Embryology: Trilaminar Germ Disc and Organogenesis

Questions and Answers

What structure does the notochord and prenotochordal cells extend cranially to?

Primitive pit

Amniotic cavity

Oropharyngeal membrane (correct)

Cloacal membrane

Which axis is formed at the cranial end of the endoderm?

Cranio-caudal axis

Anterior-posterior axis (correct)

Left-right axis

Dorso-ventral axis

What is the role of FGF8 during the development of mesoderm?

It influences ventralization crucial for kidney formation. (correct)

It contributes to the formation of the notochord.

It assists in bone development.

It initiates formation of the cloacal membrane.

What connects the amniotic and yolk sac cavities temporarily?

Neuromeric canal Signup and view all the answers

When does the allantoenteric diverticulum appear during embryonic development?

Around the 16th day Signup and view all the answers

What structure is formed by the cranially migrating epiblast cells?

Oropharyngeal membrane Signup and view all the answers

Which of the following describes the developmental process occurring at the cranial region of the streak?

Invagination of the epiblast cells Signup and view all the answers

At what stage do the definitive endoderm and mesoderm begin to form?

15 days into development Signup and view all the answers

Which plate is associated with the induction of the forebrain?

Precordial plate Signup and view all the answers

What is the result of the first cells moving inward from the epiblast?

Creation of definitive endoderm Signup and view all the answers

How do epiblast cells move during the second week of development?

Lateral, cranial, and caudal movements Signup and view all the answers

What anatomical structure is formed as a consequence of epiblast migration at the end of the second week?

Germ disc Signup and view all the answers

What role does the hypoblast play during the migration of epiblast cells?

It is displaced to create the definitive endoderm. Signup and view all the answers

What is the primary event of gastrulation occurring during the 3rd week of development?

Migration of cells towards the primitive streak Signup and view all the answers

Which factor is primarily responsible for controlling cell specification into mesoderm during the 3rd week?

Fibroblast Growth Factor 8 (FGF8) Signup and view all the answers

During gastrulation, what happens to the hypoblast cells?

They are displaced to create the endoderm. Signup and view all the answers

What structure is formed at the cephalic end of the epiblast during the development of the primitive streak?

Primitive node Signup and view all the answers

What embryonic structure is responsible for the establishment of body axes during the 3rd week?

Primitive streak Signup and view all the answers

What type of cell movement occurs when cells migrate inward toward the primitive streak?

Invagination Signup and view all the answers

Which germ layer is formed from the cells that remain in the epiblast after invagination?

Ectoderm Signup and view all the answers

Which gene expression is regulated by FGF8 to control mesoderm specification?

BRACHYURY (T) Signup and view all the answers

Which germ layer is responsible for forming the skin and muscle tissue in an embryo?

Mesoderm Signup and view all the answers

What process leads to the formation of the neural tube from the neural plate?

Neurulation Signup and view all the answers

During neurulation, what structure forms at the midline where the neural folds approach each other?

Neural tube Signup and view all the answers

Which factor is NOT mentioned as involved in the upregulation of the neural plate?

Epidermal Growth Factor (EGF) Signup and view all the answers

On which day does the anterior cranial neuropore typically close during embryonic development?

Day 25 Signup and view all the answers

Which of the following is an organ derived from the endoderm germ layer?

Liver Signup and view all the answers

What is the primary shape of an 18-day presomite embryo?

Pear-shaped Signup and view all the answers

What occurs as the lateral edges of the neural plate elevate during neurulation?

Development of neural folds Signup and view all the answers

Which mesoderm migrates at the lateral edges of the node during gastrulation?

Paraxial Mesoderm Signup and view all the answers

During which developmental stage does the growth and elongation of the embryonic disc primarily occur?

Gastrulation Signup and view all the answers

What role does the primitive streak play during embryonic development?

It supplies new cells for development until the end of the fourth week. Signup and view all the answers

Which germ layer differentiates into structures in contact with the outside world?

Ectoderm Signup and view all the answers

What process continues in the caudal segments of the embryo while cranial structures develop?

Gastrulation Signup and view all the answers

What is the primary morphological change observed in the embryonic disc as development progresses?

Elongating with a broad cephalic and narrow caudal end Signup and view all the answers

Which type of mesoderm generally contributes to the formation of extraembryonic structures?

Caudal-most Mesoderm Signup and view all the answers

Which stage in embryonic development is characterized by the formation of tissues and organs from germ layers?

Period of Organogenesis Signup and view all the answers

Study Notes

3rd Week of Development: Trilaminar Disc

Gastrulation marks the onset of the third week with the formation of the primitive streak on the epiblast surface, initiating major developmental processes.
Invagination occurs as cells migrate inward towards the primitive streak, guided by Fibroblast Growth Factor 8 (FGF8), which regulates mesoderm specification by modulating BRACHYURY (T) expression.
Cells that move inward replace the hypoblast to create definitive endoderm, while those remaining in the epiblast establish the ectoderm.
The precordial plate is formed from epiblast cells that migrate cranially, leading to future forebrain development, and the oropharyngeal membrane, which will become the oral cavity.
The notochord and prenotochordal cells extend from the oropharyngeal membrane to the primitive pit and are vital for axial skeleton development.

Establishment of Body Axes

Body axes are established early, including anterior-posterior (AP), dorso-ventral (DV), and left-right (LR) axes.
The anterior visceral endoderm (AVE) at the cranial end indicates future head region formation.
The cloacal membrane develops at the caudal end of the embryonic disc, with the allantoenteric diverticulum appearing around day 16.

Growth of the Embryonic Disc

Initially flat, the embryonic disc undergoes elongation, with continuous cell migration from the primitive streak directed toward the cephalic end.
The primitive streak regresses and shrinks as cranial structures differentiate, persisting throughout the fourth week.

3rd to 8th Weeks: The Embryonic Period

Period of Organogenesis

The period is characterized by the formation of organs and tissues from the three germ layers: ectoderm, mesoderm, and endoderm.
Ectoderm gives rise to structures interacting with the external environment, including the CNS, PNS, sensory epithelium, skin, hair, nails, and glands.
Mesoderm forms supportive tissues such as muscle (myotome), cartilage (sclerotome), and skin (dermatome).
Endoderm develops into internal organs including the gastrointestinal tract, respiratory tract, kidneys, liver, and pancreas.
Neural plate induction is driven by FGF upregulation and BMP4 inhibition.

Neurulation

Neurulation involves the neural plate elongating, with lateral edges lifting to form neural folds, creating a neural groove.
Neural folds fuse in the midline to form the neural tube, leading to the development of the spinal cord and brain.
The anterior cranial neuropore closes by day 25 and the posterior caudal neuropore by day 28, completing neurulation.
During this phase, the embryonic axis is defined, where the length is approximately 1.25 mm and width is about 0.68 mm at 18 days.

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Description

This quiz covers the stages of the embryonic period from the 3rd to the 8th week of development, focusing specifically on the trilaminar germ disc, gastrulation, and the formation of key structures such as the notochord. Dive into the crucial concepts of organogenesis during this pivotal time in embryonic development.