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.

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).

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.

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.

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.

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.

Two patients are admitted to the burn unit following an accident:

• Patient A: deep dermal burn on the forearm. Severe pain and sensation are preserved; the biopsy shows destruction of the epidermis and the papillary dermis, but the hair follicles and sebaceous glands of the reticular dermis are viable.

• Patient B: subdermal (third-degree) burn on the leg. The central area is whitish, leathery, and completely insensitive to pinpricks.

1. a) Justify at a neurohistological level the apparent paradox that patient B's injury, while being much more severe, is painless at its center, whereas patient A experiences acute pain.
2. b) Explain the origin of the cells that will regenerate the epidermis in patient A. Then, reason why spontaneous re-epithelialization is not viable in patient B and a split-thickness/full-thickness skin graft is required.
3. c) During the deep scarring process of patient B, myofibroblasts synthesize high amounts of collagen in a disorganized manner and contract intensely. Predict what biomechanical sequelae the adjacent joint will suffer if early rehabilitation therapies are not applied.

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.

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.

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.