Embryology

Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage. After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end. An ovum (plural ova, from the Latin word Ovum meaning egg or egg cell) is a haploid female reproductive cell or gamete.

The morula consisting of cells (called blastomeres) in a solid ball contained within the zona pellucida is produced by embryonic cleavage, the rapid division of the zygote. Once the zygote has divided into 32 cells, it begins to resemble amulberry, hence the name morula (Latin, morus: mulberry). Within a few days after fertilization, cells on the outer part of the morula become bound tightly together with the formation of desmosomes and gap junctions, becoming nearly indistinguishable. This process is known as compaction.  The cells of the morula then secrete a viscous liquid, causing a central cavity to be formed, forming a hollow ball of cells known as the blastocyst. The blastocyst's outer cells will become the first embryonic epithelium (the trophectoderm). Some cells, however, will remain trapped in the interior and will become the inner cell mass(ICM), and are pluripotent. In mammals (except monotremes), the ICM will ultimately form the "embryo proper", while the trophectoderm will form the placenta and other extra-embryonic tissues.

The blastocyst is a structure formed in the early development of vertebrates. It is preceded by the morula. It possesses an inner cell mass (ICM), or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta. The trophoblast surrounds the inner cell mass and a fluid-filled blastocyst cavity known as the blastocoele or the blastocystic cavity. The human blastocyst comprises 70-100 cells ( 60 cells) Zona pellucida: 1- prevent adhesion 2- non-stretchable

Trophoblast will differentiate into 2 layers: 1- Inner cellular layer (cytotrophblast) 2- Outer non-cellular layer (syncytiotrophoblast)

Cells of the inner cell mass or embryoblast also differentiate into two layers: (1) a layer of small cuboidal cells adjacent to the blastocyst cavity, known as the hypoblast layer, and (2) a layer of high columnar cells adjacent to the amniotic cavity, the epiblast layer

Together, the layers form a flat disc Together, the layers form a flat disc. At the same time, a small cavity appears within the epiblast. This cavity enlarges to become the amniotic cavity. Epiblast cells adjacent to the cytotrophoblast are called amnioblasts; together with the rest of the epiblast, they line the amniotic cavity

  DAY 0 : - Fertilization occurs at the ampulla DAY 2: - Two-cell stage at 30 hours after fertilization - Four-cell stage at 40 hours after fertilization مش مهم تعرف هالمعلومة , لانو اصلا الدكاترة عنا مباشرة " نطو " اي قفزوا الى يوم 3 بدون هالتفصيلات DAY 3: - Morula DAY 4: - Early blastocyst. DAY 5: - Late blastocyst - Inner cell mass (or embryoblast) = group of cells at the pole towards the uterine lining. - Trophoblast (or outer cell mass) = outer sphere of cells. (Will differentiate into cytotrophoblast and syncytiotrophoblast.) - Blastocyst cavity (or blastocoele) = hollow space within trophoblast. DAY 6: - Blastocyst has implanted itself on the endometrial surface of the uterus ( superficially implant ) and trophoblast cells begin to penetrate the uterine epithelium

DAY 8: - Blastocyst is partially embedded in the endometrial stroma - Amniotic cavity forms in the epiblast. - Inner cell mass has differentiated into two layers which constitute the bilaminar disc: 1) Epiblast layer 2) Hypoblast layer - Trophoblast has differentiated into: 1) Syncytiotrophoblast = outer, multinucleated zone at the juncture between the blastocyst and the endometrial stroma. 2) Cytotrophoblast = inner layer of mononucleated cells. Blastocyst cavity DAY 9: - The blastocyst is almost entirely embedded in the uterine lining, and a closing plug (or fibrin coagulum) fills in the remaining gap. - Trophoblastic lacunae appear. - Enlarged blood vessels appear in the endometrial lining. - The blastocyst cavity is now called the primitive yolk sac (or exocoelomic cavity). Its inner side is lined by the exocoelomic (Heuser’s) membrane. DAY 12: - The primitive yolk sac is now known as the exocoelomic cavity. - Endoderm cells line the inside of the primitive yolk sac in the hemisphere towards the uterus. - Extraembryonic endoderm develops between the trophoblast externally and the amnion and exocoelomic membrane internally. - Large cavities develop in the extraembryonic endoderm and when they become confluent, they form a space called the extraembryonic coelom. - The trophoblastic lacunae enlarge. The enlarging blood vessels become maternal sinusoids, which establish the uteroplacental circulation.  

DAY 13: - The hypoblast produces additional cells that migrate along the inside of the exocoelomic membrane. These cells proliferate and gradually form a new cavity within the exocoelomic cavity. This new cavity is called the secondary yolk sac or definitive yolk sac, which is much smaller than the original exocoelomic cavity or primitive yolk sac   - The extraembryonic coelom expands and forms a large cavity known as the chorionic cavity. - The only place where the extraembryonic mesoderm traverses the chorionic cavity is in the connecting stalk, which will eventually become the umbilical cord. GASTRULATION: formation of the GERM LAYERS ( day 14 ) - By the beginning of the 3rd week (day 14) we no longer speak of a blastocyst; rather, we speak of the formation of the germ layers. - Gastrulation begins with the primitive streak, which begins as an opacity at the caudal end (i.e. what will eventually become the feet) of the epiblast surface It elongates. The appearance of the primitive streak determines the orientation of the embryo. The cells of the embryo are no longer totipotent. The primitive node is the cephalic end (i.e. what will eventually become the head) of the primitive streak. The epiblast now gives rise to the three germ layers : Ectoderm , Mesoderm., Endoderm.

The second week of development is known as the "week of 2's" 1 The second week of development is known as the "week of 2's" 1. Trophoblast differentiates into 2 layers: cytotrophoblast and synytiotrophoblast 2. The embryoblast forms 2 layers; the epiblast and hypoblast 3.The extraembryonic mesoderm splits into 2 layers; the somatic and splanchnic 4. 2 cavities form; the amniotic and yolk sac cavities

Day 11 and 12

Day 13

prepered by: Yousef Elayyan