Early Embryology/Development - Checked and Complete Flashcards
Describe development of primordial germ cells
During the very early parts of embryogenesis, the primordial germs cells leave the vicinity of the developing embryo to the wall of the yolk sac .
At about 5 weeks they return to the gonadal ridge
Describe Spermatogenesis
Two types of spermatogonia are developed from the primordial germ cells at puberty - Type A and B spermatogonia.
Type A spermatogonia undergo mitosis to make more Type A spermatogonia and also Type B spermatogonia.
Type B spermatogonia complete Meiosis I to form 2 secondary spermatocytes (now with only 23 chromosomes) which go on to complete Meiosis II to form 4 spermatids.
Spermatids undergo changes resulting in sperm (process is spermiogenesis)
Define capacitation.
Sperm must be altered in female reproductive tract before capable of fertilization.
Describe oogenesis
All primary oocytes still containing 46 chromosomes are formed by the 5th month of fetal life and remain dormant in prophase of meiosis I until puberty.
During an ovarian cycle a primary oocyte will complete Meiosis I to form a secondary oocyte containing 23 chromosomes.
This secondary oocyte then enters Meiosis II and remains arrested in metaphase until (and if) fertilization occurs. At fertilization the secondary oocyte completes Meiosis II to form a mature oocyte.
** Usually only one oocyte is discharged during an ovarian cycle. At ovulation the oocyte is in metaphase of the second meiotic division and is surrounded by the zona pellucida and outer corona radiata.**
What are the three layers the sperm must penetrate to fertilize the egg?
** 1) corona radiata**
2) the zona pellucida
**3) oocyte cell membrane **
What happens after the sperm enters the oocyte?
The oocyte finishes meiosis 2
** Zona pellucida becomes impenetrable to other spermatozoa**
** Head of the sperm separates from the tail, swells, and forms the male pronucleus**
Sperm and Egg cell membrane fuse and cleavage begins
Where does fertilization normally occur?
In ampulla of uterine tube
What happens after fertilization completes?
Mitotic divisions (cleavage) result in a smaller, very compact 16-cell stage group of blastomeres called a morula
A cavity begins to appear within the morula and is then called a blastocyst
** Composed of blastocyst cavity (blastocoele), inner cell mass (the embryo) and the outer cell mass (the trophoblast)**
Implants before Week 2
What cells are necessary for implantation?
Trophoblast cells
The trophoblast develops into what 2 layers?
**Cytotrophoblast **
Syncytiotrophoblast - implants to endometrium and releases hCG; becomes placenta
The inner cell mass or embryo blast forms what 2 layers?
Epiblast
Hypoblast
The extraembryonic mesoderm splits into what 2 layers?
The somatopleure and splanchnopleure
What two cavities form during week 2?
The amniotic sace
The yolk sac
How long does an ectopic embryo typically survive?
1-2 months
Describe Gastrulation
A primitive streak is formed on the dorsal surface of the epiblast (composed of the primitive groove, primitive node and the primitive pit)
Epiblast cells migrate toward and invaginate into the primitive streak to form endoderm
** Some cells lie between the epiblast and the endoderm to form mesoderm**
The remaining cells form ectoderm
** This creates the trilaminar embryonic disc**
Describe Notochord location/formation.
The notochord extends from the primitive node (in the middle of trilaminar disk) to oropharyngeal membrane during mesoderm formation
Has inductive role in organ formation
Persists as nucleus pulposus (center of vertebrae) in adults
What 3 important orientations does gastrulation define?
**cranial : sacral **
**right : left **
** dorsal: ventral **
Describe head and tail folding.
Embryo grows at head and tails, resulting in those two places folding down like a banana.
The caudal end continues gastrulation and grows while the cranial end differentiates
Describe lateral folding
Flat embryo-disk folds into a tube and closes all body cavities
Describe Neurulation
Lateral folding contributes to neurulation
First, the notochord induces the overlying ectoderm around the center of the embryo to thicken and form the neural plate
A groove forms in the neruoectoderm, deepens, and pinches off to form the neural tube (primitive spinal cord)
Neural crest cells migrate to needed areas
List some derivatives of neural crest cells.
CNS: Schwann cells, PNS myelinated cells, dorsal root ganglia, cranial nerve ganglia, autonomic ganglia, gut intrinsic innervation-myenteric plexus and submucosal plexus, the adrenal medulla, meninges surrounding CNS,
Head and Neck: head mesenchyme, forming parts of facial bones, muscles, and ligaments, odontoblasts and the dentin producing cells of teeth,
Skin: pigment-producing cells, melanocytes
Name 2 neural crest cell defects
DiGeorge Syndrome (deletion in #22)
**Neural Tube Defects: **spina bifida occulta, rachischisis, anencephaly
Describe layers of mesoderm
Paraxial Mesoderm - close to the midline, organized into somites; also form myotome and Dermatome
Intermediate Mesoderm - will differentiate into urogenital structures.
Lateral plate mesoderm - parietal and visceral layers line the intraembryonic cavity and surround organs.
What is a visual difference between the cytotrophoblast and syncytiotrophoblast?
Cytotrophoblast has distinct cell boundaries
Syncytiotrophoblast has blurry cell boundaries that grow into the endometrium
What is the first step of gastrulation?
Formation of primitive streak.
