L6

Question 1

What are the key events of the 3rd week (trilaminar germ disc)?

Answer 1

• Gastrulation: Formation of three germ layers
• Formation of oral and cloacal membranes
• Formation of notochordal
• Growth of embryonic disc
• Establishment of body axis
• Formation of allantois
• Differentiation of intraembryonic mesoderm
• Formation of intraembryonic coelom
• Further development of trophoblast (secondary and tertiary villi)

Question 2

1. What is gastrulation?
   
   2. What is the most characteristic event that occurs during the third week?
   3. When does gastrulation start?
   4. What happens during gastrulation?
   5. What are the main steps of gastrulation?

Answer 2

1. Gastrulation is the formation of three germ layers.
   
   2. Gastrulation is the most characteristic event that occurs during the third week.
   3. Gastrulation starts at the beginning of the third week (15th day).
   4. Gastrulation is the process by which the bilaminar embryonic disc is converted into a trilaminar embryonic disc, forming three germ layers: ectoderm, mesoderm, and endoderm.
   5. The main steps of gastrulation are:
      • Development of primitive streak and node.
      • Invagination.
      • Differentiation of three germ layers.
      • Formation of oral and cloacal membranes.

Question 3

1. When does the primitive streak appear on the epiblast?
   
   2. Where does the primitive streak first appear?
   3. How does the primitive streak appear at day 15?
   4. What is the primitive node?
   5. Where is the primitive node located?
   6. What does the primitive node consist of?

Answer 3

1. The primitive streak appears at day 15.
   
   2. The primitive streak first appears on the epiblast at the caudal end.
   3. The primitive streak appears as a midline elevation with a groove inside it.
   4. The primitive node is the cephalic end of the primitive streak.
   5. The primitive node is located at the cephalic end of the primitive streak.
   6. The primitive node consists of an elevated area with a small primitive pit inside.

Question 4

What happens during invagination?

Answer 4

Cells of the epiblast migrate toward the primitive streak, become flask-shaped, detach from the epiblast, and slip beneath it. This inward movement is called invagination.

Question 5

1. What happens to the detached cells during differentiation of the three germ layers?
   
   2. What do some of the detached cells form?
   3. What do the cells lying between the epiblast and endoderm form?
   4. What do the cells remaining in the epiblast form?
   5. What is the role of the epiblast in the process of gastrulation?

Answer 5

1. The detached cells will form the three germ layers.
   
   2. Some of the detached cells displace the hypoblast, creating the embryonic endoderm.
   3. The cells lying between the epiblast and endoderm form the secondary (intraembryonic) mesoderm, spreading cranially and laterally.
   4. The cells remaining in the epiblast form the ectoderm.
   5. The epiblast, through the process of gastrulation, is the source of all three germ layers, which will give rise to all of the tissues and organs in the embryo.

Question 6

1. What is the origin of the prochordal plate in the cranial part of the embryonic disc?
   
   2. Why is the prechordal plate important during embryonic development?
   3. How is the oral (bucco-pharyngeal) membrane formed?
   4. What is the role of the ectoderm and endoderm in the formation of the oral membrane?
   5. When is the cloacal membrane formed in the embryo?
   6. How is the cloacal membrane different from the oral membrane?

Answer 6

1. The prochordal plate is derived from migrating cells from the primitive node in the midline and cranial direction.
   
   2. The prechordal plate is important for the induction of the forebrain during embryonic development.
   3. The oral (bucco-pharyngeal) membrane is formed when the ectoderm adheres to the endoderm with no mesoderm in between, cranial to the prechordal plate.
   4. The ectoderm and endoderm adhere to each other with no mesoderm between them, forming the oral membrane (future mouth).
   5. The cloacal membrane is formed later in the caudal part of the embryo.
   6. The cloacal membrane is similar to the oral membrane in that it has no mesoderm between the ectoderm and endoderm, but it is located in the caudal part of the embryo.

Question 7

1. What happens to the primitive streak by the end of the 4th week of development?
   
   2. What may happen if remnants of the primitive streak persist?
   3. What is a sacrococcygeal teratoma?
   4. What types of tissues are found in a teratoma?
   5. What are the three germ layers involved in the formation of a teratoma?

Answer 7

1. The primitive streak diminishes in size and disappears by the end of the 4th week.
   
   2. Remnants of the primitive streak may persist and give rise to a sacrococcygeal teratoma.
   3. A sacrococcygeal teratoma is a tumor that forms in the sacrococcygeal region, containing tissues derived from all three germ layers.
   4. A teratoma contains tissues derived from all three germ layers in varying stages of differentiation.
   5. The three germ layers involved in the formation of a teratoma are ectoderm, mesoderm, and endoderm.

Question 8

1. What is the notochordal process?
   
   2. How is the notochord canal formed?
   3. What is the neuroenteric canal and how is it formed?
   4. What is the definitive notochord?

Answer 8

1. The notochordal process is formed when cells from the primitive node migrate in a cephalic direction along the middle line between the ectoderm and endoderm to form a solid rod.
   
   2. The notochordal process is canalized to form the notochordal canal, which is connected to the amniotic cavity at the primitive pit.
   3. The neuroenteric canal is formed when the floor of the notochordal canal fuses with the underlying endoderm, then breaks down and disappears, creating a temporary connection between the amniotic cavity and the yolk sac.
   4. The definitive notochord is formed when the notochordal canal is closed as the endoderm is reconstructed, and the remaining part of the notochord forms a solid cord.

Question 9

What are the functions of the notochord?

Answer 9

1. It acts as an inducer for the development of the axial skeleton (vertebral column).
   
   2. It acts as an inducer for the overlying ectoderm to form the neural plate, which will develop into the central nervous system (CNS).
   3. It degenerates over time.
   4. Its remnant becomes the nucleus pulposus of the intervertebral discs.

Question 10

1. What shape does the germ disc become in the 3rd week of development?
   
   2. Where does the expansion of the embryonic disc mainly occur during the 3rd week?
   3. In which direction does the notochord elongate in the 3rd week?
   4. How does the primitive streak change during the 3rd week?
   5. What body axes are established during the 3rd week of development?

Answer 10

1. The germ disc becomes elongated, pear-shaped, and trilaminar.
   
   2. The expansion of the embryonic disc occurs mainly in the cephalic region.
   3. The notochord elongates in the cranial direction.
   4. The primitive streak becomes more caudal.
   5. The establishment of body axes occurs (craniocaudal, dorsoventral, and left-right).

Question 11

What is the formation of the allantois?

Answer 11

The formation of the allantois occurs when the endoderm of the wall of the yolk sac invades the connecting stalk and forms a diverticulum or allantois, which appears around the 16th day of development.

Question 12

1. When is the intra-embryonic mesoderm formed?
   
   2. Between which layers is the intra-embryonic mesoderm formed?
   3. Where is the intra-embryonic mesoderm absent?
   4. What are the sources of the intra-embryonic mesoderm?
   5. What is the major source of the intra-embryonic mesoderm?
   6. Into how many parts is the mesoderm divided on either side?
   7. What are the three parts into which the mesoderm is divided?

Answer 12

The intra-embryonic mesoderm is formed around the 17th day.
2. It is formed between the ectoderm and the endoderm.
3. The intra-embryonic mesoderm is absent in the oral membrane and the cloacal membrane.
4. The sources of the intra-embryonic mesoderm are the epiblast through:
• The primitive streak.
• The primitive node.
5. The major source of the intra-embryonic mesoderm is the primitive streak.
6. The mesoderm is divided into 3 parts on either side.
7. The three parts into which the mesoderm is divided are:
• Paraxial mesoderm.
• Intermediate mesoderm.
• Lateral plate mesoderm.

Question 13

1. What are the two thick longitudinal bands found on either side of the notochord in the paraxial mesoderm?
   
   2. What are the small mesodermal blocks called in the paraxial mesoderm?
   3. When does the first pair of somites appear?
   4. How many somites are formed by the end of the 5th week?
   5. In what direction do somites appear?
   6. What is the number of somites in each region: occipital, cervical, thoracic, lumbar, sacral, and coccygeal?
   7. Which somites later disappear in the paraxial mesoderm?
   8. What do the remaining somites form?

Answer 13

1. The two thick longitudinal bands are on either side of the notochord.
   
   2. The small mesodermal blocks are called somites.
   3. The first pair of somites appear by the 20th day.
   4. By the end of the 5th week, 42-44 pairs of somites are formed.
   5. Somites appear in a cranio-caudal direction.
   6. The number of somites in each region is: 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8-10 coccygeal.
   7. The 1st occipital and the last 5 to 7 coccygeal somites later disappear.
   8. The remaining somites form the axial skeleton.

Question 14

1. How do somites appear in terms of periodicity?
   
   2. How can the age of an embryo be determined during this early period?

Answer 14

1. Somites appear with a specified periodicity.
   
   2. The age of an embryo can be determined during this early time period by counting somites.

Question 15

1. What are the two parts each somite divides into?
   
   2. What does the ventro-medial part of the somite form?
   3. What does the dorso-lateral part of the somite form?
   4. What does the lateral part of the dermomyotome form?
   5. What does the medial part of the dermomyotome form?

Answer 15

1. Each somite divides into two parts: ventro-medial part and dorso-lateral part.
   
   2. The ventro-medial part, called sclerotome, will form the vertebral column and ribs.
   3. The dorso-lateral part, called dermomyotome, will be divided into lateral and medial parts.
   4. The lateral part, called dermatome, will form the dermis of the skin of the back.
   5. The medial part, called myotome, will form the skeletal muscles of the back, body wall, and limbs through migrating cells from myotomes into the parietal layer of the lateral plate mesoderm.

Question 16

1. What does the intermediate mesoderm form?
   
   2. What structures are formed from the nephrogenic cord?
   3. What is the lateral plate mesoderm divided into?
   4. What does the somatic IE mesoderm form?
   5. What does the splanchnic IE mesoderm form?
   6. What does the intraembryonic coelom form?

Answer 16

1. The intermediate mesoderm forms the nephrogenic cord.
   
   2. The nephrogenic cord forms the kidney, ureter, gonads, and parts of the genital system (urogenital structures).
   3. The lateral plate mesoderm is divided by the U-shaped cavity called the intra-embryonic coelom into:
      • Somatic IE mesoderm
      • Splanchnic IE mesoderm
      • Intraembryonic coelom
   4. The somatic IE mesoderm forms the dermis, connective tissue, blood vessels, and bones of the body wall (sternum) and limbs.
   5. The splanchnic IE mesoderm forms the connective tissue, blood vessels, and smooth muscles of the viscera.
   6. The intraembryonic coelom forms the serous cavities (pleura, pericardium, and peritoneum).