Derivatives of the Three Germ Layers

What does the ectoderm initially form in the germ disc?

Dorsal layer of the germ disc; forms the floor of the amniotic cavity.

After embryonic folding, where is the ectoderm located?

It becomes the outermost layer of the embryo.

What are the main ectodermal derivatives? (ENERGy)

Epidermis of skin (including glands, hair, nails), Nervous system (neural tube → brain, spinal cord, peripheral nerves), Sensory epithelium (olfactory epithelium, taste buds), Ear (external auditory meatus, outer ear drum), Respiratory (nasal epithelium), GI tract (anterior oral cavity, lower ½ of anal canal).

What specific structures come from ectodermal epidermis?

Skin, glands, hair, and nails.

What nervous structures come from the neural tube?

Central nervous system (brain and spinal cord) and peripheral nerves.

What sensory epithelium comes from ectoderm?

Olfactory epithelium and taste buds.

What ear structures arise from ectoderm?

External auditory meatus and outer layer of tympanic membrane.

What respiratory structure arises from ectoderm?

Nasal epithelium.

What parts of the GI tract arise from ectoderm?

Anterior oral cavity and lower half of anal canal.

What are neural crest cells and where are they located?

Two strips of ectodermal cells on both sides of the neural plate.

What ganglia arise from neural crest cells?

Sensory, sympathetic, and parasympathetic ganglia.

What other cells arise from neural crest?

Glial cells, melanocytes.

What endocrine organ arises from neural crest?

Adrenal medulla.

What major cardiovascular structure forms from neural crest?

Septum between ascending aorta and pulmonary trunk.

What does the endoderm initially form in the germ disc?

Ventral layer of the germ disc; forms the roof of the yolk sac.

What happens to the upper yolk sac after folding?

Becomes incorporated into embryo as the primitive gut.

What GI tract epithelium is derived from endoderm?

Entire GI tract except anterior oral cavity and lower ½ of anal canal.

What urinary system parts derive from endoderm?

Most of the urinary bladder and urethra.

What parts of the ear arise from endoderm?

Middle ear and Eustachian tube.

What glands/organs are derived from endodermal parenchyma?

Palatine tonsils, thyroid, liver, and pancreas.

Where does intraembryonic mesoderm originate from?

Epiblast cells that invaginate through the primitive streak and pit; located between ectoderm and endoderm.

At what four sites is mesoderm absent?

Buccopharyngeal membrane, cloacal membrane, notochord, and neural tube.

When does mesoderm differentiate and into what parts?

On Day 17 → paraxial, intermediate, and lateral plate mesoderm.

Where is paraxial mesoderm located?

On either side of notochord and neural tube.

Where is intermediate mesoderm located?

Between paraxial and lateral plate mesoderm.

Where is lateral plate mesoderm located?

The most lateral part of the mesoderm.

What does paraxial mesoderm form in the head region?

7 somitomeres → skeletal muscles of face, jaws, throat.

How many total somites form and what are the types?

42–44 somites: 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 8–10 coccygeal.

What does the ventromedial part of a somite become?

Sclerotome → surrounds neural tube and notochord → vertebral column.

What does the dorsolateral part of a somite become?

Dermomyotome → divides into:
• Dermatome → dermis of skin
• Myotome → striated muscle

What does intermediate mesoderm form?

Most of the urogenital system.

What spaces appear in lateral plate mesoderm?

Small cavities that coalesce into the intraembryonic coelomic cavity.

What is the shape of the intraembryonic coelomic cavity?

Inverted U-shape with a central part and two limbs.

What does the central part of the cavity become?

Pericardial cavity.

What do the cranial limbs become?

Pleural cavities.

What do the caudal limbs become?

Peritoneal cavity.

What does the somatic mesoderm adhere to and form?

Adheres to ectoderm → forms striated muscles and connective tissue of body wall; forms parietal layers of serous membranes (pericardial, pleural, peritoneal).

What does the splanchnic mesoderm adhere to and form?

Adheres to endoderm → forms smooth muscle and connective tissue of gut and respiratory tract + cardiac muscle; forms visceral layers of serous membranes.

SUMMARY: Ectoderm derivatives

• Epidermis, skin appendages
• Neural tube → brain, spinal cord, peripheral nerves
• External auditory meatus, outer ear drum
• Nasal epithelium
• Anterior oral cavity, lower ½ of anal canal
• Neural crest: ganglia, glial and melanocyte cells, adrenal medulla, aorticopulmonary septum

SUMMARY: Endoderm derivatives

• Epithelial lining of most GI tract (except oral cavity & lower anal canal)
• Most of urinary bladder and urethra
• Middle ear and Eustachian tube
• Parenchyma of: palatine tonsils, thyroid, liver, pancreas

SUMMARY: Mesoderm derivatives

• Paraxial:
 – Somitomeres → skeletal muscles of face/jaw/throat
 – Somites:
  • Sclerotome → vertebral column
  • Dermomyotome:
   – Dermatome → dermis
   – Myotome → striated muscle
• Intermediate: urogenital system
• Lateral plate:
 – Intraembryonic coelom: pericardial, pleural, peritoneal cavities
 – Somatic mesoderm → parietal serous membranes + body wall
 – Splanchnic mesoderm → visceral serous membranes, smooth + cardiac muscle