The skin is the first layer of the body. Regarding tissue organization it is the largest organ of the body. It belongs to a group of ANATOMIC Structures, designated as membranes of the body.
This structure is a complex cutaneous membrane, composed of two types of tissues, the EPITHELIUM and dense CONNECTIVE TISSUE. The outer layer called the EPIDERMIS, is epithelial tissue derived from the ECTODERM. The deeper layer called the DERMIS is dense connective tissue derived from the MESODERM.
It forms the entire outer surface of the body and is a specialized boundary membrane (lamina) which has prime importance in the life of an individual.
The SURFACE AREA of the skin is 1.5 to 2 square meters. It can be determined by following formula (Du Bois formula):
| Symbol | Description | Unit |
|---|
| AA | Resulting value | Depends on context |
| WW | Area | Square meters (m2m2) |
| HH | Height | Depends on context |
| 71.84 | Constant multiplier |
A = W x H x 71.84; where weight of individual in Kg, and Height of individual in Cm.
The Epidermis is an avascular keratinized epitheliura where wear and tear and replacement processes are going on simultaneously. The wear and tear occurs in the superficial cells (dead keratin containing cells) and the replacement occurs from basal cells which show mitotic activity. Five distinct layers of cells could be seen in the epidermis.
Histologically the DERMIS represents the lamina propria of the mucous membrane. The dermis contains the blood vessels, nerves and lymphatics. The two layers cemented together to form one structure the CUTANEOUS MEMBRANE.
Beneath the cutaneous membrane is the second layer called the subcutaneous tissue (the so called superficial fascia). Collagen fibres connect the dermis of the skin with the fascia in order to prevent us from loosing skin.
The thickness of the skin varies from 0.5-3 mm and depends on two factors. The horny layer of the epidermis and the dermis as a whole. The skin of the palms and soles is thick due to the thickness of the horny layer of the epidermis, while the dermis is thin.
Conversely the skin of the back is thicker than that of the front of the body due to the thickness of the dermis. Usually the dermis tends to be thicker on the extensor surface of the body than the flexor surface.
The colour of skin depends upon three factors :-
- The yellow tinge to the skin itself is due to the presence of CAROTENE.
- The reddish hue is due to the blood flowing through the dermis.
- Shades of brown due to MELANIN pigment. This pigment is elaborated by cells called MELANOCYTES.
These cells are scattered just beneath the basal layer of the epidermis and send their cytoplasmic processes in between epithelial cells. Some of them are also found in the dermis.
CHARACTERISTICS:
1. Finger Prints: are due to ridges and grooves found on finger tips, palms, toes and sole of the foot. These ridges appear during intrauterine life in third and fourth months of development.
The pattern does not change, but enlarge after birth and is peculiar to the individual and thus can be used to identify persons. The underlying structure of the dermis is responsible for the maintenance of established pattern over the epidermis.
2. Creases are flexure lines over joints. Here the skin is thin and usually attached deeper to the deep fascia.
3. Langer’s lines are tension lines, important for every surgeon. Incisions parallel to these lines heal with less scar tissue. These lines are due to the direction of the connective tissue fibres of the dermis which tend to be parallel to the surface of the skin. They are arranged usually circumferential in the neck and trunk; longitudinal in the limbs.
APPENDAGES OF THE SKIN:
Certain structures differentiate from the epidermis called the appendages of the skin. These are :-
A)Hair.
B)Nails.
C)Cutaneous Glands.
HAIR:
The hairs are elastic keratinous thread like structures which develop from the epidermis. They are present over the entire body except palms, soles and anal and urogenital apertures. Each hair consists of two parts. shaft is the free portion. The root is the embedded portion.
The later is enclosed in a tubular hair follicle. The lower end of the follicle is expanded called the hair-bulb. The bulb is indented at the basal surface by a connective tissue papilla.
The hair follicle is associated with a few sebaceous glands and smooth muscle cells called the Arrector Pilorum which causes erection of hair on contraction. The hair grow in different region by different rates. The most rapid rate after birth is about 12 mm per month. The cycle can be represented as such:-
- GROWTH
- TRANSITIONAL PHASE
- REST AND ELIMINATION OF OLD HAIR
- GROWTH
B) NAILS:
Nails are modified keratin plates lying on the dorsal surface of the distal phalanges of the fingers and toes. The parts to be noted are:-
- Nail Plate: The part which is visible.
Nail Root: The hidden part of the nail lying in the nail groove. Nail Folds: These are skin folds bounding the nail groves. Nail Bed: Beneath the nail plate is the stratified squamous epithelium called the nail bed.
Nail Matrix: Beneath the proximal portion of the nail fold is the epithelium from which the nail grows. The nail grows about 0.5 mm per week.
Hyponychium: Is the thickened epidermis under the free edge of the nail.
Eponychium: Is the extension of the stratum corneum of the epidermis of the proximal portion of the nail fold to the free surface of the nail plate.
CUTANEOUS GLANDS:
1. Sebaceous Glands: These are holocrine glands situated over the entire surface of the body except palms and soles. These glands pour their secretions in the hair follicles.
The Mebomian glands of the eyelids are sebaceous glands, discharging their secretions direct on the surface as there are no hair follicles. The secretion called sebum is an oily material acting as cold cream over the skin. They are more numerous over the face and scalp. These glands tend to increase in size under androgenic stimulation.
2. Sweat Glands: The ordinary sweat glands are simple, coiled, tubular glands. They are found over entire surface of the body except upon the nail beds, margin of lips, glands penis and ear drum. They are most numerous in the palms, forehead and soles.
The secretary portion lies deep in dermis and hypodermis and the excretory duct opens on the surface as sweat pores. The secretion is salty, clear watery fluid and is merocrine. But there are large sweat glands found in axilla, areola of the nipple, labia majora and around the anal margin which produce thicker secretion than that of ordinary sweat glands.
In these large glands the apices of the cells are broken off in the process of preparation which is an artifact and they ae traditionally called apocrine, although the secretion is merocrine. There are modified sweat glands in the body, like ceruminous gland of external acoustic meatus, the glands of Moll in the margins of the eye lids and the Mammary Glands.
BLOOD AND NERVE SUPPLY OF SKIN:
Skin is richly supplied through a network of vascular plexus by arterial blood from the subcutaneous arteries and is drained to the subcutaneous veins. There are cutaneous lymphatic plexuses draining to the larger lymph vessels in the subcutaneous tissue. The skin contains numerous nerve endings for the secretions.
FUNCTIONS OF SKIN:
The chief functions of the skin are:
| Number | Function |
|---|
| 1 | Sensation |
| 2 | Protection |
| 3 | Temperature Regulation |
| 4 | Control of Water Output |
| 5 | Absorption |
| 6 | Secretion |
| 7 | Excretion |
| 8 | Synthesis of Vitamin D |
| 9 | Storage |
| 10 | pH Regulation |
| 11 | Repair After Trauma |
| 12 | Social Communication of Feelings |
POINTS OF INTEREST ABOUT THE SKIN:
| Field | Description |
|---|
| ANATOMIST | The skin for the ANATOMIST is the first dissecting layer. |
| SURGEON | The skin for the SURGEON is the first layer to be incised on the living body. |
| PHYSIOLOGIST | The skin for the PHYSIOLOGIST is a functional membrane to control the output of body water. |
| PATHOLOGIST | The skin for the PATHOLOGIST is the part of the body more exposed to the external environment. |
| CLINICIAN | The skin for the CLINICIAN is the part of the body to be noted first on inspection to see anaemia, cyanosis, and jaundice. |
| GENETIST | The skin for the GENETIST is the part of the body, the fingerprints of which, form a useful diagnostic tool in certain diseases (subject of Dermatoglyphics). |
SUPERFICIAL FASCIA:
The subcutaneous tissue (the so called superficial fascia) is the second layer of the body beneath the skin. It is a combination of loose connective tissue and adipose tissue. Its character varies in different regions of the body.
The fatty tissue is most abundant in the buttocks and absent in the eyelids, pinna of the ear, penis and scrotum. In some regions like scalp, face and scrotum, it is characterized by presence of muscle fibers forming the subcutaneous muscles of these regions.
The subcutaneous layer is a sort of fibrous membrane attached firmly to the dermis of the skin and also to the deeper structures, like deep fascia, periosteum and the perichondrium. In it Nerves, Blood Vessels and lymphatics pass to the skin.
The layer is also termed as Hypodermis. Its main functions are:
1.Padding
2.Space filling among other anatomic structures and organs.
3.Storage of fuel in the form of fat, to be used in time of starvation.
DEEP FASCIA:
The deep fascia is the third layer, covering the body in the same manner as the skin and superficial fascia. It is the real fascia. This fascia is a sheet like connective tissue structure and is continuous everywhere in the body.
It either invests the anatomic structure or binds together two or more structures. The character of the fascia depends upon the nature of the function to be performed.
It sends partitions and thus forms fascial compartments in the body to isolate other structures, like muscles, nerves and blood vessels. The deep fascia is named according to the area of the body it covers or the function it performs. Pectoral fascia and the Extensor or flexor retinacula.
The detail of the fascia is not so simple as the general outline mentioned above. The regional description of the fascia is the detail study of LOCALISED AREA.
CLINICAL IMPORTANCE OF DEEP FASCIA AN OUTLINE:
- 1. A general understanding of the fascia of the whole body will make the clinician to know about where and how certain fluids like pus, blood accumulate, and will permit the surgeon to FIND & FOLLOW the cleavage planes for easy separation of the anatomic structures.
- 2. Since lymphatics are carried on the fascia, therefore its knowledge will aid in knowing the extent of resection in case of malignance of any organ.