Embryology (condensed) Flashcards
How does a blastocyst form?
- A fertilized ovum divides to produce 2 daughter cells (blastomeres)
- Each blastomere divides repeatedly until a spherical mass of cells is formed (morula)
- Cells undergo compaction (cells on outside become trophoblast, cells on inside become inner mass cell, cells make fluid that forms the blastocyst cavity or blasteole/primitive yolk sac, morula is surrounded by the zona pellucida)
- Embryo is now called a blastocyst (trophoblast becomes the Rauber’s layer [which disappears to become the embryonic disc], outer cells of ICM become epiblasts, cells from embryonic disc become the ectoderm and mesoderm, inner cells of ICM become endoderm, blastoceole becomes primitive yolk sac
Explain the process of gastrulation.
The stage where the three primary germ layers are formed: ectoderm (neural and non-neural ectoderm, neural crest), mesoderm (urogenital, circulatory, supportive muscular and skeletal systems), endoderm (liver, pancreas, thyroid, parathyroid, and lining of GI system, respiratory system, bladder, middle ear, and auditory tube) After these three layers are formed, gastrulation is complete.
- *Process**
1. Formation of primitive streak.
2. Epiblasts migrate to primitive streak and invaginate to become mesoderm.
3. Epiblasts that remain on the surface now become the ectoderm.
4. The inner hypoblast (inner cells of ICM) becomes the endoderm.
5. Primitive streak disappears
What do the primary germ layers differentiate to become?
Ectoderm –> neuroectoderm (neural plate), which invaginates to form the neural tube (brain, spinal cord) or neural crest cells
Mesoderm –> form the notochord (eventually becomes part of the axis of the axial skeleton, vertebral column, etc.; also differentiates into 3 zones: paraxial mesoderm (somitomeres–> somites–>sclerotome, myotome, dermatome), intermediate mesoderm (urinary and reproductive systems, and adrenal cortex), and lateral mesoderm (SOMATIC LAYER: forms the lateral and ventral body wall and extra-embyronic membranes of amnion and chorion; fuses with ectoderm to from somatopleure also SPLANCHNIC LAYER: forms the wall of the gut tube, extra-embryonic membranes of yolk sac and allantois, and the vascular system; fuses with the endoderm to form splanchnopleure)
What are the functions of the placenta?
- supply or storage of nutrients
- respiratory exchange
- excretion of wastes
- mechanical protection of embryo
- transfer of maternal antibodies (passive immunity)
- hormone production (mammals)
How are the extra-embryonic membranes formed?
- Amnion and chorion: outer trophoblastic layer extends up and over forming amniotic folds. The inner membrane results in amnion and the outer membrane results in chorion.
- Yolk sac: the blastocyst cavity (blastoceole) is called the bilaminar yolk sac. Yolk sac lines with splanchnopleure, which fuses with the chorion to form the choriovitelline membrane. Yolk sac pinches off into gut portion and definitive yolk sac portion.
- Allantois: the primitive gut forms a diverticulum caudally which becomes the allantois.
What are the different placental classifications by shape?
- Diffuse: the interface is distributed over most of the chorioallantois (horse, pig, camelid)
- Cotyledonary: interface is multifocal; placentomes composed of the maternal caruncle and the fetal cotyledon (ruminants)
- Zonary: broad band (belt) that wraps around the embryo (carnivores)
- Discoid: focal (pancake); rodents and primates
What are the different placental classifications by histologic layers of interface?
Retention of the three maternal layers:
- Epitheliochorial: all 3 maternal layers are retained (horse, pig, whale)
- Synepitheliochorial: the maternal and fetal epithelium fuse (ruminants)
- Syndesmochorial: the connective tissue and endothelial layers are retained, the epithelial layer is lost (DOESN’T EXIST)
- Endothelialchorial: the endothelial layer is the only layer retained (dogs, cats)
- Hemochorial: no maternal layers remain (primates, rodents)
What are the different placental classifications by degree of uterine invasion?
- Non-deciduate: no loss of maternal layers, no bleeding at birth, no transfer of maternal antibodies to fetus, NEED COLOSTRUM
- Deciduate: There is loss of maternal layers (endotheliochorial, hemochorial), bleeding at birth, Lesser requirement for colostrum to attain passive immunity
How does the digestive system arise?
- Endoderm becomes epithelial lining.
- Endoderm surrounded by splanchnic mesoderm that becomes CT and muscle.
- Anal membrane separates rectum from skin.
- Intestines develop and spill into the umbilical sac.
- Vitello-intestinal duct atrophies and is no longer functional
- Liver and Pancreas develop as outgrowth of proximal duodenum.
How does the respiratory system arise?
- Lungs develop as an outgrowth of embryonic gut tube
How does the urinary system develop?
- The structure branching off the end of the embryonic gut to form the allantois persists as the urachus (until birth)
- Urachus and rectum combine to form cloaca
- Cloaca divided into urorectal septum and urogenital sinus (differentiates into bladder and urethra.
- Ureteric buds arise from urachus/urogenital sinus to become ureters.
- Ureteric buds induce formation of final kidney
- Kidneys develop from intermediate mesoderm which from the right and left urogenital ridges
- Three sets of kidneys form Pronephros, Mesonephros, and Metanephros (only permanent)
How does the cardiovascular system develop?
- Splanchnic mesoderm aggregates.
- Cells flatten out to become blood vessels or blood precursors
- Different segments of blood vessels fuse forming at different sites throughout the embryo.
- The right and left cardiac tubes fuse to create the developing heart.
- Folded cardiac tube subdivides to form R. Atrium, L. Atrium, R. Ventricle, and L. Ventricle
How is fetal blood circulated?
- Oxygenated blood from mom goes through the placenta –> umbilical vein –> liver
- Most of the blood bypasses the liver via the ductus venosus and enters the caudal vena cava –>right atrium and through the foramen ovale to the left atrium.
- A small amount of blood goes through the liver and then dumps into the caudal vena cava.
- A small amount of blood goes from the R. Atrium –> R. Ventricle –> Pulmonary artery –> lungs –> pulmonary veins –> left atrium (some blood from the pulmonary artery goes through the ductus arteriosus so that it dumps into the aorta, instead of going to the lungs)
- From the L. Atrium –> left ventricle –> aorta –> umbilical arteries –> placenta –> mom to be reoxygenated
