Early Weeks Notes
Osmosis High-Yield Notes
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NOTES NOTES EARLY WEEKS HUMAN DEVELOPMENT DAYS 1–4 osms.it/human_development_days_1-4 FERTILIZATION ▪ Oocyte, spermatozoa fuse → zygote ▫ Oocyte viable 12–24 hours after ovulation; sperm cells retain fertilizing power 24–48 hours after ejaculation ▫ Coitus must occur no more than two days before/24 hours after ovulation for fertilization ▪ Ejaculation → 200 million spermatozoa enter vaginal canal → alkaline seminal ﬂuid neutralizes acidic vaginal ﬂuid ▪ Only 1% enter cervix → travel through uterus → ampullary region of uterine tube (most likely place for fertilization) ▪ Cervix → oviduct ▫ 30 minutes to 6 day journey Figure 26.1 Sperm pathway through the uterus. 196 OSMOSIS.ORG ▪ Ovulation → spermatozoa driven by chemoattractants (produced by cumulus cells of oocytes) to ampulla of uterine tube Two required processes ▪ Capacitation: epithelial interactions between sperm, uterine wall ▫ Glycoprotein coat, seminal plasma proteins covering acrosomal region removed → easier enzyme release → acrosomal reaction ▪ Acrosomal reaction: after binding to zona pellucida ▫ Release of enzymes (e.g. acrosin, hyaluronidase) needed to penetrate zona pellucida
Chapter 26 Embryology: Early Weeks PHASES OF FERTILIZATION Phase I: penetration of corona radiata ▪ Capacitated spermatozoa allowed to pass through corona radiata Phase II: penetration of zona pellucida, sperm binding ▪ Zona pellucida: glycoprotein layer surrounding oocyte; AKA “jelly coat” ▫ Facilitates binding of sperm cell, induces acrosomal reaction mediated by ligand zona pellucida sperm-binding protein 3 (ZP3) ▪ Approx. 500 spermatozoa arrive at this layer ▪ Sperm-binding initiates release of acrosin (hydrolytic enzyme) → sperm cell penetrates zona pellucida → sperm makes contact with oocyte → cortical reaction (release of lysosomal enzymes from cortical granules of oocyte) → cortical granules initiate zona reaction, prevent further sperm penetration (polyspermy) by forming protective hyaline layer, inactivate receptor sites on zona pellucida ▫ Cortical reaction also activates oocyte to prepare for second meiotic division Phase III: fusion of oocyte, sperm cell ▪ Interactions between integrins, ligands → adhesion of sperm, oocyte ▫ Fusion of sperm, egg plasma membranes ▪ Secondary oocyte completes meiosis II → forms female pronucleus, second polar body ▪ Head, tail of spermatozoa enters oocyte → travels to female pronucleus (containing 23 chromosomes) using tail, energy generated by mitochondria ▪ Tail, mitochondria detach → sperm nucleus becomes male pronucleus ▪ Male, female pronuclei move toward each other → merge into single nucleus → cell becomes diploid (zygote contains maternal, paternal genetic information) ▪ Preparation for mitotic division ZYGOTE TO BLASTOCYST IMPLANTATION Cleavage ▪ Series of fast mitotic divisions of zygote → increase number of cells, decrease size ▪ 36 hours after fertilization → ﬁrst cleavage division → two cells (blastomeres) ▫ Second division → four blastomeres; third division → eight blastomeres; etc. ▪ After third cleavage, blastomeres form compact ball of cells connected by tight junctions (compaction) ▪ Three days after fertilization, cells of compacted embryo divide again → mulberry-shaped 16-cell morula (composed of two zones: inner, outer cell mass) ▪ Four to ﬁve days after fertilization, embryo consists of approx. 100 cells ▪ Fluid accumulates within internal cavity (blastocoel) → blastocyst ▪ Blastocyst: ﬂuid-ﬁlled hollow cell, two zones ▫ Trophoblast: single layer of large ﬂattened cells, stemming from morula’s outer cell mass; gives rise to placenta ▫ Embryoblast: 20–30 pluripotent cells located on one side, stemming from inner cell mass; gives rise to embryo OSMOSIS.ORG 197
Figure 26.2 Phases of fertilization. Phase I: sperm penetrates corona radiata. Phase II: penetration of zona pellucida, sperm binding. Phase III: fusion of sperm, oocyte; pronuclei fuse to form diploid zygote cell. 198 OSMOSIS.ORG
Chapter 26 Embryology: Early Weeks Figure 26.3 Process of going from zygote to blastocyst. HUMAN DEVELOPMENT DAYS 4–7 osms.it/human_development_days_4-7 DAY 7 Implantation ▪ Trophoblast binds to uterine wall with L-selectin, integrin receptors ▫ Penetrates between epithelial cells ▪ Uterus at implantation in secretory phase ▫ High progesterone released from corpus luteum develops endometrium for implantation ▫ Blastocyst implants into decidua basalis, along superior posterior wall of uterus Figure 26.4 Implantation, syncytiotrophoblast proliferation, and development of spiral arteries under the inﬂuence of progesterone secreted by the corpus luteum. OSMOSIS.ORG 199
HUMAN DEVELOPMENT WEEK 2 osms.it/human-development-week-2 DAY 8 Trophoblast ▪ Proliferates, forms two layers ▪ Cytotrophoblast (cellular trophoblast): inner layer of mononucleated cells ▫ Produces primary chorionic villi, protrudes into syncytiotrophoblast ▪ Syncytiotrophoblast: outer multinucleated mass of cells (without distinct cell boundaries) ▫ Invades decidua basalis with ﬁnger-like processes; makes enzymes that erode uterine cells; blastocyst burrows into decidua basalis surrounded by pool of blood leaked from degraded blood vessels ▫ Human chorionic gonadotropin (hCG) maintains viability of corpus luteum → secretes estrogen, progesterone until week eight (hCG: basis for pregnancy tests) Embryoblast ▪ Differentiates into two layers, forms ﬂat disc ▪ Hypoblast: small cuboidal cells adjacent to blastocyst → yolk sac ▪ Epiblast: columnar cells ▫ Cavity forms inside → amniotic cavity ▫ Lined with amnioblasts DAY 9 ▪ Lacunar stage of trophoblast development ▫ Vacuoles appear in syncytium → vacuoles fuse → form large empty spaces (lacunae) ▪ At abembryonic pole, ﬂattened cells (from hypoblast) form exocoelomic (Hauser) membrane → line inner surface of cytotrophoblast ▫ Hauser membrane, hypoblast line exocoelomic cavity (primitive yolk sac) 200 OSMOSIS.ORG DAY 12 ▪ Progesterone levels continue to rise → decidua undergoes decidual reaction ▫ Decidual cells enlarge, become coated in sugar-rich ﬂuid (helps sustain embryo) ▫ Blastocyst embeds in endometrial stroma ▫ Lacunae form within syncytiotrophoblast (erodes endometrial sinusoids) ▫ Lacunae fuse with sinusoids → ﬁll with maternal blood → uteroplacental circulation established DAY 13 ▪ Secondary yolk sac forms within exocoelomic cavity ▪ Hypoblast cells: differentiate into extraembryonic mesoderm cells outside embryo ▫ Mesoderm cells: line inside of cytotrophoblast, syncytiotrophoblast; line chorionic cavity ▪ Epiblast: gives rise to embryo’s germ layers (endoderm, mesoderm, ectoderm) ▪ Amniotic cavity develops above bilaminar disk, becomes lined with epiblast cells
Chapter 26 Embryology: Early Weeks Figure 26.5 Summary of the growth that occurs during the second week of development. HUMAN DEVELOPMENT WEEK 3 osms.it/human-development-week-3 DAY 14 ▪ Syncytiotrophoblast cells form little protrusions called primary villi ▪ Villi form around fetus; lacunae form between villi ▪ Arteries, veins merge within lacunae → form large pool of blood ( junctional zone) ▪ Villi submerged within junctional zone Gastrulation ▪ Major event, establishes three germ layers ▪ Begins with formation of primitive groove (narrow depression into center of epiblast layer) ▫ Starts at caudal end, grows towards cranial end → cranial-caudal axis ▫ Groove forms on dorsal side of embryo → dorsal-ventral axis ▫ Two sides of groove: left, right side of body (bilateral symmetry) ▪ Primitive node forms at cephalic end of primitive groove ▫ Contains primitive pit, surrounded by slightly elevated area of ectoderm ▪ Primitive groove, node, pit → form primitive streak ▪ Epiblast cells migrate towards primitive groove → move to bottom, slide under (invagination) ▪ After invagination, cells differentiate into three new layers of embryonic disc (trilaminar disc) ▪ Cells of trilaminar disc multipotent (ability to differentiate into many tissues, organs) ▫ Some epiblast cells displace ventral hypoblast layer, form endoderm ▫ Invaginated epiblast cells between newly formed endoderm, epiblast → mesoderm layer ▫ Rest of epiblast forms ectoderm layer DAY 15 ▪ Two areas of ectoderm layer (cranial, caudal region) push ventrally, fuse with endoderm (exclude mesoderm layer) → form two bilaminar regions in otherwise trilaminar disc ▫ Cranial bilaminar region develops into oropharyngeal membrane → disintegrates in fourth week to form mouth opening ▫ Caudal bilaminar region develops into cloacal membrane → disintegrates in seventh week to form anal opening, genitourinary tracts OSMOSIS.ORG 201
Figure 26.6 Day 14: formation of the primitive streak and trilaminar disc. DAY 17 ▪ Group of mesoderm cells form solid rod (notochord) ▫ Notochord: transient embryonic structure (nucleus pulposus of intervertebral disc: remnant in adult life) ▫ Solid structure → helps inﬂuence how embryo folds ▫ Secretes protein called Sonic HedgeHog (SHH) → guides tissue differentiation penetrate primary villi → grow toward decidua ▫ Tertiary villi: form when mesodermal cells differentiate into small blood vessels → form villus capillary system → fetal contribution to placenta DAY 20 ▪ Mesoderm cells around notochord differentiate into three specialized types of cells ▫ Paraxial mesoderm, intermediate mesoderm, lateral plate mesoderm; make different tissues, organs ▪ Notochord starts process called neurulation → stimulates cells of ectoderm to form neural plate ▪ Neural plate folds, forms neural groove with edges called neural folds ▪ Neural plate continues to grow, neural folds come together, pinch off from surface of ectoderm to form neural tube between ectoderm, mesoderm ▪ Trophoblast continues to develop: vasculogenesis ▫ Primary villi: made up of cytotrophoblastic core covered by syncytial layer ▫ Secondary villi: form when extraembryonic somatic mesoderm cells 202 OSMOSIS.ORG Figure 26.7 Day 15: bilaminar regions of trilaminar disc. Figure 26.8 Day 17: formation of notochord from mesoderm cells.
Chapter 26 Embryology: Early Weeks Figure 26.9 Day 20: differentiation of mesoderm near neural plate into paraxial, intermediate, and lateral plate mesoderm; formation of neural tube in process called neurulation. Figure 26.10 During week 3, extraembryonic mesoderm cells migrate into the primary villi, forming secondary villi. The secondary villi differentiate into fetal vessels known as the villous capillary system, which is the fetal contribution to the placenta. OSMOSIS.ORG 203
Osmosis High-Yield Notes
This Osmosis High-Yield Note provides an overview of Early Weeks essentials. All Osmosis Notes are clearly laid-out and contain striking images, tables, and diagrams to help visual learners understand complex topics quickly and efficiently. Find more information about Early Weeks by visiting the associated Learn Page.