Embryo 1

Week 3 - Gastrulation

• Gastrulation: the process by which the three germ layers of the embryo are established
  • Ectoderm, Mesoderm, and Endoderm
• Bilaminar embryonic disc becomes trilaminar embryonic disc
• Embryo may be referred to as a gastrula

Process of Gastrulation

• Formation of the primitive streak appears at the beginning of the 3rd week
  • Thickened linear band in the median plane of the dorsal aspect of the embryonic disc
  • Initiates in the caudal region of the epiblast
• Cells at the cephalad end of the primitive streak proliferate to form a primitive node
• A narrow groove, the primitive groove, develops in the primitive streak
  • Continuous with a small depression in the primitive node, the primitive pit

Embryonic Mesoderm

• Cells leave the deep surface of the streak and form mesenchyme
  • Embryonic connective tissue which forms the supporting tissues of the embryo
  • Some mesenchyme forms mesoblastic cells (undifferentiated mesoderm)
  • The mesoblasts form the intraembryonic, or 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
  • Differentiate into diverse types of cells, such as fibroblasts, chondroblasts, and osteoblasts

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 1: appearance begins with fertilization, from zygote to blastocyst
  • Inner cell mass (embryoblast), trophoblast, and blastocoel
  • "Hatching" from zona pellucida, first appearance of syncytiotrophoblast and cytotrophoblast
• Week 2: 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
• Week 3: gastrulation, bilaminar disk to trilaminar disk
  • Three germ layers: ectoderm, mesoderm, and endoderm
  • Notochord forms, and then the following develop: neural groove, neural plate, early neural tube
  • Paraxial mesoderm + somites, tertiary chorionic villi, heart tube, primordial circulation develop
  • Developing villi surround elongated, disk-shaped embryo with a few somites

Introduction to Embryology

• Why learn embryology?
  • Fascinating field of biology
  • We start out as one generalized, totipotential cell to 100 trillion cells
  • Massive changes occur within minutes after birth
• Clinical relevance: essential to understanding developmental/congenital disorders
  • Immune system, cardiovascular, respiratory, nervous system, reproductive system
  • Infertility, referred pain

Implantation

• The zona pellucida degenerates (day 5)
• The blastocyst adheres to the endometrial epithelium (day 6)
• The trophoblast differentiates into two layers: 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 (days 11 and 12)
• The defect in the endometrial epithelium is repaired (days 12 and 13)
• Primary chorionic villi develop (days 13 and 14)

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### Meiosis and Fertilization
• A diploid cell (germ cell) undergoes meiosis to produce a unique haploid gamete
• Crossing-over between maternal and paternal chromatids during prophase I results in "mixed" chromatids with both paternal and maternal genes
• Meiosis is not completed in an oocyte until the sperm penetrates the oocyte

Fertilization

• Fertilization occurs when the spermatic pronucleus and the oocyte pronucleus fuse, resulting in a diploid cell
• The single diploid cell is known as a zygote
• Fertilization typically occurs in the ampulla of the uterine tube

Ovulation

• A person with ovaries is born with a certain number of diploid oocytes that have been "paused" during the first stage of meiosis
• After puberty, the ovaries release an ovum each cycle into the uterine tubes
• The oocyte will not complete meiosis II unless fertilization occurs

Early Embryology

• The blastocyst hatches from the zona pellucida and undergoes implantation in the uterine endometrium
• The blastocyst is composed of a trophoblast, an embryoblast, and a blastocoel
• The trophoblast develops into syncytiotrophoblast and cytotrophoblast
• The embryoblast develops into a bilaminar disk consisting of epiblast and hypoblast

Week 1

• Day 1-3: The zygote develops into a morula, a 12-32 cell stage
• Day 4-5: A fluid-filled cavity develops within the embryo, and four separate structures can be noted: trophoblast, embryoblast, blastocoel, and zona pellucida
• Day 5-6: The blastocyst hatches from the zona pellucida and undergoes implantation in the uterine endometrium
• Day 6-7: The syncytiotrophoblast invades into the endometrial stroma, and the embryoblast differentiates into two distinct layers: epiblast and hypoblast

Week 2

• Day 10: Implantation of the embryo is complete
• The embryo is completely embedded within the endometrium, surrounded by syncytiotrophoblast cells
• The stromal cells undergo decidualization, accumulating glycogen and lipids throughout the uterus
• The epiblast and hypoblast extend around the entire interior surface of the blastocoel

Week 3

• The epiblast enlarges and gives rise to amnioblasts
• The hypoblast extends around the entire interior surface of the blastocoel
• The mesoblasts form the intraembryonic, or embryonic, mesoderm
• Gastrulation occurs, resulting in the formation of three embryonic germ layers: ectoderm, endoderm, and mesoderm
• The notochord forms, and then the following develop: neural groove, neural plate, early neural tube, paraxial mesoderm, and somites

Extra-Embryonic Structures

• The umbilical vesicle (yolk sac) forms from the hypoblast
• The amniotic cavity forms from the epiblast
• The extra-embryonic coelom forms from the fluid accumulation between the extra-embryonic mesodermal cells
• The chorion forms from the extraembryonic somatic mesoderm and trophoblast
• The chorionic sac encloses the embryo and its cavities, and is surrounded by the syncytiotrophoblast

Development of Chorionic Villi

• The cytotrophoblast sends extensions to the lacuna to form primary villi
• The primary villi are precursors of functional placental villi
• The syncytiotrophoblast contacts endometrial blood vessels, and the blood vessel deteriorates, forming "little lakes" or lacunar networks
• The oxygenated maternal blood and glycogen/lipids from deteriorating decidual cells nourish the embryo