Lecture 1: Embryology I Flashcards
Mucosa
General term for the innermost layer of an organ/tissue
Endometrium
Uterine mucosa layer
Decidua
Uterine mucosa layer during pregnancy; changes from endometrium due to implantation (basalis, capsularis, parietalis)
Stratum functionalis (functional layer)
Apical endometrial sublayer that is built up and shed each menstrual cycle. Composed of a simple columnar epithelium and lamina propria.
Stratum basalis (basal layer)
Basal sublayer of endometrium that is not shed during menstruation. Contains stem cells to regenerate the functional layer and comprised of loose connective tissue.
Lamina propria
Loose, spongy connective tissue that lies underneath the epithelium in many mucosae
Tissue categories
Epithelia, connective, muscle, neurons
Epithelia tissue
Tissue sheets connected with junctional proteins that have apical and basal polarity. The apical side is often against a space/compartment, the basal side is often against connective tissue or another basal surface
Connective tissue
Tissue that supports, protects, and gives structure to other tissues/organs. Always comprised of fluid, fiber, and protein components with varying relative amounts (e.g. blood vs. ligaments)
Muscle tissue
Cells whose primary function is to exert force on surrounding tissues
Neurons
Electrically active cells that transmit information via electrical impulses or short-range chemical signals
Ovaries
Paired structure, site of oogenesis
Ovarian follicle
Oocytes develop within growing follicles and are then released during ovulation
Ovulation
The cyclical release of oocytes, the corona radiata, and the zona pellucida to the abdominal cavity
Fallopian or uterine tubes
Structures that pick up the ovulated mass of cells and are the site of fertilization
Uterus
Normal implantation site of the blastocyst. Supports the development of the placenta and embryo.
Cervix
Proximal end of the uterus
Fertilization age
Starts counting at the time of fertilization
Menstrual (gestational) age
Starts counting at the last menstrual period (~2 weeks prior to fertilization). More clinically useful than fertilization age.
Ectopic pregnancy
A serious medical condition where the blastocyst implants outside the normal area in the uterus
Trophoblast
Cells that line the outside of the blastocyst and interact with the uterine epithelium for implantation. Develop into definitive placental structures
Syncytiotrophoblast
Trophoblast cells that fuse into a syncytium and maintain the blood-placental interface. Develop into the placental villi
Cytotrophoblast
Trophoblast cells that remain undifferentiated beneath the syncytiotrophoblasts and are a source of new cells during development
Ovum (egg)
A fully mature oocyte
Zygote
A novel diploid cell, formed by the fusion of 2 haploid gametes through meiosis. Features 2 pronuclei and polar bodies.
Morula
Cluster of cells generated through isovolumetric mitosis after fertilization but before the blastocyst stage. Named after the mulberry
Blastocyst
First clear cell fate distinction during development. Ball of cells with a cavity, the blastocoele, and inner mass. Trophoblasts line the cavity and embryoblasts form the inner cell mass.
Embryoblast
Mass of cells within the blastocyst that eventually develops into the definitive fetal structures
Decidua basalis
Uterine mucosa directly underlying the implantation site
Decidua capsularis
Uterine mucosa that grows back to fix the defect caused by the implantation process
Decidua parietalis
The rest of the uterine lining that does not participate in implantation
Myometrium
The thick smooth muscle layer of the uterus, between the endo and perimetrium
Perimetrium
Connective tissue layer mostly comprised of serosa and some adventitia on the outside of the uterus
Bilaminar Germ Disc
Interface between the epiblast and hypoblast sheets. “2-layer” stage of embryonic development, aka week 2
Teratogens
Substances that can cross the placental membrane and disrupt embryonic development
Embryonic period
Critical development period of organogenesis from week 3 to 8 after fertilization. Increased sensitivity to developing congenital abnormalities.
Uterine body
Normal implantation site in the uterus
Implantation
Process by which the blastocyst attaches and enzymatically invades into the endometrium in order to receive nourishment from the mother
Ampulla
Enlarged region of the uterine tubes where fertilization most often occurs
Zona pellucida
ECM surrounding the ovulated oocyte that blocks implantation as the fertilized egg travels through the fallopian tubes. Also restricts cell size as the zygote undergoes mitosis.
Fimbriae
Projections at the ends of the uterine tubes that help to pick up an ovulated ovum as the tube sweeps over the ovary
Cleavage
Rapid mitosis without cell growth; occurs after fertilization but before implantation and results in the totipotent blastomeres becoming smaller
Blastomere
Cell produced by cleavage of the zygote after fertilization
Amnion
Second spherical structure that develops during week 2 from the inner cell mass of the blastocyst. Defines the epiblast
Epiblast
Cell sheet interface between the blastocoele and the amnion. Forms the columnar dorsal layer of the embryoblast.
Primary (primitive) yolk sac
Forms from the blastocoele lining, defines the hypoblast sheet where it contacts the epiblast
Hypoblast
Forms the cuboidal ventral layer of the embryoblast
Extraembryonic mesoderm
Primitive, very fluid, connective tissue that forms between the amnion/yolk sac and trophoblast cells.
Secondary yolk sac
Definitive yolk sac that develops late in week 2. Replaces the primary yolk sac
Stages of Week 1 embryo development
Day 0: Fertilization
1: Cleavage
2.5: 8-cell, compaction
3: Morula
4: Blastocyst
6: Implantation
Carnegie stages
23 stages covering the 1st 8 weeks post-ovulation. Each stage is based on morphological development and associated with the appearance of specific structures
Primitive streak
Site of ingression that determines the site of gastrulation and defines left/right, cranial caudal, and dorsal ventral axes.
Gastrulation
The stage during which the epiblast is reorganized into the trilaminar germ disc with the 3 primary germ layers
Trilaminar germ disc layers
Ectoderm, mesoderm, endoderm
Placental villi stages
Primary: cytotrophoblast core surrounded by syncytiotrophoblast
Secondary: extraembryonic mesoderm core inside trophoblastic cells
Tertiary: mesodermal cells differentiate into embryonic blood vessels
Bilaminar disc cavities
Extraembryonic (chorionic cavity) contains amnion/amniotic cavity + secondary yolk sac. Chorionic cavity surrounded by extraembryonic mesoderm/trophoblasts.
Sinusoids
Intervillous spaces in the placenta. Parental vessels penetrate the trophoblast to fill these spaces with parental blood for nutrient exchange.
Amniotic fluid
Fills the amniotic cavity and acts as a physical buffer/temperature regulator for the embryo. Fluid volume is critical for normal development and first comes from amniotic cells/maternal tissues. Later on, the fetus drinks/urinates to maintain volume.
Primitive node/pit
Elevated area surrounding a circular depression at the cranial (anterior) end of the primitive streak. Organizer for gastrulation.
Gastrulation process
- Cells delaminate from the epiblast to undergo the EMT.
- Cells ingress and dive through the primitive streak.
- Cells first displace the hypoblast to form the definitive endoderm.
- More cells migrate to form the mesoderm, aka become mesenchymal cells.
- Remaining epiblast cells become the definitive ectoderm.
Oropharyngeal membrane
Region at cranial end of primitive streak where endoderm and ectoderm contact each other. Defines future oral cavity.
Cloacal membrane
Region at caudal end of primitive streak where endoderm and ectoderm contact each other. Defines future anal cavity.
Epithelial-to-mesenchymal transition (EMT)
Process during gastrulation where cells transition from epithelial to mesenchymal phenotype in order to form the germ layers as they delaminate from the epiblast.
Notochord
The axial mesoderm. Induces formation of the neural plate and other structures
Connecting stalk
Future umbilical cord; composed of extraembryonic mesoderm. Crosses chorionic cavity to connect chorionic plate with amnion/yolk sac.
Prechordal mesoderm
Region of mesoderm cranial to the notochord that thickens and develops into the head later
Primitive streak regression
The primitive streak develops during gastrulation and then regresses around week 4 or 5. Lack of regression (excess proliferation) results in a teratoma. Lack of sufficient cell migration can cause sirenomelia (fused legs)
First 8 Carnegie stages
- Zygote
- Morula
- Blastocyst
- Implantation
- BLG
- Primitive streak / gastrulation
- TLG
- Notochord
(9. Head fold)
