Lesson 1 Flashcards
Define embryology
The study of prenatal development
What are the 3 distinct periods of embryology and how do they fit into the trimesters?
- Preimplantation Period - first week
- Embryonic period - week two to week eight (end of second month)
- Fetal Period - third month (Week 9) to ninth month
First trimester - Proliferative and Embryonic Period
Second and Third trimester - Fetal Period
________ is the action of one group of cells on another that leads to the establishment of the developmental pathway in the responding tissue
Induction
________ is the controlled cellular growth and accumulation of by products
Proliferation
Cells increase in number
_________ is the change in identical embryonic cells to become distinct structurally and functionally
Differentiation
Differentiation occurs at various rates in the embryo.
Many parts of the embryo are affected: cells, tissue types, organs, and systems.
________ is the development of specific tissue structure of differing form due to embryonic cell migration and inductive interactions
Morphogenesis
_________ is the attainment of adult function and size due to proliferation, differentiation, and morphogenesis
Maturation
Describe interstitial growth
- Type of proliferation
- Deep within a tissue/organ - e.g. soft tissue
Describe appositional growth
- Type of proliferation
- tissue enlarges its size by the addition of layers on the outside of a structure - e.g. Bone or dental tissues
_________ is the development of different cell types
Cytodifferentiation
Describe Morphodifferentiation
- The development of the differing structure or shape for each organ or system.
- This is accomplished by morphogenesis, the process of development of specific tissue structure or shape. Change in shape, form and function.
When are developmental disturbances most common?
Between the 2nd and 8th week
List the important stats about developmental disturbances
- 50% of fertilized ova lost within the first 3 weeks
- 15% recognizable pregnancies have spontaneous abortion
- Birth defects in 3-6% of all newborns
- 65% of all birth defects have no known or identifiable cause
Define embryopathology
Concerns the study of pathologic conditions and malformations like early development failure
What are the 2 categories of Teratogenic agents?
- Genetic factors - e.g. Chromosome abnormalities
- Environmental agents & factors: medications, drugs, infectious agent & radiation
Describe Genetic Teratogens
- About 20-25% of human malformations observed in the first year of life are caused by genetic agents.
- causal mechanisms underlying these processes include, but are not limited to: gene deficiency, gene abnormality, chromosome rearrangement, chromosome deletion and chromosome excess.
- Although environmental factors may modify the development of the genetically abnormal embryo, the genetic abnormality is the major contributor to the pathologic process
Describe environmental teratogens
- Approximately 10% of human malformations observed in the first year of life are caused by environmental agents
- Drugs, chemicals, infections, radiation
List common drugs that act as teratogens
- Nicotine (Tobacco)
- Ethanol (Alcohol)
- Excessive Vitamin D
- Radiation therapy(>5rads)
List common infections that act as teratogens
- Viral - Cytomegalovirus, Herpes virus, Rubella virus (German measles)
- Bacterial - Syphilis
List common metabolic imbalances that act as teratogens
- Diabetes
- Folic acid deficiency
- Iodine deficiency
- Maternal Starvation
- Obesity
- Hyperthermia
- rheumatic disease
Describe Teratogenic Outcomes
- Exposure to teratogens can result in a wide range of structural abnormalities such as cleft lip, cleft palate, dysmelia (Congenital Limb Deficiency), anencephaly (absence of a major portion of the brain, skull, and scalp), ventricular septal defect.
- Exposure to a single agent can produce various abnormalities depending on the stage of development it occurs.
- Specific birth defects are not characteristic of any single agent.
Describe the preimplantation period
- First week after fertilization
- Fertilization is completed with formation of zygote.
- The result of this process is the joining of the ovum’s chromosomes with those of the sperm.
- This joining of chromosomes from both biological parents forms a new individual with “shuffled” chromosomes.
- To allow this formation of a new individual, the sperm and ovum, when joined, have the proper number of chromosomes (from diploid number of 46= zygote).
- After a week of cleavage, the blastocyst consists of a layer of peripheral cells, the trophoblast layer, and a small inner mass of embryonic cells, or embryoblast layer.
Describe Down Syndrome
An extra chromosome number 21 is present after meiotic division.
Clinical Description:
1. Epicanthic folds
2. Oblique eyelid fissures
3. Furrowed lower lip
4. Wide mid face
5. Flat bridged nose
6. Gastroesophageal reflux
7. Varying levels of intellectual disability
8. Arched palate, (thickened palatal shelves)
9. Open bite, protusion of the tongue, 10. microdontia, malocclusion
10. Weak muscles of the tongue, fissured tongue
11. Absence, reduction of maxillary & frontal sinus
12. Difficulty with speech and mastication
Describe the end of the pre-implantation period
By the end of the first week, the blastocyst stops traveling and undergoes implantation and thus becomes embedded in the prepared endometrium, the innermost lining of the uterus on its back wall.
Describe implantation (end of first week)
- Blastocyst embeds into the endometrium
- Trophoblast (peripheral layer of cells) invades endometrium.
- Endometrial tissue grows over blastocyst to complete implantation.
Describe the embryonic period
- The embryonic period of prenatal development, extends from the beginning of the second week to the end of the eighth week.
- During the second week of prenatal development, the implanted blastocyst grows by increased proliferation of the embryonic cells
- Differentiation also occurring, resulting in changes in cellular morphogenesis
- Increased number of embryonic cells creates the embryonic cell layers (or germ layers) within the blastocyst.
- Two small cavities develop on either side of the embryoblast layer - Embryonic cell layers-Bilaminar embryonic disc
Describe the bilaminar embryonic disc that develops during proliferation
- Develops from the blastocyst and appears as a flattened, essentially circular plate of bilayered cells
- The bilaminar embryonic disc (or disk) has a superior and an inferior layer.
- The superior epiblast layer is composed of high columnar cells
- The inferior hypoblast layer is composed of small cuboidal cells.
- Two cavities develop on either side of the bilaminar embryonic disc - Yolk sac/Amniotic cavity
Describe what happens during the beginning of the 3rd week of embryonic development
- The beginning of the third week, some cells from the epiblast layer move or migrate toward the hypoblast layer only in the area of the primitive streak.
- Epiblast differentiates into ectodermal cell layer
- Hypoblast differentiates into endodermal cell layer
- Developing between ectoderm and endoderm: mesenchyme cells of the mesodermal cell layer
- Proliferation of cells in the midline area.
- The primitive streak causes the disc to have bilateral symmetry, with a right half and left half.
Trophoblast layer gives rise to the prenatal support tissues i.e. the _________
Placenta
The inner mass of embryonic cells (trilaminar disc=embryo) is called the _________
Epiblast (embryoblast) layer
The central nervous system (CNS) begins to develop in the embryo during the __________
Third week
*Beginning of the spinal cord and brain
Describe the changes in the ectoderm during the 3rd week
- A specialized group of ectodermal cells on the dorsal surface differentiate to form neuroectodermal cells
- This central band of cells that extends the length of the embryo, from the cephalic end to the caudal end.
- These cells form the neural plate of cells
- Lateral edges of the neural plate begin to elevate
- Morphodifferentiation of embryo’s flat sheet of cells which differentiate into the neural groove.
- Near the end of the third week, the neural groove deepens further and is surrounded by the neural folds.
- In addition during the third week, another specialized group of cells, the neural crest cells, develop from neuroectoderm.
- These cells migrate from the crests of the neural folds and then disperse within the mesenchyme.
What do neural crest cells differentiate into?
- Components of the nervous system
- Melanocyte pigment cells
- Connective tissue proper
- Cartilage
- Bone
- Dental tissues: all except enamel
Describe somatic development
- By the end of the third week, the mesoderm additionally differentiates and begins to divide on each side of the forming neural tube into 38 paired cuboidal segments of mesoderm, forming the somites (create the connective tissue).
- Differentiated mesoderm gives rise to the somites that are located on the sides of the developing central nervous system.
Describe Somites
- Differentiate into mesenchymal cells which differentiate into connective tissue forming cells
- Skeletal (bone & cartilage) structures of head, neck, trunk
- Associated muscles, dermis
Closure of the neuroectoderm occurs, the neural tube is formed during the __________ week by the neural folds undergoing fusion at the most superior part. It is lined by neuroepithelial cells
Fourth
Describe how the neural tube forms
- The neuroectoderm forms from the ectoderm and is located at the neural plate.
- The neural plate then thickens to form the neural groove, surrounded by the neural folds.
- Then these folds meet and fuse, forming the neural tube.
Describe the development of the brain the spinal cord
- Neuroepithelial cells proliferate and differentiate into neuroblasts
- Neuroblasts are primitive nerve cells which develop into neurons
- These cells do not have the potential for mitosis
Describe the formation of cranial nerves
- 3 weeks forebrain enlarged, sensory areas laterally located
- 4-5 weeks forebrain bent forward, cranial nerves have grown into tissues they innervate
Describe what happens to the disc during the 4th week of development
- The disc undergoes embryonic folding: The trilaminar disc has folded into the embryo as a result of extensive growth of the ectoderm
- This places forming tissue types into their proper positions for further embryonic development as well producing a somewhat-tubular embryo.
Describe the folding of the embryo
The trilaminar disc has folded into the embryo as a result of extensive growth of the ectoderm. The endoderm is thus inside the ectoderm, with the mesoderm filling in the areas between these two tissue types, except at the two embryonic membranes. Note the developing brain, heart, and digestive tract.
List the components of the ectodermal layer
- Epidermis
- Sensory of eyes, ears, nose, nervous system
- Neural crest cells - first step, eventually move to mesoderm
- Mammary glands and cutaneous glands
What components of the face and neck develop during the 4th week?
Finally, during the fourth week, the face and neck begin to develop, with the primitive eyes, ears, nose, oral cavity, and jaw areas.
The foregut gives rise to the primitive _________, which will form the _________.
- Pharynx
- Oropharynx
Facial development begins in the _________ week and completed in the __________ week
- 4th
- 12th
Describe the development of connective tissue
- Fibroblasts migrate to either side of the neural tube within the somites
- Differentiate into mesenchymal cells which differentiate into connective tissue forming cells, ie: osteoblasts (bone) and chondroblasts(cartilage)
List the components of the mesodermal layer
- Muscle
- Connective tissue
- Urinary system
- Reproductive system
- Cardiovascular system
- Lining of body cavities
The initial skeleton is made of _________
Cartilage
Describe Endochondral Ossification
Cartilage skeleton is replaced by bone to offer rigidity and strength for muscle insertion
Describe Intramembranous bone formation
Bones of the face and cranium develop from direct transformation of connective tissue into bone
Describe muscle development
- Tenth week prenatally myoblasts migrate from the myotomes following the segmental pattern of the skeleton
- Differentiate into elongated specialized cells with the property of contractility
- Skeletal, smooth and cardiac muscles
Describe the development of specialized connective tissue
- Mesenchymal cells form the endothelial cells form the capillary wall
- Mesenchymal cells in the center form the red blood cells
- Plasma, leukocytes, and lymphatic system
Describe the endodermal layer
Endodermal cells form elongated tube:
- Linings of Digestive tract, Respiratory tract, Urinary tract
- Thyroid, parathyroids, thymus
- Liver, gallbladder, pancreas
Describe developmental disturbances during the embryonic period
- Because the beginnings of all essential external and internal structures are formed during the embryonic period, this is considered the most critical period of prenatal development.
- Thus developmental disturbances occurring during this period may give rise to major congenital malformations of the embryo.
Describe ectodermal dysplasia
- A developmental disturbance that occurs during the embryonic period
- It involves the abnormal development of one or more structures from ectoderm.
- There may be partial or complete anodontia
If there is failure of migration of the neural crest cells to the facial region, __________ syndrome (or mandibulofacial dysotosis) develops in the embryo.
Treacher Collins
An example of an infective teratogen is the virus of _________, which can result in cataracts, cardiac defects, and deafness.
rubella
Another infective teratogen is the bacterial spirochete of ________, Treponema pallidum, because it produces defects in the incisors (Hutchinson’s incisor) and molars (mulberry molar) as well as blindness, deafness, and paralysis.
syphilis
An example of the result of a teratogenic drug effect during the embryonic period is _________
fetal alcohol syndrome.
Describe fetal alcohol syndrome
- Pre and postnatal growth deficiency, intellectual deficiency, anomalies: low nasalbridge, smallmidface, widely space eyes with epicanthic folds, indistinct philtrum and thin upper lip
- Crowding of dentition, mouth breathing, open bite
Describe spina bifida
Failure of fusion of the neural tube
Describe the fetal period
- The fetal period of prenatal development follows the embryonic period.
- It encompasses the beginning of the ninth week, or third month, to the ninth month, with the maturation of existing structures occurring as the embryo is enlarging to become a fetus.
Describe developmental disturbances during the fetal period
- Congenital malformations can also occur during the fetal period of prenatal development.
- The most common invasive prenatal diagnostic procedure to detect these malformations is amniocentesis, where the amniotic fluid is sampled during the fourteenth to sixteenth week after the last missed menstrual period.
- Systemic tetracycline antibiotic therapy of the pregnant female can act as a teratogenic drug during the fetal period.
- This therapy can result in tetracycline stain within the child’s primary teeth that are developing at that time.
