General embryology: Summary of different event of 1st, 2nd and 3rd week of development

1^st week of development:

The main events

1.       Fertilization  

2.        Zygote formation

3.        Initiation of cleavage division

4.       Formation of morula

5.     Morula convert into blastocyst  

6.     Implantation

2^nd week of development:

It is also known as the week of twos:

1. the trophoblast (which forms supporting structure of embryo) differentiate into two layers 

1. the cytotrophoblast    
2.  the syncytotrophoblast

2.  the embryoblast forms two layers

i) epiblast and

 ii) hypoblast

3. the extraembryonic mesoderm splits into two layers                                             i)somatopleuric mesoderm and                                                                   ii)splanchnopleuric mesoderm
4. two cavities form

 i) amniotic and

 ii) yolk sac

5. Other features:

• uteroplacental circulation begin

• primary villi of placenta is formed

• chorionic cavity is formed

3^rd week of development:

The main events-

1. Appearance of  primitive streak: it is the thickening of the  epiblast at the caudal end of the embryonic disc
2. Gastrulation : formation of three germ layers – ectoderm, mesoderm and endoderm
3. Formation of the notochord:
4. growth of embryonic disc
5. establishment of body axes
6. tertiary villi are formed by the end of the third week       

Anaphase lag

Implantation : easy and brief description

Implantation: easy and brief summary

Definition :The process during which the blastocyst attaches to the endometrium and subsequently embeds in it.

It occurs at 6 days after fertilization and

by the 11 th to 12^th days of development the blastocyst is completely embedded in the endometrial stroma.

Bleeding may occur at the implantation site as a result of increase blood flow into the lacanar spaces.

 This bleeding occur near the 28^th day of menstrual cycle, it may confused with normal menstrual bleeding.

In which phase of menstrual cycle implantation is occurred?

Ans: The secretory phase, during which time uterine glands and artery become coiled.

What are the phases of menstrual cycle?

1.     Menstrual phase

2.     proliferative(follicular)  phase

3.     secretory (progestational) phase

What is ovarian cycle?

It is a regular monthly cyclic changes in the ovaries (which start at puberty )

Phases are

i)                   follicular development

ii)                ii) LH surge (ovulation)

Normal sites of implantation: The blastocyst implants in the endometrium along the anterior or posterior wall of fundus of uterus. (Trophoblast cells of blastocyst invade the epithelium and underlying endometrial stroma with the help of proteolytic enzymes.)

Decidua: means gravid endometrium

Decidua reaction:

                 i) cells of the endometrium polyhedral and loaded with glycogen and  lipids

                 ii) intercellular spaces are filled with extravasate

                 iii) the tissues is edematous

At first it occur at the site of implantation but soon spread throughout the endometrium 

Abnormal implantation:

1.     implantation in abdominal cavity

2.     implantation in the pouch  of Douglas

3.     implantation in the ampullary region of f. tube (most common)

4.     Implantation in the internal os

5.     Ovarian implantation

6.     Interstitial implantation

7.     Tubal implantation

Fertilization : easy and brief description

Fertilization : easy and brief explanation

Definition: The process by which male and female gametes fuse to form zygote

Site of  fertilization: the ampullary region of the uterine tube

Spermatozoa are not able to fertilize the oocyte immediately upon arrival in the female genital tract but must undergo a) capacitation  b) acrosome reaction to acquire this capability.

Capacitation : is a period of conditioning in the female reproductive tract that in the human lasts approximately 7 hours

The acrosome reaction: which occurs after binding to the zona pellucida, is induced by zona proteins.

The phases of fertilization include

Phase 1: penetration of the corona radiate.

Phase 2: penetration of the zona pellucida.

Phase 3: fusion of the oocyte and sperm cell membranes.

The main results of fertilization are as follows

·        restoration of the diploid number of chromosomes

·        determination of sex

·        initiation of cleavage

Egg responses in three ways when the spermatozoon has entered the oocyte

1.     cortical and zona reactions:

a) the oocyte membrane becomes impenetrable to other spermatozoa

b) zona pellucida alters its structure and composition to prevent sperm binding and    penetration

2.     Resumption of the second meiotic division

3.     Metabolic activation of the egg

Gemetogenesis : easy and brief handout

Gemetogenesis: easy and brief description 

Definition: it is the process of formation and development of male and female gametes

·         Gametes are derived from primordial germ cells.

·          It is formed in the epiblast during the 2^nd  week

·         Then  move to the wall of the yolk sac  during the 4^th week ,

·         these cells begin to migrate from the yolk sac to the developing gonad, where they arrive by the end of the 5^th week

Meiotic cell division :

Importance of meiotic cell division:

1. Restores the normal dioploid number of chromosomes from generation to   generation
2. crossovers- interchange of chromatid segments between paired homologous chromosomes

      3. Allows random assortment of maternal & paternal chromosomes between gametes

Oogenesis :

 Definition: it is the sequence of events by which oogonia are transformed into mature oocytes. 

This maturation begins before birth and completed after puberty and continues to the menopause (the permanent cessation of the menses)

Maturation of oocytes:

Before birth:

Primordial germ cells (arrived in the gonad by the end of the 5^th week)

Differentiate into oogonia ( at 5^th month its number reaches maximum about 7 million)

Primary oocytes : oogonia enlarge to form primary oocytes.

Primordial follicle – a primary oocyte with its surrounding flattened follicular epithelium.

Near the birth, the primary oocyte remain arrested in prophase of meiosis I(diplotene stage) and do not finished meiosis I before puberty

At puberty:

Primary follicle - follicular cell became cuboidal and zona pellucida begin to form

Growing follicle: follicular cells forming the stratified layer of granulose cells and well-defined zona pellucida

Secondary follicle: fluid filled spaces developed within the granulosa cells .  

Grafiaan follicle : single cavity formed within granulosa cell

Shortly before ovulation the primary oocyte completes the first meiotic division and became secondary oocyte .

The division is unequal, the secondary oocyte receives almost all the cytoplasm but the 1^st polar body receives very little.

The secondary oocyte begins 2^nd meiotic division but arrest at metaphase.

Meiosis II is completed only if the oocyte is fertilized.

Spermatogenesis

Definition: It is the sequence of events by which spermatogonia are transformed into mature sperms or spermatozoa.

This maturation process begins at puberty and continues into old age

In male the primordial germ cells remain dormant until puberty.

At puberty, after several mitotic divisions of the spermatogonia, primary spermatocytes (diploid)  are formed. They are the largest cells of seminiferous tubules and enters a prolonged prophase (22 days)

Each primary spermatocyte is completed meiosis I to form two haploid secondary spermatocytes

Subsequently, two secondary spermatocytes complete the meiosis II to form four spermatids   

Spermiogenesis :

Definition: The spermatids are gradually transforms into mature sperm or spermatozoa by a process known as spermiogenesis

These changes include:

1. loss of cytoplasm
2. formation of acrosomes which covers half the nuclear surface, derived from the golgi region of spermatid contain enzymes that assist penetration of corona radiata and zona pellucida during fertilization
3. condensation of nucleus
4. formation of neck, middle piece, and tail
5. the mitochondria arranged in the middle piece as mitochordial sheath

The time required for a spermatogonium to become a mature spermatozoon is approximately 74 days   

Terminology related with general embryology

Terminology related to general embryology

Gestational period: The whole gestation period is subdivided into three stages:

a)       Germinal period : it extends from 1^st to 3^rd week of development

b)       Embryonic period : it extends from the 4^th to 8^th week

c)       Foetal period: it extends from the third month up to termination of pregnancy  

Oocyte: the female germ or sex cell produced in the ovaries

Sperm: the male germ cell produced in the testes.

Blastocyst : after the morula enters the uterus from the uterine tube , a fluid filled cavity –  the blastocyst cavity –develops inside it . This change converts the morula into a blastocytes

Implantation: the process during which the blastocyst attaches to the endometrium and subsequently embeds in it. It occurs at 6 days after fertilization.

Gastrulation : formation of three layered embryonic germ disc

Embryo:   the developing human during its early stages of development. It extends to the end of the eighth week

Conceptus: the embryo and its adnexa or associated membranes

Fetus: after the embryonic period the developing human is called a fetus.

Gemetogenesis: it is the process of formation and development of male and female gametes

Zygote: Definition:this cell formed by the union of an oocyte and a sperm during fertilization

Cleavage: is a series of mitotic divisions that results in an increase in cells, blastomeres, which become smaller with each division.

Morula : the 12-16 cell stage of the early embryo at 3 days post conception

Menopause: the permanent cessation of the menses

Length of the pregnancy: from 1^st day of last menstrual period: 280 days or 40 weeks

                                            After fertilization: 266 days or 38 weeks

Embryonic period /period of organogenesis: most major organ and organs systems are formed during the third to eighth weeks and this period is called embryonic period or period of organogenesis.

Growth: means increase in bulk which takes place by cells division, increasing cell size or increased accumulation of intercellular substance

Differentiation: is a complicated process in which groups of cells assume special characteristics and functions

Totipotent cells: are able to produce a separate embryo under favorable conditions

Pluripotent cells: are groups of cells which produce specific type of tissue