Human Cell Biology and Anatomy: Structure, Function, and Systems

Posted on Dec 8, 2024 in Biology

Study of Man

The human body moves, deliberates, and survives adverse conditions. Its construction is complex and survives because of its brain, which is able to think abstractly and enrich perceptions, reasoning, and actions. All life is made up of cells, tiny units that make up the body. The first human cell is the zygote, or fertilized egg. Cells breathe, eat, and are able to reproduce. In the body, there are 200,000 billion cells that collaborate in survival and reproduction. Each cell is an individual that is part of a higher organism, the human body. It has a life of its own, with organs for food, metabolizing nutrients, and excreting waste. Some, such as muscle and nerve cells, respond to external stimuli. Cells can only be seen by electron microscopy, and their size is smaller than the tip of a needle.

Parts of the Cell

  • Cell membrane
  • Cytoplasm (a sticky substance composed of water, protein, fat, and carbohydrates)

Inside, the cell contains:

  • Vacuoles
  • Mitochondria: produce cellular energy
  • Ribosomes: protein synthesis occurs
  • Centrosome: involved in cell division
  • Nucleus (with its nucleolus) surrounded by a spherical nuclear membrane of 3 millimicrons. Inside are housed chromosomes, each of which contains a number of genes, or individual units of heredity, arranged in thin filaments.

Humans have 46 chromosomes, 2 of which are sex chromosomes (XY). The organization of chromosomes is offered as a double chain-shaped protein called espiremas. These proteins that constitute DNA correspond to espiremas. The number and form of chromosomes determine the disposition of the zoological species espirema; each species has a given chromosome formula. When the formula is normal, body cells grow subject to a single purpose: to achieve and maintain the integrity of this body during development. But when disturbed, cells multiply anarchically, invading the body and forming a neoplasm, such as cancer.

The cell reproduces by division, a process that results in each daughter cell producing 2 cells with the same properties as its progenitor. During division, the chromosomes become more contracted, dividing into short, thick sections throughout their length, producing two identical halves. Each half goes to an opposite pole of the cell, and then the cell starts to split at the level of the equator. Cell division occurs continuously, which is necessary to allow growth and to replace aging and dying cells.

The reproduction process begins with the fertilization of an egg by a sperm. One or the other bearing chromosomes contain genes that direct fetal development and all the functions that will develop in the adult. Once the zygote is formed, cell division, or mitosis, begins. The zygote divides into two identical cells, these in turn into 2 others, and so on, doubling the number of cells in each reproductive process.

The accumulation of cells is called a morula. When it has obtained a certain size and the exterior nutrients cannot reach the depth, the outer cells of the morula form a layer-absorbing called the trophoblast. The rest of the morula that remains inside is called the inner cell mass. It reaches the blastula stage, consisting of a trophoblast within which there is accumulated nutrient and the germinal islet.

After this stage comes the gastrula stage, needed to digest the absorbed substance. To this end, the germinal islet cells form a closed cavity membrane, constituting the endoblast or endoderm, a cavity that is capable of digesting. The germinal islet assimilates substances for nutrition and forces other waste that must be eliminated. These accumulate at the opposite pole of the germinal islet, forming a new cavity closer to the outside. This will form the ectoderm.

To facilitate the absorption process and product conduction, a sponge base is formed between the intermediate and the germinal islet cells. These sloughed germ cells are arranged like a lattice of spongy tissue between the trophoblast, the endoderm, and the ectoderm, constituting the mesenchyme. The differential of the endoderm, ectoderm, and mesenchyme is a good rest of the germinal islet that has all the vital power and trainer we saw in the morula. It is the epiblast. The epiblast, ectoderm, endoderm, mesenchyme, and trophoblast form the embryonic blastema.

Among the blastoderm layers, there is wide differentiation, and some are smaller. The mesenchyme, which is a spongy reticulum cell padding, can give rise to tissues such as blood, vessel walls, and bone tissue. These systems are arranged in the same kind of tissue to perform a basic function. Sometimes, part of the tissue conglomerates and identifies morphologically, adopting a particular form called an organ. The meeting of all bodies of the same tissue constitutes a system. But if they are of different tissues, they constitute a complex apparatus, which can function as a complex, as occasionally happens in the locomotor system. The device consists of organs, distinct from the dominant system in a single fabric.

We must distinguish the case of the same body consisting of different tissues willing to integrate nutrition and reproductive functions. These organs are called organs. A set of differentiated cells in a special way and having a function and tissue structure is determined. There are 4 types:

  • Epithelial
  • Connective
  • Muscular
  • Nervous

Epithelial Tissue

Rating:

a) As to the number of layers of cells:

I) Simple Epithelium: a single layer of cells on secretory or absorptive surfaces. Its component cells have different forms:

  • Squamous Epithelium: Forms a delicate layer of cavities covered in pleural, pericardial, and peritoneal membranes.
  • Simple Cuboidal Epithelium: Lines ducts and tubules that may have secretory or excretory properties.
  • Simple Columnar Epithelium: Appears on absorptive surfaces, such as the small intestine.

II) Stratified Epithelium: Several layers of cells, and their primary function is to protect. This is ill-suited for absorption and secretion.

III) Glandular Epithelium: Those involved in phenomena of secretion are arranged in the so-called glands. There are exocrine and endocrine glands. Exocrine glands secrete their products through a duct to the free surface. They are classified into:

  • Mesocrine: The product of discharge is proteins.
  • Apocrine: They add to their secretion of the cytoplasm (e.g., mammary glands).

Nervous Tissue

Its function is to receive external stimuli or internal environmental information, which is gathered by sensory receptors reaching the nervous system by afferent neurons. This information received is processed in the central nervous system, producing appropriate responses that are transmitted by efferent neurons. The nervous system regulates body activities such as the functioning of the viscera, movements, and behavior. It comprises:

  • Neurons: functional cells with a central nucleus and some extensions that are called dendrites (conduct impulses toward the cell body) and axons (unique, also striking cylinder axis, is efferent, transmitting impulses from the neuron cell body to the effector organ).

It is divided into:

  • Central Nervous System (CNS): composed of the brain and spinal cord.
  • Peripheral Nervous System (PNS): nervous tissue outside the CNS.

It is further divided into:

  • Voluntary: involved in voluntary functions of the body.
  • Autonomic (Vegetative): involved in involuntary functions (e.g., digestion).

Muscle Tissue

Muscles are active organs capable of producing organized movements. Their cells are called fibers and are separated by connective tissue. They need nerve fibers to initiate movement and blood vessels to nourish these fibers.

Types:

  • Skeletal or Voluntary: Responsible for the movement of bones and organs such as the eyeball and tongue.
  • Visceral or Involuntary: Part of visceral structures like blood vessels and the bladder. It is smooth.
  • Cardiac: Responsible for the continuous rhythm and contractility of the heart. It is striated.

Features:

They consist of elongated and multinucleated cells called fibers, from 10 to 100 microns in diameter and up to 35 cm in length. The nuclei are arranged in the periphery, and the cytoplasm, or sarcoplasm, possesses large quantities of organelles (vacuoles, mitochondria, sarcoplasmic reticulum, and Golgi apparatus).

Muscle Structure and Physiology

Muscle cells are grouped in bundles with endomysial connective tissue between them. These bundles are surrounded by the perimysium. Most muscles are composed of many bundles, and all this muscle mass is surrounded by the epimysium. Fibers contract at shorter lengths longitudinally, so movements are performed. When a muscle contracts, cells shorten as actin slides over myosin. Skeletal muscles are controlled by neurons coming from the CNS.

The connection between the nerve fiber and muscle is called the motor plate or neuromuscular junction. When the impulse reaches the plate, a substance called acetylcholine is released, altering the permeability of the muscle cell. This alteration creates an action potential that spreads along the fiber so that muscle contraction occurs at a time in all the myofibrils in a cell. For the muscle to contract, a nerve stimulator is always necessary. By contracting the sarcomeres of the myofibril, the overall shortening of the myofibril is achieved, and it is performed in all fibers of contracted skeletal muscle.

For contraction, energy is needed as it consumes ATP, first stored in the cells and then produced from creatine phosphate in the muscles. Skeletal muscles have a small degree of contraction called muscle tone. Normally, muscles stretch between two bones, but sometimes they attach to ligaments, cartilage, skin fascia, or organs such as the eyeball. These bonds can be made for long fiber bundle packages called tendons or aponeuroses, called connective laminae.

According to their participation in the movement, they can be:

  • Agonists: They execute the same movement.
  • Synergists: They fix certain joints so that agonists can perform different movements better.
Visceral Muscle

Smooth. Specialized for continuous contractions of the entire muscle mass instead of the contraction of motor units individually. Their cells are small, elongated, and spindle-shaped.

Heart Muscle

Like skeletal muscle, contractions are strong and use energy. Like visceral muscle, contractions are continuous and are initiated by inherent mechanisms, although modulated by external stimuli, autonomic and hormonal. Its cells are long and cylindrical, and they include the connective tissue that holds the capillary network needed to satisfy metabolic demand due to its intense and continuous activity.

Connective Tissue

It is of mesodermal origin and provides metabolic and structural support to other tissues and organs of the body. It takes the blood vessels and regulates the exchange of nutrients, metabolites, and waste products between the tissues and the circulatory system. It provides strong support to the skin and has an important metabolic role, such as the fat deposit in adipose tissue. In addition, it is a body’s defense mechanism against pathogenic microorganisms. Its most important function is to repair body tissues. It has two elements:

a) Cells:

  • Fibroblasts: responsible for the synthesis and maintenance of the extracellular material.
  • Adipocytes: responsible for the storage and metabolism of fat (adipose connective tissue).
  • Cells with immune defense functions.

b) Extracellular material (which determines the physical properties of each type of connective tissue) with two components:

  • Ground substance: formed by mucopolysaccharides. It is amorphous and transparent. It is an important barrier to microorganisms.
  • Connective Tissue Fibers: There are 3 types:

I) Collagen: the main component is collagen, found in the extracellular matrix of most connective tissues and is the major body protein. There are various types, and type III gives rise to reticulin fibers.

III) Glycoprotein: Structural fibers are composed of chains of proteins linked to branched polysaccharides.

Cartilaginous Tissue

Connective tissue formation where there is a predominance of the fundamental substance of the extracellular matrix. Glycoproteins are responsible for the solid but flexible character of cartilage. It is formed from a primitive mesenchymal cartilage precursor cell called a chondroblast. Subsequently, each chondroblast undergoes one or two mitotic divisions, forming a group of cells called chondrocytes. At the end of growth, cartilage consists of chondrocytes, including a mass of material or extracellular matrix.

On the periphery of mature cartilage, there is a zone of condensed connective tissue called the perichondrium with cartilage potential. Most cartilage is devoid of blood vessels, so the exchange of metabolites is by diffusion of water through the dissolution of ground substance.

Types of Cartilage:

  • Hyaline: the most frequent and is found in the nasal septum, larynx, and articular surfaces.
  • Elastic: in the ear, epiglottis, and walls of the Eustachian tube.
  • Fibrocartilage: is intermediate and is in the intervertebral discs, pubic symphysis, and between certain tendons and bones.

Articular Tissue

A joint is the set of structures that connect two or more bones. Types:

I) Synarthrosis: the bones are fixed by fibrous tissue that does not allow mobility. They are:

  • Syndesmosis: The most typical are cranial sutures, where the union is connective tissue that persists after bone has formed.
  • Synchondrosis: the union between the bones is hyaline cartilage.
  • Synostosis: the medium of union is bone tissue. They are firmer, like the hip, and may be a pathological phenomenon.

II) Amphiarthrosis: they are semi-mobile joints, allowing a certain mobility.

III) Diarthrosis: a joint that allows large movements, and there is a cavity between the bones. This articular surface is covered with cartilage that protects and prevents bone erosion. All of it is coated by a capsule, which is a looser fibrous structure the more mobile the joint. Reinforcing this capsule is a series of ligaments formed by bundles of collagen fibers. Both groups are rich in nerve endings that lead to the central nervous system, providing information on the position, motion, and pain in joints and involved in postural reflexes.

Coating the inside of the capsule is a thin membrane called the synovial membrane, consisting of a tissue rich in blood and lymphatic vessels and synthesizing synovial fluid. This fluid fills the joint cavity, is yellow, and has a special component of hyaluronic acid with a lubricating effect on the joint. Its analysis provides a great deal of information in arthritic problems.

Joints can form articular disks and menisci. They appear in unstable joints where joints are not congruent (e.g., the knee). The menisci have a free extremity; the disks do not.

Classification of Synovial Joints
  • Plane: allow sliding of one over another (e.g., two continued vertebrae).
  • Cotiloideas: formed by a convex surface surrounded by a dome-shaped cavity (e.g., scapulohumeral joint, hip).
  • Hinge: movements on tours throughout a transverse axis (e.g., elbow, interphalangeal).
  • Condyle: it has different joint surfaces to prevent the decoupling of movements (e.g., knee, temporomandibular joint).
  • Trochoid: those that allow rotation, the movement is in your surroundings in a single vertical axis (e.g., radioulnar joint).
  • Saddle: connecting surfaces are complementary (e.g., carpometacarpal joint of the thumb).

Bone Tissue

It is a specialized connective tissue consisting of:

a) Cells:

  • Osteoblasts: responsible for osteogenesis, they are capable of forming organic intercellular substance, on which minerals are deposited.
  • Osteocytes: typical cells of mature bone.
  • Osteoclasts: responsible for bone resorption (osteolysis).

b) Intercellular organic substances: form most of the bone substance and are formed by crystals of calcium and phosphorus in the form of hydroxyapatite, deposited on collagen fibers, providing strength to the bone and making it visible on X-rays.

In all, there are two types of bone tissue:

  • Compact: forms a layer on the periphery of the bones and consists of concentric bony plates, tightly arranged and grouped to form cylindrical structures called osteons or Haversian systems. Lamellae are arranged around canals containing nerves and blood vessel nodes; these channels are known as Haversian canals.
  • Spongy: bone plates are shaped spaces whose network is the bone marrow.

The membrane that covers the entire bone except the articular areas is called the periosteum. It provides the bone with vascularity and innervation, besides intervening in bone growth and fracture repair.

The bone marrow is a special tissue that fills the bone cavities. It may be yellow (fat) or red (with lots of blood vessels). In pregnancy, it is all red, but it becomes yellow such that in the adult, red marrow exists in the bones of the trunk and proximal parts of the bones.

Bones can be:

  • Long: It has two epiphyses and a diaphysis. During growth, between the epiphysis and the shaft is the growth plate. The area of the shaft adjacent to the disc is called the metaphysis. The shaft is compact tissue surrounding the spinal cord. The epiphyses are spongy tissue surrounded by a thin layer of compact tissue.
  • Short: cubic shape, almost all is spongy.
  • Flat: are thin and often curved (e.g., ribs, sternum) and consist of compact tissue with spongy gills.
  • Irregular (e.g., vertebrae): spongy tissue surrounded by a thin compact lamina.

Bone tissue is metabolically active and is renewed periodically, as there is reabsorption and retraining. These two processes are balanced and are under the influence of different hormones: parathyroid hormone (PTH) stimulates bone resorption to increase blood calcium. Calcitonin stimulates bone formation. Growth hormone, estrogen, and testosterone also play roles. Bones have growth potential as long as the cartilage of conjunction remains. This ceases at 18 to 20 years.