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?

Prechordal plate Signup and view all the answers

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

Invagination Signup and view all the answers

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

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

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

They form mesoderm. Signup and view all the answers

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

Laterally and cranially Signup and view all the answers

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

To connect the placenta to the intraembryonic circulatory system Signup and view all the answers

What is formed when cytotrophoblastic cells penetrate the syncytium?

The outer cytotrophoblast shell Signup and view all the answers

What are stem or anchoring villi responsible for?

Attaching the chorionic plate to the decidua basalis Signup and view all the answers

What is the function of free (terminal) villi?

Facilitating nutrient and factor exchange Signup and view all the answers

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

It later develops into the umbilical cord Signup and view all the answers

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

Notochord and pre-chordal plate Signup and view all the answers

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

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

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

By the end of the fourth week Signup and view all the answers

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

Development of primary villi Signup and view all the answers

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

Secondary villus Signup and view all the answers

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

End of the fourth week Signup and view all the answers

What is the definitive placental villus formed from?

Primarily mesodermal differentiation Signup and view all the answers

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

Lateral plate mesoderm Signup and view all the answers

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

To act as a signaling center for inducing the axial skeleton. Signup and view all the answers

Which structure appears similar in composition to the cloacal membrane?

Oropharyngeal membrane. Signup and view all the answers

At which embryonic structure does the notochord extend cranially towards?

Prechordal plate. Signup and view all the answers

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

It is gradually replaced by endoderm cells. Signup and view all the answers

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

It connects the amniotic and yolk sac cavities temporarily. Signup and view all the answers

When does the allantois typically appear during development?

Around the 16th day. Signup and view all the answers

What distinguishes the notochordal plate from the definitive notochord?

Notochordal plate is a transient structure before the definitive notochord forms. Signup and view all the answers

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

Development of the notochordal plate. Signup and view all the answers

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.