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Takes place during the first week after conception.
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At the beginning of the first week, conception takes place where a women's ovum is penetrated by and united with with a mans sperm during fertilization.
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This union of the ovum and sperm subsequently forms a fertilized egg or zygote.
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During fertilization, the final stages of meiosis occurs in the ovum. This process joins the ovum's chromosomes with those of the sperm
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If any disturbances occur in the basic process of meiosis during feralization, major congenital malformations result from the chromosomal abnormality. An example would be Down syndrome.
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After fertilization, the zygote then undergoes mitosis, or individual cell division that splits it into more and more cells due to cleavage
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The zygote is now a blastocyst. The rest of the first week is characterized by further mitotic cleavage, in which the blastocyst splits into smaller and more numerous cells as it undergoes successive cell division by mitosis.
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By the end of the first week, the blastocyst stops traveling and undergoes implantation. It then becomes embedded in the prepared endometrium, the innermost lining of the uterus on its back wall.
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After a week of cleavages, the blastocyst consists of a layer of peripheral cells, the trophoblast layer and the small inner mass of the embryonic cells, or embryoblast layer.
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The implanted blastocyst grows by increased proliferation of the embryonic cells, with differentiation also occurring resulting in changes in cellular morphogenesis; every ridge, bump, and recess now indicates these increased levels of cellular differentiation.
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The increased number of embryonic cells creates the embryonic cell layers.
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The beginning of the second week to the end of the eighth week.
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A bilaminar embryonic disc is eventually developed from the blastocyst and appears as a 3D but flattened, essentially circular plate of bilayer cells. This disc is composed of a superior epiblast layer and a inferior hypoblast layer.
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After the bilaminar embryonic disc creation, the disc is suspended in the uterus's endometrium between two fluid-filled cavities called the amniotic cavity.
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Later, the placenta, develops from the interactions of the trophoblast layer and endometrial tissue.
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In the beginning of the 3rd week, the primitive streak forms within the bilaminar disc.
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The primitive streak causes the disc to have bilateral symmetry, with a right and left side; most of the further development of each half of the embryo mirrors the other half.
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In the third week, cells from the epiblast layer move or migrate toward the hypoblast layer and becomes the mesoderm, an embryonic connective tissue, as well as embryonic endoderm.
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By this time the disc now has a cephalic end or a head end. This membrane is the location of the future primitive mouth or stomodeum of the embryo and thus the beginning of the digestive tract. The disc also has a caudal end or tail end. At the caudal end the cloacal membrane forms, which is the location of the future anus, or terminal end of the digestive tract.
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With 3 layers present, the bilaminar embryonic disc has thickened into trilaminar embryonic disc. With the creation of the new embryonic cell layers of mesoderm and embryonic endoderm, the epiblast layer is now considered ectoderm.
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Later in the third week, the CNS begins to develop and form the beginning of the spinal cord and brain.
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First, a specialized group of cells differentiates from the ectoderm and its now considered neuroectoderm. These cells are localized to the neural plate of the embryo. This plate then undergoes further growth and thickening, which causes it to deepen and invaginate inward, forming the neural groove.
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By the end of the third week, the neural groove deepens further and is surrounded by the neural folds.
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Another specialized group of cells, the neural crest cells develop from neuroectoderm. These cells then migrate from the crest of the neural folds and joins the mesoderm to form mesenchyme. The mesenchyme in involved in the development of many face and neck structures, such as the branchial arches.
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The trilaminar embryonic disc undergoes anterior and lateral embryonic folding, which places forming tissue types into their proper positions for further embryonic development as well as producing a somewhat tubular embryo.
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During this time the rapidly growing brain of the embryo bulge over the oropharyngeal and developing heart.
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A hollow tube lined by endoderm from the cephalic end to the caudal end of the embryo or specifically from they oropharyngeal membrane, to the cloacal membrane. This tube creates the future digestive tract : the foregut, midgut, and hindgut
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After formation for the stomodeum, two bulges of tissue appear inferior to the primitive mouth: two mandibular processes.
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Stacked bilateral swellings of tissue appear inferior to the stomodeum and include the mandibular arch. These branchial arches consists of six pairs of u-shaped bars of which the central core consists of mesenchyme derived from mesoderm.
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The primitive mouth has become the stomodeum, which initially appeared as a shallow depression in the embryonic surface ectoderm as it's cephalic end.
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Also in the fourth week, the lens placodes and the nasal placodes develop. This will create the future eyes and noes.
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An adjacent swelling forms from increased growth of the mandibular arch on each side of the stomodeum. This swelling forms the maxillary process.
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A bulge of tissue forms at the most cephalic end of the embryo and is the cranial boundary of the stomodeum.
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During the fourth week of prenatal development, within the embryonic period, the tongue begins its development. The tongue development begins as a triangular median swelling, the
tuberculum impar (or median tongue bud). The single tuberculum impar is located in the midline and is formed on the mandibular arch, which is considered the first branchial arch, at the floor of the primitive pharynx within the embryo’s conjoined nasal and oral cavities. -
During the fifth week of prenatal development and still within the embryonic period, the intermaxillary segment forms. The intermaxillary segment arises as a result of fusion of the two
medial nasal processes internally within the embryo. -
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The intermaxillary segment also gives rise to the primary
palate (or primitive palate). At this time, the primary palate serves only as a partial separation between the developing oral cavity proper and nasal cavity. -
Later, two oval lateral lingual swellings (or distal tongue
buds) develop on each side of the tuberculum impar. The paired lateral lingual swellings grow in size and merge with each other.
Then the two fused swellings overgrow and encompass the
disappearing tuberculum impar to form the anterior
two thirds of the mature tongue, the body of the tongue, which lies within the oral cavity proper. -
The future nasal septum of the nasal cavity is also developing
when the palate is forming. The structure of the nasal septum, similar to the primary palate, is a growth from the fused medial nasal processes. The tissue types that form them nasal septum will grow inferiorly and deep to the medial nasal processes and superior to the
stomodeum. -
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Maxillary process fuses with each medial nasal process by proliferation of the NCCs mesenchyme. Contributes to side of upper lip and philtrum
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The two palatal shelves elongate and move medially toward each other, meeting to join, and then fusing to form the secondary palate.
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During the start of the sixth week of prenatal development the upper and lower lip begins formation when each maxillary process fuses with each medial nasal process on both sides of the stomodeum.
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During the sixth week of prenatal development, within the embryonic period, the bilateral maxillary processes give rise to two palatal shelves. These shelves grow inferiorly and deep on the inside of the stomodeum in a vertical direction, along both sides of the developing
tongue. -
Even farther posterior to the copula is the projection of a third median swelling, the epiglottic swelling. This swelling marks the development of the most posterior region of the tongue and site of the future epiglottis
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Immediately posterior to these fused anterior swellings, a pair
of swellings, the copula becomes evident. The copula gradually
overgrows the hyoid arch, which is considered the second
branchial arch. This overgrowth will form the
posterior one-third of the mature tongue, the base of the tongue. -
This stage is through week 6 to 7 of the embryonic period. The Stomodeum becomes lined with ectoderm that gives rise to oral epithelium. The Ectomesenchyme and Basement membrane is also involved in the initiation stage. The Ectomesenchyme forms neural crest cells (NCC) and the NCC differentiate to form teeth. The Basement membrane separates oral epithelium/connective tissue.
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Is part of the initiation stage and finishes in the 7th week. dDental lamina is formed as a result of OE growing deep into ectomesynchyme. Mx/Md arches. Begins at midline
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Strats at the beginning of week 8. Extensive proliferation of dental lamina starts creating three dimensional, oval masses about 10 buds per arch. The ectomesenchyme and Basement membrane is also involved in this stage. Ectomesenchyme - proliferation, basement membrane btw. dental lamina. Proliferation- dental lamina & ectomesenchyme
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The tongue is fully developed by week 8.
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This stage is from the 9-10th week (fetal period). Proliferation, differentiation and morphogenesis takes place during this stage.
Proliferation- Unequal growth, formation of cap, attached to dental lamina. Differentiation- primordium of tooth. Morphogenesis -predominant process. -
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The production of enamel, dental papilla, future dentin & pulp, dental sac such as the periodontium and tooth germ is all produced in this stage.
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Begins in the 11th week and ends in the 12th week. Deeping of the under suface happens in this stage. Proliferation, differentiation, &
morphogenesis also occur in this stage forming 4 different types of cells. -
The 4 different types of cells form the outer enamel epithelium, stellate reticulum, stratum intermedium and inner enamel epithium.
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During this stage the enamel, dentin, and cementum are secreted in successive layers. The process of induction continues to occur between the ectodermal tissue of the enamel organ and mesenchymal tissue of the dental papilla and dental sac.
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The final stage of odontogenesis. This stage is reached when the matrices of the hard dental tissue types subsequently fully mineralized to their correct levels.
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The final palate is finally finished in week 12.
