Cell is a basic functional and structural unit of life that is capable of independent existence and arises from pre-existing cells. If we sit down to classify them then there are two basic types: Prokaryotes and Eukaryotes. It is not that they are morphologically different but also vary in their metabolic system, structure and reproduction. However be it of any type they belong to any of the five kingdoms that is Monera, Protista, Plantae, Fungi, and Animalia.
Prokaryotic Cells:
In these cells the nucleus is devoid of nuclear membrane. Genetic information is generally stored in plasmids instead of chromosomal DNA. Their size is approximately 1-2µm in diameter and 10µm in length roughly size of mitochondria. They vary in shapes also and are spherical shaped, rod shaped and spiral shaped. They divide by the process of binary fission. Prokaryotes perform various functions in the environment such as decomposers, fermentations and also play a crucial role in our digestive cycle. Prokaryotes are an integral part of nitrogen cycle thus restoring nitrates back to the soil. They also have the flexibility of using sulfur for their metabolism instead of oxygen. Bacteria and cyanobacteria belong to this class.
Eukaryotic Cells:
Their most distinguishing feature is the presence of membrane bound nucleus. Apart from nucleus they also have other membrane bound structures called as organelles. Mitochondria and chloroplast being the most important organelles perform metabolic and energy conversion functions respectively. Other than these organelles there is another structure known as endoplasmic reticulum which aids in transportation and structural support. Plant cell is also an important member of eukaryotic cells; they function in a similar way only. It is just that they have three additional structures which are cell wall, vacuoles and plastids. Members of all kingdoms except that of Monera come under this type.
There is another mode of classifying cells that is on the basis of their origin.
Cells arising from ectoderm
Ectoderm is the outermost germinal layer and there is variety of cells in human body originating from this layer.
v Gland cells: Exocrine secretary cells
- Mucous cell of Salivary Glands
- Serous cell of Salivary Glands
- Mammary gland for milk secretion
- Lacrimal gland for tear secretion)
- Ceruminous gland cell of ear
- Sebaceous gland
- Seminal vesicles secreting seminal fluid components
- Prostate gland cell
- Mucous cell of stomach lining
- Clara cell of lung
v Hormone secreting cells:
There are various glands which may be endocrine or exocrine which are responsible for secretion of hormones. Hormones are responsible for different physiological as well as metabolic activities in human body.
- Anterior Pituitary Gland: secrete different hormones like
- Somatotropes
- Corticotropes
- Thyrotropes
- Lactotropes
- Gonadotropes
- Intermediate Pituitary Gland which secrete Melanocyte stimulating hormone
- Neurosecretory Cells: it secretes hormones like oxytocin and vasopressin
- Gut and respiratory tract cells secretes an array of hormones
- serotonin
- endorphin
- insulin
- glucagon
- somatostatin
- cholecystokinin
- gastrin
- secretin
- Thyroid Gland cells
- Parathyroid Gland cells
- Aderenal Gland cells which secrete steroid hormones
- Leydig cells found in testes secrete testosterone
- Ovarian follicles secreting estrogen
- Corpus luteum cells secreting progesterone
- Juxtaglomerular cell for renin secretion
- Mesangial cells of kidney
Cells arising from epithelial cells lining the body cavities
- Synovial cell found in lining joint cavities which secrete hyaluronic acid
- Serosal cell found in lining of peritoneal, pleural, and pericardial cavities
- Squamous cell lying in the lining perilymphatic space of ear
- Squamous cell in the space of ear
- Columnar cell of ear
- cell again in the lining ear
Then there are ciliated cells with propulsive task
- Ciliated cells of respiratory tract
- Ciliated cells of ovarian follicle
- Ciliated cells in brain cavity
- Ciliated cells of rete testis
- Ciliated cells of uterus lining
Cells arising from endoderm
Endoderm is the inner most germinal layer. Generally cells of Integumentary system and Nervous system are included in this. They include
v Integumentary cells
- Keratinized Epithelial Cells
- Wet stratified barrier epithelial cells
v Nervous system
- Transducer cells
- Autonomic nervous system cells
- Sense organs
- Peripheral neuron supporting cells
- Central nervous system cells
- Glial cells
- Lens cells
Cells derived from mesoderm
Mesoderm is the middle germinal layer. It includes:
v Storage and Metabolic cells
- Liver cells
- Fat cells
v Barrier Function cells: They include cells of lung, gut, exocrine cells, kidney cells, urinogenital tract cells
v Matrix Cells: They are extracellular and include:
- Ameloblast cell for tooth enamel secretion)
- Organ of Corti
- Fibroblasts
- Tendon fibroblasts
- Bone marrow fibroblasts
- Odontoblast
- Hyaline cartilage cells
- Osteoblast/osteocyte
- vitreous cells of eye
v Contractile Cells
Skeletal muscles
Heart muscles
Stem cells
Myoepithelial cell of iris and exocrine glands
v Blood and immune cells
Erythrocytes
Platelets
Monocyte
Macrophages
Langerhans cel
Osteoclast
Dendritic cell
Microglial cell
Neutrophils
Eosinophils
Basophils
Mast cell
Helper T cell
Suppressor T cell
Cytotoxic T cell
Natural Killer cell
v Pigment Cells include retinal pigment cells and melanocytes
v Germ Cells includes oocytes, spermatids and spermatocytes
v Nurse Cells: Ovarian follicle cells, Sertoli cells of testes and thymus epithelial cells come under this category
v Interstitial cells basically that of kidney
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