Histology Class Embryology - First Three Weeks of Development PDF

The third week marks the beginning of gastrulation, a process that converts the...

The third week marks the beginning of gastrulation, a process that converts the bilaminar embryonic disc into a trilaminar disc The primitive streak appears as a thickened linear band of epiblast cells on the dorsal surface of the embryonic disc It establishes the embryo's craniocaudal axis and symmetry **Primitive Streak** Epiblast cells migrate toward the streak, initiating the formation of germ layers Migrating epiblast cells displace hypoblast cells to form the definitive endoderm, which Definitive Endoderm: will contribute to the lining of the gastrointestinal and respiratory systems Development of the Embryonic Disc the result of fertilization, a single diploid cell Some epiblast cells migrate between the epiblast and newly formed Diploid cell with 46 chromosomes endoderm to create the intraembryonic mesoderm, which differentiates Mesoblast Formation: into various structures like muscles, bones, and blood vessels Surrounded by the **zona pellucida** The ectoderm thickens to form the neural plate, **Zygote** the precursor to the central nervous system Undergoes **syngamy**: merging of male and female pronuclei Neural Plate and Groove Formation: The neural groove forms as the neural plate begins to fold inward Surrounded by the zona pellucida, a glycoprotein layer that prevents premature implantation and protects the zygote **Gastrulation** Structure of the Zygote: The primitive line is the early phase of the primitive streak Inside is the cytoplasm, where the pronuclei (male and female) merge during syngamy Primitive Line and Endoderm Epiblast cells migrate through the primitive line to form the Cleavage ensures an increased number of cells definitive endoderm, displacing the original hypoblast for future differentiation Cleavage begins approximately 24 hours post-fertilization Cells migrating from the epiblast between the It occurs as the zygote travels through the ectoderm and endoderm differentiate into fallopian tube toward the uterus mesoblast cells. Mesoblast Formation: **Cleavage** Mitotic divisions occur without an increase in These cells will later form the mesodermal cytoplasmic volume (cells become structures (e.g., connective tissue, progressively smaller) cardiovascular system) Mesoblast Formation and Cloacal Membrane Series of mitotic divisions Blastomeres are totipotent, meaning each cell Located at the caudal end of the embryonic has the potential to develop into a complete disc, this area lacks mesoderm and will form the Cloacal Membrane: embryo future anal membrane. By the third day, a 16-cell stage is reached, forming the morula A small, elevated region at the cranial end of the primitive streak. First Week of Development **Inner cell mass (embryoblast)**: future embryo Primitive Node (Hensen’s Node): It serves as an organizing center for notochord Structure: and axial structure formation. **Outer cell mass (trophoblast)**: contributes **Morula** to placenta Located at the cranial end of the embryonic disc, this membrane is a region without mesoderm and will Oropharyngeal Membrane: Primitive Node and Oropharyngeal Membrane Blastomeres maximize contact with each other, forming tight junctions in the outer Compaction develop into the mouth opening layer. This process is essential for the structural integrity of the developing embryo Mesodermal cells migrating cranially from the primitive streak The morula is still surrounded by the zona pellucida Cardiogenic Zone: form the cardiogenic zone, where the heart primordium develops Around day 4–5, fluid begins to accumulate The primitive node extends a cellular structure called between the blastomeres the chordal process cranially along the midline A central cavity called the blastocoel forms Primitive Node and Chordal Process The chordal process eventually develops into the notochord, a structure critical for neural tube formation and signaling **Blastocyst Formation** Key Features: The morula transitions into the blastocyst for surrounding tissue differentiation. The embryoblast clusters to one side (pole), The chordal canal forms as a lumen within the chordal process while the trophoblast forms a thin outer layer Chordal Canal This canal facilitates communication between the amniotic Thins but remains intact, preventing the blastocyst from implanting prematurely Zona pellucida cavity and the yolk sac during early development in the fallopian tube (which would result in an ectopic pregnancy) The neurenteric duct is a temporary connection between the By the end of the first week, the blastocyst is ready for implantation amniotic cavity and the yolk sac at the level of the notochord Neurenteric Duct The zona pellucida disintegrates, allowing the **Late Blastocyst** Its function is primarily transient and part of embryonic blastocyst to interact with the uterine lining structural rearrangements Cytotrophoblast: The inner cellular layer The dorsal chord (notochord) is a rod-like Trophoblast differentiates: structure that forms along the midline. Syncytiotrophoblast: A multinucleated outer layer that Dorsal Chord: Histology Class: invades the uterine endometrium during implantation.  It acts as a signaling center to direct the development of the neural tube and somites. Embryology - First Ectoderm: Forms the nervous system and Dorsal Chord and Tridermal Disc Formation Three Weeks of Implantation in the col of the uterus begins around day 6–7 and is completed by day 12 epidermis Third Week of Development Development Mesoderm: Forms muscles, bones, blood The embryo transitions into a trilaminar disc, **Cytotrophoblast**: Cellular layer Tridermal Embryonic Disc: vessels, and connective tissues. composed of three germ layers: **Trophoblast Differentiation** **Syncytiotrophoblast**: Secretes hCG, Endoderm: Forms the lining of the establishes blood supply gastrointestinal and respiratory systems The syncytium trophoblast (syncytiotrophoblast) secretes digestive The neural plate arises from the thickened enzymes that erode the uterine epithelium and stroma (CT), creating a 1 ectoderm cranial to the primitive node secure anchorage in the uterus Neural Plate and Neuroectoderm The cells of the neural plate become neuroectoderm, which The trophoblast cells invade maternal tissues to establish a will form the brain, spinal cord, and peripheral nervous system connection with the maternal blood supply The neural plate begins to invaginate, forming a Process: A cavity appears within the epiblast, called the amniotic cavity central groove. This process is the first step Neural Groove: Amniotic Cavity Formation: toward neural tube formation Epiblast cells adjacent to the trophoblast become the amnion, which Neural Groove and Neural Crests surrounds the embryo with amniotic fluid for cushioning and protection Neural crest cells are specialized ectodermal **Implantation** cells at the edges of the neural plate. The hypoblast layer spreads and forms the Neural Crests: exocoelomic membrane, which lines the blastocoel These cells later migrate to form structures like peripheral Hypoblast Formation: nerves, melanocytes, and parts of the craniofacial skeleton. The blastocoel is transformed into the primary yolk sac, a key structure for nutrient exchange in early development The edges of the neural plate elevate and fuse to form the neural tube, the precursor to the central nervous system Changes During Nidation Somatic mesoderm: Lines the cytotrophoblast Neural Tube Formation Cells from the hypoblast spread to form the and amnion Closure begins in the middle of the embryo and extraembryonic mesoderm, which splits into two progresses both cranially and caudally layers: Splanchnic mesoderm: Surrounds the yolk sac Extraembryonic Mesoderm Formation: Para-Axial Mesoderm: Forms somites, which give rise to the vertebrae, skeletal muscles, and dermis This split creates the chorionic cavity (extraembryonic coelom), Second Week of Development which separates the developing embryo from the trophoblast Intermediate Mesoderm: Forms the urogenital system (e.g., ("Week of Twos") The mesoderm differentiates into three regions: Mesoderm Differentiation The primary yolk sac is replaced by the secondary yolk sac, kidneys, gonads) Secondary Yolk Sac:  which supports embryonic nutrition and blood cell formation Lateral Mesoderm: Forms the heart, blood vessels, and the body cavity lining The syncytiotrophoblast forms lacunae (spaces filled with maternal blood). Development of Lacunar Spaces: These spaces are precursors to the uteroplacental circulation Derivatives of the para-axial mesoderm that segment along the body axis. **Epiblast**: Forms ectoderm and amnion By the end of the second week, the embryo Somites: Bilaminar embryonic disc consists of: These segments give rise to bones, muscles, **Hypoblast**: Forms endoderm and yolk sac and dermal layers Somites and Nephrogenic Cord Amnion and amniotic cavity Part of the intermediate mesoderm, which Nephrogenic Cord: End of Second Week Secondary yolk sac forms the nephrotomes (early kidney structures) A cavity within the lateral mesoderm that eventually Chorionic cavity (extraembryonic coelom) Intraembryonic Coelom: forms the pericardial, pleural, and peritoneal cavities. Intraembryonic Coelom and Mesodermal Layers Significance: The embryo is now ready to begin Somatic Mesoderm: Associated with the ectoderm. gastrulation in the third week, where the three The lateral mesoderm splits into two layers: Somatic and Splanchnic Layers: germ layers form. Splanchnic Mesoderm: Associated with the endoderm The trilaminar disc has formed. The neural tube is beginning to close. Mesodermal structures like somites and By the end of the third week: End of the Third Week coelomic cavities have begun developing The embryonic structures are organized, setting the stage for organogenesis in the fourth week