What do primordial germ cells do during weeks 4 and 5 of development?
What is gametogenesis? What cellular process does it involve?
During the very early parts of embryo development the primordial germs cells “get out of town” or leave the vicinity of the developing embryo. They return at about 5 weeks. During weeks 4 and 5 of development primordial germ cells located in the wall of the yolk sac migrate to the gonadal ridge. Gametogenesis is the production of converting primordial cells into human gametes (oocytes and spermatozoa) and involves the process of meiosis which reduces the number of chromosomes (n = 23), redistributes chromosomal material to ensure genetic variability and exchanges a small amount of maternal and paternal DNA via crossover.
What process must ejaculated sperm must undergo to be capable of fertilization? Where does this process happen?
Ejaculated sperm are not capable of fertilization until they complete a process called capacitation which occurs in the female reproductive tract.
What layers must a spermatozoa penetrate in order to fertilize an oocyte?
What processes occur as soon as the spermatocyte has entered the oocyte?
What are the 3 results of fertilization?
If fertilization is to occur, the spermatozoon must penetrate (a) the corona radiata (phase 1), (b) the zona pellucida (phase 2), and (c) the oocyte cell membrane (phase 3). As soon as the spermatocyte has entered the oocyte, (a) the oocyte finishes its second meiotic division and forms the female pronucleus; (b) the zona pellucida becomes impenetrable to other spermatozoa; and (c) the head of the sperm separates from the tail, swells, and forms the male pronucleus.
Key concept: The results of fertilization are 1. restoration of the diploid number of chromosomes,
2. determination of chromosomal sex and
3. initiation of cleavage.
What is the normal site of fertilization?
Fertilization normally occurs in the ampulla of the uterine tube.
Explain the series of mitotic divisions that occur after fertilization. What are these mitotic divisions called?
When does the fertilized egg enter the uterus?
In order to grow and develop the newly fertilized ovum undergoes some changes. The first of these changes is a series of mitotic divisions (cleavage) that results in an increase in cells. The cells are called blastomeres and as they divide and become compacted to become a tightly grouped ball of cells. When this dividing mass of cells reaches the 16-cell stage it is called a morula. The morula enters the uterus on the 3rd or 4th day after fertilization and a cavity within the developing zygote begins to appear. At this point the morula (a.k.a. zygote) is referred to as a blastocyst.
What are the parts of the blastocyst? What do they develop into?
During what week does implantation occur?
The blastocyst has a blastocyst cavity and inner and outer layers. The inner cell mass will develop into the embryo and the outer cell mass will form the trophoblast. If all goes well implantation is completed during Week 2.
What is the function of trophoblast? What is the normal site of implantation within the uterus?
What are the 2 layers of trophoblast? What are their functions?
Through the process of Implantation the blastocyst uses the trophoblast cells to implant within the uterine cavity normally on the superior wall in the anterior or posterior position. The trophoblast develops into 2 layers:
1. cytotrophoblast has distinct cell boundaries and undergoes mitotic division to form the syncytiotrophoblast.
2. syncytiotrophoblast (cell boundaries are blurred so the whole thing looks like a blob with nuclei) will grow into the endometrium to make connection with the endometrial (maternal) blood vessels and glands producing the hormone human Chorionic Gonadrotropin (a.k.a. hCG)
HCG is detectable in the maternal blood by day 8 and maternal urine by day 10 which is the basis of pregnancy testing.
Why is week 2 known as the "week of 2's"? What processes occur during this week?
• The trophoblast differentiates into 2 layers, the cytotrophoblast and syncytiotrophoblast.
• The embryo blast forms 2 layers, the epiblast and hypoblast.
• The extraembryonic mesoderm splits into 2 layers, the somatopleure and splanchnopleure. •
Two cavities form; the amniotic and yolk sac.
What is an ectopic pregnancy? What are sites for ectopic pregnancies?
Ectopic Pregnancy - when Implantation goes wrong.
• implantation of the blastocyst outside the uterine cavity resulting in an ectopic (out of place) pregnancy.
• sites for ectopic pregnancies include the uterine tube, abdominal cavity, peritoneal covering of the intestinal tract.
• in most ectopic pregnancies, the embryo dies about the second month of gestation
What is gastrulation? How does gastrulation begin?
Describe the process of gastrulation.
GASTRULATION (a.k.a. development of the 3 basic embryonic layers (ectoderm, mesoderm and endoderm). 1. Gastrulation begins with formation of the primitive streak on the dorsal surface of the epiblast. It is visible as a narrow groove with slightly bulging regions on either side. The primitive streak is composed of the primitive groove, primitive node and the primitive pit which is located on the cephalic end of the streak.
2. Cells of the epiblast migrate toward the primitive streak and slip beneath it through a process known as invagination to form the inner-placed embryonic endoderm (inner layer)
3. Some cells lie between the epiblast and the endoderm to form mesoderm (middle layer) and the remaining cells form ectoderm (outer layer).
4. This results in the development of the trilaminar embryonic disc.
5. The trilaminar disc has 3 definitive germ layers of ectoderm, mesoderm and endoderm.
6. All adult cells can be traced back to these 3 layers.
What structures does the notochord extend btwn?
What is the function of the notochord?
What is the adult remnant of the notochord?
During the formation of what germ layer is the notochord formed?
The notochord extends from the primitive node to the oropharyngeal membrane and plays an inductive role in organ formation. In the adult it persists in the intervertebral discs as the nucleus pulposus (gel like substance that absorbs shock). Formation of the notochord occurs during mesoderm formation. The definitive notochord lies between the ectoderm and endoderm and is the first indication of an axial skeleton.
Gastrulation results in defining what 3 important orientations?
Key Concept: Gastrulation results in defining 3 important orientations :
• cranial : sacral axis.
• right : left orientation.
• dorsal: ventral orientation.
What is the result of head and tail folding?
Head and Tail Folding: The embryonic disc begins as a flat rounded structure. It will continue to grow and elongate in a cephalocaudal direction. This results in a folding of the cranial and caudal ends creating head and tail folds. The heart which originates in the most cranial position will swing down to the position of the future thorax. Thus gastrulation, or formation of the germ layers, continues in caudal segments while cranial structures are differentiating, causing the embryo to develop cephalocaudally.
What is the result of lateral folding?
Lateral folding of the embryo completes the formation of the adult form by extending laterally first and then ventrally closing of body cavities. The connection with the yolk sac will persist for a while and later be pinched off. A body cavity is formed around the tube of endoderm. The cavity is line with cells from the somatic mesoderm and persists as the parietal layer of the serous membranes. Cells from the splanchnic mesoderm develop into smooth and cardiac muscle cells.
What germ layer is the neural plate derived from? Somites?
Two important precursor structures are developed at this point: the Neural plate (from ectoderm) and Somites (from mesoderm) .
Describe the process of neuralation.
During this time the nervous system and mesoderm-derived structures begin to form. This is how it happens. The notochord induces the overlying ectoderm around the center of the embryo to thicken and form the neural plate. The central cells of the plate make up the neuroectoderm. The neuroectoderm forms a groove which deepens. The lateral edges of the group then migrate toward each other to form the primitive spinal cord. During this time neural crest cells form at the edges of the neuroectoderm. These cells migrate through the embryo to form adult structures. Neural crest derivatives are listed below (attached pic). This process is referred to as Neurulation.
Chromaffin cells of adrenal medulla, heart valves, and ganglion are some derivatives of neural crest cells.
An important concept in nervous system development is the ____ of cells and _____ to their adult location. Any interruption in this 2-step process can produce a developmental defect.
An important concept in nervous system development is the mitosis of cells and migration to their adult location. Any interruption in this 2-step process can produce a developmental defect.
Where are neural crest cells originally clocated?
What do neural crest cells form in the following parts of the body?:
head and neck
Originally, neural crest cells are located along dorsal midline of neural tube. These cells undergo mitosis and exit the neuroepithelium to migrate to their adult locations in the body. These cells become widely dispersed and differentiate into a highly diverse group of cells.
1. In the nervous system neural crest cells make important contributions to: Schwann cells, PNS myelinated cells, dorsal root ganglia, some cranial nerve ganglia and numerous autonomic ganglia, gut intrinsic innervation-myenteric plexus and submucosal plexus, the adrenal medulla, part of the meninges surrounding CNS.
2. In the head and neck neural crest cells contribute to part of head mesenchyme, forming parts of facial bones, muscles, and ligaments, odontoblasts and the dentin producing cells of teeth.
3. In skin neural crest cells contribute to making the pigment-producing cells, melanocytes.
What is DiGeorge syndrome caused by? What are the sx of this disease?
• AKA 22q11.2 Syndrome (deletion in #22)
• Characterized by subtle craniofacial anomalies, cardiovascular anomalies, and sometimes thymic and parathyroid aplasia
• Results from abnormal migration and differentiation of head and neck neural crest cells
What are neural tube defects typically cased by?
What is the range in severity of neural tube defects?
Neural Tube Defects.
• Common kind of congenital birth defects
• Multifactorial etiology- some genetic and some environmental.
• Typically caused by failure of neural tube closure and/or induction of bony neural arches of spinal cord
• Range in severity from asymptomatic spina bifida occulta to devastating and fatal anencephaly
What are the 3 divisions of mesoderm? What do they differentiate into?
Mesoderm is the middle of the 3 germ layers and is divided into:
1. Paraxial Mesoderm - these cells are close to the midline and are organized into segments called somites. The somites appear first in the cephalic region and then proceed to develop in a craniocaudal direction. There are occipital, cervical, thoracic, lumbar, sacral, and coccygeal pairs of somites. Sclerotome somites surround spinal cord to form the vertebral column and also form tendons/cartilage/bone. Dermomyotome combination of the Myotome and the Dermatome . Myotome (providing the segmental muscle component), Dermatome (forms the dermis of the back). Each myotome and dermatome retains its innervation from its segment of origin.
2. Intermediate Mesoderm - will differentiate into urogenital structures.
3. Lateral plate mesoderm - parietal and visceral layers line the intraembryonic cavity and surround organs.
What adult structures do the ectoderm, mesoderm, and endoderm give rise to?
Ectoderm (outer layer)- organs and structures that maintain contact with the outside world: nervous system, sensory epithelium of ear, nose, and eye, skin, hair, nails, pituitary, mammary, sweat glands and enamel of the teeth.
Mesoderm (middle layer) - connective tissues, tendons, cartilage, bone, muscle, some urogenital structures, blood cells, adrenal cortex
Endoderm (inner layer) - gastrointestinal tract; epithelial lining of the respiratory tract; parenchyma of the thyroid, parathyroids, liver, and pancreas, stroma of the tonsils and thymus, the epithelial lining of the urinary bladder, urethra and lining of the tympanic cavity and auditory tube.
see pg 51 of course notes!!!