what do you mean by 'inner cell mass'?

As the morula develops the blastomeres start to form different sections, outer and inner blastomeres. The outer blastomeres differentiate to early trophoblasts and form the outer ring of cells for the blastocyst. Where as the inner blastomeres from the morula stay together as a discrete cluster and form the inner cells mass of the blastocyst. There is also a fluid filled space called the blastocoele.

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Course: Class 12 Biology (India) > Unit 3

Lesson 6: Fertilisation, Pregnancy and Embryonic Development

Fertilization and implantation

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Formation of zygote, development of blastocyst, and implantation.

This article offers a quick refresher on the events and structures involved in human fertilization and implantation.

Key terms

Term	Meaning
fertilization	fusion of male and female gametes (sperm and egg)
zygote	diploid cell formed by fusion of gametes
cleavage	mitotic division of zygote
blastomere	daughter cell produced by cleavage
blastocyst	structure that develops from morula; has inner cell mass and outer trophoblast
implantation	embedding of blastocyst in endometrial lining

What happens during fertilization?

In humans, fertilization occurs when a single sperm and egg cell fuse to form a zygote.

Polyspermy, or multiple sperm cells fertilizing a single egg, will result in a zygote with several sets of chromosomes. Humans are diploid organisms, which means they have only two sets of chromosomes (one from each parent), and so have mechanisms to make sure that polyspermy does not occur.

Some amphibians and birds (for e.g. the zebra finch) allow polyspermy, and can even require hundreds of sperms to successfully fertilize an egg.

The zygote then divides rapidly as it travels to the uterus, where it attaches to the endometrial lining, and leads to pregnancy.

Let's take a brief look at the zygote's journey.

Cleavage and implantation

The image below summarizes the events that occur between fertilization (

A

) and implantation (

H

The table below lists the key features of the labelled events in the image.

$A$ ‍	$B$ ‍	$C$ ‍- $F$ ‍	$G$ ‍	$H$ ‍
Fertilization	Zygote formation	Cleavage	Blastocyst formation	Implantation
Single sperm fuses with ootid	Single, diploid cell	$2$ ‍- $8$ ‍ blastomeres, morula ( $F$ ‍) with $16$ ‍ blastomeres	Trophoblast and inner cell mass differentiated	Uterine cells dividing to cover blastocyst

test your understanding

Identify the blastocyst from the structures below.

test your understanding

The events of fertilization and implantation are depicted below, numbered in order of occurrence.

At which stage has mitotic division of the zygote begun?

test your understanding

Which of the following structures transforms into the blastocyst?

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FireFrank
Posted a year ago. Direct link to FireFrank's post “what do you mean by 'inne...”
what do you mean by 'inner cell mass'?
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(3 votes)
Answer
- Ria
  Posted a year ago. Direct link to Ria's post “As the morula develops th...”
  As the morula develops the blastomeres start to form different sections, outer and inner blastomeres.
  
  The outer blastomeres differentiate to early trophoblasts and form the outer ring of cells for the blastocyst. Where as the inner blastomeres from the morula stay together as a discrete cluster and form the inner cells mass of the blastocyst. There is also a fluid filled space called the blastocoele.
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  (4 votes)
Pragnya Mishra
Posted 10 months ago. Direct link to Pragnya Mishra's post “What is proliferation?”
What is proliferation?
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(1 vote)
Answer
- shreeyavakil
  Posted 7 months ago. Direct link to shreeyavakil's post “increase in the number of...”
  increase in the number of cells due to mitotic division is called proliferation. For eg, the granulosa cells of follicles proliferate. Hope this helps.
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  (1 vote)