B2.3 Cell Specialization

Gametes

• Gametes are reproductive cells.

• Male gametes are sperm.

• Female gametes are ova (eggs).

• Gametes have haploid nuclei. (half the chromosomes to combine w sperm)

• When gametes fuse during fertilization, a diploid zygote is formed.

Sperm

• male gametes

• Sperm are the smallest cells in the body with a length of 50 to 70µm, and a width of 2 to 3µm.

• Sperm are small cells with a streamlined shape, adapted to swimming.

• Sperm are composed of three sections, a head, a midpiece and a flagellum (tail)

• Adaptations of Sperm

  • The head of sperm contains a haploid nucleus and an acrosome.

    • The haploid nucleus contains the paternal genetic information which combines with the maternal genetic information at fertilization.

  • The acrosome contains hydrolytic enzymes, which help the sperm to fertilize the egg.

  • The midpiece contains many mitochondria that provide the sperm with ATP energy to swim

  • The flagellum allows the sperm to swim through the female reproductive system to reach the egg.

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Egg (Ovum)

• female gametes

• Ova (egg cells) are the cell with the largest volume with a diameter of approximately 100µm.

• Eggs (Ova) are large cells that contain a haploid nucleus with the maternal genetic information which is combined with the male genetic information at fertilization.

• Adaptations of Egg Cells

  • The ovum is surrounded by a glycoprotein matrix, known as the zona pellucida, which prevents polyspermy after fertilization.

  • The ovum contains many vesicles known as cortical granules which make the zona pellucida impenetrable to sperm after fertilization.

  • The cytoplasm of the egg contains many lipid droplets which provide the developing embryo with energy after fertilization.

Fertilization

• The fusion of gametes is called fertilization, which leads to the development of a zygote.

• When gametes fuse during fertilization, a diploid zygote is formed.

Zygote

• The fusion of gametes is called fertilization, which leads to the development of a zygote.

• The zygote is a totipotent stem cell.

• Totipotent stem cells can develop into all other cell types or develop into an embryo.

• The zygote develops into a blastocyst through cell division over five days.

• When gametes fuse during fertilization, a diploid zygote is formed.

Stem Cell

• Stem cells are undifferentiated cells which have an unlimited capacity to divide, and can differentiate into specialized cells.

Totipotent

• Totipotent stem cells can develop into all other cell types or develop into an embryo.

• Zygotes are totipotent.

Pluripotent

• Pluripotent stem cells can differentiate into all types of cells, but not develop into an embryo.

• Embryonic stem cells are pluripotent.

Multipotent

• Multipotent stem cells can differentiate into a limited range of cells.

• Adult stem cells are multipotent.

Embryonic Stem Cells

• The zygote develops into a blastocyst through cell division over five days.

• The blastocyst contains embryonic stem cells.

• Embryonic stem cells are pluripotent, meaning that they can differentiate into all other cell types but not into an embryo.

Adult Stem Cells

• The blastocyst develops into a foetus, as the embryonic stem cells differentiate into specialized cells.

• Some adult stem cells remain to replenish dying cells and repair damaged tissue.

• Most adult stem cells are multipotent, meaning that can form a range of closely related cells. 

• Hematopoietic stem cells are multipotent as they can differentiate into all types of blood cells, but not other cell types.

Specialized Cells

• All cells from a multicellular organism have the same genome.

• Stem cells differentiate into specialized cells by expressing some genes and not expressing others.

Cell Differentiation

• Stem cells differentiate into specialized cells by expressing some genes and not expressing others.

Morphogens

• The embryonic stem cells differentiate into specialized cells and tissues  in a controlled manner through a group of gene regulating chemicals, known as morphogens.

• Morphogens are a group of gene regulating chemicals (Transcription factors) which determine the specialized cell that develops according to their concentration.

• Morphogens are produced and released from embryo cells, and diffuse through tissues.

• The morphogens spread, creating a concentration gradient across tissue, with the highest concentration near the source cells.

• Morphogens bind to receptors on cells, resulting in the activation or repression of genes.

• The concentration of the morphogen determines which genes will be expressed, determining the type of specialized cell.

Stem Cell Niche

• Adult stem cells are multipotent stem cells that replenish dying cells, and repair damaged tissue.

• Adult stem cells are located in stem cell niches. Stem cell niches are locations in the body where stem cells can be maintained or promoted to proliferate and differentiate.

• exmaples:

  • Bone marrow is a niche for hematopoietic stem cells which can differentiate into all types of blood cells

  •  Hair follicles contain various pools of stem cells, such as epithelial, melanocyte, and mesenchymal stem cells that self-renew, differentiate, regulate hair growth, and maintain skin homeostasis

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Neurons

• Neurons have a width ranging from 4 to 100µm, and can have a length of over 1m.

• a specialized cell transmitting nerve impulses; a nerve cell.

Striated Muscle Cells

• long cylindrical cells that contain numerous myofibrils arranged in a highly ordered fashion, forming the contractile elements of skeletal muscle.

• Muscles contract and are involved in movement.

• There are three types of muscle cells:

  • Cardiac Muscle

  • Striated Muscle (also known as skeletal muscle)

  • Smooth Muscle

• Striated and Cardiac Muscle

  • Striated muscle is attached to the skeleton and is also known as skeletal muscle. Striated muscle is involved in the movement of bones

  • Both muscle types contain contractile myofibrils.

Scientific Model

• Scientific models are simplified versions of complex systems.

• It is possible to use cubes as model cells to identify the relationship between surface area of a cell and volume of a cell as the cell grows.

Microvilli

• The cells lining the proximal convoluted tubule include extensions of the cytoplasm known as microvilli.

• The microvilli increase the rate of absorption of nutrients and water as filtrate passes through the nephrons of the kidney.

Erythrocytes

• Erythrocytes (red blood cells) are thin and flat with a biconcave shape, which increases their surface area for the exchange of gases.

Proximal Convoluted Tubule

• The cells lining the proximal convoluted tubule include extensions of the cytoplasm known as microvilli.

• a segment of the renal tubule responsible for the reabsorption and secretion of various solutes and water.

Nephron

• each of the functional units in the kidney, consisting of a glomerulus and its associated tubule, through which the glomerular filtrate passes before emerging as urine.

Alveoli

• Gas exchange happens at the alveoli in the lungs. Alveolar epithelial tissue is composed of two types of cells which facilitate the exchange of gases.  

• Alveolar tissue is adapted for the rapid exchange of gases, and consists of two types of cells:

• 2 types of Pneumocytes

Type I Pneumocytes

• Type I Pneumocytes are long and extremely flat cells adapted for gas exchange.

Type II Pneumocytes

• Type II Pneumocytes are cuboid-shaped cells which secrete a surfactant that reduces surface tension in the alveoli and provides a liquid for rapid diffusion of gases.

• The type II pneumocytes contain many secretory vesicles (lamellar bodies), which release the surfactant by exocytosis.

Diffusion

• the movement of molecules from an area of high concentration to low concentration due to molecular kinetic energy; that is, the endless and random movement of molecules. Osmosis is a specialized type of diffusion: the diffusion of water.

Cardiac Muscle

• Cardiac Muscle: located in the heart, and is responsible for the continued rhythmic beating of the heart.

  • Cardiac muscle cells are branched and connected by intercalated discs to other muscle cells.

  • Cardiac muscle cells are branched and connected by intercalated discs to other muscle cells.

  • Electrical impulses trigger contraction of the cardiac muscle cells.

  • Intercalated discs are gap junctions between cells, and allow ions to flow between two cells. 

  • Both muscle types contain contractile myofibrils.

Striated Muscle Fibres

• Striated Muscle Fibres

  • Striated muscles are composed of long multinucleated fibres that are formed by the fusion of cells.

  • Since muscle fibres are formed by the fusion of cells, it is debatable if muscle fibres should be considered cells.

  • Striated muscle fibres are atypical multinucleate cells forming from the fusion of several cells. Striated muscle fibres can range in length from a few millimeters to several centimeters, and have a diameter of 10 to a 100µm.

Myofibrils

• A myofibril (also known as a muscle fibril or sarcostyle) is a basic rod-like organelle of a muscle cell. Skeletal muscles are composed of long, tubular cells known as muscle fibers, and these cells contain many chains of myofibrils.

Intercalated Discs

• Intercalated discs are gap junctions between cells, and allow ions to flow between two cells. 

Gap Junctions

• A Gap Junction is defined as a junctional complex that facilitates intercellular communication by allowing the passage of metabolites, ions, and second messengers between adjacent cells through connexin protein channels.

Multinucleate

• having many nuclei

• a eukaryotic cell that contains two or more nuclei within a single common cytoplasm

Acrosome

• The acrosome contains hydrolytic enzymes, which help the sperm to fertilize the egg.

• an organelle that develops over the anterior (front) half of the head in the spermatozoa (sperm cells) of humans and many other animals

Cortical granules

• The ovum contains many vesicles known as cortical granules which make the zona pellucida impenetrable to sperm after fertilization.

Flagellum

• flagellum (tail)

• The flagellum allows the sperm to swim through the female reproductive system to reach the egg.