Embryology

Sumamry This lesson will cover , implantation, and . The first 4 weeks of .

Pregnancy is split into three trimesters. Semesters one and two contribute to the embryonic period and semester three becomes the fetal period. The first of these trimesters is a critical time period, because this is when all the main organ systems begin to develop. The second trimester is when we start to look more human and our organ systems are near complete ready to move into the third trimester, which shows rapid fetal growth and organ function. This process takes between 38-40 weeks, which is when birth occurs.

Week 1:

Key Words:

Fertilisation EmbryoblastsReviseDental.com Implantation

Cell actions: Proliferate, Migrate and Differentiate.

Fertilisation:

The optimum time for fertilisation to occur is between 12-24hrs after ovulation (when the secondary oocyst leaves the ovary). However, due to the ability of sperm being able to remain viable for 48hrs, there is a 3 day window around the time of ovulation for fertilisation to take place.

Sperm and ova are haploid cells. This means they have half the amount of chromosomes of a human cell. Therefore, on fusion (syngamy) a diploid cell is created, now known as a zygote. The Zygote now has the correct number of chromosomes: 46.

Diagram schematicReviseDental.com of a sperm and ova The sperm cells are specially equipped to enter the ova, having acrosome enzymes to penetrate the cell wall and a powerful flagella (tail) for motility. On entrance, to prevent polyspermy (multiple sperm entering), the ova cell wall depolarises alongside deactivation of cell receptor, making the ova's zona pellucida impenetrable. Cleavage:

Mitotic cell division occurs around day 2-3 forming a morula (16 cells). When the cell population reaches 32, the mass is now known as a blastula. The cells are still within the zona pellucida and therefore show no increase in size, but cell density has increased (compaction), these cells are known as .ReviseDental.com

Around days 4-5 an accumulation of fluid (blastocoel) occurs within the morula via , alongside the zona pellucida degradation, and the blastocyst is created. The blastocoel fluid is derived from the and provides nourishment for the cells.

The blastocyst now shows two clear cell populations. The are the embryoblasts (what becomes us). The outer cell mass are the , which play an important role during implantation. Implantation:

Implantation occurs around day 6 and firm attachment to the endometrium is seen by the end of the first week of theReviseDental.com embryonic period. As the blastocyst burrows, it becomes fully surrounded by the endometrium and an increase in vascularity occurs. This area of the endometrium is called the and provides glycogen and lipid to the developing embryonic cells and later contributes to formation of the .

Week 2:

Key Words: Trophoblasts: and = Bilaminar disc: and Sinusoids

Trophoblasts:

Around day 8 the trophoblasts have differentiated into two distinct layers with distinct functions. The syncytiotrophoblasts go on to assist with the embedding of the blastocyst by secreting enzymes as well as secreting human chorionic gonadotrophin (hCG), which prevents menstruation (note: pregnancy test). Syncytiotrophoblasts have no distinct cell boundaries and also contribute to the surrounding vascular supply. The cyctotrophoblasts are clearly demarcated cells and become the out lining cells of the developing .

Bilaminar Disc:

Day 8 also shows the Embryoblast cells differentiate to form two cell layers; the hypoblasts and the epiblasts. The hypoblasts migrate and surround the original fluid filled cavity of the blastocyst, now known as the yolk sac. The yolk sac is the nutrient supply until the development of the placenta takes over, and has many other functions e.g. contains primitive stem cells which will contribute to the gut and gonads.

The layer (which will become us), also cavitates, to form a small amniotic cavity. The amniotic cavity is lined by amnion cells at its roof and the epiblasts on its floor. The amnion will eventually surround the embryo completely, and will become the large amniotic cavity as we know which contains the amniotic fluid. The amniotic fluid has many roles and is initially filled with constituents of maternal blood. It acts as a shock absorber, helps regulate body temperature, keeps the developing cells moist (preventing adhesion) and later in the pregnancy acts as a waste collector (e.g. fetal urine).ReviseDental.com Sinusoids:

As mentioned above the syncytiotrophoblasts contribute to the vascular supply to the developing cells. Around days 9-12, they do this by creating lacunae spaces, which fuse to form the lacunar networks. Endometrial capillaries dilate around these networks and form sinousoids. Nutrient and waste exchange occurs at these boundaries. Chorion:

Cells from the yolk sac migrate and form the extraembyronic . This connective tissue will eventually surround the whole embryo and cavitate to form the extraembyronic .

The chorion consists of the trophoblastic layers and the extraembyonic mesoderm. This network of cells provides nutrient supply, protection from the mother's immune cells, and secretes hCG. It fuses with the amnion and the developing coelom (mentioned above) now becomes the chorionic cavity. These structures will become the embryonic part of the future placenta.

Week two is demarcated by a clear bilaminar disc consisting of hypoblasts and epiblasts. The development of the has also occurred between the bilaminar disc and trophoblasts, which will go on to becoming the future .

ReviseDental.com ReviseDental.com Week 3:

Key words:

Gastrulation and node Trilaminar disc Mesoderm Epithelial and mesenchyme Angiogenesis Notochord Neurulation Neural plate/ groove/ fold/ tube Crest cells

General note: The development of 3 primary germ layers and the primitive nervous system. Key cell activities involve: differentiation, migration and rearrangement.

Gastrulation:

Around day 15, the epiblasts from the bilaminar disc start to migrate. This early migration is seen as the primitive streak, as they invaginate and migrate inwards to displace the layer. The epiblasts develop into the three primary germ layers: the ectoderm, mesoderm and endoderm. The disc is now a trilaminar disc. At the head end of the primitive streak, a depression is seen, known as the . The ectoderm and endoderm are epithelial cell layers, and the mesoderm is a loose connective tissue layer known as mesenchyme. ReviseDental.com

Below is a table of the key systems each will eventually develop into: Notochordal process:

At day 16 the mesoderm layer begins to develop a crucial structure called the notochord, which completes around the end of the third week. The notochord is a key influencer by inducing other cells to become active and specialised.

At the same timeReviseDental.com as the notochord develops, an oropharyngeal membrane at the head end also begins to develop, as well as the cloacal membrane at the tail end. These areas are made up of ectoderm and endoderm only. At this stage the yolk sac creates an out pouch known as the , a structure that will contribute to early vessels and the urinary system.

Somites:

During week three, the mesoderm which lies adjacent to the notochord develops into three sections. The forms adjacent to the notochord, the forms next to this and furthest away from the notochord forms the lateral plates. The paraxial mesoderm further sections into regions: myotome (muscle), dermatome (connective tissue and dermis), sclerotome (vertebrae and ribs). The intermediate mesoderm contributes to the kidneys and gonads. The lateral plates divides into two sections; splanchnic and somatic, due to the development of the . The splanchnic region which develops into the heart/ circulation (heart beat around 21days) and the somatic region which forms the bones, ligaments and many other structures.

Angiogenesis and the placenta:

Vessels develop first in the chorion, yolk sac and connecting stalk (remember the chorion is the trophoblasts and the extra embryonic mesoderm that surrounds the embryo, amniotic cavity and yolk sac). Mesoderm cells differentiate in angioblasts which group to form blood islands. Lumens begin to develop in the islands by the angioblasts rearranging to form the endothelial cells of the vessel walls, and eventually the tunic layers. Blood cells and plasma develop first in the chorion and yolk sac vessels

As mentioned in week one, during implantation lacunae were formed, which fill with maternal blood. Projections now start to emerge from the chorion, along with its new vessel network. These projects are known as the which become surrounded by the lacunae and are bathed in the mother's blood. Nutrient and waste transfer occurs here and is transported back to the embryo along the connecting stalk (umbilical cord) and the new vascular network (consisting of two main arteries and one main vein).

The placenta consists of tissue contributed by two separate individuals; the mother and future child. Its consists of the chorionic villi and the decidua (mentioned in week 1), The full structure isn't fully developed till around week 12 but its begin starts here in week 3 as explained in the paragraphs above. The placenta provides a barrier between the mother and child as well as contains, carbohydrate, calcium and iron aiding fetal development. It also plays a role in hormone secretions. However, it is not impenetrable and the foetus can be susceptible to viruses such as measles, medications (teratogens) e.g. thalidomide, and alcohol, all of which can damage and interrupt normal development. ReviseDental.com Neurulation:

The process of neurulation does continue into week four, but during week three of development, the notochord induces the overlying ectoderm to form the neural plate. These cells fold creating a groove, which will eventually fuse at its ends creating the neural tube; the early central nervous system (CNS). At the ends of the neural plate cells had formed. When the neural tube has developed, these cells migrate laterally and will contribute to the peripheral nervous system, melanocytes, adrenal medulla and importantly the majority of the skeletal and connective tissue of the head and neck (among other systems). ReviseDental.com ReviseDental.com

As we move into week 4, the primary brain vesicles become more prominent, developing into the prosencephalon, mesencephalon and the rhombencephalon. Week 4:

Key words:

Organogenesis Pharyngeal arches

General note: during weeks 4-8, rapid development and differentiation of the major organs, known as occurs.

The developing embryo undergoes numerous folds and transforms from the disc inside a sphere into the classic C-shaped embryo, with a clear head and tail end. During this period of the embryonic life cycle, rapid growth occurs and the embryo can triple is size. It is the difference between growth rates that moulds the embryo into its C-shape.

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During this time period, the stomodeum at the head end is formed (primitive mouth), as well as the primitive gut which are initially separated by the buccopharyngeal membrane. At the other end, the hindgut expands into the cloaca cavity, and eventually the annus will be formed. Around day 22, at the head end, the somites and neural crest cells have migrated to form the five pairs of pharyngeal arches (numbered 1,2,3,4,6) Each arch consists of:

Ectoderm (outer layer) Mesoderm (middle layer) Endoderm (inner layer) Artery Cranial Nerve Skeletal cartilage (support network) Skeletal muscle Separated by a pouch (inner) and a cleft (outer) - note: no mesoderm in these areas.

Each arch will develop into specific structures of the head and neck, which we will go through in more detail in the next lesson (Pharyngeal arches). In addition to the arches at day 22, the otic placodes (primitive ear) and the lens placodes (primitive eye) can be seen. Towards the end of the 4th week the 5 facial prominences have developed (frontal, mandible x2 and maxillary x2).

At week 4 the embryo has also "sprouted" outgrowths of mesoderm with an ectoderm covering (future skin). These are the first signs of the limbs being developed. At the end of week four there is a distinct cardiac prominence and tail as well.

Over the course of the next four weeks, large amounts of growth and development will occur, for example, chambers of the heart are formed, brain development is significant, as well as clear limb and facial formation. The embryo now looks more human and enters into the fetal period at week 9. The fetal stage, is mainly (but not all) about growth; therefore, the fetus becomes less vulnerable to teratogenic substances but not invincible.

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Conclusion With the fundamentals covered of how the germ layers develop, we are now able to focus on the head and neck development, starting with the pharyngeal arches. Third Party Links

References Berkovitz BK, Holland GR, Moxham BJ. Oral Anatomy, Histology and Embryology E-Book. Elsevier Health Sciences; 2017 Jul 11. Nanci A. Ten Cate's Oral Histology-e-book: development, structure, and function. Elsevier Health Sciences; 2017 Aug 15. Tortora GJ, Derrickson BH. Principles of anatomy and physiology. John Wiley & Sons; 2018 May 15. Marieb EN, Keller SM. Essentials of Human Anatomy and Physiology: Books a la Carte Edition. Benjamin-Cummings; 2011. Norton NS. Netter's head and neck anatomy for dentistry e-book. Elsevier Health Sciences; 2016 Sep 13. Logan BM, Reynolds P, Rice S, Hutchings RT. McMinn's Color Atlas of Head and Neck Anatomy E-Book. Elsevier Health Sciences; 2016 Oct 21. Atkinson ME. Anatomy for dental students. Oxford University Press; 2013 Mar 14.

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