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1st Trimester: Pre-implantation Period (1st week)
Occurs during first week after conception: Fertizulation: Women's ovum is united with sperm.
-Ovum and sperm create a fertilized egg Zygote.
-Meiosis: Ovum's chromosomes joins with sperm's 46 diploid number.
-Creates a new individual with "shuffled" chromosomes. -
Period: to
Prenatal Timeline
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1st Trimester: Preimplantation Period (1st week)
After fertilization: Zygote undergoes mitosis
-Cells split more and more due to cleavage (cell division)
-Cleavage creates Morula (Solid ball of cells)
-Morula turns into Blastocyst and travels down to uterus -
1st Trimester: Preimplantation period (1st week)
End of first week:
-Blastocyst stops traveling and undergoes implantation
-Blastocyst embeds in the endometrium of uterus
After a week of cleavage: Blastocyte consists of:
- Outer Trophoblast layer (peripheral cells)
- Inner Embryoblast layer (embryonic cells) -
1st Trimester: Embryonic Period (2nd week)
Implanted blastocyte grows by proliferation and differentiation of the embryonic cells.
embryonic cell layers are created within blastocyte.
Bilaminar embryonic disc develops from blastocyst.
-has superior epiblast (high columnar cells) layer
-inferior hypoblast layer (small cuboidal cells)
Bilaminar embryonic disc suspended in the endometrium lining the uterus between 2 fluid-filled cavities.
-Amniotic cavity
-Yolk sac -
1st Trimester: Embryonic Period (3rd week)
Primitive streak forms within bilaminar disc
Cells from epiblast layer migrate toward hypoblast layer only in the primitive streak
-Mesoderm and Endoderm are created
Bilaminar embryonic disc thickens into trilaminar embryonic disc
- Epiblast layer turns into ectoderm
- Hypoblast later turns into extra-embryonic endoderm -Mesoderm gives rise to connective tissue
-Ectoderm gives rise to skin epidermis
-Endoderm gives rise to respiratory epithelium and cells of glands -
1st Trimester: Embryonic Period (3rd week)
Trilaminar embryonic disc forms a cephalic end (head) and caudal end (terminal end of digestive tract) During latter part of week 3, CNS develops in embryo
- Group of cells from ectoderm turn into neuroectoderm
-Localized to the neural plate of the embryo
Neural plate forms the neural groove -
1st Trimester: Embryonic Period (3rd week)
End of 3rd week:
Neural groove deepens, surrounded by neural folds
Neural tube forms due to neural folds undergoing fusion
Neuro crest cells (NCCs) are formed by neuroectoderm.
NCCs from neuroectoderm migrate to the mesoderm to form mesenchyme
-Forms the somite
-distinct elevations on the sides of the embryo
-gives rise to head, neck, trunk, muscles/dermis -
1st Trimester: Embryonic Period (Week 4)
Future digestive tract starts to form:
3 major regions: Anterior Foregut (primitive pharynx), Posterior Midgut and Hindgut Face and neck structures begin to develop as well as the developing brain and heart -
1st Trimester Embryonic Period (Week 5-8)
Orofacial structures continue to develop- Eyes, Ears, Nose, Oral Cavity, Jaw
5 prominences:
2 Maxillary and 2 Mandibular processes, 1 frontonasal process Placodes:
Nasal Placodes, Lens Placodes, Otic placodes
Heart and Brain continue to develop
Tongue development -
2nd and 3rd Trimester Fetal Period (8th week to 9 months)
Final period of prenatal development
Maturation of existing structures occurring as embryo enlarges becomes a fetus. Involves maturation, proliferation, differentiation, and morphogenesis. allows newly formed tissue types and organs to function. End of 4th month: Fetal heartbeat and movements are present.