^ Chapter 6 - 3rd to 8th weeks: The embryonic period Flashcards Preview

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Flashcards in ^ Chapter 6 - 3rd to 8th weeks: The embryonic period Deck (104):
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When does the embryonic period, or period of organogenesis, occure?

It occurs from 3rd to 8th weeks

1

What happens in the embryonic period, or the period of organogenesis?

The three germ layers (ectoderm, mesoderm, and endoderm) gives rise to a number of specific tissues and organs.

2

When does the main organ systems become established, rendering the major features of the external body form recognizable by the 2nd month?

By the end of embryonic period

3

When does the germ layer have the shape of a disk that is broader in the cephalic than in the caudal region?

At the beginning of the third week of development

4

When does the neural plate form?

At the beginning of the third week

5

Neuroectoderm

- Cells of the neural plate make up the neuroectoderm
- Their induction represents the initial event in the process of neurulation

6

What cause the induction of the neural plate?

The upregulation of fibroblast growth factor (FGF) signaling together with inhibition of the activity of bone morphogenic protein 4 (BMP4), a transfoming growth factor-B (TGF-B) family member responisble for ventralizing ectoderm and mesoderm, causes induction of the neuronal plate.

7

BMP4

BMP4 permeates the mesoderm and ectoderm of the gastrulating embryo, ectoderm is induced to form epiderims, and mesoderm forms intermediate and lateral plate mesoderm

8

Which proteins inactivate BMP (bone morphogenic protein)

Noggin, chordin, and follistatin

9

Where can we find noggin, chordin, and follistatin?

They are present in the organizer (primitive node), notochord, and prechordal mesoderm

10

What cause the mesoderm to become notochord and paraxial mesoderm (dorsalizes mesoderm)?

Noggin, chordin, and follistatin neuralize ectoderm by inhibiting BMP and cause the mesoderm to become notochord and paraxial mesoderm (dorsalizes mesoderm).

11

Induction of caudal neural plate structures (hindbrain and spinal cord) depends on which two secreted proteins?

- WNT3a
- FGF

12

Retinoic acid (RA)

Appears to play a role in organizing the cranial-to-caudal axis because it can cause respecificaton of cranial segments into more caudal ones by regulating expression of homeobox genes.

13

What is neurolation?

Neurolation is the process whereby the neuronal plate forms the neural tube.

14

When does the neural plate become elevated to form neural folds and the depressed midregion forms the neural groove?

By the end of the 3rd week.

15

What does the neural crest undergo as it leaves neuroectoderm by active migration and displacement to enter the underlying mesoderm?

Neural crest undergoes an epithelial-to-mesenchymal transition as it leaves the neuroectoderm

16

Mesoderm def.

Refers to the cells derived from the epiblast and extraembryonic tissue

17

Mesenchyme def.

Refers to the loosely prganized embryonic connective tisuue

18

Dorsal pathway through the dermis

Is where the crest cells will enter the ectoderm through holes in the basal lamina to form melanocytes in the skin and hair follicles

19

Ventral pathway through the anterior half of each somite to become (the crest cells):

- Sensory ganglia
- Sympathetic and enteric neurons
- Schwann's cells
- Cells of the adrenal medulla

20

Neural crest cells that leaves the neural tube before closure contribute to the:

- Craniofacial skeleton
- Neurons for cranial ganglia
- Glial cells
- Melanocytes
- Other cell types

21

Neural crest also refered to as

the fourth germ layers, because it is important and it contributes to so many organs.

22

SNAIL and FOXD3

specify cells as neural crest

23

SLUG

promotes crest cell migration from the neuroectoderm

24

The fate of the entire ectodermal germ layer depends on....

BMP concentrations

25

High level of BMP

induce epidermis formation

26

Intermediate levels of BMP at the border of the neural plate and surface of ectoderm

Induce neural crest

27

Very low concentration of BMP

Cause formation of neural ectoderm

28

What regulates the neural crest cell migration, proliferation, and differentiation

- BMPs
- other members of TGF-B family
- FGFs

29

Ectodermal thickenings

- The otic placodes
- The lens placodes

30

Where does the structures needed for hearing and maintenace of equlibrium come from?

Otic placodes -> Otic vesicles -> structures needed for hearing and maintenace of equlibrium come from

31

Placodes --> lenses for eyes

During the 5th week

32

The ectoderm gives rise to:

gives rise to organs and structures that maintain contact with the outside world:
● The central nervous system
● The peripheral nervous system
● The sensory epithelium of the ear, nose, and eye
● The epidermis, including the hair and nails.

In addition, it gives rise to:
● Subcutaneous glands
● The mammary glands
● The pituitary gland
● And enamel of the teeth

33

Neural crest derivates

- Connective tissue and bones of the face and skull
- Cranial nerve ganglia
- C cells of the thyroid gland
- Conotruncal septum in the heart
- Odontoblasts
- Dermis in face and neck
- Spinal (dorsal root) ganliga
- Sympathetic chain and preaortic ganglia
- Parasympathetic ganlia of the gastrointestinal tract
- Adrenal medulla
- Schwann's cells
- Glial cells
- Meninges (forebrain)
- Melanocytes
- Smooth muscle cells to blood vessles of the face and forebrain

34

Paraxial mesoderm is formed by the

17th day

35

The lateral plate (thin mesoderm layer) is divided into 2 layers:

1. The somatic or parietal mesoderm layer
2. The splanchnic or visceral mesoderm layer

36

Which lateral plate layer covers the yolk sac?

The splanchnic or visceral mesoderm layer

37

intraembryonic cavity is formed by

the layers of the lateral plate

38

What connects the paraxial and lateral mesoderm?

The intermediate mesoderm

39

The intraembryonic cavity is continous with the...

extraembryonic cavity

40

When does the paraxial mesoderm to be organized into segments?

By the beginning of the 3rd week

41

Somitomeres

segments that the mesoderm

42

From the occipital region caudally, somitomeres further organize into..

somites

43

When does the first pair of somites arise?

the 20th day

44

How many pairs of somites to we have by the end of 5th week?

42-44 pairs:
- 4 occipital
- 8 cervical
- 12 thoracic
- 5 lumbar
- 5 Sacral
- 8-10 coccygeal pairs

The first occipital and the last 5-7 coccygeal somites later dissapears

45

What regulates the boundaries of each somite?

- Retinoic acid (RA)
- a combination of FGF8 and WNT3a

46

Each somite forms its own

- Sclerotome
- Myotome
- Dermatome

48

Sclerotome

- The tendon cartilage and bone component
- Differentiate into vertebrae and ribs

49

Myotome

- Providing the segmental muscle component

50

Dermatome

Forms the dermis of the back

51

Does the myotome and dermatome have their own segmental nerve component?

Yes

52

What is the noggin genes and sonic hedgehog (SHH) by?

They are produced by the notochord and floor plate of the neural tube.

53

Which transcription factor does the sclerotome cells express?

Sclerotome cells express the transcription factor PAX1

54

PAX1

Initiates the cascade of cartilage- and bone-forming genes for vertebral formation

55

What regulates the expression of PAX3?

WNT proteins from the dorsal neural tube

56

PAX3

marks the dermomyotome region of the somite

57

Muscle-specific genes

MYOD
MYF5

58

Neurotophin 3 (NT-3)

- Secreted by the dorsal region of the neural tube, to form dermis
- Direct the midportion of the dorsal epithelium of the somite

59

Intermediate mesoderm

- Differentiate into urogenital structures
- Temporarely connects paraxial mesoderm with the lateral plate

60

Where does it form segmental cell clust regions?

In cervical and upper thoracic region (future nephrotome)

61

Where does it form unsegmented mass of tissue, the nephorgenic cord?

More caudally

62

Excretory units of the urinary system and the gonads develop from.....

the party segmented, partly unsegmented intermediate mesoderm

63

Lateral mesoderm splits into..

- Parietal (somatic) layer
- Visceral (Splanchnic) layer

64

The parietal (somatic) layer

Lines the intraembryonic cavity

65

The visceral (splanchnic) layer

Surrounds the organs

66

Lateral body fold =

Mesoderm from parietal layer + ectoderm

67

What forms the dermis of the skin in the body wlal and the limbs, the bones and the connective tissue of the limbs, and the sternum?

The parietal layer of lateral plate mesodem

68

What forms the costal cartilages, limb muscles, and most of the body wall muscle?

Sclerotome and muscle precusor cells that migrate into the parietal layer of lateral plate mesoderm

69

Mesothelial membranes, or serous membranes....

- Line the perioneal, pleural, and pericardial cavities and secret serous fluid
- Thin membranes formed by mesoderm cells of the parietal layer surrounding the intraembryonic cavity

70

What forms a thin serous membrane around each organ?

Mesoderm cells of the visceral layer

71

Where does blood cells and blood vessles arise from?

mesoderm

72

Which two ways does the blood vessles form in?

1. Vasculogenesis
2. Angiogenesis

73

Vasculogenesis

Vessels arise from blood islands

74

Angiogenesis

Blood vessles forming from already existing vessles

75

When and where does the first blood islands appear?

- They appear in mesoderm surrounding the wall of the yolk sac at 3 weeks of development and slightly later in lateral plate mesoderm and other regions.

76

Where does the blood islands arise from?

The islands arise from mesoderm cells that are induced to form hemangioblasts, a common precusor for vessles and blood cell formation.

77

Hemangioblasts

- Directed to form blood cells and vessles by vascular endothelial growth factor (VEGF)
- A common precusor for vessles and blood cell formation

78

Where does the first blood cells arise in?

The first blood cells arise in islands in the wall of the yolk sac, this population is transitory (lasts only for a short period of time)

79

Where does the hematopoietic cells derive from?

They derive from mesoderm surrounding aorta in a site near the developing mesonephric kindey called aortagonad-mesonephros region (AGM)

80

A major hemopoietic organ of the embryo and fetus?

- The liver -> from the 2nd to 7th months of development

81

When does the stem cells from the liver colonize the bone marrow?

In the 7th month of gestation. After that the liver loses its blood-forming function

82

When does the liver lose its blood-forming function?

After the 7th month of gestation

83

FGF2

Induces blood island development from competent mesoderm cells that form hemangioblasts

84

Vascular endothelial growth factor (VEGF)

- Is secreted by surrounding mesoderm cells
- Stimulates proliferation of endothelial cells at points where new vessles are to be formed

85

Peripheral hemangioblasts differentiate into...

Angioblast

86

Hemangioblasts in the center of blood islands form...

Hematopoietic stem cells

87

Where is SHH secreted?

It is secreted by notochord

88

PROX1

Appears to be master gene for lymphatic vessle differentiantion.

89

What is the main organ system derived from the endodermal germ layer?

The gastrointestinal tract is the main organ system derived from the endodermal germ layer.

90

What forms the roof of the yolk sac?

The gastrointestinal tract

91

Ventral body wall defects

Failure of the lateral body folds to close the body wall results in ventral body wall defects.

92

The gut tube

The gut tube is divided into three regions:
- Foregut
- Midgut
- Hindgut

93

How does the midgut communicate with the yolk sac?

Through vitelline (yolk sac) duct

94

What is the ectodermal-endodermal membrane called?

At its cephalic end, the foregut is temporarily bounded by an ectodermal–endodermal membrane called the oropharyngeal membrane.

95

Oropharyngeal membrane

This membrane separates the stomadeum, the primitive oral cavity derived from ectoderm, from the pharynx, a part of the foregut derived from endoderm.

96

What happens with the oropharyngeal membran in the 4th week?

In the fourth week, the oropharngeal membrane ruptures, establishing an open connection between the oral cavity and the primitive gut. The hindgut also terminates temporarily at an ectodermal–endodermal membrane, the cloacal membranen.

97

The cloacal membrane

This membrane separates the upper part of the anal canal, derived from endoderm, from the lower part, called the proctodeum, which is formed by an invaginating pit lined by ectoderm.

98

Proctodeum(?)

The proctodeum breaks down in the seventh week to create the opening for the anus.

99

When is the yolk sac duct, allantois, and umbilical vessels are restricted to the umbilical region

By the 5th week, the yolk sac duct, allantois, and umbilical vessels are restricted to the umbilical region

100

Endoderm gives rise to:

● The epithelial lining of the respiratory trac
● The parenchyma of the thyroid, parathyroids,
liver, and pancreas
● The reticular stroma of the tonsils and the
thymus
● The epithelial lining of the urinary bladder
and the urethra
● The epithelial lining of the tympanic cavity and
auditory tube

101

What is the homeobox genes known for?

Homeobox genes are known for their homeodomain, a DNA-binding motif, the homeobox.

102

Homeotic cluster

Many homeobox genes are collected into homeotic clusters

103

Hom-C in drosophila

Is the name of the hemopoietic gene complex that is an important cluster of genes specifying the craniocaudal axis

104

How is the genes, which contans Antennapedia and Bithorac classes of homeotic genes, organized?

They are organized on a single chromosome as a functional unit