Introduction

Fertilization takes place when the sperm has successfully entered the ovum and fuse together resulting in a zygote.  The zygote contains the combined genetic material carried by both the male and female gametes which consists of the 23 chromosomes from the nucleus of the ovum and the 23 chromosomes from the nucleus of the sperm. These 46 chromosomes undergo changes after the mitotic division which leads to the formation of the embryo having two cells, 24 -30 hours after sperm entered an ovum.

When the assessments were done?

The assessment were done a day after the ovum or oocyte retrieval. About 16 to 18 hours after the insemination technique, ICSI or IVF.  The observation of the zygote and embryo development will be continued until the embryo divide and cleave to two to four cells embryo on day 2 (44 to 46 hours) and cleave to six to eight cells embryo (66 to 68 hours) on day 3 post insemination.

Soon after development of the blastomeres begin to form the tight junctions with one another, leading to deformation of their round shape and formation of a mulberry-shaped mass of cells called a morula, this also called compaction. The blastocyst is a structure formed in the early development of mammals.  In humans, its formation begins 5 days after fertilization.

Germinal vesicle (immature):

Germinal vesicle is a large nucleus of an oocyte before it develops into an ovum. No polar body but a germinal vesicle is seen in the cytoplasm.

Metaphase I (immature):

Immature oocyte where the germinal vesicle has broken down and no first polar body extruded yet.

Methaphase II (mature):

Mature oocyte where the first polar body is expelled into the perivitelline space.

Zygote assessment

The assessments of zygote were done after 16 to 18 hour post insemination.

PN stage (post insemination)

A fertilized ovum with 2 normal pronuclei and 2 polar bodies post 16 to 18 hours of insemination.

D2 cleavage (48hr post insemination)

Post 48 hours of insemination with 2-cells embryo without fragments but uneven size of blastomeres.

D3 cleavage (72hr post insemination)

Day 3 cleavage with 6 to 8 cells embryo

D4 morula

Embryo consisting of cells called blastomeres usually presence on day 4 post insemination.

D5 blastocyst

Picture showing hatching blastocyst presence on day 5 post insemination.