Embryology Flashcards
Types of fusion:
True fusion
Apparent fusion
True fusion:
Epithelium breaks down to allow continuity of mesenchymal layer below
Eg: palatal processes → secondary palate
Neural folds → neural tube
Apparent fusion
Tissues were originally connected with a continuous epithelium—lateral extension eliminates the furrow
Eg: facial processes fusion, formation of lateral commissures of lips
Wk 1
Fertilisation of ovum by sperm forms zygote
Zygote divides to become morula
Which divides to become blastocyst
Trophoblasts - help attach via placenta to uterus wall
Embryoblasts - become embryo
Wk 2
Bilaminar germ disc is formed
Epiblast layer → forms amniotic sac
Hypoblast layer → forms yolk sac
Wk 3
Primitive streak forms
Establishes anterio-posterior relationship; pathway for migration of epiblast cells
Trilaminar disc forms
Gastrulation: Epiblast differentiates into ectoderm, mesoderm and endoderm.
Ectoderm forms the neural plate
Neural plate invaginates, forming neural folds that deepen until it forms a neural tube which detaches from the ectoderm, and neural crests from the adjacent parts.
Neural crest cells form from the neuroectoderm
Undergo epithelio-mesenchymal transformation to become ectomesenchyme
Mesenchyme: loose cells of mesoderm
Wk 4
Rostral-caudal folding of the embryo
Stomatodeum (primitive oral cavity) forms
Neural tube expands forming primitive forebrain
Branchial arches form (branchial grooves on outside, pharyngeal pouches inside)
Tongue forms from the tuberculum impar and lingual swellings from the 1st BA (anterior) which fuses with the hypobranchial eminence (posterior 1/3rd) from the 2,3,4th BA.
Occipital somites will give rise to muscles of tongue.
Arch
Forms
1
Mx arch and
Md arch (in the form of Meckel’s cartilage)
Muscles of mastication
CN V Trigeminal nerve
2
Reichert’s cartilage
Muscles of facial expression
CN VII facial nerve
3
CN IX Glossopharyngeal nerve, CN X vagus nerve
Tongue formation
Tongue formation
From the 1st BA
Anterior ⅔: centre swelling (tuberculum impar) + two lingual swellings left and right. Innervated by trigeminal nerve → Md division lingual branch
Line in the middle remains called the median sulcus
Anterior and Posterior is separated by the terminal sulcus
IN WEEK 5:
Posterior 1/3rd
Made of BA 2,3,4
Copula forms between the 2nd and 3rd BA
Hypobranchial eminence
Very back of the tongue is a 3rd swelling from the 4th BA, innervated by CN X vagus.
Wk 5
Olfactory placodes
form from localised thickenings within ectoderm of frontal prominence
Nasal process and pit
From from proliferation of mesenchyme around olfactory placodes → forming horseshoe-shaped ridges
Lateral part of horseshoe - lateral nasal process
Medial part of horseshoe - medial nasal process
Formation of lower lip
Fusing of mandibular processes
Wk 6
Mx and Md processes fuse, forming the cheeks
Meckel’s cartilage
Acts as scaffold for intramembraneous ossification
Mandibular nerve splits into IAN (inferior alveolar nerve) and lingual nerve
Ossification of mandible: IAN splits into incisive and mental branches
Left and right parts remain separate until symphysis of Md processes after birth
[Primary epithelial bands form along Mx and Md arches]
Wk 7
Formation of upper lip
Middle part of upper lip + primary palate: fusion of medial nasal processes with frontonasal process
Lateral aspects of upper lip: fusion of anterior part of Mx process + lateral aspects of medial nasal process
Nasal septum grows downward from frontonasal process
Formation of intermaxillary segment which completes primary palate - aka premaxilla ie the alveolar arch + front part of hard palate:
Fusion of Mx process + medial nasal process
Palatine shelves extend from Mx processes
Maxilla = Palatine shelves (aka lateral palatine processes) + medial nasal process
[Primary epithelial bands grow into ectomesenchyme, forming dental lamina and vestibular lamina]
Wk 8
Formation of secondary palate
Tongue depresses, allowing palatine shelves to ascend
Palatine shelves fuse together
[Bud stage]
Wk 9-11
Wk 10 - Cap stage
Wk 12
Palatine shelves fuse with nasal septum, sealing off oral cavity from nasal cavity
[Bell stage]
Wk 14
[Stratum intermedium forms from epithelium between IEE and stellate reticulum]
[Histodifferentiation of ameloblasts from IEE and odontoblasts from OEE]
Types of CLP
Type of cleft
Caused by incomplete fusion of
Unilateral/bilateral cleft lip - Mx + medial nasal process
Oblique facial cleft - Mx process and lateral nasal process
Median cleft lip - Medial nasal processes and frontonasal process
Mandibular cleft - Md processes
Cleft of lip and alveolus aka just cleft lip
Cleft of lip and primary palate
Cleft lip and palate
Cleft palate only / Median cleft palate - Palatine shelves
Causes of CLP
Mutation of Hox (regulatory) genes
MSX1 which regulates expression of
BMP2 and BMP4 which are in charge of proliferation of palatal mesenchyme. Insufficient proliferation → shelves cannot elevate
FGF9 - fibroblast growth factor - control palatal growth and fusion, req for timely elevation
Environmental:
Lack of folic acid in diet of mother
Smoking
Alcohol
Radiation
Rubella virus
Certain drugs/hormones: topiramate (epileptic), valproic acid (psychiatric - BPD)
