Embryonic Period (Weeks 3-8)

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Gastrulation

Gastrulation is the conversion of the epiblast from a bilaminar disc into a trilaminar embryonic disc consisting of ectoderm, mesoderm, and endoderm. Gastrulation begins with the formation of the primitive streak.

Primitive Streak

The primitive streak is a linear band of thickened epiblast that first appears at the caudal end of the embryo and grows cranially. At the cranial end its cells proliferate to form the primitive knot (primitive node). With the appearance of the primitive streak it is possible to distinguish cranial (primitive knot) and caudal (primitive streak) ends of the embryo.

Figure 3 - Primitive streak and notochord

Notochordal Process

Mesenchymal cells migrate from the primitive knot to form a midline cellular cord known as the notochordal process. The notochordal process grows cranially until it reaches the prechordal plate, the future site of the mouth. In this area the ectoderm is attached directly to the endoderm without intervening mesoderm. This area is known as the oropharyngeal membrane, and it will break down to become the mouth. At the other end of the primitive streak the ectoderm is also fused directly to the endoderm; this is known as the cloacal membrane (proctodeum), or primordial anus.

Notochord

The notochord is a cellular chord that develops by transformation of the notochordal process. The notochord will eventually become the nucleus pulposis of each intervertebral disk.

Germ layers

The embryonic three germ layers give rise to the many tissues and organs of the embryo:

Table 6 - Embryonic Germ Layers and Their Adult Derivatives

Germ Layer		Adult Derivatives
Ectoderm	Surface ectoderm	Lens of eye Adenohypophysis (anterior pituitary gland) Utricle, semicircular ducts, and vestibular ganglion of CN VIII Epithelial lining of external auditory meatus Olfactory placode, including CN I Epithelial lining of: anterior two thirds of tongue, the hard palate, sides of the mouth, ameloblasts, and parotid glands and ducts Mammary glands Epithelial lining of lower anal canal Epithelial lining of distal penile urethra Epidermis, hair, nails, sweat and cutaneous sebaceous glands
	Neuroectoderm	All neurons within the CNS All glial (supporting) cells within the CNS Retina Pineal gland Neurohypophysis (posterior pituitary gland)
	Neural crest	Postganglionic sympathetic neurons within the sympathetic chain ganglia and prevertebral ganglia Postganglionic parasympathetic neurons within the ciliary, pterygopalatine, submandibular, otic, enteric ganglia, and ganglia of the abdominal and pelvic cavities Sensory neurons within the dorsal root ganglia, Schwann cells Pia mater and arachnoid membrane Chromaffin cells of the adrenal medulla Melanocytes Bony structures of the face and neck: Maxilla, zygomatic bone, palatine bone, vomer, mandible, hard palate, incus, malleus, stapes, sphenomandibular ligament, styloid process, stylohyoid ligament, hyoid bone, frontal bone, parietal bond, sphenoid bone, and ethmoid bone Odontoblasts Aorticopulmonary septum Parafollicular cells of thyroid Dilator and sphincter pupillae muscles Ciliary muscle Carotid body
Mesoderm	Paraxial mesoderm	Skeletal muscles of trunk Skeletal muscles of limbs Skeletal muscles of head and neck Extraocular muscles Intrinsic muscles of tongue Vertebrae and ribs Cranial bone Dermis Dura mater
	Intermediate mesoderm	Kidneys Testes and ovaries Genital ducts and accessory sex glands
	Lateral mesoderm	Sternum, clavicle, scapula, pelvis, and bones of the limbs Serous membranes of body cavities Lamina propria, muscularis mucosae, submucosa, muscularis externae, and adventitia of the gastrointestinal tract Blood cells, microglia, Kupffer cells Cardiovascular system Lymphatic system Spleen Suprarenal cortex Laryngeal cartilages
Endoderm		Epithelial lining of the auditory tube and middle ear cavity Epithelial lining of the posterior third of the tongue, floor of the mouth, palatoglossal and palatopharyngeal folds, soft palate, crypts of the palatine tonsil, and sublingual and submandibular glands and ducts Principal and oxyphil cells of the parathyroid glands Epithelial reticular cells and thymic corpuscles Thyroid follicular cells Epithelial lining and glands of the trachea, bronchi, and lungs Epithelial lining of the gastrointestinal tract Hepatocytes and epithelial lining of the biliary tree Acinar cells, islet cells, and the epithelial lining of the pancreatic ducts Epithelial lining of the urinary bladder Epithelial lining of the vagina Epithelial lining of the female urethra and most of the male urethra

Development of Somites

As the notochord and neural tube form, the mesoderm alongside them forms longitudinal columns called paraxial mesoderm. These columns divide into paired cubical bodies called somites. The somites develop in pairs; the first pair develops near the cranial end of the notochord around the end of the third week. Additional pairs of somites develop in a caudal direction from days 20 to 30 (period of somite development) and the number of somites is sometimes used as a criterion for determining an embryo’s age. The somites give rise to most of the axial skeleton (vertebral column, ribs, sternum, and skull base) and associated musculature, as well as to the adjacent dermis.