Third to 8th weeks, Embryology

The embryonic period begins with fertilization and extends through what week of development?

8th week

Organogenesis occurs in what weeks of development?

3-8th week

By the end of the embryonic period, what is established?

Organ systems and conceptus has external features characteristic of a fetus

The fetal period begins in what week?

End of the 8th week

this is the time when majority of birth defects are induced:

Teratogenic period

The teratogenic period is considered to occur during what weeks?

1st to 8th week

At the beginning of the 3rd week of development, the ectodermal layer has the shape of a disc. What portion is broader?

Cephalic is broader than caudal region

Appearance of the notochord induces the overlying ectoderm to thicken and form the:

neural plate

Cells of the plate make up the neuroectoderm and their induction is the initial event of:

neurulation

The ectoderm is the:

outermost germ layer

What is derived from the ectoderm?

Nervous system (CNS/PNS)

Sensory epithelium of ears/nose/eyes

Skin, hair, nails, teeth enamel

Pituitary, mammary, and sweat glands

What are the main functions of the ectodermal layer?

Protection, sensation, neural integration

This is the process of forming the neural tube (from the neural plate)

Neurulation

What are the steps of neurulation?

Neural plate formation

Neural groove formation

Neural fold elevation

Fusion

Neurulation occurs when?

begins in the 3rd week, completes in the 4th week

As the neural plate lengthens, what occurs?

1. Lateral edges elevate to form neural folds

2. Depressed midregion forms the neural groove

The neural tube gives rise to the:

brain and spinal cord

In neurulation, fusion begins in what region?

Cervical region (5th somite)

During neurulation, fusion begins in the cervical region and proceeds:

Cranially and caudally to form the neural tube

Until fusion is complete, the cephalic and caudal ends of the neural tube communicate with the amniotic cavity through:

Anterior (cranial) and posterior (caudal) neuropores

The closure of the cranial neuropore occurs on what day?

25 (18-20 somite stage)

Closure of the caudal neuropore occurs on what day?

Day 28 (27 somite stage)

Once the cranial and caudal neuropore are closed, this signifies?

Completion of neurulation

After neurulation is complete, it is a now closed tubular structure with a narrow caudal portion, which will be:

the spinal cord

After neurulation is complete, it is a now closed tubular structure with a broader cephalic portion with number of dilations, which will be:

the brain vesicles

These cells are ectodermal cells at the border of the neural tube.

Neural crest cells

Neural crest cells under go what kind of transition as they leave the neuroectoderm by active migration and displacement to underlying mesoderm?

Epithelial-to-mesenchymal transition

From the trunk, the neural crest cells leave the neuroectoderm after the closure of neural tube and migrate one of two pathways:

Dorsal pathway through dermis

Ventral pathway through anterior half

Explain the mechanism of the neural crest cells and the dorsal pathway.

though the dermis

Enter ectoderm through holes in the basal lamina to form melanocytes of skin

Explain the mechanism of the neural crest cells and the ventral pathway:

Through anterior half of each somite to become:

sensory ganglia

Sympathetic and enteric neurons

Schwann cells

Cells and adrenal medulla

Neural crest cells also migrate from the cranial neural folds, leaving neural tube before closure in this region. They contribute to:

craniofacial skeleton

neurons for central ganglia

Glial cells

Melanocytes and other cell types

NCC’s are important and contribute to many organs, often referred to as:

the 4th germ layer

Neural crest cells are involved in at least 1/3 of all birth defects and many cancers, such as:

melanoma, neuroblastomas

This is caused by the failure of the neural tube to close properly anywhere from cervical region caudally (most common site is lumbosacral region)

Spina bifida

Spina bifida occurs commonly in what region?

Lumbosacral region

When the tube fails to close in the cranial region, absence of the major portions of the brain and skull, causing a lethal defect.

Anencephaly

How can neural tube defects be prevented?

Importance of folic acid (fortification/supplementation)

Cells of the mesoderm initially form thin sheet of loosely tissue on:

each side of the midline

On the 17th day, cells close to midline proliferate and form thickened plate of tissue called the

paraxial mesoderm

The thin, lateral mesoderm layer is known as the:

lateral plate

the lateral plate will coalescence to intercellular cavities, dividing the tissue into two layers:

Somatic/parietal

Splanchnic/visceral

The somatic/parietal mesoderm is continuous with the:

mesoderm covering the amnion

The splachnic/visceral mesoderm layer is continuous with the:

mesoderm covering the yolk sac

the two layers (somatic and splanchnic) line a newly formed cavity, that is continuous with the cavity on each side of the embryo:

Intraembryonic cavity

The intermediate mesoderm connects the:

paraxial and lateral plate mesoderm

From the occipital region caudally, the paraxial mesoderm becomes segmented and organizes into:

somites

In the head- paraxial mesoderm remains:

unsegmented and contributes to the formation of bones/muscles there

The first pair of somites arises in the occipital region of the embryo on what day?

day 20

New somites appear in the craniocaudal sequence at a rate of:

three pairs per day

New somites will appear in the craniocaudal region until when?

At the end of 5th week

By the end of the 5th week, how many pairs of somites are there?

42-44 pairs present

List the pairs of somites:

4 occipital (1st disappears)

8 cervical

12 thoracic

5 lumbar

5 sacral

8-10 coccygeal (5-7 disappear)

The pairs of somites form the:

axial skeleton

Somites appear with specified periodicity, so:

the age of the embryo can be accurately determined by counting

when somites first form, they are formed from:

presomitic mesoderm (balls of mesoderm cells)

The paraxial mesoderm gives rise to:

Somites

Dermatome (dermis)

Myotome (muscle)

Sclerotome (cartilage/bone)

By the beginning of the 4th week, cells in ventral and medial walls of somites lose epithelial characteristics, become mesenchymal again, and shift position to surround the neural tube and notochord. These cells will become:

sclerotome

Sclerotome cells will become the:

vertebra and ribs

Cells at the dorsomedial and ventralateral edges of upper region of the somite form:

precursors for muscle cells

Cells between two groups form:

dermatome

Cells from both the muscle precursor groups become mesenchymal again and migrate beneath the dermatome to create the:

dermomyotome

The dermomyotome cells form the:

dermis for skin

skeletal muscles for the back

Body wall (abs, intercostals)

some limb muscles

Each myotome and dermatome retain its:

innervation

Each somite forms its own:

sclerotome (tendon, cartilage, bone component)

myotome (muscle component)

dermatome (dermis of back)