BIO102 Unit 11
Human Embryology
Glossary
allantois
extaembryonic membrane sac that will become part of the umbilical cord.
amnion
the extraembryonic membrane that appears as a thin membrane that eventually surrounds the embryo and that fills with a fluid (amniontic fluid) that cushions and provides a living environment for the fetus.
blastocyst
the second stage of embryonic development when the embryo is a hollow sphere that implants in the uterus wall. Usually this stage is present six to eight days after fertilization and is composed of twenty-three cells.
blastula stage
the blastocyst develops into this stage which consists of thirty-two to sixty-four cells and is present ten to twelve days after fertilization.
breech birth
an abnormal birth when the buttocks of the baby present itself instead of its head.
chorion
the extraembryonic membrane which is composed of blood vessel filled projections that will grow into the endometrium of the uterus and will eventually become the fetal portion of the placenta.
cleavage
rapid mitotic cell division that occurs to the zygote during the first month of gestation without adding cytoplasm or cell enlargement. Cleavage sets the stage for the development of all the body’s tissues and structures by providing stem cells to direct fetus morphology and differentiation.
differentiation
Occurs during embryonic development when cells become specialized in both structure and function; a muscle looks and acts differently than a nerve cell.
ductus ateriosus
a fetal by-pass that connects the pulmonary trunk to the aorta temporarily by passing the lungs.
ductus venous
a fetal vein that connects the umbilical vein directly to the inferior vena cava which allows the fetal circulatory system to by pass the liver.
ectoderm
the outermost embryonic germ layer which will form parts of the nervous system, the skin, and linings of the mouth and rectum.
endoderm
the innermost embryonic germ layer that becomes the respiratory tract and the lining of the digestive and urinary systems.
embryo
the developing human which is present from the time of implantation in the uterus until about the eighth week of gestation went it is usually called a fetus.
fertilization
the union of one sperm and one egg resulting in a cell called a zygote. This union usually occurs in the fallopian tubes of a female.
fetus
the term applied to the embryo after the eighth week when the baby takes on the form of a human being.
foramen ovale
a fetal “oval hole” in the septum between the right and left atria which allows the fetuses blood to by pass the lungs.
fraternal twins
these twins are the result of two different fertilizations that usually occurred during the same menstrual cycle of the female. These embryos develop independent of each other with their own placentas, own genetic makeup and can be different sexes.
gastrula stage
this stage is the result of the invagination of the blastula stage. At this stage the three embryonic germ layers are formed. (ectoderm, mesoderm & endoderm).
gestation
the forty week period during which the fetus/embryo is protected and nourished in the uterus of the mother, also commonly known as pregnancy.
homologous chromosomes
the normal 23 pairs of chromosomes found in most cells of the human body. A cell having this normal chromosome number contains a diploid number of chromosomes.
identical twins
results when the embryo divides into two separate embryos during the early cleavage stages. These embryos are the result of only fertilization and the subsequent babies will have shared the same placenta, have the same genes and will always be the same sex.
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mesoderm
the middle embryonic germ layer the will develop into the skeleton, muscles, cardiovascular system and the reproductive system.
morphogenesis
occurs during embryonic development as the embryo/fetus starts to take form into different body parts; the emergence of shape in tissues, organs or the entire embryo during development.
morula stage
early embryonic stage that occurs three days after fertilization when the embryo is a solid sphere composed of sixteen cells.
ovum
the female haploid sec cell commonly called the egg produced in the ovaries.
placenta
the seven inch in diameter structure that develops from the chorion of the fetus and the endometrium of the mothers uterus that allows for the exchange of oxygen and nutrients from the mother to the fetus and carbon dioxide and waste products from the fetus to the mother.
sperm
the male haploid sex cell produced in the testes
umbilical cord
a structure that developed from the yolk sac and the allantois from the fetus that stretches between the fetus and the placenta. The “lifeline” of the fetus containing two umbilical arteries and one vein.
yolk sac
an extraembryonic membranous sac filled with the first blood cells that becomes part of the umbilical cord.
zygote
a fertilized egg which resulted from the union of one sperm and one egg.