Genome – chromosomes (DNA), total in cell
Chromosomes – have 23 pair; have DNA in an organized fashion (strand)
Autosomes and Sex chromosomes (x,y)
2 chromatids = 1 chromosome
Consist of codons (3 nucleotides)
A series of codons is a gene
Genes make proteins!
Types of proteins:
Receptors: tyrosine kinases are what the trophic factors interact
with
Growth Factors: trophic factors tell stuff what to do
Ex. Growth of nerve cells
Trophic Factors will trigger:
1. Cell division: mitosis or meiosis
2. differentiation: make different proteins
3. Motility
An embroblast will become any tissue the body is composed of.
On chromatids there are alleles. Theses alleles control traits (characteristics)
through dominant and recessive genes.
Metacentric Chromosome: centromere is in the middle, dividing it into two equal
parts
Acrocentric Chromosome: centromere is not in the middle, off center
Telocentric Chromosome: centromere sits on top or on bottom
Cell Cycle:
Types:
Labile: continually dividing Ex. Epithelial
Stable: Cells divide when stimulated Ex. Fibroblasts
Permanent: can no longer divide Ex. Nervous, muscles
Phases of Interphase:
G0: cell is there, normal function
G1: normal cell functions, duplication of organelles, proteins, etc.
S: DNA replication
G2: protein synthesis
Mitosis:
Prophase, metaphase, anaphase, and telophase
Cytokinesis: cell splitting
Mitosis
vs.
Conservative:
Genome is identical
One Cell Division
Centromeres line up
Somatic Cells
Meiosis
Nonconservative:
genome not identical
Two Cell Divisions
Homologous(part of chromatid,
not centromere) regions
line up
Sex Cells
Chiasmata: the actual point where the homologous regions touch
Disjunction: separation of chromosomes/ chromatids
Nondisjunction: incomplete separation—too much or too little
Trisomy: one extra chromosome
Ex. Trisomy 21 – Down’s syndrome
Monosomy: one less chromosome
Gametogenesis: formation of gametes
Oogenesis:
Oocytes: at birth they are suspended in prophase
So, increased age means an increased risk for homologous regions
to cross and get stuck together, i.e. increased risk for downs
syndrome
OMI – oocyte maturation inhibitor – protein the blocks oocyte to
move on to metaphase.
During meiosis, after the first division there is an oocyte and a polar body. There
is essentially no difference between the two except that the polar body has no
cytoplasm.
Oocytes
Polar
Bodies
Epiblast – one of the cell types from which all tissue types can derive from; the
precursor to the gametes.
During formation, the yolk sac wall is there the gametes (primordial germ cells)
are formed. These cells move to the ovaries and begin to make follicles.
Page 20 (figure)
Primordial Germ Cell  Oogonia (during mitosis) primary oocytes (after
meiosis)
For spermatogenesis: primordial germ cell spermatogonia primary
spermatocyte
Oogonia are surrounded by a layer of Squamous epithelium. (4th week of
development)
During development, a loss of trophic factors occurs. Apoptosis (programmed
cell destruction) takes place.
Primordial follicle: primary oocyte surrounded by epithelial cells, called follicular
cells. (7 months of development)
At birth, all eggs are formed and are in follicles.
During puberty, FSH (follicle stimulating hormone) will excite a few follicles (5-10)
at a time. The Squamous cells will differentiate into cuboidal cells. These
cuboidal cells will secrete glycoproteins into follicle. The Zona Pellucida is a
layer of glycoproteins around the oocyte. The zona pellucida acts as a selective
barrier that the sperm must traverse to fertilize the cell. To move the oocyte to
the end of the follicle (and not in the middle), fluid is made and makes a space
called the Antrum.
Ovulation occurs as a result of LH. LH stimulates the production of
progesterone. LH also stimulates the production of collagenase (this breaks
down collagen). LH also stimulates the production of prostaglandins, which
stimulate the muscle around the ovary and uterus. LH leads to disinhibition of
OMI (oocyte maturation inhibitor).
The release of the oocyte is due to the follicle bursting. At ovulation, the primary
oocyte completes the first meiotic division becoming a Secondary Oocyte. The
follicular cells surrounding the oocyte are called cumulus oophorus cells
producing the corona radiata. The zona pellucida is still in tact. The oocyte also
has a membrane. So, these three layers the sperm must travel thru to fertilize
the cell.
The secondary oocyte does not undergo meiosis II until fertilization. The other
follicles that do not mature to ovulation are formed into the corpus atrecium.
Gonadotrophic releasing hormone (GnRH) – released from hypothalamus where
it acts on the anterior pituitary the production and release FSH and production of
LH (luteinizing hormone).
Under the influence of FSH, follicular cells secrete estrogen. Estrogen
stimulates the release of LH.
The oocytes are attracted to the finger-like projections (fimbrae) of the fallopian
tubes. There are stigma (extensions) on the cells.
After ovulation, the follicular cells become the corpus luteum (due to LH). This
structure releases progesterone. At the drop of progesterone levels (i.e. no
fertilization of cell), menses begins (sloughing off of the endometrium).
In the case of fertilization, the corpus luteum becomes corpus luteum of
pregnancy. It will continue to release progesterone. hCG (human chorionic
gonadotropin) is released by the fertilized cell.
Spermatogenesis:
Pg. 25 in text
Remember this occurs at Puberty!!
Spermatogonia (implies that it is in mitosis)
Primary Spermatocyte – after first meiotic division
These cells do not complete Cytokinesis until the very end of the cycle and
undergo morphogenesis
Secondary Spermatocytes – after secondary meiotic division
After the secondary division – the “tail” will arise from the centriole, there is an
acrosomic granule that will form a “cap” that contains enzymes that are “trypsin
like.” After this, the morphogenic stage begins where the cytoplasm is decreased
to make the shape of the sperm with the cap and tail.
Fertilization:
After the sperm enters the female reproductive system, the sperm must
undergo capacitation, which is where the seminal proteins are stripped off of the
sperm by the epithelial cells on the vaginal walls. This makes the sperm
attracted to the egg cell. ZP3 is what the sperm interact with to break down the
acrosomal cap. The acrosomal reaction occurs. This is the breakdown of the
zona radiata. Metabolic activation is the reaction of the egg membrane and
sperm, which causes the zona pellucida to change not allowing any other sperm
inside. At this point, the female pronucleus is formed. The male pronucleus is
formed when the genome is released into the oocyte. The two pronuclei
combine and will divide in half forming a zygote. The cells will rapidly divide.
Morula – 16 cell stage of zygote. After this stage, compaction will occur to make
an inner layer and outer layer. The outer layer of cells are called the trophoblast
and the inner layer is called the embryoblast. The outer layers will eventually
form the placenta. REMEMBER: The trophoblasts secrete “stuff” that tell the
female body that it is “self”. When I say that I mean that it is normal to the body
and does not need to be destroyed. A Blastocyst forms moving the embryoblast
to one end. The rest of the space makes the blastocyst cavity. The trophoblastic
cells on the polar end will “branch” out to adhere to a nearby structure, normally
the endometrium. (Ectopic pregnancy – implantation occurs outside of uterus)
Genetic diversity, obtaining a diploid state, and life formed are the main results of
fertilization.
2nd week of development:
Noted by the Bilaminar disc
The embryoblastic and the trophoblastic cells will differentiate into different
cells.
The embryoblastic cells will differentiate into epiblast and hypoblast cell
layers.
The trophoblastic cells will differentiate into cytotrophoblast (cuboidal cell
layer) and syncytiotrophoblast cells (loose layer of cells).
Bilaminar Disc:
Within the epiblast and hypoblast, an amniotic cavity is formed, where the
baby will develop. The cavity is surrounded by amnioblasts.
Implantation defect – area where blastocyst is implanted
Fibrin coagulum – tough connective tissue that surrounds the implantation
defect
After a few days 9 days : the cell grows and the amniotic cavity increases in
size. The exoceolomic cavity is formed by the hypoblasts. It is the primitive yolk
sac. The syncytium form fluid filled cavities, lacunae. Eventually these lacunae
will combine with blood vessels in the endometrium.
At 12 days: a new layer of cells if formed in-between cytotrophoblastic cells and
the yolk sac called the extra-embryonic mesoderm. This mesoderm will lead to
formation of blood vessels, blood cells, and umbilical cord.
At 13 days: the bilaminar disc is free floating in the chorionic cavity
The yolk sac was pinched off to make an exocoelomic cyst
The connecting stalk is starting ( the umbilical cord)
The epiblast will make the ectoderm, mesoderm, and the endoderm.
The mesoderm will make villi moving toward the blood circulation.
Gastrulation: formation of germ layers of the trilaminar disc (3rd week)
Trilaminar disc:
Come from the epiblast
It is elliptical
Formation of the buccopharyngeal membrane – opening of oral cavity
Formation of the primitive streak (part of gastrulation) – establishes the
cephalic and caudal axis (i.e. what is head and tail) for the streak to be made the
cells must invaginate and fall into the fold. When this happens the cells will
differentiate. Growth factor play a key role in this because they cause cells to
move, differentiate or reproduce. BMP-4 stimulate epiblasts to become
mesoderm. As these cells are pushed down towards hypoblasts by other
mesoderm cells, these cells next to the hypoblasts will become endoderm.
When BMP-4 is stopped, the epiblasts will become ectoderm. With the loss of
BMP-4, neuralation will occur. At the end of the streak are specialized cells
where the epiblasts differentiate into their appropriate cells. This is known as the
primitive node. The notochord is above the primary streak. The notochord is the
central axis of the developing fetus.
The head will develop much faster than the rest of the body during
development.
Blood vessels arise in the development of the tertiary villus (extraembryonic mesoderm), which notes the fact that the two blood supplies connect.
The endothelial cells of the blood vessels of the baby and the syncytium of the
mother are connected. The cytotrophoblasts will now surround the outside of the
embryo. They are synonymous with the syncytium. (3rd week of development)
Node = organizer
Nodal – substance that maintains the primitive streak
BMP-4 = bone morphogenic peptide -4; chordin, noggin, and follistatin block
BMP-4, which then causes neuralation to start
FGF – fibroblast growth factor
Embryonic Period (3rd-8th week of development):
Greatest probability of birth defects during this period
Formation of the organ systems
Nervous system develops very rapidly
Day 19: the neural plate with the neural folds and groove are easily seen
Day 20: Somites are seen along midline, these are just segmented
mesoderm cells, and the neural groove is very prominent.
The two neural crests fuse to make a neural tube, out of the neural groove
From here the neural crests will move outward to form the Dorsal Root
Ganglion. i.e. neural crest cells are precursors for sensory bodies in
DRG. In addition, some cells will also become parts of the
sympathetic ganglion, preaortic ganglion, and suprarenal ganglion.
The neural groove cells are precursors for the motor spinal nerves.
Ectoderm will eventually become skin (epidermis).
Somites will differentiate into bone, dermis, and muscle tissue.
Number of somites correlate to the age of the embryo.
Day 23: the neural tube is closed
Day 25: pharyngeal arches arise; these are bulges which make facial
bones. A groove exists between the arches
Day28: lens placode (eyes), otic placode (hearing), a heart bulge, and
limb ridges will arise
Fig. 5.9
Lateral mesoderm
The intermediate dermis will make the urinary system
Mesoderm covers the yolk sac, these cells will eventually become the
visceral peritoneum
The mesoderm that covers the amnion will become the parietal
peritoneum
As the amnion and the yolk sac separate, cavities are made. The yolk sac
will be pinched to make the gut tube and the peritoneal cavity.
Para-axial mesoderm:
The neural tube forms and the neural crest cells move laterally. The name
of these cells change into the sclerotome, which lie close to the neural tube, and
the dermomyotome. Sclerotome will make bones and connective tissue.
Dermatome will make the dermis and the Myotome will make muscles.
Neural Crest derivatives:
C.T. and bones of the face and skull
cranial nerve ganglia
C Cells of the thyroid gland
Conotruncal septum in the heart
Odontoblasts
Dermis in face and neck
DRG
Sympathetic chain and preaortic ganglia
Adrenal medulla
Parasympathetic
Schwann cells
Glial cells
Meningeal layers
Spina Bifida:
Types:
Occulta: failure of closure of vertebrae, no spinous process
Will have a hairy patch on skin
Cystica: failure of vertebrae to close, dura does not properly occur,
and Subarachnoid space is very large
Meningomyelocele: cord is outside of vertebrae and sits in
the space next to skin
Rachischisis: there is not a proper neural tube, therefore not a
normal spinal cord, and neural tissue can be exposed as outside
skin
Test #2:
From crown to heal length:
At 3 months: head is about ½ body size
At 5 months: 1/3 body size
At birth: ¼ body size
Blood lakes are intervillous spaces. They contain maternal blood.
At the end of the 2nd month:
Inner layer of the uterus is decidua parietalis. Extending over the amniotic
is the decidua capsularis. This crosses over the decidua basalis, which covers
the embryonic pole (where the embryo comes in contact with the endometrium).
There is a chorion frondosum. This is a wave-like structure of villi. So the
placenta is made of chorion frondosum and decidua basalis. Fusing membranes
make the amniochorionic cavity and membrane.
A Full Term Placenta:
From the cord, the placenta is flat with vessels radiating from it. On the
bottom of the placenta, cotyledon (means little partitions) are covered by decidua
septa. Under the cotyledon is the decidua basalis.
Within the cotyledon have villa and vessels. Cotyledon can communicate
with each other. Spiral arteries fill the villa with maternal blood. Each cotyledon
has an artery and vein supplying it.
Functions of the Placenta:
Exchange of gases
Via simple diffusion
Exchange of nutrients
Of amino acids, etc. via facilitated diffusion
Production of hormones
Produces progesterone and estrogen (estriol)
Somatomammotrophin – stimulates the mammaries
Stimulates glucose uptake by fetus
Transport of Immunoglobins
IgG (maternal) – passes passively, is a protein
Compliment (group of proteins found in blood)
Part of the immune system, this occurs in the fetus
Twins:
Fraternal: dizygotic twins, occurs between 7-11/1000 births, babies have
two different placentas and cavities within same uterus; OR babies could have
same placenta but separate amniotic cavities.
Identical: monozygotic twins, occurs between 3-4/1000 births, Could form
two different placentas and different cavities (this is the most common); OR one
placenta and separate cavities; OR One Placenta and One cavity.
The last option for identical twins could produce conjoined twins. That option
could also produce the vanishing twin syndrome. This is where one child
receives greater circulation and the other twin does not develop properly.
Conjoined Twin Types: they share organ systems
Thoracopegus – attached along the abdominal region
Pygopagus – attached along the back
Craniopagus – attached at the head
Pre-natal Screening for Abnormalities:
Ultrasonography – “ultrasound”, could see a 3D image of child, noninvasive, receive info about placenta (position), twins, spina bifida
Amniocentesis – removes amniotic fluid, invasive – cells are collected
about Trisomy, Monosomy, genetic abnormalities; this is done when genetic
abnormalities are pre-existing. This is also done to test for chemical byproducts
– like alpha fetal protein which detects a neuralation problem and stunted cavity
development
Chronic Villus Sampling – can culture cells and determine metabolism
disorders
Failure in Developmental Process (birth defects):
Structure of chromosomes
Change in number of chromosomes
Environmental factors
Causes of Birth Defects:
Genetic – structure of number of chromosomes
Congenital – normal genes, but still a defect from the environment
Acquired – after birth
Malformation:
Due to genetic or congenital defects
Occurs during organogenesis
Disruption – alterations in structures (after 8 weeks of development)
i.e. unilateral renal agenesis – only one kidney formed
Deformation – due to mechanical forces that mold or shape to a particular
structure; commonly associated with the skeletal system. i.e. cleft foot
Syndrome – group of abnormalities that occur together by a known cause
Association – random group of abnormalities that occur together by an
unknown cause
Association Abnormalities:
C – colobomas – absence of ocular tissue i.e. missing color in iris
H – heart defects – includes defects in valves and vessels
A – atresia of choanea – abnormality in funnel-like structures
R – retardation
G – genital
E – ear
The child needs to exhibit two or more of these.
Generalized Abnormalities:
V – vertebral
A – anal
C – cardiac
T – trachea
E – esophagus
R – renal
L – limb
Minor Abnormalities:
Pigment spots, small ears, small eyes, etc.
If a person has a minor abnormality there is a 3% of a major abnormality.
If a person has 2 minor abnormalities, there is a 10% of a major abnormality.
If a person has 3 or more minor abnormalities, there is a 20% chance of a major
abnormality.
Viruses can cause abnormalities. Ex. Cytomegalovirus, rubella, HIV
Chemicals (drugs) can cause abnormalities also. Ex. Aminopterin (inhibits folic
acid), amphetamines, alcohol, lead, etc.
Physical agents, like X-ray or hyperthermia, could also cause abnormalities.
Hormones could also cause abnormalities. Ex. Androgenic agents, maternal
diabetes.
Intrauterine Growth Retardation – child is below the 10th percentile in weight for
its age
Caused by:
Nutritional status of mother
Smokes
Diseases the mother has
Fetal Alcohol Syndrome Features:
Short palpebral fissures, flat face, short nose, thin upper lip, low nasal
bridge, epicanthal folds
Chromosomal Abnormalities:
Translocation: happens from meiosis I to meiosis II; two chromosomes
attach to each other. About 75% of cases of downs syndrome are causes by
nondisjunction by the female, but it could be a result of translocation.
Chromosome 21, 13, and 18 are common ones for nondisjunction
Results of Trisomy 18:
Low set ears, renal abnormalities, fusion of digits
1/5000 births
Results of Trisomy 13:
1/20000 births
Midline structures are affected
Known as Holoprosencephaly (or patau’s syndrome)
Cyclopsy, cleft lip, microphthalmia
Most children do not live with this defect
Klinefelter’s syndrome:
Found only in males
Usually found in puberty
XXY chromosomes, nondisjunction of XX homologue
Individuals are sterile, testicular atrophy, hyalinization of seminiferous
tubules
Most are not mentally retarded
Turner’s Syndrome:
Found in women
Absence of ovaries, short stature, webbed neck, skeletal deformities,
broad chest, widely spaced nipples,
Lymphedema – abnormality in formation of lymphatics
75% of cases caused by nondisjunction in male
Microdelitions:
Deletion of a certain chromosomal segment
Ex. Long arm chromosome 15 (maternal)
Results in angelman syndrome – cannot speak, mentally retarded,
poor motor development, unprovoked/prolonged laughter
Prader – Willi syndrome
Paternal chromosome 15 missing
Hypotonia, obesity, mental retardation, hypogonadism
Skeletal System:
Mesoderm, specifically the paraxial and lateral, is important in formation of
this system
Neural Crest cells make the bones of the face.
Endochondrial Ossification – process by which hyaline cartilage is turned
into compact bone. Fibroblast Growth Factor Receptors (FGFR) are
proteins that are responsible for growth at the epiphysis (ends) of the
bone.
Membranous Ossification – Hyaline cartilage is not used as the base,
spicules are the base. This process makes flat bones.
Osteoblasts, Fibroblasts, and Chondroblasts all arise from the mesoderm.
Skull:
Develops in two areas: Neurocranium and Viscerocranium
Neurocranium are the bones that surround the brain. Has two
subdivisions:
Membranous – form the flat bones
Cartilagenous – form the irregular shaped bones
Viscerocranium are the bones of the face. The dorsal portion is made of
the first two pharyngeal arches which forms the maxillary process. This
makes the maxilla, zygomatic and a portion of the temporal bone. The
lateral portion makes the mandibular process which makes the
mandible. The dorsal tip of mandibular notch and 2nd pharyngeal arch
make the auditory ossicles as well as the ear bones (incus, stapes, and
malleus).
Don’t forget the anterior and posterior fontanelle, which make the bregma
and lambda respectively. There is a mastoid and sphenoid fontanelle
also.
Abnormalities:
Cranioschisis – failure of the cranial vault to form, this leads to
anencephaly.
Craniosynostosis – premature fissure of the sutures
Scaphycephaly – premature fissure of the sagittal suture;
skull becomes long and narrow.
Acrocephaly – premature closure of the coronal suture; skull
becomes tall and wide.
Plagiocephaly – premature closure of the coronal and
lambdoidal suture, which skull is asymmetric.
Cloverleaf skull is a result of abnormality of FGFR-1(or 2), a part of
Pfeiffer syndrome. Abnormalities of FGFR-3 upsets the growth of the
limbs.
Formation of the limbs:
Start as buds at 5 weeks of age
At 6 weeks the pads of the hands and feet form
The development starts distal (hands or feet) and the rest of the
limb grows afterward.
Apical ectodermal ridge will cause cells to differentiate at the end of
the limb bud.
Apoptosis – programmed cell death; directed by a chemical signal;
occurs between webbing of fingers and toes
Syndactyly – fusion of the digits
Polydactyly – extra digits
Meromelia – hands but no forearms
Amelia – no limb
Micromelia – arms/legs are developed by small
Vertebral Column:
The caudal portion of the somite moves inferiorly and joins the
superior portion of the somite below it. The caudal portion of the somite
leaves a space when it moves and this space becomes the intervertebral
space. The notochord is still present and in tact. After the space is made,
the notochord will differentiate into the nucleus pulposus of the IVD. After
this process, ossification occurs of the body and then the t.p. and s.p.
For the Final:
The ventral portion of the somite becomes the sclerotome.
The dorsal portion is left behind and becomes the dermomyotome.
The dermomyotome dissociates to become the dermatome and Myotome.
Muscular System development
Derived from mesoderm
Except muscles of the iris, which come from optic cup ectoderm
Skeletal muscle derived from paraxial mesoderm
Somites from occipital to sacral region
Somitomeres in head region
Smooth muscle
Splanchnic mesoderm surrounding gut tube
Cardiac muscle
Splanchnic mesoderm surrounding heart tube
Muscular Development
Myotome cells differentiate into myoblasts
Individual cells fuse to form muscle fibers
Striations visible by 3rd month
By the end of the 5th week, Myotome is divided into two sections:
Dorsal portion – epimere; form extensors of spine
Ventral portion – hypomere; form muscles of thorax and abdomen
Nerves also divided into two sections:
Dorsal Primary Ramus
Ventral Primary Ramus
These nerves will remain with the muscle while developing
Trophic influence is needed for growth of both nerves and muscle
Myoblast of epimeres form extensor muscle of vertebrae
Myoblast of hypomeres form lateral and ventral flexor muscles
Myoblast from cervical hypomere forms
Scalenes
Geniohyoid
Prevertebral
Myoblast from thoracic segment form
External intercostal, internal intercostal, and transverse thoracic
muscles
In abdominal wall, myoblast form layers
External and internal oblique and transverse abdominis
Due to ribs, thorax muscles remain segmented
In head region all voluntary muscles are derived from paraxial mesoderm
and connective tissue from neural crest cells
Limb formation – 1st indication at 7th week
Appears as a condensation of mesenchyme near base of limb bud
Limb bud has a singer outer layer of cuboidal cells
Apical Ectodermal ridge
Cells of AER have inductive effect on underlying mesenchyme to
remain undifferentiated by rapidly dividing
Cells further from the influence of AER differentiate into cartilage and
muscle
Limb development
Abnormalities
Meromelia – partial development
Amelia – complete absence
Phocomelia – long bones area absent with rudimentary hands and
feet
Micromelia – segments present but smaller
Polydactyly – extra digit
Ectrodactyly – one digit does not develop
Syndactyly – digits fused
Congenital hop dislocation
Underdevelopment of acetabulum and head of femur
Amniotic bands – cause ring constrictions and amputations of limbs and
digits
Congenital absence or deficiency of radius
Usually associated with other abnormalities such as
craniosynostosis, radial aplasia syndrome, etc.
Vertebral defects
Scoliosis – lateral curvature of spine
Spina bifida – occulta and cystica
Central Nervous System
Development begins at the beginning of the 3rd week
Neural plate – slipper-shaped plate of thickened ectoderm; occurs with
inhibition of BMP-4
Neural folds – formed from the lateral edges of the neural plate; ultimately
becomes the neck
Neural tube – formed by fusion of the neural folds; forms the DRG,
Sympathetic ganglion, Preaortic ganglion
Cephalic end of neural tube develops three dilations called the primary
brain vesicles
Prosencephalon – forebrain
Mesencephalon – midbrain
Rhombencephalon – hindbrain
Simultaneously it forms two flexures
Cervical at the junction of the hindbrain and spinal cord
Cephalic in the midbrain region
Prosencephalon
By the 5th week prosencephalon consist of
Telencephalon – primitive cerebral hemispheres
Diencephalon – thalamus and hypothalamus
Mesencephalon separated by a deep furrow called the
rhombencephalon isthmus
Rhombencephalon
Consists of two parts
Metencephalon – forms the pons and cerebellum
Myelencephalon – medulla oblongata
Boundaries between these two parts called the pontine flexure
Lining the inner part of the neural tube are neuroepithelial cells. These cells take
on a bipolar shape and eventually become neuroblasts. As these cells mature,
they move outward towards the edge. The are acting under the influence of
nerve growth factor(NGF). With this hormone, the cells differentiate into their
proper nerve cells
Spinal cord:
Neuroepithelial cells
Neuroblasts
Characterized by large nucleus with pale staining
nucleoplasm and dark-staining nucleolus
Form the mantle layer (gray matter)
Make the alar (sensory) and basal (motor) plate of the cord
Marginal layer
Contains nerve fibers (white matter)
The roof and floor plate are areas where there are no neuroblasts.
They are areas for crossing fibers
Basal plate - contains ventral horn cells; multipolar cells
Alar plate – form sensory area
Sulcus limitans – longitudinal groove marking boundary between alar and
basal plates
Roof plate – dorsal midline portion
Floor plate – ventral midline portion
Nerve cell histological differentiation
Nerve cells
Multipolar Cells
Unipolar Cells
Bipolar Cells
Glial cells
Astrocytes
Protoplasmic
Fibrillar
Microglia
Oligodendrocytes
Ependymal cells
Neural crest cells
Neuroblasts of sensory ganglia form two processes
Central and peripheral growing process
Differentiate into
Sympathetic neuroblasts
Schwann cells
Pigment cells
Odontoblasts
Meninges
Mesenchyme of pharyngeal arches
Myelination begins at the 4th month of development
By oligodendrocytes and schwann cells
Neuroepithelial cells differentiate into:
Bipolar neuroblasts, which make multipolar neurblasts
Ependymal cells
Gliablasts – which make proplasmic and fibrillar astrocytes
They also help to make oligodendroglia, which help make
mesenchyme cell (they make microglia).
Positional Changes of the Spinal Cord
3rd month – spinal cord extends full length of embryo
With increasing age, vertebral column and dura lengthen more
rapidly than neural tube. As a result, the terminal end of the
neural tube shifts to a higher level.
At birth, the terminal end of the spinal cord ends at 3rd lumbar
vertebrae
In the adult, the dural sac and Subarachnoid space extend to S2
Below L2 and L3 is the filum terminale, a threadlike extension of pia
mater
Nerve fibers that extend below the terminal end are called cauda
equina
Brain
Rhombencephalon (hindbrain)
Myelencephalon – forms the medulla oblongata
Differs from spinal cord in that its lateral walls are everted
Alar and basal plates separated by sulcus limitans
Basal plate contains
Medial somatic efferent group:
Intermediate special visceral efferent group
Lateral general visceral efferent group
Alar plate contains:
Somatic afferent
Special visceral afferent
General visceral afferent
Metencephalon – forms cerebellum and pons
Each basal plate contains three groups:
Somatic efferent – gives rise to Abducens nerve
Special efferent – contains nuclei of the trigeminal
and facial nerve
General visceral efferent – innervate the
submandibular and sublingual glands
Alar plates contain three groups of sensory nuclei:
Lateral somatic afferent
Special visceral afferent
General visceral afferent
Cerebellum
Dorsolateral parts of the alar plates bend medially and form the rhombic
lips
Continued folding leads to formation of cerebellar plate
By the 12th week, it shows a small midline portion called the vermis and
two lateral hemispheres
Neuroepithelial cells migrate to surface of cerebellum to form external
granular layer
In the 6th month, two cell types are formed: basket and stellar cells
Mesencephalon (midbrain)
Each basal plate contains two motor nuclei:
Medial somatic efferent – Oculomotor and Trochlear nerves
A small general visceral efferent – nucleus of Edinger-Westphal,
which innervates the sphincter of the pupillary muscle
The marginal layer enlarges to form the Cru Cerebri, which serves as a
pathway for nerve fibers descending from the cerebral cortex to lower
centers in pons and spinal cord.
Prosencephalon (forebrain)
Diencephalon
Develops from median portion of prosencephalon
Consists of a roof plate and two alar plates and lacks a floor plate
and a basal plate
Roof plate consists of a single layer of ependymal cells and it is
covered by vascular mesenchyme
Choroid plexus
Caudal part of the roof plate forms into the pineal body or epiphysis
Alar plates form the lateral walls of the diencephalon
Diencephalon
Hypothalamic sulcus divides the plate into dorsal and ventral regions
Thalamus and hypothalamus
Hypophysis or pituitary gland
Develops from two different parts: Rathke’s pouch and
infundibulum
Pituitary Gland
Rathke’s pouch
Forms the anterior lobe of the hypophysis
A small extension of this lobe forms the pars tuberalis
The posterior wall of rathke’s pouch develops into the pars
intermedia
Infundibulum gives rise to the stalk and the pars nervosa or posterior
hypophysis
Telencephalon
Cerebral hemispheres – lateral outpocketings
Median portion – lamina terminalis
Autonomic Nervous System