Embryology

Question 1

First ossification of bone occur in ….period

Answer 1

o Marked by first appearance of ossification centers that form bones in fetal period

Question 2

Significance of embryonic period

Answer 2

o It is the most important & most sensitive period, because it is the period of intensive
differentiation and
development of the major organs and systems.
o Congenital malformations usually occur during this period.

Question 3

Effect of Teratogen timing

Answer 3

1)Proliferative period: IF teratogen damage so many cells, it leads to miscarriage. If few cells are damaged, they are compensated for by the remaining undifferentiated cells (Minor effect).
2) Embyronic period: High effect
numerous malformations.
3) Fetal period: Minor effect

Question 4

Yolk cavity function

Answer 4

The other cavity facing the endoderm forms the yolk sac, gives nutrition for the embryo & help in GIT development.

Question 5

4th germ layer

Answer 5

neural crest cells, 4th germ layer or ectomesenchymal cells .

Question 6

Fate of neural tube

Answer 6

The central nervous system is established,  The neural tube will form the spinal cord  Its cranial portions enlarge to form the primary division of the brain.

Question 7

Neural crest cell drivatives

Answer 7

1) Schwann cells. 2) Sensory ganglia 3) Sympathetic neurons. 4) Pigmented cells ( Melanocytes ). 5) Most of the Mesenchyme of the branchial arches and face.
6) Essential in development of all Dental tissues

Question 8

Mandibulofacial dysistoses

Answer 8

Genetic disorder characterized by: 1. Underdeveloped facial bone including the mandible.
2. Lower border of the mandible concave. 3. Face appear dropping. 4. Malformed Ear 5. Cleft palate. 6. Dentinogenesis imperfecta (Teeth are discolored (blue-gray or yellow-brown) and translucent. Teeth are also weaker than normal, prone to rapid
wear, breakage, and loss).

Question 9

Formation of pituitary gland

Answer 9

From roof of oral cavity an ectodermal pouch called Rathke’s pouch grow up into brain floor.
 Middle and posterior lobes develop from brain’s infundibulum.

Question 10

General description of branchial arches

Answer 10

5-6 pairs of parallel bars ventrally arranged in the lower part of face and neck.
• The 1st four are well developed & numbered (I, II, III and IV ) craniocaudally.
• Intially, only the 1st & 2nd extend to midline. • Next ones are smaller progressively. • The 5th is transient in human.

Question 11

Formation of tympanic membrane

Answer 11

Ectoderm in the depth + mesoderm & endoderm of the first pouch forms the tympanic membrane

Question 12

Structure that could turn to cervical cyst or fistula

Answer 12

Cervical sinus

Question 13

Derivatives of pharyngeal bouches

Answer 13

Endoderm of the pharyngeal pouches differentiate into;
 1st pouch tube.
 2nd pouch Middle ear and Eustachian Epithelium covering palatine tonsils and its crypts.  3rd pouch thymus gland.  4th pouch Superior parathyroid and ultimobranchial body Inferi

Question 14

Thymus gland fate

Answer 14

Thymus gland; large at birth, continuous to grow only until puberty, gradually atrophies and completely disappears later in life

Question 15

hormone important for the function of T- lymphocyte

Answer 15

Thymopoietin

Question 16

Nerve supply of branchial arches

Answer 16

 1st arch by • Mandibular division of Trigeminal nerve (V)
• Chorda tempani branch of Facial nerve (VII).
 2nd arch by Facial nerve (VII).  3rd arch by Glossopharyngeal nerve ( IX)
 4th arch by Vagus nerve (X).

Question 17

nasolacrimal groove, which later becomes obliterated. If not, gives …

Answer 17

oblique facial cleft

Question 18

How nasal placode becomes horse shoe

Answer 18

Mesenchymal proliferations at lower margins of each nasal placode convert it into horse-shoe shaped structure surrounding “Nasal pit” at 6th wiul.

Question 19

Lateral nasal process gives

Answer 19

• Give rise to sides ( alaa) of nose.

Question 20

Failure of fusion of 2 mand processes

Answer 20

Failure

Median cleft of the mandible.

Question 21

Tongue attached at floor of the mouth posteriorly so …

Answer 21

The tongue’s posterior end is attached to the floor, so elevation start at posterior parts pressing the tongue downward & forward to release the anterior parts

Question 22

Body of tongue is formed by union of 2 bilateral parts represented by central longitudinal line on dorsal surface called …

Answer 22

median sulcus

Question 23

Origin of tongue

Answer 23

Origin: 1st , 2nd , 3rd & part of 4th

branchial archs & occipital myotomes’ muscle fibers.

Question 24

Development of the tongue ant 2 thirds

Answer 24

Local mesenchymal proliferation give 3 elevations at ventromedial aspect of 1st branchial “Mandibular arch”
1. Two lateral lingual swellings on the internal surface of the two mandibular processes on each side of the median plane.
2. A small median elevation called tuberculum impar just posterior to two lateral lingual swelling.The two Lateral lingual swellings grow rapidly backward and forward & medially to merge with each other and overgrow the tuberculum impar forming a large mass that later form the mucous membrane of the anterior 2/3.
The line of union between the two lateral lingual swellings is indicated Externally by Median sulcus and Internally by Median raphae.

Question 25

Supply for central part of post third of tongue

Answer 25

Internal laryngeal (branch of vagus X).

Question 26

Muscle of tongue development

Answer 26

 Arise at 2nd miul (9th wiul)  From the occipital somite myotomes.
Tongue Muscles
 Migrated from developing brain forward into the tongue area
 Carrying with them their nerve supply, the 12th cranial nerve (hypoglossal nerve)

Question 27

Time frame of condylar cartilage

Answer 27

The condylar cartilage:
Carrot shaped cartilage appears at 12th WIU
at 14th WIU converted to bone by endochondral ossification
At 20th WIU, only a thin layer of cartilage remains in the condylar head
Acts as an active growth center till 20 years of age