Human Embryology: Week 3 Development

3rd Week Overview

• Embryonic development during the 3rd week involves the appearance of the primitive streak, notochord development, and the differentiation of the three germ layers
• The 3rd week occurs during the week of the first missed menstrual period, about 5 weeks after the last menstrual period
• Cessation of menstruation can be the first sign of pregnancy

Gastrulation

• Gastrulation converts the bilaminar embryonic disc into a trilaminar embryonic disc
• The embryonic disc's three germ layers (ectoderm, endoderm, mesoderm) give rise to specific tissues and organs
• Gastrulation marks the beginning of morphogenesis, which establishes body form and organ structure

Primitive Streak

• The primitive streak appears on the dorsal aspect of the embryonic disc at the beginning of the 3rd week
• The primitive streak appears as a thickened linear band
• This band results from the proliferation and migration of epiblast cells toward the embryonic disc's median plane
• Once the primitive streak appears, the embryo's craniocaudal axis, dorsal and ventral surfaces, and right and left sides can be identified
• The primitive streak elongates as cells are added to its caudal end, while its cranial end proliferates to form the primitive node
• A narrow primitive groove develops within the primitive streak
• The primitive pit marks the end of the primitive groove
• Epiblast cells migrate through the primitive groove to form the endoderm and mesoderm
• Mesenchyme is a loose network of embryonic connective tissue

Germ Layers

• Influence from embryonic growth factors causes epiblast cells to migrate through the primitive groove to form endoderm and mesoderm
• Mesenchymal cells can proliferate and differentiate into fibroblasts, chondroblasts, and osteoblasts
• The primitive streak actively forms mesoderm until the early fourth week

Notochordal Process and Notochord

• Mesenchymal cells migrate cranially from the primitive node and pit and form a median cellular cord called the notochordal process
• The notochordal process develops a lumen known as the notochordal canal
• The primitive streak diminishes in relative size, becoming an insignificant structure in the sacrococcygeal region
• The notochordal process grows cranially between the ectoderm and endoderm, reaching the prechordal plate
• The prechordal plate is a small area of cells, important as an organizer of the head region
• The fused layers of ectoderm and endoderm create the oropharyngeal membrane, which is the future site of the oral cavity (mouth)
• Caudal to the primitive streak is the cloacal membrane, which indicates the future anus site
• The notochordal canal merges with the degenerating endoderm, forming the neuroenteric canal
• Notochordal foldings close, creating the notochord and are then separated from the underlying endoderm
• The notochord serves as the embryo's axis, providing rigidity
• It serves as the basis for the axial skeleton's development, including the head bones and vertebral column
• The notochord indicates the site for future vertebral bodies
• The vertebral column forms around the notochord
• The notochord degenerates and disappears as the vertebrae form, with remnants persisting as the nucleus pulposus of each intervertebral disc

Mesenchymal Cells

• Mesenchymal cells are continuous with the extraembryonic mesoderm that covers the amnion and umbilical vesicle
• Mesenchymal cells migrate laterally and cranially between the ectoderm and endoderm, extending to the margins of the embryonic disc

Cardiogenic Area

• Some cells from the primitive streak migrate cranially on each side of the notochordal process and around the prechordal plate, meeting cranially to form the cardiogenic mesoderm
• The cardiogenic mesoderm is located in the cardiogenic area, where the heart primordium begins to develop at the end of the third week

Neurulation

• The notochord functions as the primary inductor in the early embryo
• It induces the overlying embryonic ectoderm to thicken and form the neural plate, which becomes the primordium of the central nervous system, a process called neurulation
• Neurulation includes the neural plate and neural folds as well as the closure of these folds to form the neural tube
• Neurulation is completed by the end of the fourth week
• The ectoderm of the neural plate (neuroectoderm) gives rise to the central nervous system (CNS) that entails the brain and spinal cord
• The neural plate appears cranial to the primitive node and dorsal to the notochord
• The neural plate broadens and extends cranially as the notochord elongates
• Around day 18, the neural plate invaginates along its central axis, forming a median neural groove with neural folds on each side
• Neural folds are the first indication of brain development
• The neural folds progress to move together and fuse, converting the neural plate into the neural tube
• The neural tube will become the primordium of the brain vesicles and spinal cord
• The neural tube detaches from the surface ectoderm when the neural folds meet
• The ectoderm's free edges fuse, making this layer continuous over the neural tube to differentiate into the epidermis of the skin

Neural Crest Formation

• As the neural folds fuse to create the neural tube, some neuroectodermal cells lining the crest of each neural fold lose their epithelial affinities and attachments
• Neural crest cells migrate dorsolaterally on each side of the neural tube after they separate from the surface ectoderm
• A flattened irregular mass called the neural crest forms between the neural tube and the overlying surface ectoderm
• The neural crest separates into right and left parts that migrate to the dorsolateral aspects of the neural tube
• Neural crest cells differentiate into various cell types that include, spinal ganglia and ganglia of the autonomic nervous system
• Cranial nerves V, VII, IX, and X ganglia and sheaths of the peripheral nerves as well as the pia mater and arachnoid mater also differentiate

Development of Somites

• Simultaneous to the notochord and neural tube formation, intraembryonic mesoderm forms a thick column of paraxial mesoderm
• Each column is continuous with the intermediate mesoderm, gradually thinning into the lateral mesoderm layer
• At the end of the 3rd week, paraxial mesoderm differentiates and divides into paired cuboidal bodies which are called somites
• Somites form on each side of the forming neural tube and appear as surface elevations on the embryo
• Somites give rise to the axial skeleton, associated musculature, and adjacent dermis of the skin

Allantois

• The allantois appears around day 16 as a small diverticulum from the caudal wall of the umbilical vesicle to the connecting stalk
• This structure is involved in early blood formation and is associated with the urinary bladder
• The allantois' blood vessels make up the umbilical arteries and veins

Development of Intraembryonic Coelom

• The intraembryonic coelom (body cavity) initially appears as small, isolated coelomic spaces
• These spaces are in the lateral intraembryonic mesoderm and cardiogenic mesoderm
• The spaces coalesce to form a single cavity-the intraembryonic coelom
• The coelom divides the lateral mesoderm into two layers, a somatic or parietal (somatopleure) layer and a splanchnic, or visceral (splanchnopleure) layer
• The somatic layer is continuous with the extraembryonic mesoderm covering the amnion
• The splanchnic layer is continuous with the extraembryonic mesoderm covering the umbilical vesicle
• The intraembryonic coelom divides into three body cavities the second month
• The three cavities are the pericardial cavity, pleural cavities, and peritoneal cavity

Early Development of the Cardiovascular System

• Embryonic nutrition is from the maternal blood via diffusion through the extraembryonic coelom and umbilical vesicle at the end of the second week
• The developing cardiovascular system correlates with oxygen transport and embryo nourishment from maternal circulation through the chorion
• Blood vessel of the third week formation (vasculogenesis) begins in the extraembryonic mesoderm, umbilical vesicle, connecting stalk, and chorion
• Vasculogenesis begins in the chorion
• A primordial circulation has developed at the end of the third week

Vasculogenesis and Angiogenesis

• During vasculogenesis, mesenchymal cells become angioblasts, aggregate to create blood islands and small cavities and then flatten
• Angioblasts flatten to create endothelial cells which form the primordial blood vessels, and the cavities fuse to form endothelial channels
• During angiogenesis, vessels sprout and/or bud outward into the adjacent nonvascularized parts and areas and then fuse with other like vessels
• Blood cells result from hematopoietic stem or blood vessels, that develop on the vesicle allantois at the third week
• Mesenchymal cells will form the heart and large vessels
• The paired, endothelium-lined channels called endocardial heart tubes appear during the third week, fusing to create the primordial heart tube
• The tubular heart joins with blood vessels in the embryo to include, chorion, and the vesicle to form a cardiovascular system
• At the end of the third week, blood is flowing from the heart and will begin on day 21 or 22

Primordial Heart Tube

• The cardiovascular system is the first primitive state functioning organ
• Heartbeat of embryo is determined by doppler ultrasonography in week four that is 6 weeks after menstruation

Development of Chorionic Villi

• At the end of week two, primary chorionic stem villi appear an will branch off after
• Week three, mesenchymal growth into 1' villi will form into tissue cores
• When capillaries are present, it will be called 3' chorionic villi
• Oxygen and nutrients travel from blood to walls of villi and then to the bloodstream of embryo
• Carbon dioxide and waste will diffuse through walls of villi into the maternal body