Gastrulation Overview

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Questions and Answers

At what stage does the trophoblast develop into a tertiary villus?

Beginning of the fourth week

Beginning of the second week

End of the third week (correct)

Beginning of the third week

The cytotrophoblastic cells in the villi do not establish contact with the maternal endometrium.

False

What structure connects the embryo to the trophoblastic shell?

connecting stalk

The primary villi consist of a cytotrophoblastic core covered by a __________ layer.

syncytial Signup and view all the answers

Match the following types of villi with their descriptions:

Primary villi = Consist of a cytotrophoblastic core and syncytial layer Secondary villi = Formed when mesodermal cells penetrate primary villi Tertiary villi = Have capillaries that establish connection with the embryo Stem villi = Anchoring villi that connect placental tissue to decidua Signup and view all the answers

What is the primary function of the notochord during embryonic development?

To induce the formation of the neural tube Signup and view all the answers

The notochordal canal communicates only with the yolk sac.

False Signup and view all the answers

What structure appears around the 16th day of development and extends into the connecting stalk?

Allantois Signup and view all the answers

The ________ membrane is formed at the caudal end of the embryonic disc and consists of tightly adherent ectoderm and endoderm cells.

cloacal Signup and view all the answers

Match the following terms with their descriptions:

Notochord = Acts as a precursor to the vertebral column Prenotochordal cells = Cells that migrate cranially and intercalate into the hypoblast Notochordal canal = Canal that connects yolk sac to amniotic cavity Cloacal membrane = Membrane at the caudal end with no intervening mesoderm Signup and view all the answers

What is the first structure formed by the invagination of prenotochordal cells?

Notochord Signup and view all the answers

The intra-embryonic mesoderm is found in the prochordal plate.

False Signup and view all the answers

What is the first event that occurs during the process of gastrulation?

Formation of the primitive streak Signup and view all the answers

The primitive node is located at the caudal end of the primitive streak.

False Signup and view all the answers

What three germ layers are established during gastrulation?

Ectoderm, mesoderm, endoderm Signup and view all the answers

During gastrulation, cells of the epiblast undergo __________ to become mesoderm.

invagination Signup and view all the answers

Match the following terms with their descriptions:

Ectoderm = Forms the outer layer of the embryo Mesoderm = Gives rise to muscles and skeleton Endoderm = Develops into internal organs Primitive streak = Site of cell migration during gastrulation Signup and view all the answers

Which factor is crucial for cell migration and specification during gastrulation?

Fibroblast growth factor 8 (FGF8) Signup and view all the answers

The hypoblast contributes to the formation of ectoderm during gastrulation.

False Signup and view all the answers

What structure determines the cranial and tail end of the embryo?

Prochordal plate Signup and view all the answers

The ________ is a depression located in the center of the primitive node.

primitive pit Signup and view all the answers

What change occurs to cells of the hypoblast at the cranial part of the embryonic disc?

They become tall columnar Signup and view all the answers

Study Notes

Gastrulation

The most important event during the third week of gestation
Establishes the three germ layers: ectoderm, mesoderm, and endoderm
Begins with the formation of the primitive streak on the epiblast
Primitive streak has a slightly bulging region on either side
Primitive node is the cephalic end of the streak (slightly elevated area around the primitive pit)
Primitive pit is a depression in the center of the primitive node
Cells of the epiblast migrate toward the primitive streak becoming flask-shaped, detaching and slipping beneath it (inward movement called invagination)
Cells that displace the hypoblast create the embryonic endoderm
Cells that lie between the epiblast and the endoderm form the mesoderm
Cells that remain in the epiblast form the ectoderm
Cell migration and specification are controlled by fibroblast growth factor 8 (FGF8)
Cells spread laterally and cranially after migrating between the epiblast and hypoblast layers
The epiblast is the source of all three germ layers
In the cranial part of the embryonic disc, hypoblast cells become taller and columnar, forming the prochordal plate
Prochordal plate is a thickening firmly attached to the overlying epiblastic disc on day 14
The appearance of the prochordal plate determines the embryo's pole and its central axis which becomes the cranial end
The tail end is on the opposite side

Notochord

Develops from the upper end of the primitive streak to the lower end of the prochordal plate
Notochordal process - cells multiply in the midline between the ectoderm and endoderm until they reach the lower end of the prochordal plate (day 17-18)
Notochordal canal - the primitive pit extends into the notochordal process, converting it into a canal
Floor of the notochordal canal breaks down
Notochordal canal communicates with the yolk sac and amniotic cavity through the primitive pit
Neurenteric canal - temporary connection between amniotic and yolk sac cavities
Prenotochordal cells invaginate in the primitive node and move cranially in the midline until they reach the prechordal plate
Notochord and prenotochordal cells extend cranially to the prechordal plate (area just caudal to the oropharyngeal membrane) and caudally to the primitive pit
Notochordal plate - two-cell layers in the midline of the embryo where prenotochordal cells become intercalated in the hypoblast
Definitive notochord - solid cord of cells formed by notochordal plate, lies under the neural tube
Notochord is the basis for the axial skeleton
Notochord induces the formation of the neural tube
Notochord persists in the first decade of life as the nucleus pulposus of the intervertebral disc

Trophoblast Development

By the start of the third week, the trophoblast is characterized by primary villi, which consist of a cytotrophoblastic core covered by a syncytial layer
Mesodermal cells enter the core of primary villi and grow towards the decidua, forming a secondary villus
Secondary villus mesodermal cells differentiate into blood cells and small blood vessels, forming the villous capillary system
Tertiary villus (definitive placental villus) - the villus after developing a capillary system
Villi capillaries connect with capillaries developing in the chorionic plate mesoderm and the connecting stalk - these vessels connect the placenta and the embryo
Heart begins to beat in the fourth week of development, allowing the villous system to supply the embryo with nutrients and oxygen
Cytotrophoblastic cells in the villi penetrate the syncytium until they meet similar extensions from neighboring villous stems (thin outer cytotrophoblast shell)
The cytotrophoblast shell surrounds the trophoblast and anchors the chorionic sac to the maternal endometrial tissue
Stem or anchoring villi - extend from the chorionic plate to the decidua basalis (decidual plate)
Free (terminal) villi - extend from the sides of stem villi and are involved in nutrient exchange
Chorionic cavity becomes larger by the 19th or 20th day
The embryo is attached to the shell by a narrow connecting stalk (develops into the umbilical cord)

Intra-embryonic Mesoderm

Spreads all over the disc except for the prochordal plate
Prochordal plate is very thin with adherent ectoderm and endoderm, no mesoderm between the layers
The prochordal plate will later form the buccopharyngeal membrane (oral cavity)
The cloacal membrane is formed at the caudal end of the embryonic disc - similar structure to the oropharyngeal membrane, with tightly adherent ectoderm and endoderm with no intervening mesoderm.

Intraembryonic Coelom

Caudal end of the connecting stalk forms a small diverticulum called the allantoenteric diverticulum (allantois) around day 16
The allantois extends into the connecting stalk
Mesoderm divides into:
- Somatic mesoderm (parietal mesoderm) - forms lateral body wall.
- Splanchnic mesoderm (visceral mesoderm) - forms wall of viscera.

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Description

This quiz focuses on the key events during gastrulation, particularly in the third week of embryonic development. It covers the formation of the three germ layers, the role of the primitive streak, and the process of cell migration and specification. Enhance your understanding of these fundamental developmental concepts.