Unit 3: Embryology Flashcards
Gestational period
- period when the mother is pregnant. Defined in 2 ways:
1. Embryologically: from conception / implantation until normal birth. Lasts about 38 weeks.
2. Clinically: first day of the last menstruation. For the first 2 weeks of this period implantation has not taken place. Lasts 40 weeks.
Infant mortality of premature babies
- babies born around 37-40 weeks have lowest mortality rates
- babies born before 37 weeks or after 41 weeks have higher mortality rates
Primordial germ cells
Found in yolk sac early in development and migrate to the developing ovary or testis to become oocytes or sperms
Process of fertilisation
- Sperm capacitation: a process by which spermatozoa become able to fertilise an ovum. Lasts about 7 hours
- Acrosome reaction: when in contact with ovum the acrosomal cap breaks down and releases hyalorunidase (digesting enzyme), this allows sperm to burrow between the cells of corona radiata and zona pellucida.
- Burrowing of the sperm
- The zona reaction: when the first sperm has penetrated the zona pellucida rapidly changes the ovum to be impermeable to other sperms.
- Sperm entry into the oocyte: the sperm nucleus along with some of other components swim into the oocyte.
- Oocyte forms a spindle: arrange male and female chromosomes on the spindle.
- Cell undergoes mitosis and cleavage
Product of fertilisation
Morula
Implications of fertilisation
- Beginning of a life with individual human genome
- First cell division
- Ethical implications
Blastomeres
Cells of morula, each have predestined roles
Blastocyst
Structure formed after fluid accumulates between blastomeres of the morula. This forms the trophoblast, and embryoblast which later becomes epiblast and hypoblast
Implantation
Trophoblast attaches to the surface of endometrium, trophoblastic cells invade maternal tissues and eventually become embedded within the endometrium completely.
Trophoblast separates into two layers: the other syncytiotrophoblast (cell membranes disappear) and inner cytotrophoblast. Cytotrophoblast releases proteolytic enzymes that break the mother’s blood vessels to allow a better diffusion of nutrients. The little holes they create are called lacunae and they are filled with maternal blood. Cytotrophoblast grows extensions into these lacunae which facilitate nutrient exchange between the mother and foetus - called villi.
Primary stem villi
Consists only of cytotrophoblast and syncytiotrophoblast
Secondary villi
When connective tissue has grown into embryonic villi
Tertiary stem villi
When blood vessels form in the connective tissue and has connected with embryonic circulation
Function of placenta
exchange of oxygen, nutrients and waste products. Functions as embryonic lungs, GIT and kidneys.
Rhesus incompatibility
Maternal is rhesus negative and foetal is rhesus positive, maternal immunity develops antibodies against the baby’s red blood cells.
The Bilaminar disc
- forms around 2nd week of development. Inner cell mass arranges themselves into two layers - hypoblast and epiblast.
- cephalic vs. caudal end defined
- longitudinal ridge forms near the caudal end in the midline = primitive streak
- cephalic end of primitive streak = primitive node
Gastrulation
- 3rd week of development, cells of primitive streak breaks away and migrate in all directions. Some replacing the cells of hypoblast - forming the endoderm; and some form a 3rd layer in between epi and hypoblast - called mesoderm. Epiblast is then called the ectoderm
Primary germ layers
Ectoderm, endoderm and mesoderm
Oropharyngeal membrane
Small area near the cephalic end of trilaminar disc where epiblast and hypoblast remained attached, after folding, it ends up in a position near the tonsils at the back of the mouth.
Prechordal plate
Block of mesoderm near the oropharyngeal membrane, acts to induce the overlying ectoderm into brain development
Cloacal plate
Positioned at the caudal end of embryo, similar to oropharyngeal membrane. Closes off the gastrointestinal and genitourinary systems of the embryo
Notochord
- Cylinder of mesodermal cells that runs from the primitive node to the oropharyngeal membrane.
- Formed by mesodermal cells that migrate cranially.
- Induce spinal cord formation in overlying ectoderm
- induce surrounding paraxial mesoderm into forming bodies of vertebrae
- Remnants: nucleus pulposis
PATHOLOGY: chordomas in adults. Occurs at skull base or in the vertebral column.
Fate of primitive streak
After gastrulation, primitive streak should disappear.
PATHOLOGY: remaining primitive streak forms sacrococcygeal teratoma
Derivatives of ectoderm
- epidermis of skin
- CNS
- PNS
- parts of ear and eye
Derivatives of endoderm
- epithelial linings of GIT
- GIT glands (liver, pancreas, salivary glands)
- epithelial linings of respiratory tract
Derivatives of mesoderm
- connective tissue (incl. bone, blood, cartilage)
- muscles (skeletal, smooth and cardiac)
Neurulation
- Nervous system developed from ectoderm that overly the notochord, positioned between the oropharyngeal membrane and primitive node
- Chemicals released from notochord induce this ectoderm to form a raise, thickened area = neural plate. The neural plate is wider at the cephalic end and narrower at caudal end, edges of neural plate fold up to form neural folds.
- Neural groove forms as the neural folds and lateral sides of neural plate rise up and migrate upwards.
- Closure of neural plate starts near the cephalic end and extend until a complete tube is formed. Eventually neural folds fuse in midline.
- Neural tube breaks away from surface ectoderm and is completely surrounded by mesoderm.
- At the cephalic end, neural tube forms 3 vesicles that constitute the prosencephaly, mesencephaly and rhombencephaly (fore, mid and hind brain) and the remainder of the neural tube forms the spinal cord.
Neural crest cells
Cells that are on the sides of the neural tube, after fusion of tube, they migrate away to form various structures of the body.
- dorsal root ganglia
- cranial nerve ganglia
- autonomic ganglia
- adrenal medulla
- melanocytes
SOME OF
- cartilage
- bone
- dermis
- head and neck
- part of the heart
Defects of neural tube formation
spina bifida:
a) occulta - no disability
b) meningocele - bag of meninges bulging from affected vertebra
c) meningomyelocele - involves neural tissue, often results in paraplegia
anencephaly:
- minimal brain and cranial development, often results in death within a few days after birth
- folic acid!
Coelom
Our coelom is composed of 3 cavities:
- pleural
- pericardial
- peritoneal
they surround our thoracic and abdominal organs
Lateral folding
- Differential growth of body parts
- initially takes place at the cranial and caudal ends, lateral sides of trilaminar disc migrate ventrally and fuse at midline, leaving a defect where the anterior abdominal wall will be, through this defect a tub that connects to yolk sac protrudes.
- converts trilaminar disc into three-layered tube
ANOMALIES - defects on the anterior thoracic or abdominal wall.
- ectopia cordis: exposed heart
- ectopia veesicae (extrophy of the bladder)
- omphalocoele: exposed intestinal tract
FORMATION OF COELOM: before lateral folding
mesoderm of the lateral side of trilaminar disc comes to divide into two layers, one layer attached to the ectoderm and the other attached to endoderm.
Cardiogenic mesoderm
on the trilaminar disc, cephalic to the oropharyngeal membrane. Mesoderm that eventually forms the heart
Septum transversum
cranially to the cardiogenic mesoderm, a bar of mesoderm that forms part of the diaphragm
Craniocaudal folding
Craniocaudal folding puts the oropharyngeal membrane, septum transversum and cardiogenic mesoderm where they should be.
CEPHALIC END
1. the rapidly developing neural plate and brain causes the embryo to bulge dorsally on the cranial side. The cranial end of the embryo is “pushed over the top” as this happens
- part of the disc which is most dorsal (septum transversum) will end up most inferiorly in the fully formed body (in the diaphragm)
- cardiogenic mesoderm will end up above the diaphragm
- oropharyngeal membrane flips over and points upwards. Later owing to the rapidly developing brain and sides of the face, the oropharyngeal membrane ends up in a deep pit called the stomadeum, which becomes the oral and nasal cavities
CAUDAL END
1. the cloacal plate faces dorsally, during caudal folding this migrates and ends up pointing inferiorly in contributing to the formation of anus and urogenital structures
- blood vessels and connective tissue connect the trilaminar disc with placenta form a cordlike connecting stalk. This initially connects the caudal tip of the disc, but later it migrates onto anterior abdominal wall and forms a major part of the umbilical cord
Oligohydramnios
- insufficient amniotic fluid
- may be caused by underdeveloped kidneys of the embryo
Polyhydramnios
- too much amniotic fluid
- suggests blocked oesophagus (oesophageal atresia)
Meconium
When embryo passes faeces into amniotic fluid, can occur when embryo is stressed. Composed of greenish substance that is exfoliated gut cells mixed with bile of the liver. Baby may inhale meconium-stained fluid and develop chemical pneumonia and respiratory problems.
