Gastrulation, Neurulation, and Somitogenesis

What are two types of plates in the Lateral Plate Mesoderm?

Lateral Plate Mesoderm 1. Parietal (somatic) lateral plate 2. Visceral (splanchnic) lateral plate

What is LSF?

Lateral Somitic Frontier

What is the structure (kinda like a tunnel) through which the mesenchymal cells travel into the Lateral Plate Mesoderm?

Lateral Somitic Frontier (LSF)

What is somitomeres?

Loosely organized segmented collections of paraxial mesoderm in the cranial region

What is the Somatopleure? What is the Splanchnopleure?

Somatopleure = dorsal (parietal) lateral plate + surface ectoderm Splanchnopleure = ventral (visceral) lateral plate + endoderm

What is Anencephaly?

*Anencephaly*, -failure of the cranial (anterior) neuropore to close between days 23-25, -usually only brainstem present, -little to no cerebrum develops -poor prognosis (die before birth of <2 years)

What chronic infection can occur in 20% of Situs Inversus patients? Why does this occur?

*Chronic Sinusitis*, -occurs from *abnormal cilia* throughout their respiratory system

The neural tube, once it has formed fully, is open at the cranial and caudal ends. What are these openings called? What day do these openings close?

*Cranial (anterior) neuropore*, -close day 25, *Caudal (posterior) neuropore*, -close day 28, -(remember that development shifts from cranially to caudally, so the later closing of the caudal neuropore makes sense)

What does the Dorsal (parietal) Layer of the Lateral Plate Mesoderm differentiate into?

*Dorsal (parietal) Layer* -sternum -dermis of the body wall -parietal pleura and peritoneum -bones/connective tissues of the limbs

What is Encephalocele?

*Encephalocele*, -failure of the cranial neuropore to close between days 23-25, -can affect anterior or posterior skull

What are two main TYPES of embryonic cells?

*Epithelial* cells and *Mesenchymal* (stem) cells

What are two types of embryonic cell transitions? How do the cells morphologically change in each?

*Epithelial-to-mesenchymal transition (EMT),* -cells lose polarity and become migratory, *Mesenchymal-to-epithelial transition (MET),* -cells gain polarity and form tight junctions of tubes and sheets

How is the Mesoderm formed?

*Mesoderm* is formed by Epiblast cells that invaginate to form a mesoderm layer dorsal to the endoderm

When Neurulation begins from the Notochord inhibiting BMP signaling, what structure begins to form?

*Neural Groove*

What is the Notochord?

*Notochord*, -a cartilaginous rod that forms ventral to the neural tube

Somites have mesenchymal cells that migrate to become the myotome What are the myotome cells that are more medial (close to the neural tube) called? What are the myotome cells that migrate into the Lateral Plate Mesoderm?

*Primaxial* - stayed IN the somite to differentiate, close to the neural tube -(primaxial ~= proximal) *Abaxial* - migrate OUT of the somite to differentiate in the lateral plate mesoderm

What is a Sacroccoccygeal Teratoma?

*Sacroccoccygeal Teratoma*, -failure of primitive streak to regress fully at the end of week 4, -primitive streak regresses caudally until it's gone

Each Paraxial Mesoderm forms what bilateral columns of mesenchymal tissue on either side of the notochord? What is the end product of this structure?

*Segmental Plate* End product = *Somites* -makes sense because we're developing cranially to caudally, so the segmental plate at the caudal end hasn't become somites yet Shouldn't the arrow be pointing the opposite direction?

What is Situs Inversus? What are two types?

*Situs Inversus*, -organs being mirror images of normal. 1. *Complete* - all organs mirror image, 2. *Incomplete* - only a SINGLE organ is involved

What is Spina Bifida?

*Spina Bifida* -failure of the caudal (posterior) neuropore to close between days 22-28

What are Teratomas?

*Teratomas*, -tumor cells, but they're *of germ cell origin*, -ex: teeth, hair, bone, brain, etc.

What does the Ventral (visceral) Layer of the Lateral Plate Mesoderm differentiate into?

*Ventral (visceral) Layer* -walls of the gut tube -visceral pleura/peritoneum -cardiac muscle -lung/tracheal cartilages -connective tissues

When gastrulation begins, how do Epiblasts migrate?

1. *Epiblasts* (blue cells) travel down the primitive streak, 2. Epiblasts invade the inside of the embryo -displaces the *hypoblasts* (yellow cells), 3. Epiblasts then become the *Definitive Endoderm*

What are the THREE tissues types that Paraxial Mesoderm (somites) differentiate into?

1. Dermatome 2. Myotome 3. Scleratome

What are somites composed of?

1. Dermomyotome (dermatome + myotome) 2. Sclerotome

Neural Crest cells migrate to form what structures? (11)

1. Sensory ganglia of cranial and spinal nerves 2. Autonomics 3. Schwann cells 4. Meninges 5. Connective tissue/bone of the face and skull (*NCCs develop A LOT for the face and the skull*) 6. Dermis, vascular smooth muscle of face and skull 7. *Odontoblasts* (tooth forming cells) 8. Parafollicular cells of the thyroid gland 9. Chromaffin cells of the adrenal medulla 10. Conotruncal septum in the heart 11. Melanocytes

When myotome cells leave paraxial mesoderm and cross the LSF what domain it designated?

Abaxial domain

What can mesenchymal stem cells differentiate into?

Adipocyte Chondrocytes..?? Osteoblasts Muscle cells Neurons

Notochord is the inducing structure for what?

Axial Skeleton

A 34 y/o pregnant female visits her obstetrician who directs her to the hospital for a routine ultrasound, which shows a small mass protruding from the posterior of the head. The radiologist diagnoses encephalocele. The obstetrician tells her that her baby may be fine after some reconstructive surgery. What is the cause of the infant's condition? A. Failure of the cranial neuropore to close before day 20 B. Failure of the cranial neuropore to close before day 25 C. Failure of the caudal neuropore to close before day 20 D. Failure of the caudal neuropore to close before day 25 E. Failure of the caudal neuropore to close before day 28

B. Failure of the cranial neuropore to close before day 25

A newborn female presents with gastroschesis (an opening in the anterior abdominal wall that allows viscera to protrude through). A defect concerning which of the following tissues is the primary contributor to the formation of this congenital anomaly? A. Dermatome B. Myotome C. Parietal lateral plate D. Scleratome E. Visceral lateral plate

C. Parietal lateral plate

In which direction development of somites proceed?

Cranial-to-caudal

A newborn male presents with DiGeorge syndrome (a *neural crest syndrome*) due to a deletion on chromosome 22. He displays craniofacial defects, heart defects, and his adrenal gland is underdeveloped. Which other cell type may be affected in this patient? A. Astrocytes B. Lower motor neurons C. Sensory neurons of spinal cord D. Sympathetic chain ganglia E. Upper motor neurons

D. Sympathetic chain ganglia -anything autonomic is neural crest

What day does gastrulation begin?

Day 14

Around what day Somites begin to form?

Day 20

What day does gastrulation and neurulation end?

Day 28

The Lateral Plate Mesoderm splits into two segments. What are those two segments called?

Dorsal (parietal/somatic) Layer Ventral (splanchnic/visceral) Layer

Embryonic Epithelial cells: How tightly connected are they? What do they form?

Embryonic Epithelial cells: -epithelial cells, -tightly connected, -CAN'T MIGRATE, -form *tubes* and *sheets*

Embryonic Mesenchymal (stem) cells: How tightly connected are they? What do they form?

Embryonic Mesenchymal (stem) cells: -connective tissue, -loosely connected, -form *matrices* and CAN MIGRATE and act independently, -aka: multipotent cells

What are the two main cells of Gastrulation? Which cell migration initiates Gastrulation?

Epiblasts - migrates to initiate gastrulation, Hypoblasts

How does *gastrulation* begin?

Formation of the *primitive streak* at the *caudal* part of the embryo

The most anterior paraxial mesoderm forms what tissue that gives rise to the skull and face?

Head Mesoderm

Notochordal plate separates from what to form a solid notochord?

Hypoblast

How does the Notochord persist in adults?

In the Intervertebral Discs as *Nucleus Pulposus*

Neural folds are formed by?

Individual cell shape-changes.

What are two folds that occur during embryonic development?

Lateral Fold Cephalocaudal fold (look these up on youtube)

During Neurulation, what cell type breaks away from the Neural Tube to migrate?

Neural Crest cells

The Notochord is inducing the ectoderm to form what neural tissue structure?

Neuro-ectoderm (neural plate)

How does the Notochord form?

Notochord formation: 1. Mesenchymal epiblast cells invaginate through the *primitive node*, 2. These cells migrate cranially to form *Prechordal Mesoderm*

What is formed when prechordal mesoderm cells fuse with hypoblast cells?

Notochordal plate

Next to the Neural Tube, what are the two main types of Mesoderm? Which of these types of mesoderm compose Somites?

Paraxial mesoderm - form somites Lateral Plate mesoderm NOT SURE!!!!!

Myotome cells that remain in the neural tube designate what domain?

Primatial domain

What muscles are Primaxial muscles? What muscles are Abaxial muscles?

Primaxial Muscles - *intrinsic back* and *intercostal muscles* Abaxial Muscles - *body wall* and *limb muscles* -imagine your arms having abs with axes tattooed on them

What structures are responsible for innervating Primaxial Muscles? What structures are responsible for innervating Abaxial Muscles?

Primaxial Muscles are innervated by DORSAL RAMI Abaxial Muscles are innervated by VENTRAL RAMI

What are the stages of progression of Segmental Plate to Somites?

Segmental plate --> Somitomeres --> Somite

What is Sirenomelia (caudal dysgenesis)? What is the cause?

Sirenomelia (caudal dysgenesis), -lower limb fusion, -renal agenesis, -imperforate anus, Cause: inadequate mesoderm production in caudal portion of embryo

What does "Splanch-" mean?

Splanch means "viscera"

What are three types of epithelial cells?

Squamous, Cuboidal, Columnar

How is the Ectoderm formed?

The *Ectoderm* is formed by the remaining epiblast cells from gastrulation

What structures does the neuro-ectoderm give rise to? What is this process called?

The *Neuro-Ectoderm* gives rise to CNS, PNS, and neural crest cells, This occurs through a process called *Neurulation*

What is the Primitive (Hensen's) Node?

The *Primitive Node* is a group of cells that forms at the cranial end of the primitive streak, -the *organizer* of the embryo, telling cells where to go, what to do, -helps establish the longitudinal axis (directionality) of the embryo by organizing tissues that form the notochord

What is the Primitive Streak?

The *Primitive Streak* is the groove through which epiblasts undergo EMT and migrate, -once migrated through this groove, they differentiate into three germ tissues

Why is the Notochord staying close to the Ectoderm (blue cells)?

The Notochord is *inducing* (thickening) the structure of the ectoderm to form *neural tissue*

What are the signals that the notochord is expressing to induce the ectoderm into the neural plate? If the notochord is not present to induce the ectoderm, what occurs?

The Notochord is expressing *NOGGIN* and *CHORDIN*, -NOGGIN and CHORDIN *inhibit BMP signaling* in the ectoderm, Without BMP signaling, ectoderm differentiates to neuro-ectoderm, -if the notochord is NOT PRESENT, ectoderm differentiates into *Epidermis*

What *IS* gastrulation?

The formation of the three germ layers: endo, meso, and ectoderm from the embryonic disk

What is the process by which Somites are formed? What are Somites?

The process by which Somites are formed is called *Segmentation* *Somites* are segments of the body

Sclerotome forms what?

Ventral region -Vertebrae and ribs Dorsal region -becomes dermomyotome (remains epithelial)

What is frontier?

a line/boarder separating 2 countries, a line division btw different or opposed things

What is a segmental plate

columns of mesenchymal tissue on either side of notochord formed by early paraxial mesoderm