Gastrulation

Overview

Primitive streak forms and sets the basic axes

Epiblasts move in thru primitive streak Þ ectoderm, mesoderm, endoderm

Mesodermal notochord forms at the midline

Paraxial (somites), intermediate (urogenital), and lateral (cardiogenic) mesoderm form

Bilaminar oropharyngeal (future oral opening) and cloacal membrane (future anal opening) form

Body cavities (coelomic spaces) form and are surrounded by somatopleure (parietal, wall of body) and splanchnopleure (visceral, wall of gut)

Gastrulation

Bilaminar embryonic disc Þ 3 primary germ layers

Epiblast Þ ectoderm, mesoderm, endoderm

Establishes craniocaudal axis and bilateral symmetry

The Primitive Streak and Gastrulation

~week 3 (day 15)

Epiblasts migrate to midline at caudal end Þ primitive streak

Primitive streak elongates in cranial direction

Embryonic disc becomes pear shaped

Day 16 – 22: primitive streak regresses caudally

Day 26: primitive streak disappears

Basic axes of the embryo are set

Cell Migrations During Gastrulation

Epiblasts migrate thru primitive pit into space between epiblasts and hypoblasts

Some cells displace hypoblast cells to the extra-embryonic yolk sac and form the endoderm

Others form mesoderm between epiblast and hypoblast

After gastrulation, the remaining dorsal surface epiblast cells are called ectoderm

2 areas remain bilateral after gastrulation(oropharyngeal membÞ future oral opening; cloacal membÞ future anal opening)

Gastrulation is not random, but orderly Þ obtain fate maps

Epiblasts change shape as they migrate: cuboidal Þ bottle-shaped Þ amoebic

Germ Layer Origin and Human Tissues

Endoderm Þ epithelium of: pharynx, thyroid, thymus; larynx, trachea, lungs; gut and associated glands; bladder, vagina, urethra

Mesoderm Þ Muscle; Connective tissue, cartilage, bone; Blood, bone marrow; Epithelium of blood vessels, kidney, ureters, gonads, adrenal cortex

Ectoderm Þ epidermis, hair, nails; nervous tissue; hypophysis; epith of sense organs, nasal cavity, mouth, anal canal

Formation of Mesoderm

The mesoderm Þ "controls the action"

Chorda mesoderm – midline location (future notochord)

Paraxial mesoderm – lateral location (future somite)

Intermediate and lateral mesoderm (urogenic, cardiogenic)

Chorda mesoderm Þ notochordal process Þ notochord Þ degenerates

notochord Þ rigid structure, provides support for the embryo

mesodermal notochord induces ectoderm Þ neural plate (neuralation)

Paraxial mesoderm Þ somites Þ form in cranial – caudal sequence

end of 5^th week, 42-44 pairs of somites

somites Þ sclerotomes (future vertebral column); myotomes (future axial muscles); dermatomes (future dermis)

Intermediate mesoderm Þ urogenital and gonadal ridges and kidneys

Lateral mesoderm Þ cardiogenic mesoderm (vasculature, heart, body cavities)

Formation of Body Cavities

Coelom splits lateral mesoderm into 2 layers

somatopleure (somatic and parietal mesoderm) Þ embryonic body wall

splanchnopleure (splanchnic/visceral mesoderm plus endoderm) Þ embryonic gut

These spaces coalesce to form horseshoe shaped cavity Þ 3 cavities (pericardial, pleural and peritoneal cavities)

Primary Induction

As notochord develops, embryonic ectoderm thickens Þ neural plate

Neural plate rolls up Þ neural tube Þ neurons and glia of CNS plus PNS