Embryology - Fertilization to Gastrulation (BMS 150)

Embryology Fertilization to Gastrulation BMS 150 Week 8 Introduction to Embryology Why learn embryology? Fascinating field of biology § We start out as one generalized, totipotential cell à 100 trillion cells, the vast majority of which are highly specialized § Within minutes after birth we undergo...

Embryology Fertilization to Gastrulation BMS 150 Week 8 Introduction to Embryology Why learn embryology? Fascinating field of biology § We start out as one generalized, totipotential cell à 100 trillion cells, the vast majority of which are highly specialized § Within minutes after birth we undergo massive changes: Fetus - an organism that breathes no air, does not use the digestive tract and lives in a sterile environment Newborn - an air-breathing organism with a functional GI tract that quickly becomes colonized by microbial flora Introduction to Embryology Why learn embryology? Clinical relevance – a basic knowledge of embryology is essential to understanding: § Common categories of developmental/congenital disorders (just a few listed here): Immune system – DiGeorge syndrome Cardiovascular – abnormalities of the great vessels and valvular/septal structures of the heart Respiratory – neonatal respiratory distress syndrome Nervous system/otolaryngology – neural tube defects, developmental head and neck disorders Reproductive system – gestational trophoblastic disease, ectopic pregnancy § Infertility § Referred pain Introduction to Embryology Human development is complex, so it is important to categorize your knowledge What categories? § The age of the embryo/fetus – we want you to know what’s happening at each week, especially early in development § What structures have appeared… or disappeared Some people like to think about the rough size of the embryo § If you link information all together, then it will help you form a complete picture i.e. at week 3: Introduction to Embryology A simple table won’t be enough to build your understanding, though it will help. Other strategies include: 1. Identify a particular event § i.e. gastrulation 2. Define the event in 1-2 bullet points § What does it mean? What are the key features (beginning, ending, purpose)? 3. Describe the event as a series of steps that link to each other 4. Describe the relevance of the “end product” § This could be very short or long, depending on the event Introduction to female reproductive anatomy Reproductive anatomy will be covered in much more depth in Biomedicine in Year 2 Essentials will be discussed here as they relate to fertilization and implantation of the embryo Introduction to female reproductive anatomy Ovaries: § Production of oocytes Female haploid gametes § Production of progesterone and estrogens Uterine tube (Fallopian tube): § Receives oocyte from ovaries § Site where sperm fertilizes the oocyte Uterus § Site where the embryo develops § Site where the placenta and membranes develop Placenta = site where the maternal and embryonic vasculature exchange substances Basics of ovulation and fertilization Remember meiosis? § A diploid cell (germ cell) undergoes meiosis to produce a unique haploid gamete Unique à crossing-over between maternal and paternal chromatids during prophase I to end up with “mixed” chromatids “mixed” = some paternal, some maternal genes Meiosis is not completed in an oocyte until the sperm penetrates the oocyte § Spermatic pronucleus and the oocyte pronucleus fuse, thus completing fertilization Combination of spermatic and oocyte genetic material à diploid cell § Single diploid cell = zygote Basics of ovulation and fertilization A person with ovaries is born with a certain number of diploid oocytes that have been “paused” during the first stage of meiosis – do not continue meiosis until after puberty After puberty, the ovaries release an ovum each cycle into the uterine tubes § The oocyte will not complete meiosis II unless fertilization occurs § Fertilization typically occurs in the ampulla of the uterine tube Ovulation - release of a secondary oocyte from ovarian follicle Ovulated secondary oocyte together with zona pellucida, is externally covered with granulosa cells – cumulus oophorus Cumulus oophorus will rearrange and form corona radiata Basics of ovulation and fertilization Fertilization typically occurs in the ampulla of the uterine tube Sperm cell penetrates the zona pellucida and “injects” its genetic material into the oocyte § Afterwards, the zona pellucida becomes impenetrable to other sperm cells (can’t have “double fertilization”) § After approximately 24 hours, the oocyte completes meiosis II and the zygote completes the first cell division This is the end of day 1, week 1 Key Events – Week 1 Note the locations and the level of development of the embryo in each area of the female reproductive tract Ampulla of uterine tube Overview of fertilization Corona radiata = cells that surround the oocyte, found outside of the zona pellucida Sperm cells undergo several changes (discussed later) in the uterus to prepare them to inject the pronucleus across the zona pellucida into the oocyte Embryologic terms – fertilization to week 1 Gamete – a haploid germ cell § Oocyte – gamete from ovaries § Sperm – gamete from testes Fertilization – fusion of the pronucleus of the two gametes Zona pellucida – protein coat that surrounds an oocyte as well as the early embryo Zygote – a fertilized, diploid oocyte – has not yet divided Embryo – multicellular organism, prior to fetal stage § Arbitrarily defined as period from fertilization – end of week 7 Fetus – multicellular organism, from end of embryonic stage to birth Neonate – newborn Extraembryonic = cells formed during development that do not become part of the neonatal organism, but involute or contribute to the fetal membranes Embryologic terms – fertilization to week 1 Morula – approximately 16-cell stage (12 – 32 cells) of an embryo § No blastocoel Blastocyst – a spherical mass of cells that is composed of a trophoblast that surrounds a fluid cavity (blastocoel) and an inner cell mass (embryoblast) Cleavage – cell division in the early embryo à each division does not increase the size of the embryo, instead each division results in smaller and smaller cells Blastomere – a cell that is totipotential and is present during very early development (first week) § Product of cleavage § Totipotential = a cell that can become any cell Implantation – occurs when an embryo contacts and then becomes surrounded by the endometrium of the uterus Early Embryology Overview: Week Key developmental steps 1 2 3 Appearance Begins with fertilization From zygote to blastocyst Inner cell mass (embryoblast), trophoblast, blastocoel “Hatching” from zona pellucida, first appearance of syncytiotrophoblast & cytotrophoblast Adhesion to endometrium (uterus) and beginning of implantation From a single cell to a “ball” of cells Trophoblast develops into syncytiotrophoblast and cytotrophoblast Embryo “sinks” beneath the endometrium Trophoblastic extensions begin to interface with maternal blood vessels Embryoblast develops into a bilaminar disk – epiblast + hypoblast Prechordal plate develops at the end of this week Umbilical vesicle, extraembryonic coelom develop Disk-shaped embryonic cells surrounded by extra-embryonic cavities Size – 0.2 mm Gastrulation: bilaminar disk à trilaminar disk Three germ layers Notochord forms, and then the following develop à Neural groove, neural plate, early neural tube Paraxial mesoderm + somites Tertiary chorionic villi, heart tube, primordial circulation develop Developing villi surround elongated, diskshaped embryo with a few somites Size – 0.7 mm Size – 0.1 mm Early Embryology Overview Week 1 Week 2 Week 3 From zygote à blastocyst From days 1 – 3, the embryo develops from zygote to a sphere-shaped cluster of cells surrounded by the zona pellucida § 12 – 32 cell stage = morula (because it looks like a mulberry) § Cell divisions known as cleavage, cells known as blastomeres (all genetically identical – embryonic stem cells !) A fluid-filled cavity develops within the embryo, and four separate structures can be noted at this stage (days 4 - 5): § Trophoblast – layer of cells on the outside of the sphere Trophoblast is still covered by the zona pellucida Many of these cells develop into the membranes of placenta § Embryoblast (inner cell mass) – surrounded by the trophoblast, these cells develop into the embryo § Blastocoel – the fluid filled cavity within the sphere From zygote à blastocyst Fertilization of the oocyte usually occurs in the ampulla of the uterine tube (see next slide) The epithelial cells of the uterine tube are equipped with cilia that “wave” in a single direction § Ciliary movement increases as progesterone levels increase (more later) Progesterone secretion peaks shortly after ovulation § After about 5 days, the blastocyst has arrived at the superior aspect (fundus) of the uterus Propelled by ciliary movement Key Events – Week 1 Note the locations and the level of development of the embryo in each area of the female reproductive tract Ampulla of uterine tube From zygote à blastocyst Zona pellucida (ZP) plays many important roles early in development: § Barrier that ensures that only one sperm fertilizes an oocyte § Porous – allows communication between the embryo and the maternal reproductive structures § Protects the embryo from immunologic defenses § Acts as a signal to help with differentiation of trophoblast cells § Prevents premature implantation of the embryo § Prevents the blastomeres from dissociating Zona “hatching” and implantation The ZP prevents the early implantation of the embryo § If the embryo implants too early, it could result in an ectopic pregnancy (discussed later) At day 6, the embryo will “hatch” out of the ZP § The trophoblastic cells just over the embryoblast seem to secrete/produce particular proteinases that weaken the wall of the ZP § The blastocyst “hatches” through this defect in the ZP and is ready to contact the endometrium and implant itself within it The blastocyst and the endometrium need to “cooperate” for implantation to occur Zona “hatching” and implantation The endometrial epithelium expresses different types of mucin proteins as well as small apical processes known as pinopods § The trophoblast of the blastocyst (free of the ZP) contacts the pinopods and adheres to the endometrial epithelium § Adhesion is mediated by selectin (binds to mucins) and integrin binding, and results in the blastocyst invading into the endometrium Similar to leukocyte emigration from the bloodstream Once the trophoblast contacts the endometrial epithelium and invades, it forms two layers: § Cytotrophoblast – inner layer § Syncytiotrophoblast – outer layer that covers cytotrophoblast Occurs at days 5 - 6 Implantation of the blastocyst Note the pinopods on the uterine epithelium Syncytiotrophoblast Cytotrophoblast Implantation of the blastocyst Syncytiotrophoblast develops into a multinuclear cell “mass” where the borders between individual cells are indistinct The syncytiotrophoblast invades quickly and deeply into the endometrial stroma (area under the epithelial cell) and performs a number of important functions: § Invasion into the endometrial stroma and induction/formation of villi (later becomes the placenta) Proteinases and adhesion molecules perform this role § Secretion of human chorionic gonadotropin (hCG) hCG prevents the shedding of the endometrium (and loss of the embryo) by maintaining ovarian secretion of steroid hormones (i.e. progesterone) hCG is the hormone detected in pregnancy tests End of Week 1 – days 6-7 As the syncytiotrophoblast invades into the stroma, the inner cell mass (embryoblast) differentiates into two distinct layers: § Epiblast This layer will become embryo proper § Hypoblast It will line the blastocystic cavity (coelom) and form the primary yolk sac Also known as the primary endoderm Overview of Week 2 13 14 12 9 10 Embryologic terms – Week 2 & 3 Coelom – a fluid-filled cavity Gastrulation – the process of forming three embryonic germ layers: § Ectoderm – a layer that is typically found on the “exterior” of the organism § Endoderm – a layer that is typically found on the “interior” of the organism § Mesoderm (intraembryonic mesoderm) – a layer found between the ectoderm and endoderm § Note – endoderm and mesoderm can also be extraembryonic (formation of extra-embryonic endoderm and mesoderm is not classified as gastrulation) Cephalad – towards the head region (anatomical term) Caudad – towards the “tail” region (anatomical term) Completion of Implantation Implantation of the embryo is complete at about day 10 § The embryo is completely embedded within the endometrium § Surrounded by syncytiotrophoblast cells The stromal cells (below the epithelium) undergo decidualization § Now known as decidual cells § Decidual cells accumulate glycogen and lipids throughout the uterus § The decidual cells that the syncytiotrophoblast contact undergo apoptosis, releasing stored nutrients needed for embryonic growth (until the placenta is better established) Epiblast and Hypoblast The epiblast enlarges and gives rise to amnioblasts § Amnioblast = cells that surround the developing amniotic cavity The hypoblast extends around the entire interior surface of the blastocoel § New class of cells begin to form and migrate between the hypoblast-derived cells (yolk sac or extraembryonic endoderm) and the cytotrophoblast. These cells form extraembryonic mesoderm § At this point, what was formerly the blastocoel is known as the primary umbilical vesicle / primary yolk sac The hypoblast and epiblast form the bilaminar disk Epiblast and Hypoblast – Week 2 Note the migration of hypoblast cells à line the inner surface of the cavity Note the migration of hypoblast cells à line the inner surface of the cavity Extra-embryonic mesoderm develops between the cytotrophoblast and the cells derived from the hypoblast (umbilical vesicle = yolk sac) Amnioblasts Development of Extraembryonic Structures Fluid begins to accumulate between the extraembryonic mesodermal cells to form another cavity that is known as the extraembryonic coelom At this point there are three distinct fluid-filled cavities developing in the embryo: § Umbilical vesicle (yolk sac) – as the embryo develops the primary umbilical vesicle becomes the smaller secondary umbilical vesicle § The amniotic cavity – found above the epiblast § Extraembryonic coelom – as this enlarges and develops, it will develop into the chorionic cavity Development of Extraembryonic Structures Note the accumulation of fluid spaces within the extra-embryonic mesoderm § At day 13 (bottom picture) that fluid space becomes the extraembryonic coelom § Extraembryonic coelom completely surrounds the rest of the embryo everywhere except for at the junction of the amniotic cavity and the rest of the chorionic sac This junction will become the connecting stalk à this develops later into the umbilical cord Development of Extraembryonic Structures There are now two layers of extraembryonic mesoderm, separated by the fluid in the extraembryonic coelom: § Extraembryonic splanchnic mesoderm – surrounds the umbilical vesicle(s) § Extraembryonic somatic mesoderm – found just underneath the cytotrophoblast, inner lining of the chorionic sac Top picture – day 13 Bottom picture – day 14 (end of second week) Development of Extraembryonic Structures Chorion = extraembryonic somatic mesoderm + trophoblast = wall of chorionic sac Chorionic sac encloses the embryo and its cavities, and is surrounded by the syncytiotrophoblast § By day 14, the extraembryonic coelom is called the chorionic cavity The amniotic cavity, secondary umbilical vesicle, and bilaminar disk are “attached” to the chorion via the connecting stalk Top picture – day 13 Bottom picture – day 14 (end of second week) Development of Extraembryonic Structures Where syncytiotrophoblast contacts endometrial blood vessels, the blood vessel deteriorates and blood pools § Form “little lakes”, or lacunar networks The oxygenated maternal blood + glycogen/lipids from deteriorating decidual cells nourish the embryo § Simple diffusion from lacuna & decidual cells, no circulation yet § During day 13-14, the cytotrophoblast sends extensions to the lacuna to form primary villi Precursors of functional placental villi End of week 2 – Prechordal Plate At the end of week 2, the prechordal plate appears § Thickened area of columnar cells that acts as an organization area, found in the cephalad region of the hypoblast § An embryonic organizing centre that is responsible for the induction of other structures Induction = signaling “episodes” by key areas of the embryo that stimulate differentiation and development of local structures § The hypoblast is an organizer of the head and mouth region, and helps to induce the formation of structures found at the cephalad pole of the embryo § It also prevents the formation of structures that belong at the caudal aspect of the embryo End of week 2 – Prechordal Plate The prechordal plate and the nearby anterior visceral endoderm are important organizing centers § More to be discussed later Summary of implantation Implantation of the blastocyst in the uterine endometrium begins at the end of the first week and is completed by the end of the second week. Implantation may be summarized as follows: The zona pellucida degenerates (day 5). Its disappearance results from enlargement of the blastocyst and degeneration caused by enzymatic lysis. The lytic enzymes are released from the acrosomes of the sperms that surround and partially penetrate the zona pellucida. The blastocyst adheres to the endometrial epithelium (day 6). The trophoblast differentiates into two layers: the syncytiotrophoblast and cytotrophoblast (day 7). The syncytiotrophoblast erodes endometrial tissues and the blastocyst begins to embed in the endometrium (day 8). Blood-filled lacunae appear in the syncytiotrophoblast (day 9). The blastocyst sinks beneath the endometrial epithelium and the defect is filled by a closing plug (day 10). Lacunar networks form by fusion of adjacent lacunae (days 10 and 11). The syncytiotrophoblast erodes endometrial blood vessels, allowing maternal blood to seep in and out of lacunar networks, thereby establishing a utero-placental circulation (days 11 and 12). The defect in the endometrial epithelium is repaired (days 12 and 13). Primary chorionic villi develop (days 13 and 14). Summary of week 2 Rapid proliferation and differentiation of the trophoblast occurs as the blastocyst completes implantation in the uterine endometrium. The endometrial changes resulting from the adaptation of these tissues in preparation for implantation are known as the decidual reaction. Concurrently, the primary umbilical vesicle (yolk sac) forms and extraembryonic mesoderm develops. The extraembryonic coelom (cavity) forms from spaces that develop in the extraembryonic mesoderm. The coelom later becomes the chorionic cavity. The primary umbilical vesicle becomes smaller and gradually disappears as the secondary umbilical vesicle develops. The amniotic cavity appears as a space between the cytotrophoblast and embryoblast. The embryoblast differentiates into a bilaminar embryonic disc consisting of epiblast, related to the amniotic cavity, and hypoblast, adjacent to the blastocystic cavity. The prechordal plate develops as a localized thickening of the hypoblast, which indicates the future cranial region of the embryo and the future site of the mouth; the prechordal plate is also an important organizer of the head region. Week 3 - Gastrulation Gastrulation: The process by which the three germ layers of the embryo are established Ectoderm Mesoderm Endoderm Bilaminar embryonic disc becomes trilaminar embryonic disc Embryo may be referred to as a gastrula Week 3 – Process of Gastrulation At the beginning of the 3rd week, formation of the primitive streak appears Primitive streak = thickened linear band in the median plane of the dorsal aspect of the embryonic disc Initiates in the caudal region of the epiblast It results from proliferation and movement of epiblast cells to the median plane of the embryonic disc Week 3 – Process of Gastrulation Cells at the cephalad end of the primative streak proliferate to form a primitive node Concurrently, a narrow groove — primitive groove — develops in the primitive streak This groove is continuous with a small depression in the primitive node known as the primitive pit Week 3 – Embryonic Mesoderm Cells leave the deep surface of the streak and form mesenchyme § Mesenchyme = embryonic connective tissue which forms the supporting tissues of the embryo (i.e. embryonic connective tissue) Some mesenchyme forms mesoblastic cells (undifferentiated mesoderm) § The mesoblasts form the intraembryonic, or embryonic, mesoderm Week 3 – Embryonic Mesoderm Cells from the epiblast, as well as from the primitive node and other parts of the primitive streak, displace the hypoblast § form the embryonic endoderm in the roof of the umbilical vesicle The cells remaining in the epiblast form the embryonic ectoderm Mesenchymal cells derived from the primitive streak migrate widely § These pluripotential cells differentiate into diverse types of cells – examples: Fibroblasts, chondroblasts, osteoblasts Week 3 – Primitive Streak Later, the primitive streak diminishes in size and becomes an insignificant structure in the sacrococcygeal region Disappears by the end of the 4th week Early Embryology Overview: Week Key developmental steps 1 2 3 Appearance Begins with fertilization From zygote to blastocyst Inner cell mass (embryoblast), trophoblast, blastocoel “Hatching” from zona pellucida, first appearance of syncytiotrophoblast & cytotrophoblast Adhesion to endometrium (uterus) and beginning of implantation From a single cell to a “ball” of cells Trophoblast develops into syncytiotrophoblast and cytotrophoblast Embryo “sinks” beneath the endometrium Trophoblastic extensions begin to interface with maternal blood vessels Embryoblast develops into a bilaminar disk – epiblast + hypoblast Prechordal plate develops at the end of this week Umbilical vesicle, extraembryonic coelom develop Disk-shaped embryonic cells surrounded by extra-embryonic cavities Size – 0.2 mm Gastrulation: bilaminar disk à trilaminar disk Three germ layers Notochord forms, and then the following develop à Neural groove, neural plate, early neural tube Paraxial mesoderm + somites Tertiary chorionic villi, heart tube, primordial circulation develop Developing villi surround elongated, diskshaped embryo with a few somites Size – 0.7 mm Size – 0.1 mm Cell and tissue lineages Week 3 – Intro to the Notochord Roles of the notochord: 1. Establishes the longitudinal axis of the embryo and gives it some rigidity 2. Provides signals for the development of axial MSK structures and the CNS 3. Contributes to the intervertebral discs Week 3 – Intro to the Notochord Development of the notochord: mesenchymal cells dive into the primitive pit and migrate cephalad § they form a cord called the notochordal process § The notochordal process develops a lumen known as the notochordal canal § Pictures correspond to days 16, 17, and 18