somite differentiation and neural crest

Neural Crest Formation

And Somite Differentiation

ByM.VharshiniSri Ramachandra University

Neural Crest

•Neural crest cells are a temporary group of cells unique to vertebrates that arise from the ectoderm cell layer.

•They give rise to a diverse cell lineage - including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and ganglia.

Characteristics of neural crest cells1. Multipotent capability: Ability to

give rise to several precursor cells.

2. Migratory property: Neural crest cells break free from neural folds.

3. Regulation: It’s the ability of the embryo to control the loss of cells by increasing the proliferation of the remaining neural crest cells.

Regionalization of the neural crest

The crest can be divided into four main anatomical regions:1.Cranial crest cells2.Trunk crest cells3.Vagal and sacral crest cells4.Cardiac crest cells

1. Cranial crest cells:•Cranial neural crest cells migrate to produce the craniofacial mesenchyme, which differentiates into the cartilage, bone, cranial neurons, glia, and connective tissues of the face. •These cells also enter the pharyngeal arches and pouches where they contribute to the thymus, bones of the middle ear and jaw.

2. Trunk neural crest: •Trunk neural crest gives rise to two populations of cells. One group of cells fated to become melanocytes migrates dorsolaterally into the ectoderm.•A second group of cells migrates ventrolaterally to form the sympathetic ganglia, adrenal medulla, and the nerves surrounding the aorta.

3. Vagal and sacral neural crest:•The vagal and sacral neural crest cells develop into the ganglia of the enteric nervous system and the parasympathetic ganglia.

4. Cardiac neural crest:•It develops into melanocytes, cartilage, connective tissue and neurons of some pharyngeal arches. •It also gives rise to regions of the heart such as the musculo-connective tissue of the large arteries, and part of the septum.

Somite Differentiation

Development• The mesoderm forms at the same time as the

other two germ layers, the endoderm and ectoderm.

• The mesoderm at either side of the neural tube is called paraxial mesoderm.

• As the primitive streak regresses and neural folds gather (to eventually become the neural tube), the paraxial mesoderm separates into blocks called somites.

What are somites?

•Somites are blocks of mesoderm that are located on either side of the neural tube in the developing vertebrate embryo.

•Somites are precursor populations of cells that give rise to important structures associated with the vertebrate body plan and will eventually differentiate into dermis, skeletal muscle, cartilage, tendons, and vertebrae.

Molecular regulation of somitogenesis

•Formation of segmented somites from un- segmented presomitic (paraxial) mesoderm depends on a segmentation dock established by cyclic expression of a number of genes.

•The cyclic genes include members of the NOTCH and WNT signaling pathways that are expressed in an oscillating pattern in presomitic mesoderm.

•Thus, Notch protein accumulates in to form somite and then decreases as the somite is established.

•The increase in Notch protein activates other segment patterning genes that establish the somite. Boundaries of each somite are regulated by RA and a combination of FGF8 and WNT3a.

• In humans 42-44 somite pairs are formed along the neural tube. These range from the cranial region up to the embryo’s tail.

• Several caudal somites disappear again, which is why only 35-37 somite pairs can be counted in the end.

• The number of the somites that are found is used to determine the embryo's age.

Somite differentiation

In the developing vertebrate embryo, somites split to form: 1.Dermatomes2.Skeletal muscle (myotomes)3.Tendons and cartilage

(syndetomes) 4.Bone (sclerotomes)

1. Dermatome

•The dermatome is the dorsal portion of the paraxial mesoderm somite which gives rise to the skin.

•In the human embryo it arises in the third week of embryogenesis.

•The dermatomes contribute to the skin, fat and connective tissue of the neck and of the trunk.

2. Myotome•The myotome is that part of a somite that forms the muscles of the animal.

•Each myotome divides into an epaxial part, at the back, and a hypaxial part at the front. 

•The myoblasts from the hypaxial division form the muscles of the thoracic and anterior abdominal walls.

•The epaxial muscle mass loses its segmental character to form the extensor muscles of the neck and trunk of mammals.

3. Sclerotome

•The sclerotome forms the vertebrae and the rib cartilage and part of the occipital bone.

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