Embryology: Third Week of Development
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Questions and Answers

What is the primary event occurring during the third week of gestation?

  • Implantation
  • Neurulation
  • Gastrulation (correct)
  • Organogenesis

What initiates the process of gastrulation?

  • Formation of the hypoblast
  • Formation of the primitive streak (correct)
  • Formation of the prechordal plate
  • Formation of the notochord

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

  • Ectoderm (correct)
  • Endoderm
  • Mesoderm
  • Hypoblast

Which structure is formed between the notochord and the oropharyngeal membrane?

<p>Prechordal plate (D)</p> Signup and view all the answers

What process occurs when epiblast cells migrate and detach to move beneath the epiblast?

<p>Invagination (A)</p> Signup and view all the answers

Which factor is responsible for controlling cell specification into mesoderm during gastrulation?

<p>Fibroblast growth factor 8 (FGF8) (B)</p> Signup and view all the answers

What is the fate of cells that displace the hypoblast during gastrulation?

<p>They form mesoderm. (B)</p> Signup and view all the answers

As cells migrate during gastrulation, in which directions do they spread?

<p>Laterally and cranially (A)</p> Signup and view all the answers

What is the role of capillaries in tertiary villi during embryonic development?

<p>To connect the placenta to the intraembryonic circulatory system (D)</p> Signup and view all the answers

What is formed when cytotrophoblastic cells penetrate the syncytium?

<p>The outer cytotrophoblast shell (B)</p> Signup and view all the answers

What are stem or anchoring villi responsible for?

<p>Attaching the chorionic plate to the decidua basalis (D)</p> Signup and view all the answers

What is the function of free (terminal) villi?

<p>Facilitating nutrient and factor exchange (C)</p> Signup and view all the answers

How does the connecting stalk relate to the later development of the embryo?

<p>It later develops into the umbilical cord (D)</p> Signup and view all the answers

What type of mesoderm do cells migrating through the cranial region of the node become?

<p>Notochord and pre-chordal plate (B)</p> Signup and view all the answers

What is the primary factor contributing to the elongation of the embryonic disc?

<p>Migration of cells from the primitive streak (D)</p> Signup and view all the answers

When does the formation of the germ layers continue in the caudal segments of the embryo?

<p>By the end of the fourth week (C)</p> Signup and view all the answers

What characterizes the trophoblast by the beginning of the third week?

<p>Development of primary villi (D)</p> Signup and view all the answers

What is formed when mesodermal cells penetrate the core of primary villi?

<p>Secondary villus (C)</p> Signup and view all the answers

During which week does the primitive streak show regressive changes and begin to shrink?

<p>End of the fourth week (C)</p> Signup and view all the answers

What is the definitive placental villus formed from?

<p>Primarily mesodermal differentiation (C)</p> Signup and view all the answers

Which type of mesoderm is formed by cells migrating at the lateral edges of the node?

<p>Lateral plate mesoderm (D)</p> Signup and view all the answers

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

<p>To act as a signaling center for inducing the axial skeleton. (B)</p> Signup and view all the answers

Which structure appears similar in composition to the cloacal membrane?

<p>Oropharyngeal membrane. (B)</p> Signup and view all the answers

At which embryonic structure does the notochord extend cranially towards?

<p>Prechordal plate. (D)</p> Signup and view all the answers

What happens to the hypoblast during the formation of the definitive notochord?

<p>It is gradually replaced by endoderm cells. (C)</p> Signup and view all the answers

What is the role of the neurenteric canal during embryonic development?

<p>It connects the amniotic and yolk sac cavities temporarily. (B)</p> Signup and view all the answers

When does the allantois typically appear during development?

<p>Around the 16th day. (D)</p> Signup and view all the answers

What distinguishes the notochordal plate from the definitive notochord?

<p>Notochordal plate is a transient structure before the definitive notochord forms. (D)</p> Signup and view all the answers

What is the result of the invagination of prenotochordal cells during development?

<p>Development of the notochordal plate. (D)</p> Signup and view all the answers

Flashcards

Gastrulation

The process of forming the three primary germ layers (ectoderm, mesoderm, and endoderm) during the third week of gestation.

Primitive Streak

The first structure to form during gastrulation, a thickened groove on the surface of the epiblast.

Primitive Node

The cephalic end of the primitive streak with an elevated area surrounding a small pit.

Invagination

The inward movement of cells from the epiblast through the primitive streak.

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Fibroblast Growth Factor 8 (FGF8)

A signaling molecule that controls cell specification into mesoderm during gastrulation.

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Ectoderm

The outermost germ layer, which will give rise to skin, nervous system, and sensory organs.

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Mesoderm

The middle germ layer, which will give rise to muscles, bones, blood, and connective tissue.

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Endoderm

The innermost germ layer, which will give rise to the lining of the digestive and respiratory tracts.

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Oropharyngeal membrane

A small region at the cranial end of the embryonic disc where ectoderm and endoderm cells meet tightly, marking the future opening of the mouth.

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Prenotochordal cells

Cells that migrate from the primitive node and move forward in the midline, ultimately forming the notochord.

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Notochordal plate

A temporary structure formed by two cell layers (prenotochordal cells and hypoblast) in the midline of the embryo.

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Definitive notochord

A solid rod of cells that develops from the notochordal plate, providing structural support and signaling for the axial skeleton development.

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Prechordal plate

A region just caudal to the oropharyngeal membrane where the cranial end of the notochord extends. It's crucial for brain development.

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Neurenteric canal

An indentation in the epiblast at the point where the primitive pit forms, briefly connecting the amniotic and yolk sac cavities.

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Cloacal membrane

A similar structure to the oropharyngeal membrane, located at the caudal end of the embryonic disc, marking the future anus.

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Allantoenteric diverticulum (Allantois)

A small diverticulum (outpouching) from the posterior wall of the yolk sac, extending into the connecting stalk. It plays a role in bladder development.

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What is invagination?

The inward movement of cells from the epiblast through the primitive streak during gastrulation.

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What is gastrulation?

The process by which the three primary germ layers (ectoderm, mesoderm, and endoderm) are formed.

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What germ layer is formed from cells migrating laterally at the primitive node?

Cells migrating at the lateral edges of the primitive node and cranial end of the streak form this germ layer.

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What germ layer arises from cells migrating through the midstreak?

Cells migrating through the midstreak region form this germ layer.

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What germ layer is formed from cells migrating through the more caudal part of the streak?

Cells migrating through the more caudal part of the primitive streak form this germ layer.

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What tissue is formed by cells migrating through the caudalmost part of the streak?

Cells migrating through the caudalmost part of the primitive streak form this extraembryonic tissue.

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How does the embryonic disc grow?

The embryonic disc grows primarily in the cephalic region, with the primitive streak remaining relatively stable in size.

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How does gastrulation affect the embryo's development?

Gastrulation continues in caudal segments while cranial structures differentiate, leading to a cephalocaudal development pattern.

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Trophoblast

The outer layer of cells that surrounds the embryo and forms the placenta. It begins as a single layer but develops into a complex structure with multiple layers and villi.

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Villi

Finger-like projections that grow from the trophoblast and extend into the maternal endometrium.

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Vascularization of the villi

The process by which blood vessels in the villi connect to the embryo's circulatory system, allowing the exchange of nutrients and oxygen.

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Cytotrophoblast shell

A thin layer of trophoblast cells that surrounds the chorionic sac and attaches it to the maternal endometrium.

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Connecting stalk

The part of the chorionic sac that connects the embryo to the placenta, eventually forming the umbilical cord.

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Study Notes

Third Week of Development

  • Gastrulation is the defining event of the third week of gestation, establishing the three germ layers: ectoderm, mesoderm, and endoderm.
  • Gastrulation begins with the formation of the primitive streak on the epiblast.
  • Initially, the primitive streak is indistinct, but by day 15-16, it appears as a narrow groove with slightly bulging regions on either side.
  • The cephalic end of the streak contains the primitive node, a slightly elevated area surrounding the primitive pit.
  • Epiblast cells migrate towards the primitive streak.
  • These cells detach from the epiblast and move beneath it, a process called invagination.
  • This inward movement creates the mesoderm and endoderm.
  • Mesoderm and endoderm formation displace the hypoblast.
  • Fibroblast Growth Factor 8 (FGF8) controls cell specification into mesoderm.
  • The prechordal plate forms between the tip of the notochord and oropharyngeal membrane, playing a crucial role in forebrain induction.

Formation of the Notochord

  • Notochord formation involves prenotochordal cells migrating from the primitive node to the prechordal plate.
  • Initially, these cells intercalate with the hypoblast, forming the notochordal plate.
  • As endoderm cells replace the hypoblast, cells of the notochordal plate proliferate and detach from the endoderm to form a solid cord called the definitive notochord.
  • The elongation of the notochord is a dynamic process, with the cranial end developing first and the caudal regions following.
  • The notochord extends cranially to the prechordal plate and caudally to the primitive pit. A neurenteric canal temporarily connects the amniotic and yolk sac cavities.

Formation of the Cloacal Membrane

  • The cloacal membrane forms at the caudal end of the embryonic disk.
  • It is similar in structure to the oropharyngeal membrane, consisting of tightly adherent ectoderm and endoderm layers without intervening mesoderm.
  • The posterior wall of the yolk sac develops a small diverticulum, known as the allantois, around day 16 of development.

Growth of the Embryonic Disc

  • The embryonic disc, initially flat, gradually elongates, with a broader cephalic and narrower caudal end.
  • Expansion primarily occurs in the cephalic region, while the primitive streak region remains relatively constant.
  • Cell migration from the primitive streak region drives cephalic growth.
  • Invagination and subsequent migration of surface cells in the primitive streak continue until the fourth week of development before the primitive streak regresses and disappears.

Further Development of the Trophoblast

  • By the beginning of the third week, the trophoblast develops primary villi consisting of a cytotrophoblastic core covered by a syncytial layer.
  • Mesodermal cells penetrate into the core of the primary villi.
  • Secondary villi form as mesodermal cells differentiate into blood cells and blood vessels.
  • The villi become tertiary villi, or definitive placental villi.
  • Capillaries in tertiary villi connect with the intraembryonic circulatory system, linking the embryo and the placenta.
  • Cytotrophoblastic cells extend to the maternal endometrium, forming a cytotrophoblast shell, which firmly attaches the chorionic sac to the maternal tissue.
  • Villi extend from the chorionic plate to the decidua basalis, called stem or anchoring villi. Other villi, called free villi, branch from the stem villi and are involved in the exchange of nutrients and other materials.
  • The chorionic cavity expands, and a connecting stalk forms, which later develops into the umbilical cord, connecting the embryo to the placenta.

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Description

Explore the key events of the third week of gestation, particularly the process of gastrulation and the establishment of the three germ layers. This quiz covers the formation of the primitive streak, cell migration, and the implications of fibroblast growth factors in cell specification. Test your understanding of these fundamental embryological concepts.

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