Lecture 2: Embryonic Development: Germ Layers, Notochord, and Somite Formation

What is the major concept underlying development of the embryonic body form?

The embryonic period consists of distinct phases of differential growth.

What is differential growth in the context of this lecture?

Different embryonic tissues and regions grow at different rates, producing changes in the shape and organization of the embryo.

What major transformation does differential growth produce in this lecture?

It transforms the relatively flat trilaminar embryonic disc into the three-dimensional body form of the embryo.

What are the three layers of the trilaminar embryonic disc?

Ectoderm, mesoderm, and endoderm.

What is the orientation of the three germ layers?

Ectoderm is dorsal, mesoderm is the middle layer, and endoderm is ventral.

What is the key principle concerning the three germ layers?

All bodily tissues arise from one of the three germ layers.

What process produces the three germ layers?

Gastrulation.

What happens to epiblast cells during gastrulation?

Epiblast cells migrate through the primitive streak.

Which original embryonic layer gives rise to the three definitive germ layers?

Epiblast.

What three germ layers are created by gastrulation?

Ectoderm, mesoderm, and endoderm.

Which germ layer is dorsal?

Ectoderm.

Which germ layer is in the middle?

Mesoderm.

Which germ layer is ventral?

Endoderm.

What are the major topics of early germ-layer development in this lecture?

Mesoderm differentiation, ectoderm and neurulation, endoderm, and body folding.

What is mesoderm composed of?

Mesenchymal cells.

What is mesenchyme?

Embryonic connective tissue.

What major tissue types arise from mesenchymal cells according to this lecture?

Connective tissue and musculature.

What are the four major mesodermal structures/regions emphasized in this lecture?

Notochord, paraxial mesoderm, intermediate mesoderm, and lateral plate mesoderm.

Arrange the major mesodermal regions from the midline toward the lateral body wall.

Notochord → paraxial mesoderm → intermediate mesoderm → lateral plate mesoderm.

Where is the notochord located?

At the embryonic midline as an axial mesodermal structure.

What type of embryonic tissue is the notochord?

Mesoderm.

What is the major developmental role of the notochord?

It induces neurulation in the overlying ectoderm.

What is neurulation?

Formation of the primitive nervous system.

What adult derivative of the notochord is emphasized in this lecture?

The nucleus pulposus within the intervertebral discs.

What does the notochord induce the ectoderm to form?

The neural plate.

Why is the notochord important even though most of it does not persist as an adult structure?

It acts as an embryonic inducer that initiates neurulation, and its adult remnant contributes to the nucleus pulposus.

Where is paraxial mesoderm located?

Bilaterally immediately next to the notochord.

What does paraxial mesoderm condense into?

Somites.

What are somites?

Segmental structures formed from paraxial mesoderm.

What are the three major subdivisions/derivatives of somites?

Dermatome, myotome, and sclerotome.

What does the dermatome form?

Dermis of the skin.

What does the myotome form?

Muscle.

What does the sclerotome form?

The axial skeleton, including vertebrae and ribs, and the base of the skull.

Which somite component gives rise to dermis?

Dermatome.

Which somite component gives rise to muscle?

Myotome.

Which somite component gives rise to vertebrae and ribs?

Sclerotome.

Which somite component contributes to the base of the skull?

Sclerotome.

What is the developmental sequence from mesoderm to vertebrae?

Mesoderm → paraxial mesoderm → somite → sclerotome → axial skeleton including vertebrae.

What is the developmental sequence from mesoderm to muscle?

Mesoderm → paraxial mesoderm → somite → myotome → muscle.

What is the developmental sequence from mesoderm to dermis?

Mesoderm → paraxial mesoderm → somite → dermatome → dermis.

In what direction does segmentation into somites progress?

Rostral to caudal.

Approximately how many pairs of somites form?

42-44 pairs.

How many occipital somite pairs initially form?

4 pairs.

What happens to the occipital somites?

They disappear.

How many cervical somite pairs form?

8 pairs.

How many thoracic somite pairs form?

12 pairs.

How many lumbar somite pairs form?

5 pairs.

How many sacral somite pairs form?

5 pairs.

How many coccygeal somite pairs initially form?

8-10 pairs.

What happens to many of the coccygeal somites?

The last 5-7 disappear.

What does rostral mean?

Toward the head/cranial end.

What does caudal mean?

Toward the tail end.

What happens to the innervation of dermatome, myotome, and sclerotome derivatives?

Each retains innervation from its original somite.

What is the dermomyotome?

The dermatome and myotome migrating together to form tissues of skin and muscle.

Where is intermediate mesoderm located?

Bilaterally between the somites/paraxial mesoderm and lateral plate mesoderm.

What does intermediate mesoderm temporarily connect?

Paraxial mesoderm and lateral plate mesoderm.

What major structures arise from intermediate mesoderm?

Urogenital structures.

Which mesodermal region is associated with the urogenital system?

Intermediate mesoderm.

Where is lateral plate mesoderm located?

Bilaterally along the lateral margins of the mesoderm layer.

Into what two layers does lateral plate mesoderm divide?

Somatic/parietal lateral plate mesoderm and splanchnic/visceral lateral plate mesoderm.

What does somatic/parietal lateral plate mesoderm follow during differential growth?

Ectoderm.

What does splanchnic/visceral lateral plate mesoderm follow during differential growth?

Endoderm.

What major structure/cavity is associated with the separation of the lateral plate mesoderm?

The intraembryonic coelom.

What does somatic/parietal lateral plate mesoderm line?

The body wall.

What does splanchnic/visceral lateral plate mesoderm line?

The gut tube/viscera.

What is the easiest way to remember somatic vs splanchnic lateral plate mesoderm?

Somatic follows ectoderm and body wall; splanchnic follows endoderm and viscera.

Which lateral plate mesoderm follows the ectoderm?

Somatic/parietal mesoderm.

Which lateral plate mesoderm follows the endoderm?

Splanchnic/visceral mesoderm.

Which lateral plate layer contributes to the serous membranous lining of the abdominal and chest body wall?

Somatic/parietal mesoderm.

Which lateral plate layer contributes to the serous membrane lining of the gut tube and other organs?

Splanchnic/visceral mesoderm.

What are the key derivatives/functions of the notochord?

Induction of neurulation and formation of the nucleus pulposus of the intervertebral discs.

What are the key derivatives of paraxial mesoderm?

Somites, which form dermatome, myotome, and sclerotome.

What are the key derivatives of intermediate mesoderm?

Urogenital structures/system.

What are the key derivatives of lateral plate mesoderm?

Coelomic cavity linings: somatic mesoderm lines the body wall and splanchnic mesoderm lines the gut tube/viscera.

What are the two major subdivisions of ectoderm emphasized in this lecture?

Neuroectoderm and surface ectoderm.

What type of cells compose ectoderm?

Epithelial cells.

What major structures arise from ectoderm according to the lecture?

The outer layer of the skin, nervous system, and sensory organs.

What is epithelium?

Cells that make up internal and external body linings.

Where is neuroectoderm located initially?

In the midline ectoderm.

What eventually happens to neuroectoderm?

It becomes internalized during neurulation.

What process involves neuroectoderm?

Neurulation.

What is the basic sequence of neurulation?

Neuroectoderm → neural plate → neural groove/neural folds → neural tube.

What initiates neurulation?

The notochord.

What is the first major event of neurulation?

The notochord induces midline ectoderm to thicken and form the neural plate.

What happens to the neural plate after it forms?

It grows and folds bilaterally around the neural groove.

What is the neural groove?

The midline depression formed as the neural plate folds.

What are neural folds?

The elevated lateral edges of the folding neural plate.

How does the neural tube form?

The neural folds approach and meet/fuse, creating the neural tube.

What happens to the neural tube after closure relative to the surface ectoderm?

It becomes internalized beneath the surface ectoderm.

What happens to the surface ectoderm as the neural tube closes?

It fuses over the neural tube and continues as the surface covering of the embryo.

What major structures arise from the neural tube according to this lecture?

The central nervous system and retina.

What does CNS stand for?

Central nervous system.

What is the developmental sequence leading to the CNS?

Ectoderm → neuroectoderm → neural plate → neural groove/folds → neural tube → CNS.

What mesodermal structure lies ventral to the developing neural tube?

The notochord.

How are mesoderm and ectoderm both involved in neurulation?

The mesoderm-derived notochord induces the overlying ectoderm to become neuroectoderm and form the neural plate, which folds to form the neural tube.

Where does neural tube fusion begin?

At approximately the level of the 5th somite.

In which directions does neural tube fusion proceed?

Cranially and caudally from the initial site of fusion.

What is a neuropore?

A temporary opening at an end of the developing neural tube before closure is complete.

When does the cranial neuropore close?

Around day 25.

When does the caudal neuropore close?

Around day 28.