lecture 8

Question 1

difference between holoblastic and meroblastic embryos

Answer 1

HOLOBLASTIC: all cells contribute to the embryo and the organism (humans, Xenopus)
MEROLASTIC: not all cells contribute to the embryo; there is also formation of yolk (for example in zebrafish and chicken) that provides nutrients and proteins for the embryo.

Question 2

definition of axes in xenopus

Answer 2

The animal-vegetal axis is maternally determined: animal pole on top (dark, pigmented);vegetal pole on bottom (pale, yolky,heavier). Then cleavages occur: along the AV axis, perpendicular to it and the last one perpendicular to both. Sperm only enters through animal pole.

at the 4-cell stage, Xenopus embryo is already divided into dorsal and ventral halves; the definition of the DV axis is controlled by the Nieuwkoop centre; cortical rotation occurs, which moves the proteins that define the DV axis to the side (wnt and VegT); wnt11 is a morphoge (dorsal)

Presence of the Nieuwkoop centre determines formation of a second organiser = Spemann organiser that produces mesoderm-patterning signals. It induces formation of the AP axis (neural tube, notochord, somites).

Question 3

chicken development

Answer 3

During gastrulation, epiblast cells converge on the PRIMITIVE STREAK and pass through it as individual cells.
The first morpholgical structure that anticipates the AP polarity is Koller’s sickle that arises at the boundary between the area pelucida and area opaca.
The anterior end of the streak has a condesation of cells forming Hensen’s node (equivalent of Spemann organizer in Xenopus). Hensen’s node starts moving back, towards the posterior end of the embryo.

!!!! The primitive streak outlines the direction of the AP axis of the embryo (the start of the streak marks the posterior end of the embryo and it elongates towards the anterior part)
So, primitive streak forms posterior to anterior and the Hensen’s. node moves opposite to it.