Embryology

Reference: Langman’s 13th; P-54

*** Collecting part of the kidney (From collecting tubules up to urethra) develops from the ureteric bud (i.e. mesonephric duct)
*** Secretory part of kidney (Nephron) develops from metanephros.


Derivatives of mesonephric duct: 
a. Ureteric bud: It gives rise to collecting part of the kidney-
   • Collecting tubule & collecting duct
   • Minor calyx
   • Major calyx
   • Renal pelvis
   • Ureter
b. Trigone of urinary bladder
c. Male genital ducts:
   • Prostate & part of prostatic urethra
   • Seminal vesicle
   • Epididymis
   • Vas deferens
   • Ejaculatory duct

*** There is no male to male transmission in X-linked disorders, because in male the X chromosome comes from the mother. 

Criteria of X-linked recessive disorders:
1. Affected cases are usually males carrying the gene.
2. Homozygous females can be affected, but this is rare. 
3. Heterozygous females usually do not express the full phenotypic changes because of the paired normal allele.
4. Only females are carriers.
5. If a female has the allele (female carrier)- with a normal male, off-springs will be-
    - 50% daughters carrier, 50% normal
    - 50% sons affected, 50% normal
6. If a male has the allele (affected male)- with a normal male the off-springs will be-
    - All daughters carrier
    - All sons normal (no male to male transmission)
7. Affected cases have affected brothers and maternal uncles.
8. Since females have two copies of X-chromosomes and males have only one, X-linked recessive disease are much more common among males than in females.

Gastrulation is an event of 3rd week of development.

Septum transversum gives rise to central tendon of the diaphragm & fibrous pericardium.

Anus below the pectinate line develops from proctoderm

Gametes are derived from Primordial germ cells

Prostatic utricle & appendix of testes are fates of Mullerian duct in male

Reference: Langman’s 13th; P-42

The fetal membrane that is ruptured during labor – Amnio chorionic membrane

Factors responsible for descent of testis:
1. Gubernaculum testis (main factor)
2. Differential growth of the posterior abdominal wall
3. Intraabdominal pressure
4. Intraabdominal temperature
5. Uncurling of the fetal curves
6. Arched fibres of internal oblique
7. Male sex hormones (fetal testosterone)

After fertilization, important events that take place in the first week of development-
1. Cleavage  
2. Formation of morula (16-cell stage)
3. Blastocyst formation
4. Migration of the zygote
5. Implantation

2nd week: Formation of bilaminar germ disc
The 2nd week of development is known as the week of 2's:
a) The trophoblast differentiates into 2 layers; the cytotrophoblast and syncytiotropho-blast.
b) The embryoblast forms 2 layers; the epiblast and hypoblast.
c) The extraembryonic mesoderm splits into 2 layers; the somatic and splanchnic layers.
d) 2 cavities form; the amniotic and yolk sac cavities.

3rd week: Formation of trilaminar germ disc
- Gastrulation: The process of formation of three germ layers is called gastrulation.

(Ref: medical examination review anatomy P:213)

Umbilical cord contain one vein.

Reference: Langman’s 13th; P-42

Explanation: The septum primum and the endocardial cushions fuse to close the foramen primum. Subsequently, perforations in the upper part of the septum primum coalesce to form septum secondum. It is this foramen that closes postnatally when the left atrial pressure equals right atrial pressure.

Metanephros Gives rise to nephrons i.e. secretory/excretory part of the kidneys
• Bowman’s capsule
• Glomerulus
• PCT
• Loop of Henle
• DCT

The notochord is formed by the cells of the – Mesoderm

Reference: Langman’s 13th; P-47

Explanation: An ostium secondum defects is a large opening between the atria that can be caused either by excessive resorption of the septum primum or inadequate development of the septum secondum. Depending on the size of the defect, there can be shunting of blood between the atria.

Reference-langman’s medical embryology/13th/P-86

Autosomal Dominant Disorders
• Disorders of autosomal dominant inheritance are manifested in the heterozygous state, so at least one parent in an index case usually is affected. 
• Both males and females can be affected 
• Both sexes can transmit the condition.
• When an affected person marries an unaffected one, each child has one chance in two of having the disease (50%).
• New mutation can occur
• Variable expressivity
• The age of onset is delayed
• Vertical transmission (consecutive generations are affected)
• No carrier state
• Father-son transmission possible
• Condition usually less severe than autosomal recessive
• Usually associated with structural abnormalities

Autosomal Recessive Disorders 
• They make up the largest group of Mendelian disorders. 
• Manifested in the homozygous state. They occur when both of the alleles at a given gene locus are mutants.
• The trait does not usually affect the parents
• Both the parents are carriers
• Siblings have one chance in four of being affected (i.e., the recurrence risk is 25% for each birth); 
• May be result of a consanguineous marriage. 
• The expression of the defect tends to be more uniform than in autosomal dominant disorders.
• Complete penetrance is common
• Onset is frequently early in life. 
• In many cases, enzymes are affected by the mutation (Metabolic abnormalities).

Spinal cord- Neural tube
Adrenal cortex- Intermediate mesoderm
Ependymal cells- Neural tube

Neuroectoderm derivatives: 
1. Neural Tube: 
   a. Brain
   b. Spinal cord
   c. Neuron
   d. Neuroglia
   e. Ependymal cells

2. Neural Crest:
   a. Craniofacial bones & connective tissue
   b. Dermis in the face and neck
   c. Cranial nerve ganglia
   d. Spinal ganglia
   e. Sympathetic and pre-aortic ganglia
   f. Parasympathetic ganglia of GIT
   g. Glial cell
   h. Schwann cell
   i. C-cells of thyroid gland
   j. Melanocytes
   k. Odontoblast
   l. Adrenal medulla
   m. Leptomeninges (arachnoid & pia matter)
   n. Conotruncal septum in the heart

Explanation: The foramen cecum on the tongue is the point from which the thyroid gland evaginated and began its journey down into the neck It lies at the apex of the V shape terminal sulcus, which divides the tongue into anterior two-thirds, from the lateral lingual swellings and tuberculum imper and posterior one-third, from the copula.

Development of placenta
The placenta is the only organ in the body that develops from two different individuals
     The fetal portion develops from chorion frondosum
     The maternal portion from the decidua basalis

The process that establishes all three germ layers (ectoderm, mesoderm, and endoderm) in the embryo is called- Gastrulation

During the relative ascent of the kidneys in fetal development, the kidneys rotate 90 degrees medially so that the renal hilus is normally orientated in a medial direction.

Autosomal Recessive Disorders
• They make up the largest group of Mendelian disorders.
• Manifested in the homozygous state. They occur when both of the alleles at a given gene locus are mutants
• The trait does not usually affect the parents
• Both the parents are carriers
• Siblings have one chance in four of being affected (i.e., the recurrence risk is 25% for each birth);
• May be result of a consanguineous marriage.
•The expression of the defect tends to be more uniform than in autosomal dominant disorders.
• Complete penetrance is common
• Onset is frequently early in life.
• In many cases, enzymes are affected by the mutation (Metabolic abnormalities).

Autosomal Dominant Disorders
• Disorders of autosomal dominant inheritance are manifested in the heterozygous state, so at least one parent in an index case usually is affected.
• Both males and females can be affected
• Both sexes can transmit the condition.
• When an affected person marries an unaffected one, each child has one chance in two of having the disease (50%).
• New mutation can occur
• Variable expressivity
• The age of onset is delayed
• Vertical transmission (consecutive generations are affected)
• No carrier state
• Father-son transmission possible
• Condition usually less severe than autosomal recessive
• Usually associated with structural abnormalities

Flagella formation is not a part of spermiogenesis.

Cells that form the three primitive germ layers are derived from the – Epiblast

The adherence and attachment of blastocyst to the endometrium is referred to as implantation.
• Site: Normal site is posterior wall of the uterus close to the fundus
• Time: The process begins 6 7 days after ovulation and completed by 11 12 th day when the blastocyst is completely embedded in the stroma of the compact layer of endometrium.

Fertilization occurs at ampulla of uterine tube.

Exchange of gases such as oxygen, carbon dioxide, and carbon monoxide is accomplished by simple diffusion

• Immunological competence of the fetus begins to develop at around 3 months. By this time the fetus makes all the components of the complement
• Maternal IgG begins to be transported from the mother to the fetus at approx 14 weeks (3.5 months)
• Newborns begin to produce their own antibody, but adult levels are not reached until the age of 3 years

Ectodermal placodes are focal thickenings of the cranial ectoderm that generate many different components of the sensory systems of the head. Otic, nasal & lens placodes are found.

Hemophilia is an X-linked recessive trait.

In X-linked recessive disorders-
1. Affected cases are usually males carrying the gene.
2. Homozygous females can be affected, but this is rare.
3. Heterozygous females usually do not express the full phenotypic changes because of the paired normal allele.
4. Only females are carriers.
5. If a female has the allele (female carrier) with a normal male, off springs will be
    - 50% daughters carrier, 50% normal
    - 50% sons affected, 50% normal
6. If a male has the allele (affected male) with a normal female the off springs will be
   - All daughters carrier
   - All sons normal (no male to male

Rather it facilitates fetal movement.

-Completion of second meiotic divison of the ovum
-initiation of cleavage

Anus below the pectinate line develops from proctoderm

Explanation: It is during the third week that the notochordal process and it's associated mesoderm induce the neural plate. The hollow notochordal process eventually becomes the solid notochord, the forerunner of the axial skeleton.

Reference: Langman’s 13th; P-43

The barriers of an oocyte are
1. First barrier is formed by cells of corona radiata
2. Second barrier is formed by zona pellucida made of
glycoproteins, ZP1, ZP2, and ZP3.
3. Third barrier is formed by vitelline membrane of oocyte itself

This ability of barrier penetration is achieved by two processes:
(a) capacitation and
(b) acrosome reaction

Prevention of polyspermy:
- Rapid depolarization of the oocyte
- Cortical reaction
- Zona reaction
- Destruction of ZP3 receptor