Chapter 4 of Histology: Muscle & Integument: Dermis, Hypodermis & Sensory Endings. Study notes for the Block 1.2 histology practicum: smooth, skeletal and car…

Chapter 4: Dermis, Hypodermis & Sensory Endings

Below the epidermis lies the dermis, the connective tissue that gives skin its strength and elasticity, carries its blood vessels and nerves, and houses the roots of its appendages. Below that is the hypodermis, a cushion of fat that joins the skin to the structures underneath. This chapter covers both, and then the sensory nerve endings scattered through them: the reason the skin is the body's largest sense organ.

The dermis

The dermis has two layers that differ in thickness and in the kind of connective tissue they are made of.

Skin section with blue arrows marking the thin loose layer of dermis just under the epidermis — The papillary layer of the dermis (blue arrows): the thin band of loose connective tissue just beneath the epidermis, filling the dermal papillae. Histology practicum quiz

Stratum papillare (papillary layer)

A thinner layer.
Made of loose connective tissue.
Contains:
- fibroblasts, macrophages and mast cells;
- the dermal papillae, which interdigitate with the epidermal ridges (Chapter 3);
- blood vessels (capillary loops) that nourish the avascular epidermis by diffusion;
- anchoring fibrils of type VII collagen, which tie the epidermal basal lamina to the dermis;
- Meissner's corpuscles (touch receptors) in the papillae.

Stratum reticulare (reticular layer)

A much thicker layer, making up most of the dermis.
Made of dense irregular connective tissue, mainly:
- collagen fibers (type I) in thick, interwoven bundles, which give the skin its tensile strength;
- elastic fibers, which let it recoil after stretching.
Hair follicles, sweat and sebaceous glands, and larger vessels and nerves lie here.

	Papillary layer	Reticular layer
Thickness	Thin	Thick (most of the dermis)
Connective tissue	Loose	Dense irregular
Fibers	Fine collagen (types I and III), type VII anchoring fibrils	Coarse type I collagen, elastic fibers
Contents	Capillary loops, Meissner's corpuscles, immune cells	Appendages, larger vessels, Ruffini endings

With age and sun exposure, collagen and elastic fibers in the dermis degrade, which is why skin wrinkles and loses its spring. The main directions of the collagen bundles form Langer's lines; surgical incisions made along them gape less and heal with thinner scars.

The hypodermis

Low-power section of skin with a long blue arrow spanning the deep fatty layer beneath the dermis — The hypodermis (blue arrow): the deep layer of loose connective tissue and fat beneath the dermis. Histology practicum quiz

The hypodermis (subcutis, hipodermis) is:

made of loose connective tissue;
rich in adipose cells (adipocytes), grouped into lobules;
extensively vascularized;
the home of Pacinian corpuscles, sensory endings that detect pressure.

It insulates the body, stores energy, cushions against blows and lets the skin slide over the structures beneath it. Strictly speaking it is not part of the skin (it corresponds to the superficial fascia of anatomy), but it is studied with it. Its thickness varies greatly between sites and people; the layer of fat in it is called the panniculus adiposus.

Drawing of skin with the hypodermis of fat lobules and vessels at its base — Skin and its hypodermis: the lobules of fat at the bottom, with vessels passing through and Pacinian corpuscles among them. Page 19, Integumentary System module

Sensory nerve endings

Drawing of skin showing where free nerve endings, Merkel discs, Meissner, Pacinian, Krause and Ruffini endings sit, each with a small icon — The sensory endings of the skin and where they sit: free nerve endings and Merkel discs in the epidermis, Meissner's corpuscles in the dermal papillae, Ruffini and Krause endings in the dermis, and Pacinian corpuscles deep in the dermis and hypodermis. Page 35, Integumentary System module

The skin's nerve endings are either free (bare branches of the axon) or attached to a special structure: a Merkel cell, or a capsule of connective tissue and flattened cells. The capsule shapes what the ending responds to.

Corpusculum Merkeli (Merkel's disc)

Unencapsulated.
Free nerve endings attached to Merkel cells.
Located in the basal layer of the epidermis.
A receptor for light touch / tactile sensation, particularly sustained touch and fine texture (numerous on the fingertips).

Corpusculum Vater-Pacini (Pacinian corpuscle)

Drawing of an oval Pacinian corpuscle made of many concentric layers around a central nerve ending — A Pacinian corpuscle: many concentric lamellae around a central nerve ending, like a cut onion. Page 31, Integumentary System module

Oval, onion-shaped.
A nerve ending surrounded by a capsule of concentric layers (lamellae), made of collagen fibers and flattened, endothelial-like cells, with fluid between them.
Located in the deep dermis and the subcutaneous tissue.
A receptor for crude touch, sustained pressure and vibration.
Large (up to 1–2 mm), so it is easy to spot even at low power.

Two large onion-like corpuscles in the deep dermis, each marked by a black arrow — Pacinian corpuscles (arrows): large oval structures of concentric lamellae deep in the dermis. Their onion-like pattern makes them the easiest nerve ending to identify. Histology practicum quiz

Corpusculum Meissner (Meissner's corpuscle)

Drawing of an elongated Meissner's corpuscle with a branched nerve ending inside — A Meissner's corpuscle: a cylindrical capsule with a branched nerve ending winding through it. Page 32, Integumentary System module

Cylindrical (elongated, oval).
Branched, encapsulated nerve endings.
A thin capsule of connective tissue, with flattened cells arranged transversely inside it (like a stack of coins).
Located in the dermal papillae, just beneath the epidermis.
A receptor for light touch; abundant on the lips, fingertips, palms and soles.

A dermal papilla containing an oval encapsulated nerve ending with transversely stacked cells — A Meissner's corpuscle filling a dermal papilla, with its flattened cells stacked across it. Histology practicum quiz

Corpusculum Ruffini (Ruffini ending)

Drawing of a small spindle-shaped Ruffini ending with spray-like nerve branches — A Ruffini ending: a thin capsule enclosing nerve branches spread like a spray. Page 33, Integumentary System module

Cylindrical, and smaller than Meissner's corpuscles.
Nerve endings that branch in a spray-like pattern, encapsulated.
A thin capsule of connective tissue.
Located in the dermis.
A receptor for pressure (and for stretch of the skin, which signals joint and finger position).

Corpusculum Krause (Krause end bulb)

Drawing of a round Krause end bulb with a coiled branched nerve ending inside a capsule — A Krause end bulb: a round capsule with a coiled, branched nerve ending inside. Page 34, Integumentary System module

Round / spherical.
Branched, encapsulated nerve endings.
A thick capsule of connective tissue.
Located in the dermis (also in mucous membranes such as the lips and conjunctiva).
The practicum lists it as a receptor for pressure. Older textbooks called it a cold receptor; its exact role is still debated, so give the practicum's answer in the practicum.

Free nerve endings

The simplest endings: bare branches of sensory axons reaching into the epidermis. They detect pain, temperature and itch, and they are by far the most numerous. A related type wraps around the base of each hair follicle (the root hair plexus), sensing hair movement.

Summary: the five named endings

Ending	Shape	Capsule	Location	Senses
Merkel	Disc on a Merkel cell	None	Basal epidermis	Light touch
Meissner	Cylindrical	Thin; transverse flattened cells	Dermal papillae	Light touch
Vater-Pacini	Oval, onion-like	Concentric lamellae	Deep dermis, hypodermis	Crude touch, pressure, vibration
Ruffini	Cylindrical, small	Thin; spray-like branches	Dermis	Pressure, stretch
Krause	Round	Thick	Dermis	Pressure (practicum)

Two contrasts are asked most often:

Meissner vs Pacini: Meissner is small, cylindrical and high up in the papillae (light touch); Pacini is large, onion-like and deep (pressure and vibration).
Meissner vs Ruffini vs Krause: all encapsulated and branched. Meissner is in the papillae; Ruffini is small with a spray of branches; Krause is round with a thick capsule.