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Introduction to Embryology Clinical age of pregnancy is dated from the last menstrual period, the date when the baby is actually conceived is approximately 2 weeks off of this date, missing the last menstrual period is out of sync with date of fertilisation as ovulation- the egg being released is MID CYCLE

The Prenatal Period 1)Germinal Period- Days 1-14, formation of the germ layers 2)Embryonic- Days 14-56, organogenesis 3)Foetal- Day 56-birth, growth

WEEK 1 Fertilisation to implantation Fertilisation is most likely to occur in the ampulla of the oviduct Oocyte and spermatozoa meet Fertilisation phases 1)Capacitation- giving the sperm capacity to come into the vicinity of the oocyte by changing the membrane over the acrosome- nuclear material needs to get from the sperm into the oocyte, but the oocyte is covered in a plasma membrane- the acrosome. The sperm first comes into contact with the corona radiata cells, and the membrane over the acrosome begins to change 2)Acrosome reaction-sperm burrowing in- sperm moves through outer membrane, the zona pellucida (several sperm are able to start doing this) to get the correct amount of genetic information into the egg only 1 sperm can eventually enter 3)Cell membrane fusion- as soon as cell membrane fusion with 1 sperm has occurred chemical reactions change the nature of the zona pellucida, all other sperm are now blocked Phases of cell division Day 1- gametes are formed by meiosis (50% genetic in each now together to form 100%), mitosis now needs to occur, first cell division= 2 cell ZYGOTE Day 2- 4 Cell ZYGOTE Day 3- 8 cell ZYGOTE Day 4- 12-16 cell stage MORULLA Morula reaches isthmus of the uterine tube, and contains TOTIPOTENT stem cells which are cells that can differentiate into ANY type of cell

After Day 4 cells stop being totipotent and become pluripotent Day 5- Formation of the BLASTOCYST Outer trophoblast cells form the epithelial wall Inner cells, embryoblast, migrate to one end Internal structure CAVITATES, becomes fluid filled and inner cells migrate to one end. The end with the embryoblast cells is the end that is going to migrate into the uterus

Day 6- Blastocyst prepares for implantation Blastocyst sheds the zona pellucida, if this isn't shed then this will not allow the blastocyst out and wont initiate a pregnancy Day 7- IMPLANTATION Blastocyst cells are NON ANTIGENIC but still accepted by the maternal tissues Cells of trophoblast invade the maternal endometrium without being rejected, they invade the walls between glands of spongy endometrium Blastocysts can implant in an abnormal area not in the uterus causing an ECTOPIC PREGNANCY, eg in the mesentry, fimibrae, these tissues are not designed to host a growing foetus, can cause severe bleeding and be fatal WEEK 2THE WEEK OF 2S as a lot of splitting occurs Day 8-9- Trophoblast splits into 2 Cytotrophoblast- lines the developing embryo, in time will become chorionic villi, finger like projections branching out into the placenta that bring the blood o the mother next to the blood of the baby Syncytiotrophoblast- have non antigenic capability, non antigenic invading tissue cells, cell wall barriers are broken down, a multinucleated area with no distinct cell boundaries is formed

Day 8-9- Embryoblast splits into 2 Bilaminar germ disc, 2 cavities are created Epiblast- cells nearest to the direction of implantation Hypoblast- Cells further away from direction of implantation, next to cavity created in blastocyst 2 CavitiesBlastocyst cavity- now called the primary yolk sac Amniotic cavity- developing inside epiblast cells, eventually will surround the baby with amniotic fluid

Day 10-12 Primary yolk sac splits into 2 Inner= secondary yolk sac, lined with hypoblast cells Outer- Extraembryonic mesoderm Starting to cavitate embryoblast away from maternal tissues

Day 13-14 Direction of implantation is now UPWARDS Extraembryonic mesoderm cavitates to form chorionic cavity, the amniotic cavity will eventually grow so much it will obliterate the chorionic cavity. Chorionic cavity is formed by cavitating outside of yolk sac membrane

END OF GERMINAL PERIOD

WEEK 3Moving into the embryonic period- GASTRULATION Bilaminar germ disc becomes trilaminar germ disc Trilaminar disc is formed and split into 3 germ layers Epiblast Hypoblast

Ectoderm Mesoderm Endoderm

Germ cell layers: Ectoderm- form CNS,,PNS, eye and ear, skin, epithelium Mesoderm- form the body connective tissues- blood, bone, muscle, connective tissue of the skin, GI and respiratory tracts Endoderm- Form GI tract organs, epithelium of GI and respiratory tracts, gut tube

The Primitive Streak Cell signalling is already present to give the head end- oropharyngeal membrane and tail endprimitive streak, initally forms at the primitive node in the midline, epiblast cells migrate through the primitive streak to form the 3 germ layers

Mesoderm further divides into 3 Paraxial mesoderm- Somite, Musculoskeletal system Intermediate mesoderm- urogenital systems Lateral mesoderm- visceral, muscular wall of gut , parietal- body wall Problems with gastrulationToxic insults eg drugs- chemotherapy, anti-epileptic medication and alcohol can disrupt the migration of cells and lead to abnormalities, disorders affecting the midline, affecting face and

brain development - foetal alcohol syndrome. Situs inversus can occur during this stage when cell direction is reversed, Dextrocardia is when the heart is flipped WEEK 4Neurulation and Folding How the neural tube forms As well as cells moving sideways to form the germ layers there are cells moving upwards towards the head end of the embryo to form the brain Prenotochordal cells- proliferation of cells creating the neural plate , cells that are going to create and precede the structure called the notochord Notochord cells line up the inside of the body, is a line of symmetry along the longitudinal axis Prenotochordal cells detach from the endoderm to form notochord

Formation of the notochord causes the mesoderm to start expanding- proliferation of the paraxial mesoderm, Rapid proliferation causes the ectoderm to push upwards Neural folds formed, the neural crest cells= the cells close to where the neural folds are coming together. Expansion continues and folds will move closer together come together and form the Neural grooves this then closes to form the Neural tube

Neural tube defects- Due to abnormal neurulation- anencephaly and spina bifida , caused by both genetic and environmental influences, prevented by intake of Folic Acid 0.4mg daily Embryo folding and growth- rapid growth leads to cephalocaudal and lateral folding- forms body cavities and connecting stalk

AMNIOTIC FLUID 1 litre at full term Allows for shock absorption and foetal movements Foetus practises swallowing and breathing movements using the amniotic fluid, very important to practise this to ensure correct lung and kidney development. If babies can't swallow/breathe properly then the kidneys and lungs are unable to form properly. Excess amniotic fluid, polyhydramnios or deficit, oligohydramnios can be indicative of problems with the kidneys and lungs or oesophageal dysgenesis in the baby.

SUPPORT TISSUES IN UTERO Development of placental tissues Trophoblastic lacunae are spaces in the syncytiotrophoblast where maternal blood can come into maternal sinusoids

Foetus needs to get rid of waste products and obtain nutrients from the mother by being in close contact but NOT mix with the maternal blood. Rhesus disease- is caused when the mother has a rhesus negative blood groups and the baby is rhesus positive, mother is treated with immunoglobin anti D injections, the different antibodies in the mothers blood can fight the baby's red blood cells and cause the baby to become severely anaemic Primary stem villi= protusions of the cytotrophoblast

Week 3- secondary stem villi

Week 4- tertiary stem villi- blood vessels develop and cytotrophoblast cells degenerate- blood vessels develop in connecting stalk and chorionic plate (connecting stalk will eventually become the umbilical chord)

Villi increase surface area for gas exchange, nutrients , waste Villi supply nutrients and oxygen when the heart starts beating at week 4...