2. Body Folding (Dennis) Flashcards
Embryonic growth
essentially proliferation of cells (includes division and elaboration)
Morphogenesis
shape and characteristics development of organ, body parts, and the body as a whole. It is defined and determined by 1) gene expression in a preset order. It also involves any changes to cell growth, shape, movement, and interaction with other cells and the cell’s environment.
Differentiation
process by which a cell changes from one cell type to another. When no longer changes, a cell is organizes with alike cells into a structure, tissue, or organ that can perform a precise (unique or specialized) function.
Body folding
from a flat plate to a cylinder-ish 3D shape, folding in two directions (cephalocaudal and lateral flexion; or median and horizontal) takes place simultaneously. Results from different rate of growth of the embryonic structures. Important process for correct body form.
Median plane (cephalocaudal) folding
along the sagittal plane, caudal and cranial folds, constriction at the umbilical vesicle. The body increases in length (Week 4, Day 22) but lateral edges cannot keep up.
Neural folds overgrow oropharyngeal membrane as the head bends dorsally (down and back) during head folding. Growth progression moves septum transversum, primordial heart, pericardial coelom, and oropharyngeal membrane to ventral surface.
Medial folding - endoderm and GI tract
Endoderm gives rise to the GI tract – As folding continues, the endoderm moves to the midline and fuses, incorporating primitive yolk sac to create the primitive gut tube –> foregut, midgut, and hindgut. Terminal hindgut forms cloaca. The caudal eminence will project over the cloacal membrane (future anus) during tail folding.
*Note: Primitive streak to cloacal membrane relationship
caudal –> before folding
cephalad–> after folding.
Gut tube is closed by oropharyngeal membrane - raptures at the end of Week4.
Hindgut is closed by coelomic membrane - raptures during Week7 and forms urogenital and anal openings.
Horizontal folding
form the cylinder shape, and two lateral body folds due to cells extending ventrally toward the midline. This forms the abdominal wall, which incorporates the midgut and creates the SI primordium. The space between the umbilical vesicle and the endoderm of the midgut is reduced, forming the omphaloenteric duct. With the formation of the umbilical cord, the communication btw the extra and intraembryonic cavities narrows. When the extraembryonic coelom is obliterated, the amnion becomes the epithelial covering of the umbilical cord.
Coelomic cavity
A coelom is a hollow, fluid-filled cavity that acts as a protective cushion for an organism’s internal organs.
The fluid-filled extra-embryonic coelom (cavity) formed initially from trophoblast and extra-embryonic mesoderm that forms placenta. The chorion and amnion are made by the somatopleure. The chorion becomes incorporated into placental development. The avian and reptilian chorion lies beside the egg shell and allows gas exchange. In humans, this cavity is lost during Week8 when the amniotic cavity expands and fuses with the chorion.
Allantois is incorporated into the hindgut.
Later septation of the cloacal has the allantois associated ventrally with the urogenital sinus. It lies at the superior end of the primitive urinary bladder and forms the urachus. Postnatally, the urachus degenerates to form a fibrous ligament (median umbilical ligament) between the urinary bladder to the umbilical region.
embryonic derivation and components of the intraembryonic coelom
Intraembryonic coelom (or body cavity) is a horseshoe-shaped cavity that divides the lateral mesoderm into somatic (parietal) and splanchnic (visceral) layers of lateral mesoderm.
Somatic – continuous extraembryonic mesoderm covering amnion
Splanchnic – continuous extraembryonic mesoderm covering umbilical vesicle
Somatopleure: somatic mesoderm + ectoderm –> body wall
Splanchnopleure: splanchnic mesoderm + endoderm –> embryonic gut
Intraembryonic coeloms
Intraembryonic coelom forms as a single cavity appearing in the lateral plate mesoderm during Week3. This single cavity (coelom) divides the lateral plate into the somatic and splanchnic mesoderm and will later be portioned into the three main body cavities:
1) pericardial cavity
2) pleural cavities
3) peritoneal cavity where IE and EE coeloms are continuous
Extraembryonic coeloms
Initially formed by epiblast and then lined by ectoderm and surrounding extraembryonic mesoderm. In humans, it forms the innermost fetal membrane, produces amniotic fluid expanding to eventually fuse with the chorionic membrane during Week8 of development. This fluid-filled sac initially lies above the trilaminar embryo disc and with embryonic disc folding this sac is drawn ventrally to enclose (cover) the entire embryo, then fetus. The presence of this membrane led to the description of reptiles, bird, and mammals as “amniotes”.
1) amniotic cavity
2) chorionic cavity
3) yolk sac
4) allantois
Lateral Plate Mesoderm
The lateral plate mesoderm (LPM) is one of the early mesoderm derivatives. It is located distal to the notochord and paraxial mesoderm, and splits horizontally into the dorsal somatic mesoderm, which underlies the ectoderm, and the ventral splanchnic (visceral) mesoderm, which overlies the endoderm. The LPM gives rise to heart, blood vessels, blood cells and the lining of the body cavities.
Pleuropericardial folds
cranial end
superior to developing lungs, project into pleuroperitoneal folds
Pleuroperitoneal folds
caudal end
Divide mesenchyme into outer layer (thoracic wall) and inner layer (fibrous pericardium, outer layer of pericardium)
Diaphragmatic development
The diaphragm develops from multiple embryonic sources. The muscle and its associated connective tissue and central tendon develop from three sources: the septum transversum, the pleuroperitoneal folds, and paraxial somites.
