Development+of+Trilaminar+Embryo

=**Objectives**=


 * 1. Define and describe: hypoblast, epiblast, primitive streak, primitive node, notocordal process, prochrodal plate, cloacal plate, notocord, ectoderm, mesoderm, and endoderm.**

__Hypoblast__ Constists of small cuboidal cells adjacent to the exocoelomic cavity

__Epiblast__ The thicker layer consisting of high columnar cells related to the amniotic cavity

__Primative Streak__ A thickened linear band of epiblast appearing caudally in the median plane of the dorsal aspect of the embryonic disk.

__Primitive Node__ Proliferating cells at the cranial end of the primitive streak.

__Notocordal Process__ Mesenchymal cells that migrate cranially from the primitive node and pit, forming a median cellular rod.

__Prochrodal Plate__ A small circular area of columnar endodermal cells where the ectoderm and endoderm contact, preventing further extension of the notochordal process cranially. This is the future site for deveopment of the mouth.

__Cloacal Plate__ Circular area caudal to the primitive streak where the future side of the anus will develop. Remains bilaminar here, preventing the migration fo mesenchymal cells between them.

__Notocord__ Cellular rod that develops by tranformation of hte notochordal processes. Instructive signals from the primitive streak region induce notochrodal precursors cells tof or the notocord.

__Ectoderm (Embryonic)__ Cells that remain in the epiblast after mesenchymal cells begin migrating to from the embrynoic mesoderm.

__Mesoderm (Embryonic)__ Formed from migrating mesenchymal cells that come from the deep surface of the primitive streak. These cells form mesoblast whcih eventually forms the mesoderm.

__Endoderm (Embryonic)__ Cells from the epiblast displace the hypoblast during msenchymal cell migration and from the endoderm.


 * 2. Describe the formation of the bilaminar embryonic disk from the inner cell mass.**

By the blastocyst stage, the embryo has formed a hollow ball of cells with the inner cell mass (embryoblast) off to one side and the blastocystic (exocoelomic) cavity filling up the rest of the sphere. As implantation progresses, a small space appears in the embryoblast and forms the amniotic cavity. Concurrently, morphological changes ocur in the embryoblast that result in the formation of a flat, almost circular bilaminar plate of cells, the embryonic disk, which incldues the epiblast and the hypoblast. The epiblast formsthe floor of the amniotic cavity and is continusuous with the amnion. The hypoblast forms the roof of the exocoelomic cavity and is continuous with the thin exocoelomic membrane.


 * 3. Describe the formation of the notocord.**

The notocordal process elongates by invagination of cells from the primitive gut. The primitive pit extends ot the notocordal process, forming the notochordal canal. The notocordal process is now a cellular tube that extends cranially from the primitive node to the prechordal plate. The floor of the notochordal process fuses with teh underlying embryonic endodermand gradually undergo degeneration, resulting in the formation of openings in teh floor of the notochordal process, which brings the notochordal canal into communication with the yolk sac. The openings rapidly become confluent and the floor of the notochordal canal disappears. The remains of the notochordal process form a flattened grooved notochordal plate. Beginning at the cranial end of the embryo,the notochrodal cells proliferate and the notochordal plate infolds to form the notochord. The proximal part of the notochordal canal persists temporarily as teh neuroenteric canal which forms a transitory communication between the amniotic and yolk sac cavities. When development of the notocord is complete, the neurenteric canal normally obliterates. The notochord becomes detached from teh endoderm of the yolk sac, which again becames a continuous layer.


 * 4. Descirbe the formation of the intraembryonic mesoderm.**

Formed from migrating mesenchymal cells that come from the deep surface of the primitive streak. These cells form mesoblast whcih eventually forms the mesoderm.


 * 5. List the derivatives of each of the three basic germ layers.**

__Ectoderm__ Epidermis, hair, nails, cutaneous, and mammary glands, anterior part of pituitary gland,enamel of teeth, internal ear, lens of eye Neuro crest, cranial and sensory ganglia and nerves, medulla of suprarenal gland, pigment cells, pharyngeal arch cartilages, head mesenchyme and connective tissue, buldar and conal ridges in heart Nural tube, central nervous system, retina, pineal body, posterior part of pituitary gland

__Mesoderm__ Cranium, connective tissues of the head, dentin Muscles of the head, striated skeletal muscle (trunk, limbs), skeleton except cranium, dermis of skin, connective tissue Urogenital systems including gonads, ducts, and accessory glands Connective tissue and muscle of viscera, serious membranes of pleura, pericardium, and peritoneum, primordial heart, blood and lymphatic cells, spleen, suprarenal (adrenal) cortex

__Endoderm__ Epithelial parts of trachea, bronchi, and lungs Epithelium of gastrointestinal tract, liver, urinary bladder, urachus Epithelial parts of pharynx, thyroid, tympanic cavity, pharyngotympanic tube, tonsils, parathyroid glands


 * 6. Describe a somite and its derivatives.**

As the notochord and neural tube form, the intraembryonic mesoderm on each side of them proliferates to form a thick longtitudal colum of paraxial mesoderm. Toward the end of the thrid week, the paraxial mesoderm differentiates and begins to divide into paried cuboidal bodies called somites.

Somites first appear on the future occipital region of the embryo and develop craniocaudally to give rise to most of the axial skeleton and associated musculature and adjacent dermis of teh skin.


 * 7. Briefly describe the intraembryonic coelom and its formation.**

The primordium of the intraembryonic coelom apperas as isolated coelomic spaces in the lateral mesoderm and cardiogenic mesoderm. These spaces coalesce to form a single U-shaped cavity called the intraembryonic coelom, dividing the lateral mesoderm into the parietal and viseral layer. The parietal (somatic) layer is continuous with the extraembryonic mesoderm covering the amnion. The viseral (splanchnic) layer is continuous with the extraembryonic mesoderm covering the yolk sac.

Somatic mesoderm and overlying embryonic ectoderm form the embryonic body wall or somatopleure while teh splanchic mesoderm and underlying embryonic endoderm form the embryonic gut or splanchnopleure. During the second month, the intraembryonic coelom is divided into the perichardial, pleural, and peritoneal cavities.


 * 8. Briefly describe the folding processes that occur in the embryo and the consequences of these processes.**

The growth rate at the sides of the embryonic disk fails to keep pace with the rapid growth of the long axis as the embryo increases rapidly in length. This results in the folding at the cranial and caudal ends of the embryo and relative constriction at the junction of the embryo and the yolk sac.

By the beginning of the fourth week, the neural folds in the cranial region have thickened to form the primordium of the brain. Initially, the brain projects dorsally into the amniotic cavity. Later, the developing forebrain grows cranially beyond the oropharyngeal membrane and overhangs the developing heart. Concomitantly, the septum transfersum, primordial heart, pericardial coelom, and oropharyngeal membrane move to the ventral surface of the embryo. Part of the yolk sac is incorporated into the embyro as the foregut. After folding, the septum transfersum lies caudal to the heart where it subsequently develps into the central tendon of the diaphram.

Folding of the caudal end results in the partial incorporation of the endodermal germ layer into the embryo as the hindgut. The terminal part of the hindgut dilates and form the coaca. Before folding, the primative streak lies cranial to the coacal membrane; after foldling, it lies caudal. The connecting stalk becomes attached to the ventral surface and the allantois is partially incorporated into the embryo.


 * 9. Briefly describe the changes in appearance of the embryo and the fetus during development.**

The fourth to eight weeks of development constitute most of the embryonic periods. The critical developmental events occur during the first 3 weeks such as teh cleavage of the zygote, blastogenesis, and early development of the nervous and cardiovascular systems. All major external and internal structures are established during the forth to eight weeks. By the end of this organogenetic period, the main organ systems have begun to develop. As the tissues and organs form, the shape fo teh emrby changes to a more cylindrical appearance,and by the eighth week, it has a distinctly human appearance.