Download Developmental Embryology

Developmental
Embryology
Purnomo Soeharso
Dept. 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 transformation — 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 inaccessible 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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