5.2.1 Development of the Head and Neck 1 Flashcards
What are the features of the embryo at 4 weeks old?
- No face yet
- Head and neck take up almost half of body
- Follows semental pattern
- Each segment has structures from various systems
What are the 5 parts of the pharyngeal arches?
Cartilage Bar
Cranial Nerve
Ectoderm
Artery
Endoderm
What does the cartilage bar have?
Associated muscles, and supports the arch
What does the cranial nerve do in the pharyngeal arch?
Each arch has a different nerve
Motor to muscles associated with cartilage
What does the ectoderm do?
Covers superficial arch surface
Becomes skin and nervous system
What is the arterial origin?
Mesoderm
What does the endoderm do?
Covers deep aspect of the arch
Forms GI, respiratory system and endocrine glands
Fill in the table
How many pharyngeal arches are there?
5
P1,2,3,4 and 6
No 5th arch in humans
What is formed by the pharyngeal arches?
Pharyngeal clefts and pouches
What is a pharyngeal cleft?
Invagination of ectoderm = cleft
What is a pharyngeal pouch?
Evagination of endoderm
What happens to the first pharyngeal cleft?
Very close association with first pouch, nearly meets in midline, future tympanic membrane separates the two
Forms the External accoustic meatus
What happens to the first pharyngeal pouch?
Forms the eustachian tube (tubotympanic recess in utero)
What happens to the remaining pharyngeal clefts?
Subsequent clefts disappear due to second arch descending and closing them off obliterating them
What does the second pouch form?
Palatine tonsil
What does the third pouch form?
Thymus and parathyroid III
What does the fourth pouch form?
Parathyroid IV
What happens if the second arch does not fully obliterate the clefts?
Keads to formation of branchial cysts, sinuses and fistulas
Branchial=pharyngeal
How does a branchial cyst form?
Second arch descends and covers the cleft, but inner cleft not completely obliterated and not fully closed off
No communication with other structures, fluid-filled enclosed space
How does a branchial sinus form?
Second arch does not properly cover the cleft
Cervical sinus remnant
Communicates with skin
How does a branchial fistula form?
Second arch does not cover cleft at all
Cervical sinus remnant extends to the endoderm
Connection between skin and pharynx
Why are branchial cysts, sinuses and fistulae nearly always anterior to SCM?
SCM grows up to the base of the skull
SCM sits behind the cervical sinus, and thus is posterior to the clefts