Embryonic Development Week 3

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Questions and Answers

What is the primary process that occurs during the third week of gestation?

Gastrulation (correct)

Organogenesis

Cleavage

Neurulation

What is the role of the primitive streak in gastrulation?

It defines body axes (correct)

It initiates limb development

It produces hormones

It forms the heart

Which germ layer is formed from cells that remain in the epiblast during gastrulation?

Mesoderm

Ectoderm (correct)

Chorion

Endoderm

What characterizes the primitive node during gastrulation?

A circular depression known as the primitive pit Signup and view all the answers

What does the prechordal plate contribute to during development?

Induction of the forebrain Signup and view all the answers

Which statement about invagination during gastrulation is correct?

Epiblast cells detach and slip beneath the primitive groove Signup and view all the answers

Which germ layer is created by the displacement of hypoblast during gastrulation?

Endoderm Signup and view all the answers

At which point in development does the primitive streak become visible?

At 15- to 16-day embryo Signup and view all the answers

At what point does differentiation of germ layers begin in the cephalic part of the embryo?

Middle of the third week Signup and view all the answers

What is the condition known as when alcohol causes craniofacial deficiencies during gastrulation?

Holoprosencephaly Signup and view all the answers

What happens to the primitive streak during certain developmental changes?

It shows regressive changes and disappears Signup and view all the answers

When does gastrulation continue in caudal segments of the embryo?

While cranial structures are differentiating Signup and view all the answers

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

Secondary villus Signup and view all the answers

Which defect is characterized by insufficient mesoderm formation in the caudal region?

Sirenomelia Signup and view all the answers

What do the capillaries in the tertiary villi establish contact with?

Intraembryonic circulatory system Signup and view all the answers

What are tumors associated with remnants of the primitive streak in the sacrococcygeal region called?

Sacrococcygeal teratomas Signup and view all the answers

What does the oropharyngeal membrane represent in embryonic development?

The future opening of the oral cavity Signup and view all the answers

Where is the neuroenteric canal located?

At the primitive pit Signup and view all the answers

What are the components of the cloacal membrane?

Ectoderm and endoderm Signup and view all the answers

What is the primary function of the allantoenteric diverticulum?

To facilitate the connection between the embryo and the yolk sac Signup and view all the answers

What happens to prenotochordal cells during development?

They migrate cranially and form the notochordal plate Signup and view all the answers

Which part of the embryonic disc primarily expands during development?

The cephalic region Signup and view all the answers

What do cells migrating through the caudal-most part of the primitive streak become?

Extraembryonic mesoderm Signup and view all the answers

How does the embryonic disc grow in size?

By continuous migration of cells from the primitive streak Signup and view all the answers

Study Notes

Third Week of Development

Gastrulation: The primary event during week three, transforming the bilaminar embryonic disc into a trilaminar disc by forming the three germ layers: ectoderm, mesoderm, and endoderm.
Primitive Streak: A groove forming on the epiblast surface near the caudal end of the bilaminar disc, marks the start of gastrulation.
Primitive Node: An expansion at the cranial end of the streak containing the primitive pit, a circular depression.
Invagination: The process where epiblast cells migrate towards the primitive streak, detach and slip beneath it. This movement forms the endoderm, displacing the hypoblast, and the mesoderm by placing itself between the epiblast and the new endoderm; the remaining epiblast cells become ectoderm.
Body Axes: The primitive streak defines the cranial-caudal, dorsal-ventral, and left-right body axes.
Prechordal Plate: Derived from the first cells migrating through the primitive node, it lies between the notochord tip and the oropharyngeal membrane and induces forebrain development.
Oropharyngeal Membrane: A region at the cranial end of the embryonic disc, composed of ectoderm and endoderm with no mesoderm, which will become the future opening of the oral cavity.
Neuroenteric Canal: A temporary connection between the amniotic and yolk sac cavities, located at the primitive pit.
Cloacal Membrane: Similar to the oropharyngeal membrane, it's located at the caudal end and consists of ectoderm and endoderm with absent mesoderm.
Allantoenteric Diverticulum (Allantois): A small diverticulum extending from the posterior wall of the yolk sac into the connecting stalk, appearing around the 16th day of development.

Notochord Formation

Prenotochrodal cells migrate cranially from the primitive node, reaching the prechordal plate. These cells become intercalated with the hypoblast, forming the notochordal plate.
The notochordal plate proliferates and detaches from the endoderm, forming a solid cord of cells, the definitive notochord.

Fate Map

Cells migrating through the primitive streak have defined destinations: cranial node cells become prechordal plate and notochord, lateral node and cranial streak cells form paraxial mesoderm, midstreak cells form intermediate mesoderm, caudal streak cells form lateral plate mesoderm, and caudal-most streak cells contribute to extraembryonic mesoderm.

Embryonic Disc Growth

The embryonic disc initially flat and round, becomes elongated with a broader cephalic region due to the continuous migration of cells from the primitive streak to the cephalic region.
This migration continues until the end of the fourth week when the primitive streak starts to regress.

Clinical Correlate

Teratogenic Insults: Early third week is highly sensitive to teratogens. High alcohol doses can cause holoprosencephaly by killing cells in the anterior midline of the germ disc, leading to cranial facial midline deficiencies.
Caudal Dysgenesis: Insufficient mesoderm formation in the caudal region can lead to hypoplasia and fusion of the lower limbs, vertebral abnormalities, renal agenesis, imperforate anus, and genital anomalies.
Sacrococcygeal Teratomas: Remnants of the primitive streak persisting in the sacrococcygeal region can proliferate and form tumors containing tissues from all three germ layers.

Trophoblast Development

By the start of the third week, the trophoblast consists of primary villi with a cytotrophoblastic core covered by a syncytial layer.
Mesodermal cells penetrate the core of primary villi, creating secondary villi.
Mesodermal cells in the core differentiate into blood cells and blood vessels, forming the villous capillary system, resulting in tertiary villi or definitive placental villi.
Capillaries in tertiary villi connect with chorionic plate and connecting stalk vessels, establishing contact with the intraembryonic circulatory system, connecting the placenta and embryo.
Cytotrophoblastic cells penetrate the syncytium until they reach the maternal endometrium.

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Description

This quiz focuses on the major events during the third week of embryonic development, particularly the process of gastrulation. Key concepts include the formation of the germ layers, the primitive streak, and body axes. Test your understanding of these foundational processes in embryology.