Human Embryology
Thursday, 28 October 2021 10:27 AM
Prenatal development
- From conception to birth: 3 stages
○ Germinal period
§ First 2 weeks of development
§ Formation of primitive germ layers
§ Forms the bilaminar then the trilaminar germ disc
□ 3 layers
○ Embryonic period
§ 3rd-8th week
§ Organ systems develop
□ Embryogenesis/organogenesis ; first eight week
○ Fetal period
§ Last 30 weeks
□ Begins at 9th week and continues till birth
§ Organ systems grow and mature
- Clinical age
○ Mothers last menstrual period (LMP) used to calculate age of
unborn child
- Postovulatory age
○ 14 days less than clinical age
Formation of gametes
- They are derived from primordial germ cells(PGC)
○ They are derived from epiblasts
§ Early stage embryo has two layers
□ Dorsal layer is epiblast layer
○ The PGC will migrate to yolk sac in embryo after 2nd week
- Two gametes will fuse to form a zygote
- Epiblast (PGC)
○ At 2nd week they migrate to the yolk sac
○ PGC reach yolk sac at around 4th week
○ With further migrate to the gonads
○ During 4-5th week migrate to gonads
§ Forms the testes and ovaries
§ At this stage most PGC will have reached the gonads at
5th week
○ Will then undergo gametogenesis
- Gametogenesis
, ○ During 4-5th week migrate to gonads
§ Forms the testes and ovaries
§ At this stage most PGC will have reached the gonads at
5th week
○ Will then undergo gametogenesis
- Gametogenesis
○ Spermatogenesis
○ Oogenesis
Meiosis
- First meiotic division
○ Interphase
§ DNA replicates
○ Prophase
§ Chromosomes condense
§ Pairing begins
§ Pairing of chromosomes
§ Chiasma formation
□ Recombination or crossing over occurs
○ Metaphase
§ Chromosomes line up next to each other at center of the
cell
§ Spindle attach to each kinetochore of each chromosome
§ Pulling apart of double structured chromosomes begins
○ Anaphase
§ Pulled apart
○ telophase
§ Call formation
§ Cell contains 23 doubled structured chromosomes
○ Skip DNA synthesis before next round
- Second division
○ Cells now contain 23 single chromosomes
○ Haploid
- Cytodifferentiation
○ Continue to change shape and structures
○ The development of specialized cells
- After gametes fuse they restore diploid genotype
Oogenesis
- Begins in early stage of development and lasts till puberty
- The sequence of events by which oogonia are transformed into
mature oocytes
- Occurs in the cortex of the ovary
- Differentiation of PGCs into oocytes
○ Primordial germ cells start to divide and differentiate into
oogonia after arriving at the ovary
, -
mature oocytes
- Occurs in the cortex of the ovary
- Differentiation of PGCs into oocytes
○ Primordial germ cells start to divide and differentiate into
oogonia after arriving at the ovary
§ After moving to gonads they are considered as oogonia
§ they undergo mitotic division
□ Not meiotic
§ To increase the number of cells
§ Start to form Oogonium
□ First stage of oogenesis
§ Keeps undergoing mitotic division to form primary
oocyte till the third month of development
○ Third month of development
§ Primary oocyte begins first meiotic division
□ Doesn’t complete it
□ arrests at prophase 1
□ Since they stop in meiosis, when triggered they
cannot divide mitotically
§ At birth, there are lots of primary oocytes in the
primordial follicles
□ Will remain like this till puberty since the ovary is
inactive
□ When puberty occurs, ovary will be stimulated and
form primary follicle
○ Primary oocyte forms the mature vesicular follicle
§ Formation of primordial follicle
® Whereas development involves
cytodifferentiation
□ Smaller dormant follicle with single layer of
squamous granulosa cells and a basal lamina
□ All primordial follicles are formed between 6-9
month of gestation
® And all of them are present at birth in ovary
® Remains in this status for long time until
puberty begins
□ Grows and forms more layers of follicular cells
surrounding it
§ Begin to form primary follicle after it is recruited
□ The growth of follicles is achieved by cell
proliferation and formation of follicular fluid
□ Granulosa cells begin to change shape from
squamous to cuboidal
□ Has multilayer of cells
® Zona pellucida
◊ Surrounds the primary oocyte
Thursday, 28 October 2021 10:27 AM
Prenatal development
- From conception to birth: 3 stages
○ Germinal period
§ First 2 weeks of development
§ Formation of primitive germ layers
§ Forms the bilaminar then the trilaminar germ disc
□ 3 layers
○ Embryonic period
§ 3rd-8th week
§ Organ systems develop
□ Embryogenesis/organogenesis ; first eight week
○ Fetal period
§ Last 30 weeks
□ Begins at 9th week and continues till birth
§ Organ systems grow and mature
- Clinical age
○ Mothers last menstrual period (LMP) used to calculate age of
unborn child
- Postovulatory age
○ 14 days less than clinical age
Formation of gametes
- They are derived from primordial germ cells(PGC)
○ They are derived from epiblasts
§ Early stage embryo has two layers
□ Dorsal layer is epiblast layer
○ The PGC will migrate to yolk sac in embryo after 2nd week
- Two gametes will fuse to form a zygote
- Epiblast (PGC)
○ At 2nd week they migrate to the yolk sac
○ PGC reach yolk sac at around 4th week
○ With further migrate to the gonads
○ During 4-5th week migrate to gonads
§ Forms the testes and ovaries
§ At this stage most PGC will have reached the gonads at
5th week
○ Will then undergo gametogenesis
- Gametogenesis
, ○ During 4-5th week migrate to gonads
§ Forms the testes and ovaries
§ At this stage most PGC will have reached the gonads at
5th week
○ Will then undergo gametogenesis
- Gametogenesis
○ Spermatogenesis
○ Oogenesis
Meiosis
- First meiotic division
○ Interphase
§ DNA replicates
○ Prophase
§ Chromosomes condense
§ Pairing begins
§ Pairing of chromosomes
§ Chiasma formation
□ Recombination or crossing over occurs
○ Metaphase
§ Chromosomes line up next to each other at center of the
cell
§ Spindle attach to each kinetochore of each chromosome
§ Pulling apart of double structured chromosomes begins
○ Anaphase
§ Pulled apart
○ telophase
§ Call formation
§ Cell contains 23 doubled structured chromosomes
○ Skip DNA synthesis before next round
- Second division
○ Cells now contain 23 single chromosomes
○ Haploid
- Cytodifferentiation
○ Continue to change shape and structures
○ The development of specialized cells
- After gametes fuse they restore diploid genotype
Oogenesis
- Begins in early stage of development and lasts till puberty
- The sequence of events by which oogonia are transformed into
mature oocytes
- Occurs in the cortex of the ovary
- Differentiation of PGCs into oocytes
○ Primordial germ cells start to divide and differentiate into
oogonia after arriving at the ovary
, -
mature oocytes
- Occurs in the cortex of the ovary
- Differentiation of PGCs into oocytes
○ Primordial germ cells start to divide and differentiate into
oogonia after arriving at the ovary
§ After moving to gonads they are considered as oogonia
§ they undergo mitotic division
□ Not meiotic
§ To increase the number of cells
§ Start to form Oogonium
□ First stage of oogenesis
§ Keeps undergoing mitotic division to form primary
oocyte till the third month of development
○ Third month of development
§ Primary oocyte begins first meiotic division
□ Doesn’t complete it
□ arrests at prophase 1
□ Since they stop in meiosis, when triggered they
cannot divide mitotically
§ At birth, there are lots of primary oocytes in the
primordial follicles
□ Will remain like this till puberty since the ovary is
inactive
□ When puberty occurs, ovary will be stimulated and
form primary follicle
○ Primary oocyte forms the mature vesicular follicle
§ Formation of primordial follicle
® Whereas development involves
cytodifferentiation
□ Smaller dormant follicle with single layer of
squamous granulosa cells and a basal lamina
□ All primordial follicles are formed between 6-9
month of gestation
® And all of them are present at birth in ovary
® Remains in this status for long time until
puberty begins
□ Grows and forms more layers of follicular cells
surrounding it
§ Begin to form primary follicle after it is recruited
□ The growth of follicles is achieved by cell
proliferation and formation of follicular fluid
□ Granulosa cells begin to change shape from
squamous to cuboidal
□ Has multilayer of cells
® Zona pellucida
◊ Surrounds the primary oocyte
