrium

Invasion lasts about 40 hours

Decidual cell layer restricts the blastocyst invasion

Invasion stimulates the decidual reaction

endometrial stromal cells

undergo transformation

decidual cells

- large

- pale

- rich in glycogen

By the 10th day the blastocyst is completely embedded

in the endometrium

Interstitional type of implantation

For about two days, the endometrium defect is closed

by a closing plug

Consisting of

- blood clot

- cellular debris

Implantation continues and is completed in the second week

epithelium covers the endometrium defect

by the 12th day of development

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

Implantation site is an endometrial region

where the blastocyst invasion occurs

midportion of the uterine body

on the posterior wall

Implantation in the lower uterine segment

results in the placenta previa (placental presentation)

and severe bleeding

Ectopic (abnormal) implantation

is implantation in other organs

Implantation in the uterine tube

Decidual reaction fails to occur in the oviduct

Tubular pregnancy ends with the oviduct rupture and bleeding

Embryonic period - the second week of development

The 1st stage of gastrulation

Formation of the provisory organs

- amniotic vesicle

- yolk sac

- chorion

The first stage of gastrulation

results in

- bilaminar embryonic disk formation

occurs

- on the 7th day of development

- concurrently with implantation

Only embryoblast participates in the first stage of gastrulation

Embryoblast

delamination

bilaminar disk

- upper layer – epiblast

- lower layer – hypoblast

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

Provisory organ formation

Amniotic vesicle

Yolk sac

- are associated with the embryonic disk

Epiblast forms the amniotic vesicle floor, hypoblast forms

the yolk sac roof

Amniotic wall

amniotic epithelium

Yolk sac wall

extraembryonic endoderm

Extraembryonic mesoderm

surrounds the amnion and yolk sac

forms connecting stalk

underlies the trophoblast

Chorion wall surrounds the chorionic cavity and forms villi

Primary chorionic villi

- cytotrophoblast

- syncytiotrophoblast

Secondary chorionic villi

- extraembryonic mesoderm

- cytotrophoblast

- syncytiotrophoblast

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

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

Second week of development is called “the period of twos”

Embryonic disc

- epiblast

- hypoblast

Vesicles

- amnion

- yolk sac

Trophoblast

- cytotrophoblast

- syncytiotrophoblast

Human embryology is characterized by

the early development of the provisory organs

Chorion

Amnion

Yolk sac

Chorion appears in the 2nd week

forms the chorionic sac

consists of villi

- primary

- secondary

- tertiary

is in contact with maternal blood

Chorion exists up to birth

Chorion functions

- mother-embryo exchange

- nutrient production

- enzyme secretion

(to erode the endometrium)

- immune defence

- hormone release

- placenta formation

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

Amnion appears in the 2nd week of embryonic development

is filled with amniotic fluid

consists of

- amniotic epithelium

- extraembryonic mesoderm

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

Amnion functions

Amnion provides

- fetus watery environment

- fetus mechanical defence (water cushion)

- fetus body temperature control

- fetus free movements

- fetus muscular-skeletal development

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)

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

Provisory organs of identical twins

~70%

The End

Thank you for attention!