Details the process of prenatal development through the:
1. Germinal period(Formation of gametes, Meiosis process, oogenesis, spermatogenesis, fertilization, cleavage, compaction, blastogenesis, implantation, structural development of the bilaminar embryo)
2. Embryonic period (Gastrulation, neuru...
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
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