Intro to Embryology PDF

ANATOMY-LEC: LE 2 | TRANS 7 Intro to Embryology LUIS EMMANUEL O. ESGUERRA, MD | LECTURE DATE (09/30/2024) OUTLINE I. INTRODUCTION I. Introduction...

ANATOMY-LEC: LE 2 | TRANS 7 Intro to Embryology LUIS EMMANUEL O. ESGUERRA, MD | LECTURE DATE (09/30/2024) OUTLINE I. INTRODUCTION I. Introduction B. Three Germ Layer A. DEFINITION OF TERMS A. Definition of Terms Perspective Developmental biology B. Periods of Human C. Summary: 3rd → Study/process by which organisms grow & develop. Development Week Embryology II. Gametes D. Summary: → Subfield involves the study of the development of the A. Spermatozoon Fertilization to embryo (embryogenesis), from the zygote (at B. Oocyte Formation of fertilization) to its differentiation into tissues & organs. III. Spermiogenesis Trilaminar Germ → Refers to the embryonic stage of development (up to A. Process of Disc 8th week after conception in humans) Spermiogenesis VII. Differentiation of Germ → Includes organogenesis - process of formation of B. Summary of Layers organs and organ systems. Spermiogenesis A. Notochord Fetal development IV. First Week of Formation → Process of development of the fetus (after the 8th Development B. Neurulation week) until birth; period of maturation of tissues and A. Fertilization B. Morulation C. Neural Crest Migration 📣 organs and rapid body growth occur. The term Embryology is often used to encompass both Embryology Proper and Fetal Development. C. Blastocyst VIII. Derivatives of the Three Formation and Germ layers B. PERIODS OF HUMAN DEVELOPMENT Implantation A. Ectodermal D. Summary: 1st Derivatives Week B. Neural Crest V. Second Week of Derivatives Development C. Mesodermal A. Formation of Derivatives Bilaminar Germ D. Endodermal Disc Derivatives B. Summary: 2nd IX. Origin of Germ Cells Week X. Review Questions VI. Third Week of XI. References Development XII. Appendix A. Formation of Trilaminar Germ Disc SUMMARY OF ABBREVIATIONS CNS Central Nervous System DNA Deoxyribonucleic Acid GIT Gastrointestinal Tract PNS Peripheral Nervous System Figure 1. Periods of Prenatal Human Development [Lecturer’s PPT] SRY Sex-determining Region Y gene 📣 Embryonic period → Encompasses the first 8 weeks of pregnancy or ❗️ Must know 📣 Lecturer 📖 Book 📋 Previous Trans 📣 gestation. → Further subdivided into: Pre-organogenic period LEARNING OBJECTIVES − Occurs in the first 2 weeks ✔ The student should be able to describe the normal Organogenic period human embryological development of organs and − Occurs in the 3rd to the end of the 8th week tissues: From the fertilized ovum (zygote) in the 1st 3 weeks: 📣 Fetal period → Starts at the beginning of the 9th week until birth. Full gestation of humans: ○ Fertilization and cleavage → Usually take 40 completed weeks ○ Blastocyst formation and implantation → Full term infant: once a fetus achieves 37 completed ○ Formation of the bilaminar germ disc ○ Formation of the trilaminar germ disc (gastrulation) 📣weeks at birth Preterm or Premature As derivatives of the three germ layers ○ Ectoderm, including neural crest 📣 → Born before 37 completed weeks Post-term or Postmature → If gestation is more than 42 completed weeks before ○ Mesoderm birth. ○ Endoderm LE 2 TRANS 7 TG-C19: J.N. Uy, J.G. Uy, S. Uy, P. Valentino, *E. Velas, E. TE: L. Uy, K. Valenzuela, AVPAA: A. Roluna Page 1 of 23 Velchez  📣 Chart for Periods of Human Development → Note the red and yellow bars on the figure (Figure1). These represent the different organs and organ → Has an acrosomal cap at the tip Tail (or flagellum) is composed of: → Middle piece systems. → Principal piece Red portion: spans the formation of the organs → End piece Yellow portion: spans the differentiation and Neck maturation of these organs → connects the head and the tail First bar (top bar): represents the Central Nervous B. OOCYTE System Also known as Ovum − Start to form: at the beginning of the 3rd week → Appears as a round cell with a nucleus, a nucleolus, − Continue to be formed: even until the 16th week and a clear cytoplasm − Further maturation occurs even after birth → Travels within the fallopian/uterine tube, awaiting for Second bar: represents the heart fertilization by the spermatozoa − Starts to be form: at the middle of the 3rd week → Zona pellucida: homogenous glycoprotein coat − Completion of main structures: by the end of the surrounding the ovum 6th week → Corona radiata: multiple smaller round follicular cells 📣 − Further maturation occurring unto the 8th week → The other bars represent the upper limbs, eyes, that surrounds the zona pellucida 📣 lower limbs, teeth, palate, external genitalia, and ear. → Take note: Even if the organogenesis may be occurring in some organ systems after the 8th week, we still consider the organogenic period to end at the completion of 8 weeks gestation. After which, the embryo then is considered a fetus. Exposure to harmful substances or events during: − Pre-organogenic period: At risk for spontaneous abortion and fatal congenital anomalies. − Embryonic period: Increased risk for major morphological abnormalities. − After the organ systems are formed: functional 📣 defects and minor morphological abnormalities. Figure 2. The Protagonists of Fertilization [Lecturer’s PPT] Take note: A woman who is pregnant before the III. SPERMIOGENESIS 6th week may not even know she is pregnant. If she gets exposed to these harmful substances and A. PROCESS OF SPERMIOGENESIS events, major malformations in the embryo and fetus may occur. 📣 EMBRYONIC VS. FETAL It is important to keep the distinction of the terms embryonic and fetal when referring to structures that are precursors of adult tissues and organs. → We might show or point at a structure, either histologic or from a gross specimen, and ask for its embryonic or fetal precursor. → If asked for an embryonic precursor, we are asking for a structure present in the embryo, either in the Figure 3. Process of Spermiogenesis [Langman, 2019] pre-organogenic or organogenic phase. Spermiogenesis: The transformation and maturation of → If asked for an fetal precursor, we are asking for a the male gametes from the spermatid into the mature structure found when the organism is already a fetus spermatozoon and the organs are undergoing maturation Chronological events: Ex: We may point on the navel/umbilicus of the 1. An acrosomal granule is produced in the model/cadaver and ask: Name the embryonic (or developing sperm cell by the golgi apparatus which fetal) structure attached to this pointed area. If we contains enzymes that will help the spermatozoon are asking for an embryonic structure, the answer penetrate the layers surrounding the oocyte. would be connective stalk. If we are asking for a 2. This granule goes to one pole of the cell nucleus, fetal structure, the answer would be umbilical cord. while at the same time, the pair of centrioles or centrosomes position themselves at the opposite II. GAMETES pole of the nucleus. From each parent 3. The acrosome granule then becomes the Contain half of the DNA of a regular human cell acrosomal cap, covering half of the nuclear surface Unite at fertilization to form 1 fertilized ovum or zygote that as the nucleus migrates to the cell surface. The would develop into the embryo, then into the fetus and cytoplasm and the rest of the cell organelles move ultimately into the newborn infant at birth to the opposite pole. 4. At the opposite pole, one of the centrioles elongates A. SPERMATOZOON to form the flagellum, the tail of the spermatozoon. Also known as Sperm cell 5. As the nucleus in the cell elongates, the Head mitochondria form a ring-like structure surrounding → Composed of condensed DNA material from the father the proximal part or middle piece of the flagellum. ANATOMY Intro to Embryology Page 2 of 23  6. The mitochondria would generate the necessary 📣 ACROSOME REACTION energy for the propulsion of the spermatozoon by Upon binding of the sperm cell to the zona pellucida, the flagellum. the zona proteins induce the acrosome reaction mediated 7. The nucleus of the cell, covered by the acrosomal by the ZP3 receptors or the release of the acrosomal cap, forms the head of the spermatozoon. enzymes or acrosin from the acrosomal cap which would 8. Most of the cytoplasm and other cell organelles are digest through the zona pellucida allowing the sperm cell shed by the cell as residual bodies which are later to penetrate the zona pellucida and reach the oocyte phagocytosed by the sertoli cells in the seminiferous tubule in the testes. 9. Once fully formed, the spermatozoa are released 📣 plasma membrane Once the acrosomal head of the first spermatozoon comes into contact with the oocyte plasma membrane 2 within the lumen of the seminiferous tubule and reactions occur travel along the male genital tract → Cortical reaction: 10. Although initially slightly motile, the spermatozoa Release of lysosomal enzymes from the cortical would obtain full motility while in the epididymis. granules lining the oocyte plasma membrane into the B. SUMMARY OF SPERMIOGENESIS zona pellucida In summary: → Zona reaction → Cell nucleus = Becomes the head → Golgi apparatus = Produces the acrosomal cap − 📣 Lysosomal enzymes inactivate ZP3/sperm receptors This alters the properties of the whole zona pellucida rendering it impermeable to other sperm located at the tip of the head → Centrioles = Located in the neck One of which forms the flagellum or tail − 📣 cells The other sperm cells would then be arrested and embedded within the zona pellucida allowing → Mitochondria = Located in the middle piece the first sperm cell to penetrate and fertilize the IV. FIRST WEEK OF DEVELOPMENT A. FERTILIZATION Fusion of the male and female gametes occurs normally in 📣 oocyte Prevent polyspermy -penetration and fertilization of more than one spermatozoon into the oocyte the ampullary region of the fallopian tube of the female 📣 reproductive tract FUSION OF SPERM AND OOCYTE PLASMA MEMBRANE The sperm cells deposited in the vagina would have to The initial adhesion of the sperm to the oocyte involves travel to the cervix into the uterine cavity and into the the interaction of integrins (on the oocyte) and fallopian tube by their own propulsion and uterine disintegrins (on the spermatozoon) peristaltic motion which can take as fast as 30 minutes to 2 hours or as slow as 6 days 📣 Stimulates completion of oocyte meiosis II → After adhesion, both plasma membranes would fuse since the oocyte is suspended in metaphase of the second meiotic division at ovulation, it finishes meiosis II upon the fusion of the sperm and oocyte plasma 📣membrane One daughter cell with barely any cytoplasm is extruded as a second polar body while the remaining daughter cell would become the definitive oocyte 📣 Sperm centrioles for subsequent cell division → Both the head of the spermatozoon and part of its tail 📣 with the centrioles enter the cytoplasm of the oocyte → The sperm centrioles would become the centrioles of the fertilized oocyte to direct its subsequent cell Figure 4. Process of Fertilization [Langman, 2019] divisions CAPACITATION 📣 Sperm mitochondria degradation → The rest of the tail would detach and the sperm 📣 Functional maturation of the spermatozoa The spermatozoa by themselves are not able to fertilize 📣mitochondria would disintegrate Not only does the father contribute half of the DNA the oocyte without undergoing the process of capacitation Require approximately 7 hours in female reproductive tract 📣 material of each embryo, he also contributes the centrioles All the rest of the organelles and the cytoplasm come from the mother Remove glycoproteins & seminal plasma proteins from 📣 plasma membrane that overlies the acrosomal cap Once capacitated, the sperm cells may be able to penetrate the corona radiata surrounding the oocyte, then PRONUCLEUS FORMATION the follicular cells, and then the zona pellucida to reach the 📣 oocyte underneath Aside from the peristaltic contraction of the uterus and the fallopian tube, chemoattractants from the cells of the corona radiata lead the capacitated spermatozoa towards the oocyte in the fallopian tube, making them have a 📣 direction and do not move indiscriminately The sperm cells can penetrate the coronary radiata to bind with the ZP3 receptors on the zona pellucida surrounding the oocyte Figure 5. Pronucleus Formation[Langman, 2019] ANATOMY Intro to Embryology Page 3 of 23  Occurs when the spermatozoon and oocyte plasma TRANSPORT OF MALE GAMETE IN THE FEMALE 📣 membranes fuse Once the definitive oocyte is formed by the extrusion of the second polar body at the completion of meiosis II REPRODUCTIVE TRACT (VIDEO) The spermatozoa in the triggered by the entry of the spermatozoon, the vagina travel through the chromosomes arrange themselves in a vesicular nucleus cervix towards the called the female pronucleus fallopian tube to fertilize 📣 → Female: spermatozoon entry & end of meiosis II the oocyte The nucleus of the spermatozoon then approximates or They undergo moves closer towards the female pronucleus and forms capacitation to prepare the male pronucleus the acrosome to bind with → Male: approximation with female pronucleus the zona pellucida of the oocyte AMPHIMIXIS/SYNGAMY The capacitated Union of male and female pronuclei after losing their spermatozoa that nuclear envelopes as they come into close contact reaches the oocyte pass Restoration of diploid condition freely through the corona Duplication of DNA in preparation for mitotic division radiata to reach the zona pellucida 📣Initiated to form the two-cell stage CLEAVAGE GAMETE TRANSPORT AND FERTILIZATION (VIDEO) Once acrosomal head binds with the receptors of the zona pellucida, acrosin is released in the acrosome reaction to digest the zona pellucida, allowing penetration Uterus, uterine, fallopian tube, and ovary. The spermatozoa race to The uterine cavity opens to the vagina at the cervix reach the oocyte, the first At the periphery of the ovary spermatozoon reaches there are a number of the oocyte and both ovarian follicles each plasma membranes bind containing a single oocyte. In and fuse and a series of humans, normally, one events occur ovarian follicle matures at a single time. Just before ovulation, the secondary oocyte in the mature follicle is suspended at metaphase II During ovulation, the follicle ruptures and releases the The head and part of the tail of the spermatozoon enter oocyte into the peritoneal the oocyte cytoplasm and the nuclear material would cavity form the male pronucleus The zona reaction inactivates the receptors in the zona, rendering it impermeable to further digestion due to the enzymes in the cortical granules from the oocyte in the cortical reaction preventing polyspermy. The oocyte completes meiosis II, with the extrusion of the second polar body and the formation of the female pronucleus. The male and female pronuclei combine and fuse, and the fertilized oocyte or zygote undergoes its first mitotic This ovulated oocyte is then captured by the fimbriated division end of the fallopian tube and travels within it towards the B. MORULATION uterine cavity This secondary oocyte is surrounded by zona pellucida and granulosa cells called corona radiata which are still suspended in metaphase II Figure 6. Morula Formation [Lecturer’s PPT] ANATOMY Intro to Embryology Page 4 of 23  📣 Once the zygote has reached its two cell stage, it undergoes a series of mitotic divisions–increasing the 📣 Gradually, the intercellular spaces become confluent and forms a single cavity called blastocoel/blastocele or 📣The inner cell mass (embryoblast) is pushed into one number of cells blastocyst cavity (Blasto + G. koilos, hollow) 📣 Blastomere [G. blastos, germ + eros, part] formation 📣 → Cells become smaller with each cleavage division pole and the outer cell mass (trophoblast) flattened to form 📣Zona pellucida degenerates/disappears by day 5 and → Initially form a loosely arranged clump until the eight the wall of the blastocyst 📣 cell stage → After the third cleavage, blastomeres maximize their contact with each other forming a compact ball of cells ultimately disappears such that the blastocyst hatches and can now rapidly increase in size, ready to interact directly held together by tight junctions with the endometrium for implantation → Zona pellucida persists DAY 6-9 Compaction → Mitotic divisions with decrease in cell size → These cells communicate with each other with 📣 Implantation 📣 → Blastocysts attaches to the endometrial epithelium → Trophoblast layers start to invade endometrium extensive gap junctions Morula: 16-cell stage (approx. day 3 from fertilization) → Segregation of cells: 📣 penetrating between the endometrial mucosal cells → Selectins on the trophoblast cells interacting with carbohydrate receptors on the endometrium, mediate Inner cell mass = embryoblast [G. embryon, a young one] from which the embryo develops the initial attachment of the blastocyst & further Outer cell mass = trophoblast [G. trophe, attachment and invasion of the blastocyst involves nourishment] which is the precursor of the placenta Integrins that are on trophoblast cells interacting with the extracellular matrix molecules of the endometrial EARLY CLEAVAGE STAGES OF AN EMBRYO UNTIL THE stroma FORMATION OF MORULA Laminin: promote attachment Fibronectin: stimulate migration, further burying of After fertilization, we see the first cell division into 2 separate cells or blastomeres in the 2-cell stage 📣the blastocyst in the endometrium → By the end of the first week, the zygote in the blastocyst stage should have begun implantation in the uterine wall FORMATION OF THE BLASTOCYST FROM THE MORULA We would then see the zygote (VIDEO) dividing into 4 cells by Day 2. The morula at about Day 4 is undergoing further mitotic divisions and compaction to become a blastocyst To nourish the developing zygote, Take note of the appearance of halo uterine fluid would pass through the surrounding the dividing cells. This zona pellucida to enter deep within is the zona pellucida which remains the zygote and coalesce to form a 📣 while further cell division continues After multiple divisions, the zygote will start compacting and all single blastocoel. blastocyst cavity or the cells will fuse together to form The inner cell mass or embryoblast one large mass called the morula by is pushed to one pole by the fluid Day 3 to Day 4. and the zona pellucida degenerates allowing the blastocyst to enlarge rapidly as more fluid enters the 📣 C. BLASTOCYST FORMATION AND IMPLANTATION As the zygote undergoes its mitotic divisions, it is also traveling the fallopian tube towards the uterine cavity, such blastocyst The outer cells (trophoblast) that it just about enters the uterine cavity once it reaches become a thin, flattened wall. the morula stage or the 32-cell stage The blastocyst is now ready for implantation. D. SUMMARY: 1ST WEEK Figure 7. Schematic representation of a blastocyst at 4.5 days (B) and a blastocyst at the 6th day of development (C) [Lecturer’s PPT] DAY 4-6 📣 Blastocyst formation (G. blastos, germ + kystis, bladder) By day 4, uterine fluid begins to penetrate through the zona pellucida and the outer cells into the intercellular Figure 8. Events during the 1st week of human development[Lecturer’s PPT] spaces of the inner cell mass ANATOMY Intro to Embryology Page 5 of 23  The oocytes are produced in the ovaries (oogenesis) and Zona pellucida degenerates, are expelled from them during ovulation. The trophoblast cells flatten. The fimbriae of the uterine tubes sweep the oocyte into the The blastocyst rapidly ampulla, where they may be fertilized. enlarges in size and become → Usually only one oocyte is expelled during ovulation. ready for implantation after Several hundred sperm pass through the uterus and enter Day 5 the uterine tubes such that fertilization occurs when one spermatozoon gets in contact with the oocyte, approximately 12 to 24 hours after ovulation V. SECOND WEEK OF DEVELOPMENT The oocyte completes its second meiotic division forming A. FORMATION OF BILAMINAR GERM DISC the female pronucleus which combines with the male pronucleus from the spermatozoa DNA duplication occurs and the fertilized oocyte or zygote undergoes a series of mitotic divisions or cleavage giving rise to two daughter cells or blastomeres at approximately 30 hours of age At approximately three days of age, the morula with about 16 blastomeres is formed The morula with about 32 blastomeres would reach the uterine lumen at approximately four days of age The fluid then accumulates within the morula to form the blastocyst at approximately 4.5 days of age DAY 8 The zona pellucida disappears and the blastocyst enlarges Both the Embryoblast and Trophoblast cells start to and implantation begins at approximately 6 days of age differentiate into 2 layers each. CLEAVAGE AND BLASTOCYST FORMATION (VIDEO) The Embryoblast forms the superficial epiblast and the deeper hypoblast layers at the embryonic pole. The fertilized zygote reaches → Embryoblast the 2-cell stage (1st cleavage ▪ Epiblast (G. epi, upon) stage) at approximately 30 ▪ Hypoblast (G. hypo, under) hours from fertilization → Amniotic cavity forms in epiblast Each daughter cell is called a ▪ Epiblast and hypoblast layers form a flat disc blastomere between the amniotic cavity and the blastocyst cavity. The zygote will travel through The Trophoblast at the embryonic pole differentiates into the fallopian tube towards the an outer Syncytiotrophoblast and an inner uterine cavity and will reach Cytotrophoblast layer. the 4-cell stage at about 40 → Trophoblast hours from fertilization ▪ Cytotrophoblast (G. kytos, cell) ▪ Syncytiotrophoblast (G. syn, together + kytos, cell) − Responsible for the invasion of the endometrium and the underlying connective tissue. The 8-cell stage is reached at about 2.5 days The zygote remains the same size because of the persistent zona pellucida. Each blastomere becomes smaller in size after every mitotic division. The morula is formed and at Day 3 will reach the opening to the uterine cavity. DAY 9 The early blastocyst stage is The whole blastocyst is practically implanted within the reached at about Day 4.5 endometrial wall with a fibrin coagulum covering its point The inner cell mass of entry (embryoblast) is pushed to Trophoblastic lacunae form in syncytiotrophoblast one pole by the fluid within the Cytotrophoblast layer extends around abembryonic pole blastocyst cavity. (pole opposite the embryonic pole) The outer cell mass Blastocyst cavity → primitive/primary yolk sac (trophoblast) forms the wall. → Exocoelomic (Heuser’s) membrane formed [F. exo, outside + koiloma, hollow (cavity)] Originated from the hypoblast layer Line the inner surface of the cytotrophoblast ANATOMY Intro to Embryology Page 6 of 23  Embryo suspended by connecting stalk → The future umbilical cord → Only place where the developing embryo communicates with the trophoblast layer is in the remaining Extraembryonic mesoderm forming the connecting stalk B. SUMMARY: 2ND WEEK Day 7 of Implantation Implantation of the blastocyst usually occurs 6-8 days after fertilization DAY 11-12 Uteroplacental circulation is established by invasion of the the maternal uterine vasculature to the lacunae in the syncytiotrophoblast layer Extraembryonic mesoderm forms → Between the inner surface of the cytotrophoblast and the outer surface of the exocoelomic cavity Extraembryonic coelom forms within this layer → Proliferates and eventually fills up all the space Formation of trophoblast and embryoblast between the cytotrophoblast externally and the By the end of Day 8, the blastocyst has burrowed into the amnion and exocoelomic membrane internally endometrium of the uterus → Do not give rise to any structure of the embryo At this time, it is composed of 2 main components: → Large cavities form within this layer and become → Outer cell mass (trophoblast) confluent as the extra embryonic cavity or coelom → Inner cell mass (embryoblast) Coelom separates extraembryonic mesoderm into 2 layers: → Splanchnic (or splanchnopleure) [G. splanchnon, viscus] Inner extraembryonic → Somatic (or somatopleure) [G. soma, body] Outer extraembryonic Formation of cytotrophoblast and syncytiotrophoblast As the trophoblast makes contact with the endometrium it differentiates into 2 layers: → Inner cytotrophoblast → Outer syncytiotrophoblast Formation of hypoblast and epiblast from bilaminar germ disc The embryoblast differentiates into a bilaminar embryonic disc composed of 2 cell layers: → Hypoblast DAY 13 → Epiblast Syncytiotrophoblast surrounds the embryo and primary villi form Formation of amniotic → Surrounds whole embryo with trophoblastic lacunae cavity and maternal sinusoids present in the embryonic and Soon after the embryonic the abembryonic poles → primary villi formation disc has formed, a cavity Extraembryonic cavity becomes bigger and pinches the appears between the primitive yolk sac epiblast and the Hypoblast migrates to line inside of primitive yolk sac → cytotrophoblast secondary or definitive yolk sac Remnants of primitive yolk sac → exocoelomic cysts Extraembryonic somatic mesoderm (lines the inside of the cytotrophoblast) → chorionic plate Extraembryonic coelom → chorionic cavity ANATOMY Intro to Embryology Page 7 of 23  as the the embryonic mesoderm Formation of chorionic cavity Large cavities begin to appear in the extra embryonic mesoderm Formation of exocoelomic membrane and primitive which gradually fuse to yolk sac form one single cavity Cells originating from the hypoblast begin to migrate called the chorionic cavity forming a thin membrane which covers the inner surface Formation of the of the cytotrophoblast secondary yolk sac → Termed as the exocoelomic membrane Around 13 days after The exocoelomic membrane and cells of the fertilization, a large portion hypoblast form the walls of the primitive yolk sac of the exocoelomic cavity By Day 9, the blastocyst is completely embedded in the is pinched off, forming a uterus wall smaller cavity, the Growth of secondary yolk sac syncytiotrophoblast and Formation of the cytotrophoblast connecting stalk At this stage of By the end of second development, the growth week of development, the of the syncytiotrophoblast chorionic cavity enlarges and cytotrophoblast is and the bilaminar much quicker than the embryonic disc is joined to bilaminar embryonic disc the trophoblast by a band of extraembryonic mesoderm called the connecting stalk (future umbilical cord) Formation of lacunae and lacunar networks Small holes called lacunae begin to form in the syncytiotrophoblast as it continue to expand By day 12, the lacunae stop growing and fuse to form large interconnecting spaces called lacunar networks Bilaminar germ disc by end of week 2 Formation of maternal sinusoids and uteroplacental “Week of twos” circulation establishment → Trophoblast: cyto- & syncytio- Capillaries in the endometrium surrounding the → Embryoblast: epiblast & hypoblast developing embryo dilate forming maternal sinusoid → Extraembryonic mesoderm: somatic & splanchnic As the syncytiotrophoblast continue to expand, enzymes Blastocyst forms two cavities begging to erode the lining of the sinusoids and uterine → Cavities: amniotic & yolk sac glands allowing maternal blood and secretions to flow The Bilaminar germ disc is referring to the embryoblast into the lacunar networks, establishing a uteroplacental Bilaminar germ disc circulation → ‘Bilaminar’ = two layers The blood and uterine secretions only come into close → ‘Germ‘ = structure would give rise and or form the proximity to the embryo, allowing the exchange of gasses organism 📣 and metabolites → ‘Disc’ = the shape of the structure Formation of Note: the lower figure which shows the epiblast layer extraembryonic mesoderm in blue viewing it from the amniotic cavity, the two layers Around the same time, a of the embryoblast are the epiblast layer and the new population of cells hypoblast that lies underneath. This forms a flat oval or appear between the inner elongated disc somewhat like a lady finger biscuit surface of the between the amniotic cavity and the yolk sac cavity. cytotrophoblast and the outer surface of the primitive yolk sac known ANATOMY Intro to Embryology Page 8 of 23  VI. THIRD WEEK OF DEVELOPMENT → By day 16, majority of the hypoblast would have been A. FORMATION OF TRILAMINAR GERM DISC replaced by the definitive endoderm → The hypoblast cells migrate to line the secondary or GASTRULATION definitive yolk sac. → Some of the invaginated epiblast cell will remain and spread in the space between the epiblast and the newly formed definitive endoderm The remaining cells of the epiblast form the third germ layer, called, the mesoderm/mesoblast Once the formation of the definitive endoderm and mesoderm is complete, epiblast cells no longer migrate (the process of gastrulation) towards the primitive streak → At this point, the remaining cells of the epiblast is now Figure 9. Primitive Streak Formation [Langman, 2019] referred to as the ectoderm or ectoblast forming the Primitive Streak Formation is ~15 days after fertilization. third germ layer → Primitive Streak: a thickened structure formed along → By the end of the third week, the three primary germ the midline of the amniotic surface of the epiblast near layers complete the trilaminar germ disc the caudal end of the bilaminar embryonic disc. Epiblast → Formation of the primitive streak defines and → endoderm/-blast [G. endon, within] establishes the major body axis of the embryo: ▪ First cells to migrate inward Cranial or Rostral End – towards the head ▪ Displace hypoblast cells Caudal End – towards the tail ▪ Syn., entoderm/-blast Left and Right Side of the Embryo → mesoderm/-blast [G. mesos, middle] Dorsal Aspect – amniotic surface where primitive ▪ Follow initial cells to migrate between definitive streak is seen endoderm and epiblast Ventral Aspect – yolk sac side → ectoderm/-blast [G. ektos, outside] (Hensen’s node) at cranial end 📋 → Primitive streak expands to create a primitive node ▪ Remaining epiblast cells Hypoblast the layer, epiblast 📋 Cells migrate underneath, and will spread out to form Contains the circular depression (primitive pit), → Migrate to line secondary yolk sac B. THREE GERM LAYER PERSPECTIVE continuous with the primitive groove − Primitive Groove – runs caudally along the midline of the primitive streak Figure 12. Epiblastic Immigration: Endoderm (Yellow), Mesoderm (Pink), Ectoderm (Blue) [https://embryology.ch/en/embryogenese/embryonic-disk/trilaminar-germ-disk/] Figure 10. Invagination [Moore, 2016] All of these three germ layers arise from the epiblast Invagination Throughout gastrulation, the ectoderm continues to form 📋 → Occurs after primitive groove has formed the cranial to the caudal end of the embryo, establishing → Cells of the epiblast migrate inwards towards the streak, three distinct primary germ layers detach from the epiblast and slip beneath into the Table 1. Germ layer formation interior of the embryonic disc First epiblastic cells to invaginate to the primitive Endoderm Epiblast cells invade hypoblast and displace groove, invade deeply and displace the cells of the Formation its cell. hypoblast layer Mesoderm The initial cells lie between the epiblast and Definitive endoderm: new proximal cell layer Formation newly created endoderm to form mesoderm. replacing hypoblast cells Paraxial mesoderm - specific germ layer − Synonyms for endoderm: endoblast, entoderm, where the dermis of the skin of the back entoblast originated. Ectoderm The remaining cells of the epiblast form the Formation last layer/third germ. Most of the mesodermal cells arise from epiblast cells Invaginate in the primitive groove and spread laterally and then cranially between the endoderm and overlying → 📣 ectoderm represented by the caudal red arrows. The red arrows at the midline directed towards the cranial end shows schematically the migration direction of the epiblast cells coming from the primitive Figure 11. Gastrulation [Langman, 2019] node that would form the notochordal process and later the notochord which is an important structure in ANATOMY Intro to Embryology Page 9 of 23  the differentiation of the different germ cell layers and forms the basis of the axial skeleton. C. SUMMARY: 3RD WEEK The majority of the hypoblast has been replaced. The remaining cells of the epiblast are now referred to as the Ectoderm and form the most exterior distal layer. Gastrulation → Process whereby the Bilaminar embryonic disc undergoes reorganization to form a trilaminar disc → Throughout the third week of development, this Bilaminar disc differentiates to establish three primary germ layers Some of the invaginated epiblast cells remain in the space between the ectoderm and newly formed definitive endoderm. These cells form a germ layer known as the Mesoderm. Once the formation of the definitive endoderm and mesoderm are complete, Primitive streak epiblast cells no longer migrate towards the primitive → A thickened structure forms along the midline in the streak. epiblast near the caudal end of the Bilaminar embryonic disc → The formation of the primitive streak defines the major body axis of the embryo including the cranial end towards the head and caudal ends towards the tail. As well as the left and right sides of the embryo. At the cranial end of the embryonic disc the primitive streak expands to create a primitive node which contains a circular depression known as a primitive pit. Throughout gastrulation the ectoderm continues to form from the cranial to the caudal end of the embryo establishing three distinct primary germ layers through the whole embryonic disc. The gastrulation process is finally complete. In summary… Gastrulation [G. gaster, belly] → Third week is primarily characterized by the Primitive pit continues along the midline of the epiblast formation of the trilaminar germ disc: towards the caudal end of the streak forming a primitive 1. Primitive streak groove. Once formed, cells of the epiblast migrate 2. Epiblastic Immigration/ Invagination: inwards towards the streak detach from the epiblast and 2.1. Endoderm slip beneath it into the interior of the embryo. The 2.2. Mesoderm process is called Invagination. 2.3. Ectoderm The first cells to invaginate through the primitive groove → The first cells to invaginate through the primitive invade the hypoblast and displace its cells. The hypoblast groove form the definitive endoderm cells are eventually completely replaced by a new → The remaining cells of the epiblast are called the proximal cell layer which is referred to as the definitive ectoderm endoderm. → Cells that remain in the space between ectoderm and definitive endoderm for a layer called the mesoderm Trophoblastic Formation ANATOMY Intro to Embryology Page 10 of 23  → Along with gastrulation, the concomitant Formation of germ layers development of the placenta from the trophoblast also occurs (and will be further discussed in the reproductive system module) D. SUMMARY: FERTILIZATION TO FORMATION OF TRILAMINAR GERM DISC Formation of neural tube Unfertilized egg cell in ovary Appearance of appendages Sperms cells traveling down the fallopian tube Millions of sperm travel along the fallopian tube and several will try to penetrate the egg cell but only one will “win” and enter the nucleus. Formation of head and eye Sperm cell penetrating the egg cell and entering the nucleus A reprogramming process occurs where the male and Formation of small embryo female nuclei have their genes set aside to be turned on and off for early development. Early cleavage state one of the early growth phases) as the embryo moves down the fallopian VII. DIFFERENTIATION OF GERM LAYERS tube. This will form an A. NOTOCHORD FORMATION important stage called the Notochord [G. notos, back + chordē, cord or string] blastocyst → Mesodermal formation arising from the prenotochordal Blastocyst takes about 5 days cells in the primitive pit to form → Inducing formation of the central nervous tissue from overlying ectoderm → Forms midline axis and is the basis of axial skeleton Inner cell mass (ICM) cells that make the entire animal outer cells give rise to the placenta and other supporting tissues Embryo implanted to the wall of the uterus where pregnancy is really initiated ICM forming a disc As the cells continue to grow they change their physical positions/geographical relationship to one another Figure 13. Notochord formation [Langman, 2019] “Disc” formed by ICM 📣 DAY 21 Disc is transformed into an Genesis of notochordal process through invagination embryo of epiblast cells that come from the primitive node roughly The lines represent sites on the 21st day where cells are migrating in and out ANATOMY Intro to Embryology Page 11 of 23  Figure 14. Genesis of notochordal process (1) [Lecturer’s PPT] 📣 DAY 23 Figure 16. Genesis of notochordal process (3) [Lecturer’s PPT] Notochordal process extends to the prechordal plate FORMATION OF THE NOTOCHORD (VIDEO) 📣 and consist of chordal mesoderm with a central axial canal At this point in time, the chordal process fuses with the endoderm lying below it, while the chordal process and endoderm are merged for a short time (~1 day) the amniotic cavity communicates with the yolk sac cavity via neurenteric canal. The ectoderm lying above the notochord also starts to undergo changes to form the neural plate, later becoming the neural groove Figure 17. Developing embryo [Lecturer’s PPT] The left image is a sagittal view along the midline axis of a developing embryo. The right image is a cross sectional view at the plane marked by the dotted line. Figure 15. Genesis of notochordal process (2) [Lecturer’s PPT] 📣 DAY 28 Between the 22nd and 24th day, the notochordal process cuts itself off from the endoderm. The endoderm fuses again in order to form a complete cord, the 📣 Figure 18. Formation of notochordal process [Lecturer’s PPT] From the primitive node, prenotochordal cells invaginate and move forward cranially forming a midline definitive notochord. tube that reaches the prechordal plate called the → This finds itself in the middle of the mesoderm → The endoderm under the notochord comes together again closing off the neurenteric canal and the 📣 notochordal process. These cells intercalate with the hypoblast layer to form the notochordal plate so that at midline the embryo 📣connection between the amniotic and yolk sac cavities. By day 28, the notochord is then able to elongate to the caudal regions as the primitive streak recedes towards a would have two cell layers. The notochordal plate extends cranially to the prechordal plate and caudally to the primitive pit more caudal position. → Notochord plays a role in the induction of the ectoderm that lies over it to become nervous tissue → Neural groove has partly fused to become the neural tube → Notochord also plays a role in the genesis of the vertebral body of the axial skeleton 📣 [Lecturer’s PPT] Figure 19. Formation of notochordal plate At the point where the primitive pit forms, the neurenteric canal temporarily connects the amniotic and yolk sac cavities. ANATOMY Intro to Embryology Page 12 of 23  Formation of neural fold The neural plate forms at the cranial end of the embryo and grows in a cranial to caudal direction. ○ cranial or head end of the neural plate - region of the future brain ○ caudal or tail end - future region of the spinal cord Formation of definitive notochord By the end of the 3rd week 📣 Figure 20. Formation of definitive notochord [Lecturer’s PPT] The endoderm cells from the primitive streak replace the hypoblast and the notochord cells of development the lateral edges of the neural plate detach from the endoderm. The endoderm fuses become elevated and moved closing off the neurenteric canal. The definitive together to form the neural notochord is formed in the midline between the folds. The resulting space ectoderm and the endoderm. The notochord can created by the folding of the then grow to the caudal regions as the primitive neural plate is called the streak recedes. neural groove. Formation of the neural B. NEURULATION groove It is the transformation of ectodermal cells into precursor The neural folds fuse cells of the nervous system via the inductive effect of together and the neural notochord on ectoderm plate transforms into the → Process where neural plate forms neural tube neural tube, the precursor → Neural tube later becomes the Central Nervous System to the central nervous Marks the beginning of the formation of the central system. Fusion of the nervous system and is the process whereby the neural neural tube usually begins in plate forms into a neural tube the middle of the embryo extending in both cranial and caudal directions. Formation of the neural crest During the closure of the neural tube cells on the crest of the neural folds detach forming a new cell population → These cells contribute to the formation of the peripheral nervous system → once the neural tube has completely fused the process of neurulation is complete Figure 21. Neurulation [Lecturer’s PPT] SUMMARY EVENTS OF NEURULATION Neurulation occurs during the third week of embryonic development. Formation of neural plate The transformation of the neural plate into a neural tube Thickened areas of the cell marks the beginning of the formation of the central nervous system. → A neural place of thickened cells forms in a cranial to caudal direction → The lateral edges of the neural plate elevate and fuse to form the neural tube → Neural crest cells contribute to the formation of the peripheral nervous system ANATOMY Intro to Embryology Page 13 of 23  📣 📣 C. NEURAL CREST MIGRATION Through dermis: Enters ectoderm through holes of the With the induction by the notochord on the neural basal lamina to form: ectoderm to become the neural plate, the lateral borders of Melanocytes in skin the neural plate elevate and form the neural folds. Neural Hair follicles crest cells form at the tips of these neural folds, they → Ventral pathway: (through anterior half of each somite) remain at this region and do not migrate away until the Spinal (dorsal root) ganglia neural tube closure is complete. Once the neural folds Sympathetic chain of preaortic ganglia elevate and fuse to form the neural tube, the neural crest Parasympathetic ganglia of GIT cells begin to dissociate from their neighboring neural Schwann cells ectoderm once the neural tube closes. This population of Adrenal medulla cells neural crest cells undergoes a transition from epithelial to C. MESODERMAL DERIVATIVES mesenchymal configuration as it leaves the neural 📣 MESODERMAL DIFFERENTIATION ectoderm by active migration and displacement to enter the underlying mesoderm. Changes occur simultaneous to the formation of the Mesenchyme refers to loosely organized embryonic neural tube due to the presence/development of the connective tissue regardless of origin. After migration, notochord neural crest cells would then differentiate and contribute to a heterogenous array of structures including the dorsal root ganglia, sympathetic chain ganglia, adrenal medulla and other tissues. These structures, even if they are far from the neural tube, are considered neural crest derivatives. Figure 23. Transverse sections showing the development of Figure 22. Neural crest and its derivatives [Langman, 2019] the mesodermal germ layer (A) day 17; (B) day 19; (C) day VIII. DERIVATIVES OF THE THREE GERM LAYERS 📣 20; (D) day 21 [Langman, 2019] Initially, cells of mesodermal germ layer forms thin sheet of loosely woven tissue on each side of the midline. A. ECTODERMAL DERIVATIVES The ectodermal germal layer gives rise to organs and structures that maintain contact with the outside world: → CNS and PNS → Sensory epithelia of eyes, ears, and nose → Neural crest derivatives 📣 → Epidermis and its derivatives (e.g. hair, nails) Figure 24. Mesodermal differentiation [Lecturer’s, PPT] → Mammary glands and other subcutaneous glands (e.g. Day 17: Cells close to the midline proliferate and form a sweat glands, sebaceous glands) thickened plate of tissue known as paraxial mesoderm. → Pituitary gland (both oral and neural ectoderm) Laterally the mesodermal layer remains thin and is known 📣 → Enamel of teeth as the lateral plate mesoderm. B. NEURAL CREST DERIVATIVES Intermediate mesoderm - connects the paraxial and Derivatives depend on region of origin lateral plate mesoderm From head/cranial neural folds: → Craniofacial skeleton (connective tissue & bones of face and anterior skull) → Dermis of face and neck → Neurons for cranial nerve ganglia → Arachnoid and pia mater (leptomeninges) Meningeal covering of the brain → Glial cells of the PNS → C cells of the thyroid gland 📣 [Lecturer’s, PPT] Figure 25. Mesodermal differentiation → Conotruncal septum of the heart With the appearance and coalescence of intercellular → Vascular smooth muscles of face and forebrain cavities in the lateral plate, this tissue is divided into 2 → Odontoblasts (dentin-forming cells of teeth) layers: 📣 From neural folds of the trunk region: Region becomes the spinal cord. Leaves the neural ectoderm after closure of neural tube and migrates along one → Somatic/Parietal Mesoderm - Layer continues with the mesoderm covering the amnion → Splanchnic/Visceral Mesoderm - Continues with the of two pathways: mesoderm covering the yolk sac → Dorsal pathway: ANATOMY Intro to Embryology Page 14 of 23  📣 Together, the 2 layers line a newly formed cavity: Migrate beneath the ectoderm Intraembryonic cavity - continues to the extra embryonic cavity on each side of the embryo By beginning of third week, paraxial mesoderm begins to Process: 📣 Remaining dorsal portion → dermis neck and trunk Both groups of muscle precursor cells become mesenchymal cells and migrate beneath dermatome: be organized into segments called somitomeres that are → Myotome from ventrolateral region forms hypomere further organized to somites > migrates to the parietal layer of lateral plate mesoderm > gives rise to the skeletal muscle of lateral body wall and most limbs → Myotome of dorsomedial region forms the epimere > gives rise to skeletal muscle of back and extensors of the vertebral column → Remaining dermatome cells in dorsal portion become mesenchymal cells > migrate beneath the ectoderm > forms the dermis of the neck and trunk Figure 26. Mesodermal differentiation [Lecturer’s, PPT] Figure 27. Cross-section of Developing Embryo [Lecturer’s, PPT] SOMITE DIFFERENTIATION Somites arise from the segmental differentiation of 📣 somitomeres of paraxial mesoderm First appears at the occipital region approximately on 📣 the 20th day of development New somites appear in a cranial-to-caudal formation at a rate of 3 pairs a day until a total of 42-44 pairs are present at the end of the 5th week. Forms pairs of: → 4 Occipital Figure 28. Stages of somite development [Lecturer’s, PPT] → 8 Cervical → 12 Thoracic GROSS ANATOMY OF DERMATOME AND MYOTOME → 5 Lumbar Dermatome:Cutaneous (skin) sensory territory of a single → 5 Sacral spinal nerve Myotome: Mass of muscle innervated by a single spinal 📣 → 8-10 Coccygeal First occipital and last 5-7 coccygeal somites would nerve 📣 later degenerate/disappear. Remaining somites form the spinal column: → 31 pairs of spinal nerve: C1-8, T1-12, L1-5, S1-5, and 1 INNERVATION OF DERMATOME AND MYOTOME Each myotome and dermatome retains its innervation from its segment of origin no matter where the cells migrate. 📣 coccygeal pair Because somites appear with a specific periodicity, the age of an embryo can be accurately determined by Each somite forms its own sclerotome or tendons, cartilage, and bone components, its own myotome providing the segmental muscle component, and its own counting the somites. dermatome which forms the dermis of the back. Each myotome and dermatome has its own segmental nerve 📣 STAGES OF SOMITE DEVELOPMENT Paraxial mesoderm cells exist as a ball of mesoderm > Undergoes epithelialization (arranged as a donut-shaped component. This leads to dermatome maps. around a small cavity) Sclerotome: [G. sklēros, hard + tomē, cutting or segment] → Cells from ventral and medial somite wall migrating towards neural tube and notochord as they lose their 📣 epithelial arrangement → forms the vertebrae and ribs Dermomyotome → Myotome [G. mys, muscle] Migrate beneath the dermatome Migrate from ventrolateral hypomere & dorsomedial epimeres → skeletal muscle to the body wall/limbs and back, respectively Figure 29. Innervation of dermatomes & myotomes[Lecturer’s PPT] → Dermatome [G. derma, skin] ANATOMY Intro to Embryology Page 15 of 23  📣 DERMATOME MAPS → Plexus formation: mixing of nerves from different cord Shows sensory distribution of each spinal nerve on levels by union and division of bundles; causes disparity the skin surface of the body between dermatomal map and cutaneous distribution of Based on clinical findings of deficits in cutaneous peripheral nerves. sensation Cervical plexus: C1-C5 Diagnostic aids: localization of spinal cord lesions & levels Brachial plexus: C5-T1 of spinal anesthesia Lumbar plexus: L1-L4 Limits to specificity due to overlap of dermatomes Sacral plexus: L4-S4 Figure 32. Dermatome vs Map of name peripheral nerves [Lecturer’s PPT] 📣 MYOTOME NERVE FORMATION When myotomes migrate to the trunk and limbs, they fuse and merge to form skeletal muscle at different joints while maintaining their nerve supply from their spinal 📣 segment of origin. Due to the plexus formation, the nerve fiber from different spinal cord levels make up different peripheral nerves that innervate different muscle groups. Although muscles may be innervated by different peripheral nerves, they may be responsible for the same function. These peripheral nerves carry the same fibers from the same spinal cord level or nerve root. Even though myoblast cells from the same myotome migrate to different areas, they still maintain the nerve connection to their original somitic Figure 30. Dermatome Map [Netter, 2019] 📣 segment. Ex. Despite Supraspinatus and Deltoid being innervated 📣 MYOTOME MAPS Groups of skeletal muscles innervated by specific spinal cord levels are tied to particular functions by different peripheral nerves (suprascapular nerve and axillary nerve respectively), it has the same function (abduction and lateral rotation) because it has a common →