Embryo

Embryo: The embryo is a very early stage in the development of a new individual,

but people from different medical and scientific specialties use the word to mean

slightly different stages. 

Although many people would say that an individual is conceived as soon as an ovum 

(egg) is fertilized by a sperm cell, the fertilized egg is called a zygote (ZY-goht),

not an embryo. When a zygote divides into a pair of joined cells, the organism begins

to be called an embryo by workers in the field of in vitro fertilization (also called

"test tube fertilization"), but not by other physicians or most biologists. During in

vitro fertilization or storage before implantation, a bundle of cells that has grown

to a number of cells (32, usually) that would make it a candidate for implantation in

a prospective mother continues to be called an embryo. At this early stage, however, 

the small group of cells is called a blastocyst (the cells are called blastomeres) by 

most biologists. 

When a mammalian blastocyst implants itself in the uterine wall, it is called an embryo

by biologists and all physicians. After implantation in the uterus, the growing cell 

mass continues to be known as an embryo for several weeks. Some writers wait until cells

begin to differentiate into recognizable tissues to use the term embryo. After the shape

of the new individual begins to emerge and organs begin to develop, the term fetus is 

introduced. The developing baby is then known as a fetus until birth. 

Size and location An embryo is barely visible as a speck at first, when it has just lodged

in the uterine wall. There it grows for a month or two to become an inch or two long. At 

that time, when the embryo has become a fetus, the uterus has swollen, making it possible 

to feel a slight bulge through the abdominal wall of the mother-to-be. 

Role Much has been learned in recent years about the roles of specific genes and protein

in early development, although most of the work has been with fruit flies and nematodes 

instead of with humans. The first stage of development occurs in the zygote, when some 

internal separation of cell parts and material occurs before the first division into two 

cells. As a result, the two cells in the first stage of the blastocyst are different from 

each other. These differences cause further cells also to be individualized in their 

structure and operation, leading eventually to the specialized tissues of the fetus. 

Conditions that affect the embryo Because the entire plan of development depends on correct

differentiation of cells in early stages, an error in embryonic development usually leads to

natural abortion, commonly called miscarriage. Miscarriage is common; one estimate is that it

occurs in one out of ten pregnancies. The defective embryo is expelled and pregnancy is

terminated, usually with no harm to the pregnant woman. However, complications can occur if part

of the embryo or the placenta, the organ that develops to connect the embryo to its mother,

remains in the uterus. Save solid materials expelled during a miscarriage for a physician to 

examine. 

Sometimes the embryo fails to implant in the uterine wall but continues to develop in a different

location, most often one of the tubes that connects the ovaries to the uterus. This situation is

called an ectopic pregnancy. No signs of pregnancy appear, but abdominal pain continues hour after

hour. Abdominal pain in a woman of childbearing age requires immediate medical assistance. If the

problem is an ectopic pregnancy, surgery will be required.