General Embryology 2 Flashcards
Subdivision of mesoderm
- Intraembryonic mesoderm proliferates between the ectoderm and endoderm
- 3 regions develop
3 regions of mesoderm subdivision
- Paraaxial mesoderm
- Intermediate mesoderm
- Lateral mesoderm
Paraaxial mesoderm
- Head mesenchyme
- Somites
Intermediate mesoderm
- Urogenital organs
Lateral Mesoderm
- Split by the formation of the intraembryonic coelom
- Somatic mesoderm (somatopleure)
- Splanchnic mesoderm (splanchnopleure)
Somites
- Block-like condensations of paraxial mesoderm
- Form on the sides of the neural tube
Somatogenesis
- Somite pairs begin formation on day 20
- Progresses in a cranial to caudal pattern
- 42-44 pairs form, most caudal disappear leaving 37 pairs
First 4 pairs of somites
- Contribute to head
8 pairs of somites
- Contribute to the cervical region
12 pairs of somites
- Contribute to the thoracic region
5 pairs of somites (two sets of 5)
- Contribute to the lumbar region
- Contribute to the sacral region
3 pairs of somites
- Contribute to coccyx
Somites subdivide into
- Sclerotome
- Dermamyotome
Sclerotome forms
- Meninges
- Vertebrae
- Ribs
Dermamyotome forms
- Dermis of the neck and back
- Muscle
Dermamyotome differentiates into
- Dermatome
- Myotome
Myotome splits into
- Epimere, located dorsally
- Hypomere, located ventrally
Epimere gives rise to
- Epaxial muslces of the back
- Deep back muscles
Hypomere gives rise to
- Hypaxial muscles
- Muscles of the abdominal and thoracic wall
- Limb muscles
Lateral plate divides into
- Somatic mesoderm
- Splanchnic mesoderm
Somatic mesoderm
- Lines body wall
Somatopleure
- Somatic mesoderm + ectoderm
- Forms body wall with ectoderm
Splanchnic mesoderm
- Covers endoderm
Splanchnopleure
- Splanchnic mesoderm + endoderm
- Forms gut wall with endoderm
Coelom
- Body cavity formed by lateral folding of the embryo
Folding of the embryo
- Lateral folds fuse at the ventral midline
- Forms intraembryonic coelom
Intraembryonic coelom eventually divides into 3 principal body cavities:
- Pericardial
- Pleural
- Peritoneal cavities
Craniocaudal and lateral folding
- Draw in the yolk sac (purse string)
- Close off body wall (except at umbilicus)
- Flat trilaminar embryonic disc -> cylindrical embryo
Ectopia cordis
- Failure of the thoracic body wall to close
Gastroschisis
- Failure of the abdominal body wall to close
Early development of the cardiovascular system
- Vasculogenesis begins during 3rd week
- Hemangioblast cells develop in the extraembryonic splanchnic mesoderm of the yolk sac
Hemabgioblast development gives rise to
- Hematopoietic cell progenitors
- Endothelial precursor cells
- Endothelial cells surround blood cell aggregates (blood islands)
Yolk sac involved in erythropoiesis until
- About day 60
Vasculogenesis
- Differentiating endothelial cells organize into small capillary vessels
- Fuse to form channels
Vasculogenesis events
- Vascularizes the yolk sac, connecting stalk, and chorionic villi
- Vessels from extraembryonic mesoderm -> placenta -> join embryo blood vessels (from intraembryonic splanchnic mesoderm) -> establish circulation
Intraembryonic vessels begin to develop on
- Day 18
Hematogenesis does not begin within the embryo until about
- 4th week
2 major phases of hematopoiesis
- Embryonic (weeks 1-4)
- Definitive (weeks 4-term)
Embryonic phase of hematopoiesis
- Blood cells arise from yolk sac extraembryonic mesoderm
Definitive phase of hematopoiesis
- Hematopoietic stem cells (from extraembryonic/intraembryonic mesoderm) -> seed the spleen, liver, and then bone marrow
Liver is major site of
- Fetal hematopoiesis until bone marrow takes over
Vasculogenesis definition
- Blood vessels arise de novo from “hemangioblasts” that develop into blood cells and vascular tubes
Angiogenesis
- Growth of new blood vessels from existing ones
Segmentation of endoderm
- Foregut
- Midgut
- Hindgut
Foregut
- Trachea
- Esophagus
- Stomach
- Duodenum
- Liver
- Pancreas
Midgut
- Small intestine
- Ascending colon
- Proximal 2/3 of transverse colon
Hindgut
- Distal 1/3 of transverse colon
- Descending colon
- Rectum
- Upper anal canal
Weeks 4-8
- Organogenetic period: all main organ systems have begun to develop
- Critical period of many organ systems
Teratogen exposure
- Can cause major congenital birth defects during organogenetic period (weeks 4-3)
Teratogens
- Agents that can disturb the development of an embryo (or fetus)
- Radiation, drugs, infections, chemicals
