Biomed 09

II) Mitosis: It takes place in somatic cells. It is an asexual reproduction for grow and replace damaged cells. It is differentiated into the following phases (stages):.

1- Prophase

@ Chromatin begins to coil and condense to form chromosomes which become visible. @ The nuclear membranes disappear. @ The nucleolus or nucleoli have disappeared. Paired centrioles (centrosomes) move to opposite ends of the cell. As they move apart, the mitotic spindle are formed. The mitotic spindle consists of: 1) The asters which radiate in a star like pattern away from each centrosome, and 2) The spindle fibers which go toward the equator of the cell.

2- Metaphase Spindle fibers grow and form attachments to the chromosomes at the centromeres. Chromosomes move to an equatorial plate (metaphase plate) which is formed along the midline of the cell between the poles. Chromosomes are found in the most condensed state. Remember that the chromosomes are still duplicated during metaphase.

3- Anaphase

Centromeres are divided leading to the formation of daughter chromosomes. Spindle fibers shorten and the daughter (sister) chromosomes are drawn to the opposite poles of the cell.

4- Telophase Nuclear membrane (envelope) is reformed (reassembled) and surrounds each set of daughter chromosomes. Nucleolus or nucleoli reappear inside the newly formed nucleus. Remember that the chromosomes are still duplicated during metaphase

Chromosomes are decondensed in the daughter cells to become chromatin and the cells are once again in interphase.

:(Cytokinesis (division of the cytoplasm .It is the division of the cytoplasm The result of mitosis plus cytokinesis is typically two genetically .identical daughter cells Both daughter cells are smaller than the original parent cell and have .unduplicated chromosomes

Interphase

Metaphase

Prophase

Anaphase

Prometaphase

Telophase

Meiosis: Meiosis is the process by which haploid cells are produced from diploid cells. Meiosis has several functions: @ Reduce the chromosome number from the diploid number (2n) to the haploid number (n). @ This guarantees the male and female gametes share in the hereditary characters of the formed zygote in sexual reproduction.

Prophase I Meiosis I

Metaphase I

Meiosis II

Prophase II

Telophase II

Anaphase I

Metaphase II

Telophase II

Telophase I

Binary fission

Comparison between mitosis & meiosis

Mitosis

.It is an indirect division

.It occurs in somatic cells

Meiosis It is a reduction division. It occurs in germ cells of gonads (testes or ovaries).

wo daughter cells are produced withFour daughter cells are produced with diploid number of chromosomes (2n). haploid number of chromosomes (n).

No crossing over takes place

.Crossing over takes place

Gametogenesis (creation of gametes) The formation of sperms in the testes is called spermatogenesis. The formation of eggs (ova) in the ovaries is called oogenesis. Gametogenesis includes three successive phases which are: I- Multiplication III- Maturation II- Growth phase and phase, phase. Oogenesis Spermatogenesis

I- Multiplication phase

2n

2n 2n

2n

Primordial germ cell

Primordial germ cell

2n

2n

Spermatogonium

2n

2n

By repeated mitotic cell division (i.e. by mitosis)

2n

2n

2n

2n

1ry oocyte

2n

Oogonium

2n

2n

II- Growth phase By growing

1ry spermatocyte

2n 1st meiotic division

n

III- Maturation phase

n

By meiosis

2nd meiotic division

n

n

n

2ry oocyte

2ry spermatocyte

n

Spermatid n Spermatozoon Mature ovum

1st meiotic division

n

n

1st polar body

2nd meiotic division

n

n

n

2nd polar body 2 polar bodies

So, each primary spermatocyte (or spermatogonium) gives four sperms. Also, each primary oocyte (or oogonium) gives one ovum (egg) and three polar bodies. The formed three polar bodies are degenerated (disintegrated). At puberty, a male will produce approximately 1000 sperm per second i.e. 30 billions/year. Each ejaculation should contain 200-300 million sperms. When the sperms are formed, they are moved into the epididymis where they become mature then stored. Ne ck

From puberty of a female to menopause, one egg is normally formed per month.

Tail

Fertilization

Matu re huma n sper m

It the fusion of two haploid gametes (sperm and egg) to produce a diploid zygote.

Sequence of fertilization 1- The acrosomes of thousands of sperms release their enzymes that destroy the protective barrier (a gelatinous material) around the ovum and clear a pathway (is called fertilization pathway) for other sperms to follow. 2- At the point of contact between the sperms and the ovum, the egg surface produces a conical projection known as the entrance. 3- Although thousands of sperms work to clear the fertilization pathway, only one sperm actually enters the ovum. This successful sperm binds with a receptor on the cell membrane of the ovum. So, the successful sperm is engulfed and enter the ovum.. 4- A biochemical changes occur that inhibit other sperms from penetration. 5- A change in the surface layer of the egg that preventing the entrance of other sperms.

Note: During fertilization, the head and the middle piece (midpiece) of the sperm pass into the cytoplasm of the ovum while the tail is cut off and remains outside.

Embryonic development

The embryonic development of any animal starts from the fertilized egg (zygote) which usually passes through three main stages which are: 1) Cleavage, 2) Gastrulation and 3) Organ formation (organogenesis). 1) Cleavage: After fertilization, the zygote divides repeatedly by a series of mitotic divisions. Zygote

vertical

2-blastomere stage

at right angle to the horizontal 4-blastomere stage 1st division double vertical 8-blastomere stage 16-blastomere stage double horizontal 32-blastomere stage (morula) 64-blastomere stage

A blastula

128-blastomere stage

The blastula @ It is a hollow structure formed at the end of cleavage process. @ Its wall is consisted of a single layer of cells. These cells are differentiated into micromeres at the animal pole and macromeres at the vegetal pole. @ The fluid filled cavity in its center is termed blastocoel. This blastocoel is not connected to the exterior.

2) Gastrulation:

The gastrula @ It is an elongated structure formed at the end of gastrulation process by flattening and invagination of macromeres of blastula. Invagination continues until the macromeres come in direct contact with micromeres. So, the blastocoel is disappeared while a new cavity is formed. @ Its wall is formed from a double layers of cells. The outer layer which is formed from micromeres (is known as the ectoderm) while the inner layer is formed from the macromeres (the endoderm forms). @ It has a cavity that called archenteron which is connected to the exterior through an opening called a blastopore.

Covering epithelium Simple epithelium

Cells arranged in one layer

Pseudostratified epithelium

All cells attached to cell membrane while some of them only reach to the free surface

Stratified epithelium Cells arranged in several layers

Squ amo Cuboida Colu mn Cili ated Pseu do Pseu do colu mn ar us e.g. l ar lining blood e.g. e.g. mucous e.g. lining of st ra tifi st ratifi the vessels & collecting membrane ed e.g. ed oesophagous serosa of tubules of of the male alimentary the kidney & alimentary & ovisac and ciliated oviduct of the Sq uamo us Colu mn a Cilia ted Tr an sitio n urethra & canal & wall bile ducts & canal of the e.g. lining of Bowman′s capsule

sweat and thyroid glands

toad

toad

ducts of parotid glands

e.g. column ar al r trachea epidermis of e.g. lining vas e.g. lining e.g. skin of conjunctiva deferens & ureter & vertebrates larynx urinary of eye (keratinized) bladder & lining of oesophagous of mammals (nonkeratinized)

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