Embryonic Development: Germ Layers & Early Stages

• Germ layer formation is a key process in embryonic development
• Objectives include describing the sequence of changes during the second week of embryonic development
• Gastrulation is described, which establishes the ectoderm, mesoderm, and endoderm
• The structures derived from each germinal layer are enumerated

Stages of Development

• Predifferentiation occurs from fertilization to the formation of germ layers
• Predifferentiation lasts from the 2nd to 3rd week
• The embryonic stage occurs from the 4th to 8th week
• The fetal period extends from the 3rd month to the end of intrauterine life

Blastocyst Implantation

• The blastocyst implants and begins burrowing into the endometrium
• The blastocyst has an inner cell mass (embryoblast), a blastocyst cavity, and an outer cell mass (trophoblast)

Formation of the Bilaminar Germ Disc

• The outer cell mass differentiates into the trophoblast and syncytiotrophoblast
• The inner cell mass differentiates into the epiblast and hypoblast around 7.5 days
• The amnion forms a cavity within the epiblast
• The primary yolk sac forms within the hypoblast around 9 days
• Lacunae appear in the syncytiotrophoblast
• The extraembryonic mesoderm surrounds the yolk sac
• Fibrinous plug also appears around 9 days
• Vessels start entering the lacunae around 12 days

Changes during Bilaminar Germ Disc Formation

• The trophoblast differentiates into syncytiotrophoblast and cytotrophoblast
• The embryoblast differentiates into epiblast and hypoblast
• The cavity becomes the amniotic cavity and secondary yolk sac
• The mesoderm differentiates into somatic and splanchnic layers

Trilaminar Germ Disc Formation

• Trilaminar germ disc formation occurs during the 3rd week of development
• Gastrulation involves proliferation, migration, invagination, replacement, and differentiation
• The primitive streak appears during gastrulation
• The notochord forms during gastrulation
• The mesoderm also forms during gastrulation

The End of the Second Week

• A bilaminar germ disc is present at the end of the second week

Beginning of the Third Week

• The buccopharyngeal and cloacal membranes are present
• The primitive groove is visible

Day 15

• A linea thickening along the midline is seen on the upper surface of epiblast at the caudal end
• A faint groove becomes deeper with slightly raised margins

Key Structures at the Cranial End

• The primitive node and Henson's node are present
• A deep primitive pit exists

Key Structures

• The primitive groove is present
• The entire structure is called the primitive streak

Day 16

• Epiblast cells near the primitive streak begin to proliferate
• These cells migrate to the groove and invaginate deeply

Cellular Changes on Day 16

• Cells become flask-shaped and detach from the epiblast
• They migrate through the primitive streak into the space between the epiblast and hypoblast

Formation of Definitive Endoderm

• Some cells invade and replace the hypoblast, forming the definitive endoderm
• Flask-shaped cells migrate between the epiblast and endoderm, forming the intraembryonic mesoderm or secondary mesoderm

Further Development

• Secondary mesoderm extends throughout the disc, except where the cells extend forward to form the nodtocord
• Secondary mesoderm extends across the whole disc

Applied Anatomy of Germ Layer Formation

• The primitive streak is the first landmark that reveals the polarity of the embryo
• The primitive streak acts as a primary organizer by inducing the differentiation of other cells
• It actively participates in the formation of the notochord and intraembryonic mesoderm
• The primitive streak may persist and give rise to tumors like sacrococcygeal teratoma

Notochord Formation

• The notochord is a key structure in embryonic development

Prechordal Plate

• The prechordal plate is a small circular area of columnar endodermal cells attached to the overlying ectoderm
• Later the prechordal plate forms the buccopharyngeal membrane, which will become the oral cavity

Prochodal Cells

• Prochodal cells extend between the epiblast and hypoblast

Notochordal canal formation

• The notochordal process forms as a tube-like structure known as the notochordal tube

Floor of Notochordal Tube

• The floor of the notochordal tube fuses with the intraembryonic endoderm of the yolk sac
• This fused layer degenerates, forming openings
• The process brings the notochordal canal into communication with the yolk sac

Formation of Notochordal Plate

• The opening becomes confluent and the floor of the notochordal canal disappears
• A small passage called the neurenteric canal temporarily connects the amniotic cavity and yolk sac
• The notochordal process converts to the notochordal plate

Detachment and Formation

• The notochordal plate detaches from the endoderm, forming a solid cord
• The definitive notochord is formed after detachment

Function of the Notochord

• Forms a midline axis and serves as the basis of the axial skeleton
• Indicates the future site of the vertebral bodies
• It can become a part of the intervertebral disc
• Can induce differentiation of the neural tube

Embryonic Folding

• Embryonic folding occurs during the 4th week of development, involving cephalocaudal and lateral folding

Folding Process

• Folding occurs during the 3rd week
• Longitudinal and transverse sections shown

Results of Cephalocaudal Folding

• Folding brings the cardiogenic area and primitive heart tube into position
• Structures such as the connecting stalk, yolk sac, and vitalline duct are affected

Folding By Layer

• During the 3rd week the notocord, ectoderm, mesoderm, endoderm and yolk sac are affected

Lateral Folding

• Lateral folding also occurs during the fourth week, involving the ectoderm, mesoderm, and endoderm
• Results in the formation of the gut tube

Somite Formation

• Initially, mesodermal germ layer cells form a thin sheet of loosely woven tissue
• It forms on each side of the midline
• Cells close to the midline proliferate forming a thickened plate of paraxial mesoderm
• Laterally, the mesoderm remains thin as the lateral plate
• Intermediate mesoderm connects paraxial and lateral plate mesoderm

The Process of Somite Formation

• Paraxial mesoderm cells (somites) form starting in the cranial region
• Proceed in a craniocaudal direction
• Somites form discrete blocks

Somite Formation Details

• First pairs form at the cervical region at day 20
• A craniocaudal sequence of 3-4 pairs per day forms finishing on the 5th week
• 42-44 pairs of somites are formed
• Cranial and coccygeal somites disappear

Germ Layer Derivatives

• Specific structures are derived from the three germ layers: ectoderm, mesoderm, and endoderm

Somite Differentiation

• Each somite differentiates into a dermatome, myotome, and sclerotome

Mesoderm Derivatives

• Head mesoderm gives rise to the skull, connective tissue of the head, and dentin of teeth
• Paraxial mesoderm gives rise to muscles of the head, skeleton (except skull), connective tissue, dermis of the skin, and skeletal muscles of the trunk and limbs
• Intermediate mesoderm gives rise to the urogenital system, including kidneys, gonads & their corresponding ducts
• Lateral plate mesoderm gives rise to connective tissue and muscle of viscera, serous membranes, primitive heart, blood & lymph cells, spleen, and suprarenal cortex

Ectoderm Derivatives

• The ectoderm forms the neural crest, neural groove and tube
• The central and peripheral nervous systems, sensory epithelium of ear, nose, and eye are made from ectoderm
• The epidermis, subcutaneous gland, mammary gland, pituitary gland, and enamel of teeth are made from ectoderm

Central Nervous System

• The brain, spinal cord, and peripheral nervous system are all part of the central nervous system

Neural Crest Derivatives

• Neural crest derivatives include facial skeleton, odontoblasts, truncoconal septum, leptomeninges, melanocytes, DRG, ANS, Schwann cells, and cells of the adrenal medulla

Endoderm Derivatives

• The endoderm forms the epithelial lining of the gastrointestinal tract (liver, pancreas, etc.) the urinary bladder, and thyroid etc.