Third to Eighth weeks Embryonic Period pdf - نظري

Chapter 6

Third to Eighth

weeks:

Embryonic Period

EMBRYONIC PERIOD

– 8

weeks

Period of Organogenesis

• Derivatives of the Ectodermal Germ Layer
• Clinical Correlates
• Derivatives of the Mesodermal Germ Layer
• Clinical Correlates
• Derivatives of the Endodermal Germ Layer
• Patterning of the Anteroposterior Axis:
• Regulation by Homeobox Genes
• External Appearance During the Second Month
• Clinical Correlates

The embryonic period

• Period of organogenesis
• From 3

to 8

weeks of development

• Germ layers: ectoderm, mesoderm & endoderm tissues & organs
• By end of embryonic period (end of 2

month):

main organ systems

are

established

recognizable body form

Derivatives of ectodermal germ layer

NEURULATION

• Notochord and prechordal mesoderm induce the overlying ectoderm to

thicken and form the

neural plate

• Cells of the plate make up the

neuroectoderm

, and their induction

represents the initial event in the process of

neurulation

Molecular regulation of neural induction

BMP4 ( bone morphogenic protein4)

•

Inactivated (absent) BMP4

Ectoderm becomes Neuralized  neural plate

Mesoderm becomes Dorsalized notochord & paraxial mesoderm

Inactivation of BMP4 by secretion of:
o noggin, chordin & follistatin (from organizer, notochord &

prechordal mesoderm)

Which induce formation of: Forebrain & midbrain
o WNT3 & FGF
Which induce formation of hindbrain & spinal cord
In addition, retinoic acid (RA) appears to play a role in organizing the

cranial-to-caudal axis

• Presence of BMP4
Ectoderm  epidermis
Ventralize Mesoderm  intermediate & lateral plate mesoderm

NEURULATION

• Dorsal view of a late presomite

embryo (approximately 19 days).
The amnion has been removed, and
the neural plate is clearly visible.

• Dorsal view of an embryo at

approximately 20 days showing
somites and formation of the neural
groove and neural folds.

• The process whereby the neural plate

forms the neural tube.

Dorsal view of an embryo at
approximately day

22.

Seven

distinct somites are visible on

each side of the neural tube.

Dorsal view of an embryo at
approximately day

. Note the

pericardial bulge on each side of
the midline in the cephalic part of
the embryo.

CLOSURE OF ANT NEUROPORE

- DAY 25

14-somite

embryo

Note the bulging pericardial area
and the first and second
pharyngeal arches .

Closure of post neuropore- day

-somite embryo

The first three pharyngeal

arches and lens and otic

placodes are visible

Completion of neurulation

• By closure of ant & post neuropores the central nervous system is formed:

brain vesicles & spinal cord.

Neural crest formation & migration

Figure 6.5 Formation and migration of neural crest cells in the spinal cord. A,B. Crest cells form at the tips
of neural
folds and do not migrate away from this region until neural tube closure is complete. C. After migration,
crest cells
contribute to a heterogeneous array of structures, including dorsal root ganglia, sympathetic chain
ganglia, adrenal medulla,
and other tissues (Table 6.1, p. 69). D. In a scanning electron micrograph, crest cells at the top of the
closed neural tube can
be seen migrating away from this area.

Crest cells

• Crest cells form at the tips of neural folds and do not migrate away from

this region until neural tube closure is complete.

Neural crest formation & migration

epithelial-t0-mesenchymal transition

In trunk region

• DORSAL PATHWAY:

– Melanocytes in skin

• VENTRAL PATHWAY:

– sensory ganglia,
– Sympathetic
– enteric neurons
– Schwann cells
– Adrenal medulla

Neural crest formation &

migration

epithelial-t0-mesenchymal

transition

In head region

craniofacial skeleton, cranial ganglia

neurons, glial cells, melanocytes

Induction of crest cells

• Intermedite levels of bmp4.

The fate of the entire ectodermal germ layer depends on BMP concentrations:

• High levels induce

epidermis

formation;

• intermediate levels, at the border of the neural plate and surface ectoderm,

induce the

neural crest

;

• and very low concentrations cause formation of

neural ectoderm.

• BMPs also regulate neural crest cell migration, proliferation, and differentiation,
• and abnormal concentrations of the proteins have been associated with neural

crest defects in the craniofacial region of laboratory animals

Otic placodes and the lens placodes

• By the time the neural tube is closed, 2 bilateral ectodermal

thickenings, the otic placodes and the lens placodes,

• The otic placodes invaginate and form the otic vesicles

– Structures for hearing and maintenance of equilibrium

• the lens placodes  the lenses of the eyes

Ectodermal derivatives

contact with outside

•

CNS, PNS

•

Sensory epith.: ear, nose, eyes

•

Epidermis: hair, nails

•

Subcutaneous glands, mammary gl, pituitary, enamel of teeth.

TABLE 6.1 - Neural Crest Derivatives

•

Connective tissue and bones of the face and skull

•

Cranial nerve ganglia

•

Cells of the thyroid gland

•

Conotruncal septum in the heart

•

Odontoblasts

•

Dermis in face and neck

•

Spinal (dorsal root) ganglia

•

Sympathetic chain and preaortic ganglia

•

Parasympathetic ganglia of the gastrointestinal tract

•

Adrenal medulla

•

Schwann cells

•

Glial cells

•

Meninges (forebrain)

•

Melanocytes

•

Smooth muscle cells to blood vessels of the face and forebrain

Neural tube defects (NTDs)

• result when neural tube closure fails to occur.
• If the neural tube fails to close in the cranial region, then most of the brain

fails to form, and the defect is called anencephaly

• If closure fails anywhere from the cervical region caudally, then the defect

is called spina bifida

Examples of neural tube defects

(NTDs), Anencephaly.

Spina bifida

• Most cases of

spina bifida
occur in the
lumbosacral
region

• Seventy

percent of all
of these NTDs
can be
prevented by
the vitamin
folic acid.

Mesoderm derivatives

• Paraxial mesoderm
• Intermediate mesoderm
• Lateral plate mesoderm

Paraxial mesoderm

Somites forming along the neural tube

Somites

• Head region: neuromeres-

mesenchyme of head

• Occipital---caudal region:

somites ...axial skeleton

– 1

pair: occipital region:

day

– 3pairs/day
– End of 5

week: 42-44

somites

• 4occiptal pairs
• 12thoracic pairs
• 5lumbar pairs
• 5sacral pairs
• 8-10 coccygeal pairs

Age of embryo

in days &

Number of

somites

Stages in development of a somite

Somite derivatives

• Each somite: own…
 Sclerotome: tendon, cartilage & bone
 Myotome: segmental muscles
 Dermatome: dermis of back
 Myotome+dermatome: segmental nerve

EXPRESSION PATTERNS OF GENES

THAT REGULATE SOMITE DIFFERENTIATION

Intermediate Mesoderm - urogenital structures

LATERAL PLATE MESODERM
Lateral plate mesoderm splits into parietal
(somatic) and visceral (splanchnic) layers,
Line the intraembryonic cavity and surround the
organs

VASCULOGENESIS

ANGIOGENESIS

BLOOD & BLOOD VESSELS

Blood cells and blood vessels also arise

from mesoderm

EXTRAEMBRYONIC BLOOD
VESSEL FORMATION

Capillary hemangioma

A .Focal capillary hemangioma .B .Diffuse capillary hemangioma involving the oral cavity.

Endoderm derivatives git

Cephalocaudal folding

Lateral folding

Derivatives of

endodermal germ

layers

Endodermal derivatives

• Epith lining of GIT
• Epith of respiratory tract
• Parenchyma of: thyroid, parathyroids, liver, pancreas
• Reticular stroma of thymus & tonsils
• Epith lining of urinary bladder & urethra
• Epith lining of tympanic cavity & auditory tube

Patterning of the antero-posterior axis: regulations of

homeobox genes

• Craniocaudal patterning of the embryonic axis is controlled by

homeobox

genes.

• These genes are conserved from Drosophila are arranged in

four clusters

HOXA

HOXB

HOXC

, and

HOXD

, on four different chromosomes.

• Genes toward the

3’ end

of the chromosome control development of more

cranial

structures

• Those toward the

5’ end

regulate differentiation of more

posterior structures

• Together they regulate patterning of the hindbrain and the axis of the embryo.

External appearance during

the second month

• End of 4

weeks: 28 somites-external

features: somites+pharyngeal arches

Second month

Age: crown-rump length (CRL) in mm

• Increase in head size
• Formation of: limbs, face, ears, nose, eyes

Crown-Rump Length

Limbs formation

Human embryo (CRL 21 mm, seventh
week) (X4). The chorionic sac is open to
show the embryo in its amniotic sac.
The yolk sac, umbilical cord, and vessels
in the chorionic plate of the placenta are
clearly visible. Note the size of the head
in comparison with the rest of the body .

Human embryo (CRL 25 mm, seventh to eighth

weeks). The chorion and the amnion have been

opened. Note the size of the head, the eye, the

auricle of the ear, the well-formed toes, the

swelling in the umbilical cord caused by intestinal

loops, and the yolk sac in the chorionic cavity .