In the following note, the first week of embryonic development will be described. The following key words / terminology will be defined or described.
Fertilization
Capacitation
Acrosome Reaction
Zygote
Cleavage
Morula
Inner cell mass or embryoblast
Outer cell mass or Trophoblast
Fertilization
The embryonic development first begins with the fusion of female and male gametes (with haploid, n number of chromosome). The process of fusion of male and female pronuclei is known as fertilization. Fertilization occurs in the ampullary region of the uterine or fallopian tube.
Before fertilizing the ovum, the
spermatozoa must undergo (1) capacitation and (2) acrosome reaction.
Capacitation:
- is a period of conditioning of sperms in the female reproductive tract
- lasts approximately 7 hours
- changes occur in the plasma membrane overlying the acrosomal region of a spermatozoa
- only capacitated sperm can pass through the corona cells and undergo acrosome reaction
Acrosome
reaction
- occurs after the spermatozoa bind to the zona pellucid
- induced by zona proteins
- release of enzyme require to penetrate zona pellucida
The main results of fertilization are as follows:
- Restoration of diploid number, 2n of chromosomes
- Determination of sex (XX female and XY male)
- Initiation of cleavage: without fertilization, the oocyte usually degenerates 24 hours after ovulation
Zygote
The diploid cell stage that is formed by the fusion of haploid gametes, ovum (female gamete) and sperm (male gamete) is known as zygote. Thus, a zygote contains genetic information derived from both parents.
Cleavage and formation of Morula:
The zygote is reached two cells stage approximately 30 hours
after fertilization; four cells stage is reached approximately 40 hours; the
12-16 cell stage is reached at approximately 3 days. Approximately 3 days after fertilization, cells divides to
form 16-cell morula. Late morula, 32
cells stage is reached at approximately 4 days. These series of mitotic cells
division by which a zygote divides, increase in number and becomes morula, is
known as cleavage.
Blastomere
With each cleavage division, the cells become smaller. Each cell is known as blastomere.
Blastomere
With each cleavage division, the cells become smaller. Each cell is known as blastomere.
Inner cell mass (embryoblast) and Outer cell mass (trophoblast)
After third cleavage, blastomeres maximize contacts with each other forming a compact ball of cells. This process is known as compaction. At the same time, the whole mass segregates into inner and outer cells.
Inner cell mass- inner cells of morula that gives rise to embryo proper
Outer cell mass- outer cells of morula that forms trophoblast, which later takes part in the formation of placenta.
Blastocyst:
About the time morula enters the uterine cavity, fluid
begins to accumulate into the intercellular spaces of the inner cell mass.
Gradually these spaces become confluent and finally, a single cavity, blastocele, forms.
At this time the embryo is known as blastocyst.
Cells of the inner cell mass in a blastocyst is now called embryoblast and are at one pole.
The cells of outer cell mass, now called trophoblast,
flatten and form the wall of the the blastocyst.
Zona pellucida disappears at the end of the fourth day,
allowing implantation to begin.
Trophoblastic cells over the embryoblast pole begin to
penetrate between the epithelial cells of the uterine mucosa on about sixth day.
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| Fig-1: Differentiation of fertilized embryo into inner cell mass and outer cell mass (represented by trophoblast in this figure). Image Source: "Blastocyst". Licensed under CC BY-SA 3.0 via Wikimedia Commons. |
To summarize:
By the end of the first week, the human zygote undergoes cleavage and passes through morula and blastocyst stages and has begun implantation in the uterine mucosa.
For more details, readers are recommended to go through the following references: