BHE General Embryology Oct 2024 PDF

General Embryology Chapter 29 Tortora Chapter 7 Remington Dr Suzanne Hagan [email protected] Images by Tortora Copyright © 2000 by John Wiley Sons, Inc. Helpful video (others in Module Information) https://www.youtube.com/watch?v=yXUv4MPuNTA  Nomenclature...

General Embryology Chapter 29 Tortora Chapter 7 Remington Dr Suzanne Hagan [email protected] Images by Tortora Copyright © 2000 by John Wiley Sons, Inc. Helpful video (others in Module Information) https://www.youtube.com/watch?v=yXUv4MPuNTA  Nomenclature in Embryology Rostral (cranial/superior) = head end Caudal (inferior) = tail end Dorsal (posterior) = back Ventral (anterior) = front Proximal = structures near body centre Distal = structures further from body centre (e.g. hand is distal to elbow) Human Development Timeline Stage Time frame Pre-embryonic Day 1 to Week 3 Embryonic Weeks 3 to 8 Foetal Week 8 to Birth Fertilisation Fertilisation- The process where a sperm fuses with an ovum (mature egg), creating a zygote, which eventually leads to the development of an embryo  Fertilisation Gametes are sex (or “germ”) cells: sperm and ovum Ovum: female germ cell (mature egg). Is the largest single biological cell known to scientists Haploid = contains a single chromosome set (23) Diploid: contains 2 sets of chromosomes (23 from each parent = total 46, i.e. a full set) Zygote: fertilised egg (46 chromosomes)  Fertilisation: Penetration of the oocyte sperm Using acrosomal enzymes (in the head), sperm dissolves & penetrates secondary oocyte’s zona pellucida Head of sperm fuses with oocyte's plasma membrane, sperm head then disconnects from its flagellum (tail) The ovum nucleus and the sperm head nucleus are called Pronuclei These pronuclei fuse, intermixing their chromatin, to form a diploid nucleus The diploid cell is now called a zygote  Fertilisation Fertilisation, Cleavage, Morula, Blastocyst, Implantation (12-24hr post- ovulation)  Fertilisation to Implantation (Days 1-6) Fertilisation, Cleavage, Morula, Blastocyst, Implantation  Fertilisation to Implantation Fertilisation (occurs 12-24h after ovulation, in fallopian tube) Cleavage 2 cell stage (day 1) 4 cell stage (day 2) Morula (solid ball of cells, day 4) Blastocyst (external view), day 5 Blastocyst (internal view) day 5 Implantation (6 days)  Fertilisation to Implantation Fertilisation, Cleavage, Morula, Blastocyst, Implantation  Cleavage and Morula Fertilisation Cleavage 2-cell stage (day 1) Cleavage 4-cell stage (day 2) where zygote undergoes rapid mitotic divisions (cleavage) - 1st cleavage starts at 24hrs and is completed 6hrs later - By day 2, the 2nd cleavage is completed to give 4 cells Morula - By end of day 3, there are 16 cells, called the Morula = solid ball of cells (from latin Morus = mulberry)  Fertilisation to Implantation Fertilisation, Cleavage, Morula, Blastocyst, Implantation  Blastocyst Formation Fertilisation (occurs 12-24h after ovulation, in fallopian tube) Cleavage 2-cell stage (day 1) Cleavage 4-cell stage (day 2) Morula (solid ball of cells), Day 4 Blastocyst (external view), Day 5 Blastocyst (internal view), Day 5 Embryoblast (ICM) Implantation, Day 6  Blastocyst Formation End of Day 4, Morula cell number continues to increase, as it moves towards uterus Day 4-5, Morula enters uterine cavity At the 32-cell stage, fluid enters Morula, collects between blastomeres & reorganises them around the Blastocyst Cavity (fluid-filled space) When blastocyst cavity has formed, the tissue is called a Blastocyst (which contains 100’s of cells), from which 2 cell populations arise: 1. Embryoblast (or Inner Cell Mass, ICM) = will become the embryo 2. Trophoblast (outer cell layer, is blastocyst wall) = will become outer chorionic sac (surrounds foetus, site of mother-foetus nutrient/waste exchange) Blastocyst cavity  Fertilisation to Implantation Fertilisation, Cleavage, Morula, Blastocyst, Implantation Implantation Blastocyst- start of implantation By Day 5, blastocyst “hatches” from zona pellucida (glycoprotein layer surrounding oocyte plasma membrane), by enzymatically digesting a hole and squeezing through it. This shedding of z.p. is essential to permit implantation into the endometrium  Implantation (Days 6-7) In the Blastocyst, the Inner Cell Mass (embryoblast) will ultimately form the embryo ICM is the source of embryonic stem cells, which are capable of forming all embryonic cell types Outer cell layer = Trophoblast (forms placenta and adnexa) Zona pellucida Inner cell mass disintegrates and replaced (orientates with by trophoblasts endometrium) Human BLASTOCYST (fluid- filled cavity) Implantation Blastocyst is free in uterine cavity for 2 days before attaching to endometrium (uterine wall) By Day 6, blastocyst lines up with ICM (towards endometrium) and lightly attaches to endometrium (Implantation) At Day 7, blastocyst attaches more firmly and endometrium becomes more vascularised Blastocyst then secretes enzymes and “burrows” into endometrium  Implantation- Day 6  Blastocyst attaches (“rolls”) till ICM lines up with the endometrium ENDOMETRIUM /Inner Cell Mass  External view Implantation Internal view Trophoblast Blastocyst cavity Uterine wall Inner cell mass (ICM/embryoblast) (endometrium)  Week 2 (Days 8-13) Development of: Bilaminar embryonic disc Amniotic cavity Extra-embryonic mesoderm (from yolk sac), which becomes extra-embryonic coelom (a cavity) Thickening of endometrial layers Chorion (exchange site between mother & embryo, for nutrients & waste)  Start of Week 2 (Day 8) Both the Trophoblast and Inner cell mass (ICM) layers differentiate to form 2 distinct cellular layers each: 1. Trophoblast forms: Cytotrophoblast – the inner layer of trophoblast (which eventually forms primary chorionic villi). Cytotrophoblasts considered to be stem cells for syncytiotrophoblasts Syncytiotrophoblast- the outer layer, invades maternal endometrium, so in direct contact with maternal blood. Main mediator of communication between foetus and mother. Facilitates implantation of the blastocyst. 2. Inner cell mass forms: Epiblast - forms the 3 germ layers (primitive ectoderm) Hypoblast –transient layer replaced by endoderm (primitive endoderm) Together, the epiblast + hypoblast form Bilaminar embryonic disc Endometrium Derived from trophoblast (for nutrients and implantation) will become part of chorion Epiblast + hypoblast = will become embryonic disc (e+h are derived from ICM)  Week 2 (day 8 onwards) Formation of Bilaminar Embryonic Disc (from Epiblast and Hypoblast, originate from ICM) Epiblast (“primitive ectoderm”) - forms 3 germ layers which will become embryo (also some extra-embryonic membranes). Also, a hollow appears in epiblast, called the amniotic cavity) Hypoblast (cell layer beneath epiblast, contain precursors to yolk sac) = transient epithelium Bilaminar disc = development stage which occurs after implantation and prior to embryonic folding (e.g. week 2)  Development of Bilaminar Disc Thin membrane, from epi. Suspends foetus in (from blastocyst) amniotic fluid Formation of yolk sac (from hypo, vascular, provide nutrients. Shrinks end of 3rd trimester. Ultimately, replaced by placenta Bilaminar disc: Epiblast and Hypoblast Day 8, blastocyst Day 9, blastocyst  Week 2 Extra-embryonic mesoderm (from hypo) - fills space between trophoblast and the amnion and the chorion Yolk sac Day 9 Day 12 Formation of bilaminar disc creates space on either side which develop into amniotic cavity and yolk sac  Connecting stalk Extra-embryonic mesoderm By end of week 2, Bilaminar embryonic disc is connected to trophoblast by a connecting stalk (future umbilical cord) Extraembryonic coelom Chorion 2 parts, derived from trophoblast = surrounds embr and later the foetus. Becomes part of placenta  Week 3- Gastrulation This is trilaminar embryo development. Gastrulation replaces the hypoblast layer with endoderm germ layer. Useful 3min Gastrulation video https://www.youtube.com/watch?v=3AOoikTEfeo&t=24s  Gastrulation (1st major event, week 3) Formation of trilaminar embryo Early phase in mammalian embryonic development where bilaminar embryonic disc (hypo+epi) are reorganised into 3-layered (trilaminar) structure (called Gastrula), containing 3 germ layers The 3 germ layers will form the embryo’s entire body organs & tissues: 1. Ectoderm (external layer): epidermis (skin), nervous system (PNS, CNS), retina, nose and ears 2. Mesoderm (middle layer): dermis, bones, cartilage, connective tissue, cardiovascular/reproductive/lymphatic systems, muscle (striated, smooth & cardiac), kidneys & spleen 3. Endoderm (internal layer): epithelial lining of gut, lungs & respiratory tract. Epithelia of urethra & bladder, thyroid, tonsils, thymus. Also, liver & pancreas Gastrulation also generates body axes  Gastrulation (week 3) (Formation of trilaminar embryo)  Ectoderm and endoderm=form tightly-connected epithelial sheets  Mesoderm = less organised, cells more loosely connected Gastrulation-Week 3 Initial event is Primitive streak formation (a morphological groove) by migrating epiblast cells Primitive streak = will form 2 germ cell layers (endoderm and mesoderm) Primitive streak formation also establishes embryo body axis (i.e. head-tail/left-right orientation) Invagination of epiblast cells through Primitive streak, and displaces hypoblast to become endoderm Primitive node = region at cranial end of primitive streak Invagination of epiblasts cells from primitive streak, they displace hypoblast and form the endoderm (GIT/lung epithelial lining). Cells remaining between epiblast & endoderm = mesoderm (connective tissue/mesenchyme). Cells remaining in epiblast = ectoderm (skin epidermis & nervous system)  3 Germ layers The epidermis, the central and peripheral nervous systems, and some non-neuronal cells of the head and heart are derived from ectoderm Gastrulation Dorsal surface of bilaminar disc Head end Oropharyngeal Tail Primitive membrane end streak (future mouth) Transverse plane of above, showing: Primitive node Ectoderm Mesoderm Endoderm  Embryonic disc now has 3 germ layers (ectoderm, mesoderm, endoderm) and this flat disc begins to fold ventrally at the edges, to form a hollow cylinder (tube) of the 3 germ layers Next process is Neurulation (Ocular Embryology I lecture) See Neurulation video https://www.youtube.com/watch?v=lGLexQR9xGs See Chapter 29 of Tortora Also, detailed animations on embryonic folding in Module Information of GCU Learn