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Embryology > Chapter 5: Embryonic Period > Flashcards

Chapter 5: Embryonic Period Flashcards

1
Q

Embryonic period

A

The embryonic period or period of organogenesis, occurs from the 4th to the 8th week of development.
The time when each of the three germ layers of the embryo give rise to several derivatives.

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2
Q

Central nervous system

A

Formation of the central nervous system (neurulation) is induced by growth factors secreted from developing notochord.

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3
Q

Neural plate

A
  1. Thickened median region of the ectoderm between the primitive node and prochordal membrane.
  2. Formation is induced by factors that is released from from developing notochord.
  3. Thicker plate is due to increase number of ectodermal cells becomes taller.
  4. Two strips of cells known as neural crests are present on both sides of the neural plate.
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4
Q

Neural groove

A
  1. Median depression in the neural plate with two elevated folds on both sides.
  2. Neural crests are present on both sides of the neural folds.
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5
Q

How does fusion of the three structures occur?

A
  1. Neural folds fuse together, starting at the neck and extending in cranial and caudal directions to form a neural tube.
  2. The last parts to be fused are
    - Cranial neuropore: 25th week.
    - Caudal neuropore: 27th week.
  3. Neural crest strips fuse together dorsal to the neural tube. Then it splits into two columns, dorsi lateral to the neural tube.
  4. The rest of the ectoderm fuse together to cover the neural tube forming surface ectoderm.
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6
Q

Fate of neural tube

A

It forms the central nervous system:
1. Brain: inside the skull.
2. Spinal cord: in the vertebral canal.

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7
Q

Neural crest

A

Two strips of ectodermal cells on both sides of the neural plate.

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8
Q

Neural crests development

A
  1. During fusion of the neural folds, the two neural crests bands fuse together to form a single median strip dorsal to the neural tube that is covered by surface ectoderm.
  2. Then it divides into two longitudinal columns of cells on the dorsolateral aspect of the neural tube.
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9
Q

What are the derivatives of the neural crests?

A

Neural crest cells migrate to give rise to the following derivatives:
1. Ganglia:
- Sensory
- Sympathetic
- Parasympathetic
2. Cells
- Schwann
- Glial
- Melanoblast
- Pigmented epithelium of iris
3. Adrenal medulla, arachnoid, and pia meter.
4. Some bones of the skull and enamel of teeth.
5. Septum between ascending aorta and pulmonary trunk.

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10
Q

Otic and lens placodes

A

They are ectodermal thickenings at the cranial part of the embryo.
1. Otic placode:
- Otic vesicle and then the internal ear.
2. Lens placode:
- Lens of the eye.

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11
Q

Other derivatives of ectoderm:

A
  1. Peripheral nerves.
  2. Sensory epithelium:
    - Ear
    - Nose
    - Eye
    - Epidermis of skin
  3. Pituitary gland.
  4. Anterior part of oral cavity.
  5. Lower part of anal canal.
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12
Q

Intraembyonic mesoderm

A

Layer that separates between ectoderm and endoderm.

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13
Q

Intraembryonic mesoderm origin

A

Originate from epiblast cells that invaginate through the groove of primitive streak and primitive pit.

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14
Q

Intraembryonic mesoderm site

A

Present between ectoderm and endoderm except in the following sites:
1. Buccopharyngeal membrane.
2. Cloacal membrane.
3. Median region which is occupied by developing notochord and neural tube.

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15
Q

Intraembryonic mesoderm differentiation

A

At the 17th day, the Intraembryonic mesoderm is divided into 3 parts:
1. Paraxial mesoderm.
2. Intermediate mesoderm.
3. Lateral plate mesoderm.

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16
Q

Paraxial mesoderm

A

Present on both sides of the notochord and neural tube.

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17
Q

Intermediate mesoderm

A

Present between Paraxial mesoderm and lateral plate mesoderm.

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18
Q

Lateral plate mesoderm

A

Most lateral part.

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19
Q

Paraxial mesoderm formation

A

Cells from the cranial part of the primitive streak.

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20
Q

Intermediate mesoderm formation

A

Cells from the middle part of the primitive streak

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21
Q

Lateral plate mesoderm formation

A

Cells from the caudal part of the primitive streak

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22
Q

Segmentation of Paraxial mesoderm

A
  1. Paraxial mesoderm divides transversely into segments known as somites.
  2. Segmentation starts at the occipital region and extends caudally.
  3. Smaller segments of the of the paraxial mesoderm are present cephalic to the first occipital somite and are called somitomeres.
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23
Q

Somites

A

Segmented masses of the paraxial mesoderm.

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24
Q

Time of segmentation

A
  1. First pair appears in the 20th day and then 3 pairs are formed per day till the 30th day (somite period).
  2. Segmentation continuous in a slower rate till the 35th or 40th day.
25
Q

Number of somites

A

42-44 pairs of somites.

26
Q

Determination of the eye of embryo (somite period)

A

Age in days: (number of somites - 1/3) +20

27
Q

Regional classification

A
  1. 4 occipital
  2. 8 cervical
  3. 12 thoracic
  4. 5 lumber
  5. 5 Sacral
  6. 8-10 coccygeal somites
28
Q

Each somite is divided into

A
  1. Sclerotome.
  2. Dermomyotome.
29
Q

Scelerotome

A

Ventromedial part of the somite.

30
Q

Sclerotome function

A

Migrates medially and surrounds the notochord to form:
1. Vertebral bodies.
2. Invertebral discs.
Migrates medially and surrounds the neural tube to form:
1. Neural arch of vertebrae.

31
Q

Dermomyotome

A

Dorsolateral part of the somite.

32
Q

Dermomyotome is divided into

A
  1. Dermatome.
  2. Myotome.
33
Q

Dermatome function

A

Forms:
1. Dermis of the skin.
Epidermis is formed from the ectoderm.

34
Q

Myotome

A

Divides into
Dorsal part that forms:
1. Skeletal muscles of the back of the body.
Ventral part that forms:
Skeletal muscles of the anterolateral aspect of the body and limbs.

35
Q

What is the nerve supply of Myotomes?

A

Each spinal nerve divides into dorsal and ventral primary rami to supply the divided parts of myotomes.

36
Q

Intermediate mesoderm segmentation

A

Partially segmented.

37
Q

What is formed from intermediate mesoderm?

A

Urogenital system

38
Q

Lateral plate mesoderm location

A
  1. Lies lateral to the intermediate mesoderm.
  2. Lateral plate mesoderm on both sides are continuous together, cranial to Buccopharyngeal membrane.
  3. Continuous with the extra-embryonic mesoderm at the margin of the disc.
39
Q

Intraembryonic coelom

A

A horse shoe cavity that is formed in the lateral plate mesoderm.
It’s caudal ends communicate with the extraembryonic coelom at the margin of embryonic disc.

40
Q

Intraembryonic coelom is divided into

A
  1. Pericardial cavity: medial cranial part of the coelom.
  2. Peritoneal canals: lateral part of the cranial.
  3. Pleural cavities: between pericardium and peritoneal canals.
41
Q

Lateral plate mesoderm is split by Intraembryonic coelom into:

A
  1. Somatoplueric (somatic) mesoderm.
  2. Splanchnopleuric (splanchnic) mesoderm.
42
Q

Somatoplueric (somatic) mesoderm

A
  1. Mesoderm present in contact with ectoderm.
  2. Forms:
    - Connective tissue of anterolateral walls of the body.
    - Parietal layer of pluera, pericardium, and peritoneum.
43
Q

Splanchnopleuric splanchnic mesoderm

A
  1. Mesoderm present in contact with endoderm.
  2. Forms:
    - Smooth muscles.
    - Connective tissue of gut and respiratory tract.
    - Cardiac muscles.
    - Visceral pluera, pericardium, and peritoneum.
44
Q

Cardiogenic area

A

Splanchnic mesoderm related to the pericardium.

45
Q

Septum transversum

A

Mass of mesoderm:
1. Before folding: cranial to pericardium.
2. After folding: caudal to pericardium.

46
Q

Endodermal layer gives rise to: (derivatives goody )

A

Lining of epithelium of:
1. Digestive system except it’s beginning and end.
2. Respiratory tract.
3. Most of the urinary bladder and urethra.
4. Tympanic cavity and Eustachian tube.
Grandular epithelium:
1. Parenchyma of liver, pancreas, thyroid, thymus, tonsils (palatine and nasopharyngeal), and parathyroid glands.

47
Q

Derivatives of ectoderm (goody)

A
  1. Central nervous system.
  2. Neural crest.
  3. Otic lens and placodes.
  4. Other derivatives:
    - Peripheral nerves.
    - Sensory epithelium of ear, nose, eyes, and epidermis of skin.
    - Pituitary gland.
    - Anterior part of oral cavity and lower part of anal canal.
48
Q

Derivatives of Intraembryonic mesoderm (goody)

A
  1. Paraxial mesoderm.
  2. Intermediate mesoderm.
  3. Lateral plate mesoderm.
49
Q

Folding

A

A process by which the embryonic disc becomes folded upon itself.

50
Q

Time of folding

A

Starts by the end of the 3rd week and is completed at the end of the 4th week.

51
Q

Types of folding

A
  1. Cephalo-caudal folding: head and tail folds.
  2. Lateral folding: folding of the sides of the embryonic disc.
52
Q

Causes of folding

A
  1. Increases longitudinal length of the embryonic disc due to growth of neural tube and somites leads to cephalo-caudal folding.
  2. Expansion of amniotic cavity leads to folding in all directions.
53
Q

Limitations of head and tail folds

A
  1. Head fold: is limited by relativity firm cranial end of notochord.
  2. Tail fold: limited by relatively firm primitive streak.
54
Q

First step of folding

A

Expansion of amniotic cavity that leads to ventral shift of amnio-ectodermal junction towards endoderm with subsequent incorporation of a part of the yolk sac inside the folded embryo. The embryonic disc gradually bulges into the amniotic cavity.

55
Q

Second step of folding

A

The embryo acquires a cylindrical shape:
1. Ectoderm on the outer surface of embryo.
2. A cavity lined by endoderm: gut.
3. The primitive umbilical ring is present on the ventral body wall and is surrounded by the amnio-ectodermal junction.
4. The embryo becomes completely surrounded by the amniotic cavity bathing in the amniotic fluid.

56
Q

3rd step of folding

A

Cranial-caudal elongation of the embryonic disc by the growth of neural tube and somites leads to formation of heals fold and tail fold due to cephalo-caudal folding.

57
Q

Results of folding

A
  1. The embryonic disc changes into a cylindrical shape with a body cavity.
  2. Amniotic cavity surrounds the cylindrical embryo.
  3. Formation of primitive umbilical ring by ventral shifting of the amino-ectodermal junction.
    It contains the:
    - Connecting stalk.
    - Allantois.
    - Vitelline duct.
  4. Formation of gut lined with endoderm:
    - Foregut: head fold.
    - Midgut: middle.
    - Hindgut: tail fold.
  5. Formation of definitive yolk sac which is part of the yolk sac that remains outside the abdomen in the umbilical cord. The midgut is connected to the definitive yolk sac by vitelline duct.
  6. Formation of forebrain bulge in the head fold which is separated from pericardial by stomodeum.
    Buccopharyngeal membrane separates stomodeum from forgut
  7. Peritoneal canals of the Intraembryonic coelom surround the gut with subsequent formation of the mesenteries.
  8. Reversal of position:
    - heart and pericardium become cranial to septum transversum ( before folding, septum was the most cranial).
    - connecting stalk and allantois become ventral and cranial to cloacal membrane instead of being the most caudal before folding.
58
Q

.

A

Embryology (8 decks)

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