Explain the process of fertilization and implantation
Identify main steps in blastocyst formation
I've actually written about all of this before in old posts about reproduction and pregnancy, but I didn't have as much of an understanding back then and thus my writing wasn't really that clear so I'm going to attempt again. Can't guarantee that my writing will be any clearer this time, though, but at least the concepts should be clearer in my own head so at least one person should benefit from this. Whoop-de-doop.
Anyway. Fertilisation. I'm sure you all know the general process that has to take place for sperm to get anywhere near an egg. (If you don't, then I guess that you'll have to redo year 8 sex ed.) At this stage the egg is surrounded by a couple of layers: a hard shell called the zona pellucida, and a layer of cells called the corona radiata. Part of the reason why millions of sperm need to be ejaculated is because no single sperm has enough enzymes to break through all of the layers. Firstly, the sperm release hyaluronidase which breaks down the hyaluronic acid holding together the cells of the corona radiata. Sperm then latch onto ZP2 glycoproteins in the zona pellucida and break through using zona-digestive enzymes located in the acrosome of the sperm (it's like a modified lysosome). After this is done, the first sperm to get through fuses with the membrane of the oocyte. A couple of processes then occur to stop more sperm from binding, otherwise you would end up with shitloads of chromosomes and that would be bad.
Oh and don't worry, I don't think you need to remember all of the details of how the sperm breaks through those layers. I just added them in for funsies. (Yes, I know, I have a weird idea of fun. Shut up.)
After the sperm fuses with the oocyte, a few more things happen- the female and male pronuclei (i.e. the nuclei of the egg and sperm, respectively) break down to release the chromosomes and then mitosis begins. The mitosis that happens at this stage is known as "cleavage," and involves many divisions but no cell growth in between. The cells are known as "blastomeres" and by 4 days there are around 32 of them that form a ball. This ball is known as a "morula."
Cells at the morula stage could theoretically keep dividing, but then they'd be so small it'd be absurd. Besides, you and I clearly aren't just tiny balls of cells. Something had to have happened to make us bigger.
That something is a process known as "hatching." Basically, the embryo "hatches" out of the zona pellucida. It happens around the time the morula enters the uterus, and to my understanding it's partly due to fluid seeping through the zona pellucida and into the intercellular spaces.
This fluid that seeps through forms a fluid-filled space known as the blastocystic cavity, or blastocoele. Of the remaining cells, some form a thin layer around the outside- these cells are known as trophoblasts and eventually form the placenta. The rest of the cells form a big mass which is why they're known as the "inner cell mass." The inner cell mass later becomes the embryo.
At around the end of the first week, implantation starts to occur. Firstly, the blastocyst fuses with the uterine wall. The trophoblasts then begin proliferating and differentiating (you'll find that "proliferating" and "differentiating" are pretty much the buzzwords of embryology). They differentiate into two layers: the cytotrophoblast which is pretty much just one layer of trophoblastic cells, and the syncytiotrophoblast which surrounds the cytotrophoblast and is made up of many cells fused together into one big one with several nuclei. The syncytiotrophoblast is the one that does the actual burrowing, while the cytotrophoblast maintains these cells. Blood vessels and so forth eventually start to form near the syncytiotrophoblast in order to allow for exchange of nutrients and waste products between the mother and the baby.
Outline the process of inner cell mass differentiation
The inner cell mass initially begins to differentiate into just two layers: the epiblast (upper layer) and the hypoblast (lower layer). Cavities begin to form within these layers. The cavity that forms within the epiblast is called the amnion, or amniotic cavity, whereas the cavity that forms within the hypoblast is known as the yolk sac. The area where the two layers come into contact is the "bilaminar embryonic disc."
A new layer of cells, the extraembryonic mesoderm, soon begin to form and line the rest of the cavity. Coelom (spaces) form between the cells of the extraembryonic mesoderm. These coelomic spaces then join up to form a large cavity called the chorionic cavity.
Thus, by the end of the week, you have a definite two-layer (bilaminar) embryo. There are a few more things that happen- like a secondary yolk sac forms and displaces the primary one (which eventually degenerates) and part of the hypoblast thickens to form the prechordal plate, which eventually becomes the cranial region of the embryo. (Cranial = head end, caudal = tail end.) Main point is, though, is that when the embryo is two weeks old, it's a two-layered disc.
Describe the process of differentiation of the basic germ layers
In week 2, the embryo has two layers; in week 3, the embryo will have three. (Nope, it doesn't continue beyond there: a 4-week old embryo does not, in fact, have four layers.) The process of the third layer forming is known as gastrulation.
Gastrulation begins with the formation of the primitive streak. The primitive streak is a thickening of the epiblast along the midline of the embryonic disc. It consists of a "primitive groove" with a "primitive node" at the end. The primitive node contains a depression known as the "primitive pit." It is this primitive streak that defines the cranial and caudal sides of the embryo.
For gastrulation to occur, epiblast cells near the primitive streak must first proliferate and differentiate (yes, it's those buzzwords again). These cells then migrate through the primitive streak in a process known as ingression. At first, the epiblast cells push away the hypoblast cells, replacing them with new endodermal cells to form the endoderm (endo = inside). Once the endoderm is done, the next lot of cells to migrate through become the mesoderm (meso = middle). That leaves the epiblast to become ectoderm (to my understanding it gets called "ectoderm" once the mesoderm is established).
Also of great importance during this time is the formation of the notochord. The notochord is a hollow tube that is responsible for signalling in the early embryo, and is thus important for signalling cells to proliferate and differentiate. The notochord extends cranially from the primitive node upwards to the prechordal plate (which, as I mentioned in the previous section, is a thickening of the hypoblast at the cranial end).
Formation of the notochord starts by prenotochordal cells migrating through the primitive streak and fusing with the endoderm to form the notochordal plate. Eventually this fused layer degenerates to form the definitive notochord.
Yet another important event that happens towards the end of the third week is the further development of the mesoderm. The mesoderm has three main sections. Paraxial mesoderm (i.e. "along the axis") lies next to the notochord. It is further differentiated into three types of clumps, called somites. Sclerotomes are responsible for forming the skeleton, myotomes are responsible for forming voluntary muscle and dermatomes are responsible for forming the dermis of the skin. Lateral to the paraxial mesoderm is the intermediate mesoderm, which is responsible for the formation of the kidneys, adrenal cortex and genitals.
The third and final section of the mesoderm is the lateral plate mesoderm. During development, spaces begin to form within the lateral plate mesoderm, effectively splitting it into two layers with an intraembryonic coelom in between (contrast with the extraembryonic coelom, which is a space that lies outside the embryo). One layer associates with the ectoderm and is known as the somatopleure. The other layer associates with the endoderm and is known as the visceropleure.
A final important development at this time is that the cardiovascular system begins to form. Formation of the cardiovascular system starts off with blood vessels appearing in the yolk sac, and then in the embryo itself. A tubular heart also forms at this time.
During week four of development, no new layers are created, but the layers that are present begin to grow. As they grow, they begin to fold on themselves so that the embryo finally looks less like a disc and more like an embryo. Two types of folding occur simultaneously. One is craniocaudal (i.e. "head to tail") folding, caused by rapid growth of the ectoderm, while the other type is lateral folding, caused by rapid growth of the mesoderm. Together, they result in a "tube within a tube" body plan, where there is an outer tube that represents the general outline of the embryo, and an inner tube that is the gut of the embryo.
Explain processes associated with the establishment of the neural tube
The process in which the neural tube develops is known as neurulation. In neurulation, ectoderm cranial to the primitive node begins to thicken to form the neural plate. The edges of the neural plate begin to fold up to create neural folds, while the bit in between becomes the neural groove. The neural folds continue to rise until they meet each other, forming the neural tube in the middle. After the neural folds meet and fuse, some of the cells break off to form neural crest cells- these cells become the nerves of the peripheral nervous system, among other things, as highlighted in the list below.
The meeting of the neural folds begins in the neck, and then proceeds to "zip up" in both directions (cranial and caudal). The cranial end that isn't zipped up yet is called the cranial neuropore, while the caudal end that isn't zipped up yet is the caudal neuropore. The cranial end closes before the caudal end. Failure to close can result in a serious condition called spina bifida. The risk of spina bifida can be reduced by taking folic acid during pregnancy.
Identify the major primordia associated with the germ layers
List time!
Ectoderm
- Surface ectoderm- epidermis of the skin and epidermal appendages (i.e. hair, nails, etc.), anterior pituitary gland, roof of oral cavity, enamel
- Neural tube ectoderm- central nervous system
- Neural crest ectoderm- sensory epithelia of the eyes, ears and nose, peripheral nervous system, adrenal medulla
Mesoderm
- Paraxial mesoderm (somites)- voluntary muscle (from myotome), axial skeleton (sclerotome), dermis of skin (dermatome)
- Intermediate mesoderm- gonads, kidneys, adrenal cortex
- Lateral plate mesoderm- contributes to the serous membranes lining body cavities (pericardial, pleural, peritoneal)
Endoderm
- Epithelial lining of gastrointestinal and respiratory tracts, urinary bladder and urethra etc.
- Thyroid and parathyroid glands
- Thymus, liver and pancreas
Whew! That was long...
These are really helpful summaries, thanks for writing them up!
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