General Anatomy (Embryonic And Foetal Periods)

Question 1

What is the embryonic period (3th - 8th week) about?

Answer 1

• The three germ layers give rise to specific tissues and organs (organogenesis). The cells of three germ layers: migrate, aggregate and differentiate. Growth takes place by mitosis & Complexity takes place by differentiation.
• The external body form becomes recognizable.
• Sensitive period to teratogens: results in major congenital defects.

Question 2

What happens by the end of 3rd week of gestation?

Answer 2

• By the end of 3rd week of gestation, the neural plate is formed from the ectoderm.
• The edges of the plate elevate as neural folds and the central region forms the median neural groove.
• The folds are prominent in the cranial end and are the first signs of brain development.
• By the 21st day of gestation, the neural folds fuse and form the neural tube. The fusion is cranio- caudal.
• The neural tube communicates with the amniotic cavity by the cranial and caudal neuropores.

Question 3

When does the cranial neuropore closes?

Answer 3

Day 25th

Question 4

When does the caudal neuropore closes?

Answer 4

27th day

Question 5

When does the neutral tube closes?

Answer 5

• the neutral tube soon closes and separates from the surface ectoderm
• the free edges of the surface ectoderm fuse so that this layer becomes the back of the embryo

Question 6

What is the formation of the neural crest?

Answer 6

• As the neural folds fuse, some of the neuroectodermal cells lose their attachments to the neighboring cells.
• These cells migrate dorsolaterally on each side of the neural tube.
• They form a flattened mass, the neural crest cells between the neural tube and overlying ectoderm.
• They split into right and left parts and follow 2pathways.

Question 7

What is the name of the cells that move dorsally and enter the ectoderm?

Answer 7

Melanocytes

Question 8

What are the formation of cells that move ventrally?

Answer 8

➢ Leptomeninges (arachnoid and piamater)
➢ Glial cells including Schwann cells
➢ Cranial nerves, autonomic (sympathetic and parasympathetic) and dorsal root ganglia
➢ Connective tissue and bones of the face
➢ Dermis in the face and neck
➢ Odontoblasts of teeth
➢ C cells of the thyroid
➢ Conotruncal septum in the heart
➢ Adrenal medulla

Question 9

What are ectodermal derivatives?

Answer 9

• Organs and structures that maintain contact with the outside world.
1. CNS, PNS
2. Sensory epithelium of eye, nose and ear.
3. Epidermis and appendages.
4. Subcutaneous glands, mammary glands.
5. Pituitary gland,
6. Enamel of teeth.

Question 10

What are the three segments that the mesoderm organizes on either side of the notochord (axial mesoderm)?

Answer 10

• Paraxial mesoderm
• Intermediate mesoderm
• Lateral plate mesoderm

Question 11

What does the paraxial mesoderm organizes into?

Answer 11

Organizes into segments called somites

Question 12

What are somites?

Answer 12

• Somites first appear in the cephalic region and proceed cephalo- caudally.
• Since somites are prominent during 4th and
5th week, they are used to estimate the embryo’s age.
• Somites form the bones of axial skeleton, its musculature, and adjacent dermis.
• New somites appear at the rate of 3 pairs/ day, until the 5th week forming 42-44 pairs.

Question 13

What are the three components that somites form?

Answer 13

1. Sclerotomes: Surround the spinal cord and notochord to form the vertebral column.
2. Myotome: migrates as precursors of limb and body wall muscles.
3. Dermatomes: form the overlying dermis and subcutaneous tissue of skin.

Question 14

What does the intermediate mesoderm form?

Answer 14

• In the cervical region, the intermediate mesoderm forms nephrotomes and nephrogenic cord.
• This intermediate mesoderm forms:
1. Excretory units of the urinary system
2. Gonads

Question 15

What are formed within the lateral plate mesoderm?

Answer 15

Intercellular cavities

Question 16

What is The layer continuous with mesoderm covering amnion known as?

Answer 16

Somatic or parietal mesoderm

Question 17

What is The layer continuous with mesoderm covering yolk sac known as ?

Answer 17

splanchnic or visceral mesoderm- wall of the gut.

Question 18

What is the intraembryonic coelom?

Answer 18

• The intraembryonic coelom is continuous with the extraembryonic coelom on either side of the embryo- Lateral and ventral body wall.
• The cells facing the coelomic cavity - mesothelial membranes that line the pleural, pericardial and peritoneal cavities.

Question 19

What are the derivatives of lateral plate mesoderm?

Answer 19

1. Somatic mesoderm- closest to ectoderm
   • body wall and limb osteogenic, chondrogenic and fibrogenic
   • except ribs and scapula
2. Splanchnic mesoderm - closest to endoderm
   • heart and smooth muscle of GIT and blood vessels

Question 20

What are mesodermal derivatatives?

Answer 20

• Supporting tissue such as connective tissue, cartilage and bone.
• Striated and smooth musculature.
• Blood and lymph cells and walls of the heart, blood and lymph vessels.
• Adrenal cortex
• Spleen

Question 21

What are endodermal derivatatives?

Answer 21

• The GIT is the main organ formed.
• Epithelial lining of respiratory tract.
• Parenchyma of glands, thyroid, parathyroid, pancreas.
• Parenchyma of tonsils and thymus.
• Epithelial lining of urinary bladder and urethra.
• Epithelial lining of tympanic cavity and auditory tube.

Question 22

What is the folding of the embryo?

Answer 22

• Folding of the embryo is a significant event in the establishment of body form.
• The flat trilaminar embryonic disc folds into a cylindrical embryo.
• Cephalocaudal folding: folding of the ends of the embryo ventrally produces head and tail folds that move the cranial and caudal regions ventrally.
• Lateral folding:
folding of the sides of the embryo produces right and left lateral folds

Question 23

What is the lateral folding of the embryo?

Answer 23

• Ventral fusion reduces the region of communication between the intra and extra embryonic coelom.
• The amniotic cavity expands and obliterates the extra-embryonic coelom.
• Eventually the amnion forms the epithelial covering of the umbilical cord.
• As a result of lateral folding, the connection with the yolk sac is obliterated.
• A part of the secondary yolk sac becomes incorporated into the fetus as the mid gut.

Question 24

What is the folding of neural plate?

Answer 24

• As a result of lateral folding, the
neural plate also folds and unites on either side.
• The upper and lower ends do
not fold forming the cranial and
caudal neuropores.

Question 25

What happens to the embryos in the 4th week?

Answer 25

• Embryo initially straight
• Fusion of neural folds in the cervical region.
• Cranial neuropore begins to close (25th day).
• Caudal neuropores close 2 days later(27th day).
• Optic vesicles and otic placodes appear

Question 26

What happens to the embryo in the 5th week?

Answer 26

• Upper limb paddle shaped.
• The head looks bigger due to rapid development of brain.
• Face contacts heart prominence.

Question 27

What happens to the embryo in the 6th week?

Answer 27

• Hand plate with digital rays.
• Trunk and neck begin to straighten.
• Pigmented eye
• Auricular hillocks- primordia of auricles.

Question 28

What happens to the embryo in the 7th week?

Answer 28

• Digital rays visible in foot.
• Upper lip formed.
• Trunk elongates and strengthens.
• Abdomen bulging due to liver and mid gut.
• The heart beat can be recognised by ultrasound

Question 29

What happens to the embryo in the 8th week?

Answer 29

• Upper and lower limbs well formed.
• Digits separated.
• Human appearance, with a still larger head
• Scalp vascular plexus visible.
• External genitalia have sexless appearance

Question 30

What happens at the end of the 4 weeks?

Answer 30

• C shaped curvature due to head and tail folds.
• Four pharyngeal arches.
• Upper limb buds
• Heart prominence
• Somites present

Question 31

What happens to the embryo in the 9th week until birth?

Answer 31

• Rapid growth of body.
• Maturation of tissues and organs.
• Growth in length is more during third, fourth and fifth months. (12- 20 weeks)
• Growth in weight is more during the last two months (32- 36 weeks).
• Length of pregnancy is 38 weeks after fertilisation or 40 weeks after the last menstrual period (LMP).

Question 32

What happens at the beginning of third month?

Answer 32

• Body grows rapidly. By the end of third month, the length of the foetus has doubled.
• Face is more human looking.
• Ears reach definitive position.
• External genitalia starts to differentiate.
• The liver is the site of erythropoiesis.
• Urine formation begins.

Question 33

What happens to the embryo at 12 weeks?

Answer 33

•Skin is extremely thin.
• Face more human like.
• Intestinal loops withdraw into abdominal cavity.
• Movements begin but not felt by mother.
• The spleen is the site of erythropoiesis.

Question 34

What happens to the embryo to 13-16 weeks?

Answer 34

• Rapid body growth.
• Limbs lengthen.
• Movements more, but slight not to be felt by mother.
• Bones of foetal skeleton visible in USG
• Eyes that were lateral now come to the ventral aspect.

Question 35

What happen to the embryo from 17-20 weeks?

Answer 35

•Foetus covered with lanugo hair which holds vernix caseosa.
• Skin is covered by whitish, cheesy, fatty substance vernix caseosa composed of secretory products from foetal sebaceous glands and dead epithelial cells (to protect foetal skin from amniotic fluid)
• Brown fat is formed, produces heat by oxidising fatty acids.
• Mother recognises movement (quickening)

Question 36

What happens to the embryo from 21-25 weeks?

Answer 36

• Foetal looks proportionate but lean.
• Skin of the foetus is reddish with a wrinkled appearance due to lack of underlying connective tissue.
• Rapid eye movements begin (blink- startle response)
• By 24 weeks, lung cells secrete surfactants.
• Fingernails present.
• Foetus born during the 6th month or first half of 7th month has difficulty surviving.

Question 37

What happens to the embryo from 26-29 weeks?

Answer 37

• Eyelids open.
• Toe nails visible.
• Foetus survives if born prematurely under intensive care.
• Lungs have developed sufficiently.
• NS has matured to direct rhythmic breathing movements and control body temperature.
• Smoothing out of wrinkles due to presence of subcutaneous fat.
• By 28 weeks, bone marrow site of erythropoiesis.

Question 38

What happens to the embryo from 30-34 weeks?

Answer 38

Skin is pink and smooth.
• Upper and lower limbs have chubby appearance.
• Foetus born 32 weeks and older survive if born prematurely.

Question 39

What happens to the embryo from 35-38 weeks?

Answer 39

• Term: Baby ( ◠‿◠)
✿
• Head appears smaller compared to the body
but still the largest part of the body as it has to pass through the birth canal.
• Foetus appears plump as there is deposition of subcutaneous fat.
• NS well differentiated.
• Testes descend into scrotum.

Question 40

How to calculate EDD?

Answer 40

• 266 days or 38 weeks after fertilisation.
• To calculate EDD (Naegele’s method): count back three months from first day of last menstrual period (LMP). Then add 7 days and one year.

Question 41

How to determine the age of the embryo?

Answer 41

• An accurate determination of foetal size and age is essential for managing pregnancy.
• Onset of LMP- holds good for women with regular menses.
• Ultrasound- morphological characteristics
• Foetal length and weight.
• 4th-6th weeks - somites
• 7th-14th weeks - crown-rump length (CRL)
• 16th-30th weeks -
  i) biparietal diameter (BPD)
  ii) Head and abdominal circumference
  iii) length of femur

Question 42

What is the post-maturity syndrome?

Answer 42

• Prolongation of pregnancy > 3 weeks than EDD.
• Overweight foetus. Sometimes long, thin baby indicating wasting.
• Dry-parchment like skin.
• Absence of lanugo and vernix caseosa.
• Long nails, increased alertness.
• Increased rate of mortality, labour is induced.