Developmental Embryology

Purnomo SoeharsoDept. of Medical Biology FMUI

Organisms (species) develop from primordial cells or/of previous organisms by sexual or asexual reproduction.

Multicellular organisms (including human) develop by sexual reproduction.

Sexual reproduction is carried out through the reunion of gametes (sex cells) fusion of male gametes (spermatozoa) & female gametes (ovum) — fertilization zygotes develop to become new organisms.

In humans — production of gametes, fertilization & generation of new organism (embryo) are

facilitated by sex organs (reproductive organs).

Gametogenesis

Primordial germ cells (PGC) occupy gonad during embryonic development. male spermatogonia seminiferous tubules. female oogonia ovarian cortex. Spermatogonia & oogonia are inactive during childhood and become active as individuals become puberty.

Gametogenesis process : - cell proliferation — mitosis - formation of haploid cells — meiosis - maturation — transformation & capacity for fertilization

Spermatogenesis

Spermatogonia are situated around the outer edge of seminiferous tubules next to the basal lamina.

1. Spermatogonia proliferate by mitosis — some stop to proliferate & differentiate into primary spermatocytes.

2. Primary spermatocytes (diploid cells) enter first meiotic division (leptotene, zygotene, pachytene

, diakinetic cells) secondary spermatocytes.

3. Secondary spermatocytes (haploid cells) undergo second meiotic division spermatids — each carrying half chromatide.

4. Spermatids undergo morphological transforma- tion — spermiogenesis forming mature functional spermatozoa

Oogenesis

Oogonia lie in the ovarian cortex — proliferate by mitosis before differentiate into primary oocytes

(occur during fetus development) produce primary (prophase arrested) oocytes — maintained

inactive until puberty.

Primary oocytes (diploid) finalize first meiosis by expelling 1st polar body carrying half set (haploid) chromosomes secondary oocytes.

Secondary oocytes undergo second meiosis by expelling 2nd polar body carrying half chromatides mature eggs (ovum)

Oocyte is surrounded by granulose cells (follicles) in the ovarian cortex, developing to mature follicles & produce estrogen under FSH stimulation.

Primary follicle secondary follicle tertiary follicle Graafian follicle ovulation.

Granulose cells of ovulated follicle transform to lutein cells under LH stimulation corpus luteum produce progesterone.

Fertilization

Normally take place in the ampulla — 1/3 proximal part of fallopian tube; only 1 sperm successfully penetrate (fertilize) the ovum.

Process of fertilization :- sperm capacitation by female secretion along fallopian tube increase sperm metabolism & motility.- as contact with ovum, the sperm release hyaluronidase to remove the follicle cells (corona radiata).

- the sperm binds to zona pellucida and induced to undergo acrosomal reaction the acrosome break and release acrosin which distroy zona pellucida and allow the sperm to penetrate in.

- the sperm reach and fuse with egg plasma membrane (vitelline membrane) the sperm bring the head contents (haploid chromosomes) into egg cytoplasm & leaf other organels (mid &

end piece) outside.

- the egg undergo cortical reaction — changes the

vitelline membrane and zona pellucida inaccessi- ble for other sperm penetration (preventing polyspermy) fertilization membrane.

- within the cytoplasm the sperm head becomes male pronucleus, while the egg necleus becomes female pronucleus.

- the two pronuclei come together to form the new diploid cell (zygote) — syngamy.

Cleavage

- the zygote undergoes a series of cleavage division, progressing through 2, 4, 8 and 16-cell stages, the cells in the cleavage stage are known as blastomeres.- blastomers continue to cleave simultaneusly into

smaller cells leading to the formation of morulla.- accumulation of fluids secreted by blastomers, signals the formation of blastocyst — consisting trophoblast and inner cell mass.

Inner cell mass ready for gastrulation the formation of 3 embryonic layers :

- Ectoderm- Mesoderm- Entoderm

Entoderm is formed from cells lining the blastocoel, the entoderm surrounding the blastocoel will becoming primitive yolk sac.

Ectoderm is formed from the delamination of entoderm creating the layer next to the former layer ectoderm is separated from trofoblast & create the vavity amnionic cavity.

Mesoderm is formed from the primitive streak during gastrulation process.

Implantation and the formation of placenta

- the embryo reach endometrium of lutheal phase, consisting compact and spongy layer.

- the trophoblast erodes compact layer, proliferate and penetrate the spongy layer endometrium increase secretion & vascularisation — decidual tissue (endometrium of pregnancy).- the proliferation of trophoblast produces syncytial

(multinucleated) cells that expand throughout the spongy layer syncytiotrophoblast & the former trophoblast lining blastocoel cytotrophoblast.

- cytotrophoblast & syntrophoblast grows forming a

mass & protrude outward chorionic villi

- chorionic villi facing/associated with basal decidua are growing more intensive & develop branches chorion frondosum. - the spaces between chorionic villis intervillous

spaces — into which maternal capillary supply blood containing oxygen & nutritive substances.- oxygen & nutritive substances diffuse to the chorionic villi transported to embryo via capillary vessels in the chorionic villi umbillical vein within umbillical cord.

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