A Comprehensive Guide for the Accurate Classification of Murine Hair Follicles in Distinct Hair Cycle Stages

Question 1

Hair Follicle Cycles

Answer 1

Rapid growth (anagen), apoptosis-driven regression (catagen), relative quiescence (telogen), and speculative shedding (exogen)

Question 2

Epidermis

Answer 2

The outer, nonvascular, nonsensitive layer of the skin

Question 3

Dermis

Answer 3

The thick layer of living tissue below the epidermis that forms the true skin, containing blood capillaries, nerve endings, sweat glands, hair follicles, and other structures.

Question 4

Subcutis

Answer 4

The deeper layer of the dermis, containing mostly fat and connective tissue.

Question 5

Panniculus carnosus

Answer 5

Part of the subcutaneous tissues. It is a layer of striated muscle deep to the panniculus adiposus

Question 6

Arrector Pili muscle (APM)

Answer 6

Small muscles attached to hair follicles in mammals. Contraction of these muscles causes the hairs to stand on end - known colloquially as goose bumps.

Question 7

Basement membrane (BM)

Answer 7

A thin, delicate membrane of protein fibers and glycosaminoglycans separating an epithelium from underlying tissue.

Question 8

Dermal papilla (DP)

Answer 8

Located at the bottom of the epithelial compartments and it functions as a signaling center.

Mesodermal part of the Hair Follicle, which consists of closely packed mesenchymal cells; framed by the bulb matrix during anagen

Question 9

Hair shaft (HS)

Answer 9

The hair per se, composed of trichocytes (= terminally differentiated HF keratinocytes), divided into hair cuticle, cortex and medulla

Question 10

Inner root sheath (IRS)

Answer 10

An important structure of the lower part of the hair follicle that surrounds and protects the growing hair.

Multilayered structure composed of terminally differentiated HF keratinocytes surrounded by the ORS; consists
of Henle’s layer, Huxley’s layer and cuticle; surrounds the hair shaft up to the hair canal

Question 11

Melanin (mel)

Answer 11

A dark brown to black pigment occurring in the hair, skin, and iris of the eye in people and animals. It is responsible for tanning of skin exposed to sunlight.

Question 12

Outer root sheath (ORS)

Answer 12

The outer root sheath of the hair follicle encloses the inner root sheath and hair shaft. It is continuous with the basal layer of the interfollicular epidermis.

Question 13

Sebaceous gland (SG)

Answer 13

A small gland in the skin which secretes a lubricating oily matter (sebum) into the hair follicles to lubricate the skin and hair.

Glandular structure close to the insertion of the m. arrector pili with holocrine function; composed of lipid-filled sebocytes with a foamy appearance

Question 14

Bulge

Answer 14

Contains the stem cells themselves in the stem cell niche. It maintains the ability to produce new hair progenitors

Question 15

Bulb

Answer 15

Prominent, onion-shaped thickening on the proximal end of the Hair Follicle; consists of relatively undifferentiated matrix cells, Hair Follicle melanocytes and of cells from the proximal Outer Root Sheath

Question 16

Club Hair

Answer 16

Resting hair shaft with a hollow brush of keratinized keratinocytes on the proximal end, tightly attached to the cortical cells of the hair cortex

Question 17

Connective Tissue Sheath

Answer 17

Part of the dermal connective tissue, tightly attached to the outer side of HF, composed of fibroblasts (and macrophages) and connective tissue

Question 18

Epithelial Strand

Answer 18

Column of epithelial cells between the germ capsule and the compact DP; laterally demarcated by the thickened glassy membrane

Question 19

Secondary germ capsule (secondary hair germ)

Answer 19

Bag-like structure of glycogen-free cells (germ cells) of distal ORS, surrounding the club hair

Question 20

Hair shaft medulla

Answer 20

Central part of the hair shaft, composed of large, loosely connected keratinized cells with large intercellular air spaces

Question 21

Hair shaft cortex

Answer 21

The mass of the hair shaft, composed of keratinized cells, longitudinally packed with keratin filaments (and melanin granules in pigmented hair shafts)

Question 22

Hair canal

Answer 22

Passage way between epidermal surface and the most distal part of the IRS, demarcated by surrounding ORS.

Question 23

Hyaline membrane

syn: vitreous membrane, glassy membrane

Answer 23

Outermost noncellular part of the HF; composed of basal lamina and two layers of orthogonally arranged collagen fibers; separates ORS from CTS

Question 24

Infundibulum

Answer 24

Most distal part of the HF in the dermis extending from sebaceous duct to the epidermal surface (including hair canal and distal ORS)

Question 25

Isthmus

Answer 25

Middle portion of the HF extending from the sebaceous duct to the insertion of the m. arrector pili (bulge region)

Question 26

Papillary stock syn: papillary stalk

Answer 26

Fibroblasts that link the proximal pole of the DP and the perifollicular CTS

Question 27

Basic classification criteria for HF stage

Answer 27

Can by identified via light microscopy

Question 28

Auxiliary classification for HF stage

Answer 28

Additional staining required

Question 29

Telogen basic classification

Answer 29

1. Entire hairfollicle resides in the dermis
2. no inner root sheath
3. Compact ball-shaped Dermal papilla attached to the germ cell cap

Question 30

Teogen auxilliary classification

Answer 30

1. AP+ Dermal papilla in the dermis

2. IL 1-RI+ keratinocytes in the germ capsule, germ cell cap and Outer root sheath