I week of development

Dr. G. PrabavathyAssistant Professor

Department of Anatomy

Objectives

• Gametogenesis:– Spermatogenesis– Oogenesis

• Changes in the first week:– Fertilisation- stages, effects of it– Cleavage– Morula– Blastocyst– Implantation

Fertilization

• Fertilization means the fusion of two mature germ cells, an ovum and a spermatozoa, to form a single cell, the Zygote.

• It occurs in ampulla of the uterine tube.

Fertilization

Interstitial part

Stages of fertilization

a) Approximation of the gametes – involves transport of sperms and ova – Prostaglandins, Oxytocin

b) Contact and fusion of the gametes

c) Effects of fertilization

How the sperms reachthe site of fertilization?1. By movement of their

tail.2. By the movement of

uterine cilia.3. By chemo attraction.

How the oocyte reachesthe site of fertilization?1. By the movement

fimbria of the uterine tube.

2. By peristaltic movement of the uterine tube.

Secondary oocyte

• Secondary oocyte(metaphase of 2nd meiotic division) with corona radiata enters the fallopian tube.

Vitelline membrane

Contact and Fusion of the Gametes

• Out of 200 – 300 million sperms-300-500 sperms reach ovum; only one unites with oocyte.

• 3 barriers protect female gamete – Corona radiata– Zona pellucida– Vitelline membrane

Vitelline membrane

Before the sperm fertilizes the oocyte it must undergo 2 processes

Capacitation:• Final step of maturation of the spermatozoa in female

genital tract.• Requires 7 hrs of time.• Glycoprotein &seminal proteins are removed from the

surface of the sperm.

Acrosome Reaction:• It occurs after capacitation where proteolytic enzymes

as acrosin, esterases, hyaluronidase, and neuraminidase are liberated in order to penetrate the zona pellucida.

• First barrier disintegration- Hyaluronidase• Second barrier – Acrosin-zona reaction• Third barrier – disintegrin peptides

– Completion of second meiotic division, formation of mature ovum & extrusion of second polar body.

Male pronucleus – sperm head with its nuclear envelope swollen, tail degenerates.

Female pronucleus – nucleus of mature ovum.

Female pronucleus

Effects of fertilization

a) Completion of second meiotic division – mature ovum & 2nd polar body.

b) Restoration of diploid number of chromosomes in zygote.

c) Determination of chromosomal sex.d) Initiation of cleavage division of the zygote.e) Determination of polarity of the embryo.

Assisted Reproductive TechniquesIn vitro Fertilization (IVF)

Two or three of the resulting zygotes are placed back in the uterus (embryo transfer ) after 3 days of fertilization.

Intracytoplasmic sperm injection

It is the method of choice in cases of infertility due to the male factor (severe oligospermia). It implies the same process as IVF except that the best sperm is selected and injected into the ovum cytoplasm.

Stages of Embryology

1. Germinal period : 1st to 3rd week- cleavage division, blastocyst & its implantation, differentiation of trophoblast & chorion, Bilaminar & Trilaminar germ disc.

2. Embryonic period : 4th to 8th week- shape & external appearance of embryo.

3. Foetal period : 3rd month upto birth- rapid growth of fetus, complete placental development.

Cleavage

Definition: It is the repeated mitotic divisions of the zygote, resulting into a rapid increase in the number of cells that are called blastomeres.

Site: The uterine tube medial to the ampula.

Cleavage• 2 cell stage – 30hrs after

fertilization• 4 cell stage – 40-50 hrs after ftz• 12 cell stage – 72hrs after ftz• 16 cell stage – 96 hrs after ftz.

• The zygote gradually migrates from uterine tube, reaches the uterine cavity at 12 to 16 cell stage – 72 hrs after ftz.

Morula

• 12 cell to 16 cell stage.• It has mulberry appearance.• Inner cell mass – embryo proper.• Outer cell mass – protective &

nutritive coverings of the embryo.

• Presence of Zona pellucida.Zona pellucida

Blastocyst• Shortly after the morula enters

the uterus, uterine fluid passes through the ZP to form a fluid filled cavity – Blastocele.

• The morula after the formation of blastocele is called blastocyst

• Outer cell mass – trophoblast (future placenta)

• Inner cell mass – embryoblast (future embryo)

• Blastocyst is formed within ZP on 4th or 5th day of fertilization .

• Zona pellucida disappears – end of 5th day of development

• Implantation of blastocyst – 6th or 7th day after fertilization.

Blastocele.

trophoblast.

embryoblast

Implantation

Implantation

Definition: Penetration of the blastocyst into the superficial (compact) layer of the endometrium.

Time: Implantation occurs at the 6th day after fertilization and is completed about the 11th day.

• Site: The normal site is the endometrium of the posterior wall of the fundus of the uterus in or near the middle line. The endometrium after implantation is called decidua.

Types (parts) of decidua

1. Decidua basalis (part of endometrium between the blastocyst & the myometrium ).

2. Decidua paritalis (endometrium covers the wall of the uterus).

3. Deciduas capsularis (endometrium between the blastocyst & uterine cavity).

4. Decidua marginalis (endometrium surrounding the site of implantation).

Definition: It is the compact layer of the endometrium after implantation of the blastocyst.

• By the 5th day the Zona pellucida degenerates.

• Trophoblast cells penetrate the epithelium of the endometrium.

Stages of Implantation

By 6th day the blastocyst adheres to the endometriumBy 7th day, Trophoblast differentiated into 2 layers

• Cytotrophblast, inner layer, mitotically active.

• Syncytiotrophoblast (outer multinucleated mass, with indistinct cell boundary.

By 8th day the blastocyst is superficially embedded in the compact layer of the endometrium

Implantation - embedding of blastocyst into uterine lining begins at day 7

Blastocyst - with blastocoele cavity

Trophoblast - outer layer of cells

Inner cell mass - will form embryo

Lacunae and primary villi formed by trophoblastAll of these form placental tissues

Fig 28-3

Trophoblast forms syncytial trophoblast- erodes into endometriumCellular trophoblast - carries nutrients to inner cell mass

Implantation can be detected by: 1- Ultrasonography.

2- hCG (human chorionic gonadotrophin which is secreted by the Syncytiotrophoblast) about the end of 2nd week

Abnormal sites of implantation

1. Uterine ectopic pregnancy: a. At the cornu of the uterus

(the site of attachment of the uterine tube) leading to early abortion.

b. At the lower uterine segment leading to placenta previa.

c. Cervix, leading to cervical pregnancy and early abortion.

mesentry

tubeovaryInternal

os

It is also called ectopic pregnancy. It might result in death of the embryo or early abortion with severe internal hemorrhage.

1,A2,C

1,C1,B

2,A

Extra uterine ectopic pregnancy: a. The commonest site is in the Fallopian

tube leading to tubal pregnancy that leads to early abortion or tubal rupture with severe internal haemorrhage.

b. Ovary (ovarian pregnancy), broad ligament or omentum (rare).

c. Peritoneum of Douglas pouch (abdominal pregnancy).

Ectopic Pregnancy

Tubal Ectopic Pregnancy

PLACENTA PREVIA

Definition: • Implantation of the blastocyst

in the lower segment of the uterus close to the internal os.

• Thus the placenta will precede the fetus at delivery (normally, the fetus is the lowermost and fills the lower segment). Previa = at the front of the fetus.

Important questions

• Fertilisation• Cleavage• Morula• Blastocyst• Implantation• Abnormal sites of implantation

Thank You