Human Development Week 3: Gastrulation and Embryo Formation

Study Notes

3rd Week of Human Development

Objectives:
- Understand how cell movement drives Gastrulation and the formation of a Trilaminar Embryo (3-Germ Layers)
- Learn about the formation of the Primitive Streak and Notochord
- Learn about the beginning of Neurulation
- Understand the formation of the Neural Plate, Neural Tube, Neural Crest, and the Brain

Formation of the Trilaminar Embryo

Gastrulation: conversion of a bilaminar to a trilaminar embryonic disc consisting of endoderm, mesoderm, and ectoderm
Gastrulation begins with the formation of the primitive streak
During Gastrulation, the embryo is individualized, and twins cannot form

Emergence of the Primitive Streak

The primitive streak elongates medially from the thickened dorsal epiblast surface of the bilaminar embryo
The bilaminar embryonic disc converts to a trilaminar embryonic disc as the primitive streak gives rise to the mesoderm layer
The epiblast is now called the ectoderm layer, and the hypoblast is replaced by the endoderm layer
Axial orientation is established, giving bilateral symmetry to the embryo

Formation of the Notochord

The primitive streak degenerates and shortens, but adds cells to elongate the notochordal process under the epiblast
The notochordal process induces the overlying embryonic ectoderm to form the neural plate, which gives rise to the CNS
On Day 21, the notochordal process flanked by neural folds also begins to fold, forming a tube called the notochord

Neurulation

The neural plate invaginates along its central axis to form a neural groove flanked by neural folds
The neural folds fuse to form the neural tube, which delaminates from the ectoderm
The neural tube forms the primordium of the brain vesicles, spinal cord, and retina

Neural Crest, Spinal Ganglia, and the Epidermis

The neural fold is flanked by neural crest cells
The neural crest cells migrate ventro-laterally and form the spinal ganglia
The surface ectoderm becomes delaminated and fuses to form a continuous epidermis layer

Somite Formation Defines the Axial Skeleton

Paraxial mesoderm forms between the neuroectoderm and endoderm, adjacent to the notochord
The paraxial mesoderm differentiates, condenses, and divides into somites in a cranio-caudal sequence
By Day 35, there are 44 somites, forming the axial skeleton

Development of Intraembryonic Coelom

Coelomic space fuse to form a horseshoe-shaped cavity, the intraembryonic coelom
The lateral mesoderm splits into somatic mesoderm (parietal layer) and splanchnic mesoderm (visceral layer)

Early Development of the Cardiovascular System

Day 16: Extraembryonic mesoderm forms blood islands that fuse to form blood vessels that attach to the connecting stalk
Day 18: Intraembryonic mesoderm forms blood islands that fuse to form blood vessels that attach to the connecting stalk and chorion
Cranially, intraembryonic mesoderm converts to cardiogenic mesoderm, forming the early cardiovascular system and a heart primordium

Vasculogenesis and Angiogenesis

Vasculogenesis: vascular channels form by assembly of cells called angioblasts
Angioblasts aggregate to form clusters of blood island, which form cavities that coalesce to form primordial blood vessels
Angiogenesis: budding, sprouting, and branching of new vessels from pre-existing endothelial vessels

Primordial Cardiovascular System

Day 20-21: Heart and great vessels originate from the cardiogenic mesoderm
Lacunae in the lateral mesoderm contribute to the formation of the pericardial coelom
Paired, endothelial-lined endocardial heart tubes fuse and form a primordial heart tube

Functional Cardiovascular System

The heart tube joins with blood vessels in the embryo, connecting the stalk, chorion, and umbilical vesicle to form a functional cardiovascular system
The system has three branches by Day 26: the arterial system, the umbilical vein, and the vascular plexus of vitelline veins

Summary of the 3rd Week

Initiation of gastrulation, where the embryonic disc is converted from a bilaminar to a trilaminar disc
Formation of the primitive streak, which gives rise to the mesoderm
Formation of the notochordal process, which induces the formation of the neural plate
Formation of the neural tube, which gives rise to the CNS
Development of the intraembryonic coelom, somites, and the axial skeleton### Embryo Folding and Development
18-24 days: Cylindrical human embryo forms with lateral body folds
Folding occurs in two directions: longitudinal (cephalo-caudal) and lateral (transverse)
Longitudinal folding due to rapid enlargement of cranial end of neural tube and primordial brain
Lateral folding due to rapid enlargement of somites, leading to wrapping of amnioblast around embryo and pinching of yolk sac

Hindgut Development and Germ Layer Derivatives

Day 28: Lateral folding fuses three germ layers in the cylinder
Endoderm in the center forms midgut and a primordium intestine
Foregut and hindgut are already defined earlier
Lateral folding reduces connection between midgut and yolk sac, forming an omphaloenteric duct

Morphological Changes in Early Week-4

Day 22-23: Major morphological changes occur in the embryo
Day 22: Embryo is straight with ~4-10 somites
Day 23: Medially placed neural tube has 8-10 somites, open to amniotic cavity at cranial and caudal ends

Formation of Pharyngeal Arches and Heart Development

Day 24: First pair of pharyngeal arches (mandibular arch) emerge, forming mandible and maxilla
Heart starts pumping blood; rostral neuropore closes

Development in the Late Week-4 Embryo

Day 26: Three pairs of pharyngeal arches emerge at 27-somite stage
Rostral neuropore closes, forebrain enlarges, and embryo becomes C-shaped
Lens placode, otic pit, and nasal placode develop

Emergence of Limb Buds and Chambered Heart

Day 27: Upper limb buds develop as small swellings on ventrolateral body
Day 28: Fourth pair of pharyngeal arches formed; lower limb buds emerge, caudal neuropore closes, and nasal placode prominently emerges
Primordial organ systems form; large primordial heart defines atrium and ventricular prominences

Description

This quiz covers the objectives of the 3rd week of human development, including cell movement in gastrulation, formation of the trilaminar embryo, and the development of the primitive streak and notochord.