Question

14. Describe how the time and place of gastrulation of cells result in different mesodermal tissues having the ability to pattern the ectoderm in vertebrates, and what molecules those mesodermal tissues release

Answer 1

During gastrulation, cell movements result in a massive reorganization of the embryo from a simple spherical ball of cells, the blastula, into a multi-layered organism. During gastrulation, many of the cells at or near the surface of the embryo move to a new, more interior location.The primary germ layers (endoderm, mesoderm, and ectoderm) are formed and organized in their proper locations during gastrulation.

Mesoderm gives rise to the skeletal muscles, smooth muscle, blood vessels, bone, cartilage, joints, connective tissue, endocrine glands, kidney cortex, heart muscle, urogenital organ, uterus, fallopian tube, testicles and blood cells from the spinal cord and lymphatic tissue.

During the third week a process called gastrulation creates a mesodermal layer between the endoderm and the ectoderm.This process begins with formation of a primitive streak on the surface of the epiblast.The cells of the layers move between the epiblast and hypoblast and begin to spread laterally Some of the migrating cells displace the hypoblast and create the endoderm, and others migrate between the endoderm and the epiblast to create the mesoderm.The chordamesoderm is the central region of trunk mesoderm.This forms the notochord which induces the formation of the neural tube and establishes the anterior-posterior body axis.Between days 13 and 15, the proliferation of extraembryonic mesoderm, primitive streak and embryonic mesoderm take place.

For pattern formation ,first requirements is axis.First, axis to be established in bilateral organisms is:-

1. Anterior/Posterior
2. Dorsal/Ventral
3. Left/Right

After axes formation Homeotic genes determines the fate of the subunits .Pattern formation organizes cell types into their proper locations based on positional information.Anterior-posterior patterning is regulated by the zone of polarizing activity, or ZPA. The proximal-distal pattern formation is regulated by the amount of time a cell spends in the progress zone. Dorsal-ventral patterning is controlled by the overlying ectoderm.

Molecules released by Mesoderm are as follows:- Mesoderm send signals for somite differentiation whichare Morphogens, protein SHH, protein WNT1 that expresses PAX 2 so the somite creates the myotome and dermatome and retinoic acid (RA) and a combination of FGF8 and WNT3a

Multitude of genes that encode known or putative transcription factors (e.g., Tbx6, Hox, Foxc2 (Mf1), Cdx1, Cdx4), peptide growth factors (e.g., Fgf3, Fgf4, Fgf5), and molecules for cell–cell signaling (e.g., Crabp, Notch1, Fgfr1, Epha4).