Embryonic Tissue Development Quiz

Neurulation, Folding, and Development of the Nervous System

Week 3: The Notochord

• Establishes the longitudinal axis of the embryo and gives it some rigidity
• Provides signals for the development of axial MSK structures and the CNS
• Contributes to the intervertebral discs
• Develops from mesenchymal cells that migrate cephalad and form a cord called the notochordal process
• The notochordal process develops a lumen known as the notochordal canal
• The notochordal process approaches the prechordal plate, and the floor of the process "fuses" with the endoderm
• The notochordal process is now the notochordal plate
• The notochordal plate cells proliferate and fold inwards, forming the fully-developed notochord with no canal

Gastrulation and Formation of Germ Layers

• The primitive streak appears at the beginning of the third week as a thickening of the epiblast at the caudal end of the embryonic disc
• The primitive streak results from the migration of epiblastic cells to the median plane of the disc
• Invagination of epiblastic cells from the primitive streak gives rise to mesenchymal cells that migrate ventrally, laterally, and cranially between the epiblast and hypoblast
• The epiblast is now known as embryonic ectoderm
• Some cells of the epiblast displace the hypoblast and form embryonic endoderm
• Mesenchymal cells produced by the primitive streak soon organize into a third germ layer, the intraembryonic or embryonic mesoderm
• Cells of the mesoderm migrate to the edges of the embryonic disc, where they join the extraembryonic mesoderm covering the amnion and umbilical vesicle

Neural Development

• The neural plate appears as a thickening of the embryonic ectoderm, induced by the developing notochord
• A longitudinal neural groove develops in the neural plate, which is flanked by neural folds
• Fusion of the neural folds forms the neural tube, the primordium of the CNS
• The neural tube is the primordium of the CNS
• Neuroectodermal cells form a neural crest between the surface ectoderm and neural tube
• The neural crest cells lose affinity to the epithelium and neighbouring cells, detaching and migrating widely throughout the mesenchyme
• Neural crest cells give rise to ganglia of CN V, VII, IX, X, spinal ganglia, autonomic nervous system ganglia, neurolemma sheaths of peripheral nerves, and contribute to the arachnoid and pia mater, adrenal medulla, melanocytes, craniofacial bone and cartilage, and portions of the heart

Intraembryonic Mesoderm

• During the 3rd week, the intraembryonic mesoderm proliferates to form a thick column of mesoderm on either side of the notochord
• The paraxial mesoderm forms beside the axis of the organism (as defined by the notochord)
• The intermediate mesoderm is found just lateral to the paraxial mesoderm
• The lateral mesoderm is lateral to the intermediate mesoderm
• Somites develop adjacent to the neural tube, formed from the paraxial mesoderm
• Somites give rise to most of the axial skeleton and associated musculature, as well as the dermis in those areas

Intraembryonic Coelom

• The primordium of the intraembryonic coelom (embryonic body cavity) appears as isolated spaces in the lateral mesoderm and cardiogenic mesoderm
• These spaces soon coalesce and form a single horseshoe-shaped intraembryonic coelom
• The intraembryonic coelom divides the lateral mesoderm into two layers: a somatic or parietal layer of lateral mesoderm located beneath the ectodermal epithelium and continuous with the extraembryonic mesoderm covering the amnion, and a splanchnic or visceral layer of lateral mesoderm next to the endoderm and continuous with the extraembryonic mesoderm covering the umbilical vesicle

Embryonic Folding

• Embryonic folding is the process by which a relatively "flat" embryonic disk becomes more and more cylindrical in shape
• Folding occurs in two general planes: the median plane (cranial-caudal folding) and the horizontal plane (lateral folding)
• The edges "roll" ventrally towards the umbilical vesicle
• Folding begins at the end of the 3rd week and is easy to see in the 4th week

Cranial Folding

• The brain vesicles first begin to appear
• A few somites are obvious
• The foregut lies between the brain and heart
• The oropharyngeal membrane separates the foregut from the stomodeum
• The septum transversum lies caudal to the heart, develops into the central tendon of the diaphragm, and separates the abdominal cavity from the thoracic cavity

Germ Layer Derivatives

• Ectoderm: skin, bones, CNS, PNS, sensory epithelia of the eyes, ears, and nose, epidermis and its appendages, mammary glands, subcutaneous glands, enamel of teeth, pituitary gland
• Mesoderm: connective tissue, cartilage, bone, striated and smooth muscles, heart, blood, and lymphatic vessels, kidneys, ovaries, testes, genital ducts, serous membranes lining the body cavities, spleen, and cortex of suprarenal glands
• Endoderm: epithelial lining of the digestive and respiratory tracts, parenchyma of the tonsils, thyroid and parathyroid glands, thymus, liver, and pancreas, epithelial lining of the urinary bladder and most of the urethra, epithelial lining of the tympanic cavity, tympanic antrum, and eustachian tube