Study Questions for BIOL 303 – Lecture 12
1. Why is mammalian cleavage atypical when compared to other systems?
Mammalian cleavage is atypical as it has:
-zygotic genome activated very early
-zygotic genome is used for cleavage and early development and these events do require
maternal transcripts and proteins
2. What is compaction?
It is the process where the loose blastomeres express E-cadherin and become tightly
packed. The individual blastomeres are no longer as obvious and the morula take on a
smoothened look. During compaction, cell-cell junctions such as tight junctions in outer
cells, and gap junctions in inner cells form.
3. What is the first differentiation event in the mammalian embryo? Is there any
determinant known to specify this event?
The first differentiation is cells becoming either inner cell mass or
trophoblast/trophoectoderm. This event seems to be entirely random.
4. What is the difference between totipotency and pluripotency? At what point is
totipotency lost during mammalian development?
Totipotency refers to the earliest blastomeres that can become either inner cell mass or
trophoblast cells. Pluripotency refers to the inner cell mass cells that can become any cell
type in body, but not trophoblast.
5. Sketch and label a picture of a typical blastocyst. What process forms the
Cavitation forms the blastocoel, it involves trophoblast cells secreting fluid into the
morula. This is done by having the trophoblast cells pump Na+ ions into the central
cavity, thus drawing in water and enlarging the blastocoel. 6. Following fertilization and during cleavage divisions what function is served by the
zona pellucida? For approximately how many days does the developing
morula/gastrula remain inside the zona pellucida?
The zona pellucida allows for sperm binding during fertilization and prevents the
blastocyst from sticking to the oviduct walls – preventing ectopic pregnancy. After six
days, the morula/gastrula hatches from the zona pellucida.
7. How does the blastocyst attach to the endometrium? What molecules are involved in