Human Embryology II

Embryonic period – the third week of development

The 2nd stage of gastrulation Germ layer initial differentiation and axial organ formation Primitive cardiovascular system formation Subsequent chorion development Allantois appearance Folding

The second stage of gastrulation

results in

- trilaminar embryonic disk formation occurs

- on the 14th to 15th day of development

Only epiblast participates in the 2nd stage of gastrulationHypoblast does not take part in the embryo body formation

Epiblast gives rise to embryonic

- ectoderm

- endoderm

- mesoderm

Hypoblast is displaced to

- extraembryonic regions

Primitive streak is the key structure of the 2nd stage of gastrulation

Epiblastic cells at the disk cranial end

- proliferate

- migrate along the disk margins

- converge at the disk caudal end

- turn back to the disk cranial end

towards the midline

⇓ primitive streak ⇒

cranial end

caudal end

Primitive streak anterior portion thickens to form the primitive knot or Hensen’s nodule

Primitive groove

- develops in the primitive streak

- is continuous with the primitive pit

in the primitive knot

⇔

Primitive streak is a source of the embryonic mesoderm and embryonic endoderm

Primitive streak cells migrate ⇒

- into the primitive groove

- inwardly between the epiblast and hypoblast

early-migrating cells ⇒ endoderm

later-migrating cells ⇒ mesoderm

Gastrulation is completed with the trilaminar disk formation

As soon as the primitive streak gives rise to

- embryonic endoderm

- embryonic mesoderm

remaining epiblastic cells are referred to as

- embryonic ectoderm

⇔

Duplication of the primitive streak results in twinning

Удвоение

⇐ duplication of the primitive streak

⇐ monochorial monoamniotic twins

~30% ~4%~70%

Conjoint twins (~1% of monozygotic twins) result from the primitive streak duplication

partial duplication of the primitive streak (Y-shaped)

⇓

complete duplication of the primitive streak but incomplete duplication of the germ layers

⇓

bifurcation of the spinal cord and vertebral column fusion of soft tissues (Siamese twins)

Siamese twins

may be separated surgically

⇒

Initial germ layer differentiation and the axial organ formation

Complex of the axial organs includes

- notochord

- neural tube

- mesodermal somites

Notochord is the first to appear concurrently with mesoderm

Primitive pit

- extends into the primitive knot

- forms the notochordal canal Primitive knot cells

- migrate through the canal

- give rise to the notochord

Notochordal process looks like a cellular rod

extends

- cranially from the primitive knot

- between the ectoderm and endoderm

wing-like mesoderm is on

each side of the notochord⇐

⇒

Notochord forms the embryo midline axis around which the vertebral column develops

Notochord

- disappears where it becomes surrounded by the vertebral bodies

- persists as the nucleus pulposus of the intervertebral disks

- induces the overlying ectoderm to form the neural plate

⇒

Neurulation or the neural tube formation is induced by the notochord with the adjacent mesoderm

Stages of the neural tube development neural plate (15 – 16 days) neural groove and neural folds (18 – 21 days) neural tube (23 – 25 day)

⇔

Neuroectoderm includes the neural tube and neural crest

Neural tube⇓

BrainSpinal cordRetinaOlfactory epithelium

Neural crest⇓

Neural gangliaPia mater and arachnoidSkin melanocytesAdrenal medullaThyroid gland C-cells

Surface ectoderm remains after the neural tube separation

Gives rise to

- skin epidermis

- sweat and sebaceous glands

- nails and hair

- mammary glands

- salivary glands

- tooth enamel

- oral cavity epithelium

- corneal epithelium

Mesoderm subdivision

Paraxial mesoderm ⇒ somites

- myotome

- dermotome

- sclerotome Intermediate mesoderm (somite cord) ⇒ nephrogonadotome Lateral mesoderm ⇒ parietal layer or somatopleure

visceral layer or splanchnopleure

⇐ coelom in the lateral mesoderm

Subsequent mesoderm differentiation

Myotome ⇒ skeletal muscles Dermatome ⇒ skin dermis Sclerotome ⇒ bones and cartilages

Nephrogonadotome ⇒ kidney and gonads Coelom ⇒ - pericardial

- pleural

- peritoneal cavities Somatopleure ⇒ mesothelium Splanchnopleure ⇒ - mesothelium

- myocardium

- epicardium

- adrenal cortex

⇒

Some mesodermal cells migrate and become mesenchyme

Mesenchyme gives rise to

- blood

- blood and lymphatic vessels

- all types of connective tissue

- smooth muscle cells

- microglial cells

- endocardium

Embryonic endoderm differentiation

Gastrointestinal tract epithelium Pancreas parenchyma Liver parenchyma Gallbladder epithelium Lung epithelial parts

Primitive cardiovascular system formation

Angiogenesis begins in the provisory organs

- yolk sac

- connecting stalk

- chorion

Embryonic vessels begin to develop about two days later

embryonic vessels and primitive heart

arise from the mesenchyme

Angiogenesis and hemopoiesis occur concurrently

Primitive blood cells

- differentiate from mesenchyme

- inside the embryonic vessels

⇓ intravascular hemopoiesis

Cardiovascular system is the first system to attain a functional state - by the end of the 3rd week

сhorionic and embryonic vessels

become connected via the connecting stalk

⇔

Chorionic villi become tertiary villi

Composition of tertiary villi

- syncytiotrophoblast

- cytotrophoblast

- extraembryonic mesoderm

- chorionic blood vessels ⇓

Chorionic villi provide maternal-fetal blood exchange

are bathed by maternal blood from lacunae

Chorionic villi are

- stem or anchoring villi

- branch villi

Allantois appears on the 16th day of embryonic development

is a finger-like projection

- of an embryo endoderm

- into the connecting stalk

Allantois exists for two monthsIts remnant will be a part of the umbilical cord

Allantois is involved in

- blood formation

- angiogenesis

- the urinary bladder development

Folding – the body fold formation

begins on the 21st day of development

There are two pairs of folds

- longitudinal folds

- transversal folds

Longitudinal – cranial and caudal - folds

convert the embryonic disk into C-shaped structure

⇓

Transversal folds

include

- surface ectoderm

- somatopleure

- splanchnopleure

- endoderm move down to meet each other converge below the embryo body

Transversal fold results

embryonic disk is converted to

a cylinder-like structure

⇔

Folding consequences

Embryo acquires

- cylindrical C-like body shape

- primitive gut . . . and separates

- from the yolk sac

Embryonic period from the 4th to the 8th weeks

All tissues and organs differentiate, develop,

and begin to function

The period is the most critical period of embryogenesis because

any disturbances may give rise to congenital malformations

7th week embryo

⇔

Placentation or placenta development

begins after implantation is completed by the 8th week

⇔

Embryo by the 8th week – the end of the embryonic period

is disposed in the amnion and bathed by amniotic fluid amnion fills the chorionic cavity amniotic wall underlies the chorion

chorion fills the uterine cavity uterine cavity obliterates umbilical cord connects the embryo and chorion

⇒

⇐

Umbilical cord arises from the connecting stalk

contains

- two arteries

- a vein

- mucoid connective tissue

- remnants of the yolk sac and allantois is covered by amniotic epithelium

Umbilical cord functions to connect

fetal cardiovascular system with chorionic vessels

Endometrium in pregnancy is called the decidua graviditas

Decidua basalis

- underlies the implantation site Decidua capsularis

- covers the implantation site Decidua parietalis

- remaining endometrium

Endometrium by the 8th week of development

Decidua basalis

- takes part in placenta formation Decidua capsularis

- fuses with decidua parietalis

when the uterine cavity obliterates

Chorion by the 8th week of development is subdivided into

Smooth chorion

- almost lacks villi

- is associated with the decidua capsularis

Villous chorion

- possesses large and branched villi

- is associated with the decidua basalis

Smooth chorion is one of the amniochorionic membranes

that form the fetal bladder wall

- amnion wall

- smooth chorion

- decidua capsularis

Villous chorion or chorion frondosum

takes part in placenta formation

Placenta is a combined organ

is formed by

- maternal body – the decidua basalis

- fetal body – the villous chorion

Two placental parts are involved in

the maternal-fetal circulation exchange ⇒

Human placenta is discoid in shape

It is determined by the circular form of the villous chorion

fetal part ⇒

⇐ maternal part

placenta after parturition

Fetal part of placenta

Chorionic plate Tertiary villi

Umbilical cord is attached to the fetal surface Amniotic epithelium surrounds the umbilical cord

and covers the fetal placenta part

Chorionic plate

is a layer of extraembryonic connective tissue contains blood vessels coming from the umbilical cord is covered with amniotic epithelium

gives rise to chorionic villi

⇐

Chorionic villi arise from the chorionic plate

project into intervillous spaces or lacunae are bathed by maternal blood

⇔

Chorion attaches itself to the decidua basalis

Stem or anchoring villi

- are attached to the endometrium

- form cytotrophoblastic shell at the site

of attachment

Branch or floating villi

- arise from the stem villi

- float in lacunae with maternal blood

- provide the main fetoplacental exchange

Chorionic villi are tertiary villi

syncytiotrophoblast cytotrophoblast extraembryonic mesoderm villous blood vessels

villi are bathed by the maternal blood

⇐

Fibrinoid material arises from the decidua basalis necrosis

results from the syncytiotrophoblast enzyme activity contains fibrin and immunoglobulins covers villi and the endometrium separates the fetal tissues from maternal tissues takes part in immune defence

Maternal part of placenta

Decidua basalis with decidual cells Placental septa Lacunae with maternal blood

Decidua basalis

is a layer of the lamina propria connective tissue contains ruptured blood vessels and gland remnants

is underlined by the decidual plate

- remains after parturition

- is involved in the endometrium regeneration

⇔

Decidual cells result from decidual reaction

arise from the endometrial stromal cells are rich in glycogen and lipids

Functions

- restrict the trophoblast invasion

- provide some nourishment for the embryo and fetus

- create a layer of the placenta separation in parturition

- secrete hormone relaxin responsible for the cervix softening ⇑

⇒

Placental septa are wedge-like areas of the endometrium

project from the decidua basalis to the chorionic plate

(never attach themselves) divide placenta into 15 to 20 lobules – cotyledons

Cotyledon includes

- two or more stem villi

- numerous branch villi

Cotyledons are visible on the placenta maternal surface

⇐maternal part

fetal part

placenta after parturition

Lacunae or intervillous spaces

result from syncytiotrophoblast enzymatic activity contain maternal blood surround chorionic villi communicate with each other

⇔

Uteroplacental circulatory system

Maternal blood

- enters the placenta through ruptured spiral arteries

- flows into the intervillous spaces

- circulates in the intervillous spaces

- leaves the placenta through the endometrial veins

- brings O2 and nutrients

- carries away CO2 and waste products

Fetal blood enters the placenta through

paired umbilical arteries

⇓ chorionic plate vessels

⇓ chorionic villus vessels

⇓ a single umbilical vein

capillary network in the small branch villi

- is in close association with maternal blood-filled lacunae

- takes part in fetal-maternal blood exchange

⇐

Placental barrier separates fetal blood and maternal blood

includes only fetal tissues

- syncytiotrophoblast

- cytotrophoblast

- trophoblastic basal membrane

- villous connective tissue

- villous capillary wall

⇐ fetal blood never mixes with maternal blood

⇒

Placental barrier ultrastructure Syncytiotrophoblast Cytotrophoblast Trophoblastic basal membrane Endothelium basal membrane Endothelial cells

⇓

⇒ ⇓

⇓⇑

Placental barrier by the last trimester of pregnancy becomes very thin, facilitating maternal-fetal exchange lacks

- cytotrophoblast (degenerates)

- connective tissue (disappears)

includes ⇑- syncytiotrophoblast

- villous capillary wall

⇒

Placenta functions

Selective fetal-maternal blood exchange

- gases, water, electrolytes

- nutrients, hormones, antibodies

- medicine, drugs, infection agents Synthesis of some nutrients

- glycogen, cholesterol, fatty acids Release of enzymes to erode the endometrium Hormone production

- progesterone, estrogens,

- human chorionic gonadotropin (hCG)

- human placental lactogen (hPL)

- relaxin hCG in the villus syncytiotriophoblast

⇑

The End

The End

Thank you for attention!