LECTURE 3: General Embryology III (NYUCD)

Mesoderm Derivatives
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Mesoderm Derivatives
Mesoderm is sandwiched between the ectoderm and the endoderm.

our focus:
-Notochord
-Head muscle
-Axial & limb skeleton
-Axial & limb muscles
-Body wall
-Connective tissue of the skin

future lectures:
-Circulatory System
-Heart
-Kidneys
-Ovaries
-Testes
Image: Mesoderm Derivatives
Mesoderm Regionalization
During gastrulation specific regions of the epiblast migrate through different regions of the primitive streak to form the mesoderm = FATE MAPPING
Image: Mesoderm Regionalization
Fate mapping
analyses indicate that mesodermal tissue regionalization depends on the position and the timing of cell passage through the node and the primitive streak in an anterior to posterior direction.
Image: Fate mapping
No problem I love Embryo
n - notochord
pm - paraxial mesoderm
im - intermediate mesoderm
lpm - lateral plate mesoderm
eem - extraembryonic mesoderm
Development of Mesoderm happens
Anterior to posterior
Subdivisions of the mesoderm at the end of neurulation
1-Chordamesoderm (axial mesoderm)
-Notochord


2-Paraxial mesoderm
-Head mesenchyme
-Somites
(i) Sclerotome (cartilage, bones)
(ii) Myotome (skeletal muscle)
(iii) Dermatome (dermis)
(iv) Syndetome (tendons)

Suck My D**** Science

3-Intermediate mesoderm
-Kidney
-Gonads


4-Lateral plate mesoderm
-Splanchnic (visceral) mesoderm (circulatory system)
-Somatic (parietal) mesoderm (body wall)
Image: Subdivisions of the mesoderm at the end of neurulation
Development of the mesoderm layer
- After gastrulation we have three layer --> neurulation --> different regions of mesoderm start to segregate

- Two layers of mesoderm: PARIETAL AND VISCERAL LAYER
Image: Development of the mesoderm layer
Derivative of the Notocord
- The notochord will become the nucleus pulposus (chondrocyte-like cells).
- It is later surrounded by the circular fibers of the annulus fibrosus.
- Combined, these two structures form the intervertebral disc
Image: Derivative of the Notocord
Derivatives of the Lateral Plate Mesoderm
Dorsal, ventral, coelom, somatopleure, and splanchnopleure
Image: Derivatives of the Lateral Plate Mesoderm
DORSAL component forms
SOMATIC (parietal) MESODERM:

o Body wall, lining the outside of the peritoneal, pleural & pericardial cavities
o Mesodermal components of the limbs (except muscles)
Image: DORSAL component forms
VENTRAL component formsSPLANCHNIC (visceral) MESODERM: o Covering of the abdominal organs, lung and heart o Circulatory system (heart, blood and blood vessels) o Muscle & connective tissue of digestive and respiratory tractsCOELOMspace between the 2 layers will form the BODY CAVITY o Abdominal region --> peritoneal cavity o Thoracic region --> pericardial & pleural cavitySomatopleureSomatic (parietal) mesoderm + ectoderm o Forms the ventral and lateral body wall. o Forms the membranes that line the peritoneal, pleural & pericardial cavities.SplanchnopleureSplanchnic (visceral) mesoderm + endoderm o Form the wall of the gut and the thin serous membrane around each organ.Embryo grows --> lateral flexion --> fusion of the two lateral plate mesoderm --> fuse at lateral end to SEGREGATE THE GUTo Visceral mesoderm associated with endoderm o Parietal endoderm associated with ectodermIntermediate mesoderm [lateral to the somite] gives rise toUrogenital system (their development is interconnected anatomically)Urinary Systemo Mesonephros and mesonephric duct (embryonic kidney) o Metanephros and metanephric duct (adult kidney)Genital Systemo Gonads(Testis/Ovary) o Genital tractsSomiteblocks of mesoderm that are located on either side of the neural tube in the developing vertebrate embryoSomite Development (paraxial mesoderm)- On each side of the neural tube, appears in the anterior region first --> and move to unsegmented region ( form in anterior --> posterior pattern) - Paraxial mesoderm cells become loosely organized into aggregation of cells called somitomeres. - The somitomeres eventually separate from the presomitic paraxial mesoderm to form individual somites. - Somites appear first in the anterior portion of the trunk, and new somites "bud off" form the unsegmented presomitic (PSM) at regular intervals.SomitomeresParaxial mesoderm cells become loosely organized into aggregation of cellsSomite Periodicity- Somites appear with a specific periodicity. - The periodicity is a highly regulated process (species specific) which depends on a segmentation clock established by the cyclic expression of genes of the **Notch** and **Wnt** signaling pathways. - The number of somites is usually a good indicator of the age of an embryo.Somite Differentiation- When the somite is first separated from the presomitic mesoderm, cells are not committed to a specific lineage. - As the somite matures, its various regions become progessively restricted to form only certain cell types.Somite Components(1) SCLEROTOME (cartilage, bones) > Ventral portion of somite --> delaminates & gives rise to structures all around the spinal cord > Primaxial dermamyotome: gives rise to muscles in the back (close to neural tube) > Abaxial dermamyotome: farthest form the neural tube (2) MYOTOME (skeletal muscle) (3) DERMATOME (dermis) (4) SYNDENTOME (tendons) >the last somatic compartment to form. >It forms at the boundary between the sclerotome and the myotome. >The syndetome give rise to tendon progenitors.What determine cell fate within the somite?A: Location, location, location! B. Tissues secretionAxial Skeletonskull, vertebral column, rib cageSkull developmentThe skull has a dual embryonic origin paraxial mesoderm (posterior) and neural crest (anterior)Vertebral column- Vertebrae are derived from the sclerotome portions of the somites - The sclerotome portion of each somite undergoes a process known as resegmentation - Each vertebra is formed from the fusion of the caudal half of one somite and the cranial half of its neighborLimb developmentOutgrowth of the embryonic body wall, consisting of mesenchyme derived from the somites and the lateral plate mesoderm. These precursors accumulate and proliferate under the epidermis to create: limb budLimb bud *The mesenchyme of the limb bud is derived from the mesoderm: somites (muscle precursors) and somatic lateral plate (skeletal precursors)Apical ectodermal ridge (AER)an important organizing center in the limb budProgress zone (PZ)Zone immediately adjacent to the AER Interaction between the AER and the PZ is critical to limb development > participate in the establishment of the proximal-distal axis of the limbProximal-distal axis of the limb- During outgrowth of the limb, cells of the PZ are assigned progressively more distal positional identities. - Cells leaving the PZ differentiate and their positional value will depend on the time at which they leave the progress zone. - The first cells leaving the PZ form proximal structures, the cells that have undergone numerous divisions in the PZ become the more distal structures. - As the bud grows cells leave the PZ and start to differentiate and cartilaginous structures begin to appear in the mesenchyme. - The part of the limb nearest to the body wall is the first to differentiate, and differentiation proceeds distally as the limb extends.Phocomelia- Thalidomide (sedative) prescribed in the 60's causes severe limb deformities if taken at critical times during pregnancy. - The long bones are shorter than normal and the more proximal elements are lost.ZPA- The anterior-posterior axis is specified by the posterior mesoderm of the limb bud known as the zone of polarizing activity (ZPA). - The pattern of digit formation is dependent upon the activity of the ZPA. > ZPA produces retinoid acid and sonic hedgehogAnterior transplantation of ZPAinduces duplication of the digits = polydactylyPolydactylyFormation of supernumerary digits hand is like a mirror of itselfCell death plays a major role insculpting the limb - It is essential for joints formation and for the separation of the digits.Apoptosisgenetically programmed cell deathSyndactylyAbnormal persistence of soft tissues between the digitsEndoderm DerivativesENDODERM: deepest germ layer o Digestive tract o Respiratory tract o Tonsils o Thyroid o Parathyroid glands o Thymus o Liver o Pancreas o Gall bladderGut and associated organ development- ENDODERM: forms lining/epithelia and secretory elements of the digestive tube and glands. - MESODERM: (mesenchyme) surrounds the tube and forms connective tissue and smooth muscles for peristalsis. > Gut associated organs are generated through reciprocal inductive interaction of endoderm and mesoderm (surrounding mesenchyme)Gut associated organs are generated throughreciprocal inductive interaction of endoderm and mesoderm (surrounding mesenchyme)Gut Formation- Lateral flexion: lateral plate flexes laterally & fuses to give the splanchnic & somatopleura - Longitudinal flexion: bending of the embryo in the anterior and posterior end --> individualizes the gut from the yolk sacGut Tube Regions (subdivisons)- Pharyngeal gut - Foregut - Migdut - HindgutGut Tube RegionalizationPlease Feed My HungerPharyngeal gutextends from the buccopharyngeal membrane to the respiratory diverticulum (pharyngeal pouches)Foregutextends to the liver bud (esophagus, stomach, proximal half of the duodenum, liver and pancreas) >>a ll glands associated with digestive tractMidgutextends to the proximal part of the transverse colon (distal half of duodenum, jejunum, ileum, cecum, and part of colon)Hindgutextends to the cloacal membrane (bladder, urethra, and prostate gland, the rest of the colon and rectum)Activation of transcription factorsis important in differentiation of gut > activation & specialization of gut is done by activating different regions of transcription factorsRegionalization of Gut Tube- Regional specification of the gut tube is initiated by a concentration gradient of retinoic acid. This gradient causes the expression of transcription factors in the gut endoderm that will determine the identity of the different gut regions. - The identity of the different gut regions is maintained through interactions between the gut endoderm and the surrounding splanchnic mesoderm. - Endoderm secretes SHH (sonic hedgehog) which differentially regulates HOX genes expression in the mesenchyme. >> Induces the dominant expression of HOX GENES --> providing POSITIONING IDENTITY along anterior-posterior axis - Different concentration of sonic-hedghog --> activates different concentration of HOX genesPhysiological Gut Herniation- Intestinal loops (midgut) enter the extraembryonic cavity in the umbilical cord during the 6th week. > RETRACTION is a natural process in which there is an enlargement of the umbilical cord as the gut loop is moving into the UMBILICAL CORD - Retraction of the intestinal loops is initiated during the 10th week and completed by 12th week.Development of the Respiratory System(1) The respiratory diverticulum (lung bud) appears around the 4th week. (2) Outgrowth from the ventral wall of the foregut. (3) The appearance and location of the lung bud are dependent upon an increase in retinoic acid produced by adjacent mesoderm. (4) Epithelium of the internal lining of the larynx, trachea, and bronchi, as well as that of the lungs, is entirely of endodermal origin. (5) The cartilaginous, muscular, and connective tissue components of the trachea and lungs are derived from splanchnic mesoderm surrounding the foregut. (6)Start with pharyngolaryngeal tube --> divides --> EVERYTHING FORM SPLANCHNIC MESODERM EXCEPT BUDStages of Pulmonary Development (pneumonic: every person can suck air)EMBRYONIC @ week 3-7 - lung bud formation - trachea and bronchi differentiate PSEUDOGLANDULAR @ week 5-17 - branching of the conducting airways up to the terminal bronchioles CANALICULAR @ week 16-16 - terminal bronchioles divide into respiratory bronchioles - terminal bronchioles divide into alveolar ducts - appearance oh Type-I-pneumocytes SACCULAR @ week 26-36 - terminal alveolar - surfactant detectable ALVEOLAR @ week 36-3 years - alveoli maturation - well-developed alveoli capillary interfaceStages of Pulmonary Development remember byEvery Person Can Suck AirBronchiole tree developmentCONDUCTING PORTION [trachea > bronchi > bronchioles > terminal bronchioles] > RESPIRATORY PORTION [respiratory bronchioles > alveolar ducts > alveolar sacs]