General Embryology

Reference: Langman’s 13th; P-54

Gastrulation is an event of 3rd week of development.

Gametes are derived from Primordial germ cells

Reference: Langman’s 13th; P-42

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

Umbilical cord contain one vein.

Reference: Langman’s 13th; P-42

Reference: Langman’s 13th; P-47

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).

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

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.

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

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

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

By the beginning of 4th month,cytotrophoblastic cells and some connective tissue cells disappear.the syncytium and endothelial wall of the blood vessels are then the only layers that separate the maternal and fetal circulation.

Reference: Langman’s 13th; P-47

Reference: Langman’s 13th; P-43

Primary oocyte is produced during prenatal period.

Reference: Langman’s 13th; P-47

Thyroxine is not teratogenic. Thyroxine replacement is given during pregnancy if there is hypothyroidism.

45XO- Turner's syndrome
47 XXY- Klinefelter's syndrome
47 XX/XY+18- Edward's syndrome (Trisomy 18)
47 XX/XY+13- Patau's syndrome (Trisomy 13)

Down's syndrome:
Also known as Trisomy 21 is the most common numerical chromosomal abnormality.

Cause :
- Non disjunction (94%)- 47 XX/XY+21 (Trisomy 21)
- Mosaicism (2%)
- Transloation (4%)

Criteria of ovum at the time of ovulation:
• Ovum at the stage of secondary oocyte arrested at Metaphase of Meiosis-II 
• 120-150um in diameter
• Surrounded by cumulous oophorus/granulosa cell
• Completes 1st meiotic division 3 hours before ovulation
• Expulsion of 1st polar body just occurred
• Completes 2nd meiotic division after fertilization
• Ovulation process may last 12-48 hours
• Ovulation occurs after 36 hours of LH peak
• Inj. HCG causes ovulation after 38-40 hours

Normal site of Implantation- Anterior or posterior wall of body of uterus

Reference: Langman’s 13th; P-39

Birth defect, congenital malformation, and congenital anomaly are synonymous terms used to describe structural, behavioral, functional, and metabolic
disorders present at birth.

Susceptibility to teratogens varies with the developmental stage at the time of exposure. The most sensitive period for inducing birth defects is the third to eighth weeks of gestation, the period of embryogenesis. 

Number of blastomeres in Morula – 16

Reference: Langman’s 13th; P-40

Human placenta is - Hemochoreal

** Vitamin-D resistant rickets: X-linked dominant
** Thalassaemia & Homocystinuria: Autosomal recessive

X linked Recessive Disorders

Blood:
 Hemophilia A & B
 Glucose-6-P dehydrogenase deficiency
 Chronic granulomatous disease

Musculoskeletal:
 Duchene muscular dystrophy

Immune:
 Agammaglobinemia

Metabolic:
 Nephrogenic diabetes insipidus

Nervous:
 Fragile X syndrome

CNS development does not require influence of notochord for development.

Only IgG can cross placenta

Fate of notochord -Nucleus pulposus of intervertebral disc

Explanation: Capacitation involves removing the glycoprotein coat and seminal plasma proteins from the head of the sperm, exposing the acrosome and allowing the acrosome reaction to occur. Capacitation occurs within the female genital tract, and without its occurrence, fertilization could not occur.

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