Uthsc Developmental Anatomy - Weeks 3 And 4

Front 1

What is the primitive streak?

Back 1

A thickened, longitudinal band of epiblast cells which appears in the caudal, median plane of the dorsal aspect of the embryonic disc. The repeated addition of cells to the caudal end of the primitive streak results in the elongation of the streak towards the cranial end.

Front 2

What is the primitive node?

Back 2

A bulge at the cranial end of the primitive streak that results from the proliferation of primitive streak cells. Contains cilia on its ventral surface. Also called Henson's node.

Front 3

What is the primitive groove?

Back 3

A longitudinal groove that forms in the primitive streak through the early migration of epiblast cells. The part formed in the primitive node is called the primitive pit.

Front 4

When does the primitive streak degenerate?

Back 4

By the end of week four.

Front 5

What controls the invagination process of epiblast cells migrating towards and beneath the primitive streak and by what mechanism?

Back 5

Fibroblast growth factor 8 (FGF 8). Downregulates epiblast cell adhesion (through E-cadherin) and controls specification of migrating cells into mesoderm through regulation of Brachyury (T).

Front 6

What do the first set of epiblast cells to migrate form?

Back 6

Mesenchyme

Front 7

What is mesenchyme?

Back 7

Mesenchyme is a tissue consisting of ameboid and actively phagocytic cells that are suspended in a gelatinous matrix. Forms most of the connective tissues of the embryo. Some mesenchymal cells form mesoblasts which will form the embryonic mesoderm.

Front 8

How is the embryonic endoderm formed?

Back 8

Cells from the epiblast, primitive node, and portions of the primitive streak displace the hypoblast, and form this innermost layer of the embryo.

Front 9

What is the origin of the oropharyngeal membrane?

Back 9

Close to the cranial portion of the embryonic disk, the ectoderm and endoderm remain fused in the area of the prechordal plate, which becomes the oropharyngeal membrane.

Front 10

What are the three areas in which classic mesoderm does not intervene between ectoderm and endoderm?

Back 10

The oropharyngeal membrane, the cloacall membrane, and the notochord.

Front 11

What is Nodal?

Back 11

Nodal initiates and maintains the primitive streak through local expression. A member of the transformiing growth factor beta family.

Front 12

How is the dorsal-ventral axis created?

Back 12

Interplay of several genes. Bone morphogenic protein 4 (BMP4) is secreted through the embryonic disc and in the presence of FGF, all mesoderm will be ventralized to contribute to intermediate and lateral plate mesodermal structures. Genes expressed by the primitive node will antagonize activity of BMP4 to allow for dorsalization of mesoderm.

Front 13

What is the function of the genes chordin, noggin, and follistatin?

Back 13

Antagonize BMP4 to dorsalize cranial mesoderm into notochord, somites, and somitomeres. They are also expressed later in the notochord and are important for neural induction.

Front 14

What is the function of Goosecoid?

Back 14

It is a transcription factor that activates chordin. Over or underexpression leads to severe malformation of the head.

Front 15

What is Kartagener's syndrome?

Back 15

Also known as primary ciliary dysplasia. It is an autosomal recessive condition in which the cilia that line the respiratory tract and the flagella of sperm are either missing or malfunctioning. Half of these patients have situs inversus.

Front 16

What does surface ectoderm give rise to?

Back 16

The epidermis, mammary glands, subcutaneous glands, hair, enamel, pharyngeal arch cartilages, and the adrenal medulla.

Front 17

What does neuroectoderm give rise to?

Back 17

The neural crest and the neural tube.

Front 18

What is paraxial mesoderm?

Back 18

Formed from the differentiation of intraembryonic mesoderm close to the midline. Organizes into paired segments, known as somitomeres.

Front 19

What are somitomeres?

Back 19

Blocks of mesoderm visible externally which are located on each side of the neural tube and notochord. They first appear in the cephalic region and are added sequentially in a cranio-caudal direction at a rate of approximately 3-4 per day. The first seven pairs of somitomeres do not form somites, but will give rise to the musculature of the face, jaw, and throat.

Front 20

What is intermediate mesoderm?

Back 20

Temporarily connects paraxial mesoderm with the lateral mesoderm. It will differentiate into the urinary system and part of the genital system.