Introduction to Embryology 1: Fertilisation to Week 4 PDF

Introduction to Embryology 1: Fertilisation to Week 4 Learning Objectives Describe the stages of development from fertilisation to week 4 Understand how a blastocyst differentiates to an embryoblast and trophoblast...

Introduction to Embryology 1: Fertilisation to Week 4 Learning Objectives Describe the stages of development from fertilisation to week 4 Understand how a blastocyst differentiates to an embryoblast and trophoblast Understand how an embryoblast develops into a bilaminar disc Understand what happens during gastrulation and how the three germ layers form Understand what happens during neurulation Appreciate that congenital anomalies result if embryonic development is disturbed. Fertilisation: The fusion of gametes Millions of sperm enter the female reproductive tract – few reach the uterine tube Introduction to Embryology 1: Fertilisation to Week 4 1  How are sperm moved upwards into the fallopian tube? contractions by smooth muscle cells in the uterine wall What happens after sperm enter the fallopian tube? survive in fallopian tube for up to 5 days Sperm ‘conditioned’ in the tube (capacitation) before they can fertilise an egg What is capacitation? a period of time that sperm must reside in the female reproductive tract before they acquire the ability to fertilise oocytes What happens during fertilisation? Introduction to Embryology 1: Fertilisation to Week 4 2  How is polyspermy prevented? zona pellucida becomes impermeable Clinical Relevance: Molar pregnancy What are the two types of molar pregnancy? complete mole partial mole Complete mole single sperm fertilises an egg that has no genetic material abnormal trophoblast develops, but not an embryo Partial mole normal egg fertilised by 2 sperms embryo starts to develop but cannot survive A molar pregnancy gives a _____ test. Introduction to Embryology 1: Fertilisation to Week 4 3  positive The Zygote Fertilisation produces a zygote that is genetically unique Introduction to Embryology 1: Fertilisation to Week 4 4  What is cleavage? zygote undergoes cell division immediately after it is produced At first, is there an increase in overall size as cells divide? no The Morula What process produces a morula? Cleavage What are blastomeres? cells formed from cleavage of fertilised ovum How are blastomeres arranged in the morula? Introduction to Embryology 1: Fertilisation to Week 4 5  tightly compacted What structure surrounds the morula? zona pellucida (glycoprotein coat) In IVF, a _______ can be removed for genetic testing prior to transfer into the uterus. blastomere The Blastocyst Introduction to Embryology 1: Fertilisation to Week 4 6  Morula forms a fluid-filled cavity Cavity separates the cells into two groups - an inner and outer group The inner cell mass (ICM) develops into the _____. embryo The outer cells are the trophoblast and develop into the _____. placenta Cells in the ICM differentiate into the _____ and _____. hypoblast and epiblast Introduction to Embryology 1: Fertilisation to Week 4 7  What do the hypoblast and epiblast establish? dorsoventral axis Blastocyst Hatching the process by which a blastocyst sheds its clear outer membrane and attaches to the lining of the uterus in preparation for implantation. Introduction to Embryology 1: Fertilisation to Week 4 8  Clinical Relevance: Abnormal zygotes and blastocysts What occurs in the zygote or blastocyst that commonly results in spontaneous pregnancy loss (miscarriage) at an early stage? Chromosomal anomalies Implantation: Begins around day 6 What are the three main stages of implantation? 1. Apposition 2. Adhesion 3. Invasion What is the endometrium? inner-most lining of the uterus Introduction to Embryology 1: Fertilisation to Week 4 9  What is apposition? first physical contact between blastocyst and endometrium Adhesion blastocyst adheres to endometrium Invasion Invasion of the endometrium by the trophoblast Complex signalling between the endometrium and trophoblast The blastocyst must implant adequately - but not too deeply…. The blastocyst is a foreign body as it does not have the same proteins or make up as maternal tissue – why is it not rejected by the mother? Introduction to Embryology 1: Fertilisation to Week 4 10  Implantation: Differentiation of the trophoblast Trophoblast differentiates __________ at the embryonic pole proliferates Cytotrophoblast _________ invades the endometrium. Syncytiotrophoblast Maternal vessels which form blood filled spaces called ____. lacunae Trophoblast contacts maternal vessels these will bathe the forming placental villi and allow gas exchange between the maternal and fetal circulation. Introduction to Embryology 1: Fertilisation to Week 4 11  Implantation: Changes in the inner cell mass The ICM flattens into two layers, what is this called? Bilaminar disc The two layers are the epiblast and hypoblast What develops between the epiblast and the trophoblast? amniotic cavity The blastocyst cavity forms the primitive yolk sac. Introduction to Embryology 1: Fertilisation to Week 4 12  Clinical Relevance: Implantation problems Implantation may not be successful – this might be because the: blastocyst is abnormal blastocyst does not reach the uterus endometrium is not receptive Implantation may be ectopic outside the body of the uterus Where might an embryo implant instead of in the body of the uterus? fallopian tubes, ovary, abdomen, or the cervix. Sub-optimal implantation can result in ______ problems later in pregnancy. placental Introduction to Embryology 1: Fertilisation to Week 4 13  Clinical Relevance: Invasive placentation Placenta must not invade too far May invade through the endometrium and to, or through, the myometrium (muscle wall of the uterus) +/- adjacent organs Placenta does not come away easily after birth Can result in severe _______. haemorrhage Introduction to Embryology 1: Fertilisation to Week 4 14  Gastrulation: Week 3 A critical event The bilaminar disc develops into a trilaminar disc Establishes the _______ germ layers from which all the embryo’s tissues develop three Further specification of body axes - head and ‘tail’ ends (AP), front-back (DV), left-right. Introduction to Embryology 1: Fertilisation to Week 4 15  The Primitive Streak Appears as a groove at which end of the epiblast? caudal (tail end) of the epiblast Introduction to Embryology 1: Fertilisation to Week 4 16  Epiblast cells migrate towards it and _____ through it invaginate Settle between the epiblast and hypoblast to form a third layer, the mesoderm. Clinical Relevance: Laterality anomalies Introduction to Embryology 1: Fertilisation to Week 4 17  Laterality anomalies may occur – these are conditions of ‘abnormal sidedness’ Situs inversus thoracic and abdominal viscera are ‘flipped’ (mirror image). Dextrocardia the heart is ‘flipped’ Neurulation: Week 4 process that forms the neural tube Introduction to Embryology 1: Fertilisation to Week 4 18  What happens first? appearance of neural plate - thickening of the ectoderm Neural plate formation is inducted by what structure? notochord Plate bends to form a groove Cells at the top of the folds are neural crest cells. Neurulation: Formation and fate of the neural tube Introduction to Embryology 1: Fertilisation to Week 4 19  The two sides of the groove fuse to form the neural tube The neural tube detaches from the _______. ectoderm Cranial end expands = brain Caudal end remains tubular = spinal cord Introduction to Embryology 1: Fertilisation to Week 4 20