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Human development begins at fertilization when an oocyte (ovum) from a female is fertilized by a sperm (spermatozoon) from a male. Development involves many changes that transform a single-cell, the zygote, into a multicellular human being.

Embryology is concerned with the origin and development of a human being from a zygote to birth.

Before We Are Born. Essentials of Embryology and Birth Defects. Keith L. Moore, T.V.N. Persaud, Mark G. Torchia. 9th Edition. Copyright 2016 by Saunders, an imprint of Elsevier, Inc.

Blastocyst

The hollow cellular mass that forms in the first and second weeks after the zygote and morula stages and before implantation in the uterine wall.

Consists of cells forming:

  • an outer trophoblast layer
  • an inner cell mass (embryoblast)
  • a fluid-filled cavity (blastocoele)

The trophoblast will eventually form the placenta. The inner cell mass continues to give rise to the embryo and is the source of true embryonic stem cells.

Hill, M.A. Embryology Glossary of Terms https://embryology.med.unsw.edu.au/embryology

Morula

Latin, morula = mulberry. An early stage in post-fertilization human development when cells divide rapidly, producing a solid mass of cells (12-15 cells) with a “mulberry” appearance.1

  • Morula stage of development occurs during the first week following fertilization (after zygote and before blastocyst stages)
  • For embryos created with IVF, the morula stage is when the earliest prenatal diagnostic tests can be carried out. By removing a single cell and carrying out genetic diagnosis and screening on its DNA, genetic diseases, chromosomal disorders and sex can be identified.

1Hill, M.A. Embryology Glossary of terms. https://embryology.med.unsw.edu.au/embryology

Zygote

Science consistently demonstrates when the life of a new human being begins: at the onset of fertilization (i.e., when the membranes of the egg and sperm fuse).  The “moment of conception” is the second it takes for this fusion to occur, and when the new organism in its unicellular form – the zygote – is created.

A New Cell

The zygote is a new type of cell, as its composition and behavior are distinct from the gametes from which it arose. Immediately following gamete membrane fusion, the newly created zygote’s first task is to modify its composition so as to prevent the penetration of other sperm.

And, a New Organism

The zygote is also a unique, living, complete organism…a member of Homo sapiens in his or her unicellular form. A zygote intelligently and predictably organizes its parts in preparation for the next stage of the known pattern of human development.

We exist in our zygotic form for only a few hours as our job is to divide as soon as possible.

Existing as a one-celled organism is not unique to the human being. Homo sapiens is one of almost a million cataloged species of organisms within the animal kingdom, all of which start life as a single cell. 

Ref: (1) When Does Human Life Begin? Condic. (2) Embryonic Differentiation in Animals. embryo.asu.edu.

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