Chapter 18. Week 3 of Development: Trilaminar Germ Disk Embryo Formation and Gastrulation - Review of Medical Embryology Book

Introduction: week 3 is a period of rapid development of the conceptus coinciding with the first missed menstrual period. By days 15-16, the embryo is 1.5 mm long, and one clearly sees the primitive streak, Hensen's node, and the notochordal process-all morphologic indications characteristic of gastrulation. The latter is the formation of the third embryonic layer, the mesoderm
Trilaminar germ disk

PRIMITIVE STREAK appears near the end of week 2 and is clearly seen about day 15 as a narrow midline, thickened linear band of embryonic epiblast in the caudal end of the dorsal aspect of the embryonic disk. Its appearance enables identification of embryonic axes, cranial and caudal ends, top and bottom surfaces, and sides of the embryo

As the streak elongates by addition of cells at its caudal end, its cranial end thickens to form the primitive knot or node (Hensen's); simultaneously a narrow primitive groove develops in it which continues into a depression in the knot called the primitive pit
About day 16, cells of the epiblast migrate medially toward the streak, enter the primitive groove, then leave the basal layer of the groove and migrate laterally between the embryonic ectoderm and entoderm to organize into a layer, the intraembryonic mesoderm

Cells migrate from the intraembryonic mesoderm to become mesenchymal cells and form mesenchyme which has a variety of differentiation: become fibroblasts, osteoblasts, and chrondroblasts. The mesenchyme functions as a supporting tissue
The epiblast layer is called the embryonic ectoderm (second germ layer) after the streak begins to form cells destined to become mesoderm

About day 16, cells migrate cranially from the primitive knot and form a midline cord, the notochordal process which grows between ectoderm and entoderm to reach the prochordal plate (small circular area of columnar entodermal cells). The latter is firmly attached to overlying ectoderm forming the oropharyngeal or buccopharyngeal membrane

Other cells of the streak and notochordal process migrate laterally and cranially to reach margins of the embryonic disk and there join the extraembryonic mesoderm covering the yolk sac and amnion. Thus, most extraembryonic mesoderm comes from trophoblast, but some may originate from the primitive streak
Some cells of the primitive streak migrate cranially, pass on each side of the notochordal process, around the prochordal plate, and meet cranially in the cardiogenic area where the heart will be formed

The cloacal membrane is a circular area caudal to the primitive streak. The embryonic disk remains bilaminar here and at the oropharyngeal membrane because embryonic ectoderm and entoderm fuse at these sites and prevent mesenchymal cells from migrating between them
By the middle of week 3, intraembryonic mesoderm separates the ectoderm and entoderm everywhere except at the oropharyngeal membrane cranially, the cloacal membrane caudally, and in the midline, cranial to the primitive knot, where the notochordal process extends
The embryonic disk is at first flat and circular but eventually elongates and becomes pear-shaped as the notochordal process grows. The disk expands mostly in the cranial region; the caudal region remaining mostly unchanged

The growth and elongation of the disk are caused by the continuous migration of cells from the primitive streak

The primitive streak actively forms the intraembryonic mesoderm until the end of week 4 and then slows down as it diminishes in relative siz It usually undergoes degeneration and disappears but may in some cases persist and give rise to a teratoma