Notochord Development Overview

Notochord Development

• The notochord is a rod-like structure that forms from notochordal precursor cells induced by signals from the primitive streak.
• The notochordal process elongates by invagination of cells from the primitive pit.
• An indentation forms and extends into the notochordal process, forming the notochordal canal.
• The floor of the notochordal process fuses with the underlying embryonic endoderm, leading to the degeneration of the fused layers.
• Openings form connecting the notochordal canal with the umbilical vesicle.
• As the openings in the floor of the notochordal process become confluent, the floor disappears, forming the notochordal plate.
• The notochordal plate cells proliferate and undergo infolding, creating the definitive notochord.
• The proximal part of the notochordal canal temporarily persists as the neurenteric canal, forming a communication between the amniotic cavity and the umbilical vesicle.
• The neurenteric canal eventually obliterates as development progresses.
• The notochord detaches from the endoderm of the umbilical vesicle, becoming a continuous layer.
• The notochord extends from the oropharyngeal membrane to the primitive node.
• As it degenerates, the notochord contributes to the formation of the nucleus pulposus of each intervertebral disc.

Functions of the Notochord

• The notochord defines the primordial longitudinal axis of the embryo, providing rigidity and serving as the initial scaffold for body structure.
• The notochord acts as a primary signaling center, inducing the overlying embryonic ectoderm to thicken and form the neural plate, the primordium of the CNS.
• The notochord sends signals necessary for the development of axial musculoskeletal structures and the CNS.

Allantois

• The allantois appears as a small outpouching from the caudal wall of the umbilical vesicle around day 16 of embryonic development.
• The allantois extends into the connecting stalk.
• The allantoic mesoderm expands beneath the chorion and forms blood vessels that eventually supply the placenta.
• The proximal part of the allantoic diverticulum persists as a stalk called the urachus, which extends from the bladder to the umbilical region.
• In adults, the urachus is represented by the median umbilical ligament.
• The blood vessels of the allantoic stalk develop into the umbilical arteries.
• The intraembryonic part of the umbilical veins has a separate origin from the allantois.

Neurulation

• Neurulation is the process of forming the neural plate, neural folds, and closure of the folds to form the neural tube.
• Neurulation is completed by the end of the fourth week when the caudal neuropore closes.
• The notochord induces the overlying embryonic ectoderm to thicken, forming the neural plate.
• The neural plate consists of thickened epithelial cells called neuroectoderm, which gives rise to the CNS.
• The neural plate initially matches the length of the notochord but eventually extends cranially to the oropharyngeal membrane.
• Around the 18th day, the neural plate begins to invaginate along its central axis, creating the neural groove flanked by neural folds.
• The neural folds, prominent at the cranial end, are the first indicators of brain development.
• By the end of the third week, the neural folds begin to fuse, transforming the neural plate into the neural tube, the primordium of the brain vesicles and spinal cord.
• The neural tube detaches from the surface ectoderm as the neural folds meet.
• As the neural folds fuse, neural crest cells undergo an epithelial-to-mesenchymal transition and migrate away.
• The surface ectoderm fuses over the neural tube, later differentiating into the epidermis.
• Neurulation is completed during the fourth week.

Neural Crest

• As the neural folds fuse to form the neural tube, some neuroectodermal cells along the inner margin of each fold detach from neighboring cells.
• These cells form a flattened, irregular mass called the neural crest, located between the neural tube and the overlying surface ectoderm.
• The neural crest soon separates into right and left parts, shifting to the dorsolateral aspects of the neural tube.
• Neural crest cells give rise to the sensory ganglia of spinal and cranial nerves.

Development of Somites

• Somites are segmented blocks of mesodermal tissue that form along the sides of the neural tube during embryogenesis.
• Somites develop from the paraxial mesoderm, derived from the primitive node.
• The paraxial mesoderm is continuous laterally with the intermediate mesoderm and thins out into the lateral mesoderm.
• Toward the end of the third week, the paraxial mesoderm begins to differentiate, condense, and divide into paired cuboidal bodies called somites.
• Somites form in a craniocaudal sequence, developing from the head region downward along the body.
• During the somite period (days 20 to 30), about 38 pairs of somites form.
• By the end of the fifth week, there are 42 to 44 pairs of somites.
• The number and appearance of somites are used as criteria to determine the age of the embryo.

Functions of Somites

• Somites contribute to the development of:
  • Most of the axial skeleton (bones of the head and trunk)
  • Associated musculature
  • The adjacent dermis of the skin
• Motor axons from the spinal cord innervate the muscle cells in the somites, guided by specific pathways.

Development of Intraembryonic Coelom

• The intraembryonic coelom begins as isolated coelomic spaces in the lateral intraembryonic mesoderm and cardiogenic mesoderm.
• These spaces merge to form a single horseshoe-shaped cavity, the intraembryonic coelom.
• The intraembryonic coelom divides the lateral mesoderm into two layers:
  • Somatic (Parietal) Layer: Located beneath the ectoderm, continuous with the extraembryonic mesoderm covering the amnion.
  • Splanchnic (Visceral) Layer: Located adjacent to the endoderm, continuous with the extraembryonic mesoderm covering the umbilical vesicle.
• The somatic mesoderm and the overlying ectoderm form the embryonic body wall (somatopleure).
• The splanchnic mesoderm and the underlying endoderm form the embryonic gut (splanchnopleure).
• During the second month, the intraembryonic coelom divides into three major body cavities:
  • Pericardial Cavity
  • Pleural Cavities
  • Peritoneal Cavity

Early Development of Cardiovascular System

• Initially, the embryo obtains nutrition from maternal blood through diffusion via the extraembryonic coelom and umbilical vesicle.
• Blood vessels begin to form in the extraembryonic mesoderm of the umbilical vesicle, connecting stalk, and chorion.

Third Week of Embryo Development

• The beginning of the third week marks the period of gastrulation.
• This period is characterized by the development of the primitive streak, notochord, and three germ layers.
• The third week corresponds to 5 weeks after the first day of the last normal menstrual period.
• Cessation of menstruation is often the first indication of pregnancy.

Gastrulation

• Process by which the three germ layers are established in embryos.
• Bilaminar embryonic disc transitions into a trilaminar embryonic disc.
• Involves significant cell shape changes, rearrangement, movement, and alterations in adhesive properties.
• Beginning of morphogenesis (development of body form).
• Most significant event during the third week.
• The embryo is referred to as a gastrula.

Germ Layer Derivatives

• Ectoderm: Epidermis, nervous system (central and peripheral), eyes, internal ears, neural crest cells, head connective tissues.
• Endoderm: Epithelial linings of respiratory and digestive tracts, glands, liver, pancreas.
• Mesoderm: Skeletal muscles, blood cells, blood vessel linings, smooth muscle, serosal linings of body cavities, reproductive and excretory systems, cardiovascular system.

Primitive Streak

• First morphological sign of gastrulation.
• Appears on the epiblast surface of the bilaminar embryonic disc.
• Thickened linear band of epiblast.
• Crucial for establishing the embryo's craniocaudal axis (cranial and caudal ends, dorsal and ventral surfaces, right and left sides).
• Elongates as cells are added to its caudal end.
• Forms the primitive node at its cranial end.
• Develops a groove (primitive groove) and a small depression (primitive pit).
• As cells move between the epiblast and hypoblast layers, they form mesenchyme.
• Produces extraembryonic mesoderm covering the yolk sac and amnion.
• Diminishes in size and disappears by the end of the fourth week.

Notochord Formation

• Critical structure for signaling the formation of central nervous system and axial skeleton.
• Mesenchymal cells migrate through the primitive streak and acquire mesodermal cell fates.
• Cells migrate cranially from the primitive node and pit, forming the notochordal process.
• Notochordal process develops a lumen, becoming the notochordal canal.
• Notochordal process continues to grow cranially between the ectoderm and endoderm, reaching the prechordal plate.
• Prechordal plate is crucial for signaling cranial structure development.
• Mesenchymal cells from the primitive streak and notochordal process migrate laterally and cranially, contributing to the formation of the cardiogenic mesoderm.

Neurulation

• Process of forming the neural plate, neural folds, and neural tube.
• Completed by the end of the fourth week.
• Notochord induces the overlying embryonic ectoderm to thicken, forming the neural plate.
• Neural plate consists of thickened epithelial cells (neuroectoderm), which gives rise to the CNS.
• Neural plate begins to invaginate, creating the neural groove and neural folds.
• Neural folds begin to fuse, transforming the neural plate into the neural tube.
• Neural tube detaches from the surface ectoderm.
• Neural crest cells detach from the folds, forming a flattened, irregular mass.
• Neural crest cells migrate and differentiate into sensory ganglia of spinal and cranial nerves.

Development of Somites

• Segmented blocks of mesodermal tissue.
• Form along the sides of the neural tube.
• Develop from the paraxial mesoderm.
• Form in a craniocaudal sequence.
• About 38 pairs of somites form during the somite period (days 20 to 30).
• Number and appearance of somites are used to determine the age of the embryo.

Functions of Somites

• Most of the axial skeleton (bones of the head and trunk).
• Associated musculature.
• Adjacent dermis of the skin.
• Motor axons from the spinal cord innervate the muscle cells in the somites.

Development of Intraembryonic Coelom

• Initially isolated coelomic spaces in the lateral intraembryonic mesoderm and cardiogenic mesoderm.
• Spaces merge to form a single horseshoe-shaped cavity.
• Divides the lateral mesoderm into two layers:
  • Somatic (Parietal) Layer: beneath the ectoderm.
  • Splanchnic (Visceral) Layer: adjacent to the endoderm.
• Somatic mesoderm and overlying ectoderm form the embryonic body wall.
• Splanchnic mesoderm and underlying endoderm form the embryonic gut.
• Divides into three major body cavities during the second month:
  • Pericardial Cavity
  • Pleural Cavities
  • Peritoneal Cavity

Early Development of Cardiovascular System

• Nutrition initially obtained by diffusion through the extraembryonic coelom and umbilical vesicle.
• Blood vessels form in the extraembryonic mesoderm.
• Embryonic blood vessels develop to deliver oxygen and nutrients.

Development of the Vascular System

• Vasculogenesis: Formation of new vascular channels by the assembly of angioblasts.
• Angiogenesis: Formation of new blood vessels by budding and branching from existing vessels.
• Blood Islands: Clusters of angioblasts that differentiate into endothelial cells and form vascular channels.
• Mesenchymal cells differentiate into muscular and connective tissue components of blood vessels.
• Blood cells first develop from specialized endothelial cells.
• Hematopoiesis begins in the fifth week.
• Heart and great vessels develop from mesenchymal cells in the cardiogenic area.
• Paired endocardial heart tubes form and fuse into the primordial heart tube.
• Heart tube connects with blood vessels, forming a primitive cardiovascular system.
• Blood circulation begins by the end of the third week.
• Heart starts beating on the 21st or 22nd day.
• Cardiovascular system is the first functional organ system.
• Heartbeat detectable by Doppler ultrasonography during the fourth week.

Development of Chorionic Villi

• Primary chorionic villi appear at the end of the second week and branch.
• Mesenchyme grows into these villi, forming secondary chorionic villi.
• Mesenchymal cells differentiate into capillaries and blood cells, forming tertiary chorionic villi with visible blood vessels.
• Capillaries in the villi fuse to form arteriocapillary networks, connecting with the embryonic heart.