Human Embryology: Segmentation and Blastula Changes

Questions and Answers

What is the primary role of the notochord during early embryonic development?

To facilitate nutrient absorption

To form the first layer of skin

To serve as a source of blood cells

To act as a primary longitudinal structural element (correct)

At which stage does the notochordal canal form during embryonic development?

On the 16th day

On the 17th day

On the 15th day

On the 18th day (correct)

What structural change occurs to the embryonic plate during the third week of embryonic development?

It changes from oval to circular shape

It transforms from circular to pear-shaped (correct)

It undergoes no significant changes

It becomes elongated only

What is formed at the cranial end of the embryonic plate around the 14th day?

The Prochordal Plate Signup and view all the answers

During which day of embryonic development does the floor of the notochordal canal fuse with the endoderm?

On the 19th day Signup and view all the answers

What is the primary function of the hypoblast in the embryonic disc?

It forms the primary yolk sac. Signup and view all the answers

Which of the following describes the role of somatic mesoderm in the embryonic development?

It lines the trophoblast and participates in placental formation. Signup and view all the answers

At what stage does the amniotic cavity first appear during embryonic development?

On the 7th or 8th day. Signup and view all the answers

What occurs as a result of the formation of the extraembryonic coelom?

A mesodermal mass connecting the two layers at the caudal end is formed. Signup and view all the answers

Which statement about the division of extraembryonic mesoderm is true?

A mesodermal mass remains connecting the two layers at the embryonic caudal end. Signup and view all the answers

Study Notes

Segmentation or Cleavage

The embryonic disc forms from the inner cell mass, which organizes into two layers: the hypoblast (primary endoderm) and epiblast (primary ectoderm).
Two cavities appear:
- Amniotic cavity, formed on the 7th or 8th day.
- Primary yolk sac, arising after the amniotic cavity.
Extraembryonic mesoderm forms as a connective tissue layer between the trophoblast and yolk sac/amniotic cavity.
Isolated spaces within the extraembryonic mesoderm fuse to create the extraembryonic coelom, dividing mesoderm into somatic mesoderm (lining trophoblast) and splanchnic mesoderm (covering yolk sac).
The connecting stalk connects the two mesodermal layers at the embryo's caudal end, which will form the future umbilical cord.
The chorion, formed by the trophoblast and somatic mesoderm, together with structures it encloses, is called the chorionic vesicle.

Third Week Development

Characterized by changes in the bilaminar embryonic plate and the formation of three germ layers.
The embryonic plate transitions from circular to oval, and finally to pear-shaped as the notochord develops.
Notochord serves as the primary longitudinal support structure and plays an organizing role in the nervous system.
Prochordal plate forms from increased height of endodermal cells at the cranial end around the 14th day.
Primitive streak appears as columnar ectodermal cells multiply around days 15-16, leading to the formation of the primitive node.

Notochord Formation

The notochordal process develops on the 17th day from cells originating in the primitive node, extending cranially.
By the 18th day, a notochordal canal forms from an invaginated primitive pit, eventually connecting to the yolk sac.
On the 19th day, the floor of the notochordal canal fuses with the endoderm, leading to the formation of the neuroenteric canal.
On the 20th day, the notochord detaches from the yolk sac endoderm, solidifying its structure.
The cranial part of the notochord contributes to the base of the occipital bone and sphenoid bone, while other parts degenerate or contribute to the nucleus pulposus in intervertebral discs.

Gastrulation

A process that transforms the bilaminar embryonic disc into a trilaminar embryo, starting around the 15th day.
Cells from the primitive streak migrate laterally and cranially, forming the intraembryonic mesoderm, which merges with the extraembryonic mesoderm.
The intraembryonic mesoderm separates ectoderm from endoderm, except at key areas: the oropharyngeal membrane, cloacal membrane, and median plane cranial to the primitive node.
The three primary germ layers are established:
- Ectoderm (outer)
- Mesoderm (middle)
- Endoderm (inner)

Differentiation of Germ Layers

Ectoderm derivatives include the central and peripheral nervous system, sensory epithelium, skin epidermis, and dental enamel.
The notochord stimulates ectoderm to thicken into the neural plate, initiating neural tube formation.
Neural folds develop from lateral margins of the neural plate and converge to form the neural tube by the end of the 3rd week.
The neural tube separates from the surface ectoderm, with neuroectodermal cells forming the neural crest.

Folding

Folding converts the flat trilaminar embryo into a cylindrical shape, beginning in the 4th week.
Cephalo-caudal folding is driven by rapid growth of the central nervous system, while lateral folding is influenced by somite shape.
The result of folding creates the foregut, midgut, and hindgut divisions and results in the formation of the umbilical cord.
The foregut and hindgut correspond to sections of the yolk sac included in the head and tail folds, respectively, altering the positions of various embryonic structures.

Description

Explore the fascinating aspects of human embryology, focusing on the segmentation or cleavage stages and the changes occurring in the blastula, specifically the chorionic vesicle. This quiz covers the arrangement of cells in the embryonic disc and the two-layer formation in the inner cell mass.