Third Week of Development, Embryology

This is the process of transforming bilaminar germ disc into a trilaminar structure.

Gastrulation

Gastrulation will occur to form what three primary germ layers?

1. Ectoderm

2. Mesoderm

3. Endoderm

The formation of the three primary germ layers begins in what week of development?

Begins at the 3rd week of development

At the end of the second week of development, what occurs to the germ disc?

1. Amniotic cavity opens

2. Hypoblast and epiblast in contact

3. The primitive streak forms a shallow groove in the caudal region of the embryo

4. The oropharyngeal membrane is now at the end of the disc, which will be the future opening cavity

Where does the cloacal membrane form?

Forms at the end of the caudal end of the embryonic disc

The cloacal membrane will give rise to:

urogenital and anal openings

The cloacal membrane is similar in structure to the oropharyngeal membrane because:

it is tightly adherent to the ectoderm and endoderm cells with no mesoderm

This is the thickened line on the epiblast surface

primitive streak

What is the main function of the primitive streak?

It defines the major body axes and initiates the cell migration for germ layer formation

This is a circular structure at the cranial end of the primitive streak (PIT)

primitive node

during gastrulation, the epiblast cells migrate and go in through the primitive streak, the cells will displace the hypoblast to form the:

Endoderm

During gastrulation, the epiblast cells will migrate toward and in through the:

primitive streak

During gastrulation, cells spreading between the epiblast and newly formed endoderm will become the:

Mesoderm

During gastrulation, remaining epiblast cells after the migration process form the:

ectoderm

Completion of the trilaminar germ disc has what:

3 layers (ectoderm, mesoderm, endoderm)

This is a rod-like structure that is crucial for the development of the axial skeleton and neural tube.

Notochord

The notochord is formed from:

cells migrating from the primitive node to to the cranial end

The notocord will form the:

midline axis, underlies neural tube, serves as a signaling center for inducing axial skeleton

During the formation of the notochord, the epiblast cells will move:

laterally and cranially

In a cephalic direction, the epiblast cells move from pit toward the prechordal plate, which the epiblast cells will:

displace the hypoblast cells

The anterior-posterior (A-P) can be known as the :

cranial-caudal axis

the establishment of body axes determines the:

body plan of the embryo

What are the 3 body axes?

A-P

D-V

L-R

when does the establishment of the body axes occur?

In early embryogenesis, initiated during late morula to blastocyst stages (L-R is LAST)

The fate map is established during gastrulation, which predicts?

The future development of different embryo regions

This layer will give rise to epithelium, nervous tissue, nails, and hairs

Ectoderm

This layer will give rise to blood, muscle, bones, and connective tissue

Mesoderm

this layer will give rise to the epithelial lining of the gut and respiratory system, and other internal organs

Endoderm

What causes holoprosencephaly?

Genetic mutations, environmental factors

What is holoprosencephaly?

the failure of the forebrain to divide into two hemispheres

What are the consequences of holoprosencephaly?

Facial anomalies

intellectual disabilities

What are the characteristics of the holoprosencephaly?

deficiency of the midline

fused eyes (synophthalmia)

Proboscis with single nasal openings (fusion of the lateral nasal processes)

This disorder is known as ‘mermaid syndrome’, where the legs are fused.

Sirenomelia

What causes sirenomelia?

Failure to form mesoderm by gastrulation in the lumbosacral region, resutling in the fusion of the limb buds and other defects

What are the consequences of sirenomelia?

Severe malformations, often incompatible with life

This tumor is associated with gastrulation and is the most common tumor in newborns (1 in 37,000)

Sacrococcygeal teratoma

What causes sacrococcygeal teratoma?

Remnants of primitive streak persists in the sacrococcygeal region. the clusters of pluripotent cells proliferate and form tumors, often containing tissues derived from all three germ layers

This is the complete reversal of organ replacement in the body.

Situs inversus

What causes situs inversus?

Genetic factors affecting left-right axis establishment

The embryonic disc will change shape from oval to more elongated, with the:

broad cephalic and narrow caudal end

The expansion of the embryonic disc mainly occurs in what region?

Cephalic region

What causes the growth/elongation of the cephalic portion of the disc?

continuous migration of the cells from primitive streak region in the cephalic direction

By the end of the 4th week, what occurs to the primitive streak?

The streak will shrink and disappear

Gastrulation will continue in the caudal segments by the end of the 4th week, while what occurs in the cranial portion?

The cranial segments are differentating, causing the embryo to develop ‘cephalocaudally’

What is the trophoblasts role in development?

Provide nutrients and support to the embryo

The trophoblast will eventually differentiate into the:

primary, secondary, and tertiary villi

The trophoblast will differentiate into the primary, secondary and tertiary vilili, which will establish what organ?

Placenta

this villi is made from cytotrophoblast cells that proliferate into the syncytiotrophoblast.

primary villi

this villi is from the extraembryonic mesoderm that generates the core primary villi (mesoderm core)

Secondary villi

In tertiary villi, blood vessels are developed within the:

mesoderm core

What is the function of tertiary villi?

Efficient nutrient and gas exchange between mother and fetus