Building a Human

The first cleavage of a zygote occurs immediately after [blank_start]fertilisation[blank_end]. Before the zygote is implanted into the uterine wall at day 7-10, it undergoes a series of subdivisions without [blank_start]growth[blank_end]. The cleavage generates a [blank_start]blastocyst[blank_end] - a hollow ball of cells consisiting of an outer layer of cells known as the [blank_start]trophoblast[blank_end]. This outer layer will interact with the [blank_start]uterine[blank_end] wall. Inside the [blank_start]blastocyst[blank_end] is the [blank_start]inner cell mass[blank_end] which will develop into the embryo.

After implantation, the cells of the inner cell mass in the zygote rearrange to form what?

In what structure of the embryo does the primitive streak develop?

What is gastrulation?

Fill in the blanks below to describe the process of gastrulation. 1. The primitive streak develops in the [blank_start]epiblast[blank_end]. 2. The end of the primitive streak expands to form the [blank_start]primitive node[blank_end]. 3. Cells of the epiblast migrate inwards towards the streak, detach from their layer and migrate [blank_start]beneath[blank_end] it. 4. The first cells of the epiblast to migrate invade the [blank_start]hypoblast[blank_end] layer, displacing its cells. 5. Eventually, the hypoblast cells will be entirely replaced by a new layer. This layer is the definitive [blank_start]endoderm[blank_end]. 6. Some of the invaginated epiblast cells remain in the space between the layers. This layer is the [blank_start]mesoderm[blank_end]. 7. The remaining cells of the epiblast form the definitive [blank_start]ectoderm[blank_end].

The epiblast cells no longer migrate after the definitive germ layers have formed.

What is secondary neurulation?

What name is given to the formation of the neural tube in the embryo?

Using the image, fill in the blanks to describe each stage of neurulation. 1. Neuroectodermal cells differentiate to form the [blank_start]neural plate[blank_end] with a border to separate it from regular ectoderm. 2. The neural plate folds upwards with its apex extending [blank_start]dorsally[blank_end]. The two edges join at the [blank_start]neural plate borders[blank_end], forming the [blank_start]neural crest[blank_end]. 3. The closure of the tube disconnects the neural crest from the wall. The cells of the neural crest then differentiate to form the [blank_start]peripheral nervous system[blank_end]. 4. The [blank_start]notochord[blank_end] degenerates and other mesoderm cells differentiate into the [blank_start]somites[blank_end].

What happens to the notochord following the formation of the neural tube?

What will result from an absence of joining of the neural folds at the cranial end of the neural plate?

Label this image showing the organisation of the embryo following neurulation.

Fill in the blanks to describe what each of the primitive tissues will develop into. The [blank_start]lateral plate[blank_end] mesoderm will form the splanchnic and somatic nerves. The [blank_start]intermediate[blank_end] mesoderm will form the kidneys and the gonads. The [blank_start]paraxial[blank_end] mesoderm will form the head and the somites.

Why do the neural crest cells only occur in the anterior half of the vertebral column?

Fill in the blanks to describe the process of neural crest migration. The first wave of migration forms the [blank_start]autonomic[blank_end] ganglia. The parasympathetic ganglia migrate to the walls of the [blank_start]organs[blank_end]. The sympathetic ganglia migrate in front of the [blank_start]aorta[blank_end]. The second wave of migration forms the [blank_start]posterior root[blank_end] ganglia either side of the spinal cord. It also forms the [blank_start]Schwann[blank_end] cells and satellite cells. The final wave of migration forms the [blank_start]melanocytes[blank_end] throughout the epidermis. These give pigmentation to the hair, skin, iris etc. The neural crest cells at the cranial end form the [blank_start]sensory ganglia[blank_end] of the cranial nerves. They also migrate to form the [blank_start]mesenchyme[blank_end] of the face and neck.

Label this image to describe the body axes of the developing embryo.

How does a cell in a particular position in the developing embryo 'know' what tissue it will differentiate into?

The notochord directs the development of the ventral spinal cord.

The Sonic Hedgehog gene and subsequent protein mediates the development of what?

What embryonic structure secretes the SHH protein?

Somites do not respond to the SHH protein.

A mutation in the [blank_start]SHH[blank_end] gene can result in holoprosencephaly. This results in a lack of development of the two hemispheres of the [blank_start]forebrain[blank_end]. It often presents with microcephaly and/or a cleft palate/lip. [blank_start]Cyclopia[blank_end] is the most extreme form of this condition whereby the orbits of the eye do not divide into two cavities, leaving a single central eye.

These diagrams show the blastocysts at different stages of their development. Label them.

Failure of the neural tube to close at a particular point along the spine results in what birth defect?

Failure of the neural tube to close at any point results in what birth defect?

The paraxial mesoderm forms the head and the somites. The somites will then differentiate into... The [blank_start]sclerotome[blank_end] - this will form the vertebrae and the rib cartilage. The [blank_start]myotome[blank_end] - this will form the muscles. The [blank_start]dermatome[blank_end] - this will form the skin and connective tissue.

What will cells in the dorsal portion of the neural tube develop into?

What will cells in the ventral portion of the neural tube differentiate into?

The different regions of the anterior neural tube receive different morphogens in different concentrations to determine what tissues they differentiate into based on location.

What will you see if you remove the notochord during development?

What will you see if you transplant an additional notochord during development?

In what direction do somites develop?

The somites organise the peripheral nerves.