Unit 3: Embryology

Question 1

Gestational period

Answer 1

• 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.

Question 2

Infant mortality of premature babies

Answer 2

• babies born around 37-40 weeks have lowest mortality rates

- babies born before 37 weeks or after 41 weeks have higher mortality rates

Question 3

Primordial germ cells

Answer 3

Found in yolk sac early in development and migrate to the developing ovary or testis to become oocytes or sperms

Question 4

Process of fertilisation

Answer 4

1. Sperm capacitation: a process by which spermatozoa become able to fertilise an ovum. Lasts about 7 hours
2. 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.
3. Burrowing of the sperm
4. The zona reaction: when the first sperm has penetrated the zona pellucida rapidly changes the ovum to be impermeable to other sperms.
5. Sperm entry into the oocyte: the sperm nucleus along with some of other components swim into the oocyte.
6. Oocyte forms a spindle: arrange male and female chromosomes on the spindle.
7. Cell undergoes mitosis and cleavage

Question 5

Product of fertilisation

Answer 5

Morula

Question 6

Implications of fertilisation

Answer 6

1. Beginning of a life with individual human genome
2. First cell division
3. Ethical implications

Question 7

Blastomeres

Answer 7

Cells of morula, each have predestined roles

Question 8

Blastocyst

Answer 8

Structure formed after fluid accumulates between blastomeres of the morula. This forms the trophoblast, and embryoblast which later becomes epiblast and hypoblast

Question 9

Implantation

Answer 9

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.

Question 10

Primary stem villi

Answer 10

Consists only of cytotrophoblast and syncytiotrophoblast

Question 11

Secondary villi

Answer 11

When connective tissue has grown into embryonic villi

Question 12

Tertiary stem villi

Answer 12

When blood vessels form in the connective tissue and has connected with embryonic circulation

Question 13

Function of placenta

Answer 13

exchange of oxygen, nutrients and waste products. Functions as embryonic lungs, GIT and kidneys.

Question 14

Rhesus incompatibility

Answer 14

Maternal is rhesus negative and foetal is rhesus positive, maternal immunity develops antibodies against the baby’s red blood cells.

Question 15

The Bilaminar disc

Answer 15

• 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

Question 16

Gastrulation

Answer 16

• 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

Question 17

Primary germ layers

Answer 17

Ectoderm, endoderm and mesoderm

Question 18

Oropharyngeal membrane

Answer 18

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.

Question 19

Prechordal plate

Answer 19

Block of mesoderm near the oropharyngeal membrane, acts to induce the overlying ectoderm into brain development

Question 20

Cloacal plate

Answer 20

Positioned at the caudal end of embryo, similar to oropharyngeal membrane. Closes off the gastrointestinal and genitourinary systems of the embryo

Question 21

Notochord

Answer 21

• 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.

Question 22

Fate of primitive streak

Answer 22

After gastrulation, primitive streak should disappear.

PATHOLOGY: remaining primitive streak forms sacrococcygeal teratoma

Question 23

Derivatives of ectoderm

Answer 23

• epidermis of skin
• CNS
• PNS
• parts of ear and eye

Question 24

Derivatives of endoderm

Answer 24

• epithelial linings of GIT
• GIT glands (liver, pancreas, salivary glands)
• epithelial linings of respiratory tract

Question 25

Derivatives of mesoderm

Answer 25

• connective tissue (incl. bone, blood, cartilage)

- muscles (skeletal, smooth and cardiac)

Question 26

Neurulation

Answer 26

• 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.

Question 27

Neural crest cells

Answer 27

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

Question 28

Defects of neural tube formation

Answer 28

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!

Question 29

Coelom

Answer 29

Our coelom is composed of 3 cavities:

1. pleural
2. pericardial
3. peritoneal

they surround our thoracic and abdominal organs

Question 30

Lateral folding

Answer 30

• 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.

1. ectopia cordis: exposed heart
2. ectopia veesicae (extrophy of the bladder)
3. 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.

Question 31

Cardiogenic mesoderm

Answer 31

on the trilaminar disc, cephalic to the oropharyngeal membrane. Mesoderm that eventually forms the heart

Question 32

Septum transversum

Answer 32

cranially to the cardiogenic mesoderm, a bar of mesoderm that forms part of the diaphragm

Question 33

Craniocaudal folding

Answer 33

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

2. part of the disc which is most dorsal (septum transversum) will end up most inferiorly in the fully formed body (in the diaphragm)
3. cardiogenic mesoderm will end up above the diaphragm
4. 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

2. 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

Question 34

Oligohydramnios

Answer 34

• insufficient amniotic fluid

- may be caused by underdeveloped kidneys of the embryo

Question 35

Polyhydramnios

Answer 35

• too much amniotic fluid

- suggests blocked oesophagus (oesophageal atresia)

Question 36

Meconium

Answer 36

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.