Developmental Period: Week 3&4 Flashcards
(112 cards)
1
Q
What major developmental process occurs in Week 3 of embryogenesis?
A
Gastrulation – formation of the three germ layers: ectoderm, mesoderm, and endoderm.
2
Q
What is the origin of all three germ layers in the embryo?
A
Epiblast cells.
3
Q
On which day does the primitive streak appear?
A
Day 14.
4
Q
Where does the primitive streak form?
A
On the floor of the amniotic cavity, at the caudal end of the embryonic disc.
5
Q
What is the sequence of germ layer formation during gastrulation?
A
- Endoderm (epiblast cells replace hypoblast)
- Mesoderm (epiblast cells move between endoderm and epiblast)
- Ectoderm (remaining epiblast cells)
6
Q
What does the endoderm primarily give rise to?
A
The gut tube and linings of respiratory and digestive tracts.
7
Q
What does the mesoderm give rise to?
A
Cardiovascular system (CVS), muscle, bone, blood, and connective tissues.
8
Q
What does the ectoderm form?
A
Neural tube (CNS) and epidermis (skin).
9
Q
What is the prochordal plate and when does it appear?
A
A thickening at the cephalic end of the embryonic disc, appears on Days 14–15, marking the future mouth.
10
Q
What is the direction of growth of the primitive streak?
A
From the caudal to the cephalic end – called caudocephalic growth.
11
Q
Which gene family regulates the cephalo-caudal axis formation in the embryo?
A
HOX (Homeobox) genes.
12
Q
Which structure marks the start of gastrulation?
A
The primitive streak.
13
Q
What is the shape of the embryo in dorsal view during this period?
A
A disc-shaped embryonic plate.
14
Q
What are the three germ layers present by Day 18?
A
- Ectoderm
- Mesoderm
- Endoderm
15
Q
What embryonic structure forms the brain and spinal cord?
A
Neural tube (from ectoderm)
Cranial end → Brain
Caudal end → Spinal cord
16
Q
What is the primitive streak and where is it located?
A
It is a thickening of epiblast cells on the dorsal midline of the embryonic disc (caudal end) where gastrulation begins.
17
Q
What does the mesoderm give rise to by Week 4?
A
Heart tube ❤️
Somites (muscle blocks) 💪
Pharyngeal arches
Urogenital structures
Blood vessels
18
Q
What structure begins forming the mouth?
A
Mouth bud from ectoderm + endoderm contact (oropharyngeal membrane).
19
Q
What structures arise from endoderm in Week 4?
A
Gut tube (Foregut, Midgut, Hindgut)
Lung bud
Liver bud
Future urinary bladder
20
Q
What is the vitelline duct and where does it lead?
A
A diverticulum from the midgut that connects to the umbilical cord.
21
Q
What is the allantois and where does it arise from?
A
A diverticulum from the hindgut extending into the umbilical cord; important in early urine collection and bladder development.
22
Q
What happens to the yolk sac during Week 4?
A
It begins to disappear as the embryo develops and the gut tube forms.
23
Q
What is the amnion, and what does it surround?
A
A membrane derived from epiblast that surrounds the amniotic cavity; protects the embryo.
24
Q
What is the embryonic shield?
A
A flat disc-shaped bilaminar structure (epiblast + hypoblast) seen early in development.
25
What do neural crest cells (NCCs) originate from and what do they give rise to?
Originate from ectoderm (edges of neural plate)
Give rise to:
Peripheral nervous system (PNS)
Melanocytes
Facial cartilage & bone
Adrenal medulla
26
What is the relationship between cardiovascular tube and embryonic folding?
The heart tube initially lies anterior, but shifts ventrally and caudally as the embryo folds.
27
What structures are seen externally during Week 4?
Heart bulge
Pharyngeal arches
Eye buds 👁️
Somite blocks (muscle precursors)
Tail region
Umbilical cord
28
What forms the foregut, midgut, and hindgut?
The gut tube formed from endoderm.
29
What is the timeline for gastrulation?
Begins in Week 3 of development.
30
What is the sequence of germ layer formation from epiblast cells during gastrulation?
1. Endoderm (lines yolk sac)
2. Mesoderm
3. Ectoderm
4. Neural Crest Cells (NCCs)
31
What is the first type of cell to be specified during gastrulation?
Primordial Germ Cells (PGCs) – arise by the end of Week 2 from the epiblast.
32
What is the origin of all three germ layers in the embryo?
Epiblast cells
33
What is the correct location of gastrulation initiation in the embryo?
At the caudal end, where the primitive streak forms.
34
Q. Which of the following is NOT TRUE regarding gastrulation?
A. Occurs during 3rd week
B. Epiblast cells of inner cell mass form all germ layers
C. Establishes all three germ layers
D. Occurs at the caudal end of the embryo prior to its cephalic end
Gastrulation begins at the caudal end, but not prior to cephalic end – this option is misleading in wording.
35
What and where Primordial Germ Cells (PGCs) originate from?
From epiblast cells at the primitive streak (end of Week 2).
36
Describe the migration path of PGCs.
1. Originate near primitive streak
2. Migrate to yolk sac
3. Then to hindgut
4. Travel via mesentery
5. Reach mesonephros → genital ridge
37
What does the ectoderm give rise to?
Surface ectoderm → Skin, lens, anterior pituitary
Neuroectoderm → CNS
Neural crest cells → PNS, adrenal medulla, melanocytes, craniofacial structures
38
What does NCC (Neural Crest Cells) originate from?
From the edges of the neural plate (ectodermal origin).
39
What are the two types of trophoblast cells and their roles?
Syncytiotrophoblast → Invasive, secretes hCG
Cytotrophoblast → Mitotically active inner layer
40
What is the structure of the bilaminar disc (Week 2)?
Epiblast (E.P.) → Dorsal, columnar cells
Hypoblast (H.B.) → Ventral, cuboidal cells
Amniotic cavity above epiblast
Yolk sac below hypoblast
41
What are the extraembryonic layers derived from?
From trophoblast and hypoblast, forming:
Extraembryonic mesoderm (XE-M)
Extraembryonic endoderm (XE-E)
42
How do extraembryonic structures arise from hypoblast if it's replaced during gastrulation?
Hypoblast-derived extraembryonic structures form before gastrulation, and only the embryonic region hypoblast is replaced by epiblast during endoderm formation.
43
What is the first derivative of the epiblast in temporal sequence?
Primordial Germ Cells (PGCs) by Day 14–15
44
Where do PGCs first migrate after their formation?
To the endodermal wall of the yolk sac
45
During which week do PGCs migrate towards the genital ridge?
Week 4
46
By which week do PGCs reach the genital ridge?
End of Week 5
47
Through which path do PGCs migrate to the genital ridge?
1. Yolk sac
2. Hindgut
3. Dorsal mesentery
4. Genital ridge
48
What does abnormal migration of PGCs lead to?
Teratomas
Oropharyngeal
Sacrococcygeal
49
What is the controversial fact about PGC origin?
Derived from epiblast
Reside temporarily in endoderm of yolk sac
50
When does longitudinal folding of the embryo begin?
Week 3–4
51
What happens to the head and tail ends of the embryo during folding?
They fold ventrally and move towards each other
52
What happens to the amniotic cavity during folding?
It expands dorsally, enveloping the embryo
53
What happens to the yolk sac during folding?
It's incorporated into the embryo to form the gut tube
54
What lines the primitive gut tube?
Endoderm (from incorporated yolk sac lining)
55
What two diverticula enter the umbilical cord?
Vitelline duct (from midgut)
Allantois (from hindgut)
56
What is the relationship between cardiovascular tube and embryonic folding?
The heart tube initially lies anterior, but shifts ventrally and caudally as the embryo folds.
57
What structures are seen externally during Week 4?
Heart bulge
Pharyngeal arches
Eye buds 👁️
Somite blocks (muscle precursors)
Tail region
Umbilical cord
58
What forms the foregut, midgut, and hindgut?
The gut tube formed from endoderm.
59
What is the timeline for gastrulation?
Begins in Week 3 of development.
60
What is the sequence of germ layer formation from epiblast cells during gastrulation?
1. Endoderm (lines yolk sac)
2. Mesoderm
3. Ectoderm
4. Neural Crest Cells (NCCs)
61
What is the first type of cell to be specified during gastrulation?
Primordial Germ Cells (PGCs) – arise by the end of Week 2 from the epiblast.
62
What is the origin of all three germ layers in the embryo?
Epiblast cells
63
What is the correct location of gastrulation initiation in the embryo?
At the caudal end, where the primitive streak forms.
64
INICET-style Concept Check MCQ:
Q. Which of the following is NOT TRUE regarding gastrulation?
A. Occurs during 3rd week
B. Epiblast cells of inner cell mass form all germ layers
C. Establishes all three germ layers
D. Occurs at the caudal end of the embryo prior to its cephalic end
Correct Answer: ❌ D
→ Gastrulation begins at the caudal end, but not prior to cephalic end – this option is misleading in wording.
65
What do Primordial Germ Cells (PGCs) originate from?
From epiblast cells at the primitive streak (end of Week 2).
66
Describe the migration path of PGCs.
1. Originate near primitive streak
2. Migrate to yolk sac
3. Then to hindgut
4. Travel via mesentery
5. Reach mesonephros → genital ridge
67
What does the ectoderm give rise to?
Surface ectoderm → Skin, lens, anterior pituitary
Neuroectoderm → CNS
Neural crest cells → PNS, adrenal medulla, melanocytes, craniofacial structures
68
What does NCC (Neural Crest Cells) originate from?
From the edges of the neural plate (ectodermal origin).
69
What are the two types of trophoblast cells and their roles?
Syncytiotrophoblast → Invasive, secretes hCG
Cytotrophoblast → Mitotically active inner layer
70
What is the structure of the bilaminar disc (Week 2)?
Epiblast (E.P.) → Dorsal, columnar cells
Hypoblast (H.B.) → Ventral, cuboidal cells
Amniotic cavity above epiblast
Yolk sac below hypoblast
71
What are the extraembryonic layers derived from?
From trophoblast and hypoblast, forming:
Extraembryonic mesoderm (XE-M)
Extraembryonic endoderm (XE-E)
72
How do extraembryonic structures arise from hypoblast if it's replaced during gastrulation?
Hypoblast-derived extraembryonic structures form before gastrulation, and only the embryonic region hypoblast is replaced by epiblast during endoderm formation.
73
What is the first temporal derivative of the epiblast?
Primordial Germ Cells (PGCs) – appear by Day 14–15
74
From where do PGCs originate and where do they initially migrate?
Originate from epiblast
Migrate to endodermal wall of yolk sac
75
How do PGCs reach the genital ridge?
Week 4: Begin migration
Path: Yolk sac → hindgut → dorsal mesentery → genital ridge
By Week 5: Reach genital ridge
76
What happens if PGCs migrate abnormally?
They can cause teratomas in mislocated areas:
Oropharyngeal teratoma
Sacrococcygeal teratoma
77
What is the controversial MCQ regarding PGC origin?
Most accepted:
→ Epiblast-derived, reside temporarily in endoderm of yolk sac
78
What is the first derivative of the epiblast in temporal sequence?
Primordial Germ Cells (PGCs) by Day 14–15
79
Where do PGCs first migrate after their formation?
To the endodermal wall of the yolk sac
80
During which week do PGCs migrate towards the genital ridge?
Week 4
81
By which week do PGCs reach the genital ridge?
End of Week 5
82
Through which path do PGCs migrate to the genital ridge?
1. Yolk sac
2. Hindgut
3. Dorsal mesentery
4. Genital ridge
83
What does abnormal migration of PGCs lead to?
Teratomas
Oropharyngeal
Sacrococcygeal
84
What is the controversial fact about PGC origin?
Derived from epiblast
Reside temporarily in endoderm of yolk sac
85
When does longitudinal folding of the embryo begin?
Week 3–4
86
What happens to the head and tail ends of the embryo during folding?
They fold ventrally and move towards each other
87
What happens to the amniotic cavity during folding?
It expands dorsally, enveloping the embryo
88
What happens to the yolk sac during folding?
It's incorporated into the embryo to form the gut tube
89
What lines the primitive gut tube?
Endoderm (from incorporated yolk sac lining)
90
What two diverticula enter the umbilical cord?
1. Vitelline duct (from midgut)
2. Allantois (from hindgut)
91
What happens to the vitelline duct and allantois eventually?
They regress and become part of the umbilical cord
92
What does neuroectoderm form?
CNS → Brain + Spinal Cord
93
What does mesoderm form?
Cardiovascular system, bones, muscles, urogenital structures
94
What lines the entire gut tube?
Endoderm
95
96
What two major cavities are present on Day 14?
1. Amniotic cavity (dorsal)
2. Yolk sac cavity (ventral)
97
Which germ layer lies beneath the amniotic cavity?
Ectoderm (surface ectoderm)
98
What forms the dorsal supporting rod of the embryo?
Notochord (midline structure)
99
What structure is continuous with the gut tube during folding?
Ventral yolk sac cavity
100
What happens to the yolk sac cavity by Day 28?
It disappears, leaving behind the endodermal gut tube
101
What is the name of the cavity that forms within the embryo by Day 21?
Intra-embryonic coelomic cavity
102
What does the intra-embryonic coelom later differentiate into?
1. Pericardial cavity
2. Pleural cavity
3. Peritoneal cavity
103
What lines the peritoneal cavity?
Peritoneum
104
What is the name of the peritoneal fold suspending the gut tube dorsally?
Dorsal mesentery
105
What forms from the neural ectoderm?
Neural plate → Neural groove → Neural tube → CNS
106
What are neural crest cells derived from?
Borders of the neural plate ectoderm
107
What does the lateral plate of mesoderm form?
Splanchnic mesoderm layer
108
What does splanchnic mesoderm differentiate into in heart development?
Myocardial cells → Myocardium
109
What forms the cardiovascular tube?
Splanchnic mesoderm → Myocardial cells → Endocardial tube + Cardiac jelly = Heart tube
110
What suspends the developing heart tube within the pericardial cavity?
Dorsal mesocardium
111
What are the angiogenic cell clusters and where are they located?
Precursors of blood vessels
Found in splanchnic mesoderm around the heart region
112
What is the relationship of dorsal aorta to neural crest cells?
Neural crest cells contribute to the formation of dorsal aorta smooth muscle
