MedicoPlexus
Embryology
Arjun Ajithan
Human Development
| Event | Day |
| Zygote formation | day 1-2 |
| Blastocyst formation | around day 5 |
| Implantation | around day 7 |
| Amniotic cavity formation | day 8 |
| Yolk sac formation | day 9 |
| Decidual reaction | day 12 |
| Formation of primary villi | day 14 |
| Formation of lacunae, junctional zone, bilaminar embryonic disc | week 2 |
| Gastrulation | day 14 |
| Formation of cranial and caudal bilaminar layer | day 15 |
| Notochord formation | day 17 |
| Formation of intraembryonic coelom | day 19 |
| Heart tube formation | day 20 |
| Neurulation, formation of somites | day 20 |
| Formation of septum transversum | day 22 |
| Formation of villentine duct, allantois | week 3 |
| Closing of cranial neuropore | day 25 |
| Blood flow | day 25 |
| Closing of caudal neuropore | day 28 |
| Formation of umbilical cord | week 4 |
| Re-segmentation of somites | week 4 |
| Start of respiratory system development | week 4 |
| Formation of primitive gut tube | week 4 |
| Formation of urorectal septum | week 4 |
| Metanephros | week 5 |
| Aortic arches | from weeks 4 to 6 |
| Start of gonadal differentiation | week 7 |
| Formation of nephrons | week 8 |
| Development of alveoli | week 36 to after birth |
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Spermatogenesis, oogenesis, fertilization
Spermatogenesis
Spermatogenesis starts in puberty as a result of the hypothalamus secreting GnRH which then stimulates LH and FSH. Leydig cells get stimulated to produce testosterone by the action of LH and Sertoli cells produce androgen binding protein by the action of FSH.
Spermatogenesis begins when a diploid spermatogonia undergoes mitosis to produce a primary spermatocyte and a spermatogonium which can act as reserve spermatogonia. The primary spermatocyte undergoes meiosis I, giving two cells with 22 chromosomes and an X or Y chromosome. These cells are called secondary spermatocytes which undergo meiosis II and turns into sperm. The sperm then matures in the seminiferous tubules and moves to the epididymis.
Sertoli cells are responsible for the inhibition of spermatogenesis by producing an enzyme- inhibin which decreased FSH production. While Sertoli cells inhibit FSH, Leydig cells are responsible for the inhibition of LH production.
Oogenesis
At birth, the ovary has about 2 million diploid primordial follicles- composed of a primary oocyte at the center surrounded by follicular cells. The maturation of the cell can be classified into three stages:
- Infancy to puberty stage
The primary oocytes stays in the prophase of meiosis I and does not undergo change while the primordial follicle turns into a primary follicle.
- Puberty
The primary follicle develops into secondary and tertiary follicle -Graafian follicle). The primary oocyte is still in prophase but has theca cells and more granulosa cells surrounding it. These two cells are responsible for producing estrogen. Later on, in this process, an antrum forms in the Graafian follicle.
- Follicular phase
The follicular cells grow, and granulosa cells produce more estrogen. The primary follicle completes meiosis I due to decreased FSH and LH and releases the secondary oocyte- a process called ovulation. The secondary oocyte is released into the fallopian tube and waits for fertilization after stopping at meiosis II.
Fertilization
The egg released by the ovary gets fertilized by the sperm in 12 to 24 hours to form a zygote. Then the single-celled zygote continues to divide to form a new human being.
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