Basic concept of Cell junctions

2. 
 Intercellular space in closely packed tissue is about
20nm. The cells are bound together by the specific
adhesive glycoprotein.
 Epithelial cells adhere to each other through
glycoproteins called Cadherins
 Modified cell membranes contributing in cohesion
and communication are called Cell junctions
Cell Junctions

3. 
 There are three types of Cell Junctions
1. Occluding Junctions
2. Adhering Junctions
3. Communicating Junctions
Types of Cell Junctions

4. 
 Found in epithelial tissues
 Also known as “Tight Junctions”
 Do not allow passage of small molecules form
impermiable membrane.
Types:
 Zonula Occludens
 Fascia Occludens
Occluding Junctions

5. 
 Encircles the entire cell perimeter
 Occludes the intercellular space
 Series of focal fusions
 The adjacent cell membranes approach each other,
outer leaflets fuse, diverge again then fuse again
 At fusions sites specific trans membranous proteins
named (Occludins, and Claudins) perform the
binding function
 Less in PCT and more in the intestinal mucosa
Zonula Occludens

6. 

7. 
 A strip like tight junction of limited extent
 Found between the endothelial cells of the blood
vessels
Fascia Occludens

8. 
 Anchoring junctions
 Provide cell-cell or cell to basal lamina adherence
Types:
 Zonula adherens
 Fascia adherencs
 Macula adherens (Desmosomes)
 Hemidesmosomes
Adhering Junctions

9. 
 A belt like junction
 No fusion of cell membranes
 Trans membranous glycoprotein “E-cadherin”
occupies intercellular gap
 E-cadherin links to adherent proteins in cytoplasm
which are:
 Catenin
 Vinculin
Zonula Adherens

10. 
 Structurally it is similar to Zonula adherence
 But its cell junction is strip-like and (not ring-like or
belt-like)
 i.e. Cardiac muscle cells.
Fascia Adherens

11. 
 Macula adherins are commonly known as desmosomes
 “Spot-weld” like junctions
 Randomly distributed along lateral plasma membranes
of the cells in simple epithelium
 In stratified epithelium it is distributed throughout the
plasma membrane
 It is also found in cardiac muscle cells
Desmosomes

12. 
 Cell membrane in the region of junctions are seen
further apart (30mm) than the usual gap
 Electron dense attachment plaques are located
opposite to each other on the cytoplasmic aspects
 Intermediate filaments of the cytoskeleton are
anchored to the attachment plaques
 Two types of transmembranes glycoproteins named
Desmocolins and Desmogleins provide adherence
Desmosomes

13. 

14. 
 These junctions serve to anchor the epithelial cells to
the basal lamina
 A hemidesmosome is a spot like adhering junction
which gives appearance of a half desmosome
 In hemidesmosome transmembrane linker proteins
are integrins
 The cytoplasmic intermediate filaments of keratin
are inserted in to the attachment plaque
Hemidesmosomes

15. 

16. 
 Characterized by presence of minute tubular
passageways
 Provide direct cell to cell communication
 Tubular passages allow movement of ions and other
small molecules between adjacent cells
Communicating
Junctions

17. 
 Gap junction also called the “Nexus” which are
communication junctions, occur frequently between
the epithelial cells
 Also found in cardiac muscle cells, smooth muscles,
neurons, astrocytes, and osteocytes
 Plasma membrane of the adjoining cells are closely
opposed with a gap of only 2nm
 The gap junction contains closely packed numerous
tubular intercommunicating channels
Gap Junction

18. 
 The lumens of the channels of gap junction have an
average diameter of 1.5nm
 These channels permit free passage of ions, sugar
and amino acids
 In cardiac and smooth muscles the gap junction
provides electrical coupling of the adjacent cells
 Gap junctions are frequently found in embryonic
cells
Gap Junctions

19. 

20. 

21. 
 Static cell population
The cells incapable of mitotic division i.e.
 Neurons
 Cardiac Cells
 Stable Cell population
Division occurs on stimulation as a result of an
injury or a disease
 Renewing Cell population
Undergoes regular mitotic division i.e. epidermis of
skin and epithelial lining of GIT
Cell Renewal

22. 
 Cells belonging to renewing population undergoes a sequence of
events which are repeated over and over again
 The cycle is divided in to two parts
 M PHASE: in which mitosis occurs (30 to 60 minutes)
 INTERPHASE: it is intervening period between two cell divisions
consist of three sub phases
1- The G1 Phase (GAP-1)
- During this phase synthesis of RNA and proteins occur s
- Cell size is restored to normal
- The duration of G1 is about 8 hours
Cell Cycle

23. 
2- The S-Phase:
- During this synthesis of DNA takes place
- It results in preparation of exact replica of genetic material
and duplication of centrioles
- Duration is 8 hours
3- G2 (GAP-2) Phase:
- It is period between the end of S phase and beginning of
mitosis
- During this process production and accumulation of
energy for mitosis takes place
- Duration is 2 to 4 hours
Cell Cycle

24. 

25. 

27. 
Mitosis
• The process of cell division which results in the
production of two daughter cells from a single parent cell.
• The daughter cells are identical to one another and to the
original parent cell.

28. 
Mitosis can be divided into stages
1. Interphase
2. Prophase
3. Metaphase
4. Anaphase
5. Telophase

29. 
Interphase
The cell prepares for division
 DNA replicated
 Organelles replicated
 Cell increases in size

30. 
 Chromosomes become visible under LM
 Threads become shorter and thicker consist of two
chromatids joined by centromere
 Nucleoli disappears
 Centrioles separates and migrate
to each pole and starts giving out
mitotic spindle
Prophase
The cell prepares for nuclear division

31. 
 Chromosomes line up at the center of the cell
 Spindle fibers attach from daughter cells to
chromosomes at the centromere
 Equatorial plate is formed
 Microtubules of mitotic spindle
are attached at centromere
 Microtubules exert pull on
chromosomes
Metaphase
The cell prepares chromosomes for division

32. 
 Spindle fibers pull chromosomes apart
 ½ of each chromosome (called chromatid) moves to
each daughter cell
 Chromatids separate and move to respective poles as
an independent chromosome
 In human cell two identical sets of
46 chromosomes move to the
opposite poles
Anaphase
The chromosomes divide

33. 
 A constriction called cleavage furrow appears in
the middle of elongated cell
 Nuclear envelop is formed enclosing
chromosomes
 2 nuclei form
 Cell wall pinches in to form the
2 new daughter cells
Telophase
The cytoplasm divides

34. 

35. Mitosis -- Review
Interphase Prophase
Metaphase Anaphase
Telophase Interphase

37. 
• Meiosis is the type of cell division by which germ
cells (eggs and sperm) are produced.
• One parent cell produces four daughter cells.
• Daughter cells have half the number of chromosomes
found in the original parent cell
Meiosis

38. 
 During meiosis, DNA replicates once, but the
nucleus divides twice.
 Four stages can be described for each division of the
nucleus.
Meiosis

39. First division of meiosis

40. 
 Prophase is much longer consisting of five stages
1. Leptotene: Chromosomes becomes visible in the nucleus
2. Zygotene: Homologus chromosomes come together along
their entire length and synapses are formed
3. Pachytene: Chromosomes become thicker and shorter
Each chromosome pair is called bivalent
4. Diplotene: Chromosomes began to separate along their
length. Each bivalent consists of four chromatids
5. Diakinesis: Separation of chromosomes continue.
Nucleolus and the nuclear envelop disappears
Prophase

41. 
 A spindle of microtubules is produced by centrioles
 Equatorial plate is formed
 The bivalent chromosome pairs align in the centre of
the spindle
Metaphase

42. 
 Chromosomes of homologous pairs completely
separates and move to the opposite poles
 No division of centromere occurs and the whole
chromosomes move to opposite poles
Anaphase

43. 
 Nuclei are reconstructed
 The parent cell is divided in to two daughter cells
 Each daughter cell contains haploid (23)
chromosomes
 Each chromosome is double structured consisting of
two sister chromatids
Telophase

44. Second Division of Meiosis

45. 
Meiosis II

46. 
Differences in Mitosis & Meiosis
 Mitosis
 Asexual
 Cell divides once
 Two daughter cells
 Genetic information is
identical
 Meiosis
 Sexual
 Cell divides twice
 Four haploid daughter
cells
 Genetic information is
different