Trophoblastic Cleavage

Cleavage Fertilization takes place in the ampulla of the oviduct Cleavage takes place as the zygote moves down the oviduct towards the uterus The morula enteres the uterus 3 days after fertilization The morula is 12 – 16 cells forming a solid ball of cells and is still surrounded by the zona pellucida As cleavage divisions continue cavities appear in the interior of the morula

Blastocyst formation The cavities become filled with fluid from the uterine cavity to become the blastocyst cavity The embryo is now a blastocyst, consisting of the trophoblast, surrounding the blastocyst cavity with the inner cell mass/embryoblast/ formative cells at the embryonic pole The inner cell mass is basophilic - stains bright when stained with basic dyes. The trophoblast gives rise to the extra‑embryonic membranes, while the embryo proper develops out of the inner cell mass. The zona pellucida (fertilization membrane) is still intact around the blastocyst and the blastocyst lies free in the uterine cavity.

Attachment The zona pellucida starts to degenerate and eventually disappears. This enables the blastocyst to become attached to the endometrial epithelium. The embryo is now referred to as a conceptus. The conceptus orientates itself in such a way that the embryonic pole lies against the endometrium of the uterus. The trophoblast attaches itself to the endometrial epithelium in the following way:

Superficial implantation The trophoblast cells in contact with the endometrial epithelium undergo multiple divisions It differentiates into two layers: inner cellular layer = cytotrophoblast and an outer = syncytotrophoblast. Cell membranes of the outer layer disappear to form a multi‑nucleated protoplasmic mass = a syncytium The syncytotrophoblast forms finger-like processes which grow into the endometrium. The embryo is superficially implanted by the end of the first week.

Attachment and superficial implantation A new layer becomes visible during attachment and superficial implantation A flat layer of cells possibly originating from the trophoblast arranges, through a process of migration, into a thin layer of flat cells lining the inner surface of the inner cell mass to form the hypoblast. The hypoblast therefore forms a ceiling to the blastocyst cavity, separating the inner cell mass from the blastocyst cavity.

Formation of Bilaminar disc The inner cell mass + hypoblast arrange into a double layered disc = the bilaminar or embryonic disc, consisting of a top layer of columnar cells = the epiblast and a second layer of cuboidal cells = hypoblast. The epiblast + hypoblast develops into all three germ layers of the embryo proper. The cells of the hypoblast develop into endoderm or part of it. It is believed that its contribution is mostly extra -embryonic in that it mainly contributes to the endoderm of the extra embryonic membranes.

Interstitial implantation The syncytotrophoblast invades deeper into the endometrial stroma. The cells of the stroma degenerate in the area of the penetrating syncytotrophoblast. The cytotrophoblast continue to divide producing cells that migrate into the increasing mass of the syncytotrophoblast where they soon loose their cell membranes. The formation of the syncyto trophoblast expands around the cytotrophoblast of the conceptus as implantation progresses.

The formation of the amnion Cavities form between the inner cell mass and the trophoblast. These spaces join each other to form the amniotic cavity. The part of the cytotrophoblast overlying the epiblast splits into 2 layers. The inner layer consists of amnioblast cells The amnioblasts becomes the amnion. The cells of the epiblast form the floor of the amniotic cavity and are continuous peripherally with the amnion.

Formation of the exo-coelomic membrane Delamination of the cytotrophoblast surrounding the blastocyst cavity also takes place It is a thin membranous layer called the exocoelomic membrane The exocoelomic membrane around the blastocyst cavity is continuous with the hypoblast The blastocyst cavity is now referred to as the exocoelomic cavity.

The further implantation of the embryo The syncytotrophoblast invades deeper into the endometrial stroma as the cytotrophoblast continue to produce cells that migrate into the syncytotrophoblast where they soon loose their cell membranes. The cells of the endometrial stroma degenerate in the area of the penetrating syncytotrophoblast. The syncytotrophoblast expands around the cytotrophoblast and the conceptus penetrates deeper into the stroma until it is completely embedded. A closing plug (blood clot and cellular debris) remains on the surface for a short while until the regenerating epithelium covers over the conceptus. This type of is called interstitial implantation.

Formation of the primitive uteroplacental circulation Lacunae appear in syncytotrophoblast filled with maternal blood from the ruptured capillaries + secretions from the eroded endometrial glands. = Nutritive fluid is embryotroph and reaches the embryonic disc by diffusion. A lacunar network is established giving the syncytotrophoblast a sponge‑like appearance. The lacunae become connected with maternal blood vessels & a primitive uteroplacental circulation is established. Oxygenated blood reach the lacunae through the endometrial arteries, and the deoxygenated blood is removed from the lacunae by the endometrial veins. The lacunar network is initially formed in the area of the embryonic pole, but increases and spreads throughout the whole endometrium to surround the conceptus completely.

The formation of the extra-embryonic mesoderm   The delamination of cytotrophoblast continues and the extra embryonic mesoderm is laid down between the cytotrophoblast and the exocoelomic membrane or primary yolk sac, and between the cytotrophoblast and the amnion.

The development of the extra-embryonic coelom & the primary yolk sac The extra‑embryonic mesoderm continues to increase Isolated cavities appear in the mesoderm forming the extra embryonic coelom The hypoblast spread out along the inner surface of the exocoelomic membrane, partially surrounding the exocoelomic cavity to form the extra embryonic endoderm. The part surrounded by only the exocoelomic membrane is now termed the primary yolk sac The exocoelomic membrane + the extra-embryonic endoderm resemble the yolk sac of the bird embryo although it is filled with fluid- this part is the secondary yolk sac forming The formation of a yolk sac together with the fact that the embryo develops out of a blastodisc indicate that the ancestors of the mammals had large yolky eggs with meroblastic discoidal cleavage.

Formation of the layers of the extra-embryonic mesoderm As the extra‑embryonic coelom becomes continuous and the extra embryonic mesoderm splits into: - extra-embryonic somatic mesoderm, lining the trophoblast and - extra-embryonic splanchnic mesoderm around the yolk sac and the amnion. The extra embryonic somatic mesoderm and the cytotrophoblast together form the chorion. The chorion envelops the entire embryo with its amnion and yolk sac and is called the chorionic sac. The embryo with amnion and yolk sac is suspended in the chorionic sac. The extra embryonic coelom now becomes the chorionic cavity. The amnion remains attached to the chorionic sac in one area to become the connecting stalk

Formation of the yolk sac The primary embryonic yolk sac decreases in size. The extra-embryonic endoderm lines only a section of the exocoelomic membrane. Together the - exocoelomic membrane, - extra-embryonic splanchnic mesoderm - extra-embryonic endoderm form the wall of the secondary yolk sac The remaining remnant of the primary yolk sac is 'pinched off, leaving only the secondary yolk sac

The formation of the chorionic villi and the prochordal plate Towards the end of the 2nd week the chorionic villi make their appearance. The cytotrophoblast produces finger-like protrusions = primary chorionic villi that push deep into the syncytotrophoblast. Cells of the hypoblast situated opposite to the connecting stalk become columnar. This thickened disc‑shaped area is called the prochordal plate. This plate indicates the head or cranial region of the future embryo as the endoderm of the mouth and pharynx develop from it. The opposite side is the caudal region of the developing foetus.

The formation of the trilaminar disc through gastrulation Gastrulation is the process by which the bilaminar embryonic disc is converted into a TRILAMINAR embryonic disc.

The formation of the primitive streak Some of the cells of the epiblast detach from the rest of the epiblast on caudal side and move toward the midline. These cells converge either side of the midline to form two ridges, The new structure called the primitive streak, consisting of the primitive groove that ends anteriorly in a thickening known as the primitive pit or primitive knot.  As the cells converge progressively towards the primitive streak, the migrating cells get forced down and move in between the epiblast and the hypoblast to form a loose network (a mesenchyme) of cells the mesoblast. Some cells of the mesoblast, once part of the epiblast spread out sideways as well as anteriorly in the direction of the prochordal plate to form the embryonic mesoderm.

The primitive streak extends in an anterior direction as more cells converge into the two ridges The section of the mesoblast tissue that migrates over the anterior part of the primitive streak and the primitive knot into the space between the epi‑ and hypoblast converge on the middorsal line to form the notochordal process. This develops into the notochord while the intra embryonic mesoderm flanking it on either side develops into the somites. Other mesoblastic cells join the hypoblast, displacing the original hypoblastic cells laterally to form a new layer with the cells of the hypoblast. This layer is the embryonic endoderm. The remainder of the epiblast that is left behind on the surface now become the embryonic ectoderm.

The notochordal process becomes longer as more mesoblast cells roll over the primitive pit, and will eventually reach the prochordal plate More of the migrating cells move into the interior between the epi‑ and hypoblast until the convergence of the cells start to decrease. The ridges of the primitive streak therefore become shorter and the primitive streak eventually disappears. The epiblast is therefore the origin of the embryonic ectoderm, the embryonic mesoderm and probably most of the embryonic endoderm. The migration of the mesoblast cells into the primitive streak and in between the epi‑ and hypoblast and the subsequent formation of the notochordal process causes the elongation of the embryonic disc. By the end of the third week it is no longer disc shaped but quite elongated.

Neurulation The embryo is now known as the TRILAMINAR embryonic disc and is triploblastic, which means that it consists of three germ layers. After the formation of the notochordal process the ectoderm overlying the notochord and the mesoderm on either side of it, thickens to form a neural plate and the process of neurulation i.e. the development of the neural tissue now commences, exactly as in Branchiostoma.