Embryology Timeline

Sources:

Fehrenbach, M., & Herring, S. (2017). Illustrated Anatomy of the Head and Neck (5th ed.). Elsevier S̀̀̀̀̀̀̀aunders.

Fehrenbach, M., & Popowics, T. (2016). Illustrated Dental Embryology, Histology, and Anatomy (5th ed.). Elsevier S̀̀̀̀̀̀̀aunders.

Fetal development: The 1st trimester. (2022, June 3). Mayo Clinic. https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/prenatal-care/art-20045302

Professional, C. C. M. (n.d.). Fetal Development. Cleveland Clinic. https://my.clevelandclinic.org/health/articles/7247-fetal-development-stages-of-growth

Embryology Timeline

Makayla Larsen

Embryonic Period

Pre-Implantation Period

Fetal Period

The Fetal Period: the maturation of existing structures occurring as the embryo enlarges to become a fetus

The Pre-Implantation Period: takes place during the first week after conception

5 Physiological Processes

• Induction - action of a group of cells that leads to a developmental pathway in responding tissue
• Proliferation - controlled levels of cellular growth
• Differentiation - cells become structurally & functionally different
• Morphogenesis - process of development of specific tissue morphology
• Maturation - attainment of adult size, form, & function

Week 7

Week 11

Week 8

Week 12

Week 9

Week 10

Week 3

Week 4

Week 5

Week 6

Week 7

Week 8

Week 12

Week 11

Week 9

Week 10

Week 1

Week 2

Week 3

Week 4

Week 5

Week 6

Week 1

Week 2

• Taste Buds Develop

• Fusion of Final Palate begins

• Muscles of Mastication
• Upper Lip Formation

• Final Palate Completed
• Nasal Cavity Completed

• Muscles of Mastication
• Filiform Pappillae

• Upper Lip Formation
• Secondary Palate Formation

• Nasal Cavity Complete
• Tongue Fusions Complete
• Lingual Papillae Appear

• Bilaminar Disc forms from Blastocyst
• Placenta develops

• Fertilization takes place when the ovum & sperm are united forming a zygote
• After fertilization the zygote becomes a blastocyst
• Implatation occurs in uterus

• Embryonic Folding
• Face & Neck Development
• Primitive Mouth
• Nasal Placodes
• Maxillary Process
• Tongue development starts
• Branchial Arch Formation

• Primitive Muscle cells prep to form Masticatory Mucles
• Formation of Palate Begins
• Formation of Nasal Cavity Begins

• Primitive Streak forms
• Mesoderm
• Trilaminar Disc
• Cephalic & Caudal Ends
• CNS starts to develop
• Somites

• Mandibular muscle masses are organized into the 4 muscles of mastication (Temporalis, Masseter, Medial Pterygoid, & Lateral Pterygoid)
• Nerve branches from trigeminal nerve (CN 5) are incorporated into muscles

• Embryo is recognizable at 8 weeks

• Final palate is completed - fusion of swellings/tissue from different surfaces
• Nasal cavity formation is completed
• Fusion of the vertical nasal septum & horizontal final palate is complete

• At the start of the 6th week the upper lip begins when each maxillary process fuses with each medial nasal process on both sides of stomodeum
• The maxillary processes lead to the sides of the upper lip
• The medial nasal processes lead to the philtrum

• Fusion of the vertical nasal septum & horizontal final palate begins

• Ovum & sperm are haploid cells, meaning they each only contain half the chromosomes required
• They are formed by Meiosis
• Meiosis results in unique daughter cells
• Each has 23 chromosomes
• Then, when ovum & sperm are joined together during fertilization, the newly formed zygote is a diploid cell, containing all the chromosomes necessary
• It has 46 chromosomes
• Fertilization occurs in the Fallopian Tubes

• Mandibular muscle mass has grown larger and its cells have begun to migrate to areas where they will begin to differentiate into the 4 muscles of mastication

• Primitive muscle cells from mesoderm of the mandibular arch begin to differentiate
• These muscle cells orient at the origin & insertion sites of the masticatory muscles they will later form

• Taste buds beging developing
• By inductive interaction between epithelial cells of tongue & invading nerve cells from the facial nerve (CN 7) and the glossopharyngeal nerve (CN 9)

• Blastocyst continues to grow through proliferation, differentiation, & morphogenesis creating a bilaminar embryonic disc
• Bilaminar disc has 2 layers
• Epiblast layer - superior layer of high columnar cells
• Hypoblast layer - inferior layer of small cuboidal cells
• The disc is suspended in the endometrial lining of the uterus between 2 fluid filled cavities
• Amniotic cavity
• Yolk sac - nourishes disc

• Primitive streak forms within bilaminar disc
• Caused by increased proliferation of cells at the midline
• Creates bilateral symmetry
• Mesoderm forms as some epiblast cells migrate towars the hypoblast cells in the area of the primitive streak

• Embryonic folding
• The trilaminar disk goes through embryonic folding which puts the tissue types into their proper positions for further development & produces a tubular embryo
• After folding
• Endoderm is inside the embryo, the ectoderm is on the outside, and the mesoderm is inbetween
• Forms 1 long tube, lined by endoderm, from the cephalic to caudal end (from oropharyngeal membrane to cloacal membrane)
• This tube is the future digestive tract, which is separated into 3 regions
• Foregut
• Midgut
• Hindgut

• Filiform papillae develop

Nasal cavity is completed

• The zygote goes through mitosis, or cell division, to form a clump of cells called a morula
• Mitosis results in identical daughter cells
• Mitosis continues as cells split and secrete fluids withing morula
• Then the zygote becomes a blastocyst and the process of mitosis continues

• Formation of palate begins
• Intermaxillary segment forms
• Result of the fusion of the medial nasal processes
• Intermaxillary segment gives rise to primary palate (aka primitive palate)
• Serves as a partial separation between developing oral cavity proper and nasal cavity

• Secondary palate formaiton
• Bilateral maxillary processes give rise to 2 palatal shelves
• Shelves grow inferiorly & deep inside of stomodeum in vertical direction along both sides of tongue

• At the end of the 7th week, the paired medial nasal processes fuse internally & inferiorly on the inside of the stomodeum forming the intermaxillary segment
• Fusion of the maxillary process & medial nasal process forms upper lip

• The face & neck begin to develop with primitive eyes, ears, nose, oral cavity, & jaw areas
• The rapidly growing brain of the embryo bulges over oropharyngeal membrane & developing heart
• The area of future face is now squeezed between the developing brain & heart
• All 3 layers (ecto-, endo-, and meso- derm) are involved in facial development
• Facial development includes the formation of the
• Primitive mouth
• Mandibular arch
• Maxillary process
• Frontonasal process
• Nose

• Placenta develops joining pregnant woman & embryo
• Umbilical circulation allows for selective exchange of substances including:
• Oxygen
• Carbon Dioxide
• Nutrients
• Hormones

• The trilaminar embryoninc disc forms as a result of the bilaminar disc thickening into 3 layers
• Trilaminar disc layers:
• Ectoderm
• skin epidermis, & central nervous system
• Mesoderm
• connective tissues - skin dermis, cartilage, bone, blood, & muscle
• Endoderm
• respiratory epithelium & gland cells

• At the beginning of week 4, the primitive mouth becomes the stomodeum
• Limited in depth by oropharyngeal membrane
• Later in week 4, the oropharyngeal membrane disintegrates
• Increase in depth

• Tongue has completed fusions of swellings
• Copula merges with anterior swellings (base & body of tongue merge)
• Sulcus terminalis - demarcation from fusion
• Tongue then contracts & moves anteriorly & inferiorly to avoid being an obstacle to developing palatal shelves

• At the end of the first week, implantation occurs as the blastocyst is embedded into the endometrium (inner lining of the uterus)
• The blastocyst now has 2 layers
• Trophoblast layer - outer layer of peripheral cells
• Embryoblast layer - inner mass of embryonic cells

• Formation of the nasal cavity begins

• By the end of the seventh week, the last four branchial grooves are obliterated
• This obliteration gives the neck a smooth surface

• The disc now has a cephalic end (head end) & caudal end (tail end)
• Oropharyngeal membrane forms @ cephalic end
• just ectoderm & endoderm (no mesoderm)
• location of future primitive mouth
• Cloacal membrane forms @ caudal end
• just ectoderm & endoderm (no mesoderm)
• location of future anus

• Lingual papillae appear
• First circumvallate & folate
• Then fungiform

• Central nervous system (CNS) development begins
• Cells from ectoderm differentiate into neuroectoderm cells and localize @ neural plate (runs from cephalic to caudal end)
• Neuroectoderm cells continue to grow and invaginate creating neural tube (future spinal cord)
• Neural crest cells (NCCs) form from neuroectoderm
• NCCs migrate from neural folds to mesoderm to create mesenchyme

• Mandibular processes - 2 bulges of tissue that appear inferior to the primitive mouth (core of mesenchyme & NCCs)
• These processes fuse at midline to form the mandibular arch (developmental form of mandible)
• Mandibular arch is considered the 1st branchial arch
• During growth of mandibular arch - Meckel cartilage forms within each side of arch
• Most cartilage disappears as bony mandible form

• Frontonasal process forms as bulge of tissue @ most cephalic end
• Placodes (rounded areas of specialized thickened endoderm) form
• Lens, otic, and nasal
• Tissue around nasal placode grows & starts development of nasal region & nose
• These placodes then became submerged forming a depression called nasal pits
• The nasal pits the deepen into nasal sacs
• Nasal sacs are separate from stomodeum until oropharyngeal membrane disintegrates
• The superior, middle & inferior nasal conche are developing on lateral walls of developing nasal cavities
• Medial nasal process - 2 crescent shaped swelling located between nasal pits
• Lateral nasal processes - 2 other crescent shaped swellings

• Mesoderm differentiates & forms 38 paired segments called somites

• Maxillary process & midface formation
• Maxillary process - tissue swelling forms from increased growth of mandibular arch on either side stomodeum
• Formed by mesenchyme

• Branchial Arch Formation
• Stacked bilateral swelings of tissue appear inferior to the stomedeum & include the mandibular arch
• 6 pairs of U-shaped bars
• Central core - ectomesenchyme
• Covered externally by ectoderm & lined with endoderm
• Each arch has it's own developing components
• cartilage
• nerve
• vascular
• muscle
• Arches:
• 1st - Mandibular arch
• 2nd - Hyoid arch
• 3rd
• 4th
• 5th - so rudementary it may be absent or included with 4th arch
• 6th