Lecture - Week 1 and 2 Development

2010 Lecture | Zygote | Blastocyst | Implantation

Objectives

Week 1 and 2 Development

Understand the events during week 1 of development (Zygote, Blastomeres, Morula, Blastocyst)
Understand the events during week 2 of development (Trophoblast, Syncytiotrophoblast, Cytotrophoblast, Embryoblast, Implantation)
Brief understanding of early placentation
Brief understanding of maternal changes

Zygote Formation

zygote is the first diploid cell formed following fertilisation.
male and female pronuclei, 2 nuclei approach each other and nuclear membranes break down.
DNA replicates, first mitotic division
sperm contributes centriole which organizes mitotic spindle

Movie - Pronuclear Fusion | Movie - Parental Genomes

Conceptus - the term refers to all material derived from this fertilised zygote, includes both the embryo and the non-embryonic tissues (placenta, fetal membranes).

Links: Carnegie stage 1

Cleavage of Zygote

cleavage of zygote forms 2 blastomeres and is also cleavage with no cytoplasm synthesis.
- special "embryonic" cell cycle S phases and M phases alternate without any intervening G1 or G2 phases (MSMSMSMS, adult MG1SG2) therefore individual cell volume decreases.
cell division is initially synchronous, then asynchronously
- slow- centre cells, larger fast- peripheral cells
zona pellucid still intact (division occurs within the ZP)

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Links: Carnegie stage 2 | Movie - Early Cell Division

Morula


Human Embryo (day 2)	Human Embryo (day 3)

about day 4 is a solid ball of 16-20 cells with peripheral cells flattened against zona pellucida
compaction occurs forming a cavity and leading to the next blastocyst stage

Links: Figure 8.19. Changes in DNA methylation during mammalian development

Blastocyst

Blastocyst (day 5)

about day 5 have 2 identifiable cell types and a fluid-filled cavity (blastoceol)
- outer cell layer - trophoblast, peripheral flattened cells, forms the placenta and placental membranes
- inner cell mass - embryoblast, mass of rounder cells located on one wall of the blastocoel, forms entire embryo

Blastula Cell Communication

Two forms of cellular junctions Junctions

gap junctions, allow electrically couple cells of epithelium surrounding a fluid-filled cavity
tight junctions, close to outer surface create a seal, isolates interior of embryo from external medium

Blastocyst Hatching

Blastocyst Hatching - zona pellucida lost, ZP has sperm entry site, and entire ZP broken down by uterine secretions and possibly blastula secretions. Uterine Glands - secretions required for blastocyst motility and nutrition

Links: Carnegie stage 3 | Figure 21-69. The blastula

Week 2 - Implantation

The second week of human development is concerned with the process of implantation and the differentiation of the blastocyst into early embryonic and placental forming structures.

implantation commences about day 6 to 7
Adplantation - begins with initial adhesion to the uterine epithelium
- blastocyst then slows in motility, "rolls" on surface, aligns with the inner cell mass closest to the epithelium and stops
Implantation - migration of the blastocyst into the uterine epithelium, process complete by about day 9
- interaction between trophoblast cells and endometrial epithelium (apoptosis and decidualization)
coagulation plug - left where the blastocyst has entered the uterine wall day 12

Normal Implantation Sites - in uterine wall superior, posterior, lateral

File:Week2_001_icon.jpg</wikiflv>

Movie - Implantation

Endometrial Receptivity

In humans, receptivity occurs 6 days after the post-ovulatory progesterone surge and lasts about 2 to 4 days.
- Similar "receptivity window" in other species (rat day 5 and mouse day 4.5).
Many studies have looked into identifying markers for this receptivity period both to optimise and to block this process.

Abnormal Implantation

Ectopic tubal pregnancy

Abnormal implantation sites or Ectopic Pregnancy occurs if implantation is in uterine tube or outside the uterus.

sites - external surface of uterus, ovary, bowel, gastrointestinal tract, mesentry, peritoneal wall
If not spontaneous then, embryo has to be removed surgically

Tubal pregnancy - 94% of ectopic pregnancies

if uterine epithelium is damaged (scarring, pelvic inflammatory disease)
if zona pellucida is lost too early, allows premature tubal implantation
embryo may develop through early stages, can erode through the uterine horn and reattach within the peritoneal cavity