Introduction to Human General Embryology Developmental Genetics
E. Cellular Differentiation
Download 3.46 Mb.
|
- Navigate this page:
- ****************************************What is an Embryonic Primordium (Primordia)
| E. Cellular Differentiation
During embryonic development there are some cellular differentiation events in which mesenchymal cells become epithelial cells and at other times epithelial cells differentiate into mesenchymal cells. Following epithelial-mesenchymal transition, cells can migrate away from an epithelium and then associate with other similar cells in a new location. During embryonic development, cells are restricted to different layers due to differential affinities. One of the ways this can occur is when cells share the same cell-to-cell adhesion molecules. For instance, homotypic cell adhesion can maintain boundaries between groups of cells that have different adhesion molecules. Furthermore, cells can sort based upon differences in adhesion between the cells, so even two populations of cells with different levels of the same adhesion molecule can sort out. The molecules responsible for adhesion are called cell adhesion molecules (CAMs) and several types of cell adhesion molecules are known and one major class of these molecules are cadherins. There are dozens of different cadherins that are expressed on different cell types. Cadherins bind to other cadherins in a like-to-like manner: E-cadherin (found on many epithelial cells) binds preferentially to other E-cadherin molecules. Mesenchymal cells usually express other cadherin types such as N-cadherin. ****************************************What is an Embryonic Primordium (Primordia)? An embryonic primordium is the earliest indication of an organ or part of it, during embryonic development and differentiation. It is otherwise an aggregation of cells in the embryo indicating the first trace of an organ or structure. Examples of embryonic primordium include: The connecting or body-stalk which gives rise to the umbilical cord. The notochord: most of which disappears but has remnants that remains as the nucleus pulposus of the intervertebral disc. The Neural tube, which develops from the neural plate. The brain and spinal cord develops from it. The Somites, which develops from the paraxial mesoderm. It remnants remains as most of the intervertebral disc except the nucleus pulposus. However, it has two components i.e. the sclerotome from which bones, tendon and cartilage develop; and Dermo-myotome from which the dermis of skin and segmental muscles develop. The intraembryonic coelom a cavity formed within the intraembryonic mesoderm. The pericardial, pleural and peritoneal cavities develop from it. The primitive gut tube which develops from the yolk sac following its incorporation into the developing embryo during embryonic folding. It is subdivided into the foregut, midgut and Hindgut. The primitive gut gives rise to structures of the gastrointestinal, and respiratory systems, as well as some of the structures of the urinary. NB: Several buds or diverticular emerge from the primitive gut tube and these include: The Hepatic Bud from which most part of the liver develops except the part that develops from the primordium called the Septum Transversum or the transverse septum. The dorsal and ventral pancreatic buds which fuses to form the pancreas. The Lung Bud from which the lungs develops. The Laryngo-Tracheal Diverticulum from which the larynx, trachea, bronchi, and bronchioles develops. The Caecal Bud from which the caecum and appendix develops. The allantoic diverticulum (allantois), though primarily a diverticulum of the yolk sac before gut formation, is secondarily seen as the diverticulum of the hindgut. It obliterates and atrophies to become Urachus (median umbilical ligament) in post natal life. Note however, that a portion of the allantois is incorporated into the body of the embryo to form the Cloaca which is later partitioned into two i.e. the dorsal and ventral portions. The dorsal portion becomes the recto-anal canal from which the rectum and anal canal develops, while the ventral portion becomes the Urogenital Sinus from which several urinary and genital structures develop. The limb buds from which the upper and lower limbs develop. The Septum Transversum derived from the intraembryonic mesoderm. It contributes to the development of the Liver and Diaphragm. The neural crest from which several structures develop. These include: The Stomatodaeum or Stomodaeum from which the mouth develop. The Rathkes pouch which forms the anterior lobe of the pituitary gland or adenohypophysis. From the anterior lobe arises the pars tuberalis and from the posterior lobe arises the pars intermedia. The proctodaeum from which the anus develop. The Otic Placodes which invaginates to form the otic vesicles from which structures needed for hearing and maintenance of balance (equilibrium) develop. The Lens Placode from which eye lenses develop. The Nasal or olfactory placodes invaginates to form the Nasal Pits from which the nasal cavities develop. The indifferent gonad from which the testis or ovary develop. The Phalus from which the penis or clitoris develop. The Branchial or Pharyngeal apparatus which gives rise to most structures of the Head and Neck. The Heart tube from which structures of the heart develop. The pronephros, mesonephros and metanephrosis transformed in phases to become the kidney. The Sinovaginal bulb gives rise to a portion of the vagina. The Genital Ducts (mesonephric and paramesonephric ducts) gives rise to the uterus, fallopian tube, duct of the testis, ureter etc. The Labioscrotal swelling which forms the Scrotum in males and Labia majora in females. The Ureteric Bud an outgrowth of the mesonephric duct. It gives rise to the ureter, renal pelvis, calyces and the entire collecting system. The Lingual Swellings, Tuberculum impar, and copula, or hypobranchial eminence contributes to the development of Tongue develop. Download 3.46 Mb. Share with your friends:
|