Mammalian development Flashcards
What features of development are unique to mammals?
- cleavage is slow: 12-24 hr per division
- little to no yolk
- holoblastic (full cleavage)
- rotational
- asynchronous - do not occur at the same time
- initial steps depend on mRNA and proteins stored in the egg
What is cleavage?
Sequence of early cell divisions that transforms the diploid zygote into a mass of undifferentiated cells which will develop as the embryo
• each cell is a blastomere
What is meant by rotational cleavage?
- first cell division is parallel
* second cell division - two blastomeres divide at right angles
What is meant by holoblastic cleavage?
• cells are equal in size and completely separate
What happens with the activation of the genome activity?
- destruction of pre-stored mRNA
* but pre-stored proteins may continue to function and regulate development for some time
What does compaction involve and when does this occur?
- 8-cell embryo - becomes a compact mass of cells
- cadherin mediated adhesion between blastomeres
- cell outlines coalesce to form a ‘morula’
- defines polarity of blastomeres (inside and outside)
What is cadherin?
A Ca2+ dependent molecule which assists cell adhesion during compaction (8-cell embryo)
Describe the pathway of an egg after fertilisation
- fertilisation in mammals occur in the upper oviduct
- cleavage occurs as zygote travels downs the oviduct
- when blastocyst arrives at the uterus - hatches from zona pellucida
- trophoblast adheres to lining of uterus, endometrium (implantation)
What are the effects of early implantation? How is it prevented?
- results in an ectopic pregnancy - dangerous condition
* this is prevented by the zona pellucida
What happens in blastulation?
- formation of a blastula from a morula
- fluid accumulates between blastomeres - cavitation
- results in the formation of a blastocoel
- outer and inner layer of cells form
Define blastocoel
Fluid-filled cavity that forms in the embryo which is called the blastula
What are the 2 layers of cells formed in blastulation?
Outer layer of cells - trophoblast/trophectoderm
• later becomes the placenta
Inner cells - inner cell mass
• becomes the embryo
What is zona hatching?
- zona prevents cell-cell contact of the embryo and oviduct wall
- hatching occurs in uterus just before implantation
- blastocyst secretes proteases that weaken the zona
- failure to hatch can cause infertility
How do zygotes generate energy?
- low QO2 (low oxygen consumption rate)
- limited capacity to utilise glucose
- generates energy from low levels of oxidation of pyruvate and/or lactate with aspartate
How do blastocysts generate energy?
- high QO2 - metabolically very active
- high capacity to utilise glucose
- generates energy from both aerobic glycolysis and the oxidation of glucose
