Embryology Lecture 2 - Second Week of Development

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Questions and Answers

What are the two distinct layers that the embryoblast separates into?

Epiblast and trophoblast

Epiblast and hypoblast (correct)

Cytotrophoblast and endometrial glands

Hypoblast and syncytiotrophoblast

Which component of the trophoblast helps in maternal-fetal communication?

Embryonic disc

Syncytiotrophoblast (correct)

Amniotic cavity

Cytotrophoblast

What is the initial function of the extraembryonic mesoderm?

Maternal-fetal protection and communication (correct)

Development of blood cells

Connection of placenta to uterus

Formation of amniotic fluid

What is the role of the placenta after the blastocyst is implanted?

Connects the embryo to the uterine wall Signup and view all the answers

During implantation, what happens to the endometrial lining?

Thickens and becomes more vascular Signup and view all the answers

What develops as part of the syncytiotrophoblast after implantation?

Vacuoles filled with blood Signup and view all the answers

What are the fluid-filled cavities that develop within the embryonic unit?

Amniotic cavity and yolk sac Signup and view all the answers

What is the first week of gestation primarily concerned with in terms of embryonic development?

Complex biochemical and mechanical process of implantation Signup and view all the answers

What is the primary purpose of the trophoblast cells in the blastocyst?

To develop into the placenta Signup and view all the answers

What important structure is established during the second week of development?

Fetomaternal interactions Signup and view all the answers

What is formed on day eight of embryonic development?

Bilaminar embryonic disc Signup and view all the answers

What happens to the zona pellucida during implantation?

It degenerates to expose the trophoblast layer Signup and view all the answers

Which stage precedes the formation of the blastocyst?

Blastulation Signup and view all the answers

What mechanism allows the blastocyst to partially collapse during implantation?

Proteolytic enzymes of syncytiotrophoblast Signup and view all the answers

What is NOT a characteristic of the second week of development?

Development of somatic tissues Signup and view all the answers

Which of the following statements about the blastocyst is false?

Its primary role is to directly form all fetal tissues. Signup and view all the answers

Study Notes

Second Week of Development

Completion of implantation and establishment of fetomaternal interactions is a key feature during this week.
Blastulation is the stage of early embryonic development resulting in the blastula, which later forms the blastocyst consisting of dividing cells (blastomeres).
The blastocyst has an outer trophoblast layer that develops into the placenta and an inner cell mass (embryoblast) that forms the embryo.

Stages of Blastocyst Development

By day eight of development, the inner cell mass differentiates into a bilaminar embryonic disc composed of two layers.
The blastocyst partially collapses and changes shape to integrate into the endometrium due to the action of proteolytic enzymes from syncytiotrophoblast cells.

The Bilaminar Embryonic Disc

Referred to as the week of twos due to the separation into two distinct layers: the embryoblast, extraembryonic mesoderm, and trophoblast.
The embryoblast forms:
- Epiblast (upper layer)
- Hypoblast (lower layer)
Two fluid-filled cavities develop: the amniotic cavity and the yolk sac.
The trophoblast develops into two sub-layers:
- Cytotrophoblast
- Syncytiotrophoblast

Implantation

Implantation is a critical biochemical and mechanical process spanning the first and second weeks of gestation.
Following ovulation, the endometrial lining thickens, with elongated secretory glands and increased vascularity in preparation for embryo acceptance.
Upon implantation, the placenta forms, connecting the embryo to the uterine wall.
Syncytiotrophoblast forms vacuoles that fill with maternal blood, facilitating uteroplacental circulation as it erodes maternal capillaries and endometrial glands.
Contents from endometrial glands are released into lacunae, contributing to the developing placenta.

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Description

This quiz covers essential concepts from the second week of embryonic development, focusing on implantation and fetomaternal interactions. Key processes like blastulation and the formation of the blastocyst are discussed, giving insights into early developmental stages.