স্পেশাল বিসিএস (স্বাস্থ্য) - লং কোর্স

After formation, the spermatozoa are stored within the head of the epididymis. 

Spermatogenesis: The process of maturation of primordial male germ cell to mature sperm.
Onset: At puberty and continues up to old age.
Site: The wall of the convoluted seminiferous tubule 
Temperature required: 32 C.
Storage site: Head of epididymis.
Duration:
In man, the process of spermatogenesis requires 74 days, and passage through the epididymis approximately another 12 days. Therefore, it takes (74 days + 12 days) for a sperm for complete maturation and to be available for ejaculation.

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

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

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.

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

*** 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

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