Gastrulation occurs during weeks 2-4 of gestation and involves the inward migration of cells from the blastula to form the three germ layers - ectoderm, mesoderm, and endoderm. Each germ layer then gives rise to different tissues and organs. Neurulation follows gastrulation and occurs during weeks 3-4, forming the neural tube from the ectoderm. Somites form from the paraxial mesoderm on either side of the neural tube and differentiate into structures like bone, cartilage, muscle and tendons. Neural crest cells form at the border of the neural plate and migrate to form parts of the nervous system, face and pigmentation.

1. NAME AADAR NALSE ID 20-1-01237
1. Define GASTRULATION. When does it occur (time frame)? Discuss the process.
● The stage of embryonic development during which a gastrula is created by the inward
migration of cells from the blastula.
● It occurs during weeks 2 and 4 of gestation.
● During the early stages of most animals' embryonic development, the single-layered
blastula is rearranged into a trilaminar (three-layered) structure called the gastrula. The
ectoderm, mesoderm, and endoderm are the three germ layers. It occurs following
cleavage, blastula development, and the formation of the primitive streak. Organogenesis
follows, in which particular organs develop inside freshly created germ layers. In the
growing embryo, each layer gives rise to unique tissues and organs.
● Sequences of gastrulation: (i) The embryo becomes asymmetric. (ii) The primitive streak
forms. (iii) Cells from the epiblast at the primitive streak undergo an epithelial to
mesenchymal transition and ingress at the primitive streak to form the germ layers.
● The ectoderm gives rise to the epidermis, and also to the neural crest and other tissues
that will later form the nervous system. The mesoderm is found between the ectoderm
and the endoderm, giving rise to somites.
● The somites form muscle, the cartilage of the ribs and vertebrae, the dermis, the
notochord, blood and blood vessels, bone, and connective tissue.
● The endoderm gives rise to the digestive and respiratory epithelium, as well as the
organs associated with the digestive system, such as the liver and pancreas. Following
gastrulation, the body's cells are organized into either a sheet of connected cells (as in
epithelia) or a mesh of isolated cells (as in mesenchyme).
2. Define NEURULATION. When does it occur? ( time frame). Discuss the process
● The creation of the neural tube from the embryo's ectoderm is known as
neurogenesis. In all vertebrates, it occurs after gastrulation. During gastrulation, cells
migrate to the core of the embryo, generating the endoderm (deepest layer),
mesoderm (middle layer), and ectoderm (surface layer), from which all tissues and
organs will emerge.
● This phase of development begins in the third week and lasts until the fourth week.
● The skin and nervous system are produced by the ectoderm, the digestive organs by
the endoderm, and the remainder of the organs by the mesoderm.
● The notochord—a flexible, rod-shaped structure that runs along the back of the
embryo—is generated from the mesoderm after gastrulation. The notochord provides

2. signals to the underlying ectoderm during the third week of pregnancy, causing it to
become neuroectoderm. A strip of neural stem cells runs along the back of the fetus
as a result of this procedure. This strip is known as the neural plate, and it is where
the nervous system begins.
● The neural groove is formed when the neural plate folds outwards. The neural tube is
formed when the neural folds of this groove close together in the future neck area
(this form of neurulation is called primary neurulation). The basal plate is located in
the anterior (ventral or front) part of the neural tube, while the alar plate is located in
the posterior (dorsal or back) part. The neural canal is the name for the hollow
interior. The open ends of the neural tube (the neuropores) shut off towards the end
of the fourth week of pregnancy.
● When primary neurulation in vertebrates stops, secondary neurulation begins. It is
the process by which the lower levels of the neural tube, as well as the caudal and
mid-sacral regions, are produced.
● In general, the neural plate's cells produce a cord-like structure that migrates into the
embryo and hollows out to form the tube. To varying degrees, each organism uses
primary and secondary neurulation.
3. Explain the formation of somites and their significance.
● Somites are masses of mesoderm that can be found on both sides of the neural tube
in the developing vertebrate embryo. They will eventually develop into the dermis
(dermatome), skeletal muscle (myotome), vertebrae (sclerotome), tendons, and
cartilage (sclerotome) (syndrome).
● The paraxial mesoderm is the mesoderm found lateral to the neural tube. It is distinct
from the chordamesoderm, which is located beneath the neural tube. In vertebrates,
the paraxial mesoderm is known as the unsegmented mesoderm, whereas in chick
embryos, it is known as the segmented mesoderm.
● The paraxial mesoderm separates into blocks termed somites as the primitive streak
regresses and the neural folds assemble before the creation of the neural tube.
Somites serve a crucial function in early development by assisting in the specification
of neural crest cell and spinal nerve axon migration routes.
· Somites separate into four compartments:
(i) Dermatome
● The dorsal region of the paraxial mesoderm somite is known as the dermatome. It
appears in the third week of development in the human embryo.
● The dermatomes contribute to the skin, fat, and connective tissue of the neck and
trunk, however, the lateral plate mesoderm is responsible for the majority of the skin.

3. (ii) Myotome
● The myotome is the muscle-forming portion of a somite. Each myotome is divided
into two parts: a back epaxial part (epimere) and a front hypaxial part (hypomere).
● The muscles of the thoracic and anterior abdominal walls are formed by myoblasts
from the hypaxial division. The extensor muscles of the neck and trunk of mammals
are formed when the epaxial muscle mass loses its segmental nature.
(iii) Sclerotome
The sclerotome is responsible for the formation of the vertebrae, rib cartilage, and a portion
of the occipital bone. It is responsible for the musculature of the back, ribs, and limbs.
(iv) Syndetome
The syndetome is responsible for the formation of tendons and blood vessels.
4. What are neural crest cells. What structures do they ultimately form?
Neural crest cells are multipotent cells induced at the border of the neural plate that
subsequently migrate throughout the embryo and later differentiate into multiple cell types
contributing to most of the peripheral nervous system and the craniofacial cartilage and
bones, as well as pigment and endocrine cells.
5. Describe the paraxial mesoderm and name the structures it gives rise to.
The Paraxial Mesoderm is responsible for forming a mesodermal layer between the
endoderm and the ectoderm during gastrulation. The formation of the mesodermal and
endodermal organs proceeds simultaneously with the formation of the neural tube. From the
base of the skull to the tail, the notochord runs beneath the neural tube. Mesodermal cells
form thick bands on either side of the neural tube. The segmental plate (in birds) and the
unsegmented mesoderm are two types of paraxial mesoderm bands (in mammals). The
paraxial mesoderm splits into blocks of cells called somatic cells as the primitive streak
regresses and the neural folds begin to assemble at the embryo's center. The paraxial
mesoderm splits into blocks of cells termed somites as the primitive streak regresses and
the neural folds begin to collect at the embryo's center. Although somites are only temporary
structures, they play an important role in the organization of the segmental pattern in
vertebrate embryos.
The paraxial mesoderm first form somites Then The somites differentiate into bone,
ligaments and tendons, cartilage, and skeletal muscle, which are rigid structural components
of the body. They are also responsible for the formation of the dermis.