An organ is a specialized section of the body composed of several different types of
tissues. Organogenesis is the formation of the different organs from the three germ
layers: ectoderm, endoderm, and the mesoderm. The three kinds of morphogenetic
changes (folds, splits, and dense clustering of cells) are usually the first evidence of
organ formation. Early organ formation in vertebrates includes the formation of the
notochord by condensation of dorsal mesoderm, development of the neural tube from
folding of the ectodermal neural plate, and formation of the coelom from splitting of the
lateral mesoderm. The three germ layers will eventually differentiate into various
organs. The endoderm cells specialize into the lining of the digestive system, the lining
of the respiratory system, the liver, the pancreas, thyroid, parathyroids, thymus, lining of
the urethra, urinary bladder, and reproductive system. The ectoderm cells makes the
skin and its derivatives, the lining of the mouth and the rectum, the sensory receptors in
the epidermis, cornea and the lens (parts of the eye), the nervous system, adrenal
medulla, tooth enamel, the pineal and pituitary glands. The mesoderm cells make the
notochord, skeletal system, muscular system, circulatory and lymphatic systems, the
excretory systems, parts of the reproductive system (except the actual germ layers),
dermis of the skin, the lining of the body cavity, and the adrenal cortex. Vertebrate
ectoderms have three parts: the external ectoderm, the neural crest, and the neural
tube (the second and third is eventually known as neuroectoderm). The external
ectoderm makes most of the stuff that originates from the ectoderm (ex: skin, mouth,
nasal cavity, tooth enamel, pituitary glands, parts of the eyes, and other stuff). The
neuroectoderm makes the lining of the nervous system and the brain. The endoderm
and the meosderm do not split into various parts until they begin to form the individual
organs. (10, 13)
Some Highlights of Organ Formation During Embryonic Development (9)
• At six weeks the embryo begins to form the pulmonary primordium which is the first
trait of a lung. The hepatic plate is also formed which is the beginning of the liver
• At week eight the actual lungs begin to develop and the lymphatic system also begins
to develop
• By week nine most of the vital organs have begun to form and will continue to develop
• Between thirteen and sixteen weeks the fetus' pancreas and liver produce fluid
secretions
• By week twenty the fetal heartbeat can be heard by a stethoscope
• By week 23 the alveoli have formed and the by week 27 gas exchange is possible
even if the respiratory system is still developing
Close up: Heart Formation
The rudimentary heart in vertebrates develops from the vental edges of the mesodermal
mantle in the anterior part of the body. Some mesodermal cells break away from the
ventral edge of the lateral plate. They position themselves under the endoderm and
then arrange in the form of a thin walled tube, which will become the endocardium, the
lining of the heart. The mesoderm actually breaks into two parts called the myocardium
and the endocardium. These cells make a heart tube that is formed along the
anterior/posterior axis for the chambers of the heart. The heart tube folds to make five
dilations of the primitive heart which will eventually develop into the adult heart. To
make these five folds, the heart goes through looping, the folding of the tube from left
to right. Until the heart is done looping, the different parts of the heart cannot be
differentiated. Then a primitive atrium and bentricle form along with the pulmonary and
systemic veins and arteries. The heart continues to form as the fetus develops more.
(11)
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2861293/?tool=pubmed
http://www.britannica.com/EBchecked/topic/25677/animal-development/63742/Organformation
Book:
Campbell, Neil A., and Jane B. Reece. Biology. Sixth ed. Boston, MA:
Pearson Custon/Benjamin Cummings, 2002. 637, 1005. Print.
Answers to Multiple Choice Questions
Sruti Parvataneni
Cori Plesko
Madhu Prakash
1. D
2. B
3. A
4. C
5. A
6. E
7. B
8. D
9. A
10. D
Question: explain the organ formation of the heart. Start with implantation and go through
gastrulation and then talk about the organ formation
Answer: The heart does not get specialized till after implantation and towards the end of
gastrulation.
There are three stages of implantation: adplantation/apposition, adhesion, and invasion. Once it
is oriented correctly towards the endometrium (apposition) the blastocyst breaks out of its
protective covering (Zona Pellucida). The Zona Pellucida exists before this stage to prevent
premature implantation. In this time, the endometrium changes shape, and absorbs uterine fluid,
creating a sort of vacuum to bring the blastocyst closer to its epithelial layer.
Because the blastocyst is still not firmly embedded in the uterus, it can still be "flushed out"and
eliminated.
Then, the inner cell mass adheres to the uterine wall (adhesion), with the help of an exchange of
hormones and structures on the blastocyst, in a period of days called the "implantation window",
about 6 days after the peak of the hormone LH in the body. This is usually the 20-23rd day of the
menstrual cycle. In adhesion, the microvilli of the outermost trophoblast of the blastocyst interact
with the glycoproteins on the epithelial layer of the endometrium to firmly attach the blastocyst.
To prevent miscarriage, the endometrium becomes thicker and the cervix is sealed by mucus. At
this stage, the blastocyst can no longer be flushed out of the body system. Soon after adhesion,
the trophoblast differentiates into two categories of cells: ST (syncytiotrophoblast) cells and CT
(cytotrophoblast) cells. Finally, the blastocyst penetrates the epithelial layer and invades the
stroma (invasion).
Gastrulation is dramatic rearrangement of the cells of the blastula. And this cell movements
result in a massive reorganization of the embryo from a simple spherical ball of cells, the
blastula, into a multi-layered organism. Gastrulation is when layers of embryonic tissues
develops into adult body parts. The resulting development stage is called gastrula. Invagination
is when a sheet of cells bends inward. This causes the three germ layers; endoderm, ectoderm,
and mesoderm. The mesoderm is what creates the heart. Within three weeks, the blastocyst cells
begin to grow as clumps of cells within that little ball, and the baby's first nerve cells have
already formed.