All About The Carnegie stages..نمو الجنين من عمر يوم الى عمر 52 يوم

مراحل نمو الجنين من عمر يوم الى عمر 52 يوم
Human Embryology.
The Carnegie stages is commonly known as a staging scheme which remains widely used today.
Carnegie stage 1: Zygote.
1 day after fertilization, cell size 120–150 μm in diameter.
At fertilization, the oocyte completes meiosis II, forming the female pronucleus. The spermatozoa
nucleus in the oocyte cytoplasm decompresses, forming the male pronuclei. These two
pronuclei fuse to form the first diploid cell, the zygote. The first mitosis occurs during the 24
h after zygote formation. The term “conceptus” is now used to describe all the cellular products
of the zygote.
Carnegie stage 2: Morula.
1.5–3 days after fertilization, conceptus 0.1–0.2 mm in diameter.
The zygote forms two blastomeres. Mitosis of these blastomeres forms a solid ball of 16 cells,
then 32 cells, still enclosed by the zona pellucida. This cleavage stage divides the large zygote
cytoplasm into sequentially smaller cells. The term “morula” means berry, referring to the
appearance of the solid ball of cells.
Carnegie stage 3: Free blastocyst
4 days after fertilization, conceptus 0.1–0.2 mm in diameter.
Cell division continues after the 32 cell stage occurring more rapidly at the surface and slower
in the center cells. This and directional fluid transfer leads to a cavity, the blastocoel
conceptus. The surface cells form an outer squamous trophoblast layer linked by both tight
and gap junctions. The larger inner cells form the inner cell mass or embryoblast.
Carnegie stage 4: Attaching blastocyst.
5–6 days after fertilization, conceptus 0.1–0.2 mm in diameter.
The blastocyst hatches from the zone pellucida, still floating in uterine secretions of the
secretory phase of the menstrual cycle. The surface trophoblast cells can now initially adhere
to the endometrial epithelium at the site of implantation. The trophoblast cells proliferate and
differentiate into two layers. The outer cells fusing to form syncytiotrophoblasts, the inner
close remain as single cells, cytotrophoblasts.
Carnegie stage 5: Implanted but previllous.
7–12 days after fertilization, conceptus 0.1–0.2 mm in diameter.
This stage was originally divided into three (a, b, and c) substages based on trophoblast
differentiation status before outgrowth (villi) appears. 5a is the initial solid trophoblast cell
layer; 5b, lacunar trophoblast with the appearance of spaces (lacunae) within the trophoblast
layer; 5c, maternal blood-filled lacuna as capillaries and uterine glands are opened into the
trophoblast spaces.
Carnegie stage 6: Chorionic villi and primitive streak.
13 days after fertilization, conceptus 0.2 mm in size.
Trophoblast cells extend into the maternal uterine stroma (decidua) forming chorionic villi.
The extra-embryonic mesoderm arises, lining the conceptus cavity and forming the chorionic
cavity. Three separate cavities or extra-embryonic coeloms form outside the embryonic disc:
the chorionic, amniotic, and yolk sac cavities. Toward the end of this stage, the primitive streak
appears on the embryonic disc; this is the site of gastrulation.
Carnegie stage 7: Notochordal process.
16 days after fertilization, embryonic disc 0.4 mm in length
The embryonic disc establishes axes and has an initial central primitive node (Hensen’s node,
primitive pit) with the primitive streak extending caudally to the disc edge where the connecting
stalk will later form. Gastrulation occurs here forming endoderm and mesoderm that
spread laterally and rostrally from the primitive streak. Above the primitive node, cranially,
the notochordal process develops in the mesodermal layer. The length of this process increases
from 0.03 to about 0.3 mm. The embryonic disc increases in size and the amniotic cavity
enlarges over the yolk sac.
Carnegie stage 8: Primitive pit, neuenteric canal.
18 days after fertilization, embryonic disc 1.0 mm in length
The embryonic disc is pyriform, tapering caudally, and now has cranio-caudal axis, measured
from this stage onward by crown-rump length (CRL). The stage shows three key features.
1-primitive pit,
2- the notochordal canal,
3- the neurenteric canal.
1-The notochordal canal: is
marked by the cavity extending from the primitive pit into the notochordal process. The floor
of the canal is lost to form a transient passage, neurenteric canal, between the amniotic cavity
and the yolk sac. The notochord process will differentiate into the notochord or axial mesoderm.
The remainder of new mesoderm layer has not yet segmented and is called the presomitic
stage.
Carnegie stage 9: 1–3 pairs of somites.
20 days after fertilization, embryo 1.5 mm in crown-rump length (CRL)
The mesoderm either side of the notochord now segments into paired somites. Segmentation
of paraxial mesoderm only occurs at the level of the trunk, not the head, and proceeds in a
cranial-caudal direction. Note that the sequential appearance of somite pairs can also be used