EMBRYOLOGY- LESSON 2

Zygote-

Fertilized egg. 

-The result of the union of an egg cell and sperm cell.

-After fertilization occurs,  the zygote begins to divide rapidly and form smaller segments called blastomeres.

-The daughter cells of the zygote

: the layer of cells surrounding the cavity, responsible for implanting the bastocyst in the uterine

- it releases strong digestive enzymes, so it can “eat its way in” for nourishment before fetal membranes can fully supply it

– will help form the placenta

a cluster of cells on one side of the blastocyst cavity

The process of rapid cell division

- 72 hrs after fertilization (zygote nears the middle section of the uterine tube),

-the cleavage generates a solid cluster of blastomeres  called the morula

-The sac formed by an extension of the epiblast .

-The fluid buffers the developing embryo and fetus from physical shock until the time of birth.

- the sac forms from an extension of the hypoblast

-Holds a small amount of yolk

-human digestive tube forms from part of it

-tissue surrounding the yolk sac gives rise to the earliest blood cells and blood vessels.

- results from gasturlation (cell movement) the embryo grows from a two-layered disk to a three-layered disc.

- Starts on day 14 & 15 when a raised groove called the primitive streak appears on the dorsal surface of the epiblast.

-originates from epiblast

-defines the bodies axis

-extends the length of the body

-is the site of the future vertebral column

-appears on day 16 and reaches future head region on day 18

originate from ectodermal cells on the lateral ridges (neural folds) on the neural plate

-they lie just external to the closed neural tube.

-Forms sensory nerve cells, melanocytes (pigment producing cells) , and bones of the face.

A mesodermal segment of the body of the embryo.

Each divides into 3 parts: Sclerotome (hard piece) Dermatome (skin piece) (later portion) Myotome (muscle piece)

(hard piece) cells migrate medially, gather around notochord and neural tube and produce the vertebra and rib at the associated level.

(skin piece)(later portion) cells migrate externally until they lie directly deep to the ectoderm where they form the dermis of the skin in the dorsal part of the body. 

(muscle piece) stays behind after the sclerotome and dermatome migrate.

Each myotome grows ventrally until it extends the entire dorsal-ventral height of the trunk

Become segmented trunk musculature of the body wall.

Ventral parts of the myotomes grow into the limb buds and for the muscles of the limbs.

-Epitheal lining and glands of digestive and respiratory tracts

Notochord:

- nucleus pulposus of intervertbral discs

Somite- (3parts)

A mesodermal segment of the body of the embryo.

Intermediate Mesoderm:

-kidneys

-gonads

Lateral Mesoderm: (2 parts)

1.somantic mesoderm =

- dermis of ventral portion of body

-parietal serosa

-connective tissues of limbs (bones joints and ligaments)

 

2. Splanchnic Mesoderm=

-wall of digestive and respirator tracts except epithelial lining

-visceral serosa

-heart

-blood vessels

Neural crest:

-sensory nerve cells

-some sensory nerve structures

-pigment cells

-portions of skeleton

-blood vessels in head and neck

- epidermis

-hair

-nails

-glands of skin

-brain and spinal chord

- Conception- the fertilization of an oocyte by a sperm ( generall in later 3^rd of uterin tube.

* Fertalized oocyte is now called a ZYGOTE

- Zygote moves towards the uterus, along the way it divides rapidly producing multiple cells called BLASTOMERES.

* Because there isn’t much time for intervals of cell growth, the blastomere cells become smaller and smaller, whole process called CLEAVAGE.

- 72 hours after fertilization, cleavage generates a solid cluster of 12-16 blastomeres called MORULA

- Day 4 the late Morula (consisting of about 60 cells) enters the uterus.  It takes up fluid, which gathers into a central cavity.

- This new Fluid filled cavity is called a BLASTOCYST

- Blastocyst lasts from Days 4-7'''

-Blastocyst lasts from Days 4-7

Two types of Cells:

1. Inner cell mass: a cluster of cells on one side of the blastocyst cavity=Will for the embryo

2. Trophoblast: the layer of cells surrounding the cavity=will form the placenta (what transfers nutrients from the mother to the fetus.

Day 6, the Blastocyst burrows into the wall of the uterus= IMPLANTATION

* Takes about a week to complete--- The trophoblast layer erodes inward until the entire blastocyst is embedded in the uterine wall.

The embryo grows from a two-layered disc to a three-layered disc.  The process is called GASTRALATION forms the three primary germ layers:

Endoderm

Ectoderm

Mesoderm

- Germ layer formation occurs on days 14 &15 when a raised groove appears on the dorsal surface of the epiblast called PRIMITIVE STREAK .

Epiblast cells migrate inward at this streak

-Day 14 & 15 -The first cells that migrate through the primitive streak displace the cells of the underlying hypoblast to be come the ENDODERM.

- Day 16-  the ingressing epiblast cells form a new layer between the epiblast and endoderm called the MESODERM

- The epiblast cells that remain on the embryos dorsal surface make up the ECTODERM. 

** all germ layers derived from epiblast cells.

- Ectoderm in the dorsal midline thickens into a neural plate -- starts to fold inward as a NEURAL GROOVE ---the groove deepens until a hollow NEURAL TUBE is pinched off into the body.

-Closure of the neural tube begins at the end of week 3 in the region that will become the neck and then proceeds both cranially and caudally.

-   The cranial part becomes the brain, and the rest becomes the spinal chord.

as the notochord develops it signals the overlying ectoderm to start forming the spinal chord and brain. 

-most severe neural tube defect, which is linked to low maternal levels of the B vitamin, folic acid.

- results from a failure of the distal/caudal end of the neural tube to close.

-child is born with a lesion called CELE (usually in the lower lumbar)

Results usually in paralysis of both lower extremities and incontinence.

-can also have a problem with the cranial end of the tube as well called : HYDROCEPHALUS “water on the brain”

-can be managed with early detection and intervention.

-Deprived of oxygen at birth

- exposed to tetragons after week 8

-Iodine deficiency

-difficulties in development in the womb

-Downs Syndrome

- genetic mutation

FAS:

Fetal alcohol syndrome ( most common nonheradity cause)

-unusually shaped eyes and lips, short nose, receding chin

- problems with vision and hearing

- possible heart defects.

-Below average intelligence and cognitive ability

-the hard palate fails to fuse completely.

-The hard palate becomes fused at 13–16 weeks (month 4) after fertilization.

- Disruption at this stage could also interfere with kidney development, bone formation, and the appearance of joint cavities.

-The fetus would still be disproportionate in appearance in that the head is larger than the rest of the body, which would also not be elongated. -Functionally, the sucking motion of the lips and eye flinching would not occur as well.

-inherited disease, a defective gene

-causes thick, sticky mucus to build up in the lungs and digestive tract.

-It is one of the most common chronic lung diseases in children and young adults, and may result in early death.

-Millions of Americans carry the defective CF gene, but do not have any symptoms.

- person with CF must inherit two defective CF genes -- one from each parent.

-The presence of all or part of an extra 21^st chromosome

-Can be identified by amniocentesis or at birth

-Individuals with Down syndrome tend to have a lower-than-average cognitive ability

-ranging from mild to moderate disabilities.

-A small number have severe to profound mental disability.

-More common in older women

-small chin, almond shaped eyes, round head, overlapping eyelids.

-Congenial heart defects are present at birth

-It is a defect in the structure of the heart and great vessels which is present at birth

-typically will affect the blood flow, and rhythm of heart, if serious enough it may require surgery or cause death.

-can be genetic or environmental

- maternal infections (rubella) , drugs, maternal illness (diabetes)