2 - Chronology in the development of the maxillofacial region by weeks Flashcards
Periods of Prenatal development
Proliferative Period - First 2 weeks when cell division is prevalent
Embryonic Period - weeks 2 - 8
Fetal period - 8th week to birth
Week 1
- Sperm + Ovum = Zygote
- zygote moves to uterus + undergoes mitotic divisions = formation of morula
- Morula grows into blastocyst
Blastocyst - 2 layers of cells
- Inner layer = Embryoblast ⇢ Becomes embryo
- Outer layer = Trophoblasts ⇢ forms embryonic part of placenta
day 5 - Blastocyst attaches to the endometrium in the posterior wall of uterus
day 6 - Trophoblasts differentiate + penetrate endometrial epithelium
By the end of week 1 superficial implantation of Blastocyst is complete
Week 2
Rapid proliferation + differentiation of trophoblasts
Day 9 - Blastocyst completely embedded in endometrial epithelium
Day 11 + 12 - Primary yolk sac forms + part of trophoblasts differentiate ⇢ extraembryonic mesoderm
By the end of week 2:
- Primary yolk sac replaced by secondary yolk sac
- Amniotic cavity appears as a space between trophoblasts + inner cell mass
- inner cell mass ⇢ Bilaminar embryonic disk
- Epiblast/Outer layer ⇢ related to amniotic cavity
- Hypoblast/Inner layer ⇢ lies adjacent to blastocyst cavity
Week 3
Formation of the trilaminar embryonic disk via gastrulation
Gastrulation
- Towards 14 days: Primitive ectoderm thickens ⇢ Primitive Plaque
- Primitive plaque ⇢ Primitive groove
- Within cranial edge is formed Primitive Node
- Cells of primitive groove + node acquire mitotic activity:
- They divide + penetrate between ectoderm + endoderm
Neural Crest Cell Formation
- Neural plate invaginates ⇢ Neural Folds
- Neural Folds fuse at junction between brain + spinal cord ⇢ Neural tube
- Neural tube separates from overlying ectoderm
- Ectodermal cells ⇢ Neural crest
- Neural crest cells migrate throughout developing embryo + differentiate into many cell + tissue types
- Neural tube fuses cranially ⇢ Mesencephalic flexure ⇢ Major divisions of brain can begin to be distinguish
By end of 3rd week further growth of neural tube gives rise to ⇢ frontal nasal process + branchial arches
Week 4
embryo begins to form a tubular structure
- Infront of Primitive streak is the Primitive node ⇢ Mesodermal cells that form notochord
- Differentiation of cells from the ectoderm ⇢ Neuroectoderm ⇢ Neural plate ⇢ Neural Groove, surrounded by 2 neural folds
- Neural crest cells develop from neural folds
Neural crest
- as neural tube forms - group of cells separate from neuroectoderm ⇢ migrate + differentiate within developing embryo ⇢ basis for nerves structures
- Neural Crest cells in head region differentiate to form most of the CT in the head
- Embryonic CT aka Mesenchyme is derived from mesoderm
- Neural Crest cells divide rapidly + form 3 cell flow w/different biological functions
- 1st group - develops at site of its formation
- 2nd group - forms a migratory flow
- 3rd group - intended to be absorbed + stimulates differentiation of surrounding tissue
Week 5
- The base thickens ⇢ forms Cartilaginous cranial base via neuroectodermal condensation of mesenchyme
- Calvaria develops via direct membranous ossification
Week 6
- Medial nasal processes approach each other ⇢ forms single globular process that eventually gives rise to ⇢ Nasal tip, frenulum + 1° palate
- Meanwhile frontonasal processes collapse inwards ⇢ forms nasal septum
- Continued growth of maxillary mass = fusion w/lateral nasal process
- As fusion occurs ⇢ epithelial groove arises ⇢ Becomes nasolacrimal duct
- Maxillary process fuses w/Medial nasal fold = nostril
- developing nasal floor is open to the oral cavity
- within primitive stomodeum ⇢ lateral palatine processes develop from maxillary process ⇢ give rise to 2° palate
At this stage:
- developing tongue nearly fills oronasal cavity completely + reaches nasal septum
- eye continues its fevelopment as neural crests differentiate into early cornea + sclera
- external ear develops from 6 mesenchymal swellings that surround 1st branchial cleft
Week 8
- Primitive upper + lower jaws formed via complete fusion of maxillary + mandibular processes
- upper lip + nasal regions better defined
- Eyes begin to take a more medial position as frontocranial region loses overhang
- Development of skull continues ⇢ ossification centres arising within mesenchyme adjacent to developing brain ⇢ forms flat bones of calvarial vault, 2 parietal and 2 frontal bones
in Face
- Nasal bones appear above nasal capsule
- On lateral surface of nasal capsule ⇢ premaxillary + maxillary ossification centres appear
- Ossification centres arise within zygomatic + squamoid portion of temporal bone
- mesenchyme around Meckel cartilage ⇢ membranous ossification ⇢ body of mandible
- onset of bone formation = end of embryonic period
Fetal Period
Continued from 8th week until birth
During this period formation of:
- Tissues
- Organs performed differentiation
- embryo grows + prepares for function
