embryology

Embryology

Oral embryology: is the study of the development of the oral cavity, and the structures within and arround it, during the formation and development of the embryo in the first 8 weeks of pregnancy.
After this point, the unborn child is referred to as a fetus.
The correct biological term for this time period before birth is prenatal.
Histology: is the study of the specialized biological area concerned with the microscopic structure and function of tissues.
Period of prenatal development:
1: proliferated period: started from the day of fertilization to the end of two week include fertilization, implantation and formation of embryonic disk.
2: embryonic period: during this period the all tissues develop and organize to form organ systems that completed in the end of eighth week.

3: fetal period: this period extend until birth. The tissues that developed during embryonic stage enlarged differentiate and become capable of function.

Proliferated Periods:

Zygote: it is the fertilized female ovum by the male sperm. These two cells are known as germ cells. The fertilization occurs at the distal region of uterine (fallopian) tube
morula: (blastomeres) the continuous division of the zygote until reach 16 cells during 3-4 day now known as morula.
Blastocyst: the continuous growth of morula in 4-5 day formed blastocyst that contain hollow in the center and lateral cell mass and it pass to the uterine to start implantation.
Implantation: the blastocyst attach to the dorsal surface of uterus that lined by sticky endometrium membrane. The blastocyst digests this membrane and allows deep implantation to form future placenta. In this stage it known as trophoplaste.

Embryonic disk: two small cavities (sac) develop on rather side of the inner cell mass. When these two cavities come in contact, the embryonic disk develop between them that composed of two cells layers that responsible for formation of embryo.
Amniotic cavity (sac): small cavity lined by ectodermal cells that form ventral surface of embryonic disk.
Yolk cavity (sac): this cavity larger than amniotic sac lined by endodermal layer that form dorsal surface of embryonic disk.


The embryonic disk at 7th day composed of ectodermal and endodermal layers.
In the second and week there are three events occur synchronously:
1: neural plate, neural crest and neural tube formation from ectoderm.
2: development of mesoderm between two layers of embryonic disk
3: endodermal tube formation that responsible for gastrointestinal tract formation.
1: neural plate, neural folds, neural crest and neural tube:
In the second week the embryos body form a sheet of cell. The ectodermal (external) layer, known as neural plate, this plat show elevation on bilateral edge of neural plat to form neural fold. The apex region of neural folds composed of specialized cell known as neural crest cells. These folds continue on growth to union on midline to form neural tube.

Function of neural tube:

1: anterior region form primary brain vesicles lead to formation of forebrain, midbrain and hindbrain. The other part of brain develops from these three structures like frontal, temporal and occipital loop derived from forebrain.
2: the distal portion of neural tube form spinal cord.
3: formation of cranial nerves: the neural tubes lined by neuroepithelium cells that differentiated to neuroblasts cells which are developed to neuron that considered as primitive of cranial nerves.
4: formation of connective tissues: the ventral surface of somite (newly developed embryo) differentiated into sclerotome. The seclerotome differentiated to mesenchymal cell which become osteoblast, chondroblast , and fibroblast and large number of embryonic cells (stem cells).

Mesoderm development: the ventral surface of somite (newly developed embryo) differentiated into sclerotome but the dorsal part differentiated into dermatomes.
Other division developed between them known as intermediate mesoderm or myotome.
Nutrition of embryo: in first three week the nutrition come from yolk sac until developed the cardiovascular system.
Cardiovascular system: developed in the third week from cluster of cells in yolk sac known as angioblasts that form elongated tube to develop into blood vessels and this process continue to form many blood vessels that connect to the placenta to form umbilical system responsible for embryonic respiration and nutrition. The union of head region of bilateral side of tube arises from angioblast cells form two champers of the heart. The hart start to beet at fourth week.

Know the embryo start to organization by producing may processes and folding.