THE SEXUAL CELLS
EMBRIOGENESIS
ASSOCIATE PROFESSOR
IOLANDA BLIDARU
MD,PhD
The sexual cells = gametes
The spermatozoon
Origin - spermatogenesis
seminiferous tubule epithelium of
the testis
- fibromuscular wall
- Sertoli cells
- germ cells in different stages
- vessels
- Leydig cells (steroidogenesis)
The spermatozoon
Spermatogenesis: 74 days / continuously
Spermatogonia (46XY)
▼
mitosis
Spermatocyte I (46XY)
▼
meiosis
Spermatocytes II (23X) + (23Y)
▼
mitosis
Spermatides (23X) + (23Y) + (23X) + (23Y)
▼
metamorphosis
Spermatozoa (4)
The spermatozoon morphology
The head
 The
acrosome: hydrolytic enzymes
(hyaluronidase, acrosine)
mecanisms of fertilization
The connective piece
The flagellum (the tail)
 microtubular
complex
 sliding  motility (wave  180° rotation 
wave)
The pre-fertilization transformations of
the spermatozoa
 Motility
 Fertilization
capability
 Capacitation – in the female genital tract
 increased motility
 loosing material from the acrosome
surface
 exposing the receptors
 The acrosome reaction - a final
maturation
The passage of the spermatozoa in the female
genital tract
The vagina
 pH = 5 (semen - pH = 7)
 5 min – 1 hour (the vagina → the tube)
The cervical canal
 filter & reservoir (200.000 – 400.000, 24 hours)
 cervical mucus (tricot-like) – permisivity
The utero-tubal junction
 filter & reservoir
 constant concentration (1000 → a few hundreds in
the ampullary part of the tube, 2-34 hours)
The ovum
Ovogenesis & Folliculogenesis
The embrionic-fetal life
germinal epithelium (the cords) = primordial follicles
The primordial follicle

ovogonia 20 microns
▼ mitosis

oocyte I
▼ blocked in the prophase of the first
meiotic division !

granulosa cells layer
basal membrane Slavjanski

Ovogenesis & Folliculogenesis
At puberty - 300.000 follicles in the ovaries
Folliculogenesis
follicle maturation - 3 months
The primary follicle
 oocyte
I (30-60 microns)
 granulosa cells layer
 zona pellucida
 Slavjanski membrane
Ovogenesis & Folliculogenesis
The secondary follicle

oocyte I (45-70 microns)
The tertiary follicle (Call – Exner follicle)





oocyte I (60-80 microns)
granulosa cell massif
zona pellucida
theca interna (cellular)
theca externa (fibrilar)
The antral follicle



oocyte I (90 microns) +
cumulus proliger – cAMP
corona radiata
Ovogenesis & Folliculogenesis
The mature follicle (de Graaf)
15-20mm, unique/cycle
oocyte I (100 microns), peripheral
 granulosa membrane
 Slavjianski membrane
 theca interna  E
 theca externa
 follicular cavity - follicular fluid


15-50 years → 13 ovulations / year
Folliculogenesis
The follicular development
1.
recruitment
2.
selection
3.
dominance
Ovogenesis &
Folliculogenesis

The dominant follicle → increased 17β
estradiol (8-th day) → atresia of the rest of
follicles (both ovaries)
→ LH & FSH peak → ovulation

Before the ovulation – meiosis restarts
▼
oocyte II (22x) + the first polar body
Ovulation
24 h
16-40 h
 E2 peak  LH peak

ovulation
The ovum can be fertilized 24 ore postovulation

The effects of the LH peak
 the continuation of the meiosis
 the release of the first polar body
 OMI inhibition (OMI  cumulus cells  meiosis inhibition
←cAMP)


luteinization
ovum release
Ovulation
The process of ovum transfer from the ovary to the
place of fertilization.



The phenomena: complex
Follicular apex → pellucida membrane rupture →
stigma formation → ovulation (oocyte II +
cumulus + granulosa cells + follicular fluid
release) → grasped by the fimbriated extremity of
the tube
The granulosa and theca interna cells → luteal
cells (corpus luteum)
Fecundation
Fertilization = a diploid egg = zygote
1. The ovum transfer from the ovary to the
external ⅓ of the tube - 3 mecanisms
1. intra-tubal negative pressure
2. the contraction of the tubal fimbria
3. the contact between fimbriated tubal
extremity and the cumulus
2. The spermatozoa transfer in the external ⅓ of
the tube
3. The granulosa cells (cumulus oophorus)
resorbtion
Fecundation
4. The sperm interaction with zona pellucida and
its penetration
5. Transformation of the sperm head into male
pronucleus
6. Transformation of the ovum nucleus into
female pronucleus (release of the 2-nd polar globe)
+ pronuclei attachment
7. Cromosome union ► the zygote (44+XX / XY)
Fecundation
Fecundation
Fecundation
Transformation of the head into male pronucleus. Transformation of
the ovum nucleus into female pronucleus (release of the 2-nd polar
globe) + pronuclei attachment
Segmentation. Migration

Tubal migration + segmentation (cleavage)
(3-4 days)
↓
4 blastomeres (four cell stage)
↓
8 blastomeres (eight cell stage)
↓
unequal division
(small, clear, outer mass ► trophoblast)
macromeres (large, dark cells ► embryo)
↓
micromeres
morula (12-16 blastomeres, fine zona pellucida)
↓
blastocist cavity fomation (ZP disappears)
macromeres  embryo button
Segmentation
Segmentation. Migration
Segmentation. Migration
Segmentation. Migration
Segmentation. Migration
Migration (tubal transport)
muscle contractility
 epithelial cilia activity
 tubal fluid

Implantation
Post-conceptional – 7 days – up to the implantation
- 3 days – the egg is in uterus
The stages of implantation
1. Preimplantation
2. Attachment (apposition) – to the
endometrium
3. Nidation – the blastocyst penetrates into the
endometrium → decidual transformation
4. Placentation – a connection between the
endometrial vessels and the trofoblastic
lacunae
Implantation
1. The preimplantation stage
the apical membranes are not in contact
 the blastocyst → nurturition by “grasping”
mechanism

2. The attachment stage



zona pellucida  dettachment in the day 6
membrane attachment  day 6-7
syncronization of the blastocyst - endometrium
alterations  the endocrine profile of the
implantation
The preimplantation stage. The attachment stage
The preimplantation stage. The attachment stage
Nidation. Placentation.
Nidation. Placentation.
Implantation
The 1-st week
superficial blastocyst implantatation at the
fundus
 abnormal implantation  ectopic pregnancy,
placenta praevia

The 2-nd week
Fulfilling of the implantation
 Enlargment of the contact with endometrium
  trophoblast diferentiation

Endometrium  decidua (caduca)
Implantation
Implantation
The decidua consists of three layers:
1. The superficial compact layer - decidual cells
2. The spongy (deep) layer - with glands
3. The thin basal layer.
The separation of placenta occurs through the
spongy layer
 While the endometrium regenerates again from the
basal layer.

Implantation. Development of the egg
2-nd Week
- embryonic button  2 layers = embryonic disc
(endoderm + ectoderm)
- amnionic cavity (between endoderm + ectoderm)
- Heuser membrane  delimits primitive yolk sac
 lecytocel
- lacunae in syncytiotrophoblast) - embryotroph fluid
 diffusion  embryonic disc  maternal blood
(endometrial capillaries) + eroded glands secretions
Implantation. Development of the egg
Day 10 – 2-nd week
 the
egg is completely included inside the
endometrium (protrudes)
 onset of the utero-placental circulation =
opening of the uterine vessels into ST
lacunae – fusion = network – intervillous
space
 up to the end of week 2 – proliferation of
CT inside ST the solid primitive villi
Implantation. Development of the egg
Development of the egg

Week 3
gastrulation – the mezoderm appears
► trilaminar embryo
 embryonic disc ►a tube + umbilical cord
 secundary villi
 tertiary villi - conexion with the embryonic
heart
 cytotrophoblastic shell – at the
boundaries with the endometrium

Development of the egg
Development of the egg
Week 4
amnionic cavity 
 lecytocel ddivides into the umbilical vesicle +
primitive gut (connected by vitelline duct)
 Allantois + mesenchymal tissue ► umbilical
vessels  anastomosys with vascular network
from the villi ► feto-placental circulation

Development of the egg
Development of the egg
 Weeks 4-8 (the embryonic period)
 Ectoderm → nervous system, epidermis,
adrenal medulla
 Mezoderm → skeleton, connective tissue,
muscles, cardiovacular and urogenital
systems
 Endoderm → gastrointestinal tract, liver,
pancreas, gonads, dermis, respiratory
system
 Trophoblastic invasion → extravillous
trophoblast – miometrium, spiral arterioles