UNSW Embryology

Beginnings, Growth and Development - Practical 3

© Dr Mark Hill (2008)

Acknowledgements

10. Notochord

The embryonic structure which establishes body axes and patterns surrounding tissues is called the notochord. The notochord is a midline column of cells running in a rostrocaudal direction (head-tail) within the mesoderm layer. It exists as a transient developmental patterning structure with a role in molecular signaling (patterning) and controlling the direction of embryonic disc folding (mechanical).

Stage 7

	Carnegie Stages 7 Features: embryonic disc, primitive node, primative streak, primitive groove, yolk sac Facts: Week 3, 15 - 17 days, 0.4 mm View 1: embryonic disc, showing the epiblast viewed from the amniotic (dorsal) side. Events: Gastrulation is continuing as cells migrate from the epiblast, continuing to form mesoderm. Mesoderm lies between the ectoderm and endoderm as a continuous sheet except at the buccopharyngeal and cloacal membranes. These membranes have ectoderm and endoderm only and will lie at the rostral (head) and caudal (tail) of the gastrointestinal tract.
	Axes: embryonic disc is shown rostral (head) to top and caudal (tail) to bottom. Left and right are the lateral margins of the disc as shown.
	Folding: all edges of the embryonic disc will fold ventrally, forming a rostro-caudal "C" shaped tube.
	Gastrulation: Through the primitive streak cells migrate continuously through week 3 into week 4. Initial cells replace hypoblast as an epithelial layer the endoderm. Later migrating cells spread between the two epithelial layers to form mesoderm.
	Axial Process: From the primitive node a tube extends under the ectoderm in the opposite direction to the primitive streak. This tube forms first the axial process, then notochordal process, and finally the notochord. The notochord is a key to embryonic folding and regulation of ectoderm and mesoderm differentiation. It lies in the rostrocordal axis and the embryonic disc will fold either side ventrally, pinching off a portion of the yolk sac to form the lining of the gastrointestinal tract. Next: Carnegie Stage 8 Unlabelled | Stage 8 Labelled

 

Notochord Development

Diagrams showing formation of the axial process which then forms the notochord (also called axial mesoderm). This mesodermal column of cells determines the direction of embryonic disc folding and signals the differentiation of nearby mesoderm (somites) and overlying ectoderm (neural plate then tube).

The notochordal process begins as a fold of ectoderm extending cranially toward the prechordal plate region. The sequence of differentiation: notochordal process -> notochordal plate -> notochord.

(a) Elongation of the notochordal process cranially from the primitive pit as a hollow tube (notochordal canal) in the midline of the embryonic disc underlying the ectoderm.

(b) The notochordal canal may appear to break down on the endodermal side forming a notochordal plate continuous with the endodermal layer.

(c) Notochordal plate folds to form notochord. The notochord (also called axial mesoderm) is an embryonic structure that regulates differentiation of surrounding structures including the overlying ectoderm (neural plate) and mesoderm (somites).

(figures- Moore and Persaud, 1998)

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Terms

• bilaminar- having 2 layers
• blastocyst- the developmental stage following morula, as this stage matures, the zona pellucia is lost allowing the conceptus to adplant and then implant into the uterine wall.
• blastomeres-the cells resulting from the initial rounds of mitotic division of the zygote. These cells become smaller (in cytoplasmic volume) with each division.
• corona radiata- Layer of follicle cells of cumulus oophorus remaining attached to zona pellucida of oocyte after ovulation.
• inner cell mass- the clump of cells found inside the blastocyst. These cells will go in to form the embryo, these are the "stem cells" (we here about in the media) that are totipotential, they can form any tissue in the embryo. Mature oocyte-the female germ cell released at ovulation from the ovary.
• morula &endash;(L. morus = mulberry) early stage of development (12-15 cells) Followed by formation of a cavity in the mass (blastocyst stage). (More? Week 1 Notes)
• parental genomes- the male (sperm) and female (oocyte) DNA which contributes to the embryo's cells.
• polar bodies- 3 exclusion bodies which contain the DNA not used by the embryo. Contributed to initially by the meiotic division of the oocyte.
• pronuclei- the male (sperm) and female (oocyte) nuclei within the fertilized oocyte, prior to their combination to form the new embryo's nuclei.
• trilaminar embryonic disc- the 3 layered embryo stage.
• Trophoblasts- (Gr. trophe = nutrition) outer layer of cells on blastocyst that will generate the embryonic part of the placenta.
• zona pellucida- glycoprotein shell that surrounds the oocyte through to blastula stage of development.
• Zygote- The first cell stage following fertilization of the oocyte by the sperm. This is the first cell of the conceptus which will divide into blastomeres.

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