IMMS embryology Flashcards
At the end of Week 8 what happens to the embryo?
It becomes a foetus
The kidney, liver, brain and lungs are all beginning to function.
Fingers and toes are separate and the external genitalia are formed
What is the function of the Fallopian tube? (Uterine tube)
Allows the passage of the egg from the ovary to the uterus
What are the different parts of the Fallopian tube?
Infundibulum- With its associated fimbriae are near the ovaries
Ampulla- Widest part of the tube, represents major portion of the lateral tube
Isthmus- Narrower part of the tube that links to the uterus
Intramural part (interstitial part)- transverses the uterine musculature
Tubal ostium- Point where the tubal canal meet the peritoneal cavity
Uterine opening- Entrance to the uterine cavity, the utero-tubal junction
Ovulation and fertilisation process in females
With each ovarian cycle, a number of primary follicles begin to grow, but usually one (the dominant follicle) reaches full maturity, and only one oocyte is discharged at ovulation
Ovulation is inducted through a surge in the leutenizing hormone (LH) from the pituitary gland
Tubal fibriae, carry the oocyte into the uterine tube through its sweeping action
Fertilisation takes place in the fallopian tube, at the junction of the ampulla and the isthmus (the ampullary-isthmic junction)
What is the length of a pregnancy?
280 days (40 weeks or 9 months) after the onset of the last normal menstruation period
More precisely 266 (38 weeks) after fertilisation
Fertilisation takes place on day 15 following the onset of the last normal menstrual cycle
From the time of fertilisation to the end of the 8th week is called the embryonic period
From the beginning of the 9th week to birth is known as the fetal period
Changes during embryonic period?
The 1st week - Fertilisation and formation of the blastocyst [F for first = fertilisation]
The 2nd week - Implantation and formation of bilaminar embryonic disc [2nd week = 2 = bilaminar]
The 3rd week - Further development of the embryo and formation of trilaminar embryonic disc [3rd week = 3 = trilaminar]
The 4th week - Folding of the embryo [F for 4th = F for folding]
The 5th - end of 8th week - Development of all organs
End of 8th week - all organs have developed and the embryo looks like an adult and is called a foetus
What is the function of the male reproductive system?
Produce, maintain and transport the sperm and semen
Discharge sperms within the female reproductive tract during sex
Produce and secrete male sex hormones
Process of fertilisation?
Union of male and female reproductive cells (gametes) to produce a fertilised reproductive cell (zygote)
Results of fertilisation:
- Restoration of diploid number of chromosomes
- Determination of chromosomal sex
- Initiation of cleavage/division of cells
The fertilised ovum, a zygote will undergo rapid mitotic divisions i.e. 2-cell, 8-cell and a 16-cell morula
As the morula enters the uterus on the 3rd or 4th day after fertilisation, a cavity begins to appear and the blastocyst forms
Implanatation of the blastocyst in the endometrial stroma of the uterine wall occurs at the end of the 1st week
By the end of the 2nd week, the blastocyst is completely embedded
The blastocyst implants in the endometrium along the anterior or posterior wall (the uterus at the time of implantation is the secretory phase)
Morula (16 cells), enters the uterine cavity on day 3 or 4 —> Blastocyst (32 cells) —> penetrates uterine wall on day 8
Cilia in the fallopian tube beat ovum towards the uterus
At the end of the 2nd week the endometrium wall has surrounded the embryo
The cells of the blastocyst differentiate into two cell masses:
- Inner cell mass, called the embryoblast at one pole, it develops into the embryo proper and the blastocyst cavity
- Outer cell mas, call the trophoblast, the cells surround the embryoblast
The embryoblast (inner cell mass (ICM) goes on to differentiation into two layers:
* Epiblast
*Hypoblast
(the two layers form a flat disc- bilaminar disc)
What does the epiblast give rise to?
Amnioblasts that line the amniotic cavity superior to the epiblast layer
What does the hypoblast give rise to?
Gives rise to cells that line the blastocyst cavity and the inner surface of the trophoblast
Cells that originate from the hypoblast and line the blastocyst cavity and the inner surface of the tropoblast form the exocoelmic membrane (primitive yolk sac)
What does the trophoblast differentiate into?
Cytrophoblast
Synctiotrophoblast (outer layer that erodes maternal tissue)
What happens by day 8?
Lacunae develop in the syncytiotrophoblast resulting in the maternal sinusoids (vessels) to be eroded meaning maternal blood can enter the lacunar network and a primitive utero-placental circulation is established
A new population of cells appear between the inner surface of the cytotrophoblast and the outer surface of the exocoelomic membrane of the primitive yolk sac
These cells, derived from yolk sac cells form a fine, loose connective tissue the extraembryonic mesoderm
What is the extraembryonic mesoderm? (chorionic plate)
Eventually fills all of the space between the cytotrophoblast and exocoelomic membrane
Large cavities develop in the extraembryonic mesoderm forming a new space called the extraembryonic cavity or chorionic cavity
The extraembryonic mesoderm forms two layers:
- Splanchnopleuric (splanchnic or
visceral) layer - Somatopleuric (somatic or parietal) layer
THE EXTRAEMBRYONIC MESODERM IS RESPONSIBLE FOR THE FORMATION OF THE BLOOD VESSELS THAT WILL CONNECT THE EMBRYO TO THE PLACENTA
The extraembryonic mesoderm looks and acts just like the embryonic mesoderm but found outside of the embryo
Hypoblast?
Forms additional new cells that migrate along the inside of the exocoelomic membrane, they proliferate gradually and form a new cavity within the primitive yolk sac - these are the ENDODERMAL CELLS
This new cavity is knows as the secondary yolk sac (definitive yolk sac), it replaces the primative yolk sac
Trophoblast?
On further development of the trophoblast, the connecting stalk appears
The connecting stalk contains capillaries which are connected to the chorionic plate (extraembryonic mesoderm) and the embryo
Chorion?
A membrane that exists during pregnancy between the developing foetus and mother
Formed by extraembryonic mesoderm and the two layers of trophoblast (cytotrophoblast & syncytiotrophoblast)
Chorionic villi emerge from the chorion and invade the endometrium and allow for the transfer of nutrients from maternal blood to fetal blood - this initiates the formation of the placenta
In this way the embryo starts getting its nutrients and oxygen through the connecting stalk
Gastrulation?
During this phase, the embryoblast develops into a TRILAMINAR STRUCTURE known as the GASTRULA
Occurs during the third week
The three germ layers are known as:
- Ectoderm
- Mesoderm
- Endoderm
It begins with the primitive streak (an elongating primitive groove with a node anteriorly)
The primitive streak appears on the dorsal mid-sagittal surface of the epiblast, along the anterior-posterior axis of the embryo
Cells of the epiblast migrate toward the primitive streak, they are detached from the epiblast and slip beneath it (invagination)
Invaginating cells give rise to MESODERM
Other invaginating cells displace the hypoblast and give rise to the ENDODERM
The epiblast remains as the ECTODERM
Prenotochordal cells invaginating in the the node of the primitive streak form the notochord
The notochord forms a midline axis which serves as the basis of the axial skeleton
During development the three germ layers differentiate into their own tissues and organ systems.
What does the ectoderm give rise to?
Structures that are in contact with the outside of the body:
Central nervous system
Peripheral nervous system
* Parietal Mesoderm
Ectoderm
Sensory epithelium of the
nose, ear & eye
Epidermis of skin, hair and nails
* Endoderm
Pituitary, mammary & sweat glands
Enamel of teeth
What does the mesoderm give rise to?
Formed of three parts:
Paraxial plate mesoderm: * gives rise to Somites * Somites give rise to the supporting tissue of the body: A. Myotome (muscle tissue) B. Sclerotome (cartilage and bone) C. Dermatome (dermis of the skin)
Intermediate plate mesoderm - UROGENITAL SYSTEM:
- Kidneys
- Gonads & their respective duct systems
Lateral plate mesoderm - found at the periphery of the embryo:
- Splits into two layers:
1. Somatic (parietal) layer: forms the future body wall
2. Splanchnic (visceral) layer forms: - Circulatory system
- Connective tissue for the glands
- Muscle, connective tissue and peritoneal components of the way of the gut
What does the endoderm differentiate into?
Epithelial lining of the GI tract, resp tract & urinary bladder
Parenchyma (functional part) of the thyroid gland, parathyroid gland, liver &
pancreas
Epithelial lining of the tympanic cavity & auditory tube
Differentiation of paraxial mesoderm and somites formation
During development, the paraxial mesoderm is divided into paired segments called somites on each side of the notochord, they develop in a cephalocaudal direction (head to tail)
The first pair of somites appear at day 20 and then at a rate of around three pairs per day until 42-44 pairs are formed
At the beginning of the 4th week, somites
differentiate into:
1. Myotomes (form skeletal muscle)
2. Sclerotomes (form bones and cartilages) - some cells from the sclerotomes surround the notochord and spinal cord and give rise to the vertebral column
3. Dermatomes (form the dermis of the skin)
Oropharyngeal membrane
Located at the cranial end of the embryonic disc
Consists of a small region of tightly adherent ECTODERM & ENDODERM cells
Represents the future opening of the oral cavity
There is no MESODERM located between the ectoderm & endoderm
Cloacal membrane?
Formed at the caudal end of the embryonic disc
Consists of tightly adherent ECTODERM & ENDODERM cells - there is no
intervening MESODERM
When this membrane appears the posterior wall of the yolk sac forms a small diverticulum, the Allantois which extends into the connecting stalk
CAUDAL FLEXION brings the cloacal membrane onto the ventral surface of the embryo
Cranial area?
Contains the buccopharyngeal (oropharyngeal) membrane, the cardiogenic
area (heart) & septum transversum
CRANIAL FLEXION brings the buccopharyngeal membrane, cardiogenic area & septum transversum ventrally, forming the ventral surface of the future face, neck & chest. It brings the heart into its thoracic position and septum transversum to the diaphragm
Lateral folding?
Results in the incorporation of a portion of the yolk sac (which is lined with ENDODERM) into the embryo to form the primitive gut
It also leads to the formation of
body cavities
The remaining part of the yolk sac and Allantois remain outside the embryo
Primitive gut?
Blind-ended tube
Divided into foregut, midgut & hindgut
Midgut: remains connected temporarily to the yolk sac by means of VITELLINE DUCT (yolk stalk)
Foregut: Extends from oropharyngeal membrane to the liver bud
Midgut: from liver bud to the end of the right 2/3 of transverse colon
Hindgut: from beginning left 1/3 of transverse colon to the cloacal
membrane
