Exercises: Animal tissues

Define tissue and order these levels of organisation of the animal body from lowest to highest complexity: system, cell, organ, and tissue, giving an example of each level.

Name the four types of animal tissue and state a characteristic function of each, indicating a place in the body where it is found.

Explain the difference between tissue and organ using the heart as an example, and indicate which tissues work together in its function.

Distinguish between covering (lining) epithelium and glandular epithelium, give an example of each (the epidermis of the skin, a salivary gland) and explain the general function of each type.

Name the three types of muscle tissue (skeletal, cardiac and smooth), indicate where each one is found and whether its contraction is voluntary or involuntary.

Explain what the extracellular matrix of connective tissue is and how differences in its composition (gelatinous, calcified, liquid) explain the properties of cartilage, bone and blood.

What is a stem cell? Relate its presence to the renewal capacity of tissues and discuss why the skin renews itself constantly whereas nervous tissue hardly regenerates.

Explain the functions of adipose tissue (energy reserve, thermal insulation, mechanical protection, hormone secretion) and discuss why it is not a simple "passive filler" of the body.

Observe the four numbered microscopic sections in the image. a) Identify which type of animal tissue each corresponds to, reasoning from its appearance (cells stacked in layers, cells scattered in a mineral matrix, elongated and striated fibers, cells with long processes). b) State a function and body location for each tissue.

Relate structure and function in these four cases: the epithelium lining the mouth, the femur bone, the biceps muscle, and the brain. Explain what cellular feature of each tissue allows it to fulfill its role (lining, support, contraction, and signal conduction).

The stomach contains all four types of tissue working simultaneously. Locate each one — lining epithelium, supporting connective tissue, muscle layers that churn the contents, and neural networks that regulate them — and explain what each tissue contributes to digestion.

Blood is classified as a connective tissue. Justify this classification in terms of its cells and its matrix (the plasma), and name its three cellular components with one function of each.

Compare cartilage and bone: structure, blood supply (does cartilage have blood vessels?), hardness and capacity for regeneration. Apply this to explain why a torn meniscus takes so long to heal compared with a bone fracture.

The intestinal epithelium must be at once a selective barrier and an absorbing surface. Explain which adaptations make this possible (a single layer of cells, tight junctions, microvilli) and what would happen if those junctions failed.

Nervous tissue is not just neurons: describe the functions of glial cells (support, nutrition, myelin, defence) and discuss why they are indispensable even though they do not conduct impulses.

A superficial cut in the skin goes through epidermis and dermis. Describe which tissues and structures are damaged in each layer (vessels, nerve endings, glands) and relate the bleeding and the pain to the depth of the wound.

A biomechanics laboratory analyzes four unidentified adult animal tissue samples (A, B, C, and D):

• Sample A: dense extracellular matrix with collagen bundles oriented in a single parallel direction, sparse compressed fibroblasts, and virtually no vascularization.

• Sample B: stratified epithelium with polygonal and balloon-shaped surface cells that flatten elastically depending on luminal tension, resting upon thick layers of smooth muscle.

• Sample C: stratified squamous epithelium keratinized at its surface, with a mitotically active basal layer and abundant desmosomes and hemidesmosomes.

• Sample D: neuronal cell bodies, myelinated axons, and a dense network of supporting glial cells with extremely scarce extracellular matrix.

1. a) Reasonably associate each sample with one of these locations: Achilles tendon, urinary bladder wall, epidermis, and spinal cord.
2. b) A bioengineer proposes replacing the muscular coat of the urinary bladder with dense regular connective tissue to "make it more resistant to rupture." Refute this proposal biologically by predicting what would occur during the bladder filling and emptying phases.

Ehlers-Danlos syndrome encompasses genetic alterations that prevent the proper assembly and cross-linking of collagen fibers. In a biomechanical test, a strip of dermis and a section of aorta from an affected patient are subjected to longitudinal traction against samples from a healthy control: the patient's samples elongate with minimal force, but undergo mechanical failure under a load three times lower than that of the control.

1. a) Interpret these biomechanical results by contrasting the structural function of collagen with that of elastin in the extracellular matrix of connective tissue.
2. b) Justify at a histological level why this defect simultaneously causes spontaneous bruising (capillary rupture), markedly delayed wound healing, and joint hypermobility, identifying which specific variety of connective tissue is compromised in each case.
3. c) A student claims that, being a collagen defect, the patient's bones will not be affected because "bone tissue is exclusively a rigid inorganic mineral." Refute their claim by detailing the composition of the bone matrix and predicting how the mechanical properties of bone are altered if its organic fraction fails.

In a seminar on tissue evolution, the following statement is made: "Both plant xylem vessel elements and animal myelinated motor neuron axons have reached the maximum degree of specialization for long-distance transport, in both cases entailing the irreversible loss of viability of the conducting cell itself."

1. a) Critically analyze this statement: state with justification whether it is true or false for each cell type, describing the functional state of its cytoplasm and its organelles at maturity.
2. b) Compare how both tissues resolve mechanical support and metabolic maintenance along their path: contrast the role of glial cells (Schwann cells and astrocytes) with that of the accompanying parenchyma and sclerenchyma cells in the xylem.
3. c) Predict what immediate biophysical consequences on conductive function a microfracture with air entry into the cell wall of a xylem vessel (cavitation) and the localized autoimmune demyelination of an axon in nervous tissue will have, respectively.

A third-degree burn destroys the full thickness of the skin. a) Explain why the center of the burn barely hurts while the edge hurts intensely. b) Discuss why these burns do not regenerate on their own and need grafts (what happens to the stem cells and the skin appendages?). c) Describe which tissues take part in the healing of a more superficial wound.

Following a transmural myocardial infarction, necrotic cardiomyocytes of the left ventricle are reabsorbed by macrophages and progressively replaced by dense scar connective tissue.

1. a) Compare the morphological and functional properties of cardiomyocytes with those of scar fibroblasts. Explain why this tissue replacement prevents the immediate rupture of the cardiac wall, but irreversibly impairs both the systolic (contraction) and diastolic (relaxation and filling) function of the ventricle.
2. b) Adjacent cardiomyocytes are joined by intercalated discs rich in desmosomes and gap junctions. Predict the electrophysiological impact of inserting a mass of non-excitable fibrous tissue into this syncytial network and what clinical risk of arrhythmia it entails.
3. c) In regenerative medicine, the injection of bioprinted patches with stem cell-derived cardiomyocytes is being investigated. Deduce which two essential histological and functional requirements this patch must meet when integrating into the patient's myocardium to avoid fatal arrhythmias and pump blood effectively.

In a study on bone metabolism, turnover markers are measured in three groups of adult female rats:

• Group 1 (Control): normal levels of serum alkaline phosphatase (marker of osteoblastic activity) and urinary type I collagen telopeptides (CTX, marker of osteoclastic resorption).

• Group 2 (Ovariectomized, menopause model): CTX levels increase by 90 %, while alkaline phosphatase barely changes. Micrographs reveal perforation and loss of connectivity in trabecular bone.

• Group 3 (Ovariectomized treated with an experimental drug): CTX levels decrease by 60 % compared to Group 2, and alkaline phosphatase increases by 35 %.

1. a) Deduce on which target cell type the experimental drug acts to inhibit its function and on which one to stimulate osteogenesis. Justify it by analyzing the markers.
2. b) Explain why the cessation of estrogens in Group 2 triggers a net loss of bone mass, detailing the coupled role between osteoblasts and osteoclasts in continuous bone remodeling.
3. c) An astronaut who spends six months on the International Space Station in weightlessness suffers a trabecular bone loss similar to that of Group 2, despite having normal hormone levels. Justify at a cellular level why the removal of mechanical loading alters the balance between bone formation and resorption.

A radiolabeled nucleoside analogue (which is incorporated into DNA only during the S phase of duplication) is injected into cultures of four adult human tissues with an identical cell mass: intestinal epithelium, liver parenchyma, brain nervous tissue, and hematopoietic bone marrow.

1. a) Predict with reasoning which two tissues will show intense and immediate uptake of the tracer under basal conditions and which ones will show almost undetectable incorporation, relating your answer to the existence of adult stem cells and the percentage of cells in the G0 phase.
2. b) Following a partial surgical resection of 40 % of the liver, it recovers its original mass within weeks; in contrast, an equivalent loss of tissue in the brain leaves a permanent neurological deficit. Explain the cellular difference between both processes: compensatory hyperplasia of quiescent hepatocytes versus scar gliosis without functional neurogenesis.
3. c) Analyze the relationship between tissue proliferation kinetics and carcinogenesis: deduce why more than 80 % of cancers in adult humans are carcinomas (originating in epithelia) and primary neuronal tumors in adults are extremely rare.

A bioengineering team attempts to fabricate via 3D bioprinting two artificial tissues 1.5 cm thick for surgical implants: an articular cartilage block and a skeletal muscle block. After incubating them for three weeks in a conventional bioreactor with oxygenated nutrient medium without a vascular bed:

• The cartilage construct presents viable chondrocytes throughout its thickness and a homogeneous matrix rich in type II collagen and proteoglycans.

• The skeletal muscle construct presents viable fibers only in a peripheral surface layer of 0.15 mm (150 µm); the entire central core has undergone massive cell necrosis.

1. a) Justify this disparate outcome at a biophysical and histological level. Base your reasoning on the metabolic demands of each cell type and on the physical limit of passive diffusion of nutrients and oxygen in the absence of blood capillaries.
2. b) Why does the avascular nature of cartilage represent a major advantage for its in vitro culture, yet constitute a severe physiological limitation in vivo when a sports injury occurs in the joint?
3. c) Design with reasoning two biomechanical or biological modifications (in the scaffold or in the cell types used) that would allow overcoming the diffusion limit and keeping the deep cells viable in the muscle construct.